ISSN:0012-9658    

DOI:10.2307/1935212

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

Of eight species of Drosophila (ananassae, funebris, melanogaster, nebulosa, pseudobscura, simulans, virilis, and willistoni), the number of progeny produced per culture vial increases with the number of female parents for D. melanogaster, D. simulans, D. willistoni, D. ananassae, and D. nebulosa; in contrast, the progeny yield is a constant that is independent of the number of parental females per vial for D. funebris, D. pseudoobscura, and D. virilis. Forall species, however, the per capita yield decreases more or less exponentially with the square—root of the number of parental females per vial. The outcome of mixed—species competition depends largely upon the two species concerned. Drosophila simulans seemingly benefits by increases in number of its own females; four or five D. simulans females yield more progeny in the presence of D. funebris, for example, than produces D. simulans. Drosophila simulans alsoproduces greater number of progeny per vial as the number of competing D. funebris females is increased. Drosophila melanogaster and D. simulans females competing in individual culturesyield mixed progeny; the two species share the resources of each vial. In contrast, the progeny of D. pseudoobscura and D. virilis females in individual mixed—culture vials tend to be of one species or the other many cultures containing these two types of females yield few or no flies of either species. For most species—pairs the logarithm of the ratio of the two species among offspring varies directly with that among parents (slope of the regression = 1.00). For three species—pairs, however, the slope of this regression is significantly less than 1.00. If the same relationship were to hold among freelybreeding populations of the two species, the species should establish stable equilibrium proportions. Analyses of the competing species by means of replacement series reveals that the members of individual pairs may enhance or interfere with each other's development, or do neither. Similarly, the competitive abilities of the species differ considerably. The ease with which each species uses different resources within the culture vials differs. Thus, intra— and interspecific competition differentially affect the per capita progeny production of conspecific and heterospecific females. Because the competition between conspecific females differs demonstrably from that between heterospecific ones, it appears that inter—specific frequency—dependent selection may reflect the relative proportions of resources that are used with contrasting ease by the competing species. If so, frequency—dependent selection is precisely the sort of evidence needed to demonstrate that two species are not competing identically for identical resources, the type of competition that is expected to lead to competitive exclusion.

ISSN:0012-9658    

DOI:10.2307/1935213

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

The effects of rainfall and temperature on the distribution and certain behavioral characteristics of Larrea tridentata (Creosote—bush) at and near its northern limits in the northern Mojave desert of southern Nevada, were investigated at 39 sites with Larrea and 20 sites without Larrea in eight drainage basins at elevations of 915—1,770 m over a 2,600—km2 area of the Nevada Test Site. Data used were (1) rainfall records for 9 yr (1963—71) for each site; (2) maximum and minimum air temperature records for each site, November 1962—February 1972; (3) percentage cover by all shrubs species and by Larrea; (4) height and density data for Larrea; and (5) percentage of germinable seeds from 29 of the Larrea populations for three seasons (1963—65) in relation to the seasonal rainfall for each site. Total percentage cover by all shrubs species is highly correlated with mean annual rainfall, less well correlated with elevation. Percentage cover by Larreafollows two patterns of relationship with rainfall: (1) where mean rainfall is low to intermediate, the same pattern as total shrub cover in relation to rainfall, and (2) on sites of high mean rainfall, consistently low cover, a function of low density of Larrea on these sites. In general, in undisturbed communities, the taller the Larrea plants the fewer there are of them, but the relationship is not strictly linear. Using height as an index to plant volume, numbers of Larrea plants are highly correlated with total plant volume. Mean height is not strongly correlated with mean annual rainfall or temperature parameters, but is well correlated with the ratio of mean precipitation/mean temperature. Tall plants (>1 m) occur in low density and on sites with high rainfall (mean 160—183 mm). The prevailing low minimum air temperatures and their extremes in the lowlands of enclosed drainage basins are inferred to be the primary cause of the absence of Larrea in three discrete vegetation zones (Atriplex confertifolia, Lycium pallidum, and L. shockleyi) in Frenchman Falt, and over most of the basin floor of Yucca Flat, where the communities are Atriplex and Grayia—Lycium andersonii. The year—round low minima in the lowlands of these basin result from nocturnal cold air drainage phenomena and formation of a cold air layer of variable depth.Average extreme minima on these sites were mostly below O° F; the extreme minimum was —18° F in one of the Atriplex communities of Frenchman Flat. Larrea occurs over the bajadas of Frenchman Flat on sites above the lower layers of cold air. In Yucca Flat, at its northern limits, it is restricted to certain upper bajada sites and notably one site on the basin floor. Average extreme minimum air temperatures on all Larrea sites were above 1°F; the absolute minimum was —8°F. Upper altitudinal limits of Larrea apparently are not determined by minimum temperatures since minima (including the extremes) in Coleogyne vegetation, which replaces Larrea altidudinally on the slopes, are well within the range of those recorded on Larrea sites. There is no pattern of relationship between maximum temperatures and the distribution of Larrea, although the highest extreme maxima usually occur on non—Larrea sites in the lowlands of Frenchman Flat. Mean annual rainfall on the Larrea sites ranged from 118 to 183 mm. Altitudinal and latitudinal limits of Larrea coincide with a maximum mean rainfall of 183 mm. Mean altitudinal rainfall of 160—183 mm appears to be critical to the behavior of Larrea. Germination trials support the inference of a deleterious effect of high rainfall on Larrea populations through time: there were high correlation coefficients (negative or positive, depending on the year) between the rainfall of the effective rainfall season and the percentage of germinable seeds; highest mean germination percentages (20%—60%) occurrred with 80—150 mm of seasonal rain, and either lower or higher seasonal rainfall resulted in lower percentages of germinable seeds (0%—20%).

ISSN:0012-9658    

DOI:10.2307/1935214

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

The duckweed Lemna perpusilla in a small, alkaline lake was studied during 1970 and a map of its distribution was prepared. Duckweed, the only floating, unrooted higher aquatic plant present, was distributed around most of the edge of the lake but its density was extremely patchy. Experiments were made involving manipulation of the duckweed density, modification of its distribution, and measurement of the factors which experiments suggested were significant influences on its pattern of abundance. Attention was particularly focused on the importance of other plants in molding the duckweed pattern. Duckweed was absent from the central area of the lake primarily because there was nothing to prevent the plants from being transported by waves generated by the prevailing winds from the southwest. Growth on water taken from the center of the lake was poor compared to growth on water from near the shore. Duckweed can exist in the Potamogeton zone at a much higher density, and the low density does not result from grazing by animals. Growth of plants exchanged between the zone and a zone nearer the shore was depressed in the Potamogeton zone and stimulated in the other. Several experiments suggested that the lowered growth rate was attributable to the higher pH of the water, probably produced by the photosynthesis of other aquatic plants. Where it was dense enough, duckweed was able to overcome this effect by shading the plants beneath. It can be reasoned, from the structure of plants that retain their progeny inserted in the reproductive pockets, that the average ability per frond to produce further progeny is thereby impaired. Also, plants broken up into single fronds produced progeny at a faster rate than unbroken plants. Dense patches of duckweed occurred among the Scirpus plants and along the edge of the shore. Duckweed growth was more rapid on mud than on water but plants transplanted to selected places around the lakeshore did not reveal any differences in the ability of the mud to support their growth. Dense populationswere associated with breaks in the dense cover of Scirpus stems, which reduced the light intensity beneath them by 80%—90%. Comparison of growth of plants on mud in situ, in shaded and unshaded areas, indicated that these openings in the cover are responsible for allowing dense populations to develop. Development of dense patches along the unshaded shore of the lake was limited by their inability to grow fast enough to keep pace with the receding water and hence appeared only where the slope was steep. Most dense patches were eventually stranded on the mud and died, drastically reducing the density because very few new plants (growing from turions or seeds) could be recovered when water was added to dried mud. Therefore the patches must be built up again the following summer. Traditional methods of distribution study would not detect the two distinct causes of patchiness of duckweed in this lake.

ISSN:0012-9658    

DOI:10.2307/1935215

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

Mark—recapture studies were made on a grid over an extended period of time to investigate home range utilization by small mammals, especially Peromyscus leucopus, the most numerous small mammal on the study area. Since captures were largely concentrated around a home range center, Calhoun's (1963) bivariate normal density, which predicts a more diffuse capture pattern, did not adequately describe space utilization in Peromyscus. Mark—recapture results showed a similar pattern as a continual removal regimen: 50% of the total Peromyscus contrary to Calhoun's (1963) predictions that as dominant species are removed (here Peromyscus), subordinate" species (here Blarina and Pitymys) enlarge their home ranges and are caught more often. My results can better be explained by invoking behavior—mediated differences in trappability between species and/or ecological differences in the use of the habitat (fossorial vs. runaways in undergrowth, etc.). Finally, my evidence suggests that Peromyscus are arranged in family groups consisting of one adult female, several adult males (one dominant, several subordinate), and a number of juveniles.

ISSN:0012-9658    

DOI:10.2307/1935216

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

The theory of optimal reproductive strategies has traditionally been studied in two ways: formal analysis of simple models that neglect the effects of age structure, and computer studies of complex life histories. Each of these approaches has disadvantages. The consequences of simple models sometimes dependmore on the nature of the simplifying assumptions than on the biological issues in question. On the other hand, computer simulations are only as general as the examples considered. The present study seeks to extend the formal analysis of optimal life histories to complex cases. I show that an optimal life history maximizes for each age class the expected fecundity at that age plus the sum of all future expected parameters. This result enables us to determine, at least inthe case of a three—stage life history, the manner in which the optimal reproductive effort at each age depends on the efforts made at the other ages, and thus, the coevolution of the various age—specific efforts. Three cases are distinguished: (1) If fertility and post—breeding survival and growth are concave functions of reproductive effort (i.e., have second derivatives that areeverywhere negative) there is a single set of age—specific reproductive rates towhich the system evolves regardless of initial conditions. This set of reproductive rates corresponds to an iteroparous life history (repeated breeding at different ages). (2) On the other hand, if fertility and subsequent growth and survival are convex functions of effort (positive second derivatives), semelparity (a single, herculean reproductive effort, followed by death) will most often evolve. However, an alternative, iteroparous life history sometimes exists, although stability considerations suggest that it may be transitory. (3)More realistic fertility and growth survival functions can generate alternative reproductive strategies that are stable since each represents a local maximum infitness. Often one of these alternatives corresponds to semelparity, the second to repeated reproduction. In such cases, the evolutionary outcome depends on initial conditions. This suggests that related species, with similar ecologies, may have very different life histories, the differences resulting from historical accidents that have trapped each on a different adaptive peak. The Salmonid genera, Salmo and Oncorhynchus, are suggested as possible examples.

ISSN:0012-9658    

DOI:10.2307/1935217

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

Two species of legless, fossorial skinks (Typhlosaurus) are partially sympatric in the Kalahari Desert. Typhlosaurus lineatus occur both in the sandridge and flatland geographic provinces of the Kalahari, in dune streets and on sandridgesmicrogeographically in sympatry, and primarily under log and leaf litter microhabitats. Typhlosaurus gariepensis, always microgeogrphically, and primarily in basal roots of bunch grass microhabitats. Microhabitat overlap in sympatry is low. Morphologically, T. lineatus differ from T. gariepensis in being longer and in having a longer and wider head. Snout—vent lengths, head dimensions, and proportional head lengths of sympatric T. lineatus are larger than those of allopatric T. lineatus; thus T. lineatus displace from T. gariepensis in sympatry. Both species are viviparous, have one brood per year, and give birth in summer after a 5—mo gestation period. Maturity is reached at minimum ages of one and two—thirds yr. Mean litter size of T. lineatus is 1.6, whereas T. gariepensis have but one young. A response to competition between the two species is suggested by the facts that offspring of sympatric T. lineatus are significantlyheavier than those of allopatric females, and that fewer sympatric T. lineatus females are reproductive than allopatric females. Because Typhlosaurus are termite specialists (92.4% of diet by volume), termitesin guts were identified to species and caste. Compared with sympatric T. lineatus, T. gariepensis eat more Psammotermes workers, fewer Allodontermes major workers, and scarcely any Hodotermes. Sympatric T. lineatus eat fewer Psammotermes workers and more Allodontermes major workers than do allopatric T. lineatus. Prey size differences parallel lizard size differences. Because of dietary shifts by T. lineatus in sympatry, dietary overlap with T. gariepensis is substantially reduced for females and immatures, although not for males. Strong morphological and dietary evidence support, for T. lineatus females and immatures, the hypothesis that behavioral and morphological character displacement has occurred which reduces dietary overlap with T. gariepensis. Dietary data for male T. lineatus are possibly conflicting.

ISSN:0012-9658    

DOI:10.2307/1935218

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

Measurements of the web height and counts of the number of webs on randomly selected plots in lespedeza fields reveal that immature Argiope aurantia and Argiope trifasciata, very large araneid spiders, place their webs at different heights. In late summer this vertical stratification disappears, while the numbers of the two species on plots become positively correlated. Coexistence of the two spider species may depend in part upon the usual occurrence of high mortality during the immature stages. Invasion of webs by araneids is reported as possible competitive interference. The two larges species of European araneids also show vertical stratification only as immatures. However, the niches of most Polish Araneus species in fields seem differentiated by parameters related to prey size: season of breeding and size differences between species.

ISSN:0012-9658    

DOI:10.2307/1935219

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

Theories of resource allocation in sympatric heteromyids have often been based on the idea that body size is a key factor in determining the size of resource packages an animal takes. Considerable field data have supported this idea. In this study, however, two coexisting heteromyid species of different sizes are shown to be selecting the same set of seed sizes. These findings and other conflicting data show a need for a new theoretical basis for the correlation of body size with seed size in heteromyids. Both species tested take a more specialized diet at high seed densities than low. This supports the hypothesis that these seed eaters are in some sense adaptively, flexible selective. It also supports the notion that the selectivity is inversely proportional to the time required to find a seed.

ISSN:0012-9658    

DOI:10.2307/1935220

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY

摘要:

A technique is used which permits simultaneous tracing of several foods in the field without the use of radioactive isotopes. Seeds are tagged with different stable (nonradioactive) isotopes of rare elements and then distributed in the environment. Free—living rodents that have been exposed to these seeds are then trapped and their feces are collected. The relative amounts of the several tagged foods consumed by the rodents are determined by subjecting the feces to neutron activation analysis in the laboratory. In activation analysis stable isotopes are rendered radioactive by permitting them to capture neutrons in a nuclear reactor. The elements emit characteristic wave lengths of gamma radiation, allowing not only detection but precise quantification. The technique presents many technical problems, enough of which have been solved to allow its application to the investigation of some interesting ecological questions.

ISSN:0012-9658    

DOI:10.2307/1935221

出版商:Ecological Society of America    

年代:1974

数据来源: WILEY