摘要:

Human—caused fragmentation of forests is increasing, yet the consequences of these landscape changes to vertebrate communities are poorly understood. Although bird community response to forest fragmentation caused by agricultural or urban development has been well studied, we have little understanding of these dynamics in landscapes undergoing intensive forest management, where late—seral forest stands are separated by younger forest stands of varying ages and are part of a spatially and temporally dynamic forest landscape. We investigated the relationship between landscape structure and breeding bird abundance in the central Oregon Coast Range. We sampled vegetation and birds in 30 landscapes (250—300 ha) distributed equally among three basins. Landscapes represented a range in structure based on the proportion of the landscape in a late—seral forest condition and the spatial configuration of that forest condition within the landscape. We computed a variety of landscape metrics from digital vegetation cover maps for each landscape. Using analysis of variance and regression procedures, we quantified the independent effects of habitat area and configuration on 15 bird species associated with late—seral forest. Species varied dramatically in the strength and nature of the relationship between abundance and several gradients in habitat area and configuration at the landscape scale. Landscape structure (composition and configuration) typically explained<50% of the variation in each species' abundance among the landscapes. Species' abundances were generally greater in the more heterogenous landscapes; that is, they were associated with the more fragmented distribution of habitat. Only Winter Wrens showed evidence of association with the least fragmented landscapes. These results must be interpreted within the scope and limitations of our study. In particular, the scale of our analysis was constrained by the lower and upper limits of resolution in our landscapes, as set by minimum patch size and landscape extent, respectively. Thus, our results do not preclude much stronger and different relationships at finer and/or coarser scales. In addition, our community—centered habitat classification scheme and artificially discrete representation of patch boundaries may not have captured the functionally meaningful heterogeneity for each species. Finally, our analysis was limited to relatively common and widespread diurnal breeding bird species. Species sensitive to habitat fragmentation at the scale of our analysis may have been rare already and therefore not subject to the parametrical statistical approach that we employed.

ISSN:0012-9615    

DOI:10.2307/2937059

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Our approach to testing nonlinear population theory is to connect rigorously mathematical models with data by means of statistical methods for nonlinear time series. We begin by deriving a biologically based demographic model. The mathematical analysis identifies boundaries in parameter space where stable equilibria bifurcate to periodic 2—cycles and aperiodic motion on invariant loops. The statistical analysis, based on a stochastic version of the demographic model, provides procedures for parameter estimation, hypothesis testing, and model evaluation. Experiments using the flour beetle Tribolium yield the time series data. A three—dimensional map of larval, pupal, and adult numbers forecasts four possible population behaviors: extinction, equilibria, periodicities, and aperiodic motion including chaos. This study documents the nonlinear prediction of periodic 2—cycles in laboratory cultures of Tribolium and represents a new interdisciplinary approach to understanding nonlinear ecological dynamics.

ISSN:0012-9615    

DOI:10.2307/2937060

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The makeup of parasite communities is the result, among other factors, of interactions between the evolutionary history and ecological characteristics of hosts. This study evaluates the relative importance of some ecological factors (host body size, diet, habitat, latitude, and the mean number of parasite individuals per host) as determinants or correlates of parasite community richness in vertebrates, before and after controlling for potential effects of host phylogenetic relationships. Data were obtained from the literature on 596 parasite communities belonging to one of four distinct types: gastrointestinal parasite communities of fish, birds, or mammals, and ectoparasite communities of fish. There were positive correlations between the number of hosts sampled and mean species richness of the parasite community of each genus. In analyses treating host genera as independent statistical observations and using estimates of parasite species richness corrected for host sample size, positive correlations were observed between richness and host body size in gastrointestinal communities of all three groups of vertebrates. The mean number of parasite individuals per host also was correlated positively with species richness. In fish, richness increased with increases in the proportion of animal food in the host diet. Aquatic birds had richer parasite communities than their terrestrial counterparts, whereas marine fish had richer gastrointestinal parasite communities than freshwater fish. The richness of ectoparasite communities on fish showed no association with any of the ecological variables investigated. Using host genera as independent points in the analyses may lead to biased results since some host lineages are descended from recent common ancestors, and are therefore not truly independent. The comparative analysis was repeated using phylogenetically independent contrasts derived from the phylogeny of hosts. Once the effects of host phylogeny were removed, somewhat different results were obtained: host body size showed no relationship with parasite species richness in birds, and there was no evidence that habitat transitions resulted in significant changes in parasite species richness in any of the types of communities studied. Of the ecological factors studied, the comparative analyses suggest that only host body size can be an important determinant of parasite community richness in certain host groups. This study illustrates clearly the need to control for phylogeny in investigations of host—parasite interactions.

ISSN:0012-9615    

DOI:10.2307/2937061

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

One possible response of plant populations to heterogeneous environments is genetic adaptation resulting in the formation of distinct ecotypes. Genetic adaptation to stressful environments may affect both the limits to species boundaries and the potential for response to a changing environment. Reciprocal transplant experiments have frequently been used to describe ecotypic differentiation and to infer the role of natural selection when there is evidence for home site advantage. The demonstration of a home site advantage, however, does not reveal which plant characters are responsible for conferring increased fitness on populations planted in their native site. Here, we combine the classic reciprocal transplant experiment with multivariate regression analysis of selection to ask a series of questions relevant to understanding adaptive genetic differentiation in natural plant populations. Impatiens pallida plants from a mesic floodplain and a dry hillside site were reciprocally transplanted. We initially presumed the hillside to be a stressful site for Impatiens given its sparser population of consistently smaller individuals. This study describes the two environments from the perspective of the plant to ask whether it is stressful. In addition, we investigate genetic differentiation between populations and ask whether the two populations are distinctly adapted to their home sites. To identify traits that may be important for conferring home site advantage, we quantify present—day natural selection in these sites and ask whether the observed selective forces can explain genetic differences. Finally, because phenotypic correlations may play an important role in a population's response to its environment, we investigate relationships among traits to determine the extent to which they are genetically and/or environmentally controlled. The large reduction in total seed production when plants from both populations were grown on the hillside supported our initial bias that this site was stressful to Impatiens. In addition, the higher relative fitness of each population planted in its native site demonstrated that these populations represent distinct ecotypes. Genetic differences between populations were observed for several life history and morphological characters. In particular, plants from the hillside population were smaller and produced cleistogamous flowers earlier than floodplain plants. Selection analysis revealed that, while there is strong selection favoring early flowering on the hillside, there is no advantage to early flowering for plants grown on the floodplain. An increased developmental rate, which allows plants to produce seeds before they succumb to drought stress, appears to be the most important mechanism responsible for the greater relative fitness of the hillside population in its native site. While greater total plant leaf area is favored by selection on the floodplain, there is no evidence for selection on this trait on the hillside. Phenotypic covariances among traits differed between sites and populations, resulting in differences in the action of indirect selection. There is evidence that indirect selection on correlated traits is responsible for some of the observed genetic differences.

ISSN:0012-9615    

DOI:10.2307/2937062

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Succession was studied in a cold—temperate forest in the Boundary Waters Canoe Area Wilderness (BWCAW) of northeastern Minnesota. The 13 x 18 km study area comprises a complex forest mixture of jack (Pinus banksiana) and other pines, quaking aspen (Populus tremuloides), paper birch (Betula papyrifera), black spruce (Picea mariana), balsam fir (Abies balsamea), and white cedar (Thuja occidentalis) on thin soils over the Canadian Shield bedrock. The main objectives of this study were to examine the relationship between spatial patchiness, spatial scale, and canopy succession in the southern—boreal forest of the BWCAW, and to evaluate under what conditions successional direction may remain stable, converge, or diverge. Knowledge of the successional direction of old forests in the BWCAW that are undergoing demographic transition from even—aged to uneven—aged is important because the landscape now has many old stands as a result of reduced fire frequency. Rotation periods for fires have changed from °50—100 yr in presettlement times to>1000 yr since 1910. Analyses were conducted at spatial scales ranging from the individual tree (0.01 ha) to the large stand (16 ha). Two permanent mapped plots (of area 0.53 and 0.56 ha) were established in stands of different age. Fine—scale age structure, successional change, transition from one species to another, and development of small patches (of area<0.25 ha) were studied by means of stand history reconstruction with increment cores, spatial autocorrelation, and analysis of replacement trees in canopy openings. Spatial processes at nested scales of 1, 4, and 16 ha were examined on 15 square 16—ha tracts of upland forest, which are distributed among forests ranging from 15 to 190 yr old. Canopy species composition and patch development over time on these 15 tracts were interpreted on air photos taken in 1934, 1961, and 1991. Thus, the study includes a chronosequence approach, with verification of chronosequence validity by checking patch development processes at more than one time point. This checking was done at small spatial scales by reconstruction of stand history on permanent mapped plots, and at larger spatial scales with sequential air photos of the same locations spanning a 57—year period. Results show that the reduced fire frequency in recent years has changed the dominant successional pathways. When fire frequency was high, jack pine or aspen stands usually burned while still in the even—aged stage of development, and the new trees after the burn were the same species as before. Currently, many stands are undergoing demographic transition from even—aged stands of catastrophic fire origin to uneven—aged stands. This transition parallels a change in canopy composition from jack pine (occasionally red pine (Pinus resinosa)) or aspen to an old—growth multi—aged mixture of black spruce, balsam fir, paper birch, and white cedar. The mechanism that moves this successional path forward is canopy openings, 10—30 m across on average, caused by wind, insect, disease, that gradually chip away at the relatively uniform canopy of pines and aspen. Successional direction is individualistic in the sense that time and rate of transition from pine/aspen to other species depends on the action of heavy windstorms, insect infestation, and senescence of old pines that create canopy openings. Canopy openings are often filled with one of several species, but if more than one species invades an opening, monodominant patches of each species generally result. Understory—overstory interactions are very weak; the dominant species within each patch is apparently independent of the overstory species that died when the opening was created or the species dominating surrounding patches. During succession, the spatial structure of the stands at the 1—16 ha scales generally changes from a matrix heavily dominated by pine or aspen to a mosaic with relatively large mono—dominant patches that may be remnants of the extensive original matrix, and finally to a mixture with small patches (mean area 35 m2, maximum °0.1 ha) of black spruce, balsam fir, white cedar, and paper birch. Thus, at 1—16 ha spatial scales, succession leads to convergence on a mixture of species. At smaller spatial scales (e.g., 0.01—0.1 ha) successional pathways appear to diverge into four community types. The same successional pathways can be reconstructed from historical analysis of individual stands as from a chronosequence of stands; therefore, chronosequences in this area have been stable at least during the lifetime of the current generation of trees.

ISSN:0012-9615    

DOI:10.2307/2937063

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Throughout western North America, riparian ecosystem function has been transformed by anthropogenic influences on riverine environments. Modified flood frequency, duration, or intensity; depressed floodplain water tables; and increased alluvium salinity have contributed to change in riparian forest communities formerly dominated by Populus fremontii and Salix gooddingii. The invasion of the naturalized arborescent shrub, Tamarix ramosissima, potentially alters competitive hierarchies and disturbance regimes in these riparian ecosystems. We evaluated the structure and function of two southwestern riparian communities that differed in the degree of streamflow perturbation to which they had been subjected: the highly regulated lower Colorado River and the less tightly regulated Bill Williams River. Ordination analyses provided evidence that these riparian communities are structured along gradients relating to moisture, salinity, disturbance from fire, and community maturity, with Colorado River sites being more xeric and saline than those on the Bill Williams River. Foliar elemental analyses revealed high sodium concentrations in Tamarix (Na:K ratio = 1.87) and in the native shrub Tessaria sericea (Na:K = 1.56). Evaluation of tissue water relations parameters showed that Tamarix had lower osmotic potentials than sympatric woody taxa, helping to confirm that Tamarix is halophytic and probably capable of greater osmotic adjustment than native species. Carbon isotopic discrimination (△) provided evidence for higher water use efficiency in Tamaris than in Populus, Salix, and Tessaria. Tamarix △ averaged over 1% less than that of the other riparian taxa. Experimental removal of Tamarix from stands where Salix was codominant resulted in growth augmentation, less negative water potentials, and higher leaf conductance in Salix, all providing evidence of interspecific competition. The persistence of Salix, but not Populus, on the Colorado River appears to be due to greater water— and salinity stress tolerance in Salix than in Populus. A preponderance of senescent Populus along the Colorado River is an indication that this formerly dominant species is effectively approaching local extinction in parts of this ecosystem.

ISSN:0012-9615    

DOI:10.2307/2937064

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY