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The oviposition behaviour of the butterfly Anthocharis cardamines has been examined, using the methods of strong inference to investigate foodplant choice. Adaptive explanations for females ovipositing mainly on unshaded, young and large individuals of Alliaria petiolata are rejected in favour of explanations based on 'apparency' to searching females. Floral characters shown to influence intraspecific foodplant apparency are then examined in comparisons between crucifer species, and are shown to explain well the observed deposition of A. cardamines eggs. Cruciferae such as Barbarea vulgaris and Hesperis matronalis, although poor for larval survival, receive many butterfly eggs as a result of large, persistent inflorescences. The contrasting and opposing effects of hostplant apparency and defence are discussed.

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The spotted owl (Strix occidentalis) is a threatened species in many areas of its western North American range. Concomitant with its decline has been a rapid invasion of its range and habitat by barred owls (Strix varia), a native species that was restricted, until relatively recently, to eastern North America. We assess the theoretical potential for negative interactions between these two owls by examining size dimorphism and ecological relationships within various owl assemblages throughout the world. We then review the anecdotal, natural history, modeling, and experimental evidence that suggest barred owls may negatively affect spotted owls with at least a potential for the competitive exclusion of spotted owls by barred owls throughout all or part of the former's range. While it is widely accepted that barred owls are either causing or exacerbating declines of spotted owl populations, there are confounding factors, such as habitat loss and bad weather that also may contribute to declines of spotted owls. Both theory and empirical information suggest that barred owls are likely to have negative effects on spotted owl range and density, but the degree of the impact is not predictable. There is a conservation conundrum here, in that the barred owl is a native species that has expanded its range westwards, either naturally or with a degree of human facilitation, and now constitutes a major threat to the viability of another native species, the threatened spotted owl. We propose that only through carefully designed experiments involving removal of barred owls will we be able to determine if recent declines in spotted owl populations are caused by barred owls or by other factors. It is rare in conservation science that replicate study areas exist for which we also have long-standing demographic information, as is the case with the spotted owl. Removal experiments would take advantage of the wealth of data on spotted owls, and allow ecologists to assess formally the impacts of an invasive species on a threatened species, as well as to suggest mitigation measures.

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