A research team led by biology professor Mark A. McPeek recently provided evidence arguing for neutral community dynamics involved in shaping insect assemblages. The findings were published in Ecology.

The mechanisms that create and maintain high levels of biological diversity observed in many communities still remain elusive. A widely used ecological theory argues that “niche” differences between species promote their coexistence. Abundance of each species is dictated by their ability to respond to various density-dependent ecological interactions with other community members. Phenotypic differences, thus, cause interspecific trade-offs that maintain diversity.

An alternative theory of neutral community dynamics argues that communities are composed of ecologically identical species. As a result, the species in such communities experience the same strength of density dependence as determined by the total abundance of all species, rather than the relative abundance of each. Thus, these species show strongly measurable density-dependent responses to changes in total abundance but no response when relative abundance is manipulated and total abundance is held constant.

In this study, the researchers examined whether or not ecological differences among species shape Enallagma damselfly assemblages in lakes with fish. Previous field experiments suggest that density-dependent mechanisms regulate the structure of these assemblages. However, the researchers studied whether or not these mechanisms also regulate individual species or the assemblage as a whole.

The team studied two Enallagma species  to determine if manipulating their total and relative abundances leads to different responses to ecological factors. The results indicated that Enallagma in fish lakes are nearly ecologically equivalent species representing a functional group embedded in the food web. Mechanisms of coexistence promote the persistence of each functional group and regulate the total abundance of all species. The researchers discovered that total Enallagma abundance was the variable determining demographic rates as it resulted in large increases in per capita mortality and large decreases in growth for both species. On the other hand, the relative abundances of each species showed little correlation with major environmental gradients.

The researchers also pointed out that these results do not imply that all damselflies are ecologically equivalent. For example, the Ischnura species, a sister genus, is ecologically differentiated from Enallagma. In this case, niche-based coexistence dominates diversity patterns.

In sum, the study stresses the importance of unifying neutral dynamics and niche-based coexistence in shaping the structure of biological communities. Food webs are comprised of interacting functional groups, meaning that niche structure is important in regulating species diversity. However, the ecological regulation present within each taxa is governed by neutral community dynamics and is equally important in understanding the structure of biological communities.