摘要:

This study investigates the relationships among species diversity, community structure, and convergent evolution among divergent fish faunas. Morphological traits can be used as surrogates for ecological data in the comparative study of community niche relationships. In the present study I examined 30 morphological features related to the ecology of the dominant fish species from lowland stream and backwater habitats in five widely separated geographic regions: nearctic Alaska, temperate North America, Central America, South America, and tropical Africa. The study regions exhibited a general gradient of species richness from a minimum of 6 dominant species at one of the high—latitude sites (65° N) to a maximum of 43 numerically dominant species at a neotropical site (8° N). Fishes from Alaskan sites near the edge of the polar circle tended to be larger than fishes at other sites. Mean values for most morphological characters varied little between regions, indicating similar faunal centroids in morphological space. Morphological diversification within fish assemblages was estimated from species similarities based on Euclidean distances plus species projections on the principal axes from multivariate analysis. The total morphological space encompassed by ichthyofaunas in both stream and backwater sites was generally concordant with the latitudinal and species—richness gradient, with low—diversity nearctic assemblages exhibiting little morphological diversification relative to high—diversity tropical faunas. The Central American assemblages showed a greater range of ecomorphological diversification than African assemblages that contained a few more species, and this pattern may be related to greater seasonal stability at the Central American site. Phenetic patterns of dispersion reflect ecological relationships in which greater numbers of coexisting species are associated with higher levels of niche diversification and ecological specialization, leading to enhanced resource partitioning. Without additional ecological information, a community morphological analysis cannot directly determine whether or not increased ecological specialization is associated with the addition of new resources on the fringes of resource space or with increased subdivision of previously utilized core resources. Based on ecological information gathered concurrently with the fishes used in this analysis, I conclude that close species packing in morphological space is associated with niche generalists rather than with niche compression. With the possible exception of the two high—latitude sites, assemblages that contained more fish species generally did not exhibit tighter packing in niche space than species—poor assemblages, and this result was observed for comparisons both within and between regions. In contrast with several earlier studies, I interpret the lack of correlation between species richness and the average minimum distance between species in assemblage morphospace as being entirely consistent with the observed expansion of morphospace with species richness and latitude. Morphological patterns show that average spacing is held more or less constant as the variety of ecomorphological configurations increases in more—speciose fish assemblages. Even though morphological characters reflect community relationships, tight packing near the center of faunal morphospace actually indicates the opposite of species packing (niche compression) in the traditional sense of resource—utilization curves. The analysis supports the thesis that species interactions are important determinants of community morphological features, particularly in species—rich tropical regions. Relative to assemblages in the other four regions, the two Alaskan fish assemblages were hyperdispersed within a comparatively small morphological space. The Alaskan fish assemblages probably formed via geologically recent, and perhaps repeated, colonizations of polar freshwaters by species with evolutionary histories in more diverse southern and coastal fish faunas. I interpret evidence of greater niche diversification at lower latitudes within a habitat type as being derived primarily from the influence of competition and predation, whereas differences between habitats within regions seems to be associated with the combined effects of biotic interactions and differences in habitat volume and heterogeneity. Even though different regions within ecomorphological space were dominated by different fish orders, numerous ecomorphological convergences and several one—for—one ecological equivalents were identified within different biotic regions.

ISSN:0012-9615    

DOI:10.2307/2937046

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

A detailed and relatively evenly resolved food web of Little Rock Lake, Wisconsin, was constructed to evaluate the sensitivity of food—web patterns to the level of detail (degree of resolution) in food—web data. This study presents definitions (e.g., ecosystem food webs) and methods for constructing and reducing the resolution of food webs to provide relatively pragmatic and rigorous touchstones for consistency in future food—web studies. This analysis suggests that food—web patterns such as the scale—invariant links—per—species ratio, short chain lengths, and limited number of trophic levels are constrained by the resolution of food—web data rather than by ecological factors. Patterns less sensitive to changes in resolution such as directed connectance (the proportion of observed directed links to all possible directed links) may be robust food—web attributes. The food web of Little Rock Lake appears to be the first highly and evenly resolved food web of a large natural ecosystem originally documented for the purpose of examining quantitative food—web patterns. This ecosystem food web contains roughly twice as many species as the largest web to date. It also may provide the most credible portrait available of the detailed trophic structure of a whole ecosystem. The 93—trophic—species web of Little Rock Lake differs from previously published trophic—species webs by having more links per species (L/S = 11), longer chain lengths (average: ≥10, maximum: ≥16), species at higher trophic levels (maximum: = 12), higher fractions of intermediate species, and smaller fractions of top species and links to top species. The sensitivity of quantitative food—web patterns to changes in resolution was examined in several series of tropically aggregated Little Rock Lake webs. Each of the series starts with a highly and relatively evenly resolved web with 182 consumer, producer, and decomposer taxa and ends with low—resolution webs with 9 aggregates of taxa. Taxa were aggregated based on the proportion of predators and prey shared by the taxa. Different series of webs were generated using different criteria for linking aggregates to evaluate the sensitivity of food—web patterns to linkage criteria. The sensitivity analysis revealed that several, but not all, quantitative food—web patterns are very sensitive to systematic aggregation of the web. Sensitive patterns include number of links per species, linkage complexity, the distributions of chain lengths and species among trophic levels, and the proportions of top species and links to top species. Less—sensitive patterns include connectance, the ratio of predators to prey, the proportions of intermediate and basal species, and the proportions of links that are between intermediate and basal species. Directed connectance is the only pattern examined that is both very robust to trophic aggregation and generally comparable to other community webs. Quantitative food—web patterns in published community webs are generally similar to highly aggregated Little Rock Lake webs (versions with 9—40 aggregates). These findings suggest that previously described community food webs are severely aggregated versions of more elaborate webs similar to that of Little Rock Lake.

ISSN:0012-9615    

DOI:10.2307/2937047

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

It is often claimed that predation pressure on plants and animals increases toward the equator. Here I address the specific hypothesis that predation pressure is greater on lower latitude populations of the lizard Uta stansburiana. Standard mark—recapture techniques (10 997 captures of 5371 individuals) were used to estimate mortality rates for seven populations that lie along a transect of nearly 15° latitude (central Washington to extreme southeastern California, USA). Mortality was estimated for both the early (i.e., spring to summer) and late (i.e., summer to late summer or fall) portions of the activity season. As in previous studies of this species, I found that emigration was negligible for adult lizards. Thus, disappearance rates serve as reliable indices of mortality rates. Although predation is known to be a major source of mortality for populations of U. stansburiana, direct measurement (i.e., observation) of the demographic impact of predators is impractical. My strategy was to examine the potential demographic importance of mortality sources other than predation (e.g., senescence, physiological stress). If alternate sources of mortality can be shown to be of trivial importance, then overall mortality rates can be used to infer variation in predation pressure. To assess the importance of these mortality factors I examined variation in mortality within sites (e.g., among seasons, years, and individuals) in relation to data about the lizard (e.g., age, physical condition) and about their environment (e.g., recent precipitation). My results suggest that predation is the only source of activity season mortality that is of demographic importance for these populations. For example, I found no evidence that physiological stress was ever an important source of mortality. Lizards in relatively poor physical condition (i.e., those with small length—corrected mass) did not experience a greater risk of mortality, and survival actually increased during the presumably more stressful late activity season. Mortality rates were also depressed during a drought at one site, despite the poor physical condition of the lizards. Low mortality rates during the late activity season and during drought conditions probably reflect reduced activity of U. stansburiana and their predators. Reduced activity levels discourage predator—prey encounters. Because sources of mortality other than predation appear to be of trivial demographic importance, overall mortality rates can be used to infer relative predation rates. Accordingly, I examined latitudinal variation in mortality rates to address the hypothesis that predation pressure is greater on lower latitude populations of U. stansburiana. Daily mortality rates were not higher among southern populations. Even annual losses to predators do not appear to be greater among lower latitude populations. These results were surprising because U. stansburiana is one of the species for which predation pressure was thought to increase with decreasing latitude. Higher than expected mortality rates in the north may in part be due to longer daily activity periods among northern Uta and their predators. Interestingly, several indirect measures of predation intensity (e.g., tail break frequencies, measures of predator species richness) did increase significantly from north to south. Thus, these less direct measures of predation pressure yielded a misleading impression of latitudinal variation in predation pressure.

ISSN:0012-9615    

DOI:10.2307/2937048

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Nitrogen and phosphorus pool sizes, distribution, and cycling rates were described and compared for six different ecosystem types occurring along a single toposequence in northern Alaska. The toposequence was located on a series of old floodplains of the Sagavanirktok River, in the northern foothills of the Brooks Range. From tussock tundra in the uplands, the toposequence passed through a relatively dry hilltop heath zone, a hillslope shrub/lupine/Cassiope zone, a footslope Equisetum zone, a wet sedge tundra, and a riparian shrub zone. A late—melting snowbank covered the hillslope site in early June of each year, and the sites consistently varied in soil temperature, soil moisture, thaw depth, and the seasonal pattern of soil thaw. The standing stocks of N, P, and C in soils of these six ecosystem types varied dramatically but not monotonically along the toposequence, as did the turnover rates of these elements. Several measures were used in comparisons of N and P availability, including soil solution concentrations, in situ accumulation on ion—exchange resins, and levels of KCI—extractable N and P. Annual rates of net N mineralization were assayed using a buried bag method, and ecosystem respiration was measured by trapping CO2in soda lime [NaOH + Ca (OH)2]. Soil P pools were characterized by sequential extraction methods into four major pools, including loosely bound P, Al— and Fe—bound P, primary mineral P, and organic P. Both N and P availability were low in all six ecosystems when compared with temperate forests or wetlands. Among ecosystems, however, there was considerable variation in the relative availability of N vs. P, and in the apparent relative importance of nitrate as a nitrogen source.

ISSN:0012-9615    

DOI:10.2307/2937049

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Loess—dominated ecosystems cover ≈14%(11 000 km2) of the Arctic Coastal Plain and much of the northern portion of the Arctic Foothills. Knowledge of this poorly known ecosystem is important for sound land—use planning of the expanding developments in the region and for understanding the paleoecological dynamics of eolian systems that once dominated much of northern Alaska. A conceptual alkaline—tundra toposequence includes eight common vegetation types and associated soils that occur near the arctic coast. A model of the regional loess gradient describes soils and vegetation downwind of the Sagavanirktok River. The addition of calcareous loess affects numerous soil properties, including bulk density, pH, water retention properties, concentrations of soil nutrients, and seasonal thaw depths. Many plant taxa, particularly cryptogams, increase in abundance downwind of the river, apparently in response to higher amounts of nutrients and moisture associated with finer soil—particle sizes and greater organic content. For example, the highest extractable P values (8—12 µg/g) occur in areas with circumneutral pH and finer grained soils, and low P values (<2 µg/g) occur in acidic soils north of the loess region. Early, minerotrophic stages of tundra succession are maintained by loess blown from the Sagavanirktok, Canning, and other large braided—river floodplains. Areas downwind of these rivers provide analogues for vegetation that existed in unstable areas of the Alaskan Coastal Plain during and following full glacial conditions. Total aboveground phytomass in wet acidic sites at Prudhoe Bay (163 ± 21 g/m2) is close to values from similar sites at Barrow and Devon Island. Only a small amount of data is available for alkaline areas, but there is indication of lower biomass near the major rivers, suggesting a response to lower nutrient regimes. Properties of loess tundra important for land—use planning include: (1) its high ice content, which contributes to its susceptibility to thermokarst; (2) high salinities, which hamper revegetation efforts; and (3) presence of certain plant species such as Dryas intergrifolia, which are particularly sensitive to disturbance. The loess gradient provides a natural analogue for road dust, an extensive disturbance associated with oil—field development.

ISSN:0012-9615    

DOI:10.2307/2937050

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY