摘要:

On Trinidad (4520 km2), 12 bird species in foothills of the Northern Range (11 hummingbirds and Coereba flaveola, a passerine) often use floral nectar for carbohydrate food. A subset of 6 (5 hummingbirds and Coereba) occur on Tobago, a smaller (295 km2), more isolated island. For 13 mo we compared these assemblages at study sites having similar climates and similar sets of bird—visited flowers (34 species at Trinidad sites, 31 on Tobago). Density of nectar resources for birds varied seasonally>100—fold on each island. During peaks in total flower density, much nectar available to birds went unused. In contrast, during a flower—poor 3—mo period (September—November 1977), nectar available in flowers was held near zero because supplies were consumed as fast as they were secreted. Competition for this limited supply of nectar appeared to be intense on both islands. Diets of coexisting bird populations diverged, especially at the Tobago site, where only 3 nectar—feeding bird species, with disparate morphologies, persisted: 1 short—billed hummingbird, 1 long—billed hummingbird, and Coereba. At other seasons, though, a variety, of bird species consumed nectar on both islands, and often diets of different species were nearly identical. Despite the twofold difference in species richness, the two bird assemblages used nectar resources similarly, contrary to the theory that species—rich and species—poor guilds should differ in patterns of resource use. There were no statistical differences between the islands in the ratio of demand for nectar to nectar supply (expected by theory to be lower on the average on species—poor Tobago), monthly variation in that ratio (expected to be greater on Tobago), or breadth of the entire guild's diet (expected to be lesser on Tobago). One reason may have been that supplies of nectar at both sites fluctuated so widely that demand for nectar in neither bird assemblage could change quickly enough to keep pace. Second, foraging by the hummingbird Chrysolampis mosquitus, a migrant common on Tobago for the 9 mo of the year outside the period of food shortage, made up in part for nectar use by the 6 hummingbird species found on Trinidad but not Tobago. C. mosquitus on Trinidad rarely entered our primary study site, instead shifting to a type of habitat not available on Tobago and relatively little used by other hummingbird species. We found few significant differences in the intensity of competition on the two islands. One hummingbird species, Amazilia tobaci, won most interspecific aggressive encounters over nectar on both islands. There were no significant differences in the intensity of exploitative competition (measured as mean interspecific crowding) experienced by most species on different islands. C. mosquitus, though, may have left Tobago during the 3—mo nectar shortage due to a combination of interference competition (from A. tobaci) and exploitative competition (from all 3 resident species), whereas at that time the Trinidad C. mosquitus population apparently left the island due to a shortage of appropriate flowers in its preferred habitat. Species composition on Tobago can partly be explained by examining traits of species present on Trinidad, the principal source pool. Although individuals in all 5 Tobago hummingbird populations were significantly larger in one or more morphological dimensions (bill length, wing length, mass, or wing disc loading) than conspecifics on Trinidad, the adaptive significance of these size increases is obscure in all but one case (A. tobaci). In addition to Chrysolampis mosquitus and Coereba flaveola, a passerine that consumes fruits and insects as well as nectar, the other birds that used nectar at the Trinidad study sites were: 2 large—bodied, stout—billed hummingbird species that consume many arthropods and that, like C. mosquitus, may sometimes migrate to the mainland; and two distinct 4—species sets of resident hummingbirds, one set with short bills (28 mm), often curved, bills. Only one species from each of these 4—species sets was found on Tobago. We examined Trinidad's 4—species sets closely to see if the Tobago representative from each, short—billed A. tobaci or long—billed Glaucis hirsuta, was a random draw with respect to population traits thought to enhance persistence on small islands. Diets of A. tobaci and G. hirsuta on Trinidad were not particularly broad, nor was either Trinidad population particularly free of exploitative competition, relative to all other species there. At the Trinidad study site, though, A. tobaci maintained the highest and least variable demand for nectar (hence, presumably, the highest and least variable population density) of any short—billed species, and G. hirsuta maintained a higher and less variable demand than 2 of 3 other long—billed species. We argue that the relatively low seasonal variation in demand for nectar (and, presumably, in population density) within the second—growth habitat is the most straightforward explanation for persistence on Tobago of A. tobaci and G. hirsuta, rather than other species from their respective groups. The result implies that the other 6 Trinidadian populations wander relatively more than A. tobaci or G. hirsuta, respectively, among different habitat types and thus depend more heavily on a regional habitat mosaic not likely to occur on a small island such as Tobago.

ISSN:0012-9615    

DOI:10.2307/1942523

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

Factors affecting water and nutrient outflow beyond the rooting zone were studied during a 3—yr period, using data from eight contrasting stands of lodgepole pine (Pinus contorta ssp. latifolia) forest in southeastern Wyoming and the output of a hydrologic simulation model (H2OTRANS) based on tree physiology. Nutrient outflow during a specific time period was estimated by multiplying simulated water outflow times element concentrations in the soil solution, the latter determined from samples collected periodically near the bottom of the rooting zone. Estimates of actual evapotranspiration (ET) for the period from early spring to late fall ranged from 21 to 53 cm, which was 33—95% (x = 73%) of total annual precipitation. For all stands and years, transpiration accounted for 50—61% of ET, and 9—44% of the transpiration occurred during the spring drainage period (vernal transpiration, VT). Estimated VT and outflow varied considerably among the stands, with VT accounting for 4—20% of the snow water. Outflow occurred only during the snow melt period and accounted for 0—80% of the snow water. Snow water equivalent varied annually by 300% or more. Nutrient outflow from the different stands also varied greatly. Ratios between simulated annual outflow and atmospheric inputs (bulk precipitation) were consistently>1.0 for Ca, Na, and Mg; were consistently<1.0 for N; and ranged from 0.3 to 2.0 for P and from 0.2 to 3.3 for K. Much of the variability in water and nutrient outflow can be attributed to the degree of biotic control, with water outflow affected by a different combination of factors than nutrient outflow. H2OTRANS was used to simulate the effects on outflow of different snow water equivalents and different total leaf areas. One result of the simulations was that nitrogen appears to be retained even at the highest levels of water outflow. Another was that increases in water outflow following reduction in leaf area were proportional to the leaf area removed. The results indicate that stands differing in site or habitat type experience different rates of water and element losses at different times during the snow melt season, and contribute differentially to streamwater quality and hydrograph shape. Factors affecting outflow are discussed in the context of successional trends, common perturbations including timber harvest, and hypotheses pertaining to nutrient conservation in terrestrial ecosystems. Nutrient retention in the snow—dominated lodgepole pine ecosystem appears to be primarily dependent on evergreen leaf area, duration of the VT period, and high carbon/nutrient ratios of the forest floor. Net losses of limiting nutrients probably occur primarily in pulses after abiotic perturbations such as fire.

ISSN:0012-9615    

DOI:10.2307/1942524

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

Pollination, fecundity, dispersal, establishment, and survival of Lithospermum caroliniense (Boraginaceae) were examined over five years on sand dunes of different ages at Miller Dunes, Indiana, USA, to determine the effect of various environmental factors on the relationship between reproduction and other life history stages. Lithospermum caroliniense is a self—incompatible, iteroparous, distylous herb that depends on cross—pollination by insects for fertilization. Level of pollination was highly variable from year to year. For each of the two floral morphs, a high proportion of pollen was derived from the other morph in years of abundant pollination. In years when pollen loads were extremely low, fruit—set was limited by pollinator activity. Even with abundant transfer of compatible pollen, fecundity of L. caroliniense averaged less than one nutlet per flower. Individuals were consistent in level of fruit production from year to year. Emergence of seedlings of L. caroliniense was greatest when propagules were covered by 1—4 cm of sand in experimental field and garden studies. Propagules exposed on the sand surface were unlikely to obtain sufficient moisture for germination, while hypocotyls of seedlings that germinated at depths of @>5 cm were incapable of elongating sufficiently to reach the sand surface. Most propagules germinated after the first winter, but results from experimental plantings showed that many propagules remained dormant until after the second winter. First—season seedling survival averaged 7—34%, except in 1981, when frequent rainfall and moderate temperatures doubled the percentage that survived. The highest survival was among seedlings emerging near Lake Michigan or on sites with a shallow water table. Mortality of 1— and 2—yr—old plants was low. Individual plants grew faster and flowered at younger ages at sites closer to Lake Michigan. At the site closest to the lake, °83% of the 1978 cohort flowered at age 3 or 4 yr. At greater distances from the lake, most members of the 1978 cohort had not flowered by 1983. Except for some members of the 1981 cohort established near the lake, few plants in younger cohorts had flowered by 1983. In light of the slow growth rates of 1978—1982 cohorts and of many marked adults, large individuals of L. caroliniense are probably very old. Heavy nutlets typically fell within 60 cm of the parent plant. Dispersal was primarily downslope of adults, since adult foliage was lodged in a downslope direction by the time of propagule maturation; dispersal by wind or rolling was limited. Nutlets of L. caroliniense were consumed in large quantities, apparently by rodents, since the shelled remains of nutlets were common around adults. Occasional dispersal of intact nutlets by herbivores could be important in colonization of recently formed sand dunes. Although L. caroliniense is clearly dependent on newly formed sand dunes or disturbances, colonization is rare because dispersal of propagules is extremely limited. Large propagules are essential for establishment, but are not conducive to dispersal and colonization. Long adult life—span facilitates occasional colonization. Despite traits favoring establishment, seedlings suffered high mortality, except in years of abundant and evenly distributed rainfall. Since adult mortality was generally low, the life history of L. caroliniense resembles the pattern in which high juvenile mortality favors evolution of iteroparity and a long life—span. However, in contrast to species that produce a small number of offspring in any one year, the indeterminate, modular growth form of L. caroliniense means that increasing numbers of propagules are produced over the lifetime of an individual, until habitat change results in decline of adults. The combination of life—history characteristics occurring in this species argues against strict application of current life—history categorizations.

ISSN:0012-9615    

DOI:10.2307/1942525

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

Plants and animals that inhabit the intertidal zone of wave—swept shores are generally small relative to terrestrial or subtidal organisms. Various biological mechanisms have been proposed to account for this observation (competition, size—specific predation, food—limitation, etc.). However, these biological mechanisms are constrained to operate within the mechanical limitations imposed by the physical environment, and these limitations have never been thoroughly explored. We investigated the possibility that the observed limits to size in wave—swept organisms are due solely or in part to mechanical, rather than biological, factors. The total force imposed on an organism by breaking waves and postbreaking flows is due to both the water's velocity and its acceleration. The force due to velocity (a combined effect of drag and lift) increases in strict proportion to the organism's structural strength as the organism increases in size, and therefore cannot act as a mechanical limit to size. In contrast, the force due to the water's acceleration increases faster than the organism's structural strength as the organism grows, and thus constitutes a potential mechanical limit to its size. We incorporated this fact into a model that predicts the probability that an organism will be destroyed (by breakage or dislodgement) as a function of five parameters that can be measured empirically: (1) the organism's size, (2) the organism's structural strength, (3) the maximum water acceleration in each wave, (4) the maximum water velocity at the time of maximum acceleration in each wave, and (5) the probability of encountering waves with given flow parameters. The model was tested using a variety of organisms. For each, parameters 1—4 were measured or calculated; the probability of destruction, and the size—specific increment in this probability, were then predicted. For the limpets Collisella pelta and Notoacmaea scutum, the urchin Strongylocentrotus purpuratus, the mussel Mytilus californianus (when solitary), and the hydrocoral Millepora complanata, both the probability of destruction and the size—specific increase in the risk of destruction were determined to be substantial. It is conjectured that the size of individuals of these species may be limited as a result of mechanical factors, though the case of M. complanata is complicated by the possibility that breakage may act as a dispersal mechanism. In other cases (the snails Thais canaliculata, T. emarginata, and Littorina scutulata; the barnacle Semibalanus cariosus), the size—specific increment in the risk of destruction is small and the size limits imposed on these organisms are conjectured to be due to biological factors. Our model also provides an approach to examining many potential effects of environmental stress caused by flowing water. For example, these methods may be applied to studies of: (1) life—history parameters (e.g., size at first reproduction, age at first reproduction, timing of reproductive cycles, length of possible reproductive lifetime), (2) the effects of gregarious settlement on the flow encountered, (3) the physical basis for patterns of disturbance, (4) the optimum (as opposed to the maximum) size of organisms, and (5) the energetic cost of maintaining a skeleton with an appropriate safety factor. A definitive answer regarding the possibility of mechanical limits to size depends both upon an accurate measurement of the probability of encountering a wave of specific flow parameters and upon factors that are external to the model considered (e.g., life—history parameters). Further, due to their ability to move with the flow, organisms that are sufficiently flexible can escape the size limits imposed on more rigid organisms. Thus, some macroalgae attain large sizes (2—3 m in maximum dimension). The precise role of these factors awaits further research.

ISSN:0012-9615    

DOI:10.2307/1942526

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

Disturbance—perturbation, dispersal—reaction, and contemporaneous disequilibrium are similar theories used to explain the maintenance of species diversity in communities. These theories explicitly predict that in patches, on certain time—space scales, there should be substantial shifts in the order of species dominance. There is good evidence that these theories may explain species coexistence in terrestrial and marine systems of sessile organisms. We have tested this set of theories in a mobile pelagic system by examining the order of dominance of copepod species in samples separated in time and space, collected from °30 min to 16 yr apart, and from hundreds of metres to thousands of kilometres apart. We could not detect significant changes among the samples in rank order or in percent similarity of species abundance on any time scale, or on any space scale up to °800 km, either when all 175 species or when only the 30 most abundant were considered. There was small—scale, mesoscale, seasonal, and interannual heterogeneity in physical properties during the time we made our measurements. Although the theories are satisfactory explanations of diversity maintenance in sessile systems, our results fail to validate them in our mobile pelagic system. Because there were episodes of significant physical variability and because of the long—term species equilibrium and constancy of dominance, we believe our highly diverse community to be resilient and robust, rather than fragile. The regulatory forces are strong and almost certainly biological, rather than physical, but we cannot identify them.

ISSN:0012-9615    

DOI:10.2307/1942527

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

The most common system responses attributed to microfloral grazers (protozoa, nematodes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased CO2evolution, increased N and P mineralization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microfloral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua gracilis) was grown in gnotobiotic microcosms containing sandy loam soil low in inorganic N, with or without chitin amendments as a source of organic N. The soil was inoculated with bacteria (Pseudomonas paucimobilis or P. stutzeri) or fungus (Fusarium oxysporum), with half the bacterial microcosms inoculated with bacterial—feeding nematodes (Pelodera sp. or Acrobeloides sp.) and half the fungal microcosms inoculated with fungal—feeding nematodes (Aphelenchus avenae). Similar results were obtained from both the unamended and the chitin—amended experiments. Bacteria, fungi, and both trophic groups of nematodes were more abundant in the rhizosphere than in nonrhizosphere soil. All treatments containing nematodes and bacteria had higher bacterial densities than similar treatments without nematodes. Plants growing in soil with bacteria and bacterial—feeding nematodes grew faster and initially took up more N than plants in soil with only bacteria, because of increased N mineralization by bacteria, NH4+—N excretion by nematodes, and greater initial exploitation of soil by plant roots. Addition of fungal—feeding nematodes did not increase plant growth or N uptake because these nematodes excreted less NH4+—N than did bacterial—feeding nematode populations and because the N mineralized by the fungus alone was sufficient for plant growth. Total shoot P was significantly greater in treatments with fungus or Pelodera sp. than in the sterile plant control or treatments with plants plus Pseudomonas stutzeri until the end of the experiment. The additional mineralization that occurs due to the activities of microbial grazers may be significant for increasing plant growth only when mineralization by microflora alone is insufficient to meet the plants' requirements. However, while the advantage of increased N mineralization by microbial grazers may be short—term, it may occur in many ecosystems in those short periods of ideal conditions when plant growth can occur. Thus, these results support other claims in the literature that microbial grazers may perform important regulatory functions at critical times in the growth of plants.

ISSN:0012-9615    

DOI:10.2307/1942528

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY