Ricklefs 2004

Ricklefs 2004

A comprehensive framework for global patterns in biodiversity. 2004. Ricklefs, R. E. Ecology Letters 7: 1-15.

Blog Author: Laura Segura-Hernandez

Paper Author: 

Robert Ricklefs:

·     PhD University of Pennsylvania, Biology, 1967 

·     Curators' Professor of Biology, Department of Biology, University of Missouri at St. Louis.

·     My interests focus on diversity in ecological systems at several levels of organization and scales of time and space. A long-standing interest is the evolutionary diversification of avian life histories, emphasizing comparative and theoretical analyses of variation in life tables, including patterns of senescence, and physiological and experimental studies of growth, development, and parental care. On a higher level of ecological organization, I am interested in the historical development of ecological communities and regional species richness, using comparative analyses of diversity patterns and molecular analyses of genetic divergence and phylogenetic relationship. Through these studies, I would like to understand how factors that promote diversification, such as selection, speciation, and dispersal, are balanced by constraints that limit the response to selection or the coexistence of species.

Summary/Main points

1. Main Question: 

·     During decades, many hypothesis and explanations have been proposed in order to explain patterns of diversity in large-scales.

·     Due to limitations to prove or reject these hypotheses, biologist took divergent views regarding this matter. And for a while these two views were perceived as incompatible with each other:

·     On one side, they thought diversity was a result of historic and geographic processes. : diversity was gained after evolution processes in a long time period and at a large scale.

·     Later on, another side believed that diversity could be explained by interactions between species, that lead to define communities, and therefore, diversity patterns. However, this interactions happen at a small scale during ecological time.

·     Three things had a heavy role in supporting the second view, increasing the gap between the two explanations: 1. historical hypothesis were considered untestable. 2. ecological interactions were demonstrated (by models and experiments) to be faster at getting a steady condition than evolutionary processes at a local scale. 3. strong correlations between local ecological conditions and diversity lead to support the idea that interactions constraint diversity.

·     Goal: Bring together these two views. He states that “local and regional mechanisms that influence diversity interact on a continuum of time and space, which both creates a regional effect on diversity and establishes a relationship between regional and local diversity”. 

2. A brief story of the diversity issue:

·     Iniatially, explanations regarding diversity patterns centered on the idea of large scale processes that happened during long periods of time in large areas. By this time, diversity patterns were not really mentioned in the literature.

·     After population processes were integrated in community ecology in the mid 1960’s, the knowledge that interactions between species could shape how communities are composed, led to relate species interactions as the mechanism behind species diversity patterns.  After this ideas started to bloom, diversity patterns became a topic of interest for researchers.

·     This also lead biologist to believe that the differences between local and regional diversity could be explained by changes in the species turnover between habitats of particular regions (Beta diversity).

·     The idea of local determinism raised, thanks to models showing that local interactions rapidly reach an equilibrium. Large scale processes tend to be slow, so they were practically discarded to influence local equilibrium in local communities.

·     Local equilibrium proposes that diversity patterns are due to the influence of environment in the interaction between species. This idea has three predictions: 1. Diversity is strongly correlated with physical aspects of environmental. 2. Local diversity in comparable habitat of different regions, should be similar. 3. Local diversity should be independent of regional diversity (above ecological saturation level).

·     The way local determinism explain diversity patterns is ironic in that: 

◦     1. This idea came at a time in which ecologist were debating whether communities are closed, defined units vs. open, boundless units. The models used to explain ecological interactions assumed communities to be closed units, while the open units ideas was already more popular back then.

◦     2. As local determinism was gaining ground, MacArthur & Wilson equilibrium theory was also becoming popular, in which it proposes a strong external influence as a driver in island’s diversity. 

·     After 1960’s, ecologists views expanded to incorporate movement of individuals between communities. However, these new developments are still tied to the idea of a local equilibrium dependent on population processes within a heterogeneous setting.

3. Testing local determinism: 

The author provides evidence to refute the three predictions related to Local determinism:

·     Prediction 1:

◦     Diversity-environmental correlations can be explained from evolutionary processes as well as from diversity limitation by ecological interactions.

◦     These correlations make it difficult to reject hypothesis regarding ways in which the niche space is divided among species living in areas with different diversity. Correlations don’t imply mechanism.

◦     Also, explanations regarding climatic relationships are related to local productivity of the habitat to support species, or the physiological tolerance of organisms to environments with low energy. But this can also be explained by evolutionary processes.

·     Predictions 2 and 3 are not supported by the evidence gained so far. Prediction 2 have been argued to be untestable, however, the author disagrees. Prediction 3 has been rejected by data showing the opposite to what was predicted (but there are also disagreements regarding this test).  

4. Testing the influence of regional processes and historical events:

·     In cases where Prediction 2 have been tested, regional processes are rarely rejected. The author mentions the examples of tree species diversity, in which the diversity patterns between the different regions can be explained by their climatic history.

·     Another example addresses mangroves, in which the diversity patterns can be explained by factors driving how species can invade the mangroves instead of local determinism.

5. Reconciliation of local and regional perspectives:

·     If one accepts that diversity-environment relationships can arise historically, regional effects on local diversity can not be easily rejected and that regional anomalies can be translated to the local level, one has to reconcile both local and regional scale processes.

·     Also, there are two ideas that have hindered the advances in community ecology: 

◦     1. That local communities become ecologically saturated. However, niches are not fixed and species can be added by reducing the average niche size of the already existing species. Populations are compressible.

◦     2. That local processes come to equilibrium rapidly compared to evolutionary processes involved in species production. The author argues that we have relied too much in models that consider communities as closed, defined and simplified units, to explain natural processes. Even if these processes are present, their real role in defining diversity can not be confirmed. Also, the real time scale over which these processes operate is considerably larger than what the models predict.

·     Considering the individuals move within populations within regions, these interactions between populations can play a role over the entire region. The spatial variation in the environmental conditions within the region can define how competitors divide their niche space, through evolvedspecialization and population interactions. 

·     Hubbell (2001) proposed the possibility that species are ecologically and competitively equivalent, so their existence is not limited by competition. He proposes that diversity is defined by the equilibrium between speciation and extinction, that is also influenced by the size of the region. This hypothesis requires modification, however, as species are not equivalent, but his ideas point towards the regional/historical view.

·     Speciation and environment-specific variation in competitive ability among species have to be included in the models to make them more real. Uniqueness and variation in size of the geographical distribution of species can show the speciation of population with respect to the local factors present in the region, and variation in size can reflect the adjustment of populations to competitive pressures. 

·     The author proposes that as variation in geographical and environment extents of populations can be addresses between species within the same genus, it can not necessarily be explained by their evolutionary history but by their relationship to other biological factors, such as pathogens. He proposes that being a rare species might be beneficial in order to evade predators and pathogens.

·     Competitive exclusion can be extended to the time where species are produced. The speciation rate in within a region influences the number of species present. Regional and local diversity are connected by adjustments of the geographical and ecological extend of populations. Alfa and Beta diversity are spatially referencesampling properties of the overlapping distributions of species populations within a particular region.

·     Regarding extinction: is it really a strong force in defining diversity? Extinction can have a strong effect if they are associatedwith catastrophic environmental changes, diversification of new lineages of competitors or biotic invasions. These are highly selective: they affect some organisms more than others. 

·     Species production is related to the characteristics ofthe region such as geography or the geological processes. The  balance between species production and extinction is not stable across time within the region, therefore, species richness is unlikely to approach a steady state.

·     On the relationship between species richness and environment: environments have different extends and histories, that would lead the evolution of species and allow them to support different populations. Therefore, both contemporary-ecological origin and historical-evolutionary origins are compatibles. 

6. The relationship between diversity and environmental:

·     It is difficult to distinguished when a patterns is caused by tha capacity of the environment to support a biota, or to the ecological and evolutionary origins of the species. These two thing are not incompatible either.

·     Diversification in a regionincreases with age. This could be used to argue against local determinism.

·     To clarify the effect of extinction on diversity, one can address the phylogenetic effect of extinctions, by looking at the diversification rate before and after the extinction. 

7. Conclusions/Recommendations

·     Patterns of diversity can only be properly described when regional and historical processes are taken into account. 

·     Recommendations:

◦     Ecologist should abandon certain concepts or community ecology (local communities do not exist, except in cases of very discreet resources).

◦     Regional and historical factors should be addressed, as well as local determinism, when assessing diversity-environment relationships.

◦     Ecologist should assemble data according to the scale of their study.

◦     Ecologist should quantify population distributions within regions, according to heir environment and space.

◦     They should include that dispersal occurs within regions and focus also on how populations begin to form.

◦     They should construct phylogenies of large clades to estimate diversification rates and the relationship between speciation and extinction.

◦     They should also use phylogenies and compared them to ecological zones, including rates of diversification and shifts.

◦     Instead of continue to try and prove whether local or large-scale processes explain the diversity patterns, studies should focus on having an integrative view of both perspectives when addressing the causes of these patterns. This could lead to innovative and measurable experiments that could explain the patterns in a more complete way.

8. Questions/comments: 

1.  I liked how this paper addressed the issue of two diverging ideas regarding the same process. I am amaze at how ecologist seemed to be so stubborn and were so motivated to look at the theories as opposing processes when they could be related.

2.  I found the paper a little bit repetitive, especially in the introduction, but overall I think it was clear in its explanations.

3.  The author provides concrete suggestions on what and how studies regarding diversity patterns should be done. Or at least, he gives some ideas. I wonder if people have followed his ideas. I found the one regarding using phylogenies to assess diversification rates and the effect of extinction in diversity patterns very interesting.

4.  What do you think about his conclusion/recommendation that local communities is a concept that should be addressed only to very specific communities?