The need for resources in competition theory
One central thesis of this book is an idea that was more widely accepted four decades ago than it is today; i.e. that understanding competition requires an understanding of its underlying consumer-resource interactions.
Resources are substances that are required for survival and/or reproduction. For example, in predators, resources always include their prey, but they may also include sources of water, shelter, or camouflage. This means that a proper ecological understanding of competition between predators requires, at a minimum, some study of prey population dynamics, the rates of consumption by predators, and the effects of that consumption on the predators’ birth and death rates. The last of these requires some knowledge of the supply and workings of those other factors influencing the predator’s birth and death rates. Competition between plant species involves exploitation of water, light, and mineral nutrients. However, all of these are tied to the space where they occur, and the nature of the consumer-resource interaction may differ greatly with the physical characteristics of the place where the water, light, and nutrients are found. Detri- tivores, parasites, pathogens, and other groups are also consumers, and they often require different types of models of competition. This is particularly true of the latter two groups, which inhabit their ‘resource’, and their survival or reproduction may be adversely affected by overly high consumption rates. Consumer-resource interactions are also an essential component of many mutualistic interactions between species. A more complete theory of competition, based on a better understanding of consumer-resource interactions should help to provide a foundation for predicting future changes in distribution and abundance of all of these groups. A theory that includes resources explicitly is also needed for understanding the evolution of ecologically important characteristics in any species.Acknowledging the centrality of consumer-resource interactions in competition leads directly to the question of the adequacy of existing consumer-resource theory. Thus, another theme of this book will be that, for both competition theory and consumer-resource theory, there has been an over-emphasis on historical models and overly simplistic formulations for understanding the interactions. In competition theory this has meant a continuing reliance on the Lotka-Volterra model, which is discussed below. In consumer-resource theory, the legacy of simple models with a long history means, inter alia, a concentration on cases with a single resource, or just two resources in the minority of studies that consider more than one. For those branches of competition theory that explicitly include resources, the most common assumption is that there are just two consumers and two resources, and the resources are assumed not to interact with one another. Simple models also omit adaptive behaviour and phenotypic plasticity from the set of processes determining the shape of consumption rate functions. Existing consumer-resource and competition theories have additionally tended to ignore the impacts of the abundances of species occupying higher and lower trophic levels on a focal consumer-resource pair. Linear relationships between resource abundance and consumption rate are often assumed as the default, as are linear relationships between resource consumption and per individual birth and death rates.
In competition theory, there is still a large body of new work produced every year using the oldest and simplest possible model of competition in which each species has an instantaneous per capita growth rate that declines linearly and independently with the abundance of every one of its competitor species. The result is that many ecologists have expectations based on theoretical systems that are likely to have properties that are extremely uncommon, even within the vast domain of different natural communities.
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