Why Disturbances Can Be Predictable: A Perspective on the Definition of Disturbance in Streams Author(s): N. LeRoy Poff Source: Journal of the North American Benthological Society, Vol. 11, No. 1 (Mar., 1992), pp. 86-92 Published by: The University of Chicago Press on behalf of the Society for Freshwater Science Stable URL: https://www.jstor.org/stable/1467885 Accessed: 26-08-2019 01:31 UTC REFERENCES Linked references are available on JSTOR for this article: https://www.jstor.org/stable/1467885?seq=1&cid=pdf-reference#references_tab_contents You may need to log in to JSTOR to access the linked references. JSTOR is a not-for-profitSociety for service Freshwater that helps Science, scholars, The researchers, University and of students Chicago discover, Press are use, collaborating and build upon with a wide JSTOR to digitize, preserve and extend access to Journal of the North American range of content Benthologicalin a trusted digital Society archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms This content downloaded from 24.9.112.12 on Mon, 26 Aug 2019 01:31:04 UTC All use subject to https://about.jstor.org/terms J. N. Am. Benthol. Soc., 1992, 11(1):86-92 ? 1992 by The North American Benthological Society Why disturbances can be predictable: a perspective on the definition of disturbance in streams N. LEROY POFF Department of Zoology, University of Maryland, College Park, Maryland 20742-4415 USA Abstract. Resh et al. (1988, J. N. Am. Benthol. Soc. 7:433) assert that lotic ecosystems are unique in that disturbances are necessarily "unpredictable" events. Acceptance of this "predictability clause" in the definition of disturbance introduces problems that influence our ability to identify, describe, and discuss important aspects of disturbance in streams. I focus on four specific aspects of Resh et al.'s treatment of predictability, particularly as it relates to hydrologic disturbance (e.g., spates). First, Resh et al. offer only purely statistical criteria for identifying hydrologic disturbance, an approach that has important limitations for characterizing predictability of the disturbance regime. I suggest adoption of more physically based measures of disturbance (e.g., flow at which movement of bed is initiated) to which specific ecological responses can be determined. Second, lotic species are assumed to be "adapted" to predictable hydrological events, despite the general absence of supporting empirical evidence. I suggest that this assertion be reframed as a testable hypothesis. Third, defining disturbances as necessarily unpredictable can result in a tautology, in which bio- logical response to disturbance and predictability of disturbance are expressed in terms of one another. Fourth, a more explicit distinction between ecological and evolutionary time scales of response to disturbance allows predictability to be properly considered as a separate (and non- tautological) component of the disturbance regime. In sum, disturbances (including predictable ones) always have ecological effects; however, the magnitude of ecological response to a particular disturbance may be constrained by evolutionary (historical) adjustments of the biota if the distur- bance regime is characterized by high predictability. Key words: disturbance, predictability, streams, hydrology, geomorphology. Communication among scientists often de-group consisting of 10 participants, the paper pends on careful definition. This is certainly by Resh et al. attempted to distil a "consensus" true in ecology, where many terms or phrases view on a complex issue. However, the authors attempt to generalize across diverse systems expressed and alternative views throughout their levels of organization. Differences in definition paper, and they invited further discussion of may arise from discernably different philo-these views. In this paper, I wish to address a sophical starting points, or they may arise componentmore of Resh et al.'s definition of distur- subtly from ambiguities associated with com-bance, viz. the assertion that lotic systems are plex phenomena and frame of reference. Some- unique in that disturbances must be unpre- times, ambiguities arise because terms or dictable.con- I critically examine this assertion and cepts implicit in the definition are themselves explore its implications to argue that acceptance subject to variable interpretation (e.g., discus- of this "predictability clause" unnecessarily re- sion of "niche" often relies on some common stricts our ability to identify, describe, and dis- notion of "resource"). Regardless of the cuss under- important aspects of disturbance in streams, lying reasons, we may speak at cross-purposes primarily because it blurs the distinction be- because definitional assumptions have nottween been the ecological and evolutionary time adequately specified. scales of biotic response to disturbance (both of The word "disturbance" is becoming whichincreas- are important in disturbance studies). ingly important in the ecologist's lexicon. This Theis an important issue because stream ecol- recent seminal paper by Resh et al. (1988) ogists has need to explicitly evaluate the assump- played a significant role in drawing attention tions, ambiguities, and implications that un- to this concept as applied to lotic ecosystems. derlie our use of the word "disturbance", a Furthermore, it has greatly helped stream relativistic ecol- concept that changes with scale of ogists to focus on many basic issues relevant observation to (Rykiel 1985). Hereafter, distur- the role of disturbance in structuring these bance sys- will refer to hydrologic disturbance un- tems. Produced during a workshop discussion less otherwise indicated. I will focus primarily 86 This content downloaded from 24.9.112.12 on Mon, 26 Aug 2019 01:31:04 UTC All use subject to https://about.jstor.org/terms 1992] DEFINITION OF DISTURBANCE 87 on spates; however, similar arguments could be proach appropriately considers the impact of made relative to other types of disturbance as disturbance in the context of normal environ- well (e.g., "droughts", chemical pollution, etc.). mental variation, I wish to emphasize some spe- Two widely cited general definitions of dis- cific limitations associated with such a purely turbance exist. Sousa (1984) defines disturbance statistical approach to defining predictability of in a population-dynamics context as "a discrete, disturbance. Resh et al. indicate that any purely punctuated killing, displacement, or damaging statistical definition of disturbance is arbitrary of one or more individuals (or colonies) that because, in their example, all months would directly or indirectly creates an opportunity for have the same expected disturbance frequency new individuals (or colonies) to become estab- (i.e., 5% assuming a normal distribution) given lished". White and Pickett (1985) adopt a broad- a sufficiently long period of record. However, er definition of disturbance as "any relatively because all months have equal disturbance fre- discrete event in time that disrupts ecosystem, quencies, disturbances would also be equally community, or population structure, and likely for each month, leading to the unex- changes resources, substrate availability, or the pected conclusion that disturbances (as defined) physical environment". Note that both defini- have no seasonal predictability (i.e., they are tions consider disturbances as "discrete" events, uniformly distributed throughout the year). the impact of which can be measured in terms Thus, using this (or other) arbitrary statistical of ecological responses. Further, according both criterion to define "disturbance", one could ar- to Sousa and to White and Pickett, a given dis- gue that all streams are similar in having no turbance regime is characterized by the statistical seasonal predictability of disturbance. distribution (based on historical record) of in- Resh et al. partially address this issue in their dividual events in terms of such attributes as second approach, where they apply a formal frequency, intensity (magnitude), duration, measure and of predictability (Colwell 1974) to predictability. monthly maximum and minimum daily dis- Resh et al. (1988:434) modify the White charges. and They show convincingly that temporal Pickett definition for streams to include events distribution of these extreme flows varies among that are only "... outside a predictable rangestreams (fig. 3, tables 2, 3). However, maximum ... [because] ... organisms are adapted to or pre- minimum monthly extremes are arbitrary in- dictable seasonal fluctuations of discharge, sofar tem- as they do not necessarily represent actual perature, dissolved oxygen, etc. ...". Further, hydrologic disturbances, so calculated predict- they state that "... when measured by the ability im- scores do not necessarily describe pre- pact on a community, we feel that it is the dictability un- of disturbance per se (a point not ex- predictable event that typically constitutes plicitly a addressed by the authors). Resh et al. disturbance". Although intuitively appealing appropriately conclude that such a quantitative on its face, this "predictability clause" intro- analysis is useful both for considering temporal duces significant methodological and concep- hydrologic