Browsing by Subject "Functional Response"

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    Multiple scale spatial dynamics of the moose-forest-soil ecoystem of Isle Royale National Park, MI, USA
    (2008-11) De Jager, Nathan R.
    Moose (Alces alces) make foraging decisions at scales that range from plant stems (centimeters) to home ranges (kilometers). These decisions determine the spatial distribution of forage consumption and the consequent impacts on plant communities and nutrient cycles. From the fine scale changes in the size and density of plant stems to the distribution of plant patches and community assemblages across landscapes, the effects of moose browsing at one spatial scale may amplify spatial heterogeneity at scales that are orders of magnitude larger than the scales at which the interactions originally took place. In this thesis, I focused on how gradients of moose browsing in two valleys at Isle Royale National Park, MI, USA influenced plant fractal geometry and how such changes to plant geometry feedback to moose foraging across larger previously browsed landscapes. I also examined changes in larger scale patterns of forage availability, plant community composition, and soil fertility in response to recent declines in island-wide moose population density. Increasing moose browsing influenced the geometry of deciduous and conifer species differently. The fractal dimension of bite density, bite mass, and forage biomass of aspen saplings all responded quadratically to increasing moose browsing and were greatest at intermediate browsing rates. In contrast, fractal dimension of bite density, bite mass, and forage biomass of balsam fir all declined steadily with increasing moose browsing. These different responses of plant canopies to increased browsing as well as seasonal changes in bite mass altered the distribution of foraging mechanisms across larger previously browsed landscapes. At the larger scale, recent (2005-2007) landscape patterns of available and consumed browse became decoupled from each other and distributions of available forage, plant species composition, and soil fertility were qualitatively different from patterns documented in the early 1990's. These changes are coincident with and likely driven by recent declines in the island-wide moose population of Isle Royale. Collectively, these two studies suggest that large scale landscape patterns of browse availability, species composition, and soil fertility may emerge from finer scale impacts of browsing on plant geometry and the feedbacks to larger scale foraging decisions that moose make in addition to population density. The long-term dynamics of landscape patterns in boreal forests are therefore dependant on both fine scale foraging decisions and large scale population dynamics.
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    Temporal changes in spatial patterns of moose browse, causes and consequences.
    (2010-03) Hodgson, Angela Lynne
    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.