Abstract

Simple SummaryCertain moths defend themselves with toxic substances, and they show this to predators with bright and contrasting coloration. At high elevations, fewer birds, bats and other predators are present that feed on these insects. We therefore expected a decreasing proportion of colorful tiger and lichen moths with increasing elevation. Our study was carried out in forests between 1040 and 2670 m in the Ecuadorian Andes. We scored all 353 observed species according to their warning coloration, and whether they mimic bees and wasps or potentially poisonous beetles. We also measured forewing length of all species. From these data community-weighted means were calculated and related to the elevation of their collection sites. As predicted, the communitywide incidence of all three defensive traits decreased substantially from tall premontane forest to open upper montane forest. In parallel, moth size increased with elevation. Moreover, the systematic composition of tiger and lichen moth assemblages changed substantially. These findings support the idea that different selection regimes favor warning coloration at lower sites with higher predation pressure, while cryptic appearance is more advantageous at higher elevations.On tropical mountains, predation pressure decreases with elevation. Accordingly, one expects an elevational decay in the prevalence of costly defensive traits such as aposematic coloration. Using light-trap catches of Arctiinae moths (353 species, 4466 individuals), assembled along a forested gradient in the megadiverse tropical Andes of southern Ecuador, we show that the incidence of aposematic coloration decreases strongly between 1040 and 2670 m asl. While over 60% of Arctiinae moths were warningly colored at lowest sites, this fraction decreased to less than 20% in montane forest, yet increased slightly again at the highest sites in the very open Purdiaea nutans forest. In parallel, the incidence of hymenopteran mimics and of species that mimic chemically defended beetles decreased with elevation. Hymenopteran mimics accounted for less than 5% of Arctiinae moths at sites above 2100 m, and beetle mimics were essentially lacking at high elevations. These patterns coincide with a change in gross taxonomic composition of Arctiinae ensembles and with an increase in average body size towards higher elevations. Representatives of Euchromiina and Ctenuchina became scarce with altitude, whereas the prevalence of Lithosiinae increased. Our findings suggest that the variable selective pressures along the elevational gradient favor warning coloration primarily at lower sites, whereas cryptic appearance of adult moths dominates in the tropical upper montane forest.

Highlights

  • Introduction distributed under the terms andPredation pressure is an important selective force in all food webs

  • If predation pressure varies in a predictable manner along an ecological gradient, one expects to see this mirrored by gradients in the prevalence of defensive traits, across species, at the community level

  • The incidence of warning coloration among megadiverse tropical Andean Arctiinae moths continually and strongly decreased towards higher elevations. This pattern is in line with expectations based on studies in other tropical realms which indicated that predation pressure, especially by insectivorous birds, declines with elevation [36,37]

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Summary

Introduction

Introduction distributed under the terms andPredation pressure is an important selective force in all food webs. Resources need to be allocated between defense and reproduction, sequestered plant secondary compounds need to be handled, modified and stored in the body, and certain traits may be advantageous against some predators, but not against others. For each species the balance between the costs and benefits of expressing defensive traits needs to be met [3]. Potentially suitable secondary plant compounds to be sequestered for defense are not available in all host plants. Potential prey species in a food web vary in their level of defense. If predation pressure varies in a predictable manner along an ecological gradient, one expects to see this mirrored by gradients in the prevalence of defensive traits, across species, at the community level

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