15.2: Intraspecific (Single Species) Competition

Figure \(\PageIndex{1}\): Male hartebeest locking horns and fiercely defending their territories, an example of direct competition.

Intraspecific competition is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to repro​​duce. By contrast, interspecific competition occurs when members of different species compete for a shared resource. Members of the same species have rather similar requirements for resources, whereas different species have a smaller contested resource overlap, resulting in intraspecific competition generally being a stronger force than interspecific competition. 

Intraspecific competition does not just involve direct interactions between members of the same species (such as male deer locking horns when competing for mates) but can also include indirect interactions where an individual depletes a shared resource (such as a grizzly bear catching a salmon that can then no longer be eaten by bears at different points along a river).

When resources are infinite, intraspecific competition does not occur and populations can grow exponentially. Exponential population growth is exceedingly rare but has been documented, most notably in humans since 1900. Elephant (Loxodonta africana africana) populations in Kruger National Park (South Africa) also grew exponentially in the mid-1900s after strict poaching controls were put in place. However, prolonged exponential growth is rare in nature because resources are finite and so not every individual in a population can survive, leading to intraspecific competition for scarce resources. Individuals can compete for food, water, space, light, mates, or any other resource which is required for survival or reproduction. The resource must be limited for competition to occur; if every member of the species can obtain a sufficient amount of every resource then individuals do not compete and the population grows exponentially. 

When resources are limited, an increase in population size reduces the quantity of resources available for each individual, reducing the per capita fitness of the population. As a result, the growth rate of a population slows as intraspecific competition becomes more intense, making it a negatively density-dependent process. 

The falling population growth rate as population increases can be modeled effectively with the logistic growth model. The rate of change of population density eventually falls to zero, the point ecologists have termed the carrying capacity (K). However, a population can only grow to a very limited number within an environment. The carrying capacity, defined by the variable K, of an environment is the maximum number of individuals or species an environment can sustain and support over a longer period of time. 

The logistic growth equation is an effective tool for modeling intraspecific competition despite its simplicity and has been used to model many real biological systems. Here x is the size of the population at a given time, r is the inherent per-capita growth rate, and K is the carrying capacity. At low population densities, x(t) is much smaller than K and so the main determinant for population growth is just the per capita growth rate. However, as N(t) approaches the carrying capacity the second term in the logistic equation becomes smaller, reducing the rate of change in population density. Eventually, the population size equals the carrying capacity, and the population growth rate equals zero.

\[ \frac{d N}{dt}=r N\left(\frac{K-N}{K}\right) \nonumber\]

dN/dt = rate of change of population density

N = population size at time t

r = per capita growth rate

K = carrying capacity

Figure \(\PageIndex{2}\): This figure shows the growth of a population following a logistic curve, resulting in the S-shaped graph. This model reaches a stable equilibrium, sustaining the population at the carrying capacity as time continues. Developed by Nchisick under CC-BY-SA.

The logistic growth curve (Figure \(\PageIndex{2}\)) is initially very similar to the exponential growth curve. When population density is low, individuals are free from competition and can grow rapidly. However, as the population reaches its maximum (the carrying capacity), intraspecific competition becomes fiercer and the per capita growth rate slows until the population reaches a stable size. At the carrying capacity, the rate of change of population density is zero because the population is as large as possible based on the resources available. Experiments on Daphnia growth rates showed a striking adherence to the logistic growth curve. The inflection point in the Daphnia population density graph occurred at half the carrying capacity, as predicted by the logistic growth model.

In summary, the resources within an environment are limited. Therefore, the environment can only support a certain number of individuals before its resources completely diminish. Numbers larger than this will suffer a negative population growth until eventually reaching the carrying capacity, whereas populations smaller than the carrying capacity will grow until they reach it.