摘要:

We investigated web relocation and habitat selection in Latrodectus revivensis (Theridiidae), a widow spider that constructs webs on scattered shrubs in the Negev desert. We used repeated observations of individual spiders' habitat preferences, movements to new web sites, growth, fecundity, and survival under natural and manipulated habitat conditions to examine relationships between movement, habitat, and components of fitness. Our main objectives were to determine the spatial and temporal patterns of movement, and the causes, costs, and fitness consequences of shifting habitat. Repeated censuses of individually marked females showed that web relocation in L. revivensis is associated with spider growth. Spiders tended to move larger shrubs after one or two molts, and the size of web components and web—site features scaled to body size. Moves occurred mainly in Spring (March—May), and most moves (74%) were by juveniles. The main cost of web relocation was increased mortality: there was a 40% chance of death during a move. A comparison of occupied shrubs with those available in the habitat indicated strong site selection involving seasonally varying and age—dependent preferences for particular shrub species, and a general preference for taller shrubs. Prey availability at shrubs (as measured by pitfall traps, and by prey remains in nests) varied spatially and was affected by shrub species and size. However these effects were small compared to habitat—wide temporal variation in prey availability, suggesting that web—site relocations would not result in substantially higher prey capture. Nonetheless, movement to larger shrubs resulted in improved spider body condition and, ultimately, greater reproductive success. The advantages of web—site relocation in this species may relate to architectural features of shrubs that provide support for the web, and minimize exposure by thermal extremes and to predators. However, experimental manipulation of exposure by trimming shrubs did not decrease spider fitness. We suggest that web—site selection in this species is less precise than expected because of the risk of dying during a move.

ISSN:0012-9658    

DOI:10.2307/1940835

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We investigated the response of two species of lady beetles, Coccinella 7—punctata and Hippodamia variegata, to spatial variation in the density of aphids on fireweed, Epilobium angustifolium. Our approach involved experimental manipulations of aphid densities and field observations of lady beetle foraging at three different scales: (1) Individual lady beetles on individual fireweed stems (2) populations of lady beetles on individual fireweed stems, and (3) populations of lady beetles in populations of fireweed stems. At the finest scale, we found that individual lady beetles remained on single fireweed stems longer in the presence of larger aphid colonies. However, this behavioral response to aphid density was weak, explaining only 11 and 4% of the total variance in the length of time C. 7—punctata and H. variegata remained on the stems. To examine directly the population—level effects of this individual lady beetle behavior, we conducted a second series of experiments that focused on the response of populations of lady beetles; we released large numbers of lady beetles into field plots in which we had manipulated the number and size of aphid colonies. By sampling the distribution of lady beetles, we showed that the number of lady beetles found on fireweed stems depended significantly on the presence and size of aphid colonies. The average number of both C. 7—punctata and H. variegata found on a stem was 10—20 times greater when the stem contained a large aphid colony that when the stem contained no aphids. Finally, we examined the consequence of these patterns at the scale of populations of lady beetles and populations of fireweed in 25—m2field plots. We found that the number and size of aphid colonies within the plots explained 50 and 90% of the variation among plots in the average length of time that C. 7—punctata and H. variegata remained. Taken together, these results demonstrate that although individual lady beetle response to aphid density is extremely weak, the cumulative effect of many individuals can produce strong population—level aggregation of lady beetles in areas of high aphid density.

ISSN:0012-9658    

DOI:10.2307/1940836

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Within—plant variation in food nutritional quality is a fundamental feature in the foraging environment of herbivores, but little is known as to how such small—scale variability affects individuals or structures populations of herbivores. The selective pressure favoring choosy herbivores may depend on the relative performance of random foragers, which will experience chronic changes in food quality, vs. non—random foragers. To determine the effect of involuntary variation in nitrogen intake on larvae of a polyphagous insect, gypsy moth (Lymantria dispar (L.)), I reared individuals on one of three diet treatments: (1) Constant nitrogen: larvae received 3.0% N diet continuously; (2) Low variance: larvae were switched each day in an alternating sequence of 2.25% and 3.75% N diet; and (3) High variance: as in Low variance, except larvae were alternated between 1.25% and 4.75% N diet. Despite equal relative consumption rates (RCR) and nitrogen consumption rates (RNCR) across all treatments, larvae experiencing variation in the diet suffered reduced pupal mass and extended development time. Evidently growth performance cannot be predicted based solely on mean diet nitrogen concentration, which was equal among all treatments (3.0% N). Further, growth on a mixture of foods cannot be predicted based on known responses to the individual foods. Reduced growth associated with the chronic variation likely was caused, in part, by nonlinearity in the relationship between nitrogen and food utilization efficiency, and by disruption of the compensatory feeding response. These results suggest that intraplant variation in food quality has the potential to exert strong selective pressure for regulation of variation in the total diet.

ISSN:0012-9658    

DOI:10.2307/1940837

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

I studied sources of density dependence in a dragonfly population to reveal how population size is controlled. Larvae of the dragonfly Aeshna juncea, inhabiting rock pools on the Lake Superior shoreline at Isle Royale, Michigan, USA, showed clear evidence of competition when natural densities were high. After 2 yr of larval growth, individuals in crowded pools were developmentally delayed and survived less well than individuals in sparsely populated pools. The stunted larvae caught up with individuals in less crowded pools and reached the final instar in sufficient time to emerge in the summer of their 3rd yr, but even so their final body size remained relatively small. A 5—wk field experiment in natural pools demonstrated the causal connection between density and fitness: feeding activity and growth rates were significantly reduced under crowded conditions, but survival showed no response to density. In unmanipulated rock pools, however, natural increases in density from one year to the next were met with slightly overcompensatory decreases in annual survival. The results illustrate that density dependence operates in a way that could contribute to population regulation in A. juncea, and a 6—yr time series from one of the two study areas showed significant evidence for density—dependent dynamics.

ISSN:0012-9658    

DOI:10.2307/1940838

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Mate—searching has been hypothesized to influence the host preferences of small grazers, yet no previous field data have examined this idea. I examined the host preferences of Cyphoma gibbosum, a generalist gastropod that feeds on gorgonian corals, and tested if mate searching or oviposition preferences contributed to variation in host preferences. I constructed experimental reefs containing three species of gorgonian corals at uniform densities and placed snails on these reefs either alone or in group of four. Half of the groups of snails were single—sexed while the other half contained two snails of each sex. I observed snail movements by conducting daily censuses of three reefs and constructed transition matrices describing the movements of snails in each treatment among the three species of coral. Snails showed preferences for some hosts over others, and these preferences varied due to both mate searching and oviposition. Snails preferred to occupy Briareum asbestinum over Plexaurella dichotoma or Pseudopterogorgia acerosa, but these preferences were strongly influenced by previous experience. Moreover, females tended to move from B. asbestinum to Plexaurella dichotoma to oviposit, and back to B. asbestinum afterwards. Finally, both males and females moved frequently towards members of the opposite sex, in order to copulate, which influence their distribution among hosts. These data reveal that behaviors associated with mating can influence host preferences and need to be considered in understanding host utilization patterns.

ISSN:0012-9658    

DOI:10.2307/1940839

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Cost of reproduction measured under laboratory conditions may have little bearing on actual cost observed in the field, because both environmental conditions and behavior can modify the manifestation of these costs. I studied survival, growth, and reproduction in a wild population of southern water skinks (Eulamprus tympanum) over 3 yr to measure costs of reproduction in females, to examine how the environment influences these costs, and to assess the extent to which females are able to reduce costs. The proportion of mature females that reproduced each year varied from 30—60%. Frequent reproduction appeared costly in terms of both survival rates and growth. Growth rates and survival rates were negatively correlated with reproduction during the 3—yr study. In the coolest and wettest year of the study, when food intake by females was relatively low, gravid females and females that had reproduced in the previous years survived well than nonproducing females. There was no evidence of a survival cost of reproduction in the other two years of the study. Reproduction also imposed a fecundity cost, as overall, mean growth rates of reproducing females were reduced relative to those of nonreproducing females of the same body size. Reduced growth rate translates to reduced litter size in these skinks, because fecundity increases with body size. These results suggest that reproduction can be costly in terms of both survival and future reproduction in these skinks, but that survival costs are not consistent from year to year and are mediated by environmental conditions. Females appear to adjust their frequency of reproduction to minimize these costs, as those females that skipped opportunities for reproduction were those expected to gain the greatest growth and fecundity benefit by skipping.

ISSN:0012-9658    

DOI:10.2307/1940840

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

While density dependence has been well documented in many animal species, the effects of absolute size of habitat have not been thoroughly explored. This study investigates the effects of habitat size by examining the response of the distribution of several fitness correlates. I used populations of tadpoles of Bufo americanus and Rana clamitans in artificial ponds as a model system. The natural histories of these species suggested that they might respond differently to variation in habitat size, which was defined in terms of pond surface area and depth. These dimensions were factors in two completely crossed factorial experiments, one for each species. Ponds were of three areas: 529 cm2, 2116 cm2, and 8464 cm2and three depths: 10 cm, 20 cm, and 40 cm. Tadpole populations were established at a single density (number per unit volume). The use of artificial ponds controlled variation in nutrient content, shape, and vegetation structure that may be confounded with pond size in nature. Increases in pond area resulted in Bufo populations that had lower survival, growth rates, and mass at metamorphosis. Populations in ponds of large area showed greater within—population variation in larval period, although the mean of this parameter was only weakly affected. Size distributions of individuals showed positive skewness in small—area ponds. Increasing depth led to lower survival but greater mass at metamorphosis. In experiments with Rana, increased area and depth both led to lower survival but greater growth by survivors. There were statistically significant interactions between area and depth in their effects on early Bufo growth and on Rana survival. The fitness of Bufo showed a significant positive relationship with increasing surface—area—to—volume ratio while Rana showed a mixed response. The results suggest that competition among Bufo tadpoles intensifies with habitat size under conditions of constant initial density by volume because of a decreasing ratio of edge to interior habitat. The form of competition appears to shift towards stronger interference competition in smaller habitats. Individuals may compensate for early, negative effects of habitat size. The variability of the several fitness correlates, while different among populations, showed no consistent pattern with population size. Theory based on assumptions of equal levels of variation in fitness in large and small populations may not be widely applicable to populations in nature. Evidence of changes in competition and fitness distributions should be sought in other systems characterized by variation in habitat size.

ISSN:0012-9658    

DOI:10.2307/1940841

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Geographic variation in life history phenotypes between populations of a single species is often assumed to reflect genetic divergence caused by natural selection. The relative contribution of genetic and environmental sources of phenotypic variation is rarely determined, especially for vertebrates. However, distinguishing between phenotypic plasticity induced by proximate environmental variation and genetic divergence is fundamental to understanding the ecological and evolutionary significance of geographic variation. We used a reciprocal transplant experiment to uncover the relative importance of population—specific (genetic) and environmental sources of variation in individual growth rates between two populations of the fence lizard, Sceloporus undulatus. Our study revealed a population ° environment interaction, consistent with a genotype ° environment interaction that would result if differences between populations were genetically based. The growth rates of Nebraska lizards, normally twice that of New Jersey lizards, were reduced by half when the animals were transplanted to New Jersey. However, New Jersey lizards showed no increase in growth rates when transplanted to Nebraska. Comparisons of relative food availability (arthropod abundance) indicated that more food was available in New Jersey during the experiment. Estimates of potential activity day length using hollow copper models were from 2 to 2.5 h longer in Nebraska. This result suggests that the thermal biophysical environment may have reduced growth rates of Nebraska lizards transplanted to New Jersey, perhaps reducing foraging time or by limiting physiological processes supporting growth. Our results are consistent with previous studies of S. undulatus that assumed genetic differences in life histories between populations and implicated the thermal biophysical environment as an ecological source of variation.

ISSN:0012-9658    

DOI:10.2307/1940842

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

McNab (1986, 1988) has hypothesized that mammals using food with low energy content should exhibit basal metabolic rates (BMR) lower than those expected on the basis of their body mass (mb). That is, those species that exploit food with low energy content and/or high cost of digestion tend to have low, mass—independent metabolic rates. To date there is not an experimental test of this pattern. The aim of this work was to examine experimentally the effect of diet quality on BMR, digestive efficiency, and the relationship between digestion and energy expenditure in a small herbivorous mammal. We used as a model the herbivorous caviomorph burrowing rodent Octodon degus (mbnearly 200 g), an inhabitant of semi—arid and mediterranean communities of northern and central Chile. Individuals maintained during 27 wk with a diet high in dietary fiber showed significantly lower BMRs (28%) than those feeding on low fiber. Daily food intake and ingestion rates (energy and dry matter) of individuals under a high—fiber diet were significantly higher than animals maintained with a low—fiber diet. The same pattern was obtained for total feces production and rate of feces production. The total intake and rate of ingestion of proteins were not significantly different between treatments. However, a significantly higher amount of protein was excreted by the individuals exposed to a high—fiber diet. Apparent digestibility of dry matter, energy, and protein were consistently lower in individuals maintained with high fiber. However, nonsignificant differences were observed between gut contents in the two treatments (P>.58), but significantly higher digesta turnover rate was observed in animals exposed to a high—fiber diet (P>.05). A significant correlation was found between digestibility and the basal metabolic rate of individuals (rs= 0.781, P>.01), suggesting that elevated digestibilities on high—quality diets allow increased basal rates of metabolism. We suggest that, although small mammals like degus may select sparsely distributed plants of high quality in their habitat, their capability to drop their metabolic demands may help them meet their nutritional and energy requirements when nutritional conditions in the environment deteriorate.

ISSN:0012-9658    

DOI:10.2307/1940843

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We investigated how resources are allocated to reproduction and how variations in resource availability influence reproductive allocation, offspring number, and offspring size in guppies (Poecilia reticulata). Our goal was to evaluate how plastic these variables are in response to environmental variation and to characterize the nature of this plasticity. Female guppies which had just given birth (litter 1) were assigned to either high or low levels of food availability until they gave birth to their next litter (litter 2, interval 1). They were then randomly reassigned to either high or low food with the constraint that there be equal numbers of individuals in each of four treatments: high—high, high—low, low—high, and low—low. They were maintained on this level of food availability until they produced their next litter (litter 3, interval 2). We analyzed variables that characterized the female after the birth of the third litter and the offspring in the third litter. These were two—way analyses, with intervals 1 and 2 as the main effects and high vs. low food as the levels of each effect. The qualities of the third litter were influences by both interbrood intervals, indicating that the resources used for producing the litter were derived from both intervals. Specifically, higher food availability during either interval resulted in a significant increase in the number of offspring in litter 3, independent of the size of the mother. This result indicates that the number of offspring produced in a litter will be a function of both the immediate and the past environment. Lower food during either interval resulted in an increase in the number of days between the second and third litters, indicating that, if resource availability is low, the female may delay the initiation of the next litter, allowing her to acquire more resources. Resource availability during both intervals also influences how resources were allocated to individual offspring. Females responded to low food during the first interbrood interval by producing heavier offspring in litter 3. This increase in mass was almost entirely attributable to an increase in fat reserves. Such a result could represent adaptive plasticity, if it can be demonstrated that maternal fitness increases through the production of heavier offspring a low—food environment.

ISSN:0012-9658    

DOI:10.2307/1940844

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY