You are studying the populations of two species on the Penn State campus. Robins (species 1) and moles (species 2) live in the same habitat and compete with each other for earthworms, an important food item for both species. The carrying capacity for moles in this environment is 1000. The competition coefficient (α21) for the effect of robins on moles is 0.8. This means that 100 robins in this community have the same impact as ______ moles on the population growth rate of the moles. Group of answer choices 8 80 800 1000 Individuals of one species cannot affect the population growth rate of another species.

In the Lotka-Volterra model of the interspecific competition, two species interact competing for a specific resource, in this case, food. The magnitude of influence of one species over the other depends on how equivalent are individuals in resource use. The more the species superpose, the more they consume and the more they compete.

Competition coefficient α means how many individuals of one species are equivalent to one individual of the other species. α levels the competitive effect per capita of one of the species on the growth rate of the other species. This is N1=α12*N2

In this example,

Robin: Species 1

Moles: Species 2

The competition coefficient (α21) for the effect of robins on moles is 0.8

The robin (species 1) population size is 100

N2=α21*N1

N2=0.8*100

N2=80.

As α21<1 (0.8), it means that intraspecific competition is stronger than interspecific competition. The growth rate is more affected by individuals of the same species (moles, species 2) than by individuals of the other species (robin, species 1). Robin individuals do not affect the population growth rate of moles. The intraspecific component is stronger than the interspecific one.

This makes sense as 80 moles individuals have the same impact as 100 robin individuals in the population growth rate of the moles.