Temporal changes in spatial patterns of moose browse, causes and consequences.

Abstract

Ecologists determine mechanisms by observing spatial and temporal patterns of abundance and distribution in natural systems. While there has been a long history of research on techniques for describing temporal patterns of abundance, and their causes and consequences, there is still a need for ecological research to focus on the causes and consequences of spatial patterns. Progress on this goal, though, has been hindered by the lack of long-term data on spatial patterns in natural ecosystems. I present findings from one of the first long-term studies of changes in spatial patterns of plants in response to herbivory, and discuss causes and consequences. My research was conducted in the southern boreal forest on Isle Royale, Michigan, and focused on temporal changes in the spatial pattern of woody browse species that are consumed by a large herbivore (moose). I concluded that browsed woody sapling biomass is aggregated within moose feeding stations and the degree of aggregation has changed over a 20-year period. The cause of this fluctuating pattern of aggregation is due to the competing influences of inverse density dependent browsing by moose, causing an increase in aggregation, and inverse density dependent growth, causing a decrease in aggregation. Annual changes in aggregation are determined by the relative contribution of consumption and growth to changes in spatial pattern in any given year. Next, I developed a simulation model to determine the consequence of aggregation of browse on the intake rate of large herbivores. I found that the spatial aggregation of browse within feeding stations can decrease the intake rate up to 30% for herbivores feeding on low density browse (<15 g/m2). Both density and the spatial arrangement of browse, therefore, is important to consider when determining the functional response of large herbivores. Finally, I used long-term data on consumption and spatial pattern of browse to test whether a mechanistic modified contingency model could predict observed moose diet selection. This model assumes that moose select their diet in order to maximize short-term intake rate. Model predictions were consistent with observed diet selection during both summer and winter in two study sites.