Patterns in Population Change and the Organization of the Insect Community Associated with Goldenrod

The extent to which insect communities are organized is poorly understood because few studies have examined both population-level processes and community-level patterns. Furthermore, our view of phy- tophagous insect populations is probably biased by the more frequent attention given to economic pests whose dynamics might be expected to be somewhat exceptional. Herein, we report on the population dynamics and community-level features in a diverse, native fauna of phytophagous insects associated with goldenrod (Solidago altissima), a native perennial plant that forms a dominant, long-persisting element in old-field succession. The data consist of censuses taken in six consecutive years at 16 old fields in the Finger Lakes Region of New York; five additional stands were followed for four consecutive years. Our analyses address two questions: (a) to what extent is the functional structure of the community (as reflected in such attributes to the herbivore load, guild spectrum, and dominance hierarchy) maintained by compensatory changes in the densities of the member species and (b) are certain traits associated with a species' tendency to fluctuate in density or to dominate the community? In general, populations of goldenrod insects fluctuated less than those of insects reported in the literature. Few were abundant; only 7 of the 138 species in the goldenrod fauna ever reached densities at which their biomass exceeded 0.1% of the leaf biomass. The functional structure of the community, as reflected in the total herbivore load and the proportions of that load contributed by various guilds, varied widely in space and time. A small subset of the fauna remained dominant throughout the study; these species, whose populations occa- sionally irrupted, retained their high rank in relative abundance even during those periods when their populations were waning. There was no evidence for compensatory changes in the densities of species within guilds, as would be expected if a relatively distinct set of limiting resources were available to insects with different feeding styles. Species were no more likely to exhibit negative correlations with their guild mates than they were with members of other guilds. There was also little evidence of sporadic or diffuse competition within the fauna as a whole; even during population outbreaks, the dominant species rarely engendered decreases in the biomass or diversity of the remaining species, and increases in the total herbivore load were seldom associated with losses of species from the community. Several species were positively associated with one another on the basis of similar habitat requirements or the use by one species of conditions created by the presence of another. The community was predictable only in the sense that dominant species remained dominant and rare species rare. The abundance and population variability of species were not correlated with such traits as body size, generation time, or host range. Taxon-related trends, on the other hand, stood out; coleopterans tended to be abundant whereas lepidopterans were rare, and hemipterans tended to fluctuate widely whereas dipterans remained relatively constant. Furthermore, species that were clumped in space (onto stems within fields) tended to be abundant and variable. The "boom and bust" dynamics of these aggregating species may be related to the ability of colonies to survive the inroads of generalist predators. The low degree of regional synchrony in the performance of populations at different sites suggests that large-scale, weather-driven fluctuations are un- common. Our analysis of this well-developed, native insect fauna reveals a community that has a somewhat predictable structure that reflects the idiosyncracies of the component species. These species do not fluctuate in an integrated community-structuring manner, but as an outcome of each species performing at its "individualistic" level, the community displays a characteristic dominance hierarchy. The members of this diverse fauna rarely achieve densities at which they compete with one another. Our results are consistent with Hairston, Smith, and Slo- bodkin's hypothesis (1960) which predicts that terrestrial herbivores rarely deplete plant resources to levels at which there is widespread interspecific competition.