摘要:

Fruit production and patterns of seed dispersal by birds were studied at two elevations in the mediterranean scrublands of southern Spain. Fleshy—fruit—producing species represent a very prominent fraction of woody plants in terms of cover (57—76%) and species number (49—66%). Fruit production occurs year round in the lowland site but is confined to August—February upslope. Ripe fruits are most abundant (>105ripe fruits/ha) in November—December. Fruit abundance fluctuates widely between years at the highland locality but only slightly in the lowlands. In both communities, the dominant species ripen fruits in autumn—winter, display the highest within—plant fruit densities, and tend to have the most lipid—rich fruits. Fruits differ in pulp nutritive value, seediness, and relative amount of pulp among species but are remarkably uniform in size (mostly 5—10 mm transverse diameter). Two—thirds of the passerine species at each site eat some fruit. Of these species, 69% (highland) and 26% (lowland) are resident "fruit predators," feeding on either pulp or seeds alone, and damaging the seeds when eating pulp and seeds together. The rest are overwintering or migratory seed dispersers that ingest whole fruits without damaging seeds. Seed dispersers are most common in late autumn—winter, coincident with the peak in fruit abundance and the predominance of lipid—rich fruits. A few small (12—18 g body mass) disperser species (Erithacus rubecula, Sylvia atricapilla, Sylvia melanocephala) account for most of the frugivory at each site and disperse the majority of seeds. Fruit predators either are relatively scare or eat fruits infrequently, or fruits represent a negligible fraction of their diets. Fruit removal was very high (89—100% of crops) among species with fruits smaller than the gape width of the abundant small—sized dispersers, and very low among species with fruits larger than gape width. Removal success was negatively correlated with fruit size among species having fruits smaller than dominant dispersers' gape width. No relation has been found between removal success and fruit quality, fruiting time, ripening rate, or within—plant fruit density. The principal dispersers at each site ate mainly the most nutritious fruits, although not to the exclusion of less nutritious fruits. Substantial pairwise plant—bird reciprocity is not common. (The avian species disperses a substantial fraction of a plant's seeds, which in turn provide the bulk of the bird's energy supply.) Current bird—plant seed dispersal interactions are the result of evolutionary, climatic, and geographical factors in the Mediterranean. Mutualistic congruency largely is, in these cases, an epiphenomenon of these factors, not resulting necessarily from mutual adaptations (coevolution). It is suggested that actual coevolution involving a smaller set of bird and plant species may facilitate the persistence of noncoevolving (or very slowly coevolving) plant species, thus favoring the existence of a chronic "anachronism load" (with regard to dispersal) in the plant community.

ISSN:0012-9615    

DOI:10.2307/1942454

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

Habitat and life history are critical elements in assessing the production dynamics of invertebrates and their role in aquatic ecosystems. We studied invertebrate productivity at two sites in a subtropical blackwater river (the Satilla) in the Lower Coastal Plain of Georgia, USA, and found that submerged wooden substrates, or snags, are heavily colonized by aquatic insects. We compared invertebrate productivity on the snag habitat with productivity in the sandy benthic habitat of the main channel, and the muddy benthic habitat of the backwaters. The size—frequency method was applied to individual taxa in order to determine total invertebrate productivity. Emphasis was placed on the importance of the length of larval life, or the cohort production interval, in determining biomass turnover rates. The diversity of taxa was much higher on the snag habitat than in either of the benthic habitats. Filter—feeding caddisflies (especially Hydropsyche spp.) and black flies (Simulium spp.) were the major consumers on the snag habitat. Several species of midges, mayflies, and beetles also were abundant. Total densities, standing stock biomass, and production were very high for primary consumers on snags. Annual production was 51.9 and 67.1 g°m—2°yr—1(dry mass per surface area of snag, or effective habitat) for the two sites. Hellgrammites, dragonflies, and stoneflies were the major insect predators colonizing snags, and their production was 5.5 and 5.2 g@mm—2°yr—1(effective habitat). Annual production/biomass ratios (P/B) were usually 5—10 for insects that had univoltine or bivoltine life cycles. Annual P/B estimates were very high for midges (>100) and black flies (>70), since length of larval life was estimated to be very short. The sandy—substrate benthos consisted almost exclusively of very small midges with oligochaetes of lesser abundance. Densities were quite high (>20 000/m2), but biomass was very low (° 100 mg/m2or less). Production of primary consumers was>11 g°m—2°yr—1with a very high estimate of annual P/B (166—227). The major predators were Ceratopogonidae (biting midges) larvae with an annual production of 1.6—2.6 g°m—2°yr—1. The muddy—substrate benthos consisted primarily of oligochaetes (Limnodrilus) and midges. Annual production was °7—10 g°m—2°yr—1for primary consumers. The major predators were larger Tanypodinae midges. On a substrate surface area basis, standing stock biomass on snags was 20—50 times higher than in the sandy habitat and 5—10 times higher than in the muddy habitat. Production on snags was only 3—4 times higher than production in the benthic habitats, with higher annual P/B in the latter. The production estimates for the snag habitat are among the highest yet reported for lotic ecosystems, and it appears that production on snags is limited by available substrate. Habitat areas per length of shoreline were estimated so that we could approximate relative amounts of biomass and production for a stretch of river. Although the snag habitat accounted for only °6% of the effective habitat substrate over a stretch of river, it was responsible for over half of invertebrate biomass, and °15—16% of production. Taxa within each habitat were categorized to functional feeding groups, and habitat—specific functional groupings were evaluated using numbers, biomass, and production. Filtering collectors predominated on snags, and gathering collectors in benthic habitats. When corrected for habitat abundance, the distribution of biomass among filtering collectors, gathering collectors, and predators was very close. However, the distribution of production was °12% filtering collectors, 71% gathering collectors, and 17% predators. We suggest that production is the most meaningful parameter to consider in functional group analysis and that the use of numbers or biomass alone can sometimes result in misleading conclusions. As a middle order (5th—6th) stream, the distribution of production or biomass among functional groups in the Satilla River differs considerably from that predicted by the river continuum concept, predicting a high percentage of grazing consumers.

ISSN:0012-9615    

DOI:10.2307/1942455

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

A computer model was developed to predict the feeding selectivity of planktivorous white crappie (Pomoxis annularis) from a known distribution of zooplankton. The model was based on the assumption that each predation event could be subdivided into a series of independent steps: prey location, pursuit, attack, and retention. The probability that white crappie successfully completed each step was determined for potential zooplankton prey species in a series of laboratory experiments. The four steps were then incorporated into a stochastic model where the probability of a particular prey type being consumed is equal to the product of the probabilities of the individual steps. The model was field tested by sampling fish, zooplankton, and physical parameters from discrete depth strata in a small reservoir on nine dates from October 1978 through November 1979. The model proved to be very accurate at predicting the species and size distribution of the ingested prey across the range of light intensities, turbidities, temperatures, and zooplankton densities encountered. Prey consumption could not be characterized as simply size selective; rather, it reflected the selectivity expressed at each step in the feeding cycle.

ISSN:0012-9615    

DOI:10.2307/1942456

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

The species composition of pools in the intertidal zone on the coast of Washington State varies greatly from pool to pool and from time to time. While assemblages change somewhat predictably from the low— to the high—intertidal zone (presumably owing to different stress tolerances of the species), the variance among pools at a given tidal height cannot be ascribed to such physical factors. Some pools at each height are dominated by one species that monopolizes space on the rock or in the water column and modifies the pool environment. Each dominant species, once established, can spread rapidly through a pool (either by vegetative growth or by enhanced recruitment of its conspecifics) and is thus potentially self—perpetuating. When abundant, most dominants appear to prevent potential competitors from settling and surviving by monopolization of resources, abrasion of the substratum, and/or collection of sediment. Six such dominants were identified for Washington tidepools: from low to high pools, these are (1) the surfgrass Phyllospadix scouleri, (2) articulated coralline algae, (3) the mussel Mytilus californianus (exposed shores), (4) the cloning anemone Anthopleura elegantissima (more protected shores), (5) the red alga Rhodomela larix, and (6) the green alga Cladophora sp. Colonial diatoms also appear capable of dominating low pools in the absence of wave disturbance. However, each dominant monopolizes only 20—50% of the pools at any height. Disturbances, defined here as a loss of biomass exceeding 10% cover of a sessile species within 6 mo and caused by extrinsic forces, were observed frequently in regularly censused tidepools. Disturbance agents included waves, excessive heat, wave—driven logs or rocks, and unusual influxes of predators and herbivores. Severe disturbances (those affecting a large proportion of the organisms in a pool) tended to occur in high pools in the summer (due to heat stress) and low pools in the winter (due to wave damage). Overall, a disturbance occurred in every pool studied an average of every 1.6 yr. About half of the 231 observed disturbances affected one of the six dominant species. The frequencies of these disturbances ranged from one every 2—5 yr, and recovery of the species to its original level required 3 mo to>2 yr. Some species (e.g., Rhodomela) were disturbed frequently bu recovered quickly because of rapid vegetative growth. However if asexual propagation was not possible, such as when the entire population of a species was removed from a pool, the slowness and irregularity of recruitment of sexual propagules greatly impeded recovery. Experimental manipulations involving the total removal of dominant species from pools showed that such large disturbances often require>3 yr for recovery. The irregularity of planktonic recruitment can be compounded by the presence of herbivores, which can remove most settling organisms from the substratum, or by the absence of other organisms that are necessary for the settlement of a dominant (e.g., seed—attachment sites for Phyllospadix). The combination of high disturbance frequency and slow rates of recovery makes it impossible for any dominant to occupy all the pools in its tidal range at any one time. Disturbance is viewed in these habitats as the stochastic factor overlying other, more predictable, community—structuring factors such as tidal height, pool size, wave exposure, and levels of herbivory, predation, and competition. Thus combined deterministic processes and random events operate to produce a complex mosaic of species assemblages in pools in one region. None of the tidepool assemblages is "stable" over many generations; rather, they seem to exist in a dynamic state where disturbances are an integral structuring factor.

ISSN:0012-9615    

DOI:10.2307/1942457

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY

摘要:

An Annual nitrogen budget is presented for a small steam draining Watershed 10, H. J. Andrews Experimental Forest, Oregon. The role of allochthonous debris in the input, flux, and export of nitrogen is emphasized in the material balance budget. All material entering the stream channel was presumed to enter the water sometime during the year. Material estimates are based on total channel area. The major annual nitrogen input (1974—1975) was subsurface flow (11.06 g/m2) as dissolved organic nitrogen (10.56 g/m2) and nitrate (0.50 g/m2).Biological inputs of nitrogen amounted to 4.19 g/m2as direct terrestrial inputs of: litterfall (1.35 g/m2), lateral movement (1.78 g/m2), and throughfall (0.30 g/m2). Nitrogen fixation on fine wood debris contributed an additional 0.76 g/m2based on rates from a nearby watershed. Total nitrogen input was 15.25 g/m2. The nitrogen pool was dominated by large amounts of particulate organic matter. Coarse wood constituted 32% of the nitrogen pool (3.80 g/m2) and fine wood fractions 18% (2.18 g/m2). The coarse wood fraction greatly influenced stream morphology. Fine organic particulates constituted an additional 40% of the nitrogen pool (4.77 g/m2). DON (dissolved organic nitrogen) export (8.38 g/m2) was less than input, presumably due to biological uptake associated with litter mineralization, sorption, and chemical flocculation. Due to effective retention of particulate inputs by debris dams, biological processing in the particulate nitrogen pool, and uptake and sorption of DON, most particulate organic inputs increased in nitrogen concentration prior to export. Particulate organic nitrogen input (3.13 g/m2) was greater than export (2.53 g/m2). Total annual nitrogen output was 11.36 g/m2, resulting in a gain of 3.89 g°m—2°yr—1to the stream. Thus, the stream was not operating on an annual steady state, but on an input—output regime related to the processing of refractory wood debris and resetting by major storms. Although particulate and dissolved nitrogen loss per hectare was small for the 10—ha watershed, these losses passed through or were accumulated in a pool encompassing<1% of the watershed area. This concentration of N in the stream allowed establishment of a separate ecosystem whose processing efficiency and capabilities for nutrient cycling were related to the retention capacity of the channel and nutrient quality of inputs within the reach.

ISSN:0012-9615    

DOI:10.2307/1942458

出版商:Ecological Society of America    

年代:1984

数据来源: WILEY