摘要:

Field experiments were conducted to determine the foraging behaviors of granivorous desert ants and rodents and to ascertain the impact of these foraging efforts on the density and distribution of seeds in the soil. Foraging experiments were performed which involved providing seeds within taxon—specific enclosures in scattered and clumped distribution on the soil surface and 1.5 cm below the surface. These experiments indicated that ants only foraged on the surface, whereas rodents were able to garner seeds from below the ground. Ants foraged °85% of the experimental seed distributions on the surface and were able to retrieve °45% of the seeds they detected during any 24—h experimental run. Rodents detected 100% of all distributions and got °96% of the seeds from all distributions except scattered/belowground, from which they gathered only 75% of the seeds. Three—year experiments using exclosures which excluded either ants, rodents, both taxa, or neither taxon, indicated that either taxon alone, or both taxon together, have a severe impact on the density of seeds in the soil. When neither is present, seed densities remain high. Furthermore, any combination of granivores greatly reduces the number of samples with either high numbers of seeds or high biomass of seeds, although the ants tend to leave a greater number of large clumps of seeds than the rodents. Clumping indices are also extremely high in the absence of granivores. In opposition to the apparently detrimental effect of seed use by the granivores, surface seed—caching by rodents appears to enhance seed germination.

ISSN:0012-9658    

DOI:10.2307/1936954

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The average hydrogen ion input to a mountain watershed in Colorado increased by a total of 0.142 meq.m—2.wk—1over a 150—wk study period. Over the same period, the output of bicarbonate ions from the watershed, as determined by weekly stream chemistry measurements, decreased by 0.162 meq.m—2.wk—1after correction for a slight decrease in bicarbonate input. The output of sulfate, nitrate, ammonia, and dissolved organic matter all increased significantly. These increases are not enough to constitute a major change in watershed mass balance, but the nitrate and ammonia changes in particular could be important to the stream biota because of the low background levels of inorganic nitrogen. Output of calcium, magnesium, sodium, potassium, phosphate, and hydrogen ions showed no significant individual trends, but there is some evidence of a decrease in total cation output. It is postulated that essentially all of the hydrogenon ions entering in the form of acid precipitation combine with bicarbonate ions in the soil, and this accounts for the reduction in bicarbonate output of the watershed. This hypothesis is well supported by the match between rates of change in hydrogen ion input and bicarbonate output. Since carbonates are not present in the soil, a combination of acid with bicarbonate would lead to the production and release of free CO2in amounts equivalent to the acid which is neutralized. Acidification of precipitation thus appears to cause a stoichiometric reduction in bicarbonate output and smaller but potentially important shifts in output of other ions as well.

ISSN:0012-9658    

DOI:10.2307/1936955

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The stability, regulating factors, and dynamics of an experimental community of protozoans are examined here. The predator Didinium nasutum is found to coexist in microcosms with its prey, Colpidium campylum. Prey are limited by the quantity of bacteria and nutrients available. Didinium proves to be an ineffective predator and is limited by the availability and/or quality of prey as food. Enrichment of this community with bacteria or nutrients results in the extinction of prey and starvation of the predator. Increasing diversity of the lower trophic level by adding alternative prey species destabilizes this community, also causing extinction. Stability depends here on the characteristics of the particular species serving as prey and not simply the diversity of species present.

ISSN:0012-9658    

DOI:10.2307/1936956

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Previous studies suggest three hypotheses to explain the importance of dispersal in small mammal populations: (1) regulation of density, (2) promotion of outcrossing, and (3) avoidance of competition for mates. These hypotheses were examined in an experimental study of the California ground squirrel (Spermophilus beecheyi). One ground squirrel colony was supplied with supplemental food for 3 mo, while two other colonies were monitored for reference. More adult ground squirrels moved to the supplemented colony than to the reference colonies. All young males disappeared from their natal colonies, even on two areas with different adult sex ratios (.09 and .56 male per female). Most young females remained in their natal colonies through the last month of the study. More young male immigrants were trapped at a reference colony than at the supplemented colony. More young females moved to the supplented colony than to the reference colonies; both via exploratory visits from nearby colonies and by immigration. All young resident males of the colony nearest to the supplented colony disappeared from both colony areas. The supplemented colony received visits from several young females who resided at the nearest adjacent colony. Eleven of these females took up a regular "commute" between their natal colony and the supplemental food. These results support the importance of the regulation of density relative to critical resources via young female dispersal, and the promotion of outcrossing via young male dispersal.

ISSN:0012-9658    

DOI:10.2307/1936957

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

This paper analyzes how cumulative injury data in age—structured populations can be used to determine some properties of the injury—producing process, such as predation. If the injury—producing process is the only source of mortality or injury, raw frequencies of animals free from injury (e.g., tail breaks) are shown in a theoretical analysis to equal the efficiency of the injury—producing process (e.g., the fraction of predation attempts that result in death of the prey); they are not affected by the intensity of the injury—producing process. If alternate sources of mortality exist, this conclusion may still be approximately true unless those sources are large. If there exists alternative sources of injury that never result in death, raw frequencies of uninjured animals actually increase with increasing intensity of the process that produces both injury and death (e.g., predation). To estimate the intensity of the injury—producing and the per—unit—time rate of accumulating injuries in the simplest case, one needs both injury and survival frequencies. Formulae for these estimations are provided.

ISSN:0012-9658    

DOI:10.2307/1936958

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Annual patterns in hydrology, phosphorus circulation, and sediment dynamics were studies in a southern Illinois, USA floodplain swamp dominated by bald cypress (Taxodium disyichum) and swamp tupelo (Nyssa aquatica). The study emphasized the swamp's interactions with the adjacent river. For the year, major inputs of water to the swamp were throughfall (74.3 cm) and runoff (69.4 cm) with minor contributions due to groundwater (21.6 cm). Outflows were by evapo—transpiration (72.3 cm), surface outflow (56.5 cm), and groundwater (21.0 cm), with the latter two draining primarily to the river. A flood occurred during the study period, passing 1.6 x 107m3of river water over the swamp and depositing 0.06 cm of sediments. An annual phosphorus budget was developed for the swamp from field measurements. The greatest input of phosphorus to the swamp was 3.6 g P.m—2.yr—1, due to deposition of high—phorsphorus sediments during the flood. This was 10 times greater than the outflow of phosphorus to the river, 0.34 g P.m—2.yr—1, and 26 times greater than the throughfall input of 0.14 g P.m—2.yr—1. Total tree uptake from sediments was estimated to be 0.87 g P.m—2.yr—1of which 0.77 g P.m—2.yr—1returns as litterfall to the swamp sediments. Duckweed productivity was estimated to take 3.3 g P.m—2.yr—1from the water column and deposit this in the sediments during die—off. For the period 1937—1967, cypress growth, based on tree ring analyses, was closely correlated with several measurements of flooding frequencies and magnitude, all obtained from past river data. Three ring data prior to 1937 showed poor correlation with flooding, probably because of logging activity. Cypress growth has decreased dramatically in recent years, corresponding to the rise in water level caused by beaver activity.

ISSN:0012-9658    

DOI:10.2307/1936959

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Breeding seasons and birth cohorts were defined for two populations of mice (Peromyscus polionotus). Cohort—specific differences in reproductive effort were correlated with residual reproductive value. In one population, mice lived nearly 2 yr, reproduced at moderate rates, and showed few differences in cohort—specific reproductive strategy. Mice from the other population lived<1 yr, and spring—, summer—, and fall—born females exhibited markedly different reproductive strategies. Cohorts able to breed in a future season had moderate reproductive efforts, which did not increase risk of mortality. Extraordinary efforts (being pregnant and lactating simultaneously), which seriously increased mortality rate, were undertaken when residual reproductive value became zero. Nestling mortality increased with litter size when there were more than three young per litter, but larger litters still resulted in more weaned young than did smaller litters. These observations provide additional confirmation of the model of Pianka and Parker (1975). It is possible that populations of very short—lived mammals characteristically have seasonal cohorts operating under different reproductive strategies.

ISSN:0012-9658    

DOI:10.2307/1936960

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

We challenge Diamond's (1975) idea that island species distributions are determined predominantly by competitions canonized by his "assembly rules." We show that every assembly rule is either tautological, trivial, or a pattern expected were species distributed at random. In order to demonstrate that competition is responsible for the joint distributions of species, one would have to falsify a null hypothesis stating that the distributions are generated by the species randomly and individually colonizing an archipelago.

ISSN:0012-9658    

DOI:10.2307/1936961

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Plutonium inventories were determined in two old—field ecosystems, Fields 1 and 2, located near a nuclear—fuel reprocessing facility that had released approximately 640 mCi (24 GBq)238Pu and 57o mCi (21 GBq)239,240Pu to the atmosphere in the 20 yr preceding sampling. Field 1 was 230 m from the point of release (a 62—m stack) and contained 57 nCi (2.1 kBq)238Pu/m2and 225 nCi (9.4 kBq)239,240Pu/m2. Field 2 was 420 m from the stack and contained 15 nCi (0.56 kBq)238Pu/m2and 79 nCi (2.9 kBq)239,240Pu/2. The distribution of Pu between soil and vegetation components was similar in the two fields with only 1.3% of the238Pu and 0.2% of the239,240Pu occuring in pines, herbaceous vegetation and litter. More of the Pu could be suspended from the soil into the atmosphere by a 6 m/s wind on Field 1 than on Field 2 due to the lower biomass of herbaceous vegetation and litter on Field 1. Approximately 2% of the238,240Pu and 0.5% of the239,240Pu could be resuspended on Field 1, whereas only 0.5% of the238Pu and 0.1% of the239,240Pu could be resuspended on Field 2. Plutonium contamination of the vegetation occurred primarily by (1) direct deposition of recently released Pu onto vegetation, and (2) resuspension of Pu from the soil to vegetation surfaces. Root uptake apparently made negligible contributions to the Pu content of above—ground vegetation.

ISSN:0012-9658    

DOI:10.2307/1936962

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The growth of subtidal algae on calcareous cobbles is restricted to a short (2—4 mo) growing season. Most algae are destroyed during rainy season storms due to surge and cobble tumbling; algal density is low throughout the year. Significant interspecific competition does not occur on cobbles, and no species/area relationship could be demonstrated. The number of species per cobble is correlated with cobble moment of inertia (resistance to tumbling) during the rainy season. Species diversity on cobbles is significantly higher than on neighboring rocky reefs. Available evidence suggests an annual floristic cycle, although biomass and diversity are closely related to short—term weather conditions. Ephemerals, annuals, pseudoannuals, and prennials appear sequentially during the growing season. Species with greater potential size reach peak abundance late in the growing season, although many never reach maximum size or reproduce on cobbles. These longevity and size trends cannot be said to constitute adaptive strategies on cobbles. The high species diversity observed on cobbles is adequately explained by seasonal disturbance.

ISSN:0012-9658    

DOI:10.2307/1936963

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY