Pattern and Structure in an Oceanic Zooplankton Community

An analysis of pattern can reveal a great deal about the processes that regulate the species structure of a community, especially in those cases where experiments are not possible. It is predicted that differing mechanisms of regulation should result in different patterns in the distribution and abundance of potentially competing species and in the ways in which they utilize limiting resources. In this paper we describe these patterns in the zooplankton community of the North Pacific central gyre. The gyre community is old, stable and diverse and past studies have suggested that it is regulated by in situ processes. An analysis of samples collected over many years shows that the species are close to equilibrium. There are not large spatial, seasonal or year to year changes in abundance and disturbances do not seem to be large enough to perturb the species structure and prevent competitive exclusion. Food and predation are the two main limiting factors. Competition, by inference, appears to be strong, even between relatively dissimilar species. In spite of this the species do not seem to be specialists and niche separation is not apparent in either space or food utilization. This is surprising considering the age, stability and high diversity of the system. It is concluded that either some other mechanism, such as frequency dependent predation, regulates the species structure or that the mechanisms of niche separation are much more subtle than is suspected. Some of the implications of regulation through predation are discussed.