摘要:

Nine years of demographic data for two unmanipulated populations of banner—tailed kangaroo rats (Dipodomys spectabilis) in Arizona provide measures of survival, fecundity, age structure, and density. We use these data to search for costs of reproduction in two important components of fitness: survival and future fecundity. We also examine the interaction effects among reproductive effort, age, sex, and density on survival and future fecundity. None of the analyses reveals a significant negative effect of reproductive effort on survival or future fecundity. This result may be seen as evidence that reproduction has no measurable costs in fitness for bannertails. However, another explanation is that bannertails adjust their reproductive effort to individual circumstances so as to minimize costs.

ISSN:0012-9658    

DOI:10.2307/1940579

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We examined the effect of predators, competitors, and conspecifics on the choice of calling sites by male gray treefrogs, Hyla chrysoscelis, and the correlations between choice of calling sites by males and choice of oviposition sites by females. The experiment was conducted in an array of wading pools and utilized a replicated block design for variance analysis. Pools were open to natural colonization by male and female treefrogs. The treatments consisted of controls containing no predators or competitors, four treatments containing one species of predator (either Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, or Tramea carolina larvae), and two treatments containing one species of competitor (either Rana catesbeiana or Hyla chrysoscelis). Male and female treefrogs avoided pools containing conspecific tadpoles and pools containing adult black—banded sunfish, Enneacanthus chaetodon. Females also avoided pools containing larval spotted salamanders, Ambystoma maculatum. Correspondence between calling sites and oviposition sites was examined in response to treatment, block (consisting of a location and a time), and location. Correspondence between male choices and female choices was weak. Number and location of calling males showed no significant correlation with the number and location of eggs along any of the axes of choice. Both males and females chose sites based on the species present, and both showed preferences in regard to location and time. The differences between male and female choices indicate that the potential choices have different values to each sex, or that different criteria are used to rank potential choices. Anuran reproductive behavior is not a simple consequence of the direct interactions between males and females, but is a result of complex interactions between the behavior of the two sexes and the environment in which their behavior is embedded.

ISSN:0012-9658    

DOI:10.2307/1940580

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We used data from a 12—yr study of dispersal and breeding success to investigate how female Red—winged Blackbirds (Agelaius phoeniceus) make decisions about movements between nests, both within and between years. Females were studied on eight separate marshes that were sufficiently close to permit inter—marsh movement. Our analyses showed that (1) a female's previous success affects her subsequent nesting decision, especially between years; (2) females exhibit strong marsh fidelity; and (3) the presence of familiar male neighbors affects female movement decisions. Little support was found for the ideas that females move in response to different types of nest predators, familiarity with current mates, or nesting success of other females on their marshes. Our results suggest that (1) there is more social constraint on changing territories and marshes, especially within breeding years, than previously suspected, and (2) although mate fidelity is relatively weak and unimportant, male "neighborhoods" or "breeding groups" affect female nesting success and, hence, influence movements.

ISSN:0012-9658    

DOI:10.2307/1940581

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

David Lack's “brood reduction hypothesis” to explain asynchronous hatching has recently been tested experimentally on several altrical species of birds. However, for most studies it is not known whether a food shortage occurred or not. We induced food shortage by removing the male parent shortly after hatching, in Pied Flycatcher (Ficedula hypoleuca) broods with experimentally reduced and increased asynchrony. The broods were adjusted to an initial size of five. Broods with reduced and increased asynchrony showed a similar degree of partial mortality, fledging on average 3.3 and 3.5 young, respectively. In broods with both parents present and a normal level of asynchrony, all the young fledged. Mean fledgling body masses in reduced (12.6 g) and increased asynchronous (13.1 g) uniparental broods were also similar, and significantly lower than that of biparental broods (14.6 g). Pronounced nestling size hierarchies developed in both types of experimental broods, and female body mass losses during the nestling period were similar. The results demonstrate that, in the Pied Flycatcher, asynchronous hatching is not necessary for the achievement of brood reduction. The differences between broods with reduced and increased asynchrony in numbers and body masses of fledged young were not statistically significant, but were in the direction predicted by the brood reduction hypothesis. The results are inconsistent with Lack's original formulation of the hypothesis, but not with a recent life history modification, which states that asynchrony increases parental efficiency rather than within—season nestling survival.

ISSN:0012-9658    

DOI:10.2307/1940582

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

The food hypothesis of population regulation was tested using two predation—free populations of feral donkeys in tropical northern Australia. Growth, body condition, and age—specific rates of fecundity and mortality were compared between the two populations. One of the populations was roughly one—half the density of the other, following an experimental reduction 3—4 yr previously. Fecundity was high and independent of density, with>70% of mature females pregnant in both populations. Age at maturity and juvenile mortality displayed some density dependence. Populations conformed to predictions of the food hypothesis, abundance being ultimately regulated by food—related juvenile mortality. At high density, juvenile mortality was three times as great as at low density, and was associated with poorer juvenile body condition and slower growth. Differences in rates of juvenile mortality are associated with nutritional status of lactating females, which is in turn determined by density—dependent competition for quality forage. The effects of competition for forage upon juvenile mortality synchronize lactation with an annual flush of quality forage at the onset of the monsoonal wet season.

ISSN:0012-9658    

DOI:10.2307/1940583

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

We studied numerical and functional responses of breeding European Kestrels (EK) (Falco tinnunculus), Short—eared Owls (SO) (Asio flammeus), and Long—eared Owls (LO) (Asio otus) during 1977—1987 in 47 km2of farmland in western Finland. The pooled mean yearly breeding density varied from 0.1 to 2.4 pairs/km2. The number of nesting EKs (range 2—46 pairs), SOs (0—49), and LOs (0—19) fluctuated in close accordance with the spring density of Microtus (M. agrestis and M. epiroticus) voles. The mean yearly number of fledglings produced per pair ranged from 0.4 to 3.8 and, for each species, was positively correlated with spring density of Microtus voles. Due to their high degree of mobility, EKs, SOs, and LOs were able to track the population fluctuations of their microtine prey without time lags. An increase in microtine densities caused a rapid immigration into the study area and a decrease caused a rapid emigration from the area. Microtus voles were the most important prey group by mass in the diet of each species. Water voles, bank voles, shrews, and small birds were the most frequent alternate prey. The spring density of Microtus spp. was positively correlated with the percentage of these voles in the diet of EK, SO, and LO. The pooled functional response curve of these three raptor species to the fluctuating densities of Microtus spp. was close to linear, indicating that consumption rates are independent of vole densities. Breeding EKs, SOs, and LOs seemed to take a larger proportion of voles available in peak years than in low ones.

ISSN:0012-9658    

DOI:10.2307/1940584

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Is the ability to function as a keystone predator a property of a species or an emergent property of the community? We discussed this question in a temporary—pond community where the broken—striped newt Notophthalmus viridescens dorsalis is known to act as a keystone predator on larval anurans. We independently manipulated the initial density of adult Notophthalmus (two or four) and the presence or absence of one adult Siren intermedia in a set of 20 artificial ponds to determine if this additional predator affected the ability of Notophthalmus to function as a keystone predator. Each pond received a diverse assemblage of larval anuran prey: a spring—breeding assemblage of 275 Rana utricularia, 100 Pseudacris crucifer and 25 Bufo americanus tadpoles, and a summer assemblage of 200 Hyla chrysoscelis and 150 Gastrophryne carolinensis tadpoles. Two additional pond received neither predator, to assay the outcome of competition among the tadpoles. The effect of the salamander Siren on Notophthalmus survival and fecundity depended on the density of Notophthalmus. At low newt density Siren had no effect on survival, but at high newt density competition reduced the survival and growth rates of Notophthalmus. The density of Notophthalmus and the presence of Siren interacted to determine the fecundity of Notophthalmus. At low newt density Siren reduced Notophthalmus reproductive success by preying on larvae. At high newt density Siren indirectly enhanced reproductive success by reducing survival of adults, thus releasing larvae from intraspecific competition and cannibalism. Notophthalmus density had no effect on adult Siren survival or growth rate in this experiment, but other evidence indicates that competition with Notophthalmus reduces the growth rates of Siren. Notophthalmus acted as a keystone predator on the assemblage of spring—breeding anurans; it increased the number of metamorphs of the weak competitor Pseudacris crucifer by releasing them from interspecific competition. Siren preyed on tadpoles in a nonselective manner. An additive model was sufficient to describe the effects of Siren and initial density of Notophthalmus on the structure of the tadpole assemblage. In our system of experimental ponds, the strong interaction between Siren and Notophthalmus density did not extend its effects to lower trophic levels, and the direct and indirect effects of Siren did not alter the role of Notophthalmus as a keystone predator.

ISSN:0012-9658    

DOI:10.2307/1940585

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

Because hummingbirds are extremely sensitive to energy stress, yet often face conditions when energy intake is reduced or energy expenditures must be increased, they should have flexible energy budgets. I experimentally measured the ability of two syntopic tropical hummingbird species to manage daytime energy budgets under conditions simulating natural foraging constraint, defined here as low nectar secretion rate, low flower density, or both. I tested a territorial species, Amazilia saucerottei, and a traplining species, Chlorostilbon canivetii, individually in a large flight cage at two rates of food delivery (HIGH = ad libitum, LOW = 64% of ad libitum) and two perch—to—feeder distances (NEAR = 4 m, FAR = 20 m). Both species increased flight time when distance was increased (FAR>NEAR by 44%) and when rate of food delivery was reduced (LOW>HIGH by 23%). However, birds on the HIGH food delivery rate did not increase food intake to compensate for increased flight expenditures to the FAR feeder. Birds on LOW food maintained total rates of energy expenditure that were less than birds on HIGH food, and LOW food birds experienced no effect of feeder distance on expenditure. These results appear to be due primarily to short—term reductions in perching metabolic rates (PMRs) by birds on LOW food or with the FAR feeder. Reduced daytime PMRs helped to mitigate the potentially negative impacts of foraging constraint: rates of mass loss of the LOW food birds were an estimated 48% less than if birds had maintained normal PMRs. Reduced PMRs also enabled birds visiting the HIGH FAR feeder to maintain rates of mass gain equal to unconstrained birds (HIGH NEAR feeder). Relative differences between species in energy management reflected their foraging models. The low—reward trapliner paid the greatest energy costs at LOW food, regardless of distance, due to its overall tendency for high flight time. In contrast, the territorialist did poorly in the HIGH FAR treatment, due to flight expenditures associated with defending a rich but dispersed resource. Although individual birds exhibited considerable flexibility in energy management, the foraging mode represented by each species appeared to be energetically specialized for particular levels of food availability and dispersion.

ISSN:0012-9658    

DOI:10.2307/1940586

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

In order to quantify the response of birds to experimental manipulations of seed availability and densities of granivorous rodents and ants, we counted granivorous birds and measured diurnal and nocturnal seed removal on 24 plots during winter months in the Chihuahuan Desert. Removal of single, widely spaced millet seeds provided a reliable measure of bird and rodent foraging activity. Avian foraging activity increased in response to supplemental seeds, but decreased in response to long—term removal of all rodents and all ants. Although birds potentially compete for seeds with rodents and ants, these results suggest that, in the long term, indirect facilitation dominates the interactions among all three taxa. The positive effect of the other granivores on birds may be mediated through changes in habitat structure (e.g., reduction in the density of annual plants). The increase of avian foraging in response to seed addition and the high spatial and temporal variance in the patterns of seed removal by birds indicate that in desert habitats granivorous birds use their mobility to find and exploit high concentrations of seeds. Thus, birds may reduce the spatial variation in seed abundance and reduce the densities of seed available to other granivores. The foraging behavior of birds and rodents revealed by these experiments clarifies the response of granivores to experimental manipulations reported earlier, such as the lack of biomass compensation by rodents in response to seed addition.

ISSN:0012-9658    

DOI:10.2307/1940587

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY

摘要:

This study examined temporal and spatial variability in competition as well as the relative importance of competition and natural disturbance to Hydropsyche oslari and Simulium virgatum in a southern California stream. These two filter—feeding insects dominated the community studied, accounting for 99% of all individuals. I maintained four field treatments every 2 wk for a year: (1) removal of Simulium; (2) removal of Hydropsyche; (3) removal of both taxa and subsequent scrubbing of the rock surfaces; and (4) unmanipulated controls. Competition from Hydropsyche oslari restricted the spatial distribution of Simulium virgatum in all seasons except late winter in upstream but not in downstream sections of the study site. Downstream abundance of Simulium was probably limited by food availability, not competition from Hydropsyche. During the winter, when floods occurred and densities were low, competition between these insects had no significant impact on spatial distributions. Although competition was seasonally variable, it occurred frequently and was not rare. Although disturbance mediated competition, competition was more important in determining the abundance of Simulium in 1983—1984, a drought year. Disturbance caused a 13% increase, while competition caused a 34% decline in the simuliid population over the year. It was not possible, however, to rank the importance of competition and disturbance when considering both species together. The relative importances were taxon specific. The abilities of Simulium and Hydropsyche to recover quickly after disturbance were inversely related to their ability to withstand competition. In addition, the interaction between the two factors was extremely important in determining the relative abundance of both taxa through the year. Competition and disturbance both played an important role in regulating this community, at times accounting for up to 60% reductions in the numbers of individuals present in the stream. The relative importance of competition and disturbance in Refugio Creek should vary from year to year depending on the timing and intensity of winter storms.

ISSN:0012-9658    

DOI:10.2307/1940588

出版商:Ecological Society of America    

年代:1991

数据来源: WILEY