Michael A. Huston Biology Department Texas State University San Marcos, TX 78666 (512) 245-2129 hustonma@txstate.edu
Herbivores consume ten percent or more of total Net Primary Production (NPP) in many ecosystems, yet their impact is difficult to detect because it is distributed uniformly over the entire landscape. Nonetheless, long-term studies have demonstrated that abnormally high rates of herbivory (e.g., following removal of predators) can have dramatic effects on the biodiversity and structure of ecosystems, leading to the disappearance of palatable species such as orchids and lilies, or preventing forest regeneration. In national parks, rural areas, and suburbs across the country, human actions have led to excessively high densities of herbivores such as deer, with potential long-term consequences for ecosystem processes such as carbon storage and hydrologic function. In addition to the economic costs of over-grazing and its impacts on rare species, herbivory is one of the fundamental controls on species diversity, and there is no accepted theory to explain and predicts its impact across the full range of environmental conditions.
The NEON Herbivory Experiment is designed to quantify the impact of two major classes of herbivores (ungulates and rodents) on the vegetation, landscape structure, and ecosystem dynamics of all major ecosystem types across the United States. The basic design is simple: at each of the approximately 120 primary NEON sites across the country, a large (2 ha) fenced area will be established in typical vegetation to exclude deer, cattle, elk, and other large herbivore, and within it, two smaller (30 x 30m) exclosures will be built to exclude burrowing and surface rodents, as well as rabbits. These two types of exclosures will provide controls, in which biomass consumption by these two classes of herbivores is completely eliminated, for evaluation of the effects of these herbivores on the open landscape surrounding the fenced area.
Basic monitoring will include estimation of NPP (biomass clipping and allometry) inside and outside the exclosures, which will allow estimation of biomass removal by herbivory, as well as provide direct, in-situ estimates of NPP, which can be compared to estimates from other methods, such as remote sensing and eddy-flux towers. NPP is of particular theoretical interest in relation to the effects of herbivory on plant species diversity, since two recent meta-analyses have suggested that the effect of increased herbivory on species diversity completely reverses between oligotrophic and eutrophic environments, particularly in aquatic and marine environments. The range of NPP conditions across the NEON network should allow a test of this and related hypotheses in terrestrial environments.
Virtually all aspects of community structure and ecosystem processes could be considered to be dependent variables in relation to herbivory, which operates as an ecological disturbance by killing part or all of individual plants. This experiment will focus on a subset of dependent variables that includes: 1) plant species diversity and vegetation structure; 2) carbon sequestration and tissue chemistry (total C and N); 3) albedo and bare soil; 4) higher trophic level responses (diversity and population density of birds, small mammals, selected invertebrates). Over time, as strong contrasts develop between the exclosures and surrounding landscape, a wide variety of community and ecosystem process comparisons will be possible.
In addition to the system of exclosures at all 120 NEON sites, we recommend establishment of 5 or 6 intensive experimental grazing sites across the country, which would manipulate grazer and browser type and density in areas surrounding the same basic exclosure types.