We review and synthesize recent developments in the study of the spread of invasive species, emphasizing both empirical and theoretical approaches. Recent theoretical work has shown that invasive species spread is a much more complex process than the classical models suggested, as long range dispersal events can have a large influence on the rate of range expansion through time. Empirical work goes even further, emphasizing the role of spatial heterogeneity, temporal variability, other species, and evolution. As in some of the classic work on spread, the study of range expansion of invasive species provides unique opportunities to use differences between theory and data to determine the important underlying processes that control spread rates.

Biological invasions represent both an increasingly important applied problem and a tool for gaining insight into the structure of ecological communities. Although competitive interactions between invasive and native species are considered among the most important mechanisms driving invasion dynamics, such interactions are in general poorly understood. The European honey bee (Apis mellifera) is a widespread and economically important invader long suspected to competitively suppress many native bee species. Yet the extent to which this introduced species alters native communities remains controversial, reflecting ongoing debate over the importance of resource competition in regulating pollinator populations. I experimentally tested the effects of competition with Apis on colony foraging behavior and reproductive success of a native eusocial bee, Bombus occidentalis Greene, in coastal California. B. occidentalis colonies located near experimentally introduced Apis hives had lower mean rates of forager return and a lower ratio of foraging trips for pollen relative to nectar. Both male and female reproductive success of B. occidentalis were also reduced with greater proximity to introduced Apis hives. Reproductive success correlated significantly with measures of colony foraging behavior, most strongly with the relative allocation of foraging effort to pollen collection. This pattern suggests that B. occidentalis colonies exposed to competition with Apis experienced increased nectar scarcity and responded by reallocating foragers from pollen to nectar collection, resulting in lowered rates of larval production. These results provide evidence that Apis competitively suppresses a native social bee known to be an important pollinator, with the potential for cascading effects on native plant communities. This work also contributes to a greater understanding of the role competitive interactions play in pollinator communities, particularly for social bees.

Time series of abundances are critical for understanding how abiotic factors and species interactions affect population dynamics, but are rarely linked with experiments and also scarce for bee pollinators. This gap is important given concerns about declines in some bee species. I monitored honey bee (Apis mellifera) and bumble bee (Bombus spp.) foragers in coastal California from 1999, when feral A. mellifera populations were low due to Varroa destructor, until 2014. Apis mellifera increased substantially, except between 2006 and 2011, coinciding with declines in managed populations. Increases in A. mellifera strongly correlated with declines in Bombus and reduced diet overlap between them, suggesting resource competition consistent with past experimental results. Lower Bombus numbers also correlated with diminished floral resources. Declines in floral abundances were associated with drought and reduced spring rainfall. These results illustrate how competition with an introduced species may interact with climate to drive local decline of native pollinators.

Developing tools for rapid assessment of introduced species impacts is one of the most important challenges in invasion ecology. Most assessments of impact rely on correlational data or other indirect measures. Yet few studies have evaluated invasion effects using multiple, simultaneously applied monitoring and experimental approaches, in order to compare easily obtained metrics with more difficult but direct measures of reproductive success or population dynamics. In this study, I use data from an experimental test of introduced honey bee (Apis mellifera) impacts on native bumble bees (Bombus spp.) to address two major questions: 1) how well did observational data on niche overlap and spatial correlations between Apis and Bombus predict the results of experimental tests of competitive effects? and 2) how well did effects of the experimental Apis manipulations on Bombus foragers, which are easy to observe, predict changes in reproductive success of colonies, which are difficult to measure? Niche overlap between Apis and Bombus varied substantially, but increased to levels as high as 80–90% during periods of resource scarcity. Correlations between numbers of Apis foragers and numbers of Bombus foragers were also highly variable, but I detected a significant negative relationship in only one of the seven months observed. In contrast, the experimental results showed that mean numbers of Bombus foragers observed on a given transect increased significantly with greater distance from introduced Apis colonies. Of these three measures (niche overlap, correlations in abundances, and effects of experimental introductions), only the experimental data on forager abundances accurately estimated competitive effects on colony reproductive success previously reported for the same experiment, and the correlational data in particular completely failed to predict the effects observed in the experimental study. This work suggests that great caution is warranted in making assessments of invasion impact on the basis of spatial or temporal correlations between invasive and native species. Thus, investing in even small and limited experimental studies may be more valuable than extensive observational work in quantifying invasion impacts.

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