Introduction

The most general definition of associate is 'to keep company with', and the term association has accordingly been used in ecology for a wide range of situations in which different species occur together. In the early twentieth century, the term was first used to refer to a group of plant species that occurs predictably together under a given set of environmental conditions, for example, the oak-hickory forest of the southeastern USA. The plant association was the forerunner of the modern concept of the ecological community, which includes all types of organisms found together at a given place and time. Ecological associations vary in both the intimacy of interaction between the species and in the types of benefits accruing to them (Figure 1). In general, associations between species can be understood in terms of the dynamic balance between fitness costs and benefits to the parties involved. Casual and intimate associations, parasitism, and mutualism are points along a continuum, and either altered environmental conditions or evolutionary changes in the interacting species can shift the balance, moving a given association along that continuum.

Crop plant/ pollinator

Crop plant / grasshopper

Vertebrate host/ tapeworm

High

Intimacy

Figure 1 Schematic illustration of two axes of association, and the positions of some common associations in this space. The benefits axis ranges from parasitism (positive fitness consequences for parasite, negative for host) to mutualism (benefits to both parties). The intimacy axis ranges from casual and fleeting to intimate and obligatory for at least one of the associates.

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