Landscape-level bird loss increases the prevalence of honeydew-producing insects and non-native ants

Abstract

Bird exclusion experiments consistently show that birds exhibit strong top-down control of arthropods, including ants and the honeydew-producing insects (HPIs) that they tend. However, it remains unclear whether the results of these small-scale bird exclosure experiments can be extrapolated to larger spatial scales. In this study, we use a natural bird removal experiment to compare the prevalence of ants and HPIs between Guam, an island whose bird community has been extirpated since the 1980s due to the introduction of the brown tree snake, and two nearby islands (Rota and Saipan) that have more intact bird assemblages. Consistent with smaller-scale bird exclosure experiments, we show that (1) forest trees from Guam are significantly more likely to host HPIs than trees from Saipan and (2) ants are nearly four times as abundant on Guam than on both Saipan and Rota. The prevalence of HPIs varied slightly based on tree species identity, although these effects were not as strong as island-level effects associated with bird loss. Ant community composition differed between Guam and the other two islands. These results corroborate past observational studies showing increased spider densities on Guam and suggest that trophic changes associated with landscape-level bird extirpation may also involve alterations in the abundance of ants and HPIs. This study also provides a clear example of the strong indirect effects that invasive species can have on natural food webs.Electronic supplementary materialThe online version of this article (10.1007/s00442-018-4273-5) contains supplementary material, which is available to authorized users.

The evolutionary ecology of bird–ant interactions: a pervasive but under-studied connection

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The functional roles of psyllid abundance and assemblage on bird-associated forest defoliation

“…Outbreaks of insect herbivores and the types of damage they cause to their hosts are key factors affecting tree health and trophic interactions (Strong et al 2000;Hall et al 2015;Steinbauer et al 2015;Freedman et al 2018). Tree dieback is a phenomenon affecting forests globally, which urgently needs to be better understood (Fischer et al 2010).…”