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Experimental test of resource competition and keystone predation theories: Predicting the success of invasions and biological control.
Bunker, Daniel*,1, Carson, Walter1, 1 University of Pittsburgh, Pittsburgh, Pennsylvania
ABSTRACT- Resource competition theory predicts that the species that reduces the availability of the limiting resource (R*) to the lowest level will win in competition. Keystone predation theory (sensu Grover) predicts that a specialist herbivore will reverse the outcome of competition if it reduces the resource competitive ability of its host (i.e., raises the R* of the host above that of its competitors). We experimentally tested these predictions using invasive purple loosestrife (Lythrum salicaria), native broad-leaved cattail (Typha latifolia), and Galerucella calmariensis, a specialist beetle released to control loosestrife. To measure how species and herbivory affect resource availability (R*), we established 6 cattail and 24 loosestrife monocultures (1000 l cattle tank mesocosms), at low and high fertility (0, 16 gNm-2 year-1 ) (60 total). We manipulated beetle density manually, thereby creating an herbivore damage gradient. We then measured light availability at the soil surface to measure species and herbivore impacts on light availability. Without herbivory, loosestrife reduced light availability to lower levels than cattail at both low and high fertility, and therefore was the superior competitor for light. Herbivory reversed this pattern. When herbivory reached 30% (low fertility) and 60% (high fertility) leaf-area damage, cattail reduced light availability lower than loosestrife. Thus, our monocultures predict 30% (low fertility) and 60% (high fertility) leaf-area damage will reverse the outcome of competition. We tested these predictions experimentally in 48 additional mesocosms where adult loosestrife and cattail competed with and without herbivores. After two years of competition, relative abundance of loosestrife was higher than cattail in the absence of herbivory. Competitive outcomes were reversed when herbivory levels reached 40% (low fertility) and 55% (high fertility) leaf-area damage, very similar to our predictions from monoculture. Our results suggest that if Galerucella damage does not reach these high levels in the field, biocontrol will not succeed.
Key words: invasive species, herbivory, plant competition, resource competition