摘要:

The Snail Kite (Rostrhamus sociabilis), an endangered hawk, has a unique mating system in Florida (Beissinger and Snyder 1987): when food is abundant, males or females desert their mates at nearly equal frequency (ambisexual mate desertion) in the midst of a nesting cycle. I examined the demographic and environmental constraints selecting for a clutch size that permits one parent to desert, yet optimizes the number of offspring produced by each parent. Demographic studies, conducted from 1979—1983, indicated that kites have (1) a very high nest failure rate (° = 68%) due most often to unstable nest sites and predation, (2) a variable nesting season (5—10 mo/yr), (3) an early age of first reproduction for a bird this size (10 mo), (4) a high degree of iteroparity (double and potentially triple clutching within a season), and (5) unstable populations. Both nesting success and population size were directly related to Everglades water levels and resultant snail densities. Kites responded to large annual changes in food abundance, not by adjusting clutch size but by deserting their mates and presumably attempting to renest. Kite demographic traits appear to be adaptations to or results of an uncertain environment. Based on 67 yr of Everglades water levels, environmental predictability, measured by spectral analysis and Colwell's (1974) index, was low and influenced by water management regimes: (1) water levels were lowered, (2) annual variation in levels increased and annual cycles became stronger, (3) the period length of long—term drought—flood cycles shifted from 10 or more yr toward 5 yr, and (4) levels became a less predictive cue for favorable nesting conditions. A potential evolutionary pathway from biparental care to mate desertion in Snail Kites is proposed. It is unlikely that mate desertion evolved solely from a context of conflict between the sexes, because kite nesting success is so low that it is probably maladaptive for a parent to desert and jeopardize the survival of any of its first brood. Instead, mate desertion behavior probably evolved in response to a smaller average clutch size; this would allow females to be highly iteroparous and avoid the costs of overinvestment, and should be strongly favored in a highly uncertain environment. Analysis of clutches in museum collections suggests that an apparent decline in clutch size may have occurred in Florida during the past century. The potential for ambisexual mate desertion to occur in other vertebrates is during the past century. The potential for ambisexual mate desertion to occur in other vertebrates is examined.

ISSN:0012-9658    

DOI:10.2307/1939076

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Foraging strategies of Glaucous—winged Gulls (Larus glaucescens) were studied in rocky intertidal habitats of the western Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles (Balanus glandula) and mussels (Mytilus edulis) comprised most of the gull's diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins Strongylocentrotus polyacanthus), chitons (Katharina tunicata, and limpets (Collisella pelta and Notoacmaea scutum). Sea urchins, chitons, and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters (Enhydra lutris). With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Preference experiments were conducted in the field, in which the common species and sizes of prey were made equally available to foraging gulls, thus eliminating search and capture times. In comparison with natural food choice, where sea urchins were most preferred, chitons became most preferred. We suggest that chitons are infrequent in natural diets because they adhere more strongly to the substratum than do sea urchins. Benefits of selective foraging were determined by comparing the net rate of energy gain of simulated random foragers with energy gained by selection of intertidal zones, prey species, and prey sizes. Observed selection patterns provided increased energy as gulls became more selective, and averaged 155% more than that obtained by the simulated random foragers. Results of the study support the two main predictions of optimal foraging theory in that (1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and (2) gulls became more selective foragers when energetically more profitable prey were more available.

ISSN:0012-9658    

DOI:10.2307/1939077

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Morphology, diets, and activity budgets of four co—existing sea ducks, Harlequin Ducks, (Histrionicus histrionicus), Oldsquaws (Clangula hyemalis), Black Scoters (Melanitta nigra), and Common Eiders (Somateria mollissima), were studied during the fall and winter of 1983—1984 at Cape St. Mary's, Newfoundland. The diets and behavior of these species were related to body size. The smaller species, H. histrionicus and C. hyemalis, had diets with higher energy densities and spent more time feeding than did the larger M. nigra and S. mollissima. Multiple regression models were used to determine how the proportion of time spent feeding varied with five environmental and two temporal variables; we found that the two smaller species showed little flexibility in adjusting their activity budgets. This could become critical to survival of these diurnal feeders during periods of severe winter weather or ice conditions. As suggested by Bergmann in 1847, closely related species, such as these syntopic sea ducks in a harsh marine environment, adjust their behavior and diets to compensate for the thermodynamic differences associated with body size.

ISSN:0012-9658    

DOI:10.2307/1939078

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The influences of weather and food supply on Eastern Kingbird (Tyrannus tyrannus) reproduction were examined using data on annual (1980—1983) and seasonal variation in timing of breeding, clutch size, and egg mass in a population of kingbirds breeding in eastern Kansas, USA. Timing of breeding differed significantly among years and appeared to be related to insect abundance. Clutch size declined significantly with breeding date in all years, but differed significantly only between 1980 and 1981. A severe drought in 1980 likely depressed food availability, whereas an emergence of periodical cicadas in 1981 provided a superabundant food source. Across all years, however, clutch size varied inversely with the mean breeding date of the population, whereas average egg mass varied directly with food availability. Maximum annual differences in mean clutch and egg size were small compared to variation in food availability. I examined 10 hypotheses that have been proposed to explain seasonal changes to clutch size in birds. All but three rejected as possible explanations for kingbirds. The tendency of older females to lay both earlier and larger clutches is probably responsible for most of the seasonal decline of clutch size, but reduced probability of survival for offspring fledged late in the breeding season and avoidance of stress in adults just prior to migration may also be contributing factors. Reduced clutch size late in the breeding period should permit females to hatch young sooner, to raise them faster, and to fledge them at higher masses. These features should increase survival of both juveniles and adults since postfledging care is long in kingbirds.

ISSN:0012-9658    

DOI:10.2307/1939079

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Ctenophores and other gelatinous predators in the ocean are ephemeral in their spatial and temporal distributions, but often have a dramatic impact on the dynamics of their prey populations. Considerable information is currently available on the functional and numerical responses of several nearshore ctenophore species; however, an understanding of their potential for shaping the structure of natural zooplankton assemblages further information on the selective nature of their predation. Prey selection by the tentaculate ctenophore Pleurobrachia bachei was examined in the laboratory with predation experiments and videotaped observations of predator and prey behavior. Prey types offered to the ctenophore included each of the developmental stages of Calanus pacificus, adult Acartia clausii, and adult Pseudocalanus sp. Pleurobrachia exhibited an unusual, bimodal pattern of prey selection on the different developmental stages of Calanus. Calanus' vulnerability increased through the naupliar stages, dropped at the first copepodid stage, and then rose again throughout the subsequent copepodid stages prior to another decline at the adult stage. Adult Acartia and Pseudocalanus were found to be among the most vulnerable of the prey types offered to Pleurobrachia. We conclude that Pleurobrachia's diet is determined by the relative availability and vulnerability of prey, and that vulnerability to the ctenophore can be predicted from prey swimming speeds and the susceptibility of prey after encounter.

ISSN:0012-9658    

DOI:10.2307/1939080

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We tested the hypothesis that dominance and suppression are more likely to occur in plant populations as light becomes limiting, by growing greenhouse populations of cultivated radishes at two densities and three light levels. Growth rates and mean mass decreased with decreasing light and increasing density. We used standard deviation, skewness, and Gini coefficients (adjusted for mean mass) to measure the development of size distributions. These measures showed more hierarchical size distributions at higher density, but only at the lowest light level. In full sun the development of size hierarchies could be explained entirely by variance in exponential growth rates; dominance and suppression became important only when light was limiting.

ISSN:0012-9658    

DOI:10.2307/1939081

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Competitive interactions were more important than facilitative processes in controlling seedling growth in primary succession on the Tanana River floodplain in interior Alaska. Low soil and plant nitrogen levels in early succession were ameliorated by stands of alder associated with nitrogen—fixing bacteria. Facilitation was demonstrated in the greehouse where alder (Alnus tenuifolia) soil enhanced growth of willow (Salix alaxensis) and poplar (Populus balsamifera) seedlings. Similarly, in the field, nutrient addition experiments and plant nutrient analyses demonstrated that all field—grown seedlings were least nutrient limited in alder stands. However, naturally occurring spruce (Picea glauca) seedlings and transplanted seedlings of alder and poplar grew least in alder stands where nitrogen levels were highest. Trenching and clear—cutting experiments demonstrated that alder stands inhibit seedling growth by root competition and shading. Physiological differences among the study species help explain their changing dominance through succession. Seedlings of species that establish in early succession (willow and poplar) grew more rapidly than alder. Late successional spruce seedlings had the slowest rates. In addition, spruce seedlings were more tolerant of the shaded conditions in alder stands than were early successional species. Consequently, only spruce seedlings continued to establish in later stages. We conclude that this floodplain succession is a result of a complex interaction of life history, facilitative, competitive, and stochastic processes. To understand succession in such sites, the relative importance of these interactive processes must be investigated.

ISSN:0012-9658    

DOI:10.2307/1939082

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Growth, flowering, and nutrient content of Eriophorum vaginatum were observed annually over 4 yr at 34 sites spanning 5.5° latitude and 1050 m elevation in northern and central Alaska. We found a strong correlation between the average number of thawing degree—days during the growing season and the peak—season leaf mass per tiller. However, the results from reciprocal transplant gardens established at five sites suggested the correlation was due more to genetic differences among populations than to direct climatic effects. Other variables showed little correlation with long—term degree—day averages, and leaf N and P concentrations were site specific. A year of high inflorescence density at any site along the latitudinal transect was likely to be a high flowering year at all other sites. However, a year of above—average growth north of the Brooks Range did not always coincide with above—average growth to the south, and vice versa. The greatest year—to—year variation was in inflorescence density; least variation was in leaf mass per tiller an N and P concentrations. We conclude that yearly variation in weather affects plants on a broad regional scale, but specific controls over flowering versus growth differ in their geographic extent.

ISSN:0012-9658    

DOI:10.2307/1939083

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The influence of human visitor traffic on geographic distribution of the flora on 10 coral cays in the southern Great Barrier Reef region was investigated. We examined the composition of the flora and identified a substantial invasion of the region by an alien flora comprising mainly annual and perennial agrestal weeds found commonly on the adjacent mainland. The native regional flora, on the other hand, comprised mainly perennial and woody species. Anemochorous and epizoochorous species most prevalent in the alien flora, whereas the native flora comprised mainly endozoochorous and hydrochorous species. The native and alien floras could not be further delineated with respect to photosynthetic pathway. Nevertheless, we found that life mode was conditional on photosynthetic pathway and inferred a tendency of the annual and perennial species to use the C4pathway, i.e., to have the physiological capacity to colonize light—saturated and moisture—stressed habitats characteristic of the cay periphery. We postulated subsequently that the observed geographic distribution of alien species was the consequence of human visitor traffic by either (1) inadvertent dispersal of alien diaspores or (2) anthropogenic habitat disturbance and subsequent colonization by wind—dispersed alien diaspores. We found that the percentage of alien plant species recorded on a cay is a sensitive nonlinear response to the frequency of human visitor traffic to that cay and, most importantly, is independent of cay size. Moreover, the porportion of alien species on the cays has increased consistently since 1912, consonant presumably with increasing frequency of traffic to the cays. There was no substantial evidence that alien species invasion of these cays was a consequence of anthropogenic habitat disturbance and subsequent diaspore dispersal by inanimate agents, nor that the native flora was being displaced by the alien flora. We have concluded that inadvertent human visitor dispersal of alien diaspores (i.e., anthropochory) has been the principal dispersal syndrome responsible for the alien species invasion of these cays, and therefore we assert that anthropochory is an important determinant of the geographic distribution of the alien on these coral cays.

ISSN:0012-9658    

DOI:10.2307/1939084

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

Little is known of the factors that engendered the classic pine forests of Michigan in the period before settlement by Europeans. Some of the forests were relatively open savannas of jack pine and red pine. Other stands were composed of large red and white pines, occasionally with an admixture of hemlock and hardwoods. The early Government Land Office Survey records of Ros—common and Crawford Counties were used to determine the composition and the disturbance history of the pine forests of northern lower Michigan. The abundance of pine (mainly Pinus banksiana, P. resinosa, and P. strobus) in this area that was between lobes of the Wisconsin Laurentide ice sheet was correlated with the presence of coarse—textured soils derived from outwash and ice—contact deposits. These soils promoted a vegetation type which was extremely susceptible to fire. Records suggest the average return time for severe crown fires ranged from 80 yr in the case of the jack pine forest type to 120—240 yr for the mixed pine type to 1200 yr for the hemlock—white pine—northern hardwoods type. Blowdowns had a much longer return time and reached their greatest frequency in the swamp conifer and hemlock—white pine—northern hardwoods types. A three—dimensional diagram summarizes the role of fire, windthrow, substrate, and topography in the formation of Michigan's pine forests.

ISSN:0012-9658    

DOI:10.2307/1939085

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY