The Role of Disturbance in Stream Ecology

TLDR

Disturbance in stream ecosystems is defined as a discrete event with unpredictable frequency, intensity, or severity that alters ecosystem structure, and its effects are framed by dynamic equilibrium, intermediate disturbance, and equilibrium hypotheses, with variations across lentic versus lotic systems, channel forms, and regional responses, while experimental studies face spatial, logistical, and ethical challenges. The study proposes general questions for stream ecologists and identifies hydrologic regimes and comparable geomorphology as key criteria for selecting streams in comparative disturbance research. The authors describe a method to quantify hydrologic regime predictability from long‑term flow records and outline structural and functional components for comparing disturbance effects across regions and biomes.

Abstract

We define disturbance in stream ecosystems to be: any relatively discrete event in time that is characterized by a frequency, intensity, and severity outside a predictable range, and that disrupts ecosystem, community, or population structure and changes resources or the physical environment. Of the three major hypotheses relating disturbance to lotic community structure, the dynamic equilibrium hypothesis appears to be generally applicable, although specific studies support the intermediate disturbance hypothesis and the equilibrium model. Differences in disturbance frequency between lentic and lotic systems may explain why biotic interactions are more apparent in lakes than in streams. Responses to both natural and anthropogenic disturbances vary regionally, as illustrated by examples from the mid-continent, Pacific northwest, and southeastern United States. Based on a generalized framework of climatic-biogeochemical characteristics, two features are considered to be most significant in choosing streams for comparative studies of disturbance: hydrologic regimes and comparable geomorphology. A method is described for quantifying predictability of the hydrologic regime based on long-term records of monthly maximum and minimum stream flows. Different channel forms (boulder and cobble, alluvial gravelbed, alluvial sandbed) have different responses to hydrologic disturbance from spates. A number of structural and functional components for comparing disturbance effects within regions and across biomes are presented. Experimental approaches to studying disturbance involve spatial-scale considerations, logistic difficulties, and ethical questions. General questions related to disturbance that could be addressed by stream ecologists are proposed.