untitled

Competition and facilitation

between species

9

Species do not exist alone. They live in a community of several other species and

some of these will interact. There are various forms of interaction between species;

competition, commensalism (facilitation), mutualism (symbiosis), predation, and par-

asitism are the main ones. These are defined by the way each species affects the other,

as is shown in Table 9.1. In competition each species suffers from the presence of

the other, although the interaction need not be balanced. With commensalism or

facilitation one species benefits without affecting the other, while in mutualism both

benefit. These can be thought of as the converse of interspecific competition. With

predation and parasitism one species benefits to the disadvantage of the other. We

shall discuss predation in Chapter 10 and parasitism in Chapter 11 and will confine

ourselves here to interspecific competition and mutualism.

Interspecific competition is similar to intraspecific competition. It occurs when indi-

viduals of different species utilize common resources that are in short supply; or, if

the resources are not in short supply, competition occurs when the organisms seek-

ing that resource nevertheless harm one or other in the process (Birch 1957).

Interspecific competition deals with the cases when there are two or more species

present, and we should be aware of a number of implications arising from this definition.

1 Competition must have some effect on the fitness of the individuals. In other words,

resource shortage must affect reproduction, growth, or survival, and hence the

ability of individuals to get copies of their genes into the next generation.

2 Although it is necessary for species to require common resources (i.e. overlap in

their requirements), we cannot conclude there is competition unless it is also known

that the resource is in short supply, or that they affect each other.

3 The amount of resource such as food that is available to each individual must be

affected by what is consumed by other individuals. Thus two species cannot com-

pete if they are unable to influence the amount of resource available to the other

species, or to interfere with that species obtaining the resource.

135

9.1 Introduction

9.1.1Definition

9.1.2Implications

Species 1

Species 2 + 0 −

+ Mutualism Commensalism Predation/parasitism

0 Commensalism (Amensalism) Competition

− Predation/parasitism Competition Competition

Table 9.1Types of
interaction.