Assistant professor Karl Cottenie of the University of Guelph has a bone to pick with the way some ecologists calculate an ecological community’s complexity.
Complexity is a measure of how many species can be found in a habitat and how these species interact with one another. A general rule is that complexity decreases as distance from the equator increases. For example, compare the ecological density of a tropical rainforest to that of the arctic tundra.
To understand how complex a habitat is, one can take a reductionist approach and look at a single food-chain stratus, also known as a trophic level. Alternatively, a larger local guild—all the organisms sharing a common resource or niche—can be analyzed. The challenge for ecologists is to combine trophism and local guilds to evaluate the complexity of an entire community.
This is necessary because community ecologists are interested in understanding which community-shaping factors can be linked to variations in complexity. For instance, how does the depth of a pool influence the complexity of the community living within it? This requires the ability to assess complexity under different conditions.
Many researchers in this field use the calculated value “species richness”—a number calculated from a variety of factors, including the number of species found, the area of the habitat, and other factors influencing complexity—to describe the complexity of a community. Dr. Cottenie disagrees with this approach. His work disputes the validity of the species richness term in an effort to “formalize a rant [he’s] had for the last decade.”
Cottenie explains that he can’t see the advantage of reducing a complex web of species interactions to a single number. He believes that the species richness formula ignores a great deal of information about community dynamics that is critical to a comprehensive understanding of community ecology.
In a seminar room filled with students and faculty of the Department of Ecology and Evolutionary Biology, Cottenie asked who in the audience had used species richness as a dependent variable in a graph. A few sheepish hands rose amid laughter.
“There are certain aspects of that reduction that [when performed intelligently] make sense,” he said. But throughout his talk, Cottenie tried to convince listeners that a more detailed analysis is required to properly explain community ecology.
Cottenie believes scientists should use a multi-variate approach, breaking communities down into components such as environment, the number of species that can be found around a habitat, and the connectivity to other habitats. Only then can the complexity of a community be analyzed for the diversity contributed by a number of variables. By breaking down species richness into various sub-factors, Cottenie believes that more information can be extracted about a community’s structuring forces than by reducing the set to a single “species richness” calculation.
Cottenie confessed that he is one of the few ecologists who hates field work, so his case studies draw upon work performed by collaborators, including work from the fields of Belgium, the marshlands of Spain, and the wetlands of Southern Ontario. In total, he’s analyzed 156 datasets using both the species richness and the composition approach.
Through this statistical analysis, Cottenie showed that the composition approach was often able to find patterns of variation the species richness approach would miss. He found that the composition approach is better equipped to detect the nuances of influence imparted by each of the contributing variables.
Cottenie admitted that species richness was able to predict some aspects of population variation, but it was ineffective at capturing the effects of spatial variables. Therefore, applying species richness as a dependent variable can prevent researchers from observing important processes.
“My take from this is that species richness is often fiction,” stated Cottenie.
Instead, he believes ecologists need to embrace the complexity they are trying to study, not hide behind the single, catch-all figure of species richness. To understand complexity, it must be studied as a whole.
While this analysis is useful to community ecologists trying to understand existing communities and the forces that shape them, it has practical applications as well. This type of examination is important in planning the expansion of threatened habitats and dealing with the devastating affects of invasive species.
“Community ecology is in an exciting time,” said Cottenie. Describing the discovery of new tools that make variation decomposition easier than ever, Cottenie wants to convince ecologists to banish species richness from their y-axes.
