Introduction

Chapter 8 showed there are many pathways by which temporal resource partitioning can make coexistence more difficult. Regardless of its effects on coexistence, temporal variation alters the quantitative effects of one competitor on another.

This chapter will examine several other types of temporal variation, and their effects on the nature and outcomes of competition, again largely in the rather special context of two consumers and one resource. It will concentrate on systems in which there are differences in the shape of the two consumers’ functional or numerical responses. Such differences were not present in the models in Chapter 8, but were a key component of the earliest models that established the possibility of coexistence of two consumers on a single resource. The implications of these simple models for more realistic systems are also discussed.

The conclusion of this chapter will again be that different types of temporal variation can have a wide range of different effects on the outcome(s) of competition between species that differ in their functional or numerical response shapes. Four more specific results will be stressed. First, temporal variation can make exclusion either more likely or less likely depending on the details of the mechanism(s) driving that variation. Secondly, mutual invasibility of single consumer subsystems is often not required for coexistence of two consumers. Third, alternative attractors are a frequent occurrence. Finally, different period lengths for the temporal environmental variation can result in qualitatively different effects on coexistence. These findings also arose from the analyses of interspecific differences in the responses to seasonal environmental variation considered in Chapter 8. The combined findings of both chapters argue for greater understanding of the mechanistic details of consumerresource interactions as a prerequisite for predicting the consequences of variable environments for interspecific competition.

Much recent work on competition in temporally varying environments has dealt with classifying mechanisms of coexistence into the two categories in Chesson’s (1994) three-way categorization of all coexistence mechanisms that deal specifically with temporal variation (e.g. Letten et al. 2018; Ellner et al. 2019). The first mechanism is the storage effect, which is usually described in terms of a positive covariance of competition and the environment, measured when each of the species is a very rare invader in a system in which the other species is/are characterized by their

Competition Theory in Ecology. Peter A. Abrams, Oxford University Press. © Peter A. Abrams (2022).

DOI: 10.1093∕oso∕9780192895523.003.0009 limiting dynamics. All of the examples of coexistence in Chapter 8 would be classified as examples of the storage effect. The second category of coexistence associated with temporal variation is ‘relative nonlinearity’, in which variation affects two species in different ways because of differences in the shape of their functional or numerical responses. The analysis of simple models in this chapter suggests that these two mechanisms interact with each other in a wide variety of ways and are difficult to separate. The combination of differences in linearity with sustained variation in population sizes can hinder or promote coexistence. Thus, models with differently shaped consumer demographic and ecological functions provide more arguments against a research focus on a single simple classification of coexistence mechanisms. Such a focus is likely to restrict rather than expand our understanding of the full range of possible competitive outcomes’ and the pathways underlying these outcomes.

The environmental variation considered in Chapter 8 had differing effects on consumer species that were identical in their mean abilities to capture or convert resources. This chapter will take up a similar exploration of how variation affects competition’ but it will consider consumer species that differ in the shape of their relationship between resource abundance and immediate per capita growth rate.

Such species cannot coexist on a single resource in the absence of temporal variation. However, because of the differences in the shapes of their responses to resource abundance, temporal variation changes the nature of the interaction. This chapter looks at how variation affects both the nature of the interaction and the conditions for coexistence. The three forms of temporal variation treated here are: (1) variation arising as a consequence of an unstable consumer-resource interaction; (2) variation in the environment that only directly affects resource growth parameters; and (3) variation in the environment that affects neutral consumer parameters (for example, per capita mortality rates in most of the simple models in this chapter). These three types of variation are all scenarios under which differences in the nonlinearity of consumer per capita growth rate functions alter competitive effects.

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Source: Abrams Peter A.. Competition Theory in Ecology. Oxford University Press,2022. — 336 p.. 2022

Introduction

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