Spatial extent of neighboring plants influences the strength of associational effects on mammal herbivory. Insights from a meta-analysis

There is high variability in the level of herbivory between individual plants from the same species with potential effects on population dynamics, community composition, and ecosystem structure and function. This variability can be partly explained by associational effects i.e. the impact of the presence of neighboring plants on the level of herbivory experienced by a focal plant, but it is still unclear how the spatial scale of plant neighborhood modulates foraging choice of herbivores; an inherently spatial process in itself. Using a meta-analysis, we investigated how spatial scale modifies associational effects on the susceptibility to browsing by herbivores with movement capacities similar to deer. From 2496 articles found in literature databases, we selected 46 studies providing a total of 168 differences of means in damage by herbivores or survival to woody plants (mostly) with and without neighboring plants. Spatial scales were reported as distance between plants or as plot size. We estimated the relationships between the effect sizes and spatial scale, type of associational effects and nature of the experiment using meta-analysis mixed models. The strength of associational effects declined with increasing plot size, regardless of the type of associational effects. Associational defences (i.e. decrease in herbivory for focal plants associated with unpalatable neighbors) had stronger magnitude than associational susceptibilities. The high remaining heterogeneity among studies suggests that untested factors modulate associational effects, such as nutritional quality of focal and neighboring plants, density of herbivores, timing of browsing, etc. Associational effects are already considered in multiple restoration contexts worldwide, but a better understanding of these relationships could improve their use in conservation, restoration and forest exploitation when browsing is a concern. This study is the first to investigate spatial patterns of associational effects across species and ecosystems, an issue that is essential to determine differential herbivory damages among plants.