摘要:

The abundance of two planktonic grazers (Holopedium gibberum and Daphnia catawba) fluctuated seasonally in a nonparallel manner in Lake Lacawac, Pennsylvania. Holopedium gibberum was abundant in early and midsummer, while D. catawba was common only in spring and late summer. I sought to explain this seasonal succession by identifying seasonal factors which regulate each population. I monitored population abundance, individual energy reserves (lipid content), egg production, and body size distributions in plankton samples from April to October in three years. Food resources and vertebrate predator abundance were also examined seasonally. Results of experimental manipulations of food availability and predators, conducted seasonally in both small (6.4—L) and large (6000—L) enclosures, supported interpretations drawn from descriptive data. Contrasting seasonal abundance patterns of the species are explained by differences in the intensity of food stress and predation affecting each species. Food supply restricted population growth of H. gibberum at most times. In the spring of each year, starvation was a major cause of mortality; during summer, individual energy reserves and fecundity were near zero for H. gibberum. Predation losses were negligible for adult H. gibberum, but neonate (smallest age class) mortality by fish was occasionally large. Daphnia catawba experienced a seasonal change in the intensity of regulating factors; food supply restricted population growth only in spring and late summer, but predation controlled growth during summer. During late summer, predation on D. catawba was reduced and its population abundance increased sharply. Experimental removal of fish caused a comparable increase of D. catawba during midsummer. At peak animal densities in late summer, both species were food stressed Life table experiments conducted at this time suggest that competition for food occurs between D. catawba and H. gibberum, and results in a reduction of H. gibberum density.

ISSN:0012-9658    

DOI:10.2307/1938573

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We combined our data on abundance, biomass, and diet of benthic animals in Mirror Lake with estimates of annual P/B (production/biomass) ratios to construct a model of energy flow through the zoobenthos of this small, oligotrophic lake. Because we included the rarely study meiofauna (all metazoans<0.5 mm), our model provides the first accounting of energy flow through an entire zoobenthic community in freshwater. In Mirror Lake, about half of the zoobenthic assimilation of carbon (14 g°m—2°yr—1) is due to the meiofauna. Although detritivory supports about half of community assimilation, benthic algae are an important energy source to the zoobenthos, especially in the littoral zone. We suggest that the importance of benthic algae in lacustrine energy budgets has been underestimated. Major fates of zoobenthic production in Mirror Lake include invertebrate predation, insect emergence, and fish predation. Invertebrate predation is the dominant fate and accounts for 80% of zoobenthic production.

ISSN:0012-9658    

DOI:10.2307/1938574

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We examined the nature and adaptive significance of substrate selection by larvae of the sessile rotifer Collotheca gracilipes. In a small artificial pond the preferred substrate was the anatomical undersurfaces (abaxial) of Elodea canadensis leaves, although four other macrophytes were present (Ceratophyllum demersum, Lemna minor, Myriophyllum spicatum, and Nymphaea odorata). Density of adults on Elodea at times reached>6 individuals/mm2, with>98% attached to abaxial surfaces. However, larvae offered plants in pairwise combinations selected substrates in the following order; Lemma>Elodea>Myriophyllum>Nymphaea. No larval loyalty to parental substrate was exhibited. Larvae preferentially selected abaxial over adaxial surfaces of Elodea leaves (91% on abaxial) in continuous illumination experiments, but the larvae did not discriminate between the two surfaces in total darkness (48% on abaxial). In the absence of a plant substrate, cell debris from any one of four aquatic and two terrestrial plant species induced larval settlement on the bottom of plastic well—depressions. Activity could not be attributed to carbohydrate, lipid, nucleic acid, or protein moieties within these extracts. However, alpha—amylase induced rapid larval settlement in the depressions. In the presence of this calcium chelator, larvae did not distinguish between ad— and abaxial leaf surfaces to the same degree as without the enzyme. Phospholipase—C and EDTA induced similar effects. Larval preference for abaxial surfaces could be reduced slightly when PH was stabilized at<7.0. Because living Elodea in neutral—to—alkaline water can remove Ca+2from beneath its leaves and release it from adaxial surfaces while photosynthesizing, we hypothesized that larvae initiate substrate selection activities on any surface when they are in microhabitats having ambient Ca+2concentrations below a threshold concentration. Short—term, in vitro growth experiments showed that rotifers attached to abaxial surfaces of Elodea leaves grew significantly taller and produced more eggs per female than those which had been induced to settle on adaxial surfaces.

ISSN:0012-9658    

DOI:10.2307/1938575

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We investigated size—selective predation upon Daphnia pulex and D. magna by the mosquitofish (Gambusia). The independent variables were the relative size of predator and prey, prey density, and predator satiation. Gambusia actively chose to attack small prey (which we showed to be more profitable). Gambusia's preference increased monotonically, or decreased, or peaked at an intermediate prey size, depending upon fish size, prey size, and fish satiation level. These results contrast with the hypothesis the "gape—limited predators" select the largest prey available, and "size—limited predators" select small or intermediate—sized prey. Optimal diet theory predicts that a predator should drop less profitable prey items from the diet at higher prey densities. In contrast, Gambusia maintained a mixed diet and its preferences did not change in response to changes in either the absolute or the relative abundance of prey. Moreover, the ingestion rate would have been higher if less profitable (larger) prey types had been ignored. Gambusia did sharply alter its size—selective feeding behavior in response to its own satiation level, and we expect that this factor is correlated with prey density in the field. Well—fed fish concentrated their attacks, to a greater degree than starved fish, on small, more profitable prey. They also ingested less biomass per unit time. This last result suggests that maximizing food intake per unit time may not be the goal of well—fed fish, and that different criteria may determine the profitability of prey at different prey densities.

ISSN:0012-9658    

DOI:10.2307/1938576

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

In Dynamite Lake, Illinois, the cladocerans Bosmina, Ceriodaphnia and Diaphanosoma coexist with planktivorous sunfish. An in situ enclosure experiment revealed that release of the cladocerans from fish predation resulted in an increased mean body size of Diaphanosoma and Ceriodaphnia and a slightly increased density of Diaphanosoma, the largest species. Increased body size of the cladoceran community resulted in a greater estimated grazing pressure on the phytoplankton community. The net result was greater phytoplankton density in the presence of fish, relative to when fish were excluded; there were also differences in phytoplankton community structure between treatments. The hypothesis that the increased in food (phytoplankton) arising from fish predation results in elevated survivorship and/or reproduction of the cladoceran individuals that managed to avoid predation was tested by conducting two life table experiments on the three cladocerans using, as media, water with natural phytoplankton densities from the enclosures with and without fish. The hypothesis was supported. Fecundity (in both experiments) and survivorship (in one experiment) of Diaphanosoma were increased on water from the fish enclosure, and fecundity of Ceriodaphnia was increased in one experiment. Significant increases in fecundity always led to significant increases in finite rate of increase (l). Reproduction of Bosmina was unaffected by the elevation in food; survivorship could not be accurately determined because of difficulties in handling this small species. The significance of these indirect demographic effects of fish on the age and size structure of the cladoceran populations was assessed using the life table data in Leslie matrices and obtaining stable and distributions when water from the fish enclosure (high phytoplankton) or the control enclosure (low phytoplankton) was used. For both Diaphanosoma and Ceriodaphnia, the populations projected under the conditions of fish enclosure water contained a greater proportion of younger individuals relative to the control at the stable age distribution. Since age and size are correlated in cladoceran populations, this analysis indicates that fish predators may indirectly alter zooplankton size structure through their effects on the food supply of the zooplankton, as well as directly modify size structure by removing the largest individuals.

ISSN:0012-9658    

DOI:10.2307/1938577

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The effects of a reduction in the size of a roach (Rutilus rutilus) population on a perch (Perca fluviatilis) population were studied. The population size of roach was reduced by 70% and the biomass by 40%. The biomass of the perch population increased by 140%, which was a result of an increase in young—of—the—year (0+) perch. The consumption of the adult perch population increased more than two—fold compared to the situation before the removal experiment. 0+ perch did not switch in August from a diet of zooplankton to a diet of macroinvertebrates, as was the case before the removal experiment, and they also expanded their habitat. This resulted in a reduced age class competition, and the limitations set by the macroinvertebrate bottleneck phase in the life period of perch were reduced. Though the removal experiment lacked a control, the considerable response of both the zooplankton community and the perch population strongly suggests that roach greatly influenced the resource intake and resource utilization of perch.

ISSN:0012-9658    

DOI:10.2307/1938578

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

We manipulated the habitat composition and structure within territories of birds breeding in the Artemisia—dominated shrubsteppe of central Oregon in order to assess how closely individuals track habitat in features such as territory placement of size or behavioral budgeting and space. We removed 75, 50, 25, and 0% of the shrub individuals from 625—m2blocks in a checkerboard designs. Over the following 7 yr we monitored territorial locations and sizes and breeding densities of Horned Larks (Eremophila alpestris), Sage Sparrows (Amphispiza belli), and Brewer's Sparrows (Spizella breweri) on the manipulated area and adjacent unaltered control area. Activity and substrate use patterns and intensity of use of the different treatment blocks within the manipulated area were also determined for the two sparrow species. Sage Sparrows, which from our previous studies appear to be more closely linked to shrub (sagebrush) coverage than the other species, did not respond to the manipulation at a local population level. The manipulation apparently did affect territory placement and densities of Brewer's Sparrows and Horned Larks in 1980—1982, but these trends were not consistent over the entire postmanipulation period. Densities of all species varied among years. For Brewer's Sparrows and Horned Larks these variations did not parallel those in nearby census locations, but Sage Sparrow densities varied in the same ways over all the locations. These variations appeared to parallel variations in bioyear (October—April) precipitation, with a 1—yr time lag. Individuals of both sparrow species altered the details of their activity budgeting, but not their substrate—use patterns, in response to the manipulation. Both species clearly expressed an affinity for unmanipulated blocks within the manipulation area. This experimental manipulation is complicated by influences of time lags on individual and population responses that may stem in part from site tenacity by breeding adults, leading them to return to previous breeding locations in years following the manipulation despite the habitat changes. Further, the spatial scale on which the manipulation was conducted may have been inappropriate to gauge responses at the population level. We suggest that these complications may plague many field experiments in ecology.

ISSN:0012-9658    

DOI:10.2307/1938579

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The body mass (or fat reserves) maintained by a wintering bird can be viewed as a trade—off between the risk of starvation and the risk of predation. This follows from the fact that fat reserves after survival in very different ways. From the starvation point of view, a bird should be as fat (or heavy) as possible in order to minimize its probability of starvation during weather—related food unavailability because fatter birds can survive longer without food than leaner birds. From the predation risk point of view, however, a bird should be as lean as possible to minimize its probability of being killed. Leaner birds will incur a smaller cost of existence than heavier birds and thus spend less time feeding, potentially exposed to predators. In addition, lean birds are likely to be more adept at escaping predators once attacked. If a bird is attempting to minimize its probability of death during the winter season, the fat reserves and thus the body mass it maintains will reflect a trade—off between starvation and predation risk. A simple stochastic simulation model developed to explore the nature of this trade—off indicates that body mass should: (1) increase with the frequency and/or harshness of periods of food unavailability (due to snow, ice, wind, etc.); (2) decreases with increasing predation risk; (3) decrease with increasing temperature; and (4) increase with food abundance. The analysis of the model readily explains why wintering birds maintain fat levels lower than those of which they are capable. The tendency for individuals of a given bird species to be heavier in more northerly populations and during midwinter is also readily interpreted in terms of the above trade—off. Implications for foraging behavior and population regulation in wintering birds are discussed.

ISSN:0012-9658    

DOI:10.2307/1938580

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The ability of free—ranging Brant (Branta bernicla) and free ranging and captive Canada Geese (B. canadensis) to digest various constituents of marsh plants was studied using lignin as a nondigestible tracer in plant food and feces. These constituents included total organic matter, soluble carbohydrates, protein, total ether—soluble material, soluble phenolics, cell wall polysaccharides, and phenolic acids esterified to plant cell walls. The digestion of total organic matter averaged 37% in both species of geese, indicating that geese are less efficient at digesting plant material than most other vertebrate herbivores. The low total organic matter digestion was largely a result of inefficient digestion of cell wall polysaccharides. Geese digested °28% of the cellulose and 25% of the hemicelluloses present in their plant food. In contrast, the apparent digestibility of soluble carbohydrates were 69—85%, and the digestibility of protein (61—80%) was similar to that of most other vertebrate herbivores or graminoid diets. The two cells wall polysaccharides still provide up to 31% of the energy geese extract from plants despite their low digestibility, because they are present in relatively high concentrations (together, °40% of dry mass). Geese digested cellulose as efficiently as predicted by the previously reported relationship between herbivore body size (or metabolic requirement) and cellulose digestion.

ISSN:0012-9658    

DOI:10.2307/1938581

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY

摘要:

The foraging behavior of a color—marked population of the polgynous, colonial Yellow—rumped Cacique, Cacicus cela (Icterinae), was studied throughout the breeding cycle in southeastern Peru. Females changed their foraging rates and microhabitat use at each stage of the nest cycle, apparently in response to changes in protein needs, travel costs, and time constraints. Nonnesting females, with few such constraints, searched for insects slowly and inconspicuously high in the forest canopy where they caught prey (mostly small larval lepidopterans) at a low rate. When protein needs were high (i.e., during egg—laying and fledgling—feeding) or foraging time was restricted (i.e., during incubation), females accelerated their searching rates in all microhabitats. During the nestling—feeding period, when both protein needs and travel costs to and from the nest were high, females searched at the fastest rate and foraged more in the forest understory where they caught relatively more large orthopterans. These data suggest that females with few constraints minimize their exposure to predators, while energetically constrained females maximize their rate of energy intake.

ISSN:0012-9658    

DOI:10.2307/1938582

出版商:Ecological Society of America    

年代:1986

数据来源: WILEY