摘要:

The insular body size trends for different vertebrate families are compared. Certain groups such as lagomorphs, bats, artiodactyls, elephants, foxes, raccoons, snakes, and teiid and lacertid lizards are habitually represented by relatively smaller forms on islands. On the other hand, cricetid rodents, iguanid lizards, tortoises, and bears often have races with larger body sizes on islands. Contrary to conventional niche theoretic concepts, in many instances knowledge of the body sizes of some of these animals' insular and mainland competitors does not help explain the difference in that species body size in the 2 places. To account for these divergent size changes I examine optimum body size models that use as the optimization criterion the net energy gained by an organism over a given time period. These models predict that increases in the mean amount of available food should lead to evolutionary increases in body size, but only if body size is not tightly constrained by additional physical or biotic factors: such additional factors might be important if a change in body size alters an animal's effectiveness in finding or handling preferred food items or increases competition with its neighbors. Next using arguments derived from simple non—age—structured 2 species predator—prey models, the availability of food for a given consumer species at equilibrium is compared in theoretical island and mainland situations. Because islands usually contain fewer competitors and the insular physical environment is often more moderate, food availability for colonists is initially expected to be high. On the other hand, as the population grows resources will become depleted. Further, the loss of many predator species on islands may allow consumer densities to increase to such an extent that at equilibrium food may become relatively more limiting for consumers on islands than on the mainland. Whether the supply to demand ratio (S:D) of consumers for their food is ultimately greater or lower will depend on the relative magnitude of these various factors. Within this framework, a necessary condition for island S:D ratios to be greater than on the mainland is that the consumers maintain individual feeding territories. For animals whose body sizes are not tightly bound within narrow limits by physical or competitive restraints, an increase in S:D should lead to an evolutionary increase in body size. Accordingly, a good association is found between the presence or absence of territoriality and the direction of the insular body size shift in a number of different vertebrate groups. Yet there are exceptions which fall into 2 categories: First, if a species' mainland predators preferentially take larger individuals, selection favoring small size may override selection based on optimizing energy input. Such may have been the case for the now extinct mainland relatives of certain giant relictual insular reptiles. Secondly, an animal's body size may be tightly constrained by physical or competitive factors. The body size of island foxes, rattlesnakes, and some lizards appears to be primarily adjusted to the competitive milieu along typical niche theoretic lines. That is, body size may be predicted quite well from knowing the size class of competitors which are absent from an island or from differences in the species' prey—size distribution between island and mainland sites.

ISSN:0012-9658    

DOI:10.2307/1936628

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Paleolimnological techniques were used at Linsley Pond, North Branford, Connecticut, USA to compare changes in the biota with changes in watershed land use as documented by municipal land records. Fossil diatoms, midges, zooplankton, and chemical composition were examined in a core 238 cm long, which was dated by(210)Pb,(14)C, and changes in fossil pollen assemblages. The beginning of farming in 1700 caused only minor changes in the lake biota. In 1915 changes in agricultural activity at a nearby farm caused a rapid shift to eutrophic diatom and midge assemblages. In 1960 major changes occurred in the zooplankton community in response to the hypereutrophication caused by the construction of a suburban housing development. Fluxes of Fe, Mn, Ca, Cu, and P to the lake sediment seem to be controlled by changes in soil erosion–not by changes in the trophic status of the lake.

ISSN:0012-9658    

DOI:10.2307/1936629

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

A preliminary simulation model of primary productivity and carbon allocation in creosotebush (Larrea tridentata [DC] Cov.) is described. The model utilizes a systems approach in which movement of assimilate within the plant is in response to changes in source—sink strengths of leaves, stems, roots, early reproductive buds, maturing reproductive buds, flowers, and fruits. Two distinct compartments are defined per organ or developmental stage to separate assimilate into a pool fraction (labile or translocatable) and a structural fraction (nonlabile). The changes in magnitude (within upper and lower limits) of a pool compartment during the course of a simulation (i.e., growth and development of the plant) are a function of the rates of maintenance respiration and growth as well as a priority scheme governing allocation of assimilates; the increases and decreases in dry weight of a structural compartment are a function of aging and the magnitude of its associated pool (which determines structural growth and physiological death). A 1—yr simulation of a hypothetical Larrea plant shows that the model exhibits a reasonable behavior, although no validation is attempted at this stage in its development. The heuristic value of the model is illustrated in the sensitivity analysis, which shows the need for detailed knowledge ofpriority carbon movement during both vegetative and reproductive growth periods, the importance of substrate controlled respiration rates, and the need for further studies of the dynamics of labile pools in the plant. The model has been an excellent tool in our initial attempt to integrate the voluminous information on Larrea into a complete functional description of the autecology of the species. Further refinement of this model as data from our current research and that of other investigators become available should lead us to be better understanding of the ecological role of Larrea in desert ecosystems.

ISSN:0012-9658    

DOI:10.2307/1936630

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

A transient behavior analysis of the population dynamics of an herbaceous plant species (teasel, Dipsacus sylvestris Huds.) was developed to contrast with the long—term limiting response analysis of an earlier study. Following the arrival of a cohort of seeds at a site, there is typically an immediate increase in the rate of growth of rosette cover and the rate of seed production, followed by a decline several years later. This decline, caused by shifts in the stage structure of the population, may be critical in determining success or failure of a colonization episode. Simple calculations of the rate at which a population of Dipsacus might expand in space show that density effects on numbers of adult plants will become important in 15—20 yr. This conclusion supports the use of transient analysis in addition to the standard limiting behavior analysis, which assumes a constant environment over long time intervals. The reproductive value of Dipsacus increases exponentially from one stage to the next, and as a power function of accumulated biomass. A hint of a sigmoid response of reproductive value to biomass suggests that the transition from the vegetative rosette to the flowering stage may have evolved in response to the rate of accumulation of reproductive value as plant biomass increases. The response of Dipsacus to a variety of hypothetical changes in life history parameters was examined. A general method of calculating the sensitivity of population growth rate (lamda)_m was developed; when applied to the teasel model it showed that (lamda)_m is most sensitive to changes in the growth and reproduction of early stages. Delayed seed germination is not important to long—term population growth; the complete elimination of delayed germination from the model caused no more than a 4% decrease in (lamda)_m. It may be much more important in terms of transient behavior; elimination of delayed germination depressed the initial decline in rosette cover by up to 72%. If this decline delays the colonization process, the most important populational effect of delayed germination would be an increase in the probability that an attempted colonization will be successful. We also examined the optimality of the current biennial life history of Dipsacus, relative to both annual and perennial alternatives. The shift to an annual strategy is rendered suboptimal by the low probability of germination and survival of seeds relative to the probability of survival of rosettes, and by the high growth potential of rosettes. This growth potential increases the energy available for seed production by an order of magnitude over the hypothetical annual strategy, greatly increasing the reproductive value of the biennial flowering plants. The switch to a perennial habit is suboptimal because of the cost, in annual reproductive output, of maintaining a perennial flowering plant. This effect is greatest in rapidly growing populations. There seem to be no important differences between the transient and the limiting behavior analyses of life history optimality. Our direct, rather than inferred, demonstration of the optimality of teasel's life history supports the development of current theory based on optimality assumptions

ISSN:0012-9658    

DOI:10.2307/1936631

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Existing overlap indices are examined and are judged to be inadequate on the grounds (1) that they lack simple and appropriate biological interpretations, and (2) that they ignore possible variation among resource states in abundance or availability (a_i). Two indices interpretable in terms of encounters are proposed. Niche overlap (L) is measured as the degree to which frequency of interspecific encounter is higher or lower than it would be if each species utilized each resource state in proportion to its abundance (a_i). Directional overlap (Z_(xy)) is measured as the density of species Y encountered, on the average, by an individual of species X. When resource states are equal in size, L is equivalent to Lloyd's `interspecies patchiness,' and Z_(x(y)) is equivalent to his `mean crowding on species 1 by species 2.' Indices which in corporate variation in resource state abundance are also developed for mean crowding, patchiness and niche breadth.

ISSN:0012-9658    

DOI:10.2307/1936632

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

The energy dynamics of 2 colonies of yellow—bellied marmots (Marmota flaviventris) were studied in the Rocky Mountains of central Colorado in 1969 and 1970. The Intake—Rejecta and Maintenance—Production models, which included an analysis of seasonal variations in energy flow parameters, yielded similar estimates of population energy flow. Colony energy flow ranged from 64.0 to 94.6 KJ°m(—2)°yr(—1). Differences between colonies in annual energy flow can be explained by variations in biomass. Peak energy flow occurred at different times in the 2 colonies and the timing was related to reproductive conditions. The marmot populations consumed 94.6 to 119.2 kJ°m(—2)°yr(—1), which represented 0.8 to 3.1% of the aboveground primary production. The efficiency with which the marmot populations exploited the available net primary production was 2 to 6.4%. Seventy—one to 75% of the energy ingested by the populations was assimilated; only 77% of the assimilated energy went into maintenance of the population biomass. Tissue growth efficiency averaged 16.8%, °5x greater than typical homeotherms. The production/maintenance ratio averaged 29.6%. The marked differences in the respiration efficiencies, tissue growth efficiencies, and production: maintenance ratios between the heterothermic marmot and typical homeotherms suggest that heterothermy represents a distinct strategy in secondary production.

ISSN:0012-9658    

DOI:10.2307/1936633

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

This study examines microhabitat configurations for 3 small mammal species occupying second—growth mesic forest on Walker Branch Watershed in eastern Tennessee. Small mammal populations were sampled in oak—hickory, chestnut oak and pine forest types, with three 0.36—ha live—trapping grids per forest type. There were 255 total captures for 4 forest—floor species during 9,696 trap nights between 1 May and 16 August 1973. These captures included 2% Blarina brevicauda, 66% Peromyscus leucopus, 5% Ochrotomys nuttalli and 27% Tamias striatus. Blarina was captured most frequently in the pine forest type, Peromyscus and Tamias most frequently in the oak—hickory, and Ochrotomys exclusively in the pine. Twenty—nine variables describing proximate habitat structure were measured at each sample mammal capture site. There were significant overall species differences on 18 of these variables. A subset of variables was identified for each of the 3 most frequently observed species, consisting of only the variables of particular significance for distinguishing the capture sites of that species from those of the other species collectively. Discriminant analysis of these few variables both described the average microhabitat configuration, for a species and indicated the relative importance of each variable for characterizing or describing that configuration, subject to the assumptions of the analysis. The observed differences between species microhabitat configuration are consistent with both the composition by forest type of each species sample and the literature information for each species. Microhabitat segregation among these species is discussed with reference to recent theories of habitat selection, ecological, specialization and resource partitioning in equilibrium faunas.

ISSN:0012-9658    

DOI:10.2307/1936634

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Six species of darters (Etheostoma) were studied in the field and laboratory to relate respiration to habitat selection. The species ranged in habitat preference from a fast—water form (Etheostoma rufilineatum) to a slow—water form (Etheostoma fusiforme). Field measurements showed that winter O_2 tensions were not limiting the distribution of the darters when these tensions were compared with the critical O_2 determinations made in the laboratory at a low temperature (10 degrees C). However, summertime ambient O_2 levels were as low as 3.3 ppm [= 7.33 kilopascals or 55 mm Hg], a level that was 1.0—1.7 ppm below the wintertime critical O_2 tension (P_c) of several of these species. The fishes compensated for this by lowering P_c in the summer. This finding is contrary to previously reported results on the relationship between P_c and temperature. While not all darters selected habitats such that the downward shift of P_c in the summer was of ecological consequence, this was the case in 1 of the slow—water stream forms (E. boschungi) which inhabits waters in the summer that would be uninhabitable without such a shift in P_c. Additionally, one of the darters that typifies fast—water habitats (E. rufilineatum) had a P_c of 6.1 ppm [= 14.0 kilopascals or 105 mm Hg), which prevented it from occupying slow waters in the summertime. However, such respiratory exclusion from available habitats is not the case in the other stream forms studied (Etheostoma flabellare, Etheostoma squamiceps, and Etheostoma duryi), nor do these other stream forms occupy waters as slow as they are capable of on the basis of respiratory abilities. Therefore, a limited tolerance to low O_2 levels at summertime temperatures is not the general cause of the preference of Etheostoma for streams, although this is the case in some specific instances. In addition, the stillwater from (E. fusiforme) does not have any respiratory advantage over many of the stream forms in terms of a low metabolic rate or low P_c. However, darters as a group do show a rather low metabolic rate when compared to freshwater fishes in general.

ISSN:0012-9658    

DOI:10.2307/1936635

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Binary discriminant analysis (BDA) is a readily understood, easily used technique for identifying binary variables and their common trends which are most important for discriminating between groups. For the plant ecologist, the technique can be used on species lists to reveal similar patterns of preference or avoidance among species responding significantly to a multistate environmental parameter such as soil type, rock type, or aspect. In Q—mode BDA, orthogonal canonical factors are obtained which represent uncorrelated floristic trends best separating the groups. Scores of species on the factors can be plotted in multidimensional hyperspace, showing how each species responds to the floristic trends. In R—mode BDA, groups of species with similar responds to the environmental parameter are identified. These groups may be interpreted as statistical associations or community components comprised of species with similar ecologies. An example using lists of woody species from the Maryland Piedmont and Coastal Plain sorted according to underlying rock type produces floristic trends which are easily interpreted and species groups which are readily understandable.

ISSN:0012-9658    

DOI:10.2307/1936636

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

A linear model is developed for species dissimilarity. The model, which potentially can be used in conjunction with any species dissimilarity index, is designed for data sets which involve both multiple species and multiple environmental variables. The total species dissimilarity is divided into components with 1 component being assigned to each environmental variable or interaction of environmental variables. Unbiased estimates for the variance of each component are provided.

ISSN:0012-9658    

DOI:10.2307/1936637

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY