Contrasting dynamics in the same plant–herbivore interaction

Abstract

Long-term studies of two-species interactions under field conditions are unusual; most long-term field studies are of single species dynamics (1-6). Concurrent long-term studies on the dynamics of the same two interacting species in different locations are very rare. This result has led to the tacit assumption that different cases of the same two-species interaction would involve essentially quantitative differences (e.g., context-specific differences in the numeric values of demographic parameters like fecundity or death rates). Here, we show that for one of the best-known two-species systems (ragwort and cinnabar moth), this finding does not hold. The interaction between the plant and its herbivore is fundamentally different in coastal dunes in The Netherlands and in grasslands in Southeast England. In the first case, the dynamics are cyclic and the interaction involves both direct and delayed density dependence; in the second case, the insect has little impact on plant dynamics and there are no time lags in density dependence. The difference is caused by differences in the importance of seedlimitation in plant recruitment in the two locations. R ecent developments have highlighted the way that abiotic and biotic factors interact in the regulation of animal [vertebrate (1, 2) and invertebrate (3, 4)] and plant (5, 6) single-species dynamics. In reality, of course, species interact with resources, competitors, mutualists, and natural enemies (7). Although the theory of multispecies interactions is relatively well developed (8-10), there are very few tests of this theory using field data. Comparative studies of the dynamics of the same interaction in different ecological settings have not been carried out. Using a replicated plant-herbivore system, we compare and contrast the population processes and mechanisms that influence the observed dynamical patterns. We focus on the interaction between a resource and its consumer. By developing nonlinear statistical models, we illustrate how crucial dynamical effects are contingent on the precise structure of the ecological interaction between a plant and its herbivore. We evaluate the strength and change in plant density dependence (through plant recruitment) and herbivore density dependence (through numerical and functional responses) as the principal determinants of the mechanisms underlying the dynamics of the plantherbivore interaction. MethodsBiology and Population Census Methods. The system involves the cinnabar moth (Tyria jacobaeae) L. (Lepidoptera: Arctiidae) and its food plant, ragwort (Senecio jacobeae) L. (Asteraceae). The basic biology of the system is of a food-limited specialist herbivore feeding on a host plant where the dynamics of the plant are principally determined by interspecific competition for recruitment sites. This system is unique in that comparative time series data are available for the cinnabar moth-ragwort interaction over relatively long periods of time: a mesic grassland in Silwood Park (20 yr) and a coastal sand dune in Meijendel (26 yr...

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