Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe) |
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Authors: | David G Knochel Nathan D Monson Timothy R Seastedt |
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Institution: | (1) Department of Ecology and Evolutionary Biology, Institute of Arctic and Alpine Research, University of Colorado at Boulder, Campus Box 450, Boulder, CO 80309-0450, USA;(2) Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, USA |
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Abstract: | Spotted knapweed (Centaurea stoebe) is found in over 3 million ha of rangeland and forests across North America, and evidence supporting the use of biological
control as a regional method to reduce infestations and their associated impacts remains inconclusive. Several species of
insects have been reported to reduce plant densities in some areas; however, rigorous studies that test combinations of these
species and the influence of resource availability are lacking. We examined the singular and combined effects of herbivory
by a root weevil (Cyphocleonus achates) and a flower head weevil (Larinus minutus) on the growth and flower production of C. stoebe. We also manipulated soil resource fertility as an additional factor that could explain the outcomes of contradictory biological
control herbivore effects on C. stoebe. In a greenhouse study, herbivory by C. achates decreased flower production for plants across all resource environments. In a caged common garden study, the negative effects
of herbivory also did not interact with soil nutrient status. However, the presence of plant competition further decreased
knapweed growth, and the negative effects of concurrent herbivory by C. achates and L. minutus on plant biomass and flower production were additive. Derived within the context of variable levels of soil nutrient availability
and competing vegetation, these results support the cumulative stress hypothesis and the contention that combined above- and
belowground herbivory can reduce spotted knapweed densities and reduce the ecological and economic impacts of this species
in rangelands of western North America. |
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