The shifting habitat mosaic of river ecosystems

摘要

The essence of ecology is to understand the distribution and abundance of biota ( Andre wartha & Birch 1954). In the same vein, a cornerstone of ecology is quantifying how and why organisms are dependent on specific biophysical space (habitat) to complete one stage or another in their life cycles (Southwood 1977). On the one hand, phenotypic plasticity promotes successful growth and reproduction in variable habitats, but on the other hand habitat fidelity over several to many generations may constrain (adapt) the species or life stage to a habitat with quite specific spatial or functional attributes. Conservation biologists sometimes refer to these locally adapted populations with habitat-specific life cycles as ecologically significant units. Such populations have been accorded special protection and management if they are rare or declining in numbers. However, habitat intrinsically is not static, owing to constantly changing successional (or gradient) states as landscape is mediated by interactive physical (e.g. flood, drought, fire) and biological (e.g. disease, predation, invasion) drivers. Thus, physical and biological attributes vary in time and space and interact to determine quantity and quality of specific habitat per life stage. Sufficient quality habitat is required to permit a positive life history energy balance to sustain a population over the long term, otherwise extinction occurs (Hall et al. 1992). Particular species, and even particular populations of species, either adapt to the dynamic nature of habitat or they fail to persist in that landscape. Of course, a given landscape is composed of n-dimensional gradients and species responses, and feedbacks are complex and nonlinear, making habitat per life stage of each species in the landscape very difficult to define. Nonetheless, quantifying habitat for species in very specific spatial and temporal terms is fundamental to conservation of biodiversity world wide.