Literature Review: Biodiversity, Evolution and Time

by Carl Strang

Today I hope to illustrate how different studies can be made to illuminate one another. The starting point is a paper that I think has the potential to be profoundly influential in the field of community ecology.

Lowland tropical forests are renowned for their high biodiversity. They also are old.

Lowland tropical forests are renowned for their high biodiversity. They also are old.

Reich, Peter B. 2012. Impacts of biodiversity loss escalate through time as redundancy fades. Science 336:589-592.

Studies of biodiversity in plant communities have suggested that the number of species climbs to a saturation point, after which additional species do not add more biomass productivity and apparently could be removed without affecting ecosystem function. In contrast, this long-term study shows that the added species refine their niches over time, and by sorting out and separating niches they eventually produce a unique portion of niche space for every species.

This result reminds us that our focus on the day-to-day or year-to-year dynamics of ecosystems needs to be tempered by the fact that ecological roles are flexible in evolutionary time. Species that begin as competitors, but which persist together, can subdivide whatever resources are the basis for their competition, as selective pressures nudge them apart. This process provides a mechanism which in part explains the results of the next featured study.

Jetz W, Fine PVA (2012) Global Gradients in Vertebrate Diversity Predicted by Historical Area-Productivity Dynamics and Contemporary Environment. PLoS Biol 10(3): e1001292. doi:10.1371/journal.pbio.1001292

(Also interpreted in an accompanying commentary by another author). They found that 80% of the variation in vertebrate species diversity between different terrestrial biomes is accounted for by a combination of ecosystem area, age, productivity and temperature, with the highest diversity in warm tropical forests. Thus evolutionary as well as ecological factors are important. Area alone provided a poor fit. Age was considered up to 55 million years, and age combined with area improved the fit greatly (for instance, grassland ecosystems are extensive but relatively young at 8 million years). Productivity and temperature separated deserts from tropical forests, for example, and further improved the model. Productive areas have more individuals, and thus greater potential for evolutionary diversification, and also have greater ecological space for niche diversification. Finally, they looked at ecological influences by dividing biomes into finer-grained divisions (down to a 110 km grid), and found that this further refined the results, again by variation in productivity of the smaller areas. This points to ecological interactions having an influence on local biodiversity.

The connection to the ideas in the first paper should be clear. The connection to the next one is less obvious.

Franzén M, Schweiger O, Betzholtz P-E (2012) Species-Area Relationships Are Controlled by Species Traits. PLoS ONE 7(5): e37359. doi:10.1371/journal.pone.0037359

They studied Lepidoptera on islands off the coast of Sweden to consider species richness-island area relationships and the niche and other ecological or physiological traits that may contribute to the overall pattern. They found a number of traits that had an impact on species-area relationships, and in general these made ecological sense. Examples of traits which were particularly sensitive to island area were low reproductive potential, small range size, narrow diet breadth, and low abundance. Thus classical island biogeography, which simply measures species area relationships and considers gross immigration and extinction rates, is improved in its explanatory power by consideration of traits that contribute to ease of immigration and resistance to extinction.

Here the communities under consideration are on islands, and once immigration has occurred the stage is set for the kinds of ecological-evolutionary processes outlined in the first paper above. This produces phenomena like the famous Darwin finches of the Galapagos, and innumerable other examples.

Moreno-Mateos D, Power ME, Comín FA, Yockteng R (2012) Structural and Functional Loss in Restored Wetland Ecosystems. PLoS Biol 10(1): e1001247. doi:10.1371/journal.pbio.1001247

Schmitz OJ (2012) Restoration of Ailing Wetlands. PLoS Biol 10(1): e1001248. doi:10.1371/journal.pbio.1001248

These two papers provide an interesting contrast in interpretations. The first is a review of studies in which wetland restorations were compared to undisturbed baselines. The authors concluded that restoration has some success but falls significantly short of the return to ecosystem structure and function that is the goal of restoration. Thus they caution that wetland destruction should not be regarded as something that can be mitigated by restoration. Schmitz looks at their results and comes to the opposite conclusion, suggesting that wetland restoration is a thoroughly successful process, but makes no attempt to reconcile his conclusion with theirs.

Here the value of the first paper at the top of this post is to remind us that there is a pitfall in thinking about results over too narrow a time frame. Restoration is valuable and good, but in the short term it cannot be thought of as completely replacing the original. There is no way to re-create all the intricacies of the co-evolved relationships Reich wrote about. It’s much better to preserve the original. Nevertheless, restoration is worth doing, and given enough time there is hope that something as good as the original will evolve. If we can at least preserve as many species as possible, that will minimize the time needed for such a dynamic endpoint to be reached.

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