Is there more diversity in marine species?

I was reading about how life emerged from oceans, and the following question occured to me. Do land species suffer from the founder effect and is there more (genetic) diversity in marine species? A quick google search pointed to the fact that a majority of species live on land. Is that a paradox?

I know that marine habitats have a higher phylogenetic diversity (see also Faith 2006) than terrestial habitats, even though they host far fewer species, which is a result of their deeper evolutionary history. Unfortunately, I cannot find a good reference to this claim right now (see for some support though).

I wouldn't really call the difference in species numbers a paradox though, since speciation rates depend on habitat complexity and dispersal rates (which are generally higher in marine habitats). In either case, the knowledge of marine habitats and species living there are much poorer than for terrestial habitats, so differences in species numbers are highly tentative.

The estimated diversity, in this case species richness, is higher on land than in the ocean. To resume the state of knowledge of biodiversity, here's an excerpt from an open-access paper by Mora et al. (2011) How Many Species Are There on Earth and in the Ocean?

"… (our analysis) predicts ∼8.7 million (±1.3 million SE) eukaryotic species globally, of which ∼2.2 million (±0.18 million SE) are marine. In spite of 250 years of taxonomic classification and over 1.2 million species already catalogued in a central database, our results suggest that some 86% of existing species on Earth and 91% of species in the ocean still await description."

What is marine biodiversity?

Ώ] Biodiversity is an all-inclusive term to describe the total variation among living organisms of our planet. In its simplest form, biodiversity or biological diversity is therefore 'Life on Earth' and includes marine biodiversity 'Life in the Seas and Oceans.` The marine environment has a very high biodiversity because 32 out of the 33 described animal phyla are represented in there.

ΐ] Biodiversity includes four main components:

  • Genetic diversity refers to the genetic variation that occurs among members of the same species.
  • Species diversity (taxonomic diversity) refers to the variety of species or other taxonomic groups in an ecosystem.
  • Ecosystem diversity refers to the variety of biological communities found on earth. With ecosystem diversity we generally consider its two levels, that is, communities and ecosystems.
  • Functional diversity refers to the variety of biological processes, functions or characteristics of a particular ecosystem.

Genetic, species and ecosystem diversity are also often grouped as ‘structural diversity’. An example of species diversity is the number of all fish species in the North Sea genetic diversity indicates for instance the differences in genes between different populations of the same fish species and ecosystem diversity is for instance the number of communities living in different habitats/ecosystems (rocky shores, sandy beaches, soft subtidal, …).

An example of functional diversity is the number of filter feeders in an ecosystem compared to the number of grazers. Functional diversity is thought to be one of the main factors determining the long-term stability of an ecosystem and its ability to recover from major disturbances.

Biodiversity encompasses many levels of organization including genes, species, habitats, communities and ecosystems. Although species diversity is the most commonly used measure of taxonomic diversity (or diversity between types of organisms), other measures of taxonomic diversity exist, the most common of which is phyletic diversity. Phyletic diversity is the variation in the working body plans (phyla) of organisms. An example of a phylum is the Arthropoda, which includes organisms such as crabs, lobsters, shrimp as marine animals and insects and spiders as terrestrial organisms.

Why is marine biodiversity important?

Β] Marine organisms contribute to many critical processes that have direct and indirect effects on the health of the oceans and humans. What is obvious is that there are specific species and functional groups that play critical roles in important ecosystem processes, and the loss of these species may have significant influences on the whole ecosystem.

Β] Primary and secondary production are important mechanisms by which marine communities contribute to global processes. It has been estimated that half the primary production on earth is attributable to marine species. Without primary producers in surface waters, the oceans would quickly run out of food, but without planktonic and benthic organisms to facilitate nutrient cycling, the primary producers would quickly become nutrient limited.

Ώ] The marine ecosystem provide us a lot of goods and services like food provision, nutrient cycling, gas and climate regulation, … .Looking at ecosystems in terms of the goods and services they provide, allows us to realize their full value and our dependency on those systems in the broadest sense.

9 - Why are coral reef communities so diverse?

Extrinsic and intrinsic factors, physical and biological factors, and the interactions of all these are hypothesised to affect the diversity of coral reef-associated faunas. Extrinsic factors include environmental change at large spatial (regional) and temporal (geological) scales, and small (local or landscape) scale variation over ecologically relevant time. The latter also impinge on aspects intrinsic to the organisms, e.g. ecological specialisation, key innovations and life-history attributes, especially larval biology of sedentary benthic invertebrates. Certain species-rich genera and families of invertebrates and fishes contribute importantly to high species diversity of Indo-Pacific coral reefs. Naturally occurring gradients in diversity, ecology and life-history within these taxa can serve as dependent variables to test hypotheses of diversification. Gradient analyses of the most diverse genus of Indo-Pacific reef invertebrates, the gastropod Conus , support the ecological determinism and life-history hypotheses of diversity. The data do not refute other hypothesised mechanisms, which probably also operate. Broader application of gradient analysis to diverse taxa in the future may sharpen understanding of processes leading to modern patterns of diversity.

Although geographically restricted to tropical seas and occupying only 0.1% of the earth's surface, coral reefs have globally important implications for marine biodiversity. Reefs support unusually diverse animal communities with distinctive taxonomic structure and geographical distribution patterns. Coral reefs are oases of high primary productivity in barren tropical seas, and reef-building organisms have changed the face of the earth by creating entire archipelagoes of islands and, over geological time, landforms that have become incorporated in continents.

Is there more diversity in marine species? - Biology

Marine biodiversity – a vital resource

> For a long time the significance of biological diversity in the world’s oceans was unclear. It is now known to play a vital role in maintaining the functionality and productivity of ecosystems. It also makes habitats more resilient to environmental change. But the well-balanced species communities are becoming increasingly unstable.

The rapid disappearance of species

Why is marine biodiversity important?

Seagrass and kelp itself have relatively long life spans because they are poor food sources for grazing crustaceans and molluscs. They store nutrients in their biomass for a long time, including nitrogen and phosphorous compounds transported by rivers from agricultural areas to the sea. Seagrass and macroalgae thus function as a kind of biological purification system in coastal ecosystems.

Scientists have addressed the question of whether the dramatic decline in biological diversity has consequences for the stable functioning of ecosystems. After 10 years of intensive study, the answer is clear – yes, it does. Experiments in coastal ecosystems, particularly seagrass meadows and kelp forests, have shown that biological diversity in the oceans is essential for maintaining the ecosystem functions described above. Species diversity was decreased in various ways during these experiments in order to compare the ecosystem functions of species-rich with species-poor areas. In one field experiment, for example, the number of seaweed species was artificially reduced by removing some at the beginning of the growth period. The total algal biomass in this species-poor area did, in fact, decrease, thereby resulting in a decline in the food for consumers as well as the number of available habitats. In another experiment, the number of grazing species that feed on the microalgae growing on seagrass was reduced. It was found that the species-poor grazer communities consumed fewer microalgae than species-rich communities. The shortage of grazing species resulted in a slower growth of seagrass because the increased growth of microalgae repressed photosynthesis in the seagrass. These two experiments indicate that a decrease in biological diversity has a negative impact on the structure of the habitat, regardless of whether the number of species of producers (macroalgae) or consumers (grazers) is reduced. 5.11 > Hundreds of fish species live in kelp forests like this one off California. These include the yellowtail rockfish or “greenie” Sebastes flavidus.

Kelp forests Dense forests of algae where kelp is predominant are called kelp forests. These are characterized by long, thin, brown and red algae that can grow up to several metres long. Kelp forests mainly occur off the west coast of America, the coast of Argentina, the west coast of Africa, and off Australia and New Zealand. Kelp forests are unique ecosystems with characteristic species associations.

How does biological diversity work?

On the other hand, a particular ecosystem function such as grazing on seagrass is often performed by individual, very efficient species. For example, isopods and gastropods, two invertebrates that feed on algae, each have different nutritional preferences. Grazing gastropods have a strong rasp-like tongue which they use to graze on thin layers of microalgae, while isopods prefer the larger forms of filamentous algae. If the algal flora on blades of seagrass is dominated by thin growths of microalgae, then the seagrass is mostly grazed by the gastropods. If the water has a higher nutrient content, then fibrous algal forms predominate, and isopods work to keep the seagrass free of algae. Which of these two varieties of grazer performs this job depends on the ambient environmental conditions. If an ecosystem function is carried out primarily by a single species rather than several, it is referred to as the “selection effect”. The particular environment selects, so to speak, the current optimally functioning species.

Not only is the number of species important. Also significant is how many individuals of each species are present, or which species is predominant. Because of the selection effect, natural communities are usually composed of a few predominant species and a larger number of species with fewer individuals. Under stable environmental conditions, ecosystem functions, such as the creation of plant biomass, are often sustained by predominant species with optimal traits. The numerous but less abundant species play a subordinate role in these functions. But if the environmental conditions change, they are often called to task. A previously unimportant species can suddenly become predominant.

In the oceans also, a location often has only a few predominant species. There are even extreme cases where a single species prevails over all others. These ecosystems include seagrass meadows and kelp forests. In these cases biological diversity is achieved not by the abundance of species, but through the genotypic diversity of seagrass plants within a single species. Although the plants all belong to the same species, there are hidden differences in their genetic makeup.

Where in other situations species diversity sustains the ecosystem, in the seagrass meadow the genotypic diversity fills this need – that is, the invisible genetic differences between individuals of the same species. In fact, in seagrass meadows where several different genotypes were experimentally planted, the result was a greater density of shoots and a greater total biomass. The number of grazers also increased. So because of increased genotypic diversity, the general ecosystem function of biomass production was enhanced. More seagrass was present and there was an increase in food availability for predatory fish due to the abundance of grazers. Even the ecosystem’s ability to resist certain disturbances and environmental changes can be improved through genotypic diversity. In one case a seagrass area with high genotypic diversity recovered after an extreme heat wave more quickly than areas with lower diversity.

In a world experiencing climate change the diversity of less abundant species or genotypes will presumably become increasingly important. These represent a kind of potential “biological insurance” for the sustainability of ecosystem functions. They may possess as yet unknown traits or genetic information that would make them capable of adapting to the new environmental conditions, and therefore be more productive and resilient than the original predominant species or genotypes.

To what extent is biodiversity under threat?

The European bladderwrack forests, in turn, are being displaced by species-poor communities predominated by filamentous algae. Filamentous algae are a poor habitat for juvenile fish and many other organisms. For one thing, they produce less oxygen and for another, they only store nutrients for a short time because, in contrast to bladderwrack forests, they are relatively short-lived, and are a favourite food for gastropods and crustaceans. This is further exacerbated by the fact that the filamentous algae and massive phytoplankton blooms resulting from higher nutrient concentrations effectively block light from the new bladderwrack seedlings. As a result, their growth is severely hampered.
The disappearance of a species that provides an important ecosystem structure, in this case the bladderwrack, can alter the environmental conditions and thus also the habitat to the detriment of other species. One important eventual consequence is the further decline of biological diversity, so that in the future the ecosystem can no longer perform its function.

Expanding marine protected areas by 5% could boost fish yields by 20%, but there's a catch

Sweetlips shoal in the Raja Ampat marine protected area, Indonesia. Credit: SergeUWPhoto/Shutterstock

Marine protected areas, or MPAs as they're more commonly called, are very simple. Areas of the sea are set aside where certain activities—usually fishing—are banned or restricted. Ideally, these MPAs might be placed around particularly vibrant habitats that support lots of different species, like seagrass beds or coral reefs. By preventing fishing gear such as towed seabed trawls from sweeping through these environments, the hope is that marine life will be allowed to recover.

When used well, they can be very effective. MPAs have been shown to increase the diversity of species and habitats, and even produce bigger fish within their bounds. A new study argues that by expanding the world's MPAs by just 5%, we could boost future fish catches by at least 20%. This could generate an extra nine to 12 million tons of seafood per year, worth between USD$15-19 billion. It would also significantly increase how much nutritious fish protein is available for a growing human population to eat.

Spillover versus blowback

The scientific rationale is sound. We already know that MPAs can increase the numbers of fish living inside them, which grow to be bigger and lay more eggs. The larvae that hatch can help seed fish populations in the wider ocean as they drift outside the MPA, leading to bigger catches in the areas where fishing is still permitted. We know fish can swim large distances as adults too. While some find protection and breed inside MPAs, others will move into less crowded waters outside where they can then be caught. Together, these effects are known as the spillover benefits of MPAs.

The study is the first to predict, through mathematical modeling, that a modest increase in the size of the world's MPAs could swell global seafood yields as a result of this spillover. But while the predictions sound good, we have to understand what pulling this off would entail.

A minority of the world’s MPAs are strict no-take zones. Credit: Marine Conservation Institute/Wikipedia, CC BY-SA

The study maintains that the new MPAs would need to be carefully located to protect areas that are particularly productive. Locating MPAs in remote areas offshore, which are hard to access and typically unproductive, would have much smaller benefits for marine life than smaller, inshore MPAs that local fishing vessels can reach. Just 20 large sites in the remote open ocean account for the majority of the world's MPAs. As the low hanging fruit of marine conservation, these MPAs are often placed where little fishing has occurred.

The MPAs themselves would also need to be highly protected, meaning no fishing. Only 2.4% of the world's ocean area has this status. Increasing this by a further 5% would mean roughly trebling the coverage of highly protected MPAs, and that's likely to provoke a great deal of resistance. Many fishers are skeptical that spillover can boost catches enough to compensate for losing the right to fish within MPAs and tend to oppose proposals to designate more of them.

People in the UK are often surprised to learn that fishing is allowed in most of the country's MPAs. While 36% of the waters around the UK are covered by them, only 0.0024% ban fishing outright. Increasing the number and size of highly protected MPAs from just these four small sites to 5% of the UK's sea area would represent more than a 2,000-fold increase. This would be strongly resisted by the fishing industry, snatching the wind from the sails of any political effort ambitious enough to attempt it.

Keeping fishers on board

Gaining the support of local fishers is crucial for ensuring fishing restrictions are successful. That support depends on fishers being able to influence decisions about MPAs, including where they'll be located and what the degree of protection will be. Assuming that designing highly protected MPA networks is mostly a matter of modeling is a mistake, and implies that fishers currently operating in an area would have little say in whether their fishing grounds will close.

But this study is valuable. It provides further evidence for how MPAs can serve as important tools to conserve marine habitats, manage fisheries sustainably and make food supplies more secure. It's important to stress the political challenges of implementing them, but most scientists agree that more MPAs are needed. Some scientists are pushing to protect 30% of the ocean by 2030.

As evidence of the benefits of MPAs continues to emerge, the people and organizations governing them at local, national and international scales need to learn and evolve. If we can start implementing some highly protected MPAs, we can gather more evidence of their spillover benefits. This could convince more fishers of their vital role in boosting catches, as well as keeping people fed and restoring ocean ecosystems.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The secret behind coral reef diversity? Time, lots of time

Strap on a diving mask and fins and slip under the crystal-clear water near a coral reef in Indonesia, Papua-New Guinea or the Philippines, and you'll immediately see why divers and snorkelers from across the world flock to the area. Known as the Coral Triangle, the region is famous for its unmatched diversity of reef fish and other marine creatures.

Fish of all shapes and colors dart in and out of crevices created by the dazzling shapes of corals, colorful sponges and other reef-building organisms. With a little luck, a diver might catch a glimpse of a shark patrolling the reef or a turtle soaring across the landscape of colors.

While underwater enthusiasts have long known and cherished the biodiversity in the Central-Indo Pacific Ocean, scientists have struggled for more than half a century to explain what exactly makes the region the world's No. 1 hot spot of marine biodiversity and sets it apart from other marine regions around the world.

Several hypotheses have been put forth to explain the Central-Indo Pacific region's extraordinary diversity. Some researchers suggested species emerge at a faster rate there compared to other parts of the world's oceans, while others attributed it to the region's central location between several species-rich swaths of ocean in the broader Indo-West Pacific. Still others pointed to the region's low extinction rates.

Now, a study led by University of Arizona doctoral student Elizabeth Miller has revealed that Indo-Pacific coral reefs have accumulated their unrivaled richness of fish species not because of some unknown, elusive quality, but simply because they had the time.

"People used to think that new species evolve more quickly in tropical marine areas, so you get the high diversity we see today very quickly," Miller said. "Instead, we found that diversity in the Central-Indo Pacific has slowly built over a long time."

The study, published in the journal Proceedings of the Royal Society of London on Oct. 10, is the first to show a direct link between time and species richness, according to Miller.

Until now, Miller explained, it was widely believed that tropical coral reefs, similar to tropical rain forests, are hot spots of biodiversity because of an intrinsic propensity to diversify into more species than other regions. Her research showed that wasn't the case.

The team discovered that speciation rates are actually higher in cold marine areas such as the Arctic and Antarctic. However, while changes in biodiversity in the Central-Indo Pacific region could be compared to a slow but long-burning flame, in colder ocean regions, they are more like fireworks.

"There, species evolve relatively quickly, but each glaciation period clears out much of what was there before," Miller said. "Once the glaciers recede, they leave empty niches waiting to be repopulated by new species."

Frequent environmental upheaval results in overall biodiversity being lower in colder ocean regions.

In the Coral Triangle, on the other hand, new species have evolved less rapidly, but because conditions have been much more stable over long periods of geological time, they were more likely to stick around once they appeared and slowly accumulate to the biological diversity we see today.

"This suggests that a region may need long-term stability to accumulate high species diversity," Miller said. "According to our study, the magic number appears to be 30 million years."

In the Central-Indo Pacific, plate tectonics created a wide platform of shallow ocean, while its central location made it a target for colonization. It was the right place at the right time for the fishes that colonized the region.

"Things haven't changed much there in the past 30 to 35 million years," Miller said. "In contrast, other marine regions, such as the Caribbean, underwent periods of instability and isolation, and therefore fewer colonizations and higher rates extinction of the lineages that were there previously -- all those factors add up to less evolutionary time."

For the study, Miller and her team used distribution data of almost all spiny ray-finned fishes -- 17,453 species in total, representing about 72 percent of all marine fishes and about 33 percent of all freshwater fishes. They used several different statistical methods to reconstruct the causes of underlying species richness patterns among global marine regions.

To disentangle how marine fish diversity unfolded over time, the team then used a published evolutionary tree of this fish group and performed biogeographic reconstructions.

"Biogeographic reconstructions help us understand where ancestors were living at various places back in time, based on where species live today and how they are related," Miller said. "It's easy if you only compare two species that live in the same place, but if you have thousands of species and go back further and further in time, more ancestors come into play and things become more difficult."

Evolutionary biologists rely on sophisticated computer algorithms to manage and interpret the extremely large data sets. The method used by Miller and her team created many hypothetical scenarios of where species evolved. The researchers then used these scenarios to test how different models explain today's biodiversity.

"It's like drawing family histories, each slightly different," Miller said. "You start out with analyses and repeat them hundreds of times, each time based on some possible history to try and encompass uncertainty to see how they play out. In our study, it turned out the uncertainty is low, which is reassuring. It means it's a really robust result."

The general idea that patterns of diversity can be explained by how long a group has been present rather than how quickly they proliferate is relevant to lots of different systems, according to the researchers. For example, biologists have observed that the timing of colonization explains the high diversity of certain animal groups in terrestrial ecosystems, such as treefrogs in the Amazon rainforests, salamanders in the Appalachian Mountains and lizards in the desert Southwest.

"The general takeaway is that these patterns of high diversity may take tens of millions of years to arise, but can be wiped out in a few years by human impacts," said John Wiens, senior author of the paper and a professor in the UA Department of Ecology and Evolutionary Biology. "Unfortunately, the high diversity of reef fish in the Coral Triangle may soon disappear because of the impacts of human-induced climate change on coral reefs. The diversity that gets lost in the next few years may take tens of millions of years to get back."

The classification of marine organisms is always in flux. As scientists discover new species, learn more about the genetic makeup of organisms, and study museum specimens, they debate how organisms should be grouped. More information about the major groups of marine animals and plants is listed below.

Some of the most well-known marine phyla are listed below. You can find a more complete list here. The marine phyla listed below are drawn from the list on the World Register of Marine Species.

    - this phylum contains segmented worms. An example of a segmented marine worm is the Christmas tree worm. - Arthropods have a segmented body, jointed legs and a hard exoskeleton for protection. This group includes lobsters and crabs.
  • Chordata - Humans are in this phylum, which also includes marine mammals (cetaceans, pinnipeds, sirenians, sea otters, polar bears), fish, tunicates, seabirds and reptiles. - This is a diverse phylum of animals, many of whom have stinging structures called nematocysts. Animals in this phylum include corals, jellyfish, sea anemones, sea pens and hydras.
  • Ctenophora - These are jelly-like animals, such as comb jellies, but they don't have stinging cells. - This is one of my favorite phylums. It includes such beautiful animals as sea stars, brittle stars, basket stars, sand dollars and sea urchins.
  • Mollusca - This phylum includes snails, sea slugs, octopuses, squids, and bivalves such as clams, mussels and oysters. - This phylum includes sponges, which are living animals. They can be very colorful and come in a diverse array of shapes and sizes.


The LDG is a noticeable pattern among modern organisms that has been described qualitatively and quantitatively. It has been studied at various taxonomic levels, through different time periods and across many geographic regions (Crame 2001). The LDG has been observed to varying degrees in Earth's past, possibly due to differences in climate during various phases of Earth's history. Some studies indicate that the LDG was strong, particularly among marine taxa, while other studies of terrestrial taxa indicate the LDG had little effect on the distribution of animals. [2]

Although many of the hypotheses exploring the latitudinal diversity gradient are closely related and interdependent, most of the major hypotheses can be split into three general hypotheses.

Spatial/Area hypotheses Edit

There are five major hypotheses that depend solely on the spatial and areal characteristics of the tropics.

Mid-domain effect Edit

Using computer simulations, Cowell and Hurt (1994) and Willing and Lyons (1998) first pointed out that if species’ latitudinal ranges were randomly shuffled within the geometric constraints of a bounded biogeographical domain (e.g. the continents of the New World, for terrestrial species), species' ranges would tend to overlap more toward the center of the domain than towards its limits, forcing a mid-domain peak in species richness. Colwell and Lees (2000) called this stochastic phenomenon the mid-domain effect (MDE), presented several alternative analytical formulations for one-dimensional MDE (expanded by Connolly 2005), and suggested the hypothesis that MDE might contribute to the latitudinal gradient in species richness, together with other explanatory factors considered here, including climatic and historical ones. Because "pure" mid-domain models attempt to exclude any direct environmental or evolutionary influences on species richness, they have been claimed to be null models (Cowell et al. 2004, 2005). On this view, if latitudinal gradients of species richness were determined solely by MDE, observed richness patterns at the biogeographic level would not be distinguishable from patterns produced by random placement of observed ranges called dinosures(Colwell and Lees 2000). Others object that MDE models so far fail to exclude the role of the environment at the population level and in setting domain boundaries, and therefore cannot be considered null models (Hawkins and Diniz-Filho 2002 Hawkins et al. 2005 Zapata et al. 2003, 2005). Mid-domain effects have proven controversial (e.g. Jetz and Rahbek 2001, Koleff and Gaston 2001, Lees and Colwell, 2007, Romdal et al. 2005, Rahbek et al. 2007, Storch et al. 2006 Bokma and Monkkonen 2001, Diniz-Filho et al. 2002, Hawkins and Diniz-Filho 2002, Kerr et al. 2006, Currie and Kerr, 2007). While some studies have found evidence of a potential role for MDE in latitudinal gradients of species richness, particularly for wide-ranging species (e.g. Jetz and Rahbek 2001, Koleff and Gaston 2001, Lees and Colwell, 2007, Romdal et al. 2005, Rahbek et al. 2007, Storch et al. 2006 Dunn et al. 2007) [5] [8] others report little correspondence between predicted and observed latitudinal diversity patterns (Bokma and Monkkonen 2001, Currie and Kerr, 2007, Diniz-Filho et al. 2002, Hawkins and Diniz-Filho 2002, Kerr et al. 2006).

Geographical area hypothesis Edit

Another spatial hypothesis is the geographical area hypothesis (Terborgh 1973). It asserts that the tropics are the largest biome and that large tropical areas can support more species. More area in the tropics allows species to have larger ranges and consequently larger population sizes. Thus, species with larger ranges are likely to have lower extinction rates (Rosenzweig 2003). Additionally, species with larger ranges may be more likely to undergo allopatric speciation, which would increase rates of speciation (Rosenzweig 2003). The combination of lower extinction rates and high rates of speciation leads to the high levels of species richness in the tropics.

A critique of the geographical area hypothesis is that even if the tropics is the most extensive of the biomes, successive biomes north of the tropics all have about the same area. Thus, if the geographical area hypothesis is correct these regions should all have approximately the same species richness, which is not true, as is referenced by the fact that polar regions contain fewer species than temperate regions (Gaston and Blackburn 2000). To explain this, Rosenzweig (1992) suggested that if species with partly tropical distributions were excluded, the richness gradient north of the tropics should disappear. Blackburn and Gaston 1997 tested the effect of removing tropical species on latitudinal patterns in avian species richness in the New World and found there is indeed a relationship between the land area and the species richness of a biome once predominantly tropical species are excluded. Perhaps a more serious flaw in this hypothesis is some biogeographers suggest that the terrestrial tropics are not, in fact, the largest biome, and thus this hypothesis is not a valid explanation for the latitudinal species diversity gradient (Rohde 1997, Hawkins and Porter 2001). In any event, it would be difficult to defend the tropics as a "biome" rather than the geographically diverse and disjunct regions that they truly include.

The effect of area on biodiversity patterns has been shown to be scale-dependent, having the strongest effect among species with small geographical ranges compared to those species with large ranges who are affected more so by other factors such as the mid-domain and/or temperature. [5]

Species-energy hypothesis Edit

The species energy hypothesis suggests the amount of available energy sets limits to the richness of the system. Thus, increased solar energy (with an abundance of water) at low latitudes causes increased net primary productivity (or photosynthesis). This hypothesis proposes the higher the net primary productivity the more individuals can be supported, and the more species there will be in an area. Put another way, this hypothesis suggests that extinction rates are reduced towards the equator as a result of the higher populations sustainable by the greater amount of available energy in the tropics. Lower extinction rates lead to more species in the tropics.

One critique of this hypothesis has been that increased species richness over broad spatial scales is not necessarily linked to an increased number of individuals, which in turn is not necessarily related to increased productivity. [9] Additionally, the observed changes in the number of individuals in an area with latitude or productivity are either too small (or in the wrong direction) to account for the observed changes in species richness. [9] The potential mechanisms underlying the species-energy hypothesis, their unique predictions and empirical support have been assessed in a major review by Currie et al. (2004). [10]

The effect of energy has been supported by several studies in terrestrial and marine taxa. [7]

Climate harshness hypothesis Edit

Another climate-related hypothesis is the climate harshness hypothesis, which states the latitudinal diversity gradient may exist simply because fewer species can physiologically tolerate conditions at higher latitudes than at low latitudes because higher latitudes are often colder and drier than tropical latitudes. Currie et al. (2004) [10] found fault with this hypothesis by stating that, although it is clear that climatic tolerance can limit species distributions, it appears that species are often absent from areas whose climate they can tolerate.

Climate stability hypothesis Edit

Similarly to the climate harshness hypothesis, climate stability is suggested to be the reason for the latitudinal diversity gradient. The mechanism for this hypothesis is that while a fluctuating environment may increase the extinction rate or preclude specialization, a constant environment can allow species to specialize on predictable resources, allowing them to have narrower niches and facilitating speciation. The fact that temperate regions are more variable both seasonally and over geological timescales (discussed in more detail below) suggests that temperate regions are thus expected to have less species diversity than the tropics.

Critiques for this hypothesis include the fact that there are many exceptions to the assumption that climate stability means higher species diversity. For example, low species diversity is known to occur often in stable environments such as tropical mountaintops. Additionally, many habitats with high species diversity do experience seasonal climates, including many tropical regions that have highly seasonal rainfall (Brown and Lomolino 1998).

Historical/Evolutionary hypotheses Edit

There are three main hypotheses that are related to historical and evolutionary explanations for the increase of species diversity towards the equator.

The historical perturbation hypothesis Edit

The historical perturbation hypothesis proposes the low species richness of higher latitudes is a consequence of an insufficient time period available for species to colonize or recolonize areas because of historical perturbations such as glaciation (Brown and Lomolino 1998, Gaston and Blackburn 2000). This hypothesis suggests that diversity in the temperate regions has not yet reached equilibrium and that the number of species in temperate areas will continue to increase until saturated (Clarke and Crame 2003).

The evolutionary rate hypothesis Edit

The evolutionary rate hypothesis argues higher evolutionary rates in the tropics have caused higher speciation rates and thus increased diversity at low latitudes (Cardillo et al. 2005, Weir & Schluter 2007, Rolland et al. 2014). Higher evolutionary rates in the tropics have been attributed to higher ambient temperatures, higher mutation rates, shorter generation time and/or faster physiological processes (Rohde 1992, Allen et al. 2006), and increased selection pressure from other species that are themselves evolving. [11] Faster rates of microevolution in warm climates (i.e. low latitudes and altitudes) have been shown for plants (Wright et al. 2006), mammals (Gillman et al. 2009) and amphibians (Wright et al. 2010). Based on the expectation that faster rates of microevolution result in faster rates of speciation, these results suggest that faster evolutionary rates in warm climates almost certainly have a strong influence on the latitudinal diversity gradient. More research needs to be done to determine whether or not speciation rates actually are higher in the tropics. Understanding whether extinction rate varies with latitude will also be important to whether or not this hypothesis is supported (Rolland et al. 2014).

The hypothesis of effective evolutionary time Edit

The hypothesis of effective evolutionary time assumes that diversity is determined by the evolutionary time under which ecosystems have existed under relatively unchanged conditions, and by evolutionary speed directly determined by effects of environmental energy (temperature) on mutation rates, generation times, and speed of selection (Rohde 1992). It differs from most other hypotheses in not postulating an upper limit to species richness set by various abiotic and biotic factors, i.e., it is a nonequilibrium hypothesis assuming a largely non-saturated niche space. It does accept that many other factors may play a role in causing latitudinal gradients in species richness as well. The hypothesis is supported by much recent evidence, in particular, the studies of Allen et al. (2006) and Wright et al. (2006).

Biotic hypotheses Edit

Biotic hypotheses claim ecological species interactions such as competition, predation, mutualism, and parasitism are stronger in the tropics and these interactions promote species coexistence and specialization of species, leading to greater speciation in the tropics. These hypotheses are problematic because they cannot be the ultimate cause of the latitudinal diversity gradient as they fail to explain why species interactions might be stronger in the tropics. An example of one such hypothesis is the greater intensity of predation and more specialized predators in the tropics has contributed to the increase of diversity in the tropics (Pianka 1966). This intense predation could reduce the importance of competition (see competitive exclusion) and permit greater niche overlap and promote higher richness of prey. Some recent large-scale experiments suggest predation may indeed be more intense in the tropics, [12] [13] although this cannot be the ultimate cause of high tropical diversity because it fails to explain what gives rise to the richness of the predators in the tropics. Interestingly, the largest test of whether biotic interactions are strongest in the tropics, which focused on predation exerted by large fish predators in the world's open oceans, found predation to peak at mid-latitudes. Moreover, this test further revealed a negative association of predation intensity and species richness, thus contrasting the idea that strong predation near the equator drives or maintains high diversity. [14] Other studies have failed to observe consistent changes in ecological interactions with latitude altogether (Lambers et al. 2002), [1] suggesting that the intensity of species interactions is not correlated with the change in species richness with latitude. Overall, these results highlight the need for more studies on the importance of species interactions in driving global patterns of diversity.

There are many other hypotheses related to the latitudinal diversity gradient, but the above hypotheses are a good overview of the major ones still cited today. It is important to note that many of these hypotheses are similar to and dependent on one another. For example, the evolutionary hypotheses are closely dependent on the historical climate characteristics of the tropics.

The generality of the latitudinal diversity gradient Edit

An extensive meta-analysis of nearly 600 latitudinal gradients from published literature tested the generality of the latitudinal diversity gradient across different organismal, habitat and regional characteristics. [1] The results showed that the latitudinal gradient occurs in marine, terrestrial, and freshwater ecosystems, in both hemispheres. The gradient is steeper and more pronounced in richer taxa (i.e. taxa with more species), larger organisms, in marine and terrestrial versus freshwater ecosystems, and at regional versus local scales. The gradient steepness (the amount of change in species richness with latitude) is not influenced by dispersal, animal physiology (homeothermic or ectothermic) trophic level, hemisphere, or the latitudinal range of study. The study could not directly falsify or support any of the above hypotheses, however, results do suggest a combination of energy/climate and area processes likely contribute to the latitudinal species gradient. Notable exceptions to the trend include the ichneumonidae, shorebirds, penguins, and freshwater zooplankton.

Data robustness Edit

One of the main assumptions about LDGs and patterns in species richness is that the underlying data (i.e. the lists of species at specific locations) are complete. However, this assumption is not met in most cases. For instance, diversity patterns for blood parasites of birds suggest higher diversity in tropical regions, however, the data may be skewed by undersampling in rich faunal areas such as Southeast Asia and South America. [15] For marine fishes, which are among the most studied taxonomic groups, current lists of species are considerably incomplete for most of the world's oceans. At a 3° (about 350 km 2 ) spatial resolution, less than 1.8% of the world's oceans have above 80% of their fish fauna currently described. [16]

Conclusion Edit

The fundamental macroecological question that the latitudinal diversity gradient depends on is "What causes patterns in species richness?". Species richness ultimately depends on whatever proximate factors are found to affect processes of speciation, extinction, immigration, and emigration. While some ecologists continue to search for the ultimate primary mechanism that causes the latitudinal richness gradient, many ecologists suggest instead this ecological pattern is likely to be generated by several contributory mechanisms (Gaston and Blackburn 2000, Willig et al. 2003, Rahbek et al. 2007). For now, the debate over the cause of the latitudinal diversity gradient will continue until a groundbreaking study provides conclusive evidence, or there is general consensus that multiple factors contribute to the pattern.

While still ostensibly ‘on leave’ (side note: Does any scientist really ever take a proper holiday? Perhaps a subject for a future blog post), I cannot resist the temptation to blog about our lab’s latest paper that just came online today. In particular, I am particularly proud of Dr Camille Mellin, lead author of the study and all-round kick-arse quantitative ecologist, who has outdone herself on this one.

Today’s subject is one I’ve touched on before, but to my knowledge, the relationship between ‘diversity’ (simply put, ‘more species’) and ecosystem resilience (i.e., resisting extinction) has never been demonstrated so elegantly. Not only is the study elegant (admission: I am a co-author and therefore my opinion is likely to be biased toward the positive), it demonstrates the biodiversity-stability hypothesis in a natural setting (not experimental) over a range of thousands of kilometres. Finally, there’s an interesting little twist at the end demonstrating yet again that ecology is more complex than rocket science.

Despite a legacy of debate, the so-called diversity-stability hypothesis is now a widely used rule of thumb, and its even implicit in most conservation planning tools (i.e., set aside areas with more species because we assume more is better). Why should ‘more’ be ‘better’? Well, when a lot of species are interacting and competing in an ecosystem, the ‘average’ interactions that any one species experiences are likely to be weaker than in a simpler, less diverse system. When there are a lot of different niches occupied by different species, we also expect different responses to environmental fluctuations among the community, meaning that some species inherently do better than others depending on the specific disturbance. Species-rich systems also tend to have more of what we call ‘functional redundancy‘, meaning that if one species providing an essential ecosystem function (e.g., like predation) goes extinct, there’s another, similar species ready to take its place.

The evidence is out there, so why am I so chuffed about our latest paper? Much of the past evidence has focussed solely on alpha diversity (a simple inventory of how many species are in a particular location), which is not often a great measure of biodiversity. Indeed, using alpha diversity to quantify the diversity-stability hypothesis is potentially problematic because sampling problems mean that ‘missed’ (undetected) species might degrade the true underlying relationship.

Instead, we used a measure of species ‘turnover’ (beta diversity), which has been largely overlooked in the past, to determine whether the relationship was supported broadly across an entire landscape – in this case, the entire breadth of Australia’s Great Barrier Reef. Based on the hypothesis that greater beta diversity should lead to a wider range of responses to environmental fluctuations over time, there should be a stabilisation in the entire community, with fewer invasions, disease events or die-offs from extreme climatic events. Examining reef fish beta diversity across the Reef, we show that across most species groups, greater spatial turnover leads to lower temporal fluctuations in abundance over 16 years of data collection.

For the sticklers amongst you, we did also use several different metrics of spatial and temporal turnover, and we controlled for any potential alpha diversity sampling artefacts and scale dependencies. In essence, the relationship is robust, meaning that the more fish species out there (well, the faster communities change in constituent species as you move horizontally across the Reef), the less likely they are to fluctuate in population size over time, meaning that there is a lower likelihood of going extinct.

Now for that enticing anomaly I mentioned earlier – in what situation(s) did the relationship not follow expectations? Although it generally held true when we looked at all species groups together, we did find that for one family of fishes (the Acanthuridae surgeonfishes) the relationship was reversed! In other words, when there was high surgeonfish turnover, there was MORE temporal fluctuation in abundance.

How could this be? Well, surgeonfishes are highly mobile roving grazers of seaweeds that can respond quickly to changes in resources. As such, they are good opportunists that can swoop in and change the entire coral reef structure by munching away the colonising algae, thereby increasing the niches available for other fish. This ‘roving grazer’ life history makes them a key component of many near-shore coral reefs exposed to high environmental variability.

Finally, we also tested the expectation that areas conserved for their species richness in the Great Barrier Reef would generally have lower temporal fluctuation (i.e., greater resilience), as is assumed under most reserve-selection criteria. While spatial turnover was indeed higher in protected areas, temporal turnover was only slightly lower in these for all species considered. As expected, surgeonfish turnover was actually higher in protected areas. This means that protected areas are in general doing their intended job, but not necessarily for all species.

I think that’s enough to whet the appetite, but do read the article for more juicy ecological meat if you’re keen. Back to my ‘holiday’.

Ecological Mechanisms that Contributes to Biodiversity

A number of simple and complex ecological mechanisms contribute to biodiversity. According to one view, the so – called local or deterministic view, the biodiversity is determined principally by biological interactions such as competition and predation.

The other view considers the importance of other environmental factors such as soil type, moisture, temperature, gradient, productivity, niche and habitat diversity and some other factors like stability, disturbance, immigration, extinction and species differentiation and movement at the regional level and the interaction between local and regional processes.

Thus, there are a variety of determinants of species richness, some of which are discussed below.

1. Competition:

The ecological effects of inter specific competition are too many. These effects, however, depend on the scale of competition. On a small scale we can observe the co-occurrence of only a few species with complementary ecological niches but on a broad scale the community will contain more species, occurring within a patchwork, with each patch supporting only a few. In nature, competition is often avoided by differential resource utilization, e.g., different species of fish feeding at different depths.

When exotic animal or plant species are introduced to a new habitat, they sometimes prove to be better competitors and many indigenous species suffer. For example, introduction of common carp to some reservoirs of Punjab and elsewhere has adversely affected the native populations of major carps.

In brief, the significance of inter specific competition depends on how widespread are its evolutionary and ecological consequences. Inter specific competition tends to affect communities and their biodiversity in many ways some of which are less understood even today.

2. Predation:

Predation affects both prey populations and whole ecological communities. When predation promotes the coexistence of species among which there would otherwise be competitive exclusion, this is called predator- mediated coexistence. The effect of predation on a group of competing species depends on which species suffer most. If it is subordinate species, then these may be driven to extinction and therefore the total number of species in the community will decrease.

However, if dominant species suffer most, heavy predation will create space and resources for other species, consequently species numbers may then increase. The numbers of species in a community are usually maximum at intermediate levels of predation.

The effects of predation on biodiversity have been extensively studied in aquatic systems, in which the introduction of a predatory fish, starfish or salamander can greatly change the community structure of primary producers and consumers (carpenter et al. 1987,1988). The effect of herbivore on plant species diversity has also been studied.

The pest pressure hypothesis suggests that seedlings are most dense close to the parent tree, but their survival is maximum at a distance from the parent, because herbivores will be more common among the dense seedlings adjacent to parent tree. Several authors suggested that herbivore promotes more diversity in tropical forests (Clark and Clark, 1984).

3. Productivity:

Nutrient input and productivity affect biodiversity. Plant productivity often depends on the nutrient or condition which is most limiting to growth. Animal productivity also depends on resource levels and some other key factors such as temperature and moisture (for terrestrial environments) and temperature, dissolved oxygen and depth for aquatic systems. However, these are not the only factors that affect productivity.

It was suggested by Connell and Orias (1964) that biodiversity should be highest in relatively stable habitats having high productivity. This is called productivity-stability hypothesis (Tilman,1982). Tilman and Pacala (1993) suggested that biodiversity does not increase monotonically with productivity for any group of species, but that species richness varies depending on what environmental factor is used as a measure of productivity and which species are being taken into consideration.

It has been also suggested that predator-prey ratios increase with the increase in productivity. However, at high productivity levels, predators consume a disproportionate share of the available production, thereby causing a decline in community biodiversity. The intertaxon competition hypothesis (Rosenweig and Abramsk 1993) holds that the peaks of species diversity for different multispecies taxa should occur in areas showing different productivity levels. But, Tilman’s (1982) hypothesis suggested that habitat heterogeneity increases with productivity to a certain point only after which it decreases. However, increase in biodiversity with productivity is not a universal phenomenon.

4. Spatial Heterogeneity:

Spatially heterogeneous habitats offer a wide spectrum of resources and food chains. Accordingly, they are expected to support more species as they provide a greater variety of microhabitats (spatial niches), a greater range of microclimates, more types of places to hide from predators and a greater variety of trophic niches.

Gould and Walker (1997) have shown a positive relationship between number of vascular plant species and index of spatial heterogeneity (ranging from 0 to 1), based on a number of things including soil pH, slope, drainage pattern and substrate types. There are several studies indicating a positive relationship between animal species richness and plant spatial heterogeneity.

5. Climate:

The effects of climatic variation on species richness depend on whether the variation is unpredictable or predictable. In seasonally changing environment, different species may occur at different times of the year. Therefore, more species may be expected living together in a seasonal environment than a completely constant one.

For example, in temperate regions different annual plants germinate, grow, flower and produce seeds at different times during a seasonal cycle. However, there is no firm relationship between species richness and climatic instability. Stable climate is likely to support more species. Tropics often showing better climatic regulation are richer in species than temperate regions.

6. Harshness of Environment:

An environment may be called harsh or extreme if organisms are unable to live there. However, some organisms do occur in very cold and very hot environments and grossly polluted rivers and lakes. But the distribution pattern of organisms generally indicates that species richness is quite lower in harsh environments. Many studies have indicated that diversity of benthic macro-invertebrates and fish was quite low in streams and lakes having low pH.

Most caves and hot springs also exhibit low biodiversity. The deepest parts of the oceans (200 m to 8000 m) also have few species of fish such as tripod fish and lizard fish adapted to living in complete darkness, low temperature and very high pressures, some of them showing interesting specialization of eyes and luminous organs.

7. Disturbance:

Many communities experience periodic physical disturbance. Anthropogenic activities that alter habitat characteristics also affect local species diversity. The intermediate disturbance hypothesis (Connell, 1978) suggested that communities are expected to have more species when the frequency of disturbance is neither too high nor too low. This hypothesis was proposed to account for patterns of species richness in tropical rain forests and coral reefs.

In upland streams disturbances are created by a number of factors including water diversion for fishing or other purposes and quarrying. These activities affect indigenous fish species, benthic invertebrates and riparian vegetation. Sometimes flash floods with enormous gushing waters carrying tons of silt load play havoc with the natural communities as the entire river bed is destroyed and boulders and fish species are washed off by speedily flowing water (Singh and Badola, 1980, Sharma and Singh 1980). Disturbance by municipal sewage also affects biological diversity (Nautiyal et al 1996,2000).

Townsend et al (1997) observed that the pattern of richness of macro-invertebrate species conformed with the intermediate disturbance hypothesis. Disturbances often keep the community in early stages of succession and therefore poor diversity.

8. Other Factors:

Other factors and mechanisms that affect biodiversity are community succession, latitudinal gradient, altitudinal gradient and depth, immigration, emigration, extinction, and evolutionary age (time) and evolutionary adaptations.

The communities may differ in species diversity because some are closer to ecological or evolutionary equilibrium and others are still evolving. For example, tropics are richer in species than temperate regions because, among other things, tropics have existed over long periods of evolutionary time, whereas the temperate regions are still recovering from the Pleistocene glaciations.

Latitudinal gradients of diversity are quite obvious, species of plants, and animals increasing toward the equator. For example, within a small region at 60° north latitude one might found 10 species of ants at 40°, there may be 50 to 100 species and in a similar area within 20° of the equator, between 100 to 200 species (Ricklefs and Miller, 2000).

Diversity in marine environments follows a similar trend. This increase in diversity from the poles to the tropics has been attributed to a number of factors including greater predation, productivity, light, temperature and water regimes.

Altitudinal gradients (Singh and Nautiyal 1990) and depth also affect species richness. In hillstreams, low diversity of macro-invertebrates was observed at higher altitudes. In terrestrial environments, a decrease in species richness with altitude is a widespread phenomenon.

Bird, mammal and vascular plant species richness declined with the increase in altitude in Himalayan mountains of Nepal (Hunter and Yonzon, 1992 Whittaker,1977). Singh and Kumar (2003) found no fish species in Garhwal hillstreams at high altitudes (2400 to 3600 m). Number of species of phytoplankton, zooplankton and fish also tend to decline with increasing depth in the ocean.

Isolation and extinctions have also played their role in affecting species richness in different parts of this earth. The extinctions of many large animals in the Pleistocene may reflect the role of human migration. It is well known that over the past 30,000 to 40,000 years, a major loss of animal biodiversity has occurred over Australia, North America, New Zealand and Madagascar.

What are Marine Water Animals?

Animals that live in marine water ecosystems are called marine animals. Enormous number of marine animal species are found in the ocean and seas than in any other ecosystem on Earth. Among the various ecosystems found in Open Ocean and deep sea, coral reef ecosystems contain the greatest number of species diversity than anywhere else in the ocean. Marine invertebrates including crabs, worms, mollusks, corals, jellyfish, etc. are found abundantly in marine ecosystems. Bony fishes and cartilaginous fishes, turtles, dolphins and whales are the marine vertebrates found in marine ecosystems. Unlike the freshwater animals, surroundings of marine animals have very high amount of salts. Because of the high concentration of salts, osmoregulators living in marine waters face the biggest problem of dehydration (water loss).To avoid this problem, these creatures uptake large amount of sea water and expel salt in that water across their gills and skin. In addition, the marine fishes expel a large amount of calcium, magnesium and sulfate ions with very small amount of water through the urine.