Species coexistence in the real world

Collaborators: Barbara Downes (lead investigator), Rebecca Lester (Deakin University), Peter Chesson (University of Arizona, USA) and Barbara Peckarsky (University of Wisconsin at Madison, and the Rocky Mountain Biological Laboratory)

Funding: Australian Research Council Discovery Grant (2017 – 2020)

littleriver

Many upland rivers have several hundred species of invertebrates coexisting in the same locations. How the more poorly competing species are not driven out of these communities is a puzzle.

How species coexist within ecosystems is a central question in ecology. Questions about coexistence focus on species that use the same resources of food and living space (called a guild). Highly diverse guilds present a real puzzle because sharing resources inevitably leads to competition in some places or at some times. It is well-established that species vary in their ability to compete for resources – so, why aren’t weak competitors within guilds eventually driven extinct? This paradox was posed explicitly in the late 1950s, and its solution is important because species coexistence underpins our understanding of many aspects of community and ecosystem structure and function.

Relatively simple explanations for coexistence, such as resource partitioning (where species shift their diet or behaviour to specialise on particular types or parts of resources and hence avoid competition) have been studied intensively. Resource partitioning and other mechanisms (e.g. frequency-dependent predation) do work, but they do not provide comprehensive explanations for coexistence across whole guilds comprising many species. Over the last 40 years, more complex theories have been developed that can explain how species can coexist within a guild even in the presence of strong competition. These coexistence mechanisms (explained further below) rely directly on variability in the environment, and hence are called fluctuation-dependent mechanisms of coexistence. Potentially, multiple mechanisms of coexistence operate simultaneously, meaning that tests must contrast the explanatory power of different mechanisms rather than simply focussing on testing for the existence of individual mechanisms. The theoretical framework for achieving this has been devised but only recently (and by Peter Chesson).

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Fig. 1 (a) Female insects prefer specific egg-laying habitat (EH) such as emergent boulders in fast flows. (b) Different densities of EH produce different densities of larvae. EH can be manipulated to produce high (c) and low (d) densities of larvae of target species (in black); larvae drifting into and out of sites can be captured (drift nets) and adult production measured with emergence traps.

Coexistence theory has galloped well ahead of the ability of empiricists to devise strong tests. Experimental tests require manipulating the densities of individual species in natural field conditions, and measuring aspects of fitness related to population growth when a species is driven down to low densities. Because this is very difficult to achieve, most fluctuation-dependent mechanisms of coexistence lack comprehensive tests for almost any ecosystem.

In this project, our recent research on riverine insects (Fig. 1) presents a unique opportunity to carry out the first full field tests of multiple mechanisms of coexistence under natural conditions.

We can control the field densities of different species by manipulating the amount of suitable within-stream habitat available to females to lay eggs (Fig. 1a) in different sites, which produces lasting effects on the densities of their larvae (Fig. 1b), meaning that we will be able to observe outcomes when species are manipulated down to low densities (Fig. 1 c vs d).

It is possible to quantify the dispersal of larvae into and out of field sites under natural conditions (dispersal is constrained by channel and catchment boundaries), and we can quantify the production of adults, thus allowing us to assess population level outcomes. The specific aims of this project are thus to test multiple mechanisms of coexistence using fully-controlled, replicated field experiments.

 

 

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