Abstract
The species-area relationship (SAR) is one of the most well-established scaling patterns in ecology. Its implications for understanding how communities change across spatial gradients are numerous, including the effects of habitat loss on biodiversity. However, ecological communities are not mere collections of species. They are the result of interactions between these species forming complex networks that tie them together. Should we aim to grasp the spatial scaling of biodiversity as a whole, it is fundamental to understand the changes in the structure of interaction networks with area. In spite of a few empirical and theoretical studies that address this challenge, we still do not know much about how network structure changes with area, or what are the main environmental drivers of these changes. Here, using the meta-network of potential interactions between all terrestrial vertebrates in Europe (1140 species and 67 201 feeding interactions), we analysed network-area relationships (NARs) that summarize how network properties scale with area. We do this across ten biogeographical regions, which differ in environmental characteristics. We found that the spatial scaling of network complexity strongly varied across biogeographical regions. However, once the variation in SARs was accounted for, differences in the shape of NARs vanished. On the other hand, the proportion of species across trophic levels remained remarkably constant across biogeographical regions and spatial scales, despite the great variation in species richness. Spatial variation in mean annual temperature and habitat clustering were the main environmental determinants of the shape of both SARs and NARs across Europe. Our results suggest new avenues in the exploration of the effects of environmental factors on the spatial scaling of biodiversity. We argue that NARs can provide new insights to analyse and understand ecological communities.
Highlights
One of the most fundamental patterns in ecology is the increase of the total number of species as the area sampled increases (Arrhenius 1921, Rosenzweig 1995, Lawton 1999)
The scaling exponents of the number of links with area ranged between 0.16 (Pannonian) and 0.77 (Alpine), meaning that communities were gaining twice as many links as species with increasing area. This was consistent across bioregions. Exceptions to this pattern were the Arctic and Boreal regions, which showed a scaling in the number of links with area (z = 0.46 and z = 0.25, respectively) closer to that observed for the number of species (z = 0.31 and z = 0.15)
The spatial scaling of biodiversity has been traditionally understood as exclusively the scaling of species richness with area size (Arrhenius 1921, Rosenzweig 1995, Lawton 1999)
Summary
One of the most fundamental patterns in ecology is the increase of the total number of species as the area sampled increases (Arrhenius 1921, Rosenzweig 1995, Lawton 1999). Galiana et al (2018) presented a theoretical framework predicting the existence of multiple network–area relationships (NARs) arising from different spatial mechanisms, such as the existence of different SARs across trophic levels, the higher prevalence of generalist species at small spatial extents due to their higher probability of finding a resource, or the effects of dispersal limitation on species spatial turnover. These findings suggest that we should expect network properties to change differently with area size depending on how environmental factors (e.g. habitat heterogeneity) affect, for instance, species’ dispersal capabilities.
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