r
e s e a r c h.. f o c u s : Grazer-plant
interactions across rainfall and soil fertility gradients
Over
50% of the earth's land surface is managed for herbivory by mammals,
either wild and/or domestic. This herbivory threatens plant diversity
and ecosystem productivity or is itself threatened by expected changes
in climate, atmosphere, and urban and agricultural land use. However,
little synthesis has emerged despite literally hundreds of studies
of mammalian herbivore effects on grassland plant communities.
In
an ongoing cross-site study funded by NSF, I work in collaboration
with 3 other scientists and several graduate students to develop
new, more synthetic hypotheses about how mammalian herbivores of
different size might affect grassland plant species composition
and diversity, and therefore ecosystem nutrient cycling and primary
productivity. Specifically, we predict how herbivores of different
size affected plants and nutrient cycling across sites that differ
in productivity, water availability, and soil fertility (Olff and
Ritchie 1998, Ritchie et al. 1998, Ritchie and Olff 1999, Olff,
Ritchie, and Prins 2002). We address these hypotheses in a collaborative
cross-site experimentof the effects of different-sized herbivores
on grassland plant species
composition, diversity, production and nitrogen cycling. This study
features a common experiment at nine sites that varied ten-fold
in productivity, including three sites of different elevation in
Utah (Deseret Ranch), and one each in South Dakota (Badlands National
Park), Colorado (Shortgrass Steppe LTER), Kansas (Konza LTER), Minnesota
(Cedar Creek LTER), and a river floodplain grassland in the Netherlands
(Junner Koeland). Results thus far support the hypothesis that grazing
by diverse grazer communities enhances plant diversity primarily
at sites with high productivity. This effect was primarily due to
effects of large grazers, which increased plant species richness
at high and low productivity, but decreased it at intermediate productivity.
The
next phase of this project will complete and intensively sample
an existing large-scale field experiment that independently manipulates
water and nutrient supply to multiple plant species
in 64 large (450 m2) plots undergoing secondary succession. This
experiment will directly test predictions about how plant resources
influence terrestrial plant communities and the impact of herbivores
on these communities. The objective is to construct herbivore exclosures
in each plot and measure soil water, nutrient supply rates, and
plant species composition and diversity in response to different
rates of water and nutrient addition. Preliminary results suggest
that production is co-limited by water and nutrients, and plant
species are most abundant at ratios of water and nutrient supply
that match their use of carbon and nitrogen for growth and herbivore
defense.
Selected
Related Publications:
Olff,
H., M.E. Ritchie, and H.H.T. Prins. 2002. Global determinants
of diversity in large herbivores. Nature 415: 901-904.
Knops,
J. M. H., M.E. Ritchie and D. Tilman. 2000. The effect of
selective herbivory on a nitrogen fixing legume (Lathyrus venosus)
on productivity and ecosystem nitrogen pools of an oak savanna.
Ecoscience 7: 166-174.
Ritchie,
M.E. and Olff, H. 1999. Herbivore diversity and plant dynamics:
compensatory vs. additive effects. Pages 175-204 In: Olff, H., Brown,
V.K., and Drent, R. (eds.). Herbivores: Between Plants and Predators.
Blackwell, Oxford, UK.
Ritchie,
M.E., D.Tilman, and J.M.H. Knops. 1998. Herbivore effects on
plant and nitrogen dynamics in oak savanna. Ecology 79: 165-177.
Olff,
H., and Ritchie, M.E. 1998. Herbivore effects on grassland
plant diversity. Trends in Ecology and Evolution, 13: 261-265.
For
more details about my other research and related publications, please
select from the following:
diversity
and ecosystem productivity or is itself threatened by expected changes
in climate, atmosphere, and urban and agricultural land use. However,
little synthesis has emerged despite literally hundreds of studies
of mammalian herbivore effects on grassland plant communities. 
species
composition, diversity, production and nitrogen cycling. This study
features a common experiment at nine sites that varied ten-fold
in productivity, including three sites of different elevation in
Utah (Deseret Ranch), and one each in South Dakota (Badlands National
Park), Colorado (Shortgrass Steppe LTER), Kansas (Konza LTER), Minnesota
(Cedar Creek LTER), and a river floodplain grassland in the Netherlands
(Junner Koeland). Results thus far support the hypothesis that grazing
by diverse grazer communities enhances plant diversity primarily
at sites with high productivity. This effect was primarily due to
effects of large grazers, which increased plant species richness
at high and low productivity, but decreased it at intermediate productivity.
species
in 64 large (450 m2) plots undergoing secondary succession. This
experiment will directly test predictions about how plant resources
influence terrestrial plant communities and the impact of herbivores
on these communities. The objective is to construct herbivore exclosures
in each plot and measure soil water, nutrient supply rates, and
plant species composition and diversity in response to different
rates of water and nutrient addition. Preliminary results suggest
that production is co-limited by water and nutrients, and plant
species are most abundant at ratios of water and nutrient supply
that match their use of carbon and nitrogen for growth and herbivore
defense.
