Disentangling vegetation diversity from climate-energy and habitat heterogeneity for explaining animal geographic patterns

Ecology and Evolution

Disentangling vegetation diversity from climate-energy and habitat heterogeneity for explaining animal geographic patterns

Introduction

One of the main aims of biogeography and ecology is to understand spatial diversity patterns and their major determinants. From a plethora of hypotheses focused on explaining geographic variation in species diversity, those related to climate-energy and habitat heterogeneity have received major empirical support. The climate-energy hypothesis

Abstract

Broad-scale animal diversity patterns have been traditionally explained by hypotheses focused on climate-energy and habitat heterogeneity, without considering the direct influence of vegetation structure and composition. However, integrating these factors when considering plant-animal correlates still poses a major challenge because plant communities are controlled by abiotic factors that may, at the same time, influence animal distributions. By testing whether the number and variation of plant community types in Europe explain country-level diversity in six animal groups, we propose a conceptual framework in which vegetation diversity represents a bridge between abiotic factors and animal diversity. We show that vegetation diversity explains variation in animal richness not accounted for by altitudinal range or potential evapotranspiration, being the best predictor for butterflies, beetles, and amphibians. Moreover, the dissimilarity of plant community types explains the highest proportion of variation in animal assemblages across the studied regions, an effect that outperforms the effect of climate and their shared contribution with pure spatial variation. Our results at the country level suggest that vegetation diversity, as estimated from broad-scale classifications of plant communities, may contribute to our understanding of animal richness and may be disentangled, at least to a degree, from climate-energy and abiotic habitat heterogeneity.

roots in the concept of productivity, proposing that the availability of water and energy controls plant productivity, which in turn has an influence on the diversity of herbivores and associated carnivores through bottom-up forcing. A complement to this view is the ambient-energy hypothesis that states that climatic factors may also directly influence the physiology of animals, especially endotherms. In addition, habitat (environmental)

heterogeneity has been proposed as an important driver of species diversity, with similar or higher predictive power than climate and energy. In its simple form, the habitat heterogeneity hypothesis posits that the spatial variation of abiotic or biotic factors shapes the realized niches of plants and animals in a given territory.

The impacts of climate-energy and habitat heterogeneity on animal diversity are obviously linked to plant diversity, as stated by Hutchinson: "The extraordinary diversity of terrestrial fauna is clearly due largely to the diversity provided by terrestrial plants." This relationship has been extensively tested, and a meta-analysis provided strong support for the use of plant species richness as a predictor of animal diversity, emphasizing the importance of cross-taxon correlates for understanding biodiversity patterns. However, the role of plants in determining patterns of animal diversity might also be linked to the attributes of plant communities in nature. Plant community processes, such as environmental filtering, interspecific interactions, dispersal limitation, biogeographic history, and neutral processes, are all to a large extent influenced by plant-animal interactions, including herbivory, pollination, and seed dispersal. Therefore, the diversity of plant community types (defined at any level of organization in a geographic area) is expected to correlate with animal diversity by reflecting different attributes of vegetation in ecosystems. This view was introduced as the vegetation structure hypothesis, stating that the vegetation physiognomy may shape the availability of niches for animals, and later expanded by studies arguing for a stronger influence of vegetation composition or floristics, opening an unresolved debate about the relationship between vegetation and animal diversity.

The complexity of plant-animal relationships creates a conceptual difficulty since it is far from trivial to disentangle the role of plant communities as a causal driver of animal diversity or as a coexisting counterpart controlled by broad-scale abiotic factors. Although plants and animals alike are influenced by spatial and historical factors, plant communities are at the same time a source of food and shelter for the latter, thereby affecting animal richness. However, we do not know of any rigorous tests looking at the conceptual integration of vegetation diversity (i.e., structure and composition of plant communities), climate-energy, and habitat heterogeneity hypotheses. Here, we propose a conceptual framework by which vegetation diversity (including both structure and composition)

represents a necessary bridge between abiotic factors and animal richness. According to this hypothesis, plant populations respond to abiotic factors, forming plant communities that vary in functional characteristics such as productivity and functional diversity (at this point, we intentionally disregard the important role of soil biota for the sake of simplification). The structure and floristic complexity of the plant communities provide biotic niches for animals, including bidirectional plant-animal interactions. In addition, animals may also be directly influenced by climate (as suggested by the ambient-energy hypothesis) and the abiotic habitat heterogeneity (through abiotic niches). This conceptual framework integrates the general expectations of both the climate-energy and the habitat heterogeneity hypotheses (but it contrasts with the current trend that considers vegetation diversity as a surrogate of habitat heterogeneity). Thus, we presume that biotic effects of vegetation result from not only the structure (physiognomy) but also the composition of plant communities (as predicted from previous studies at different scales).

In this paper, we attempt to disentangle the effect of vegetation diversity from the effects of climate-energy and abiotic habitat heterogeneity as explanations of animal geographic patterns. In our investigation, we analyze regional drivers of animal diversity in four vertebrate (mammals, birds, amphibians, and reptiles) and two invertebrate (beetles and butterflies) groups across large European regions. We considered species richness (regional number of species), the most common estimate of diversity, and regional dissimilarity (variation, or turnover in species composition) to identify spatial diversity patterns at broad scales. We expected that, at least for certain animal groups tightly dependent on plant communities (e.g., those with short-distance dispersal and narrower ecological niches), predictors of vegetation diversity might account for some variation not explained by factors related to climate-energy and abiotic habitat heterogeneity. We also expected that vegetation-animal relationships at the regional scale might change across different animal groups and across the two facets of diversity (richness and dissimilarity).