摘要:

Convergence in community structure, under similar long—term climatic conditions, were investigated using taxonomically unrelated lizard communities in Mediterranean climatic areas in central Chile and southern California. The hypothesis of convergence, i.e., physiognomically similar sites in the two continents should have lizard community structures that are more similar to each other than to the structure of lizard communities in nearby areas in the same transect, was tested by comparing the structure of lizard communities in an altitudinal–vegetational transect in each continent. Each transect consisted of a coastal sage, chaparral, and forested montane site. Structure of the lizard communities was characterized by the patterns of habitat, food, and time of activity of the lizard species. For each of these dimensions the total range of resources used by the community as a whole was determined, as well as the utilization pattern of individual species. Utilization patterns of individual species were expressed in a multidimensional vector in which the angle between vectors representing these patterns was used as a measure of ecological similarity between species. The vector method allowed determination of species pairs exhibiting the most similar patterns of resource utilization. The comparison strongly supports the convergence hypothesis. For habitat and time of activity, range of resources used as well as utilization functions of individual species were more similar in equivalent sites in the two continents than in nearby sites in the same continent. Thus, on the average the most similar species (species analogue) is on the equivalent site in the other transect and not in the same continent. Also, in general, the proportions of lizards found, irrespective of species, are the same on analogous resources in the two continents. Habitat species analogues found by comparing sites also similarly partitioned food resources. However, there are some differences in food categories used, since two of the Chilean species eat some vegetation (fruits), but their Californian size—habitat counterparts do not. In addition, the two chaparral sites show very similar mechanisms of generation replacement such that adults go into aestivation by the time hatchings appear. Evidence supporting the hypothesis of a seasonally shrinking food supply as the cause for this pattern is also given.

ISSN:0012-9658    

DOI:10.2307/1936394

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Changes in the plant community following a selection timber harvest in an all—aged pine—hardwood ecosystem were studied for 11 yr on 218 hectares of rolling uplands in central Louisiana, USA. Pine and selected hardwoods were harvested. Prior to the study, stands were harvested at 10—yr intervals by the single—tree selection method. Fire had been excluded for 30 yr. Principal tree species were Pinus taeda, Pinus echinata, Quercus alba, Quercus falcata, Nyssa sylvatica, and Liquidambar styraciflua. In the shrub layer pertinent species were Callicarpa americana, Vitis rotundifolia, Acer rubrum, Vaccinium elliottii, and Ostrya virginiana in association with reproduction of the principal tree species. In the absence of fire and hardwood arrest, periodic selection harvest, primarily of pine, was enhancing a definite shift from pine to hardwood. In the postharvest stand hardwoods averaged 84% of the tree density and 49% of the basal area. Residual pine in the dominant and codominant canopies will be harvested at the next cutting with hardwoods largely being potential sucessors. The density of young pine<21.72 cm dbh (diameter at breast height) in the transgressive layers averaged 67.0%. Of the major hardwood species 93.5% of the average density consisted of young replacement trees<21.72 cm. Among trees<36.96 cm dbh, the percentage importance of hardwoods, as a group, was higher than the value for pine. In the shrub layer the relative dominances of pine regeneration progressively increased for 9 yr then declined appreciably as the species became intolerant to the high level of community competition. The net primary productivity (NPP) of herbaceous and woody species in the herb and shrub layer crested 2 yr after timber harvest. At this time NPP of the relatively sparse herbaceous dry matter averaged 111.9 kg/ha and that of woody species was 327.6 kg. Thereafter, NPP decreased progressively as many stems of tree reproduction grew above the shrub layer and the density and basal area of the tree stratum increased. Within 11 yr after the stand disturbance herbaceous growth was very sparse, with a NPP of 21.0 kg dry matter/ha. NPP of woody species declined to 116.6 kg dry matter/ha. The vegetative response among woody species in the herb and shrub layer after logging was primarily a general growth decline with increasing competition and not an elimination of established species nor an introduction of new species.

ISSN:0012-9658    

DOI:10.2307/1936395

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The well—established methods of network construction and analysis are adapted to the problem of modeling single populations. A major advantage of the resulting approach is that it allows explicit incorporation of key processes in the life cycle of the organism being modeled, with feedback loops providing economy of representation where they are allowed. Thus, network structures provide heuristic vehicles by which population models can be developed and modified. When a model is linear and has parameters that do not vary with time, a characteristic dynamic function can be derived by inspection from a simple transform of the network representation. The zeros of the function can be found (analytically or by commonly available numerical methods) and used directly to deduce the modeled population's dominant growth pattern and its propensity to sustain oscillations. In addition, under certain conditions (i.e., that the network model not contain both time delays and integrators), a straightforward mathod (partial fraction expansion) is available for deduction of the modeled population's specific responses to a variety of perturbations.

ISSN:0012-9658    

DOI:10.2307/1936396

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Three colonies each of two species of leaf—cutting ants (Atta colombica Guer. and Atta cephalotes L.) were studied for 1 yr in the Guanacaste Province of Costa Rica to determine the following: (1) Do colonies of Atta cut leaves from a limited number of species (i.e., are they selective)? (2) What determines which plant species are attacked? (3) Do leaf—cutters optimize foraging in terms of energy expended by attacking palatable plants closest to the nest or do they distribute their efforts more or less evenly throughout their foraging territory? Several line of evidence support the hypothesis that A. cephalotes and A. colombica are selective in terms of plant material attacked. The ants sampled a large majority of the plant species present, but concentrated their foraging on a restricted subset of species. The mature leaves of only 31.4% and 22.0% of the species present were readily acceptable to A. colombica and A. cephalotes, respectively. The new leaves of another 12—16 plant species were acceptable. Different colonies of the same Atta species consistently attacked the same plant species if they were available, cut leaves from the same species at similar rates, and even attacked the same species of plants at the same time of year. The amount of material cut from host plant species was not correlated with host plant abundance in four of the six colonies studied, and was even slightly negatively correlated in two of these four. An analysis of plant distance from the nest versus amount harvested shows that palatable plant species close to the nest have a greater probability of being visited by the ants, but do not necessarily suffer more defoliation than other such plants within 50—60 m of the nest. Amount harvested decreases greatly for plants farther than 60—80 m from the nest. Thus colonies of Atta do not normally concentrate their efforts on plants closest to the nest, but foraging cannot be described as evenly distributed either. The factor responsible for selectivity in colonies of Atta is probably the internal chemistry of the material selected. An Atta colony must provide the fungus garden when a proper balance of nutrients and moisture without overlapping it with secondary compounds from the plants selected. The ants' attempt to solve this problem may explain some of the complexities of leaf—cutter foraging behavior.

ISSN:0012-9658    

DOI:10.2307/1936397

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

In a 2—yr study of the survival of Glaucous—winged Gull (Larus glaucescens) chicks, pecking of trespassing chicks by neighboring adults was the major cause of chick mortality. In years of both low and high food availability chick survival was strongly correlated with growth rates. Chicks that grew slowly were more likely to be killed by neighbors than fast—growing chicks. In the year of low food availability, among slow—growing chicks, those hatched early in the season on large territories had better survival rates than chicks hatched late in the season on small territories. In the year of high food availability, timing of breeding and territory size had little bearing on chick survival. A model of chick survival in relation to timing of breeding predicts that when neighbor interference is the major cause of chick mortality, chicks hatched early in the season will have the highest probability of surviving. When predators are the major cause of chick mortality, chicks hatching in the middle of the breeding season will have highest survival. A second model relates chick survival to territory size. Chicks raised on large territories will most likely avoid neighbor interference, while predation will select for either small or large territory size, depending upon the effectiveness of group mobbing against the predator.

ISSN:0012-9658    

DOI:10.2307/1936398

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Resource exploitation and competition were measured for 13 herbivorous stem—boring insects inhabiting a climax prairie in Illinois. Nine species overlapped>70% in their exploitation of plant species, stem sizes, and locations within stems, but competition was detected between only two species. In this case the competition resulted from aggression rather than exploitation for limited resources, and exclusion may be delayed or prevented by specific behaviors or refuges. To quantify how closely the guild members were packed upon the available resources, probabilities of coccurrences between species were calculated using resource abundances, population sizes, and overlaps in resource exploitation. The species packing averaged over the entire guild was 30%, but some species had several potential competitors and probabilities of cooccurrences were>60%. This species structure offers the conditions for diffuse competition, but competition seldom occurred because stems could support several individuals and resources were generally nonlimiting. Although the potential for competition is high and species probably compete occasionally, the high resource overlap and species richness that competition has not been a major organizing force within this guild. When factors known to influence competition are considered, it seems possible that competition has been too infrequent, mild, or nondirectional to cause strong selection for competitive avoidance or displacement. This guild demonstrates that overlap values may not equal competition coefficients and that high overlap may exist because competition is rare. This guild is probably not unique in lacking competitive organization; herbivorous insects may be among the least likely groups to exhibit the patterns predicted by competition theory.

ISSN:0012-9658    

DOI:10.2307/1936399

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Soil arthropod response to altered litter production in a Wisconsin prairie was examined after a long established biennial spring burning program was modified by substituting raking for burning in some experimental blocks, and substituting no disturbance for burning in others. Very few species were favored by increased surface litter in undisturbed blocks. After two biennial burnings or rakings, a drop in species numbers, density of individuals, and diversity of herbivore—decomposers occurred in undisturbed, and to a lesser extent, in raked blocks. Predatory soil arthropod species showed no response. These results suggest that soil detritus levels dropped slowly in response to decreased plant productivity in undisturbed blocks. The drop in soil arthropod community diversity which occurred in undisturbed blocks indicates a direct productivity—diversity relationship. Other field experiments show the same direct productivity—diversity relationship among consumers or terrestrial producers, but an inverse relationship among aquatic producers.

ISSN:0012-9658    

DOI:10.2307/1936400

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

A 29—compartment model of the kinetics of calcium in an east Tennessee Liriodendron forest is developed. The model structure is based on the functional components of the Liriodendron forest determined after more than a decade of intensive studies on the structure and function of the forest. Parameters for the model and initial conditions for the state variables were determined from coordinated ecological studies in the Liriodendron forest. These studies included characterization of chemical pools, quantification of ecosystem processes and analyses of mineral transport dynamics using radioactive tracers. The calcium model was analyzed for stability and controlability using frequency response analysis, a classic control theory technique. The model was stable for each of the 29 possible feedback loops connecting an ecosystem monitoring point (e.g., Ca content of soil H2O) to an input source (e.g., atmospheric deposition). Certain monitoring points in the system, although they resulted in stable responses, caused natural variations in the system input to be amplified. The results proscribe arbitrary assignment of components as ecosystem monitoring points, and identify this potential problem in ecosystem management.

ISSN:0012-9658    

DOI:10.2307/1936401

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Data on dry matter and N, P, K, Ca, and Mg contents and movements in soil and ground vegetation in a 30—yr—old natural Pinus banksiana Lamb. stand in northern Ontario are presented, together with previously published aboveground data. Dry matter and N, P, K, Ca, and Mg contents of trees were 90,700; 165; 14; 82; 112; and 18 kg/ha, respectively–less than in many Pinus spp. of comparable age in other parts of the world. A comparison with nutrient contents in younger (20 yr) and older (65 yr) stands on the same site indicated that most nutrient accumulation in vegetation takes place in the first decades of stand development. Annual nutrient uptake by all vegetation was 32 kg N, 2 kg P, 18 kg K, 21 kg Ca, and 3 kg Mg per hectare. Between 71% and 89% of the elements taken up annually by trees were returned to the soil by litterfall and leaf wash. There was a buildup of forest floor organic matter with stand age and N, P, and Mg accumulated in this material faster than the other elements. Soil reserves of plant—available or mineralizable nutrients were maintained by nutrient replenishment from precipitation and vegetation—soil nutrient cycling. Thus, we conclude that removal of elements from the ecosystem by logging or burning should not result in significant impoverishment of this site.

ISSN:0012-9658    

DOI:10.2307/1936402

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

The total daily O_2 consumption rate of the intertidal oyster reef community in Georgia, USA was found to range between 0.6 x 104and 5.0 x 104mg O_2/m2of reef surface, corresponding to a seasonal range of ambient water temperature from 9 degrees C to 30 degrees C. Oysters, nonoyster macrofauna, microbiota, and chemical oxidation were estimated to account for 48.1%, 10.0%, 21.9%, and 20.0%, respectively, of total O2consumed. Reef macrofaunal biomass averages 1,108 g/m2(ash—free dry weight) of which Crassostrea virginica comprises 87.5%. Growth of individual reef oysters, and, therefore, entire reefs, appears to be extremely slow, probably due to high maintenance costs and limited inundation time characteristic of the intertidal zone. The oyster reef community occupies only 0.06% of total marsh—water surface area in a salt marsh estuary unit (the Duplin River marsh—estuary system), but it can theoretically degrade 1% of the estimated excess annual net primary production in the system.

ISSN:0012-9658    

DOI:10.2307/1936403

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY