Nested hierarchies and scale-dependence of mechanisms of flow refugium use

We outline a hierarchical framework of classes of mechanisms, operating at different spatial and temporal scales, by which populations may exploit environmental heterogeneity to ensure persistence in environments subject to physical disturbance. Examples are provided for stream systems subject to high flow disturbances. Genotypic heterogeneity, encompassing morphological and physiological adaptations, operates at evolutionary and biogeographic scales, and ultimately constrains mechanisms at smaller scales. On ecological scales, organisms may exploit temporal or spatial heterogeneity, or both, to maintain population sizes larger than possible by morphological adaptations alone. For 2 classes of mechanism operating at large scales (>1 generation and >1 habitat patch), individuals do not survive disturbances and population persistence depends on recruitment from external sources. Where generations overlap and populations exploit temporal refugia in complex life cycles, dispersal occurs between habitat patches of different types. Alternatively dispersal occurs between habitat patches of similar type. For 2 other classes of mechanism, operating at smaller scales (<1 generation and less than or equal to 1 habitat patch), individuals survive disturbances by moving between microhabitat patches of different types, or by changes in habitude which reduce the negative effects of disturbance. A simple mathematical model is used to explore the efficacy of 4 mechanisms of refugium use operating through microhabitat heterogeneity within a single habitat patch. Simulations of population changes over a series of disturbance events showed that each of 4 mechanisms could maintain a viable population under certain conditions. Total refugium area and proportion of the population lost at each disturbance event had a strong effect on final population size for all mechanisms. Doubling disturbance frequency had little effect on final population size if movements of individuals into refugia were facilitated by small accessible refugium patches, or if individuals remained in refugia during benign inter-disturbance periods. Empirical evidence supports the efficacy of these mechanisms: organisms with short life cycles and, perhaps, poor mobility may be lost from erosive patches but persist in refugia; macroinvertebrates may accumulate in refugia during disturbance events and redistribute throughout the stream after the disturbance. More empirical data on the spatial attributes of refugium patches and the mobility of stream organisms are required to test and to add realism to these models.