Describing and explaining rarity: pattern vs. process
The field of ecology distinguishes between pattern and process: patterns are observable, repeated tendencies , whereas processes are the mechanisms that drive patterns. Historical approaches to understanding rarity can be divided along this pattern–process dichotomy: the causal theories of Stebbins and Fieldler & Ahouse sought to identify the fundamental processes that result in rarity, whereas the Rabinowitz framework took the approach of describing different patterns of rarity.
While patterns are linked to ecological processes, they are also significantly affected by a second class of causal factors: contingencies, which are external factors that impact ecological patterns . Examples of contingencies include climate change, ecological drift, availability of specific habitats, and human or natural disturbances. Here, we use an expanded definition of contingencies that also accounts for geographic factors (e.g., dispersal barriers) that can influence patterns of occurrence in ways that could not be predicted from a strictly process-based perspective.
Ecological patterns should thus be understood as arising from the interaction between ecological processes and contingencies. Contingencies can complicate and obscure the search for general patterns in ecology ; however, by recognizing the separate, but interrelated roles of both contingency and process, we hope to identify the key processes that lead to rarity despite the vast diversity in patterns of rarity that stem from the influence of contingencies.