摘要:

Experimental manipulations of densities of the predatory salamanders Notophthalmus viridescens dorsalis and Ambystoma tigrinum significantly altered relative abundances of six species of larval anurans in 22 artificial—pond communities. One competitively inferior anuran, Hyla crucifer, was virtually excluded from predator—free control communities but survived best and occurred at greater relative abundances in communities containing high densities of Notophthalmus. A second competitively inferior species, Hyla gratiosa, survived best at intermediate intensities of predation. Each of four competitively superior species (Scaphiopus holbrooki, Rana sphenocephala, Bufo terrestris, and Hyla chrysocelis) exhibited inverse relationships between relative abundance and Notophthalmus density. Ambystoma eliminated the entire anuran guild from tank communities and had a much greater per capita impact on anuran guild composition than did Notophthalmus. In most anuran species, maximum and mean mass at metamorphosis were positively correlated with predator density, suggesting an inverse relationship between intensities of predation and competition among tadpoles. Low growth rates of most anuran species in the absence of predators were correlated with high abundances of superior competitors. These results indicate that predators mediated interspecific competition among larval anurans. Intermediate values of Notophthalmus density maximized the total production of anuran metamorph biomass. Biomass of metamorphs of each species varied in a species—specific manner with predation. The propensity of Rana sphenocephala for overwintering as tadpoles following a season of growth was also related to predator density. Laboratory feeding experiments demonstrated that at least two competitively superior anurans, Scaphiopus holbrooki and Bufo terrestris, were especially vulnerable to predation by salamanders. This suggested that Notophthalmus may preferentially remove competitively superior anurans from pond communities, allowing competitively inferior anurans to persist and to complete development successfully at moderate to high predator densities.

ISSN:0012-9615    

DOI:10.2307/1942491

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Seasonal patterns of decomposition and nutrient release from the major litterfall components were determined using litterbags in young (23—yr—old) and mature (180—yr—old) Abies amabilis stands in western Washington, USA. The time required for each litterfall component to decompose completely was estimated in both stands. Long—term organic matter and nutrient residence times in the forest floor were estimated using forest floor and aboveground litterfall masses. In addition, the effect of root turnover in the forest floor on estimates of organic matter and nutrient residence time was determined. Similar litter substrates lost mass at a significantly faster rate in the young than in the mature stand. An initial rapid mass loss (9—67%) for litter substrates during the winter months coincided with increased immobilization of nitrogen and calcium and mineralization of phosphorus, potassium, and magnesium. However, herb species and epiphytic lichen decomposition in both stands showed no immobilization of N, P, K, Ca, or Mg or increased mass in litterbags at all sampling dates. The remaining litter substrates showed no loss or gain in mass during summer and autumn, with continued immobilization of N and P, while K, Ca, and Mg levels remained the same or decreased. From 36 to 77% of the total 2—yr mass loss of all litter substrates occurred during the initial 4 mo. The various litter components required °6—15 and 2—24 yr for 99% decomposition to occur in the young and mature stands, respectively. The addition of roots into the estimation of organic matter residence time (MRT) in the forest floor reduced the estimate by °75% in both stands. The predicted turnover time of 1 yr's litterfall (11—12 yr) was very similar to the estimated residence time of total forest floor when roots were included in the calculations. The inclusion of roots in the estimation of forest floor MRT decreased the nutrient residence time for N by 74%, for P by 75%—86%, for K by 81—90%, for Ca by 52—55%, and for Mg by 66—85% in both stands. Addition of root nutrient input into estimates of forest floor nutrient MRT increased the annual turnover of forest floor nutrients from 1—2% to 4—16% in the young stand and from 1—4% to 4—8% in the mature stand. The element mobility series resulting from litterbag studies did not give the same pattern of mobility as obtained from the total forest floor with or without root input.

ISSN:0012-9615    

DOI:10.2307/1942492

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Seed germination in the seasonal tropical forest on Barro Colorado Island, Panama, was studied at the community level to (1) determine the seasonal timing of germination of the community, (2) identify primary selective factors controlling timing of germination, (3) determine the relative importance of dormancy vs. timing of seed dispersal as mechanisms controlling timing of germination, (4) examine inter— and intraspecific components of variance in length of dormancy, and (5) identify major seed germination syndromes. Three community—level measures of seedling emergence indicated that there was a unimodal community peak in germination within the first 2 mo of the 8—mo—long rainy season. Of the °185 dicot, mostly woody, species germinating each year, the median time of emergence of 75% of the species occurred within the 1st 3 mo. There was a unimodal peak in germination in pioneer tree species, lianas, canopy trees, wind— and animal—dispersed species, and seedlings with and without persistent seed reserves. In contrast, there was no distinct peak period of emergence in understory species and shade—tolerant tree species. Initial seedling height of species emerging sequentially throughout the rainy season did not increase. The early emergence of most species, including three—fourths of the 35 species fruiting in the late rainy season, indicated that the early rainy season was the optimal time to emerge. Maximizing the length of the first growing season was not an important factor selecting for early emergence, because later—emerging species did not compensate for the shorter growing season by being larger at germination or by having seed reserves for rapid growth. Seedling—seedling competition is a primary biotic factor selecting for early emergence in pioneer species growing in the intensely competitive light—gap habitat. In shaded understory habitats, where competition among seedlings is much less intense, understory and shade—tolerant tree species emerged throughout the rainy season. Mean length of dormancy (MLD),the time between sowing and germination, of 157 woody dicot species on Barro Colorado Island ranged from 2 to 370 d. In over half the species, MLD exceeded 4 wk; hence, delays in germination are common. The season in which seeds were dispersed and the dispersal mechanism explained small but significant portions of the variance in MLD among species; life form explained none; but differences among three germination syndromes explained two—thirds of this variance. In the delayed—rainy syndrome (18% of all species) seeds were dispersed in the rainy season but were dormant until the beginning of the next rainy season, 4—8 mo later. Dormancy is the primary mechanism controlling time of germination. In the delayed—rainy syndrome and the intermediate—dry syndrome which follows, the length of the dormant period decreased as the interval between seed dispersal and the beginning of the rainy season decreased. In the intermediate—dry syndrome (42% of all species) seeds were dispersed during the during the dry season and remained dormant until the beginning of the rainy season. Seeds are primarily dispersed 1—2 mo before the beginning of the rainy season, which reduces the number of false germination cues encountered and decreases the length of time seeds are exposed to postdispersal predation while dormancy prevents germination during dry season rains. In the rapid—rainy syndrome (40% of all species) seeds were dispersed in the rainy season and germinated during, but not early in, that season. Dormancy has been replaced entirely by timing of dispersal as a mechanism controlling time of germination. Half of these species germinated in<2 wk, the rest in 2—16 wk.

ISSN:0012-9615    

DOI:10.2307/1942493

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Community organization of the arboreal birds in two oak woodlands of western North America was examined during the course of two breeding seasons. A "natural experiment" comparing the arboreal avifaunas inhabitating oak woodlands of similar tree structure in northeastern Sonora, Mexico (referred to as Purica), and in the central coastal mountains of California, USA (referred to as Carmel), was used to address questions concerning (1) variables that account for variation in foraging behavior, (2) overall similarity in foraging behavior of the two avifaunas, (3) partitioning of resources within each avifauna, (4) guild structure, (5) foraging heights, 6) changes in foraging behavior between sites, and (7) complementarity in resource use during foraging. At Purica there were 15 breeding species, and at Carmel 10 species. Foraging technique, portion of the tree, perch substrate and height, and the substrate from which food was derived during foraging were quantified with the aid of an audio metronome. The first three axes in a Principal Components Analysis (PCA) accounted for °75% of the variation in foraging behavior at both sites, and were associated with technique, food site, and height. The rank order of the PCA loadings for all foraging variables on each PCA axis was highly correlated between Purica and Carmel, indicating that the overall pattern of foraging at both sites was very similar despite differences in avifaunal composition, density, and consuming biomass. Analysis of species breadth and overlap during foraging indicated that at both sites foraging specialization does not connote a decreased potential for species interactions. In addition, estimates of community breadth and overlap indicated that technique and food site were the most important foraging parameters in community organization at both sites. Three methods were used to develop the guild classification used in this paper: reciprocal averaging ordination, cluster analysis, and discriminant function analysis. At Purica five guilds were recognized: foliage gleaners, bark gleaners, bark probers, air salliers, and ground salliers. At Carmel four guilds were recognized: foliage gleaners, bark probers, air salliers, and ground salliers. In terms of relative guild consuming biomass (CB), foliage gleaners and bark gleaners at Purica together constitute 34% of the community. Interestingly, at Carmel a distinct bark gleaning guild is absent, and foliage gleaners constitute 32% of the community's CB. The proportion of community CB contributed by the other guilds was significantly different between sites. Although the possible functional roles performed within each community were similar (based on the kinds of guilds present), their guild structures were significantly different (based on the distribution of guild CB within each avifauna). The foraging space used by each of the guilds and the entire community was examined in detail. Foraging height of the birds was generally not related to the tree although both sites park probers derived most of their food from upper heights, foliage gleaners mostly from intermediate to upper heights, and ground salliers perched at intermediate heights. In contrast, air salliers at Purica usually perched at lower heights, while at Carmel they used upper heights most frequently. At Purica 50% of the compensatory shifts in foraging were consistent with predictions of competition—based hypotheses, whereas at Carmel none of the predictions was verified. Interspecific complementarity of resource use was demonstrated only marginally in air salliers at Purica. It is concluded that (1) the foraging behavior of some species at Purica, especially air salliers, is consistent with predictions from competition theory, although a lack of interspecific complementarity indicates that the competitive forces are rather weak, (2) at Carmel, opportunism appears to be more important in affecting the pattern of foraging behaviors, (3) other factors, especially ground cover, shrub density, and the local weather, have influenced foraging behavior and community structure at both sites, (4) similarity between sites in avian community structure is a consequence of the similarity in tree structure, whereas differences between sites probably result from short—term behavioral flexibility in microhabitat use in response to differences in prey distribution and abundance, and (5) interspecific competition is not a prerequisite for resource partitioning.

ISSN:0012-9615    

DOI:10.2307/1942494

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY

摘要:

Rates of herbivory and defensive characteristics of young and mature leaves were measured for saplings of 46 canopy tree species in a lowland tropical rain forest (Barro Colorado Island, Panama). Grazing rates were determined in the field for sample periods in the early wet, late wet, and dry seasons. Leaf properties such as pubescence, toughness, water, protein, fiber, and phenolic contents explained over 70% of the variation among plant species in the rates of herbivory on mature leaves. Leaf toughness was most highly correlated with levels of herbivory, followed by fiber content and nutritive value. Phenol content and phenol: protein ratios were not significantly correlated with damage. Mature leaves of gap—colonizing species were grazed six times more rapidly than leaves of shade—tolerant species. Gap—colonizers have less tough leaves, lower concentrations of fiber and phenolics, higher levels of nitrogen and water, shorter leaf lifetimes, and faster growth rates than do shade—tolerant species. Gap—colonizers did not escape discovery by herbivores to any greater extent than shade—tolerant species, as measured by the spatial distribution of plants or by the intraspecific distribution of herbivore damage under natural or experimentally manipulated conditions. In 70% of the species, young leaves suffered higher damage levels than mature leaves. Although young leaves are more nutritious and less tough and fibrous, they have two to three times the concentrations of phenols. The temporal appearance of young leaves was not correlated with the distribution of herbivory among individuals of a species. Interspecific patterns of defense mechanisms are discussed in terms of current theories of plant apparency, and an alternative model for the evolution of plant defenses is presented.

ISSN:0012-9615    

DOI:10.2307/1942495

出版商:Ecological Society of America    

年代:1983

数据来源: WILEY