In this review, we synthesize the current knowledge of the ecology and impacts of Rhamnus cathartica L., a shrub from Europe and Asia that is a successful invader in North America. Physiological studies have uncovered traits including shade tolerance, rapid growth, high photosynthetic rates, a wide tolerance of moisture and drought, and an unusual phenology that may give R. cathartica an advantage in the environments it invades. Its high fecundity, bird-dispersed fruit, high germination rates, seedling success in disturbed conditions, and secondary metabolite production may also contribute to its ability to rapidly increase in abundance and impact ecosystems. R. cathartica impacts ecosystems through changes in soil N, elimination of the leaf litter layer, possible facilitation of earthworm invasions, unsubstantiated effects on native plants through allelopathy or competition, and effects on animals that may or may not be able to use it for food or habitat.

Heterogeneity in quantity and quality of resources provided in the urban matrix may mitigate adverse effects of urbanization intensity on the structure of biotic communities. To assess this we quantified the spatial variation in butterfly richness and abundance along an impervious surface gradient using three measures of urban matrix quality: floral resource availability and origin (native vs. exotic plants), tree cover, and the occurrence of remnant habitat patches. Butterfly richness and abundance were surveyed in 100 cells (500 9 500 m), selected using a random-stratified sampling design, across a continuous gradient of imperviousness in Melbourne, Australia. Sampling occurred twice during the butterfly flight season. Occurrence data were analyzed using generalized linear models at local and mesoscales. Despite high sampling completeness, we did not detect 75% of species from the regional species pool in the urban area, suggesting that urbanization has caused a large proportion of the region's butterflies to become absent or extremely rare within Melbourne's metro-area. Those species that do remain are largely very generalist in their choice of larval host plants. Butterfly species richness and abundance declined with increasing impervious surface cover and, contrary to evidence for other taxa, there was no evidence that richness peaked at intermediate levels of urbanization. Declines in abundance appeared to be more noticeable when impervious surface cover exceeded 25%, while richness declined linearly with increasing impervious surface cover. We find evidence that the quality of the urban matrix (floral resources and remnant vegetation) influenced butterfly richness and abundance although the effects were small. Total butterfly abundance responded negatively to exotic floral abundance early in the sampling season and positively to total floral abundance later in the sampling season. Butterfly species richness increased with tree cover. Negative impacts of increased urbanization intensity on butterfly species richness and abundance may be mitigated to some extent by improving the quality of the urban matrix by enhancing tree cover and the provision of floral resources, with some evidence that native plants are more effective.

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