Invasive vegetation encroachment modulates dual threats faced by island-endemic scaly crickets

Abstract

AbstractThe mechanisms by which non-native species impact island-endemic populations are varied and often convoluted. For effective invertebrate conservation where capacity is limited, it is vital to understand both direct and indirect species interactions. Those interactions define specific threats and therefore highlight possible solutions. We quantified several of the non-native species threats faced by an endemic scaly cricket, Discophallus ascension (family: Mogoplistidae), found on Ascension Island’s naturally barren coastlines. We hypothesized that encroaching non-native Mexican thorn Prosopis juliflora (1) directly reduced microclimate suitability, (2) indirectly increased resource competition with non-native species, and (3) indirectly increased non-native predation. Our aim was to assess how rapidly spreading Mexican thorn might reduce endemic cricket populations. Lab trials first confirmed the temperature and diet preferences of D. ascension. Mexican thorn significantly increased ground-level temperature but not above the estimated optimum for D. ascension within key habitat. Observed interspecific resource competition was significantly greater in habitat degraded by Mexican thorn and was driven by non-native ants, house crickets Gryllodes sigillatus and cockroaches Periplaneta americana. Recorded cricket predation was almost entirely by non-native rats Rattus rattus in barren habitat but primarily by ants in Mexican thorn habitat. Threats to D. ascension were therefore modulated by invasive vegetation, but driven by non-native insects and rats at all sites. We conclude that removal of vegetation alone is likely to reduce resource competition but not predation. Managing multiple invasive species is likely necessary for successful species conservation.