摘要:

To account fully for the pattern of translocation of soil by pocket gopher activity in Mima moundfields, I investigated soil mining in relation to location along the mound—intermound topographic gradient. From September 1986 to November 1987, at Miramar Mounds National Landmark in coastal San Diego County, California, the total soil mined by pocket gophers (Thomomys bottae) was estimated by measuring the mass of soil deposited in surface heaps and counting the number of plugged surface—access tunnels on mound and intermound zones associated with 10 Mima—type mounds. Surface heap deposition was greatest in late fall and early winter, whereas the digging of surface—access tunnels was most frequent in spring and early summer. Both the mass of soil in surface heaps per unit area and the number of surface—access tunnels per unit area tended to decrease with distance from the center of mounds. When the areas of concentric zones at increasing distance from the mound center were taken into account, however, the total mas of soil mined increased with distance from mound centers. At this site, deposition of soil in surface heaps estimated to be 8.23 Mg°ha—1°yr—1and subsurface deposition to be 20.31 Mg°ha—1°yr—1, so that total soil mining equalled 28.54 Mg°ha—1°yr—1. Total mass of soil mined bore a log—linear relation to distance from the mound center. This relation was combined with that relating distance of movement of mined soil to mound height and distance from the mound center, to give a preliminary model of mound growth dynamics as influenced by pocket gopher activity. This model suggests that maximum mound size is largely determined by the pattern of soil mining and translocation by these mammals.

ISSN:0012-9658    

DOI:10.2307/1937355

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

We studied, both theoretically and empirically, the effect of intra— and interspecific competition on the foraging effort of individuals. We considered two models, one for a time—minimizer satisfying an energy requirement, the other for an animal maximizing fitness as a function of multiple inputs subject to a time constraint. The goal of satisfying an energy constraint predicts that foraging effort should increase with increased competition. The goal of maximizing fitness subject to a time constraint on multiple inputs may also predict that foraging effort should increase with increased competition because of the missed opportunity cost that results when different inputs are complementary. However, if the fitness—maximizer with multiple inputs incurs an energy cost of foraging (in addition to missed opportunity costs), then it should often reduce foraging effort in response to an increase in competition. We experimentally tested the foraging response to increased competition by two species of gerbils, Gerbillus allenbyi and G. pyramidum, over a range of manipulated population densities in field enclosures located in the Negev Desert of Israel. Our results support the cost—benefit model when the additional energy cost of foraging is important. Per capita activity (as measured by spoor) declined as a function of intraspecific density for each species and as a function of interspecific density for G. allenbyi. We detected no interspecific effect, however, on G. pyramidum.

ISSN:0012-9658    

DOI:10.2307/1937356

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The costs of reproductive effort for adult male and female Richardson's ground squirrels were compared to determine whether these costs differed in timing or magnitude in a manner related to sexual differences in mating and parental effort. Reproductive effort was assessed from 1982 to 1986 for a population of Richardson's ground squirrels in southern Alberta, Canada, by monitoring seasonal mortality schedules, fecundity, and changes in body mass and fat content. Although the adult sex ratio was female—biased, the operational sex ratio was male—biased. Males lost mass and sustained injuries as a result of male—male conflict during the mating season. From 50 to 79% of males disappeared annually during reproduction (between spring emergence and the end of the mating season), whereas<20% of females disappeared during reproduction (between spring emergence and litter emergence). Overwinter mortality rates were similar for adult males and females. Active seasons of adult males and females were of similar durations, but because reproductive effort finished earlier for males, they had a longer period in which to prepare for hibernation, and they entered hibernation (immerged) with larger fat stores. Juvenile males had a longer active season and attained a greater proportion of adult structural size than juvenile females (90 vs. 75%). Although males emerged from hibernation with more residual fat than females, fat stores were rapidly depleted during mating 2—4 wk postemergence, whereas females did not deplete residual fat until late lactation 7—8 wk postemergence. Survival rates of females that did and did not wean litters did not differ significantly, and survival rates of mothers were not significantly or negatively correlated with litter size or litter mass. Although litter size tended to regress toward the mean in consecutive years of reproduction, neither size nor mass of litter weaned negatively influenced litter size or mass in the subsequent year. Depletion of larger stored fat reserves and lower survival rate during reproduction indicate that, from both energetic and survival perspectives, mating effort is more costly for male Richardson's ground squirrels than parental effort is for females.

ISSN:0012-9658    

DOI:10.2307/1937357

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Food limitation of reproduction during the nestling stage was tested by supplementing food only during this stage in Cactus Wrens. Parents offered supplemental food fed their young more (volume per unit time) than unsupplemented parents in 2 yr of experimentation. In 1986, food—supplemented nestlings had greater mass and linear measures, whereas in 1987, most food—supplemented nestlings had greater mass, but not linear dimensions, than their controls. In 1986, survivorship of young both in the nest and during 4—6 wk after fledging, was greater with food supplementation than without. In 1987, survivorship differences were not significant. Food supplementation enhanced annual reproduction of parents because more food—supplemented Cactus Wrens laid second clutches than controls in 1987, the only year when this was examined. We conclude that food availability during the nestlings stage can limit present reproduction, enhance the likelihood of additional reproduction, or both, in Cactus Wrens.

ISSN:0012-9658    

DOI:10.2307/1937358

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Holling's (1959) disc equation is the most commonly used model of type—2 functional response. The constant—parameter version of this model makes three assumptions that are seldom likely to be satisfied: (i) attack rate per unit search time per unit prey density is independent of prey density; (ii) handling time is independent of prey density; and (iii) the relative rates of successful and unsuccessful attack are independent of prey density. Optimal foraging theory is used to suggest how each of these components of the predation process might depend on prey density. Explorations of simple models suggest that these types of adaptive variation are often consistent with type—2 responses, but generally result in shapes that differ significantly from the disc equation. The consequence of using a constant—parameter disc equation model when it is inappropriate is limited ability to predict functional responses in altered circumstances, for example, in the presence of a second prey. The potential ability to understand multispecies systems from their subcomponents cannot be assessed unless appropriate models of the functional response are used to describe the component systems; these models are likely to include adaptive behavior.

ISSN:0012-9658    

DOI:10.2307/1937359

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Individual variation in the foraging behavior of bluegill sunfish (Lepomis macrochirus) was studied in laboratory aquaria containing vegetated and open water habitats. In general fish chose to search for prey in the habitat that provided the highest foraging return, but individuals differed markedly in the degree of habitat selection. Vegetation and open water habitats require different searching techniques for efficient prey detection and individual bluegill differed in their levels of behavioral adjustment to each habitat. Fish using slower searching tactics had the highest foraging rates in the vegetation, whereas individuals searching rapidly fed more effectively in the open water. These behavioral differences in searching rates were found to be correlated with the size of the pectoral fins. Bluegill with longer pectoral fins consistently searched more slowly and spent more time in the vegetation habitat compared to bluegill with shorter pectoral fins. These results indicate that behavioral and morphological variation among individuals can influence habitat—specific feeding efficiency and habitat selection, and may also have direct consequences for models of density— and frequency—dependent habitat selection.

ISSN:0012-9658    

DOI:10.2307/1937360

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Armored catfish (Loricariidae) are the major grazers of attached algae in pools of the Rio Frijoles, Panama (9°9' N, 79°44' W). In the dry season, sunny pools were inhabited by °6 individuals loricariids per square metre of grazeable substratum. At these densities, armored catfish depleted algae and cleared sediment from bedrock substrata, leaving sparse standing crops of small, adnate diatoms (primarily Achnanthes spp.). To study the effects of armored catfish at 1/6 their natural density, I stocked four 6—7 cm (SL), 10—g Ancistrus (the most common size class of the most common species in stream pools) in each of five stream pens. Pens enclosed 4 m2of bedrock substratum, and were alternately stocked or left empty during three consecutive periods of 29, 11, and 11 d. At the end of each period, standing crops of sediment and attached algae, and rates of photosynthesis by attached algae, were measured. The attached algae that developed with sparse Ancistrus had higher standing corps with larger cells or colonies, and higher primary productivity, than did periphyton subjected to heavy grazing by unconfined armored catfish. Even heavy grazing, however, was less deleterious to attached algae than prolonged sedimentation on substrata in enclosures left empty for 11 or 29 d. The net effect of Ancistrus on their algal food changed from depletion at high grazer densities to enhancement at low grazer densities, as sedimentation became more limiting to algae than grazing.

ISSN:0012-9658    

DOI:10.2307/1937361

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Nutrient—diffusing substrata (sand—agar plates) were combined with an in situ, flow—through bioassay system to experimentally separate effects of substratum and water enrichment on algal communities in a phosphorus—poor stream in northern Michigan, USA. Glass slides and both control and fertilized sand—agar substrata (N:P ° 16) were exposed to low ambient nutrient levels in a phosphorus—limited stream (N:P ° 40) and to water amended with nitrogen and phosphorus (N:P ° 16). Quantitative cell counts of periphyton taxa and microscopic examination of intact communities on sand—agar substrata indicated that motile biraphid diatom taxa (e.g., Navicula and Nitzschia spp.) responded to nutrients in both the water and substratum, attaining spatial dominance in the upper canopy. These upperstory taxa comprised>50% total algal biovolume on sand—agar substrata and appeared to interfere with the response of sessile understory taxa (e.g., Achnanthes minutissima, Cocconeis placentula) to inorganic nutrient amendments to the water. Sessile taxa did respond to inorganic nutrients added to the water when growing on glass slides, where algal communities lacked a dense motile upperstory, indicating that substratum type can influence the nature of periphyton taxon response to nutrients. Periphyton community physiognomy and interactions between taxa are interrelating factors regulating algal response to nutrients from different sources. In addition, the form of phosphorus enrichment (inorganic vs. organic) influenced algal growth, physiognomy, and taxon composition. Algal response to the form of phosphorus was dependent on the source of the nutrient (water vs. substratum) and substratum type. Taxon diversity was greater on all nutrient treatments relative to controls on both substratum types. Diversity and species richness were highest on enriched substrata exposed to low ambient nutrient levels, relative to more homogeneous nutrient regimes where communities on enriched substrata were exposed to water amended with nutrients. Results indicate that nutrient spatial heterogeneity between substratum and water is a mechanism maintaining the species diversity of periphytic algae.

ISSN:0012-9658    

DOI:10.2307/1937362

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Two predators of zooplankton, fish (yellow perch) and larval insects (Chaoborus), were manipulated within in situ mesocosms to assess their indirect effects on phytoplankton. The two predators caused similar reductions in zooplankton size and biomass, and as a consequence, zooplankton grazing rates on phytoplankton. However, phytoplankton increased relative to predator—free controls only in enclosures with fish and not in enclosures with Chaoborus, suggesting that fish have effects on phytoplankton independent of their effects on zooplankton grazing rates. Estimation of phosphorus excretion rates of zooplankton and fish suggests that fish render P more available to phytoplankton in two ways: (1) by directly excreting and egesting P, and (2) by increasing rates of P excretion by the zooplankton community. The latter effect resulted because small zooplankton have higher biomass—specific excretion rates than large zooplankton. The importance of nutrient regeneration is further supported by the response of phytoplankton community structure. Phytoplankton taxa exhibiting enhanced biomass in the presence of fish were those with high P requirements, including green and blue—green algae and dinoflagellates; other phytoplankton groups did not respond to manipulations. These results suggest that the mechanisms underlying the tropic cascade are complex and that predators affect phytoplankton by altering nutrient recycling rates as well as zooplankton grazing rates.

ISSN:0012-9658    

DOI:10.2307/1937363

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Motivated by problems where variation among individuals is necessary to explain properties of ecological systems, we develop a mathematical model of an individual organism. The model, based primarily upon energetics, is developed specifically for female daphnids, although with appropriate modifications it should be applicable to other aquatic animals such as fish. Mimicking the life history of an individual as it progresses from egg to juvenile to adult instars, the model consists of a coupled pair of nonlinear, nonautonomous ordinary differential equations. The growth of an individual is described through the dynamics of two compartments–lipid and structure–of the organism because of the importance of lipid dynamics in aquatic animals. Energy supply and demand are handled through an energy integrator compartment. Availability of energy is represented by potential flows from the lipid and structural compartments. Energy requirements of maintenance, activity, and reproduction are the sinks. An advantage of employing an individual—based technique is that ample information relating to the individual parameters–most of which are physiological in character–can usually be obtained, and reasonable estimates for model application found.

ISSN:0012-9658    

DOI:10.2307/1937364

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY