vol. 155, no. 4 the american naturalist april 2000 Fluctuating Environments and Phytoplankton Community Structure: A Stochastic Model John M. Anderies 1,* and Beatrix E. Beisner 2, 1. Commonwealth Scientific and Industrial Research Organisation, Division of Wildlife and Ecology, G.P.O. Box 284, Canberra, Australian Capital Territory 2601, Australia; 2. Department of Zoology and the Crisis Points Group of the Peter Wall Institute for Advanced Studies, University of British Columbia, Vancouver, British Columbia V6T IZ2, Canada Submitted January 15, 1999; Accepted November 22, 1999 abstract: Spatial heterogeneity in organism and resource distri- butions can generate temporal heterogeneity in resource access for simple organisms like phytoplankton. The role of temporal hetero- geneity as a structuring force for simple communities is investigated via models of phytoplankton with contrasting life histories competing for a single fluctuating resource. A stochastic model in which en- vironmental and demographic stochasticity are treated separately is compared with a model with deterministic resource variation to as- sess the importance of stochasticity. When compared with the de- terministic model, the stochastic model allows for coexistence over a wider range of parameter values (or life-history types). The model suggests that demographic stochasticity alone is far more important in increasing the possibility of coexistence than environmental sto- chasticity alone. However, the combined effects of both types of stochasticity produce the largest likelihood of coexistence. Finally, the influence of relative nutrient levels and nutrient pulse frequency on these results is addressed. We relate our findings to variable en- vironment theory with evidence for both relative nonlinearity and the storage effect acting in this model. We show for the first time that temporal dynamics generated by demographic stochasticity may operate like the storage effect at particular spatial scales. Keywords: competition, diversity, stochastic model, temporal hetero- geneity, phytoplankton. There has been a tradition of questioning why ecological communities are diverse (e.g., Hutchinson 1961; Richer- son et al. 1970; Sale 1977; Connell 1978; Birch 1979) in * E-mail: John.Anderies@dwe.csiro.au. E-mail: beisner@zoology.ubc.ca. Am. Nat. 2000. Vol. 155, pp. 556–569. q 2000 by The University of Chicago. 0003-0147/2000/15504-0010$03.00. All rights reserved. contrast to the simple communities predicted by the com- petitive exclusion principle (Hardin 1960). The hypotheses proposed fall into three major categories: predator- mediated coexistence (Paine 1966; Lubchenco 1978; Arm- strong 1979), spatial interactions (Richerson et al. 1970; Grenney et al. 1973; Malchow 1994; Pacala and Levin 1997; Durrett and Levin 1998), and temporal variability or dis- turbance (Connell 1978; Levins 1979; Armstrong and McGehee 1980; Abrams 1984; Ebenho ¨h 1987; Chesson and Huntly 1988, 1997; Grover 1990). Temporal variability in the environment can provide temporal niche opportunities for some types of organisms. The aim of this article is to extend work on temporal variability as a possible diversity-promoting mechanism. In lake environments, nutrients and planktonic organ- isms are subject to various periodic sources of forcing. Such events operate at several characteristic spatial and temporal scales. Large-scale events include climatic forc- ing, such as seasonal shifts, and large storm events within seasons on timescales of 1–2 wk (Harris and Griffiths 1987). In the north temperate zone, these events act to stir the pelagic zones of lakes, resulting in upwelling and a vertical redistribution of nutrients and organisms. Within a lake, on daily to weekly timescales, the movement of fish on- and offshore can act as a source of nutrient redistribution (i.e., from littoral to pelagic zones; Schindler et al. 1993) and as a temporally variable source of pre- dation pressure for zooplankton. At smaller scales, within- lake heterogeneity in distribution of plankton results from the limited mobility of organisms. Variation in excretion of nutrients by zooplankton is a small-scale source of var- iability in resources for phytoplankton (Lehman 1984; Wright and Shapiro 1984). These forcing processes, es- pecially at larger scales, lead to a periodic redistribution of organisms and resources such that competing phyto- plankton observe a temporally fluctuating resource base, not a constant one. Organisms have a variety of physiological strategies to deal with variability in resource availability. In response to temporal variability in resources, there are three major life-history strategies. The first is storage, the ability of an