摘要:

In Hawaii, five species of xanthid crabs of the genus Trapezia are commensals of the branching coral Pocillopora meandrina var. nobilis. As adults, crabs are found in heterosexual pairs. Intraspecific agonistic interactions typically restrict host occupancy to one pair per species but multiple species combinations are common. Though niche overlap among the species is nearly complete, they exhibit statistically different host size requirements. Field observations suggested that the distribution patterns of the species are influenced by interference competition during host selection. A stochastic computer model of host selection produces species distributions similar to those observed in the field. In the absence of evidence consistent with alternative hypotheses, this suggests that interference competition resulting from random encounters during host selection by adults is sufficient to account for site variability in the distribution patterns of species of Trapezia. Simulation experiments were conducted to assess the relative importance of the potential sources of competitive pressure to each species of Trapezia. The suitability of the hosts available at each study site to each species was estimated 1) assuming the presence of competitors and 2) assuming the hosts were uncolonized. The decrease in suitability of the hosts due to the presence of competitors was used as an index of competitive pressure on the species. There was no significant correlation between the degree of niche overlap and the intensity of competition between species. Caution is advised in using niche overlap measures to estimate competition intensity.

ISSN:0012-9658    

DOI:10.2307/1935333

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Two cladocerans, Daphnia parvula and Holopedium gibberum, were studied to determine (a) whether the species were competing and (b) whether the observed coexistence was predictable from competitive relationships alone. Experimental manipulations of intra— and interspecific competition using14C—labelled algae, bacteria, and detritus revealed clear competitive effects only with algae. Some resource partitioning was demonstrated. A model was proposed specifying Holopedium to be the poorer competitor for mutually shared resources, but also to be less reliant on those mutual resources, owing to its larger niche. This model predicts coexistence and a higher relative abundance for Holopedium. However, the dynamics of field populations did not correspond to this prediction. Daphnia was rare and declining throughout the study whereas Holopedium exhibited a pronounced burst of growth. Analysis of birth and death processes revealed that Holopedium was relatively free from mortality throughout most of the study, while Daphnia suffered death rates of 20% per day or higher. This differential mortality explains the discrepancy between the competition model and observed densities. Finally, field studies indicated that mortality of Daphnia owing to predation by Chaoborus larvae is more than twice that of Holopedium. It is concluded that Dalphnia and Holopredium could coexist in a purely competitive system; however, because of selective predation, their observed abundances correspond little to their competitive status.

ISSN:0012-9658    

DOI:10.2307/1935334

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Two species of Pycnopsyche larvae inhabiting allochthonous organic materials in West Creek are contemporaneous and similar in size. Field studies and laboratory experiments show that about 90% of P. gentilis larvae live in fallen leaves, which they use as food and for case materials; the remaining 10% occur in detritus with P. luculenta. P. luculenta can utilize leaves or woody materials: 50% of the population is in detritus, and 50% is in more leafy habitats, but not far from detritus. The distribution of each species from September through March depends on the amount of preferred habitat space available. Larvae in leafy habitats are washed downstream during the spring thraw by habitat displacement. In 1971, two—thirds of the P. gentilis population and nearly half the P. luculenta population were displaced. The subsequent shortage of leafy habitat space and crowding of remaining larvae may result in competitive interaction and predation by grackles. In the laboratory, both species feed preferentially on leaves that decay quickly, especially on leaves infected with fungi. Dry weights of leaf tissue ingested per larva in 24 hours frequently exceed 50% of the dry body weight of the larva. P. luculenta ingests less leaf material if twigs are available. The niche size of P. gentilis is smaller than that of P. luculenta. There is some niche overlap, but it apparently does not normally result in harmful competition between the species.

ISSN:0012-9658    

DOI:10.2307/1935335

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Important research carried on currently in ecological regionalization calls for a close look at the role of classifying and mapping vegetation, as both these activities can be of fundamental significance in regionalization. A correlation of classifying and mapping vegetation with ecological regions requires an analysis of vegetation, classifications, regions, and maps. The analysis of vegetation revealed the character of biogeocenoses, plant communities, and continua and, incidentally, made it clear that the correct term for the science of vegetation studies is phytocenology. Problems of vegetation boundaries can develop when continua are compared with transition , this is important in mapping, where the nature and location of boundaries is of major significance. Vegetation is best divided into natural and cultural vegetation and further subdivided on the basis of (1) physiognomy and structure, (2) floristics, (3) community dynamism, and (4) community relations with their respective biotopes. When these units were applied to an analysis of classifications, it developed that a basic distinction must be made between highly flexible, purely descriptive and essentially classless approaches on the one hand, and clearly organized hierarchies on the other. Serious difficulties can arise when a detailed description of vegetation is related with a classification, and an important distinction emerging from these findings is between worldwide and regional classifications. Multiple mapping at large scales evolved into a particularly useful and enlightening method. However, the often demonstrated correlation between phytocenoses and environmental conditions must not lead a researcher to falsely optimistic conclusions, as it may not be applicable in the humid tropics. Aubreville, Poore, Wyatt—Smith, Koriba, Kuchler&Sawyer, etc. have illustrated the need for caution in interpreting such correlations. An analysis of some aspects of regions demonstrated that the relationships between vegetation types and biotopes must be clarified before meaningful ecological regions can be established. This need was illustrated with the map and inset maps of the Hunter Valley region in New South Wales, which proved most revealing. The chief problem of maps in ecological regionalization was found to be the map scale. Scale problems can usually be solved without much difficulty, but they be must clearly understood if the results are not to be misleading. The very nature of the biogeocenose implies by definition that the geographical distributions of biocenoses and of biotopes are most intimately related. The various analyses revealed that vegetation may be regarded as a tangible, integrated expression of the biogeocenose. Maps showing the geographical distribution of the natural (or the potential natural) vegetation do therefore and thereby also reveal ecological regions.

ISSN:0012-9658    

DOI:10.2307/1935336

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Populations of four chydorids in the littoral of Lawrence Lake, Michigan, were studied through an entire year. Three species appeared in spring after a winter absence. Their population sizes were smallest in summer and greatest in late summer or fall. Numbers of the fourth species, Chydorus sphaericus, peaked in spring and dropped to a low level in the summer, then rose through fall to a winter plateau. Rates of population processes were estimated with the egg—ratio technique. This required monitoring littoral temperature and determining developmental times of chydorid eggs over a range of temperatures. Predicted rates of birth were higher during summer when rates of observed change fluctuated considerably. A probabilistic method, based on reproductive potential, showed relatively large probabilities for changes observed from late fall to early spring in populations of Chydorus. Most population changes in the other three species, and in Chydorus during the summer, had zero probability. Birth alone could explain few population changes. Most population loss appears to be predatory; emigration and “natural” mortality are not important.

ISSN:0012-9658    

DOI:10.2307/1935337

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

The spatial and temporal distributions of 12 species of waterboatmen in a 1.2 ha pond in Northern Michigan were studied for three consecutive summers. Enclosure experiments involving the growth of one, two, and three corixid species were performed to test for interspecies competitive interactions and to determine whether the dominant species reach the upper limits to population growth. The experimental data indicate that interspecific competition between the two dominant species, Hesperocorixa lobata and Sigara macropala, in enclosures led to a significant reduction in the population size of H. lobata but not of S. macropala. From the enclosure experiments it appears that lobata failed to grow to its maximum limit because of competition, while macropala did reach its limit, at least in late summer. The experiments also show that the early summer and late summer breeding habits of Hesperocorixa lobata and Sigara macropala respectively, are not physiologically (genetically) fixed properties of these species. It is suggested that the corixid species ensemble here studied may be characterized as two sets of ecological homologues: one set of Hesperocorixa species that breeds early in the summer and one set of Sigara species that breeds later.

ISSN:0012-9658    

DOI:10.2307/1935338

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

A breeding population of red—shouldered hawks (Buteo lineatus) along the Patuxent River in central Maryland was studied during the interval 1943—71. Numbers of breeding pairs remained unchanged or increased on the PWRC (Patuxent Wildlife Research Center) and an adjoining area where habibat was not altered. A reduction in breeding pairs occurred on the third study area where large portions of the habitat had been destroyed. Basic information on 74 nests was obtained and the annual number of breeding pairs on the PWRC ranged from a low of four to a high of nine during the study. Nesting success of this highly territorial species decreased significantly as the distance between adjacent nest sites decreased (Errington's principle of inversity). Since 1960, the recruitment rate during “high” density years was 1.34 young fledged per pair as opposed to 1.95 during “optimum” (1943 and 1947 levels) density years. The 1.95 figure compared favorably with the estimated recruitment rate necessary for maintaining a stable population and with recruitment rates observed in other locations prior to the modern pesticide era. As the observed recruitment rate during the 4 years of “optimum” density was believed adequate, it is doubtful that the relatively low pesticide levels in the eggs had a detrimental effect on the reproductive performance of the population.

ISSN:0012-9658    

DOI:10.2307/1935339

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Latitudinal trends in the species diversity of North and Central American Mammalian communities are documented by examining seven habitats ranging from the tundra of central Alaska to the moist tropical forest of central Panama. The communities are compared with respect to the numbers and kinds of species in each habitat and their size, spatial, and trophic relationships. Fifteen to 16 species occur in the two Alaskan habitats examined, 31—35 species in three forests in the eastern United States, and 70 species in two Panamanian forests, Observed latitudinal changes in species diversity, ecological diversity, and community structure are primarily a result of a southward increase in the number of bat species. The greater variety and year—round availaility of food resources such as fleshy fruits and insects, rather than increased spatial heterogeneity, appear to account for the greater diversity of mammals in the tropical habitats. The structure of Old and New World tropical mammalian communities appears to be very similar. One major difference is that among the bats there are relatively more fruit—and/or pollen—eating species and fewer insect—eating species in the Panamanian community than in Malaya.

ISSN:0012-9658    

DOI:10.2307/1935340

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

On a patch of prairie near Northfield, Minnesota, four treatment factors (early mowing and raking, late mowing, raking, and fertilizing) were each applied or not applied to plots of a quasi—Latin square. Five epigeic collembolan species (Entomobrya bicolor, Orchesella ainslei, Tomocerus flavescens, Lepidocyrtus cyaneus, Isotoma viridis) were shown to have noncongruent but overlappihg distributions on the plots. Early mowing and raking raised the number of E. bicolor, Arion fasciatus (a slug), Labidostommid mite, Lycosid spiders, and spiders in general, O. ainslei and L. cyaneus decreased, as did a species of Oribatid mite (family Belbidae). Late mowing decreased the numbers of E. bicolor, O. ainslei, I. viridis, and harvestmen but increased the number of spiders. An interaction between early mowing and raking and late mowing resulted in a decrease in E. bicolor and an increase in O. ainslei. Regression analysis indicated that slope (topography) was a good predictor of the numbers of E. bicolor. O. ainslei, and T. flavescens, positively correlated for the former, negatively for the latter two species. The number of I. viridis was positively correlated with the density of other species. The number of L. cyaneus had a high, but nonsignificant, correlation with the productivity biomass ratio. Total collembola had a significant negative correlation with standing crop (but the multiple correlation coefficient was not significant). It was hypothesized that the five collembola are competitors whose areas of residence expand and contract with seasonal climatic change because of competitive interaction and changes in their microclimate and environment.

ISSN:0012-9658    

DOI:10.2307/1935341

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY

摘要:

Three species of periodical cicadas, Magicicada septendecim, M. cassini, and M. septendecula appear to remain distinct in nature by a strong prezygotic behaviorial isolating mechanism involving habitat preferences and song differences. Two of the species will cross—mate, however, when confined in a small cage. It was shown experimentally that the eggs resulting from such cross—matings hatch into viable young of intermediate size.

ISSN:0012-9658    

DOI:10.2307/1935342

出版商:Ecological Society of America    

年代:1973

数据来源: WILEY