A spatially explicit analysis of Paysandisia archon attack on the endemic Mediterranean dwarf palm

Abstract

Autochthonous plant species are heavily threatened by the increasing spread of invasive insects. The spatial distribution of invasive species' hosts is likely to play a pivotal role in the establishment and further expansion of the invading species; more specifically, distance and density dependent (DDD) processes linked to plant spatial arrangement are crucial in determining susceptibility to attack, but they have usually been overlooked in invasive research. We take a spatially explicit approach to evaluate potential DDD processes in the interaction between Paysandisia archon, a tropical lepidopteran recently introduced in Spain, and the endemic Mediterranean dwarf palm (Chamaerops humilis) in Mallorca (Balearic Islands). Specifically, we used spatial marked point pattern analyses, which allowed testing whether DDD processes affect attack probability and intensity on three dwarf palm populations corresponding to three different invasion phases (i.e., infestation core, early expansion, and expansion front). Our approach also allowed evaluation of whether and how intrinsic palm traits (size, sex) alter the proneness to P. archon attack over a range of spatial scales. The occurrence and nature of DDD effects on C. humilis performance varied largely among localities. At the infestation core, our analyses revealed positive density dependence, i.e., reduced damage at high densities or proximity to conspecific neighbors. By contrast, the early expansion locality showed negative density dependence of C. humilis performance, whereas at the expansion front, there was no evidence of DDD effects. Larger palms were consistently more prone to P. archon attack than small ones up to scales of 50 m. We found no evidence that palm sex altered the probability of attack by P. archon. Our results highlight the importance of spatially explicit analyses for assessing invasive events and point to the need of early interventions and prioritizing management efforts on larger palms in order to guarantee the conservation of autochthonous dwarf palm populations.