Impact of Predators on Arthropod Herbivores and Herbivory along Mountain Ranges on Islands Versus Mainland

Abstract

Trophic interactions between plants, herbivorous arthropods, and their predators are complex and prone to trophic cascades. Understanding these interactions is important, yet they received little attention especially for marine islands. Nevertheless, marine islands offer systems where species interactions are tractable, and their underlying drivers can be explicitly assessed. Herbivory and predation rate are expected to be lower on islands relative to mainland given the lower abundance and diversity across trophic levels. In addition, herbivory and predation are expected to be higher at low elevations due to reduced abiotic stress and more stable abiotic conditions. However, elevational clines in biotic and abiotic conditions may vary on islands relative to mainland, setting the stage for differential gradients on each landform type. Indeed, mountainous islands provide a key opportunity to test how insularity shapes trophic interactions along abiotic clines and departures from these patterns relative to mainland. To address this, we conducted a meta-analysis of predator exclusion studies on both landform types aimed to test for the strength of predation pressure on arthropod communities and herbivory. Our findings indicate that insectivorous vertebrates significantly reduce arthropod abundances and herbivore damage, and such effect was stronger overall on islands than on mainland but did not vary significantly with elevation for either landform type. This latter result could be explained by stronger effects of vertebrates on invertebrate predators at low elevations, thus dampening predator effects on herbivory at low elevation via intra-guild predation. In contrast, mean effects of invertebrate predators, i.e. ants, on herbivory did not differ between mainland and islands (though significant increases in herbivores and herbivory were found on mainland but not islands), but did show a significant decrease with elevation which was statistically indistinguishable across landform types. Combined, these results show strong contingency in responses based on the type of predator type and counter predictions of weaker top-down control on islands, at higher elevations, and further suggest no qualitative differences in elevational clines in predation pressure across environment types. These findings deserve attention in future mechanistic tests with different types of predation, herbivory, and plant traits.

Full text available upon request

Sam, K., Mrazova, A., Houska Tahadlova, M., Kollross, J., & Maraia, H. (2024). Impact of predators on arthropod herbivores and herbivory along mountain ranges on islands versus mainland. In Ecology and evolution of plant-herbivore interactions on islands (pp. 199-217). Cham: Springer International Publishing.