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12 STEPHEN T. Ross et al. 1985, 1987; Matthews et al. 1988). The avail- able literature on stream fishes indicates that per- sistence of assemblages may vary between harsh and benign habitats, but that values of well over a decade are not uncommon. However, the upper limits of temporal persistence are essentially all due to the length of the study period rather than an ac- tual limit to persistence of the assemblage (Ross et al. 1987). The actual age of assemblages (con- sidered from a persistence viewpoint) may range from a matter of years or decades-perhaps upwards to the age of river systems, on the order of 8,000- 100,000 years. For instance, the drainage character- istics of many stream systems changed significantly during the Pliocene-Pleistocene transition due to glaciation, sediment transport, withdrawal of marine transgressions, and coastal uplift (Swift et al. 1986; Mayden 1987b). Thus, many systems date from the last 10,000 years. As a further complication, all species pairs in a contemporary assemblage may (and probably do) dif- fer in the lengths of their associations. Rarely should we expect that an assemblage, once formed, has existed unchanged over time. For instance, Mayden (1987a) points out that we need to distinguish be- tween vicariant and dispersive events in the forma- tion of fish assemblages. There are probably older core assemblages, due to historical vicariant events, that have periodically gained species from disper- sive events. Thus, the degree of expected coevolu- tion should vary, even within an assemblage. The duration of individual populations is on a much shorter time scale. Most southeastern stream fishes have fairly short lifespans of 2-7 years. For instance, red shiners generally do not live beyond 3 years (Farringer et al. 1979), darters may live 3-4 years (Page 1983), topminnows may live 4-5 years (Fisher 1981), and centrarchids may live more than 10 years (Carlander 1977). Local microhabitats also may change on a scale from hours to years, often oper- ating on temporal scales that approach or exceed the life span of individuals within a species. The tem- poral duration of microhabitats varies strongly with geographical region. The second problem is the tacit assumption that present-day conditions of seemingly undisturbed systems represent the ecosystems in which the stream fauna evolved or, at least, the conditions that the assemblage has encountered for the majority of its history. In other words, we want to be able to make generalizations from our work that rest on the assumption that ours is a natural system, essential- ly unchanged from prehistoric times-the fallacy of the primeval stream. Historical changes of the last 200 years (such as widespread deforestation during the late 1800's and early 1900's) are documented well enough to argue strongly that there are vir- tually no southeastern streams that have escaped some degree of man-induced habitat change. The third problem is less an outcome of our mor- tality than of our ability to ignore certain life history stages while emphasizing others; however, this pro- blem is definitely related to time limitations-the fallacy of ontogenic stasis. In a review of resource partitioning studies of fish assemblages, Ross (1986) listed only one study that examined both larval and adult life history stages. Thus, most of what we know about fish resource use in general, and cer- tainly of stream fish resource use, is from only one or two life history stages. A quick perusal of the issues of Current Contents from 1986 to the present, as well as searches of Transactions of the American Fisheries Society and Copeia for 1985-88, indicates that there is less emphasis on larval ecology of freshwater fishes than on larval ecology of marine fishes. It is safe to generalize that the life cycle and associated habitat use are not completely known for a majority of southeastern stream fishes. The critical question is whether the lack of knowledge of early life history stages significantly weakens our under- standing of resource requirements of the fishes and any management plans based on those require- ments. Available data indicate that an under- standing of resource requirements of fishes and the subsequent management plans are substantially compromised if they do not include information from early life history stages. In summary, finite careers and budgets encourage studies of communities and stream systems on short timescales and on life history stages that are most readily collected, observed, or identified. This has led to three basic errors: (1) ignoring historical ef- fects in the structuring of communities; (2) assum- ing that present day studies deal with pristine systems; and (3) focusing on the most easily studied life history stages while ignoring stages that may, in fact, be more critical to an understanding of com- munity function. Further understanding of assemblage formation and maintenance requires a broader scope-one that considers ongoing as well as historical effects, and one that recognizes that both biotic and abiotic forces may act differently on the various life historycstages. Information Sources Baker, J. A., and $. T. Ross. 1981. Spatial and temporal resource utilization by southeastern cyprinids. Copeia 1981:178-189. ! ~