vol. 155, no. 3 the american naturalist march 2000 Disturbance Frequency and Community Stability in Native Tallgrass Prairie Scott L. Collins* Division of Biology, Kansas State University, Manhattan, Kansas tionship between disturbance frequency and community stability in 66506; and Department of Zoology, University of Maryland, terrestrial and aquatic systems. College Park, Maryland 20742 Keywords: community dynamics, community stability, disturbance, Submitted January 11, 1999; Accepted October 15, 1999 ®re, tallgrass prairie, temporal variability. It is almost axiomatic that ecological communities vary in abstract: Ecological communities are spatially and temporally var- space and time (Clements 1916; Curtis 1959; Knapp 1974; iable in response to a variety of biotic and abiotic forces. It is not Whittaker 1975). This variation results from the interplay always clear, however, if spatial and temporal variability leads to of biotic and abiotic processes operating at a variety of instability in communities. Instability may result from strong biotic spatial and temporal scales (Hastings et al. 1993; Wu and interactions or from stochastic processes acting on small populations. Loucks 1995). These interactive processes may produce I used 10±15 yr of annual data from the Konza Prairie Long-Term ªinstabilitiesº that lead to directional change in a com- Ecological Research site to examine whether plant, breeding bird, grasshopper, and small mammal communities in tallgrass prairie munity over ecological time (DeAngelis and Waterhouse exhibit stability or directional change in response to different ex- 1987). Instabilities can be ªlocal,º in response to a single perimentally induced ®re frequencies. Based on ordination and small disturbance, or ªglobal,º re¯ecting an overall re- ANOVA, plant and grasshopper communities on annually burned sponse to a large disturbance (Lewontin 1969) or perhaps sites differed signi®cantly from plant and grasshopper communities a change in the disturbance regime. Alternatively, temporal on less frequently burned sites. Breeding birds and small mammals variation in communities may occur within what De- differed among sites as well, but these differences were not clearly Angelis et al. (1985) referred to as a loose equilibrium. related to disturbance frequency. A modi®ed time series analysis indicated that plant communities were undergoing directionalchange That is, despite high year-to-year ¯uctuation in compo- (unstable) on all watersheds, regardless of ®re frequency. Contrary sition and abundance, community structure remains sta- to expectations, directional change was greatest on the annually ble, on average, over long time frames or large spatial scales burned sites and lowest on the infrequently burned sites. Unlike the (Whittaker and Levin 1977; Bormann and Likens 1979). plant communities, breeding bird, grasshopper, and small mammal Based on these notions, I consider a community to be communities were temporally stable, despite high-compositional var- globally stable if it does not exhibit a statistically de®ned iability from 1 yr to the next. Stability among the consumer com- directional change in composition and abundance over an munities within these dynamic plant communities occurs because three-dimensional vegetation structure does not change over time, ecological time frame in response to disturbance. From despite changes in plant species composition. Evidence suggests that this perspective, a community can be both highly variable instability in the plant community results from strong biotic inter- from 1 yr to the next, yet remain stable (a loose equilib- actions among temporally persistent core species and stochastic dy- rium) in terms of its average trajectory over time. namics among infrequent satellite species. Overall, community sta- Quantifying temporal variability empirically has been bility cannot be assessed if the pattern of temporal dynamics is dif®cult, however, because of the relatively short duration unknown. Long-term empirical studies of different taxa under dif- ferent disturbance regimes are needed to determine over what time of many ®eld studies (Tilman 1989). Indeed, investigations frames and spatial scales communities may be stable. Such studies of community stability have been limited primarily to the- are essential for the development of generalities regarding the rela- oretical evaluations rather than to empirical ®eld studies (Connell and Sousa 1983; DeAngelis and Waterhouse * Present address: Division of Environmental Biology, National Science Foun- 1987; Ives 1995). Determining long-term patterns of tem- dation, Arlington, Virginia 22230; e-mail: [email protected]. poral variation and factors that cause them are key research Am. Nat. 2000. Vol. 155, pp. 311±325. q 2000 by The University of Chicago. needs in community ecology. Quantifying such patterns 0003-0147/2000/15503-0002$03.00. All rights reserved. would further our understanding of the controversial re- 312 The American Naturalist lationship between diversity and stability (e.g., Tilman et the historical average ®re return interval (Anderson and al. 1997; Wardle et al. 1997) as well as increase our ability Brown 1986). Given that consumer species exist within a to predict the response of communities to both natural matrix of grassland plants that serve as food and habitat, and anthropogenic environmental change. I also tested the hypothesis that community dynamics of Grassland communities, in particular, provide an ap- consumer species (breeding birds, grasshoppers, and small propriate system for the analysis of community stability. mammals) mirrored the dynamics of the plant community Grasslands are highly variable in space and time (Roten- (Milchunas et al. 1998). That is, temporal stability of berry and Wiens 1980b; Evans 1988; Collins and Glenn breeding bird, grasshopper, and small mammal commu- 1991, 1997a; Kaufman et al. 1998). Grassland ecosystems nities will occur under historical burning regimes, and contain a complex disturbance regime composed of fre- stability will decrease at higher or lower ®re frequencies. quent large- and small-scale disturbances that interact with interannual climate variation to affect spatial and temporal dynamics (Collins 1987; Cof®n and Lauenroth Material and Methods 1988; Day and Detling 1990; Jaramillo and Detling 1992; Study Sites Milchunas and Lauenroth 1993; Bragg 1995; Steinauer and Collins 1995; Frank et al. 1998; Knapp and Seastedt Data for the following analyses come from the Konza Prai- 1998; Knapp et al. 1999). Indeed, the evolution of the rie LTER site near Manhattan, Kansas. The Konza Prairie North American grassland biome and the regional per- Research Natural Area is a 3,487-ha native grassland in sistence of grassland ecosystems have been attributed, in the Flint Hills region of northeastern Kansas. Watersheds part, to this complex disturbance regime (Axelrod 1985). at Konza Prairie have been experimentally subjected to It is unclear, however, whether grassland community var- 1-, 4-, and 20-yr intervals of ®re since 1972 (Knapp et al. iation over ecological time frames re¯ects a loose equi- 1998). Before that time, sites were burned every 2±3 yr, librium or a directional change in response to different on average, and grazed by cattle. Sampling was initiated rates of natural disturbance. 5±9 yr after cessation of grazing and the start of the ex- I used long-term data from the Konza Prairie Long- perimental burning treatment to allow the system to adjust Term Ecological Research (LTER) site to examine whether to this change in the disturbance regime. grassland communities exhibit global stability or direc- Konza Prairie occurs on topographic gradients that vary tional change in response to different experimentally in- in elevation from 320 to 444 m above sea level. Konza duced disturbance frequencies. I examined the dynamics Prairie is divided into replicated watershed units, some of of not only the vascular plant community, but also the which have been burned at 1-, 4-, and 20-yr intervals since breeding bird, grasshopper, and small mammal commu- 1972 and others that have been burned at these same nities. I used information on these four groups of organ- intervals since 1981. Konza Prairie contains continuous isms to test the similarity of responses among groups to grassland vegetation with occasional gallery forest along different frequencies of experimental burning. the larger streams. The vegetation is dominated by per- Fire is widely recognized as a critical component of the ennial C4 grasses, especially Andropogon gerardii, Andro- natural disturbance regime in tallgrass prairie ecosystems pogon scoparius, Panicum virgatum, Sorghastrum nutans, (Anderson 1990; Bragg 1995). The estimated point return Sporobolus asper, and Sporobolus heterolepis (nomenclature interval of ®re in mesic tallgrass prairie is once every 3±5 follows Great Plains Flora Association 1986). Perennial yr (Wright and Bailey 1982). Because ®re enhances dom- forbs, such as Aster spp., Ambrosia psilostachya, Artemisia ludoviciana, Solidago spp., Kuhnia eupatoroides, Salvia inance of C4 grasses and deters establishment of woody plants (Gibson and Hulbert 1987; Anderson 1990), it is azurea, and Vernonia baldwinii, are common throughout presumed to stabilize tallgrass prairie communities (An- the vegetation. Gallery forests along drainages are domi- derson and Brown 1986). This leads to an apparent con- nated by Quercus spp. The surrounding region contains tradiction with the traditional