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THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 146 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE habitats were largely on ecomorphic themes, especially for trees. The pattern suggests that landscape-level patterns of persistence and change are driven fundamentally by changes in the proportions of the basic habitats represented, not by species-level turnover, which occurred largely within habitats during the Westphalian. Major extinctions of coal-swamp plants occurred during the Westphalian-Stephanian transition. These were first recognized in miospore studies as the loss of the major Lycospora-producing lepidodendrids (Phillips et al., 1974). Recent systematic studies of the tree ferns (Lesnikowska, 1989) revealed a nearly complete extinction of species in this group in North America at the end of the Westphalian; Stephanian swamps presumably were recolonized from surrounding lowlands, where major extinctions probably also occurred, given that there is limited overlap between Westphalian and Stephanian species (e.g., Corsin, 1951; Remy and Remy, 1977). Examination of the literature on pteridosperms (e.g., Taylor, 1965; Rothwell, 1981; Pigg, 1987) and small ferns (e.g., Phillips, 1974) suggests substantial species level turnover in the coal- swamp members of these groups as well. In North America this was the first Pennsylvanian extinction to eliminate whole ecomorphic groups of trees entirely. It was these trees that defined the biotic or ecomorphic aspect of most intraswamp habitats. They also defined the biotic limits to habitat space, that is, the nature of the biotic partitioning of the habitat resources. Thus, with this extinction, the fabric of Westphalian swamp communitiesâthe persistent ecomorphic organization of several specific physical habitatsâwas destroyed. Stephanian swamps were reorganized into a different set of norms of reaction between the plants and the physical environment. We do not fully understand these at present, in part because vertical profile analysis of Stephanian coal swamps has begun only recently. The swamps did become heavily tree fern dominated and thus generally more like the surrounding clastic lowlands. Tree fern-dominated areas may have had a great deal of local spatial heterogeneity, with cordaites, sphenopsids, and pteridosperms liberally intermixed (Willard and Phillips, 1993). The lycopsids Sigillaria, a tree, and Chaloneria, a robust herb, were locally abundant in parts of many of these swamps (DiMichele et al., 1979). The degree to which any of the Stephanian habitats overlap with those of the Westphalian has yet to be determined; however, preliminary evidence suggests different patterns of resource partitioning. Changes in the Species-Level Composition of Habitats The species composition of coal swamps underwent considerable turnover during the Late Carboniferous. In analyzing the distribution and extent of that turnover through time, our objective was to relate it to changes in structure at the other levels of organization and to evaluate the locus of change with respect to habitats. In this analysis stratigraphic range data are used, which we believe is justified because most species are swamp-centered and therefore had potential access to peat swamp ecosystems throughout their existence. However, our sampling may miss some of the smaller or rarer taxa. There are numerous problems with these data: inadequate sampling, inadequate taxonomy for many groups, low sampling density in the Westphalian B and C, and regional patterns that obscure the overall pattern. Nonetheless, the basic message is consistent with the pattern seen at the landscape and habitat levels. We expect to be able to refine the data as our studies proceed. The data are drawn from our own profile and random sample analyses in order to avoid problems of inflation in the numbers of taxa known from particularly well studied coals, and to provide a uniform basis in taxonomic usage and sampling design. Thus, even though a species is known from a particular coal, if it did not appear in profile or random sample studies it was listed as absent in the raw data compilation; a species will appear as present in the data used for the turnover calculation, however, if the coal from which it is known falls between first and last occurrences in our samples. Species turnover was calculated as Ot0+1 + Et0 1/2(nt0 + nt0+1) Where Et0 = species disappearing after a coal at time t0; Ot0+1 = species first appearing in the next coal at time t0+1; nt0 = number of species at time 0; and nt0+1 = number of species in next coal in sequence. Turnover pattern is illustrated in Figure 8.9, with landscape-level breakpoint boundaries shown as dotted lines. Note that the highest turnovers occur around the major breakpoints, the Westphalian A-B boundary and the Westphalian-Stephanian boundary. Turnover within any one of the five major interbreakpoint intervals is much lower. The Westphalian D has notably higher diversity than the Westphalian A-C or the Stephanian. Analysis of species replacements by habitat suggests that species replacement patterns are not random across the landscape during the Westphalian. Species with similar ecomorphic characteristics may have complementary stratigraphic distributions, or newly appearing species may replace older ones as ecological dominants. These species tend to replace one another within a given type of biotic assemblage and habitat. As a result the ecomorphic character of the basic habitats is maintained throughout the Westphalian and, presumably, will prove to be maintained on a different set of themes during the Stephanian. Al
