Bird Communities along a Montane Sere: Community Structure and Energetics

-We examined 11 of 24 ecosystem attributes that Odum (1969) proposed would change during succession in avian communities in meadow, aspen, fir, and spruce forests in northern Utah and southern Idaho. A total of 71 bird species was observed during the 30-month study, of which 43 were known to breed. Yearly energetic relationships were calculated for the breeding bird populations using an avian community energetics model. In the different avian communities, the ratios of annual secondary production/community respiration, secondary production/biomass present, and secondary production/consumption all tended to decrease with increasing plant succession; biomass present/"unit energy flow" tended to increase. All of these trends are consistent with those postulated by Odum for the entire ecosystem. These energetics ratios differed in 1977 from those in 1976 and 1978 due to a severe drought. They also fluctuated greatly during the course of a year. Avian species diversity was highest in the preclimax fir forest during the breeding season and fluctuated widely in the nonbreeding season. Species equitability tended to be relatively high in all avian communities, although it was slightly lower during the nonbreeding season. No trends were found in male body sizes, mean clutch size, and mean length of incubation period (a crude measure of parental investment). Thus, for the bird communities, we reject the postulated trends of increased species diversity, equitability, and organism size in the climax, quantity to quality production, and rto K-selection. During the drought of 1977 the spruce avifauna appeared more stable, so we cannot reject Odum's hypothesized trend of increased stability in the climax. We conclude that the vegetational characteristics of a given sere will in most cases determine the avian population response along that sere; this in turn will influence the correspondence of the avian communities to Odum's postulates. Incorporating these avian results with other studies within our subalpine sere, some of Odum's hypothesized trends are supported while others are not, suggesting that successional relationships are more complicated than would follow from Odum's relatively simple model. Received 20 February 1980, accepted 9 June 1980. ODUM (1969: Table 1) proposed trends in 24 ecosystem attributes that change as succession proceeds from early (pioneer) to late (mature or climax) stages. He divided these ecosystem attributes into six groups: community energetics, community structure, life history, nutrient cycling, selection pressure, and overall homeostasis. The trends Odum proposed were based on the following premises concerning succession: "i) It is an orderly process of community development that is reasonably directional and, therefore, predictable; ii) It results from the modification of the physical environment by the community . ; iii) It culminates in a stabilized ecosystem in which maximum biomass (or high information content) and symbiotic function between organisms are maintained per unit available energy flow" (Odum 1969: 262; see also Odum 1971: 251-267). This viewpoint has been widely criticized (see review in Connell and Slatyer 1977). Connell and Slatyer stated that Odum's scheme is based on analogy, not evidence, and Engleberg and Boyarsky (1979) argued that even the analogy is a poor one. Few studies have attempted to test the proposed attributes simultaneously on an ecosystem-wide basis. During 1975-1978 a study was undertaken to test empirically as many of Odum's 24 attributes as possible in a relatively simple subalpine sere. Data were collected on as many aspects of the plant and animal communities that were related to Odum's I Present address: Bodega Marine Laboratory, University of California, Bodega Bay, California 94923 USA. 8 The Auk 98: 8-28. January 1981 This content downloaded from 157.55. 39.249 on Wed, 03 Aug 2016 04:13:55 UTC All use subject to http://about.jstor. org/terms January 1981] Montane Avian Community Structure 9 TABLE 1. Eleven attributes proposed by Odum (1969) that were tested by examining the spruce-fir avifauna along a subalpine sere. Ecosystem attribute Predicted trend (Odum 1969) Community energetics Production/respiration Approaches 1 Production/biomass High to low Biomass/unit energy flow Low to high Net community production High to low Community structure Total organic matter Low to high Species diversity-variety Low to high Species diversity-equitability Low to high