Ecology in Action

INTRODUCTION

The country of Tokelau in the South Pacific Ocean may seem like an unusual venue for studies of competition – interactions between individuals that have a negative effect on the fitness of the interacting individuals. The country comprises three low-lying atolls – coral islands that surround shallow lagoons – with each atoll forming numerous islands. The greatest elevation is less than 5 m above sea level, and the total land area of this country is a mere 12 km2. About 1400 humans live on these isolated islands, but of interest to us are some other types of interacting animals, the invasive yellow crazy ant, Anoplolepis gracilipes, and three species of native hermit crab in the genus Coenobita.

Islands are excellent venues for studies of ecological processes, because islands may have relatively low species diversity, which can lead to interactions involving fewer species than in a more species-rich habitat. The numerous islands on Tokelau are a natural laboratory for studying competition between the ants and the crabs. Ants have invaded some islands, but not others, so researchers can study crab foraging and habitat preferences in the presence and absence of ants. Even so, on Tokelau and elsewhere, it is often difficult to establish conclusively that competition is occurring, and to identify precisely what the interacting species are competing for. In this chapter, we discuss the possible outcomes of competition. When two species occupy very similar ecological niches, competition can result in the superior competitor excluding the inferior competitor from an area. Alternatively, ecologically similar species may coexist by changing their resource use in regions where they co-occur. The Lotka–Volterra competition model describes the conditions under which two competing species can coexist, based on the strength of competition and the carrying capacity of each species within a given environment. But competition in natural systems often involves multiple species that can interact with each other directly, or may interact with each other indirectly via their effects on other species.

Researchers on Tokelau observed that yellow crazy ants and three species of native hermit crabs tended to eat similar types of food. This set up the potential for interspecific competition, or competition between individuals of different species. Alice McNatty and her colleagues (2009) used several approaches to investigate whether competition truly existed between these two very different animals in Tokelau.