M.Sc Thesis

M.Sc StudentZach Solomon Aviv
SubjectModeling the relations between species similarity and
competitive exclusion in ecological communities
DepartmentDepartment of Civil and Environmental Engineering
Supervisor PROF. Yohay Carmel


Similar species share a large proportion of their resource requirements, and therefore the competition between them is strong. The competitive exclusion principle  suggests that the coexistence of two species that have roughly the same food requirements is not likely, and that one species will eventually exclude the other. The principle determines that when two species occupy the same niche at the same geographic territory, the species whose reproduction rate is higher will outcompete the other, and the slower species will be excluded in that area.

However, the competitive exclusion principle mechanism fails to explain the observed coexistence of similar species in the same community in many types of communities. For example, the paradox of the plankton highlights the fact that communities of many apparently similar phytoplankton species compete for the same limited resources, without any apparent exclusion.

In this study I investigate this principle in a series of computer models, and suggest an alternative view of the competitive exclusion principle. I propose that species similarity can be decomposed into two components:

(1)   Niche similarity- to what extent do the species consume the same resources.

(2)    Similarity of competitive ability, how similar are the abilities of the species to compete.

As niche overlap increases, the competition among the species become more severe. In contrast, as the competitive abilities of both species become similar, the ability of one species to completely exclude the other species from the habitat decreases. If niche similarity and competitive ability similarity are correlated, the combined effect of species similarity on competitive exclusion rate might be unimodal, where coexisting species can be either very different or very similar.

Two computer models were developed in order to examine the effect of species similarity on time to exclusion and the order of exclusion of species in the community. These are resource/consumer models that simulate indirect competition  via shared resources. Every species affects the other species by exploiting the shared resources, resulting in a decrease in the utilization ability of the competing species. The models simulate communities of microorganisms in a chemostat.  

Various combinations of competitive similarity and niche similarity were examined in order to characterize the entire range of possible combinations.  One of the four extreme points showed that when niche overlap was maximal and competitive differences were minimal, the two species strongly competed but neither of them had an advantage over the other, and exclusion rate was minimal. This indicates a possible theoretical situation in which similar species can coexist in the same community for very long periods.

Using a linear model, I found that niche length and niche overlap are strongly effecting exclusion order (32.7% and 33.6% of the variance respectively). However, when examining the similarity of excluded species in a species rich community the extinction pattern was random, thus the similarity between the coexisting species was not significantly different than random .It is possible that the random extinction pattern is a result of both initial niche size and initial niche overlap that effect exclusion order in the same magnitude but in opposite directions.