Effects of exploitative food competition on food niche dynamics — a simulation analysis

Abstract

The effects of intra- and interspecific exploitative food competition on food niche dynamics were analysed using simulation models with 2 or 5 coexisting predator species, competing for 2 or 5 prey species. For each of these combinations, food consumption by predators was calculated assuming functional responses of either type 1, 2 or 3. Two levels of competition intensity were studied: low and high intensity, with food consumption decreased by 1% or 50% as compared to a situation without competition. The reproduction of prey was either slow or rapid, potentially increasing the number of prey at the end of a simulation by either 2∗ or 100∗ initial abundances. Prey reproduction was either exponential, influenced by intraspecific interaction or by both intra- and interspecific interactions between prey. All together, separate analyses of intra- and interspecific competition between different numbers of predator species combined with different numbers of available prey species, competition intensities, functional responses and rates and regulation of prey reproduction, gave 288 different types of models. For each of these, 100 simulations were run, with prey capture rates determined randomly for each predator-prey combination. The output data analysed were frequencies of increases and decreases in food niche overlap in response to interspecific competition and expansion and contraction in food niche width in response to either intra- or interspecific competition. All changes in niche parameters resulted from pure exploitative competition, without behavioural changes, short-term selection or evolutionary processes. The results were complex, with both increases and decreases in niche width and niche overlap as results of intra- and interspecific competition. Frequencies of increases and decreases were influenced by complex interactions between number of species of predators and preys, functional response of predators and reproduction rate and density dependence in prey population growth. The effect of one species on the niche width of another, as well as on their mutual niche overlap, sometimes changed with the intensity of competition. These results are in strong contrast to the niche dynamics theory frequently used as basis for interpreting field data, according to which species are assumed to generalise in response to intraspecific competition (niche expansion), and segregate and specialise in response to interspecific competition (decreased overlap and niche contraction [resource partitioning]). These variable results were derived from models which are very simple compared to the complexity of Nature, a complexity which is reasonable to assume increases the possibility for the development of complex niche dynamics. In my opinion, niche dynamics in Nature are thus very likely to be complex and great care must be taken before concluding from field data on food niche dynamics whether or not coexisting species compete.