Apparent competition with an exotic plant reduces native plant establishment.
Biological invasions can change ecosystem function, have tremendous economic costs, and impact human health; understanding the forces that cause and maintain biological invasions is thus of immediate importance. A mechanism by which exotic plants might displace native plants is by increasing the pressure of native consumers on native plants, a form of indirect interaction termed "apparent competition." Using experimental exclosures, seed addition, and monitoring of small mammals in a California grassland, we examined whether exotic Brassica nigra increases the pressure of native consumers on a native bunchgrass, Nassella pulchra. Experimental plots were weeded to focus entirely on indirect effects via consumers. We demonstrate that B. nigra alters the activity of native small-mammal consumers, creating a gradient of consumption that dramatically reduces N. pulchra establishment. Previous work has shown that N. pulchra is a strong competitor, but that it is heavily seed limited. By demonstrating that consumer pressure is sufficient to curtail establishment, our work provides a mechanism for this seed limitation and suggests that, despite being a good competitor, N. pulchra cannot reestablish close to B. nigra within its old habitats because exotic-mediated consumption preempts direct competitive exclusion. Moreover, we find that apparent competition has a spatial extent, suggesting that consumers may dictate the rate of invasion and the area available for restoration, and that nonspatial studies of apparent competition may miss important dynamics.
Orrock
John L
JL
National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, California 93101, USA. orrock@wustl.edu
Witter
Martha S
MS
Reichman
O J
OJ
D000818:Animals; D002140:California; D017753:Ecosystem; D005247:Feeding Behavior; D008322:Mammals; D009149:Mustard Plant / Q000502:physiology*; D006109:Poaceae / Q000502:physiology*; D011157:Population Dynamics; D013045:Species Specificity; D013997:Time Factors