摘要:

This paper considers three concepts of stability as they relate to the dynamics of distinctive patch types of algal canopy guilds in southern and central California kelp communities: (1) persistence of a patch through more than one generation of the dominant species, which was evaluated by using life tables and observations of patch borders; (2) inertia or the resistance of different patches to invasion or disturbance, which was evaluated by artificially enhancing gametophytes by transplanting sporogenic material, by removing canopy, and by evaluating some important disturbance processes; and (3) resilience or recoverability of a patch following a perturbation sufficient to allow invasion of different species, which was studied by defining some of the mechanisms of successful invasion or succession. By working in distinct habitats in southern (Pt. Loma and Santa Catalina Island) and central (Pt. Piedras Blancas) California, we could evaluate different types of physical stresses as they related to these stability concepts. Taller perennial canopy guilds were dominant competitors for light, but were more susceptible to physical wave stress. Dominance hierarchies in the competition for light appeared to be reversed in areas exposed to increasing wave stress. The main causes of mortality at Pt. Loma were entanglement with storm—dislodged Macrocystis plants and, in some areas, sea urchin grazing. Mortality in central California was due to winter storms. In most cases, distinct patches resisted invasion for>10 yr. The mechanisms of resistance involved (1) competition for light and, possibly, nutrients, and (2) limits to spore dispersal. When succession occurred, it was often mediated by many factors, including seasonality of spore production, which coincided with winter storm—related mortalities; mechanisms of kelp dispersal, which were most effective via drifting plants and fragments of fertile material held against the substrate by invertebrates; and survivorship of gametophytes and small sporophytes, which was influenced by local scour and grazing. Appropriate spatial scales, stability, and succession studies in these kelp communities were determined by the size of the disturbed area, which varied from the free space resulting from detachment of single plants to the free space resulting from catastrophies such as overgrazing or unusual storms. Temporal scales were influenced by seasonality of disturbance and algal reproductive condition and aperiodic episodes of cool, nutrient—rich water advected into the patch. There appeared to be conflicting morphological adaptations of the canopy guilds: exploitation of light was enhanced at higher canopy levels, whereas the lower canopy levels were better adapted to tolerate stress from wave surge. The adaptations of the algae appeared to form four distinct groups of tactics: (1) ruderals or plants, such as Nereocystis and Desmarestia, with opportunistic life histories; (2) kelps, such as Macrocystis, adapted to exploitative competition for light and nutrients; (3) kelps (Eisenia, Dictyoneurum) adapted to physical stress such as wave surge; and (4) those algae, such as corallines and Agarum, adapted to heavy grazing. Within any given area, the relative patch stability was determined by biological relationships; between areas, the patch stability patterns were attributable to physical differences.

ISSN:0012-9615    

DOI:10.2307/1942498

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

Bearing—tree data from the original land—survey records of 1847—1850 were used to reconstruct the vegetation of the Big Woods and adjacent areas along the prairie—woodland border in south—central Minnesota. The characteristic tree taxa of the Big Woods were elm (Ulmus), basswood (Tilia americana), sugar maple (Acer saccharum), ironwood (Ostrya virginiana), bitternut hickory (Carya cordiformis), butternut (Junglans cinerea), and ash (Fraxinus). The most common tree was elm, which comprised 27% of the bearing trees. A buffer zone of fire—tolerant oaks and aspen generally lay between the Big Woods and prairie. The width of this zone depended on topography and on the presence of additional firebreaks, which in places formed sharp boundaries between the Big Woods and oak—aspen. The prairie—woodland border was characteristically a sharp boundary along firebreaks (water bodies and physiographic breaks). In some places very effective firebreaks formed sharp boundaries between prairie and the Big Woods, with no intervening oak—aspen zone. The vegetation was most strongly correlated with the fire—probability pattern, which was a function of both abiotic and biotic factors. Soils influenced the probability of fire, but they also were the major factor controlling the vegetation within areas of similar fire probability. Soil drainage was the most important factor controlling vegetation within the units of the overall pattern. Because the locations of firebreaks and the existing pattern of vegetation controlled the fire probability pattern, sites with virtually identical physical characteristics supported qualitatively different types of persistent or stable vegetation.

ISSN:0012-9615    

DOI:10.2307/1942499

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

This study compares and contrasts diets, based on 2834 prey individuals from 126 stomachs, of 16 insectivorous, aerially foraging Neotropical flycatcher species (Tyrannidae) that are seasonally sympatric in the humid Caribbean lowlands of Costa Rica. Dietary parameters examined are prey type, diversity (breadth) of prey types, heterogeneity of prey types among individuals of a species, "patchiness" of morphologically indistinguishable prey within individual stomachs, and number of prey items per stomach. An R—type factor analysis of prey taxa in flycatchers' stomachs delimited four interpretable factors (axes) based on differences in prey detectability, location, escape behavior, and substrate where caught. A Q—type cluster of flycatcher species, based on factor scores from the analysis of prey variables, tended to juxtapose congeners. Noncongeners that clustered closely included a pair of species (Todirostrum sylvia and Oncostoma cinereigulare) that tend to replace each other geographically. Some flycatchers failed to cluster with any other species. "Saturation curves" of prey—type diversity against number of stomachs sampled showed (1) adequate sampling effort for most species with 4—10 stomachs, and (2) species differences in breadth of prey types eaten. The flycatchers that did not cluster with any other species on the basis of prey taxa had the largest and smallest diet breadths of all species studied. Two species are exceptional and consistent specialists: the Ruddy—tailed Flycatcher (Terenotriccus erythrurus) ate 94% Homoptera (largely Fulgoroidea), and the Long—tailed Flycatcher (Colonia colonus) ate 67% stingless bees (Trigona species). Flycatcher species that had the most "patchy" diets (i.e., many individuals of the same prey species within a stomach) tend to hawk flying prey and/or reside in open country or forest canopy. Colonia colonus had extremely patchy stomach contents, but unlike other flycatchers that hawk flying prey, it had an exceptionally homogeneous diet. "Patch feeders," compared with other flycatchers, tended to have many more items per stomach, suggesting the selection of many, relatively small prey per unit time. Significant variation in numbers of items per stomach, even among congeners, suggested differences in feeding rate and perhaps prey size. Comparison of species with repect to these dietary parameters helps identify a "food resource" for each species. The identification of the food resource (1) helps interpretation of foraging behavior of the predators, (2) facilitates discussion of prey—type and prey—size selection, (3) delineates potential ecological and evolutionary routes of species interactions, and (4) focuses attention on the nature of the food supply available to each species. The assumption that species with patchy and heterogeneous diets feed opportunistically on relatively ephemeral prey permits the following conclusions: (1) many tropical flycatchers are not opportunistic, (2) many guilds, including tropical ones, are composed of species with a variable degree of opportunism, (3) migrants are more opportunistic while wintering (in Caribbean Costa Rica) than syntopic year—round residents, and (4) open—country and canopy flycatchers tend to be more opportunistic than forest—interior species. Variation in flycatcher diets with respect to all parameters examined necessitates multiple explanatory hypotheses, and warrants a pluralistic approach to questions of community structure in these birds.

ISSN:0012-9615    

DOI:10.2307/1942500

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

Carbon and nitrogen dynamics were analyzed during the decomposition of litter and roots of the desert ephemeral pepperweed (Lepidium lasiocarpum). We treated litter bags with the insecticide chlordane and the fungicides benomyl and captan to eliminate or restrict groups of soil biota. The mass losses of buried litter (51, 39, and 25% for untreated, insecticide—treated, and fungicide—insecticide—treated material, respectively) were higher than those of the respective root treatments (35, 18, and 15%) at 96 d. The mass loss of untreated material was correlated with numbers of detritivorous—fungivorous microarthropods, and only a small percentage of this loss was as CO2: 27 and 42% for litter and roots, respectively. In the absence of microarthropods a higher percentage of mass—loss carbon could be accounted for as CO2: 33 and 76% for litter and roots, respectively, indicating that mass loss was due primarily to litter removal by microarthropod activity and not to mineralization. Litter removal by microarthropods was less dependent on abiotic constraints such as soil moisture (r = 0.65, P<.001) than was mass loss when microarthropods were absent (r = 0.79, P<.001). In the absence of microarthropods, mass loss was more closely coupled with biomass of grazers, such as nematodes, which require free water for activity (r = 0.99, P<.0001). Unlike mass loss, carbon mineralization was highest in untreated roots, suggesting a stimulation of microbial activity by microarthropods, while in untreated litter no stimulation was observed when compared to insecticide treatments. This difference was primarily a function of fungivorous microarthropod density, with overgrazing occurring in the untreated litter. Nitrogen budgets indicated the importance of microarthropods in the turnover of root nitrogen. In the presence of microarthropods 132% of the initial root nitrogen could be accounted for after 96 d, while in the absence of microarthropods 270% could be accounted for. This net immobilization of nitrogen was primarily in the soil organic fraction around the roots and was associated with fungal development. Data from this study re—emphasize the importance of microarthropods as regulators of decomposition in deserts and suggest that predation by nematodes or protozoa on bacteria and fungi contributes to rate regulation. Nitrogen flux data suggest that when spring ephemeral plant production is high, decomposition of ephemeral roots with attendant nitrogen immobilization can reduce the nitrogen available to creosotebush, Larrea tridentata, thus reducing shrub production. Higher taxa of soil biota, i.e., nematodes and microarthropods, may thus be important regulators of nitrogen fluxes and of mass loss in decomposition.

ISSN:0012-9615    

DOI:10.2307/1942501

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY