Plant-herbivore interactions, and their influence on plant invasion rate
Plant invasion on the landscape is ultimately determined by a species’ population growth and dispersal ability. Herbivores of all types may alter the rate of invasion by affecting either or both of these processes. Herbivores can alter population growth directly through consumption, or indirectly through apparent competition. They can also alter seed movement both positively, through animal-assisted dispersal, and negatively, by altering plant and seed traits that influence dispersal. We constructed a tallgrass prairie restoration with an herbivore exclusion treatment to ask the questions: do herbivores affect invasion rates? If so, do they have a stronger influence on the population growth or the dispersal of plant species? Our experiment indiscriminately excluded herbivores from most guilds, even though we recognize that herbivores of different types (e.g., granivores, vegetative consumers, etc.) might differentially influence the invasion process. To further explore the mechanisms by which different guilds of herbivores alter plant species invasion rates, we developed a stage-structured integrodifference equation model that incorporates different guilds of herbivores affecting the transitions between stages.
Our experimental results revealed that in general, herbivores have a strong influence on the population growth, and a marginal influence on the dispersal of an early successional legume, Chamaecrista fasciculata. Herbivores decreased population growth by 75%, but decreased the mean and range of dispersal by 0.2 m and 0.75 m, respectively. These results indicate that over time, herbivores can have a drastic influence on the invasion rate of at least one native prairies species, as they decrease both processes related to invasion. Simulations of our integrodifference equation model show herbivores decrease population growth, and this subsequently influences the invasion rate. Of herbivore guilds, those that decrease the transition from seeds to adults (e.g., mice and voles), decrease the invasion rate more than those that influence adults (e.g., bison and deer). However, this effect is mediated if these herbivores simultaneously influence the dispersal distance through processes like caching. Our combined empirical and theoretical results suggest that complex interactions between herbivores and plants can influence plant movement on the landscape. This influence can come from alterations to both population growth, and dispersal distance.