摘要:

The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet—dry tropics of northern Australia during the wet season. During the dry season, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tb's) of the lizards were measured by ratio telemetry throughout the day during the wet and dry seasons. The midday Tb's during the wet season high (grand mean = 36.7°C) and typical for heliothermic lizards, but the dry season midday Tb's were significantly lower (grand mean = 32.8°C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seasons. The Te's revealed the physical possibility for the lizards to achieve much higher Tb's during the dry season than were measured. Thus, the lower Tb's in the dry season represent a shift in preference rather than an inability to attain a high Tb's during the cooler dry season. Inspection of the Tb's and Te's revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tb's. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb°4°C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregulate carefully in both seasons. Tb's were also measured in a laboratory thermal gradient during both seasons, and the Tb's selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference is acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tb's in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tb's suggests that the dry season Tb's represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.

ISSN:0012-9658    

DOI:10.2307/1940636

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Factorial manipulations of the abundance of two predator species (presence or absence of six adult Notophthalmus viridescens, and/or six larval Ambystoma opacum per pond) and a guild of three prey species (larvae of Pseudacris crucifer, Bufo woodhousii fowleri, and Hyla andersonii) tested the equivalence, additivity, and dependence on initial prey density of predation by both predator species in 1—m3artificial ponds. The two predators were functionally equivalent, eliminating Bufo and Hyla, while leaving Pseudacris to numerically dominate ponds with salamanders. Effects of both predators also cascaded down to algal primary producers, which became more abundant where predators reduced the abundance of herbivorous tadpoles. Combined impacts of the two predator species on the survival of Pseudacris were less than that predicted by an additive model of the effects of each predator measured in the absence of the other, but greater than predicted by a multiplicative model of joint predator effects. Reduced growth of Ambystoma with Notophthalmus provides one possible mechanism for the observed shift to less than additive per capita predator effects. Initial variation in prey density influenced prey species composition only in ponds without predators. Increased prey density in ponds without predators reduced the survival of Hyla, apparently through increased interspecific competition among tadpoles.

ISSN:0012-9658    

DOI:10.2307/1940637

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Larval chorus frogs (Pseudacris triseriata) and spring peepers (P. crucifer) show marked differences in their distribution patterns, with chorus frogs tending to occur in less permanent ponds containing lower predator densities, and spring peepers tending to occur in more permanent ponds with higher predator densities. In order to understand this pattern I performed experiments at two levels. First, at the community level, I evaluated the importance of competition, predation, and pond permanence for patterns of larval performance. Second, at the behavioral level, I asked whether a hypothesized trade—off between gain and risk underlies ecological interactions, and ultimately, patterns of performance and distribution of these species. I employed a field manipulation to asses the contribution of competition and predation within different ponds that varied in permanence. In order to examine the effect of interspecific competition, enclosures contained each tadpole species either alone or in the presence of an equal density of the other species; these enclosures were placed in two temporary ponds, two intermediate ponds, and two permanent ponds. Tadpole stocking densities reflected local natural densities. This competition treatment was crossed with a predator treatment manipulating the absence or presence of predators (with "presence" representing the naturally occurring density within each pond). Patterns of relative performance were consistent with relative distribution patterns across the pond permanence gradient: chorus frogs performed best relative to spring peepers in temporary ponds while spring peepers performed best relative to chorus frogs in permanent ponds (performance was measured as survivorship and growth). The dominant sources of mortality were pond drying in temporary ponds and predation in more permanent ponds. In contrast to most previous experimental studies of larval anurans, competition had small effects on tadpole performance. Results from the field experiment suggest that pond drying and predation may be important determinants of performance, and that the two species are differentially susceptible to these mortality agents. In a set of laboratory experiments I evaluated the hypothesis that a trade—off between foraging gain and predation risk mediated through activity (proportion of time spent moving) might explain patterns of Pseudacris distribution. In both species, individuals facultatively reduced activity in the present of a caged predator with a corresponding drop in growth rate. In general, larval chorus frogs also tend to be more active and faster growing and developing than spring peepers. These results provide support for the existence of the trade—off and suggest that interspecific differences in activity may contribute to patterns of larval performance and distribution among ponds. The general nature of this trade—off suggests that it may also be important in other systems.

ISSN:0012-9658    

DOI:10.2307/1940638

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The effects of predation risk from snakes on microhabitat use of kangaroo rats (Dipodomys deserti and D. merriami) was studied in the Mojave Desert. I concentrated on the effects of the predator on the foraging behavior of the rodents and eliminated effects of prey capture on prey population size. Foraging stations contained three seed trays, one in each of three microhabitats (Open, Bush, Grass). The amount of seed left in each tray after one night of foraging was used to assess the responses to predation risk and other foraging costs; additional data were collected during direct observations and by trapping. To investigate the effect of snakes on foraging and microhabitat use of kangaroo rate I manipulated snake presence at the stations. I studied the interactions between predation risk from snakes and moonlight by conducting experiments near full and new moon nights. Both species of kangaroo rats preferred to forage in the Open and avoided the Bush. The preference is opposite to the preference of the main rodent—eating snake at the study site, the sidewinder (Crotalus cerastes), which prefers the Bush over the Open. At stations with snakes, D. deserti reduced its foraging and avoided the Bush more than in control plots. However, D. merriami foraged more at stations with snakes. D. merriami thus foraged at the trays that were avoided by D. deserti and it reduced the risk of interference from the dominant D. deserti. In contrast to studies by other investigators, moonlight (which is associated with increased risk from owls) did not reduce the foraging or affect the micro habitat use of kangaroo rates in summer; moonlight effect was seen only in the fall, when snakes were not active. Unlike risk from owls, risk from snakes is high under bushes and during dark nights. The activity or rodents in summer is a combined reaction to the different predation risks posed by snakes and by owls.

ISSN:0012-9658    

DOI:10.2307/1940639

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Numerical responses of the weasel Mustela nivalis to the changes in population density of forest rodents (bank vole Cleothrionomys glareolus and yellow—necked mouse Apodemus flavicollis) and predation by weasels studied in the pristine deciduous forests of Bialowieza National Park, eastern Poland, in 1985 through 1992. The rodents experienced 5 yr of noncyclic (seasonal) fluctuations (autumn density 23—74 rodents/ha) and 2 yr of outbreak and crash (triggered by synchronous heavy crop of oak, hornbeam, and maple seeds). Autumn numbers of rodents exceeded 300 individuals/ha during the outbreak and dropped to 8 individuals/ha in the following autumn. Weasels were censused by livetrapping in summer and by snowtracking on an 11.2—km2grid of transects in winter. Radiotracking of 12 weasels in 1990 and 1991 yielded estimates of home ranges and daily movement distances, which were combined with snowtracking and livetrapping data to estimate densities during 7yr. Estimated winter density of weasels varied from 5.2 to 27.3 individuals/10km2in December and declined to 0—19.1 individuals/10 km2by early spring (March). Midsummer (July/August) indices of weasel numbers were extremely variable and corresponded to 41.9—47.6 individuals/10 km2in years with moderate density of rodents, 101.7 individuals/10 km2during the rodent outbreak and 19.1 individuals/10 km2during the crash. Increase of weasel numbers from spring to midsummer was positively related to the spring numbers of rodents. Autumn and winter decline of weasel numbers was not related to rodent density changes. During the outbreak and crash of the rodent population, the numbers of weasels and rodents (both sampled at 2—3 mo intervals) were positively correlated (P<0.0005) with no time lag. Home ranges of male weasels radiotracked during the rodent outbreak were 11—37 ha (minimum convex polygon), compared to 117—216 ha during the crash year. The predator/prey ratio varied from 0 to 2.5 weasels/1000 rodents. The ratio was highest at low densities of rodents. With increasing numbers of rodents, the ratio declined, since rodent population growth was overwhelmingly faster than weasel population growth. In the 7 autumn—winter seasons (1 October—15 April), weasels removed, on average, 1.6 to 9.5 rodents from each hectare, i.e., from 2 to 28% of autumn numbers of rodents. Winter predation by weasels was heaviest at rodent density of °20 individuals/ha. At lower densities of rodents, the number of weasels was restricted by food shortage and the role of their predation rapidly declined. At high rodent densities, the rodent numbers by far exceeded the predatory capacity of weasels and predation percentage declined again. Weasel predation in relation to rodent density has the same pattern in geographic zones ranging from Turkmen deserts to European farmlands and forests.

ISSN:0012-9658    

DOI:10.2307/1940640

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Several models for the economics of sociality demonstrate that the probability distribution of an individual's resource consumption could depend on the size of its social group. When groups share food clumps discovered by any member, expected resource consumption may increase or decline with group size, but the individual's resource consumption variance will ordinarily decline as group size increases. Hence, the formation and dissolution of social groups may represent risk—sensitive responses to foraging success. A series of field studies report greater mean prey consumption and decreased prey consumption variance as group size in colonial spiders increases. Several hypotheses attempt to explain this effect. Coloniality may promote acquisition of information concerning temporal variation in prey availability. Colony membership may permit individuals to steal food when the number of captured prey varies spatially. Finally, colonality can enhance food acquisition because prey (that might otherwise escape) ricochet from one spider's web to another group member's web. Our purpose was to quantify each hypothesis and ask if the resulting models predict reduced resource consumption variance as group size increases. We model each mechanism and conclude that stealing prey is the simplest explanation for variance reduction in spider colonies. We emphasize that variance among individuals need not provide a good estimate of the within—individual resource consumption variance, and it is the latter variance to which risk sensitivity responds.

ISSN:0012-9658    

DOI:10.2307/1940641

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984—1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were °100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density—dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluctuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and prediction rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size—dependent interactions between red scale and Aphytis.

ISSN:0012-9658    

DOI:10.2307/1940642

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

ISSN:0012-9658    

DOI:10.2307/1940643

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

Abiotic factors may constrain the functioning of species interactions such as plant—pollinator mutualisms. I investigated how thermal environment affects the interaction between the early—blooming daffodil, Narcissus longispathus (Amaryllidaceae) and its major bee pollinator (Andrena biclor; Andrenidae), focusing simultaneously on plant and pollinator sides of the interaction. I studied fruit and seed set, flower duration, and the intrafloral thermal environment of N. longispathus, and the thermal biology, foraging behavior, and thermoregulatory ability of A. bicolor, over a 6—yr period in southeastern Spain. N. longispathus flowers from February to April, when unsuitable weather often limits pollinator activity, yet most flowers are successfully pollinated in all years and sites. Fruit set was weakly pollen limited, but among flowers setting fruit the proportion of ovules developing into seeds was not. Individual flowers lasted for 17 d on average, remaining functional during the period. On sunny days, the air inside N. longispathus flowers was significantly warmer than outside. Mean temperature excess inside flowers was as high as 8°C, and was positively related to solar irradiance. Within flowers, air temperature was highest around the anthers; this intrafloral gradient was consistent with variation among perianth parts in radiation transmittance. Andrena bicolor foraged in N. longispathus flowering patches only on sunny days with air temperature>12°—13°C, and foraging behavior and flower visitation rate when temperature dependent. Bees were able to fly at relatively low thoracic temperatures (Tth; range 22°—31°C) and this was essential for successfully foraging at N. longispathus. Under the range of irradiance and air temperature found at foraging sites, A. bicolor individuals inside flowers were able to reach Tthsuitable for flight by passive means alone. Under laboratory conditions, A. bicolor was unable to raise or otherwise regulate Tthby physiological means, but free—flying individuals thermoregulated behaviorally. Basking was used to raise Tth, and intrafloral microclimate, by influencing the proportion of foraging time devoted to basking, played an important role in thermoregulaton. Flower visitation rate was positively related to the average temperature inside visited flowers, and the probability of basking immediately after one floral visit declined with increasing flower temperature. I conclude that the favorable microclimate within N. longispathus flowers, their long duration, and the thermal biology of A. bicolor, were critical elements in this early—season pollination system.

ISSN:0012-9658    

DOI:10.2307/1940644

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY

摘要:

The importance of herbivorous insects for plant demography and fitness is still controversial. The fact that their effect has not be fully evaluated within the context of the complete plant life cycle contributes to this controversy. This study is the first to test directly the influence of flower and seed consumption by a guild of inflorescence—feeding insects on the demography and fitness of a native plant over its whole life cycle within its natural environment. First, we discuss the evidence required to assess the effect of inflorescence—feeding on sequential stages in the plant life cycle. Then, we present the detailed results of two experiments that quantify the effect of inflorescence—feeding insects on seeds, seedlings, juveniles, and subsequent flowering progeny of Cirsium canescens (Platte thistle), a native, monocapic perennial species of Sandhills prairie, Nebraska. Exclusion of inflorescence—feeding insects by insecticide had four main effects. Total seed output increased (P<0.0005 in both experiments), indicating that resource were not limiting seed production. Flower heads produced later in the season contributed to the seed pool when insects were reduced (P<0.01), suggesting that insect feeding restricted the phenology of flowering and pollination. The density of seedling increased around plants protected from florescences—feeding insects (P<0.0005), showing that safe sites were not limiting. Finally, the increase in seedlings led to higher numbers of flowering adults (P<0.009), demonstrating that inflorescence—feeding insects significantly reduced lifetime fitness. Neither individual compensation for seed loss, nor density—dependent compensation for increased plant population densities, was observed when insects were excluded. Thus, we conclude that the inflorescence—feeding insect herbivores limited seed production, seedling recruitment, plant density, and maternal fitness of this plant under natural conditions. This outcome challenges current theoretical models of the effect of insects on plant population dynamics. We predict that our result will be general for short—lived perennial plants with life histories in which persistence is tightly linked to regeneration from current seed, such as for fugitive perennials with transient seed banks.

ISSN:0012-9658    

DOI:10.2307/1940645

出版商:Ecological Society of America    

年代:1995

数据来源: WILEY