Understanding the biogeography and genetics of the invasion of oceanic islands by invasive pest species can provide insights into how to limit such incursions. The invasive pasture pest weevil, Listronotus bonariensis, has established in both New Zealand and some of its offshore islands, causing significant economic damage. The opportunity was therefore taken to compare both the species’ morphology and genetics of populations collected from the New Zealand mainland, Stewart Island (30 km offshore) and Chatham Island (860 km offshore). The L. bonariensis individuals from both of the offshore populations were found to be significantly smaller than those of the mainland populations. However, based on classical and molecular taxonomy, no associated species-level differences were found, and it was concluded that the island’s smaller populations were the result of relatively poor habitat quality. Further, no significant genetic differences were detected between Stewart Island and mainland populations, indicating a close and frequent connection. Conversely, at the genomic level, the Chatham Island population was significantly different from the two other populations, and this was attributed to probable founder effects. The evidence points to a small subset of what was still a recently-established New Zealand L. bonariensis population early in the 20th century, being inadvertently transported to the Chatham Islands. Thereafter, this founding subset expanded such that more recent introductions have had little effect on the founding genetics of weevils on the island. This study indicates that irregular, unintended human transportation of species to isolated islands can lead to the occupation of previously uninhabited niches, with founder effects leading to genetic divergence from conspecific populations elsewhere.

Species must simultaneously adapt to climate stressors and other species, though available genetic variation may constrain this adaptation. Although evolutionary responses to climate can alter interactions among species, it is unknown how the intensity of selection by natural enemies influences species’ ability to withstand (i.e. survive and reproduce following) climate extremes like heat shock, and whether genetic diversity moderates these eco-evolutionary processes. Here we test whether impacts of heat shock on Drosophila simulans (host) fitness depend on their population’s history of interactions with a parasitoid or on the available host and parasitoid genetic diversity (manipulated by inbreeding). We exposed hosts to parasitoid populations over 11 host generations, then exposed their offspring and control hosts to experimental heat shocks. Heat shock more negatively affected the fitness of host populations with a history of high parasitism rates. Surprisingly, less-inbred hosts suffered more severely from heat shock, particularly when they had high historical parasitism rates. However, historically low parasitism rates were associated with a significantly reduced impact of heat shock on fitness relative to no or high parasitism, particularly for less-inbred hosts. Together these results suggest that genetically diverse host populations may retain heat-shock-vulnerable genotypes at high densities (perhaps due to a competition-tolerance trade-off), whereas lighter parasitism (at the approximate rates seen in nature) may prevent this accumulation of genotypes with low tolerance. The intensity of trophic interactions can therefore moderate species’ fitness responses to environmental change in non-linear ways.

Species diversity of consumers is known to promote high attack rates when species differ in their traits and the resources they use. Yet, despite considerable work on the role of species diversity, we lack information on how genetic diversity at different trophic levels affects rates of ecosystem processes such as trophic interactions. Here, we assess whether the available genetic diversity in three trophic levels influences parasitism rates, both within and across generations. We used experimental population bottlenecks to create a gradient of decreasing genetic diversity of hosts and parasitoids, and measured rates of parasitism, host resistance to parasitism, and host survival over 11 host generations. Less-inbred parasitoid populations exerted higher parasitism rates (analogous to known effects of species diversity), however, this effect became weaker over time. We conclude that genetic diversity can have short-term ecological impacts in trophic systems, which can be altered in the long term by evolutionary processes.