Disturbance and trajectory of change in a stream fish community over four decades.

Communities can change gradually or abruptly, and directionally (to an alternate state) or non-directionally. We briefly review the history of theoretical and empirical perspectives on community change, and propose a new framework for viewing temporal trajectories of communities in multivariate space. We used a stream fish dataset spanning 40 years (1969-2008) in southern Oklahoma, USA, emphasizing our own 1981-2008 collections which included well-documented, extreme drought and flood events, to assess dynamics of and environmental factors affecting the fish community. We evaluated the trajectory of the Brier Creek community in multivariate space relative to trajectories in 27 published studies, and for Brier Creek fish, tested hypotheses about gradual versus event-driven changes and persistence of shifts to alternate states. Most species were persistent, qualitatively, across the four decades, but varied widely in abundance, with some having unusually strong reproduction after extreme droughts. The community had an early period of relatively gradual and directional change, but greater displacement than predicted at random after two consecutive extreme droughts midway through the study (1998 and 2000). But, the community subsequently returned toward its former state in the last decade. This fish community is characterized by species that are tolerant of environmental extremes, and have life history traits that facilitate population recovery. The community appears "loosely stable" about a long-term average condition, but the impacts of the two consecutive droughts were substantial, and may foretell future dynamics of this or other communities in a changed global climate if disturbance events become more frequent or severe.