Paradoxical response of density-dependence to resource limitation

Background/Question/Methods

Density-dependence has captured the interest of both ecologists and evolutionary biologists because it is the simplest way of studying the point at which these two disciplines meet. Population density imposes selection pressure under which species can potentially diverge while also playing a role in limiting populations because – under high densities, intraspecific competition becomes more intense. Although intraspecific competition plays a significant role in ecology and evolution, the mechanisms through which it acts on populations are not thoroughly understood. Resource availability is a limiting factor in many populations and communities. The growth rates of the ciliate protozoan Colpidium sp., for example, are limited by resource availability. We expect to find that at low densities, resource availability will have very little effect on population growth rates, since the consumer to food ratio will be low, and that effects of resource availability will increase with density. We applied a fully factorial design of density and resource levels to experimental microcosms containing Colpidium sp. and their associated bacterial community and estimated per capita growth rates at three time points over a period of six days.

Results/Conclusions

We found that the overall effect of density on growth rates is strong, suggesting that per capita growth rates in Colpidium sp. are density-dependent. Increasing density by a single order of magnitude reduced per capita growth rate by about 87% on average. Our results also indicate that per capita growth rates in Colpidium sp. are only sensitive to differences in resource availability at low densities. For example, at low densities, the high resource level group had an average per capita growth rate that was about 1.5 times greater than the low resource level group. However, at high densities, the growth rate of the high resource group was not significantly different from that of that low resource level group. The response found here is contrary to what might be classically expected. These results suggest that, while resource availability plays an important role in limiting growth rates at low densities, intraspecific competition is more important at high densities. We conclude that resource availability can influence density-dependent factors in unexpected ways that will influence the ecological and evolutionary dynamics of populations.