摘要:

Observations of 21 species of open—nesting passerines breeding in contiguous field and forest habitats at Rose Lake Wildlife Research Area, Michigan, were made during 1974 and 1975. Data were collected on nest dispersion, clutch—size, and fledging success in relation to the field—forest edge. Losses of eggs or nestlings were attributed to predation, inclement weather, Brown—headed Cowbird (Molothrus ater) parasitism, nest desertion, hatching failure, and adult death. Each bird species seemed to have a preferred distance from the habitat discontinuity that was used as a nest site. Furthermore, nests were not uniformly distributed on the area. Over one—half of the nests were found within ±15 m of the habitat discontinuity. Seventy—five percent of the nests belonged to birds characteristic of mixed breeding habitats, i.e., birds requiring an open overstory canopy with elevated singing and observation perches and dense cover near the ground for nesting and feeding. These mixed—habitat species also accounted for the increase in avian species nesting near edges. Based on Kendall rank and partial rank correlation tests, increasing numbers of nests and the percentage of total clutches,<3 eggs were found to be negatively correlated with increasing distance from the habitat discontinuity. Correlation between fledging success and increasing distance from the edge was positive and highly significant. Of the several mortality factors investigated, predation and cowbird parasitism were found to be the most important. The increased predation rate with decreased distance from the edge was attributed primarily to a functional response to higher numbers of nests and a greater activity of potential nest predators in the vicinity of the habitat discontinuities. Our results indicate that habitat suitability decreases with increasing numbers of nests toward the narrow field—forest edges. Although such abrupt habitat discontinuities did attract a variety and abundance of birds characteristic of habitats with mixed life—form, these discontinuities seemed to function as "ecological traps" by concentration nests and thereby increasing density—dependent mortality. Ironically, the cowbird was also a victim of the increased predation rate. As these man—made forest edges are of recent origin, they are perhaps unrepresentative of the ecological niche in which these species evolved, and thus they may be poorly adapted to cope with the increased nest predation.

ISSN:0012-9658    

DOI:10.2307/1938540

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Granivores are likely to store food in numerous, widely scattered, small caches if they are unable to defend concentrated large caches against interspecific competitors. This scatterhoarding of seeds makes it impossible for individuals to defend all their scattered caches against intraspecific competitors as well. Optimal spacing of scattered caches should result from a balance between decreasing loss of caches to naive competitors with decreasing density and increasing cost of storage with decreasing density. A mathematical model predicting optimal density is presented. One prediction of the model is that as the (θ) of habitat suitable for seed burial surrounding a seed source is decreased, the average distance (D) a cache is taken from the source by scatterhoarders should increase. Another prediction is that increasing the number of seeds (N) at a source, either by an increase in the size of a single seed crop or by the presence of 2 or more conspecific seed producers in close proximity to one another, should increase D. One trait of trees which increases single crop sizes and average seed dispersal distance (D) is the pattern of withholding energy from reproduction some years to allow unusually large crops during mast years. Three field tests of the model and its predictions were conducted: (1) Juglans nigra seeds were buried at 3 densities and their survival in time from predation by Sciurus niger was found to increase with decreasing density; (2) average distances that J. nigra saplings occurred from 16 parent walnut trees were found to have a statistically significant correlation with the D calculated from the model and the θ of suitable habitat surrounding each tree; (3) Sciurus niger individuals were observed to scatterhoard Juglans nigra seeds in a pattern that maintained a mean cache density that gives low rates of loss to naive competitors as indicated from field test (1). The results strongly suggest maintenance of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed.

ISSN:0012-9658    

DOI:10.2307/1938541

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Body temperatures (T_b) of 7 free—ranging Abert's squirrels were monitored by intraperitoneally implanted temperature sensitive radio transmitters. Relationships between Tb, activity, solar exposure, ambient air temperature (T_a), and weather conditions were examined. T_b values ranged from 35.2 to 41.1 degrees C (° = 39.0 degrees C) and varied with behavior. A hyperthermic condition during both summer and winter developed during bouts of intense activity and the degree of hyperthermia was related to intensity of activity. Immediately following intense activity, squirrels sought shade in a tree to facilitate cooling. Squirrels exhibited hypothermia in the nest during the day and night. A diurnal rhythm of Tbwas observed in free—ranging squirrels when in the nest and similar rhythms of T_b and O_2 consumption rate (V_(O_2)) were observed in laboratory squirrels. Basking was common on winter afternoons and upon emergence from the nest in the morning throughout the year, resulting in elevated Tbvalues. Position of the tail appeared to moderate solar irradiation striking the squirrel during basking. When positioned dorsally, the tail increased reflectance. Wind and rain significantly reduced Tbin squirrels outside the nest. The relationship between Tband Tawas subtle and difficult to ascertain in the field. In the laboratory, Tbdecreased in response to falling Taand subsequently metabolic expenditures were depressed from the expected. The mean field Tbduring summer and early fall was significantly higher than during winter or spring and may indicate seasonal acclimatization. Squirrels in the field did not cache substantial quantities of food. They foraged daily and were active during all seasons, including inclement winter weather. Fresh snow appeared to restrict movements. Squirrels were seldom outside of the nests at Tavalues<—10°C. Laboratory experiments established a thermoneutral zone between 29° and 35°C. Within the zone the Vo2was 0.69 ml O2(g h)—1(standard temperature and pressure, dry) or 104% of the predicted weight specific value. The curve of thermogenesis is best described by a polynomial regression due to shifts in Vo2caused by changes in Tbin response to falling Ta. The lower lethal limit was near —20 degrees C.

ISSN:0012-9658    

DOI:10.2307/1938542

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Interspecific competition is thought to be important in determining patterns of resource use and species abundances in natural communities. However, there have been few field tests of competition—based models of community structure. In this study, experiments were conducted with 4 coexisting desert rodent species to see whether competition is a sufficient explanation for their resource use and abundance patterns. Results were consistent with 3 predictions from competition theory. (1) The 4 species differed in their use of a resource, foraging microhabitat, which is potentially limiting to their populations. (2) Each species shifted its use of microhabitats in predicted directions when competitors were removed from or added to outdoor enclosures. (3) Each species was most dense where its preferred microhabitat was abundant, and augmentation of 1 microhabitat led to an increase in the density of the appropriate microhabitat specialist. These results suggest that competition maintains interspecific differences in foraging microhabitat, and that the availability of appropriate microhabitats determines species abundances on a local scale.

ISSN:0012-9658    

DOI:10.2307/1938543

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The influence of social and demographic parameters on reproduction was examined by removing either adult ♂ ♂, breeding ♀♀, or subadults of both sexes from pairs of populations for 15 mo; the 4th pair of 8 populations was not manipulated. Removal of ♂ ♂ was associated with a decreased proportion of adult ♀♀ which became parous, low body growth rates for maturing ♀ ♀, low association of perforate and nonperforate ♀♀ with adult ♂ ♂, and a high association of adult ♂ ♂ with nonperforate ♀ ♀. Removal of parous ♀♀led to earlier maturation and breeding of ♂ ♂, negative association between perforate ♀♀ and adult ♂ ♂, and higher levels of aggression in the populations. Multiple regression techniques indicated that estrous cycling and lactation were best explained by the activity of nonperforate ♀♀ and their association with adult ♂ ♂, the number of young weaned the previous month, the percentage of adult ♂ ♂ in the populations, and the aggressive tendency of these ♂♂. Aggressive tendency was measured in ♂ ♂ removed from the populations and paired with nonaggressive control mice. Nulliparous ♀ ♀ were more sensitive than parous ♀ ♀ to changes in social] behavior or demography of a population; litters born to previously nulliparous ♀ ♀ were clustered on certain months, whereas litters of parous ♀ ♀ were born randomly. Although most ♀ ♀ failed to reproduce during the later stages of population growth, this inhibition of reproduction was not always permanent; when nonbreeding ♀ ♀ were removed from the populations, transferred to small cages, and paired with adult ♂♂ from the populations, one half reproduced within 1 mo. Nestling survival was influenced negatively by the number of aggressive acts, the proportion of adults of either sex in the population, and the proportion of perforate and lactating ♀ ♀. Only ♂ dominance had a positive correlation with litter survival. The sex ratio of recently weaned mice varied greatly and was similar to that found among adults, even when the removal of adults of 1 sex caused the adult sex ratio to be markedly unequal.

ISSN:0012-9658    

DOI:10.2307/1938544

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The simultaneous flowering of co—occurring plant species with similar pollinator affinities may result in interspecific pollen transfer and consequent fecundity reductions due to wastage of pollen, stigma surfaces, and effective pollinator visits. In such cases competition for pollination occurs and may lead to or maintain sequential flowering. Two common perennials in the mountains of west—central Colorado, Delphinium nelsoni and Ipomopsis aggregata, flower sequentially in the same meadows and are visited commonly by Broad—tailed Hummingbirds (Selasphorus platycercus). Hummingbirds carry pollen of both species and their exclusion from flowers leads to significant seed set reductions. During the brief period of flowering overlap between D. nelsoni and I. aggregata in natural meadows, hummingbirds visit both species, carry mixtures of their pollen, and appear to cause interspecific pollen transfer. Flowers of both species receptive during this period suffer significant seed set reductions relative to those receptive during nonoverlap periods. Interspecific pollinator flights and pollen transfer also occur in mixtures of potted plants, and seed set reductions consistently occur for both D. nelsoni and I. aggregata in such mixtures relative to single—species controls. Finally, seed set reductions occur for both species following interspecific hand pollination of potted plants. Fecundity reductions in natural and synthetic mixtures of D. nelsoni and I. aggregata indicate that the 2 species compete for hummingbird pollination and suggest that the competitive interaction involves interspecific pollen transfer. The observed reproductive effects represent a selective force sufficient to maintain divergent flowering times of D. nelsoni and I. aggregata in nature.

ISSN:0012-9658    

DOI:10.2307/1938545

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Annual net aerial primary production (NAPP) was estimated for Distichlis spicata (L.) Greene, Juncus gerardii Loisel., Phragmites communis Trin., Spartina alterniflora Loisel., Spartina cynosuroides (L.) Roth, Spartina patens (Aiton) Muhl. and Sporobolus virginicus (L.) Kunth utilizing the Wiegert and Evans paired plot harvest method in selected east coast salt marshes. Estimates of NAPP ranged from 616 g°m—2°m—2°yr6—1for highmarsh J. gerardii in Maine to 6.040 g°m—2yr—1for S. cynosuroides in Georgia. Estimated "turnover" based on maximum living standing crop ranged from 9.6 times/yr (Georgia, D. spicata) to 1.8 times/yr (Delaware, D. spicata). As a result of periodic to erratic tidal inundation and associated removal and/or deposit of detritus, the Wiegert and Evans method could not be adequately evaluated for its accuracy in computing NAPP. The variability of the computed instantaneous rate of detritus flux which ranged from —39 mg°g—1°day—1(Georgia, Spartina cynosuroides) to 84 mg°g—1°day—1(Maine, S. patens) was considered to be the weak component. The complexity of the system parameters and the resultant variability of the method indicated that the method may present difficulties when used in salt marsh ecosystems.

ISSN:0012-9658    

DOI:10.2307/1938546

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The pollen record at Big Meadow in northeastern Washington contains at least 5 pollen zones spanning ca. the last 12,500 yr. Initial vegetation after Pinedale deglaciation was nonarboreal. Estimates of the pollen influx and the dominance of Artemisia and Gramineae plus comparatively little pine pollen suggest a tundra—like landscape from the time of deglaciation until ca. 9,700 BP. Zone II probably lies disconformably atop this earliest zone and reveals a community dominated first by grasses plus diploxylon pines and later by diploxylon pines without prominent grasses (Zone III) between 9,700 and 3,300 BP. zone IV demarcates a short reversal of climatic conditions in which Picea and Abies are relatively prominent. Tsuga heterophylla, the present day climatic climax dominant of the area, rapidly emerged in the pollen record at ca. 2,400 BP.

ISSN:0012-9658    

DOI:10.2307/1938547

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The triclad flatworm Dugesia tigrina (Girard) is abundant in Lake Wabamum, a west—central Alberta lake receiving thermal effluent. A 14—month study was carried out to investigate D. tigrina's population ecology and the effect a thermally altered environment might have on Dugesia. Our data support the hypothesis that D. tigrina is limited by intraspecific competition for food during and after reproduction. In spring, both the normal water and heated water triclad populations grew rapidly and produced cocoons. Recruitment of young intensified food competition, causing shrinkage and mortality. The spring gain in length and weight was lost through shrinkage of individuals, and numbers of triclads in autumn were similar to prebreeding numbers. Hence the hypothesis is supported for populations in the thermally altered and the normal lake habitats. Triclads from the thermally enriched environment were found to be 2x as fecund as normal lake triclads. Cocoon production was probably increased to some extent by thermal effluent. However, the elevated mortality rate caused by the mechanical harvesting of abundant macrophyte populations in the warm water area, estimated at 1—3x the standing crop of triclads, probably accounts for much of the increased fecundity.

ISSN:0012-9658    

DOI:10.2307/1938548

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The reproductive seasons of 6 species of characoid fishes inhabiting a tropical forest stream in Panama were determined from observations on gonadal condition and size frequency distribution of the populations over an 18—mo period. In this area the relatively mild dry season lasts °4 mo, stream discharge increases moderately during the rainy season, floods are intense but brief and temperature is very stable. Bryconamericus emperador and Piabucina panamensis spawned in temporary tributaries in June with the first floods of the rainy season. Brycon petrosus and Hyphessobrycon panamensis spawned in the dry season. The majority and adult Gephyrocharax atricaudata were mature in most months, but fry appeared in several peaks scattered through both dry and rainy seasons. Roeboides guatemalensis seemed to breed throughout the year but with a dry season peak. Previous studies of tropical freshwater fishes have emphasized the widespread tendency toward a flood spawning pattern but have investigated primarily highly seasonal savanna environments. I suggest that the diversity in the present study is related to the relatively stable habitat. Five hypotheses to explain the diversity in the timing of reproduction are proposed and briefly discussed: reproductive seasonality (1) is controlled by adult or juvenile food availability, (2) is controlled by interspecific competition for food among juveniles, (3) is controlled by competition for spawning sites, (4) is a mechanism for reproductive isolation, or (5) is unrelated to local conditions but is a reflection of earlier evolution of specializations for spawning under particular conditions.

ISSN:0012-9658    

DOI:10.2307/1938549

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY