ISSN:0012-9658    

DOI:10.2307/2937183

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Complex life cycles are characterized by niche shifts at the time of metamorphosis. Current models predict optimal sizes for metamorphosis based on maximizing growth, minimizing mortality, or some balance of these goals. These models predict optimal sizes that are independent of the time of metamorphosis. Reproduction and other major events in the life history of organisms are often constrained to seasons, and the state (e.g., mass) of the organism at that time is related to fitness. Therefore, an organism's state as well as the time that that state is achieved are central variables in these time—constrained life histories. We extend earlier theory to include explicit time constraints in three, hypothetical, complex life cycles. Dynamic optimization models are constructed to determine optimal time and mass trajectories for niche shifts. First, we consider the habitat shift at emergence in mayflies, where reproduction terminates a growth period in the first habitat and is constrained to a season. Second, we consider the habitat shift at metamorphosis in amphibians, where reproduction terminates a growth phase in the second habitat and reproduction is constrained to a single point in time. Third, we combine the first two effects to allow an extended period of reproduction in amphibians. Here optimal time and mass trajectories are determined for two niche shifts–the shift from aquatic to terrestrial habitat and the shift from a growth phase to a reproductive phase. We present analytical theory that allows both quantitative and qualitative predictions. Problem constructions and solutions are presented graphically to aid intuition in interpreting our results and extending the framework to other parameter values and other life—history examples. The general conclusion is that time constraints on complex life histories lead to optimal sizes for niche shifts that vary with time. In time—constrained life histories, any variation in the state of individuals at some time prior to reproduction will be preserved to some degree at reproduction. Therefore, in time—constrained life histories, we expect optimal switches in habitat use or life history stage to depend not only on state but also on the time that state is achieved.

ISSN:0012-9658    

DOI:10.2307/2937184

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

One of the most pervasive, nonrandom evolutionary patterns is extreme domination of a taxon by one subtaxon or only a few subtaxa. Domination refers to taxonomic diversity and the fraction of the taxon that is classified in the most diverse subtaxon. We attempt to explain how subtaxa come to dominate their phyletic counterparts by examining correlations between taxonomic diversity and life history traits such as age of first reproduction, longevity, fecundity and partitioning of reproduction, and resource availability in a variety of vertebrate, invertebrate, and plant groups. Regardless of taxonomic group or rank, the number of taxa within an assemblage, or the school of taxonomy employed, diverse taxa were characterized by short generation time (early age of first reproduction and short life—span) and the ability to contact many resources (high mobility and high resource availability). We suggest that the intrinsic character of short generation time increases diversity because it promotes speciation and reduces extinction. Extrinsic factors such as resource availability and environmental complexity and variability may have a secondary influence on diversity by constraining or enhancing speciation for taxa with short generation times.

ISSN:0012-9658    

DOI:10.2307/2937185

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Trophic bottleneck, which is a common phenomenon in the pelagic food chains of large tropical lakes, was studied in Lake Valencia, a large and eutrophic lake located in Venezuela. Mechanisms of trophic bottleneck in lake Valencia were demonstrated through two years of population studies of Xenomelaniris venezuelae, a small planktivorous fish that accounts for most of the zooplankton consumption in open water. Major components of variation in abundance of Xenomelaniris included small—scale spatial (0.1—1.0 km, 70%), large—scale spatial (1—10 km, 23%), and temporal (months, seasons, 7%). The population showed a repeated annual cycle based primarily on a dominant cohort recruited during January and February. Spawning in the littoral zone continued throughout the year, but larvae survived only when rotifer densities exceeded 100 individuals/L, which occurred predictably during late seasonal mixing, but only irregularly thereafter. Steady mortality (15%/mo) of spawning fish occurred through predation. Catastrophic, predictable mortality of all fish occurred through anoxia during early seasonal mixing (57%/mo for two months). Xenomelaniris showed a strong ontogenetic shift in diet from rotifers during early development to copepods and Chaoborus during later development. Daily rations ranged from 28% of dry body mass in the smallest fish to 10% in the largest. Although Xenomelaniris was the only planktivore and was capable of consuming all major zooplankton taxa, it consumed only 3.6% of zooplankton production because it was never sufficiently abundant to exploit its food resource fully. Total growth of adult Xenomelaniris (somatic plus reproductive) conformed to the same equation as somatic growth of juveniles; growth potential was diverted increasingly to reproduction as the fish aged. Young adult fish showed 66% assimilation efficiency and 11% growth efficiency. Respiration losses decreased during development as the 0.73 power of body mass. The annual ratio of production to biomass was high (4.0), reflecting food sufficiency, a long growing season, and continuous loss of older fish to spawning mortality. Annual fresh—mass production of Xenomelaniris (31 kg/ha) is a small proportion of primary production (<0.03%) and of zooplankton production (3.6%); energy budgets show that actual Xenomelaniris production is only one third of potential production given the observed zooplankton production and the measured growth efficiency of Xenomelaniris. The explanation of this trophic bottleneck is mortality, which prevents the population from reaching carrying capacity and thus blocks energy flow between zooplankton and pelagic fish. Although there were three significant kinds of mortality, larval mortality–apparently through starvation–was the explanation of the trophic bottleneck because it restricted the reproductive output of Xenomelaniris, which could otherwise have brought the population to carrying capacity. The trophic bottleneck may be of significance beyond Lake Valencia, in view of the low diversity and abundance of pelagic planktivores in tropical lakes.

ISSN:0012-9658    

DOI:10.2307/2937186

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

I used time—invariant and time—varying matrix models to analyze the demography of Leptogorgia virgulata, a shallow—water gorgonian. For a local population in the northeastern Gulf of Mexico, I estimated monthly rates of recruitment, colony growth, and mortality in a mapped 24—m2plot for 2 yr. In a time—invariant model, average mortality and recruitment rates were nearly balanced, so the population growth rate, ln (@l), was close to 0.0. An elasticity analysis showed recruitment contributed<5% to the measured rate of population growth. The most important component of population growth rate was survivorship, particularly of the large size classes. Results were similar for a patch model that incorporated spatial variation in recruitment and colony growth rates. Several published transition matrices of forest trees, which have a similar life history, were also characterized by low elasticities for recruitment. Fluctuations in population size of L. virgulata were analyzed with a time—varying matrix model. I randomized certain elements in the 23 monthly projection matrices and simulated the population track. For models with random temporal variation in survivorship, standard deviations (and coefficients of variation) of population size were consistently larger than observed. This result suggests that temporal variation in mortality rates tended to damp population fluctuations. The damping occurred at low population sizes: models with random variation in survivorship generated significantly smaller minimum population sizes than observed. In contrast, population tracks with random temporal variation in recruitment were not consistently different from observed. Although recruitment is widely regarded as an important factor structuring marine communities, its contribution to the temporal (but not spatial) dynamics of L. virgulata was minimal. This finding may be typical of long—lived organisms with delayed reproduction and indeterminate growth, such as forest trees and many sessile marine invertebrates.

ISSN:0012-9658    

DOI:10.2307/2937187

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Many species of zooplankton display temporal changes in the size of the offspring they produce. We address the question, is offspring size variation by Daphnia a predictable response to changes in resource abundance? For each of two species of Daphnia (D. pulicaria and D. pulex), we raised several distinct genotypes at each of tree different food levels. Differences in neonate body length, dry mass, and C:N ratio were measured at each food level. The two species responded differently to food level; neonate mass decreased in D. pulex with decreasing food, but the largest (mass) D. pulicaria neonates were produced at the intermediate food level. For both species, the lightest (mass) neonates were produced at the lowest food, but these neonates were often equal (D. pulex), or longer (D. pulicaria) in body length than those produced at high food. The production of small—mass, but relatively large—length neonates in the lowest food level was associated with a lower C:N ratio of both adults and neonates compared with the higher food levels. We conclude that qualitative changes in the production of offspring can result from simple quantitative changes in resource abundance. We suggest that these responses may represent adaptive plasticity in reproduction.

ISSN:0012-9658    

DOI:10.2307/2937188

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The hypothesis that species and communities occupying vegetationally similar environments on different continents should be similar in ecology has become widely accepted, although quantitative support for this hypothesis is meagre. As an indirect assessment of the likelihood of ecological and evolutionary convergence, I quantified features of community structure and of breeding biology, behavior, and life history of birds species occupying several sites in semi—arid shrub deserts in the Great Basin of North America and the interior of Australia. Study sites on both continents were statistically indistinguishable in most quantitative measures of vegetation structure. Breeding—bird assemblages contained the same number of species per unit area, but densities of individuals in Australia averaged half those in North America. A cluster analysis based on a multivariate similarity matrix derived from 14 ecological and life—history characteristics of the species revealed little close matching of Australian species with a North American counterpart, although two such pairings suggest possible ecological convergence. The frequencies of most ecological/life—history traits among species breeding on local plots differed substantially between continents. The Australian species were characterized by longer breeding periods with more breeding attempts, greater social aggregation during breeding and feeding activities, greater use of domed nests, and sedentariness or nomadism rather than seasonal migration. The avifaunas did not differ in mean clutch sizes, the incidence of polygyny, nesting height, territory size, or general dietary patterns. Despite the close matching of vegetation structure on the two continents, differences between the species and communities are more striking than the similarities. These differences may be related to climatic differences between the areas. Although long—term mean precipitation in the Australian and North American shrub—desert regions does not differ, the monthly and yearly variance in Australia is substantially greater. Droughts may be prolonged and widespread, and this factor, together with soil—nutrient limitations, may result in generally lower and more erratic primary and secondary production in the Australian deserts. Close convergence may be lacking because, despite the similarities in habitat structure, the climates and resource levels of Australian and North American shrub deserts differ in fundamental ways, and the avifaunas are of differing phylogenetic derivation. These comparisons emphasize that (1) bird communities are frequently open, so understanding the composition and dynamics of local assemblages requires a broad—scale perspective, and (2) generalizations or explanations developed for one region of the world may not apply in other regions, even if the habitats are similar.

ISSN:0012-9658    

DOI:10.2307/2937189

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Arctic geese nest in a highly seasonal environment in which ungrazed plants reach peak nitrogen concentrations when goslings hatch (Sedinger and Raveling 1986). Grazing by geese prolongs peak nutrient concentrations but reduces food availability. This should cause nutrient availability to decline seasonally. Here, we test the hypothesis that late—hatching goslings of Black Brant (Branta bernicla nigricans) grow more slowly than those hatching early. We substracted the sizes of wild goslings from those of captive—reared goslings of the same age and regressed the differences against hatch date. Differences between wild— and captive—reared goslings for body mass, tarsus length, and culmen length were significantly negatively related to hatch date; i.e., late—hatching wild goslings were smaller than those hatching early, after accounting for age. We detected no between—year difference in gosling size, but male goslings were larger than females of the same age for all measures. Egg size was only weakly associated with size of goslings 1 mo after hatching, but we detected no effect of other brood characteristics on growth. Seasonal variation in gosling growth rate may favor early nesting in arctic geese.

ISSN:0012-9658    

DOI:10.2307/2937190

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We examined the influence of timing of reproduction and brood size on growth rates of goslings of nidifugous Lesser Snow Geese (Anser caerulescens) using data collected at La Perouse Bay, Manitoba from 1978 to the present, Gosling growth rates declined significantly during the season, and the declines were independent of the parental genotype. Goslings from larger broods grew significantly faster, but there was no significant annual variation in the seasonal growth rate patterns with either hatch date or brood size. The hatch date and brood size effects could not be accounted for by systematic differences in either egg size or body size of the female parent. Goslings that grew more slowly due to the effects of hatch date or brood size were significantly smaller as adults. Because gosling growth rates influence final adult size, they may have a significant effect on various life history traits in this species.

ISSN:0012-9658    

DOI:10.2307/2937191

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The selction of prey by two species of shore—dwelling birds was compared with the predictions of a simple optimal diet model. The preferences of African Black Oystercatchers (Haaematopus moquini) and Kelp Gulls (Larus dominicanus) for different size classes of the sand—burrowing bivalve Donax serra were examined. Gulls took only large clams, regardless of the availability of the different size classes. This preference is ascribed to the high energy expenditure required for gulls to open clam shells. Oystercatchers, which open bivalve shells relatively easily, took both small and large clams, even when large clams were abundant. The selection of small clams when large clams were abundant appears to have resulted the lack of discrimination of clam sizes by oystercatchers and the temporal segregation of availability of small and large clams.

ISSN:0012-9658    

DOI:10.2307/2937192

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY