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r
e s e a r c h.. f o c u s :
Global change
in carbon and nitrogen budgets and plant-herbivore interactions
Over
the next 40-50 years, the global concentration of carbon dioxide
in the atmosphere is expected to increase by 75% and climate should
warm by 2-4°C.  In
some areas of the world, nitrogen pollution in the atmosphere and
groundwater is expected to double the available nitrogen for plant
growth. On top of these changes, species are being lost a rate equivalent
to catastrophic extinction events in the paleontological record.
While these changes are well-documented individually, and their
consequences for plant production and nutrient cycling determined
in several experiments, their combined consequences on ecosystems
are virtually unknown. A key component of ecosystem functioning
is insect herbivory, through which primary production is transferred
to the animal food web. However, virtually no studies have explored
the effects of warming, carbon dioxide enrichment, nitrogen deposition,
and/or loss of plant diversity on insect abundance and diversity
and the impacts of insects on plants.
In
the past few years, I have conducted a series of experiments at
the Cedar Creek Natural History Area in Minnesota to address some
of these issues. A long-term experiment that explores the impact
of bird predators  on
grasshoppers, a dominant insect herbivore, shows dramatically greater
grasshopper abundance with added nitrogen, but only in years following
warmer than normal summers. These results suggest that future warming
and nitrogen pollution combined could elicit unexpected outbreaks
of insects that would detrimentally affect agriculture.
More
recent work focuses on the combined effects of enhanced carbon dioxide
and nitrogen and plant diversity loss on insects. This work centers
on the large BioCON
(Biodiversity, CO2, and N) project at Cedar
Creek, a large factorial experiment of 342 grassland plots that
receive ambient and elevated CO2 and N, and contain different numbers
of plant species. Early results show that outbreaks of aphids on
legumes are depressed by elevated CO2, as are the abundance and
species richness of all leaf-feeding insects and predators. Nitrogen
addition does not help  compensate
for these effects, and the effects of CO2 are amplified in plots
with high plant species richness. These results suggest that grasslands
with high plant species richness may be most sensitive to changes
in atmospheric CO2 and may lose a substantial fraction of their
insect diversity.
Future
work needs to address the chemical changes in plants that explain
these insect population and community responses to global change
factors. In addition the response of different insect groups, such
as those that feed on plant sap, parasitoids, and predators, need
to be better understood. Finally, there is considerable potential
to explore the effects of warming and nitrogen deposition on mutualistic
interactions, such as those between ants and aphids feeding on the
leaves of oak trees.
Selected
Related Publications:
Haddad,
N., D. Tilman, J. Haarstad, M.E. Ritchie, and J.M.H. Knops.
2001. Contrasting effects of plant diversity and composition on
insect communities: a field experiment. American Naturalist
158: 17-35.
Engelhardt,
K.A. and M.E. Ritchie. 2001. Effects of plant diversity on
wetland ecosystem function and services. Nature 410: 687-689.
[Click
here for PDF]
Ritchie,
M.E. 2000. Nitrogen limitation and trophic vs. abiotic influences
on insect herbivores in a temperate grassland. Ecology 81:
1601-1612. [Click
here for PDF]
Knops,
J.M.H., D.Tilman, N. Haddad, J. Haarstad, S. Naeem, C. Mitchell,
M.E. Ritchie, K. Howe, J. Groth, P. Reich. 1999. Cascading
effects of plant diversity on invasions, diseases, and insects.
Ecology Letters 2: 286-293.
Siemann,
E., Tilman, D., Haarstad, J. and Ritchie, M.E. 1998. Experimental
tests of the dependence of arthropod diversity on plant diversity.
American Naturalist 152: 738-750.
Tilman,
D., Knops, J.M.H., Wedin D., Reich P., Ritchie, M.E., and
Siemann, E. 1997. Functional diversity and composition influence
ecosystem processes. Science 277: 1300-16.
Tilman,
D., S. Naeem, J.M.H. Knops, P. Reich, E. Siemann, D. Wedin, M.E.
Ritchie, and J.H. Lawton. 1997. Biodiversity and ecosystem properties.
Science 277: 1866-1867.
Ritchie,
M.E. 1996. Interaction of temperature and resources in population
dynamics: an experimental test of theory. Pages 79-92 In: Floyd,
R.B., Sheppard, A.W., and DeBarro, P.J. (eds.) Frontiers in Population
Ecology. CSIRO Press, Melbourne.
For
more details about my other research and related publications, please
select from the following:
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