Of the many exotic plants that have become naturalized in North America, only a small proportion are pests capable of invading and dominating intact natural communities. In the present study, we tested the hypothesis that the most invasive plants are phytochemically unique in their new habitats. A comparison of exotic plant species that are highly invasive in North America with exotics that are widespread, but non-invasive revealed that the invasive plants were more likely to have potent secondary compounds that have not been reported from North American native plants. On average, the compounds found in the invasive plants were reported from fewer species, fewer genera and fewer families than those from non-invasive plants. Many of the unique phytochemicals from invasive plants have been reported to have multiple activities, including antiherbivore, antifungal, antimicrobial and allelopathic (phytotoxic) effects, which may provide the plants with several advantages in their new environments.

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 phytophagous 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 occasionally 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 a...

Summary 1We tested the enemy release hypothesis for invasiveness using field surveys of herbivory on 39 exotic and 30 native plant species growing in natural areas near Ottawa, Canada, and found that exotics suffered less herbivory than natives. 2 For the 39 introduced species, we also tested relationships between herbivory, invasiveness and time since introduction to North America. Highly invasive plants had significantly less herbivory than plants ranked as less invasive. Recently arrived plants also tended to be more invasive; however, there was no relationship between time since introduction and herbivory. 3 Release from herbivory may be key to the success of highly aggressive invaders. Low herbivory may also indicate that a plant possesses potent defensive chemicals that are novel to North America, which may confer resistance to pathogens or enable allelopathy in addition to deterring herbivorous insects.

We surveyed naturally occurring leaf herbivory in nine invasive and nine non-invasive exotic plant species sampled in natural areas in Ontario, New York and Massachusetts, and found that invasive plants experienced, on average, 96% less leaf damage than non-invasive species. Invasive plants were also more taxonomically isolated than non-invasive plants, belonging to families with 75% fewer native North American genera. However, the relationship between taxonomic isolation at the family level and herbivory was weak. We suggest that invasive plants may possess novel phytochemicals with anti-herbivore properties in addition to allelopathic and antimicrobial characteristics. Herbivory could be employed as an easily measured predictor of the likelihood that recently introduced exotic plants may become invasive.

ABSTRACT. Many habitat fragmentation experiments make the prediction that animal population density will be positively related to fragment, or patch, size. The mechanism that is supposed to result in this prediction is unclear, but several recent reviews have demonstrated that population density often is negatively related to patch size. Immigration behavior is likely to have an important effect on population density for species that do not show strong edge effects, for species that have low emigration rates, and during short-term habitat fragmentation experiments. We consider the effect that different kinds of immigration behaviors will have on population density and we demonstrate that only a minority of possible scenarios produce positive density vs. patch size relationships. More commonly, these relationships are expected to be negative. Our results demonstrate the importance of considering autecological mechanisms, such as immigration behavior, when developing the predictions that we test in habitat fragmentation or other experiments.