摘要:

We investigated the effects of changing sapling availability on foraging selectivity and cutting rates by beavers in large experimental enclosures. As predicted by an energy—maximizing contingency model, the mean size of saplings cut by beavers increased with distance from the lodge and was positively correlated with sapling density. Species selectivity was also positively correlated with sapling density. Trembling aspen (Populus tremuloides) was preferred to speckled alder (Alnus rugosa), and red maple (Acer rubrum) was the least preferred species. The functional responses of beavers presented with saplings of a single species and of similar size differed from those recorded in trials with a single species of saplings of variable sizes and from trials with three species of saplings of variable sizes. Size—selective foraging by beavers reduced the maximum rate of sapling cutting at high sapling densities relative to the single—size trials, but both treatments showed similar cutting rates at low sapling densities. Species—selective foraging by beavers reduced cutting rates at low sapling densities relative to the single—species trials, but both treatments showed similar cutting rates at high sapling densities. Species—selective foraging introduced a slight inflection in the functional response curve for preferred species, which could have a stabilizing effect on trophic interactions.

ISSN:0012-9658    

DOI:10.2307/1940060

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Seeds are known to serve as sources of preformed and metabolic water for desert rodents, but water loss to hygroscopic seeds has received little attention. Salivary water loss by yellow pine chipmunks (Tamias amoenus) and Merriam's kangaroo rats (Dipodomys merriami) to Jeffrey pine (Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds was measured to determine its magnitude and potential importance. Chipmunks rapidly lost salivary water to individual seeds inserted into their cheek pouches. Jeffrey pine seeds (129 mg) absorbed 9.8 mg of water and bitterbrush seeds (36 mg) absorbed 4.6 mg of water when held in the cheek pouches for 15 min. Only 3—5% of this water can be reclaimed if the chipmunk eats the seed because most of the absorbed water resides in the inedible seed coat, which the animal discards. The kangaroo rats, which possess external, fur—lined cheek pouches, lost no water to seeds held in their cheek pouches for 15 min. Chipmunk salivary water loss to Jeffrey pine and bitterbrush seeds was equivalent to 19 and 59%, respectively, of the water (performed plus metabolic water) contained in those seeds. Salivary water loss is a major and previously overlooked component of the water budgets of granivorous rodents with internal cheek pouches. The presence of fur—lined cheek pouches in heteromyid rodents (Kangaroo rats, pocket mice, and kangaroo mice) may have contributed to the great success of these rodents in the deserts of North America by solving the problem of salivary water loss to the seeds they handle and store.

ISSN:0012-9658    

DOI:10.2307/1940061

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Analysis of habitat use based on radio—tagged animals presents difficulties inadequately addressed by current methods. Areas of concern are sampling level, data pooling across individuals, non—independence of habitat proportions, differential habitat use by groups of animals, and arbitrary definition of habitat availability. We advocate proportional habitat use by individual animals as a basis for analysis. Hypothesis testing of such nonstandard multivariate data is done by compositional analysis, which encompasses all MANOVA/MANCOVA—type linear models. The applications to habitat use range from testing for age class effects or seasonal differences, to examining relationships with food abundance or home range size. We take as an example the comparison of habitat use and availability. The concepts are explained and demonstrated on two data sets, illustrating different methods of treating missing values. We compare utilized with available habitats in two stages, examining home range selection within the overall study area first, then habitat use within the home range. At each stage, assuming that use differs from random, habitats can be ranked according to relative use, and significant between—rank differences located. Compositional analysis is also suited to the analysis of time budgets or diets.

ISSN:0012-9658    

DOI:10.2307/1940062

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

To examine behavioral mechanisms of fruit selection, American Robins (Turdus migratorius) were studied foraging on Crataegus monogyna fruits in western Oregon. Robins faced a hierarchical decision—making process of fruit selection: (1) in which shrub to feed (decisions among shrubs); (2) which fruits to pick (visual decisions among fruits); and (3) which fruits to shallow once picked (within—bill decisions). Field observations of natural and experimentally manipulated populations of C. monogyna shrubs demonstrated that choices made by robins among shrubs were correlated with three plant traits (decision cues):fruit abundance, fruit size, and fruit pulpiness. These cues were themselves used hierarchically, so that when higher—ranking cues were held constant by manipulating shrubs, the next available cue became important. Once shrubs were selected, field and aviary work revealed that robins then made visual choices among fruits correlated with fruit size. However, no evidence was found to suggest that robins made within—bill decisions; once fruits were picked, additional discrimination did not occur. I suggest that a hierarchical process of fruit selection may have evolved to enable frugivores to cope with environmental uncertainty that is typical of fruiting systems. Such flexibility in foraging behavior may, in itself, promote variability within and among fruiting plants and therefore help to explain why so few general patterns have been identified in fruiting plant—frugivore relationships.

ISSN:0012-9658    

DOI:10.2307/1940063

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We present a model to examine how foragers should divide attention among different potential prey types in order to maximize their net rate of energy intake. We define attention as the brain's capacity to process information simultaneously. Our model is based on neuropsychological studies suggesting first, that predators who divide attention among an increasing number of different prey types decrease their ability to detect any given type, and second, that this decrease is larger when prey items are more difficult to detect. Our model also incorporates the effects of search rate on forager's probability to detect prey. The model predicts that foragers encountering cryptic prey should devote all attention to a single prey type. When encountering conspicuous prey, foragers should divide attention among the different prey types. For prey types that differ in conspicuousness but are equal in energy content, handling time, and density, foragers should give more attention to the more conspicuous prey if the conspicuousness values of all prey types are relatively small. However, when all prey are more conspicuous, foragers should devote more attention to the less conspicuous prey type. We suggest that our model may serve to explain and predict some of the foraging decisions of animals searching for cryptic or conspicuous prey, and especially studies on "search image" formation.

ISSN:0012-9658    

DOI:10.2307/1940064

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

The states to which multispecies communities can tend has been an important issue to ecology, but one in which rather little progress has been made at a theoretical level through lack of a tractable global theory of the dynamics. This paper explores the use of a global theory called "permanence" that indicates whether the boundary of a phase space is a repellor to orbits in the phase space. The theory is used to try to identify, at a qualitative level, the states to which solutions tend in the phase space of an arbitrary pool of species. We define a "permanent state" of the pool as a subset of the species that is permanent in its own right and univadable by any other species from the pool. A simple assembly rule for communities that stems from this is that no permanent state can be a subset of another. Data on coexistence of drosophilid species and also on that of cuckoo doves, although incomplete, are consistent with this rule. A method is given for finding the permanent states of pools of species with Lotka—Volterra dynamics. Some properties of permanent states are illustrated by means of numerical examples from regional pools of species generated with Lotka—Volterra dynamics. These examples show three kinds of dynamics: a single permanent state, two or more alternative permanent state in which none are subsets of others, and an absence of any permanent states. The statistical distribution of these outcomes in pools of four and five species indicates that a single permanent state is the most likely one to occur, but that alternative states become more probable as the number of interactions among species increases. The implications of these results for understanding and modelling the process of succession driven by population dynamics are discussed.

ISSN:0012-9658    

DOI:10.2307/1940065

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

A study of one of the simplest systems incorporating both dispersal and local dynamics, coupling two discrete time logistic equations, demonstrates several surprising features. Passive dispersal can cause chaotic dynamics to be replaced by simple periodic dynamics. Thus passive movement can be stabilizing, even in a deterministic model without underlying spatial variation in the dynamics. The boundary between initial conditions leading to qualitatively different dynamics can be a fractal, so it is essentially impossible to specify the asymptotic behavior in terms of the initial conditions. In accord with several recent studies of arthropods and earlier theoretical work, density dependence may only be detectable at a small enough spatial scale, so efforts to uncover density dependence must include investigations of movement.

ISSN:0012-9658    

DOI:10.2307/1940066

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

We have been able to clearly study the mechanisms and consequences of intraspecific competition, in contrast to most studies, because of the cave beetle (Neaphaenops tellkampfi, Carabidae) has no competitors and only a single available prey item, cricket eggs (Hadenoecus subterraneus, Rhaphidophoridae), in our study area in Mammoth Cave National Park. The sexes do not differ in size or external morphology and we have never seen interference with copulating pairs and so the likelihood of competition for mates or female choice seems low or nonexistent compared to competition for food. Both in the field and laboratory we assessed: (1) foraging effort by numbers of holes dug: (2) potential foraging success by whether the holes were deep enough to reach a buried cricket egg; and (3) time since foraging success by abdominal distension and body mass. We assessed both exploitation competition, the indirect effect of beetles on each other via cricket egg prey, and interference competition, the direct effect of beetles on each other. To demonstrate exploitation competition we showed that (1) beetles in the field reduced the density of egg prey, (2) lowered egg density in the laboratory resulted in decreased per capita harvest rate, and (3) in the field reduced harvest rates resulted in reduced fecundity at times of year when cricket numbers, fecundity, and egg—laying were lowest and beetle densities were highest. To demonstrate interference competition we showed that beetles reduced each other's foraging effectiveness. In laboratory arenas per capita foraging effectiveness decreased when egg density was held at 10 and beetle density increased from 1, to 2, to 4 per arena. With increasing beetle density there were significant declines in holes dug per beetle, depths of holes, and eggs eaten per beetle. In addition beetles spent less time foraging. In the field interference competition, as assayed by decreasing hole depths, was significantly greater in areas of high beetle density than in areas with low beetle density. Fights for holes occurred only in areas of high beetle density. Preempting holes or stealing eggs could be advantageous since bioenergetic calculations showed that the cost of digging for eggs is as high as two—thirds of a beetle's energy budget. In the field in areas of high beetle density there was no defense of holes7 mm deep. Beetles that had recently eaten eggs were found significantly more often under rocks and on the ceiling where we infer that chances of eggs being stolen are low.

ISSN:0012-9658    

DOI:10.2307/1940067

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

To examine pathways in the evolution of prey specialization in insects, we conducted a comparative experimental analysis of larval behavior in two sympatric predacious sister—species (Chrysopa quadripunctata, a generalist, and C. slossonae, a specialist on the woolly alder aphid). Responses of the larvae to food (Myzus persicae) and camouflaging material (waxy secretions from woolly alder aphids or exuviae from M. persicae) were quantified during four discrete developmental periods. Our analysis revealed species specific, ontogenetic, and environmentally induced variation in larval defensive and feeding behavior. The pattern of interspecific variation indicates that larval behavior could have subserved two processes in the evolution of prey specialization in the sister—species' progenitor: establishment on a specific prey and adaptation to the prey. First, the presence of the woolly alder aphid's secretions results in generalist larvae camouflaging themselves to a degree that rivals that of the specialist. Such phenotypic plasticity could have promoted the progenitor's initial adoption of the ant—tended woolly alder aphid as prey and its wool as a source of camouflaging material. Second, quantitative differences between the generalist's and specialist's behavior indicate that adaptation to the woolly alder aphid did not require the acquisition of novel behavioral traits in the larvae. It involved modifications in the expression of defensive behavior that existed in the generalist ancestor and alterations in the ability to attack and feed on robust prey.

ISSN:0012-9658    

DOI:10.2307/1940068

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY

摘要:

Larvae of the tortoise beetles Charidotella bicolor and Deloyala guttata carry shields formed from exuviae and feces over their bodies that are though to provide protection from natural enemies. We investigated the effectiveness of shields as a defense against three groups of invertebrate predators (mandibulate, piercing/sucking and chelicerate taxa) common in tortoise beetle habitats. Choice experiments, functional response studies, measurements of predator body size and mouthpart length, and a literature review indicated that predators with long piercing/sucking mouthparts (i.e., Heteroptera) were the most effective predators of tortoise beetles. For example, relatively large heteropterans with long, piercing/sucking mouthparts (e.g., Nabis americoferus and Podisus maculiventris) were not deterred by shields whereas small predators with short beaks (e.g., Geocoris punctipes) were. Likewise, coccinellid predators with short mandibles were deterred by the shield, while a carabid predator with long mandibles (Lebia fuscata) was effective at killing tortoise beetle larvae. No clear patterns of the effectiveness of shields against chelicerate predators were evident. Choice experiments also indicated that small tortoise beetle larvae were more susceptible to predator attack than were larger instars. Additionally, the effectiveness of the shields against different predators was quite similar between the two tortoise beetles. Field experiments indicated that predators are an important source of mortality for tortoise beetle larvae in natural settings, and high larval mortality is associated with elevated densities of predators with piercing/sucking mouthparts, such as heteropterans. Thus, shields provide some but not universal protection against the variety of predators present in tortoise beetle communities.

ISSN:0012-9658    

DOI:10.2307/1940069

出版商:Ecological Society of America    

年代:1993

数据来源: WILEY