Persistence of spatial variance and spatial pattern in the abundance of a submerged plant

Organisms usually benefit from heterogeneous conditions, but, by doing so, may reduce the degree of heterogeneity. The question therefore arises how heterogeneity is maintained. We investigated within-year spatiotemporal patterns in a monospecific stand of a submerged plant (fennel pondweed, Potamogeton pectinatus), with the novelty that we distinguished between different forms of heterogeneity: spatial variance (the frequency distribution of densities) and spatial pattern (the spatial distribution of densities). We repeatedly measured plant biomass that was affected by swan predation, winter mortality, and summer regrowth. Spatial variance was enhanced mostly by swan foraging, despite the fact that swans appear to exploit patches to the same threshold level. Spatial pattern, which had vanished after swan foraging, reestablished due to spatial pattern in winter mortality and was further enhanced by plant regrowth. We found that variance and pattern each have their own temporal dynamics and are maintained by different biological processes. We therefore advocate that it is pivotal to distinguish between variance and pattern in the study of spatial heterogeneity.