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.