摘要:

The archaeogastropod Nerita scabricosta is an abundant and extremely mobile herbivore which occurs from above mean high water through the mid—intertidal zone of exposed rocky areas along the Pacific coast of Panama. It is the principal prey of Purpura pansa, a patchily distributed, high intertidal neogastropod. Both snails exhibit activity patterns and habitat preference which appear to have evolved in response to heavy predation pressure by fish and to harsh physical factors. These behavioral mechanisms restrict movement and limit foraging time for both species, but also result in a cyclic overlap of their distributions. Potential interaction between the two gastropods is thus limited to well defined, discrete periods. Purpura elicits both an escape and an avoidance response in Nerita. The intensity of predation on the nerite by Purpura is lessened by these responses and Purpura appears unable directly to control the abundance of Nerita. However, it is experimentally shown that in a heterogeneous environment, Nerita's distribution is still more restricted and its local abundance further reduced by avoidance responses to temporally stable aggregations of Purpura. These aggregations, centered on specific fish—safe microhabitat, are thus hypothesized to result in the creation of local patches of intertidal substrate which are free from herbivory or disturbance effects by Nerita scabricosta. Evidence from field observations and manipulations is presented to support the hypothesis that in this complex, four trophic level community, a predator—prey interaction between two gastropods can limit potential resource monopolization and affect primary space occupancy.

ISSN:0012-9615    

DOI:10.2307/2937274

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Tree and shrub species composition and environmental characteristics were measured in 56 stands representing the range of natural vegetation types of the lower coastal plain of southeast Texas, and vegetation characteristics were related to the physical environment. Most of the stand—to—stand variation in species composition was expressed by the first axis of a reciprocal averaging ordination, and stand position along the first axis in turn was most highly correlated with percent sand in the surface soil (0—15 cm) (r = —.81, P<.05). The first ordination axis is interpreted as a soil moisture gradient. A more weakly expressed second axis of variation is interpreted as a soil aeration—parent material gradient (from high organic matter, low extractable Ca stands to low organic matter, high Ca stands). Overstory basal area, woody plant species richness of overstory and understory, and relative basal area of broad—leaved evergreens in both strata all varied in a regular way along the first ordination axis, having low values at the dry end, peaking in the wet—mesic range, and declining somewhat toward the wet end. Comparison of vegetation types in southeast Texas with those for the while of the southeastern. United States coastal plain showed widespread congruence of types. Major discrepancies were the lack of a pine flatwoods type in Texas and the presence in Texas of a flatland hardwood type and a pine—oak type more characteristic of the Piedmont further east. The question of fire, succession, and climax on the coastal plain is considered and it is suggested that the timing of different studies in the last 50 yr may have influenced results and interpretation.

ISSN:0012-9615    

DOI:10.2307/2937275

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

An annual organic carbon budget is presented for an 8—km segment of Creeping Swamp, an undisturbed, third—order swamp—stream in the Coastal Plain of North Carolina, USA. Annual input of organic carbon (588 gC/m2) was 96% allochthonous and was dominated by leaf litter inputs (36%) and fluvial, dissolved organic carbon (DOC) inputs (31%). Although the swamp—stream was primarily heterotrophic, autochthonous organic carbon input, primarily from filamentous algae, was important during February and March when primary production/ecosystem respiration (P/R) ratios of the flooded portions were near one. Annual output of organic carbon via fluvial processes (214 gC/m2, 95% as DOC, was 36% of total annual inputs, indicating that the swamp—stream segment ecosystem was 64% efficient at retaining organic carbon. Respiration (327 gC/m2) accounted for most of this retention, or about 57% of annual inputs, and storage accounted for the remaining 7% of inputs. Organic carbon dynamics in the Creeping Swamp segment were compared to those reported for upland stream segments using indices of organic matter processing suggested by Fisher (1977) and a loading potential index suggested here. Creeping Swamp, while loading at a high rate, retains a much larger portion of its organic carbon inputs than two upland streams. Organic carbon loading potential in Creeping Swamp is very large, primarily because of its great width and complete canopy; however, its low gradient and dense vegetation enhance organic carbon retention, especially of coarse particulate organic carbon (COPC), by maintaining low water velocities with little erosive force, tortuous flow pathways, and debris dams. Most of the retention and oxidation of organic carbon in the flooded Creeping Swamp ecosystem takes place in the floodplain. It is suggested that floodplain exchanges should also be considered in budgets of higher gradient rivers and streams. Despite the high degree of retention and oxidation of organic inputs to Creeping Swamp, there is a net annual fluvial export of 21 gC/m2, mostly in the dissolved form. Watersheds drained by swamp—streams in the southeastern United States are thought to have large organic carbon exports compared to upland forested drainages, because the stream network covers a much greater proportion of the total watershed area.

ISSN:0012-9615    

DOI:10.2307/2937276

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

In regions of mediterranean climate drought deciduousness has been considered an important adaptation for many species. This investigation focuses on the influences of a mediterranean climate on the phenology of a drought—deciduous shrub Lotus scoparius ssp. scoparius. Two research sites were chosen in the Santa Ynez mountains northwest of Santa Barbara, California for the field investigations. Phenological progressions of leaf production, leaf composition, shoot elongation, lateral branch production, and flowering in even—aged stands of Lotus scoparius were correlated with such factors as temperature, relative humidity, soil moisture, plant water relations, and photoperiod. These correlations were determined during a characteristic and uncharacteristic growing season for this mediterranean—climate region. Field correlations were the basis for the design of laboratory experiments in which the influences of photoperiod, temperature, water stress, and total daily photon flux on the phenological development of Lotus scoparius were studied. Climatic controls over Lotus scoparius phenology are complex with water relations and photoperiod having the greatest influence. Photoperiodic control over leaf abscission during water stress, and leaf production following dormancy, were two important phenological phenomena related to the unpredictably fluctuating mediterranean climate of southern California. The complex climatic control over phenology in Lotus scoparius constitutes an adaptation to the California mediterranean climate.

ISSN:0012-9615    

DOI:10.2307/2937277

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Tree squirrels in the genus Tamiasciurus are adapted to exploit conifer seeds and fungi in boreal conifer forests. The ranges of the two species in the genus are parapatric, suggesting competitive exclusion. Where their ranges are contiguous in the Cascade Mountains of southern British Columbia, Canada, the squirrels differ in at least five characters which adapt each species to be competitively superior in its range. Two of the characters (alarm calls and fur color) adapt the squirrels to avoid predation and the other three (jaw strength, body size, and reproductive rate) adapt them to exploit the resources typical of their ranges. Two or three of these differences also allow each species to be competitively superior on its side of other sections of the species boundary in coastal British Columbia and northeastern Oregon. The two species from rare hybrids, especially where logging has created disturbed habitats. The pattern of carrying food to storage in a central cache is the basis for both the evolution of territoriality in Tamiasciurus and the absence of sympatry for the two species. In most cases where congeneric territorial species are sympatric and carry food to the center of the territory, they spend most of their time searching for and catching food and relatively little time carrying it. For Tamiasciurus, on the other hand, food is easy to locate and most of the foraging time is spent in carrying food to the cache. Splitting the food niche of Tamiasciurus would involve too large an increase in time and energy for carrying food to be balanced by any gain in feeding and digestive efficiency that could result from specializing on a narrower range of food.

ISSN:0012-9615    

DOI:10.2307/2937278

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The release of phytotoxic compounds has been proposed to be involved in the inhibition of seed germination under the canopy of Adenostoma fasciculatum (McPherson and Muller 1969). McPherson et al. (1971) tentatively identified the phytotoxins as phenolic compounds; however, the soil extraction data presented in this study indicate that insufficient quantities of available phenolics are present in the soil to account for the lack of growth observed in the field. Soil from burned and unburned Adenostoma—dominated sites were exchanged and planted with herb seeds. While toxicity developed in soil from the unburned site, the resulting pattern of germination suggested that the toxicity originated from the soil and not from the shrub canopy. Treatments to disrupt the metabolism of soil microbes, and to isolate selectively the microbial fraction from the soil, as well as other experiments, lead to the conclusion that the soil toxicity was the result of phytotoxins of microbial origin. It is suggested that the role of Adenostoma in the suppression of herb growth may lie in its association with soil microbes that produce and release substances capable of inhibiting the germination and growth of plants.

ISSN:0012-9615    

DOI:10.2307/2937279

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY