Aims Species interactions regulate the invasiveness of non-native species and as declines of native tree species escalate, exotic tree species that offer supplementary resources to animal seed-dispersers should expand their distributions as they fill ecological roles. Our primary objective was to forecast impacts from an imminent biological invasion (laurel wilt disease) by quantifying resources provided by native (threatened) and exotic fruits (disease-resistant) and associated bird foraging preferences.
Methods In the southeastern USA, we tested for redundancy among the resources provided by native and exotic fruits to overwintering birds. Comparisons between abundant subcanopy species Persea borbonia (native) and Cinnamomum camphora (exotic) were paramount considering the widespread disease-induced decline of P. borbonia, and the biological and phylogenetic similarities between these species. Across two winter survey periods, we quantified fruit removal and documented bird species using motion-activated cameras in the field. Physical and chemical fruit characteristics were also quantified.
Important findings Foraging bouts on both P. borbonia and C. camphora fruits were documented for four native bird species. There was no difference in selectivity between fruit types during Year 1 of our survey, but there was a significant preference for C. camphora fruit in Year 2; the change in preference was correlated with significantly lower temperatures in Year 2. While the pulp/seed ratio and moisture content differed, the nutritional content of fruit pulp (g/100g) was similar between fruit types. Given the apparent redundancy among these native and exotic fruit resources, we forecast increases in the consumption and dispersal of exotic propagules following the widespread laurel wilt disease-induced decline of P. borbonia and other native fruit bearing members of Lauraceae. This empirically based prediction is among the first to document exotic forest pathogens as indirect threats to native bird–plant interactions and potential facilitators of exotic plant invasion.