The role of spatial scale and the perception of large‐scale species‐richness patterns

TLDR

Our knowledge of the factors shaping global species distributions remains limited, partly because perceived patterns of species richness vary and previous studies have assumed scale invariance. The review aims to show how scale effects on extent and grain size shape perceived species‑richness patterns and to identify consistent patterns across spatial and temporal scales. The authors analyze 204 published altitudinal gradient studies, using examples and quantitative methods to assess how extent and grain size affect perceived patterns. The analysis shows that hump‑shaped patterns occur in about half of studies, decreasing patterns in a quarter, but the relative frequencies shift with spatial grain and extent, and the review proposes future analytical directions.

Abstract

Abstract Despite two centuries of exploration, our understanding of factors determining the distribution of life on Earth is in many ways still in its infancy. Much of the disagreement about governing processes of variation in species richness may be the result of differences in our perception of species‐richness patterns. Until recently, most studies of large‐scale species‐richness patterns assumed implicitly that patterns and mechanisms were scale invariant. Illustrated with examples and a quantitative analysis of published data on altitudinal gradients of species richness ( n = 204), this review discusses how scale effects (extent and grain size) can influence our perception of patterns and processes. For example, a hump‐shaped altitudinal species‐richness pattern is the most typical ( c . 50%), with a monotonic decreasing pattern ( c . 25%) also frequently reported, but the relative distribution of patterns changes readily with spatial grain and extent. If we are to attribute relative impact to various factors influencing species richness and distribution and to decide at which point along a spatial and temporal continuum they act, we should not ask only how results vary as a function of scale but also search for consistent patterns in these scale effects. The review concludes with suggestions of potential routes for future analytical exploration of species‐richness patterns.