“Frequency-dependence and ecological drift shape coexistence of species with similar niches”

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Erik I. Svensson, Miguel A. Gómez-Llano, Anais Rivas Torres, and Hanna M. Bensch (June 2018)

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Ecological drift, frequency-dependence and interference competition shape coexistence of species with similar niches

How can species with similar ecological niches co-exist?

Beautiful demoiselle (Calopteryx virgo): males of this species have entirely melanized black wings, but are otherwise morphologically and ecologically similar to the close relative the banded demoiselle (C. splendens).
(Credit: Erik I. Svensson)

A research team led by Professor Erik Svensson at Lund University (Sweden) decided to experimentally investigate how closely related but ecologically similar damselflies of the genus Calopteryx (“demoiselles”) could co-exist.

The traditional explanation of why and how species co-exist are that they differ in their ecological niches, i. e. their resource use, habitats or their general way of living. This classical explanation suggest that species can co-exist without outcompeting each other, since each species might perform best in its own ecological niche, provided that niche differences between species are large. This traditional view has been challenged by the so-called neutral theory, which emphasize that many species are ecologically similar and are therefore unlikely to differ sufficiently in their ecological niches to be maintained over long time periods. According to neutral theory, species might instead go extinct for random reasons in a process called “ecological drift”, but there is limited empirical evidence for this process in nature.

Typical habitat of Calopteryx demoiselles: a slow-flowing river in southern Sweden (province of Blekinge).
(Credit: Erik I. Svensson)

In Fennoscandia, two species of Calopteryx demoiselles co-exist: the banded demoiselle (Calopteryx splendens) and the beautiful demoiselle (C. virgo). These two species are ecologically and morphologically very similar and often co-occur along slow-flowing rivers. These two species have no strong differences in resource use, habitat choice, climatic niches or thermal preferences. Using a combination of field surveys at different localities in Scandinavia, density- and frequency-manipulations in large outdoor cages, field behavioral experiments on male-male interspecific aggression and simulation modelling, the research team showed that both ecological drift and a process called negative frequency-dependence (“rare species advantage”) shape the abundance and co-occurrence of these two closely related species. This study was performed between 2008 and 2015.

These findings are important, because this is one of few documented empirical examples of ecological drift. The study shows that ecological drift can operate even if species are not completely neutral. Moreover, the mechanism of negative frequency-dependence can operate even if species do not differ strongly in their ecological niches, presumably because of interference competition and male-male aggression. These results therefore help us to understand the general problem of species co-existence with implications for how biodiversity is maintained which is important also in terms of conservation biology.

Banded demoiselle (Calopteryx splendens): males of this species has a smaller melanin wing patch, covering only 50% of the wing, but are otherwise morphologically and ecologically very similar to the close relative the beautiful demoiselle (C. virgo).
(Credit: Erik I. Svensson)


The coexistence of ecologically similar species might be counteracted by ecological drift and demographic stochasticity, both of which erode local diversity. With niche differentiation, species can be maintained through performance trade-offs between environments, but trade-offs are difficult to invoke for species with similar ecological niches. Such similar species might then go locally extinct due to stochastic ecological drift but there is little empirical evidence for such processes. Previous studies have relied on biogeographical surveys and inferred process from pattern, while experimental field investigation of ecological drift are rare. Mechanisms preserving local species diversity, such as frequency-dependence (e. g. rare-species advantages), can oppose local ecological drift, but the combined effects of ecological drift and such counteracting forces have seldom been investigated. Here, we investigate mechanisms between coexistence of ecologically similar but strongly sexually differentiated damselfly species (Calopteryx virgo and C. splendens). Combining field surveys, behavioral observations, experimental manipulations of species frequencies and densities, and simulation modelling, we demonstrate that species coexistence is shaped by the opposing forces of ecological drift and negative frequency-dependence (rare species advantage), generated by interference competition. Stochastic and deterministic processes therefore jointly shape coexistence. The role of negative frequency-dependence in delaying the loss of ecologically similar species, such as those formed by sexual selection, should therefore be considered in community assembly, macroecology, macroevolution and biogeography.