摘要:

In this paper I develop a graphical method for visualizing and analyzing the effects of nutrient enrichment on the equilibrium characteristics of 1—predator, many—prey systems. The system considered consists of 1 nonswitching, prey—limited predator feeding on an array of prey species which complete for a single resource. The replacement of 1 prey species by another as eutrophication proceeds is an integral part of this model. The model predicts that prey species which can survive at low nutrient densities should be replaced at higher nutrient densities by species with high maximum rates of increase and by species more resistant to predation. This prediction may be reversed when the predator's population size is controlled by mechanisms which inhibit the predator's ability to respond numerically to increased prey density. Both predictions can be understood in terms of shifts in the relative importances of competition and predation as the nutrient status of a system changes. The graphical method presented should be particularly adaptable to empirical studies of species change along gradients of nutrient enrichment.

ISSN:0012-9658    

DOI:10.2307/1936470

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Carbon dioxide exchanges, plant—water relations, and some morpho—anatomical characters were measured and compared between 3 elevational populations of Espeletia schultzii in the Venezuelan Andes. The paramo climate is characterized by large oscillations in temperature on a daily basis. Precipitation is strongly seasonal with rainy summers and dry winters. The elevational gradient chosen in the Sierra Nevada de Merida extends from 3450 to 4200 m and is characterized by an increase of solar radiation, wind velocity, evaporative forces, and a decrease of temperature and water availability from low to high elevation. Contrasting with temperature alpine areas, the growing season in the paramos extends throughout the entire year. Thus, the maintenance of adequate leaf temperatures on a daily basis and an adequate moisture balance on a seasonal basis are more critical than accumulating resources during a shortened growing season. Morpho—anatomical characters of E. schultzii such as thick leaves, dense pubescence, and water—storing parenchyma play important roles in regulating water loss by increasing water vapor diffusion resistance and buffering the periodic water stress during the dry season. The caulescent life form of the plant elevates the rosette of leaves above the soil surface avoiding extreme ground temperatures. Pubescence and thickness of the leaves may help to control leaf temperature by increasing albedo and heat capacity. The importance of such characteristics is evidenced by their increased prominence in plants of higher elevations. Photosynthetic rates, obtained in the field, using14CO2on mature leaves, were relatively low (2—4 mg Co2°dm—2°h—1) but correlate well with the slow growth rates and generation times reported. These low rates are not so disadvantageous in the paramos due to the extended growing season. Pubescence and density of the leaves probably have negative effects on photosynthesis by decreasing the photosynthetic active radiation (PhAR) reaching the chloroplasts and increasing resistance to CO2diffusion. Controlled conditions in the laboratory produced different responses of the populations regarding photosynthesis. Under cold acclimation, high elevation populations showed highest rates. Under warm acclimation, low elevation populations showed highest rates. Also, different degrees of homeostasis were found among the populations which were correlated with their elevation of origin. The populations of E. schultzii are further differentiated by the degree of drought resistance; the higher elevation population being the most resistant. Among the populations, differences in density—thickness and leaf pubescence are maintained even under uniform laboratory conditions. The maintenance of differences among the populations of E. schultzi even under uniform conditions indicates that they represent genetically based ecotypes.

ISSN:0012-9658    

DOI:10.2307/1936471

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Uniformly14C—labelled glucose was used to estimate maximum potential uptake kinetics (WMAX) in unfractionated and<64 μm, andaspartic acid and alanine. Glycolic acid was respired most rapidly, followed by aspartic acid, lactic acid, alanine and glucose. Most of the particulate organic carbon (POC), adenosine triphosphate (ATP), and platable bacterial cells (CFU) were in the 10 μm fraction. Glucose VMAXvalues were significantly correlated with ATP and CFU, and lactic acid VMAXvalues with CFU only. VMAXand turnover times in the unfractionated water were 57% lower and 29% greater, respectively, than in the<64 μm fraction sample, and (K / Sn) values were at times very dissimilar among fractions of the same water. These results were attributed to the physical separation of the plankton community, nutrient fluctuation in the sample, and microheterotroph diversity and distribution on particles.

ISSN:0012-9658    

DOI:10.2307/1936472

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

This study examines the relationships among 4 small mammals species in 2nd—growth mesic forest on Walker Branch Watershed in eastern Tennessee, USA. Populations of Blarina brevicauda, Peromyscus leucopus, Ochrotomys nuttali and Tamias striatus were live—trapped on 9 0.36—ha grids during the summer of 1973. Eight measures of physical habitat structure were used in discriminant analysis of the microhabitats occupied by these 4 species. Three statistically significant discriminant functions were calculated. Each discriminant function is represented as an axis in a 3—dimensional discriminant space. These 3 axes are interpreted as vegetation type, vegetation structure and litter—soil surface characteristics. The positions of the species in the discriminant space characterize the microhabitat configurations of the species relative to the ecological properties attributed to the axes. Each species differs significantly from every other species on at least 1 axis. The observed species differences are conservative estimates of microhabitat (structural niche) segregation. We propose measures of niche position (i.e., exploitation speciality) and niche breadth based on the discriminant analysis. Species i is represented by a cloud of nisample points in the discriminant space. Sample point j for species i lies the distance dijfrom the origin of the space. The average of these distances (°dj) represents the average "position" for species i relative to the origin. Because this origin represents the average of the microhabitats sampled on the watershed, and because the microhabitats actually occurring on the watershed are assumed to vary continuously, the likelihood of a species encountering favored microhabitat on Walker Branch decreases as °d increases. This ° is, thus, interpreted as an index of niche position relative to the average of the microhabitats sampled. Variability among the dijvalues for species i (vi) measures degree of specialization, a direct measure of niche breadth for species i. Our data indicate that °d and v are inversely related: breadth decreases as position becomes increasingly specialized. We incorporate measures of niche position, niche breadth and population abundance into an analysis of community "niche pattern." This niche pattern characterizes the 4 species as follows: Peromyscus is an abundant generalist, well—adapted to the watershed as a whole. Ochrotomys, the only other mouse, is a relatively rare specialist, poorly adapted to the watershed. Tamias occupies an intermediate position between Peromyscus and Ochrotomys, and exhibits intermediate abundance. Although we have little data for Blarina, this rare species appears to be poorly adapted to the watershed. Species differences in niche breadth appear to be determined more by the relative frequencies and carrying capacities of the species exploitation specialties than by the relative efficiencies with which the species exploit some critical limiting factor(s). Although we have no experimental evidence, the niche pattern observed for this community is consistent with an interference of competitive coexistence. The niche parameters of transient species which are infrequently encountered on Walker Branch are briefly discussed.

ISSN:0012-9658    

DOI:10.2307/1936473

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

In sampling 22 lakes in the Upper Peninsula of Michigan, I found that 2 species of Chaoborus, C. punctipennis and C. americanus, never co—occurred (von Ende 1975). Chaoborus punctipennis was found only in lakes with fish whereas C. americanus occurred along in stained, bog lakes without fish. Third and 4th instar C. americanus lack extensive diel, vertical migratory behavior and are found near the surface waters in stained bog lakes. Adults of this species emerge in the middle of May. Third and 4th instar C. punctipennis exhibit diel, vertical migration. They are benthic during the day. This species emerges at the end of June. In situ rearing experiments indicate the Chaoborus americanus larvae can survive in lakes with fish, when isolated from the fish. Fish (Umbra limi) added to a lake with C. americanus eliminated this Chaoborus species from the lake. It is concluded that the absence of C. americanus from lakes with C. punctipennis is due to fish predation on the older larvae of C. americanus. Chaoborus punctipennis adults are able to disperse to lakes from which their larvae are absent. In situ experiments indicate C. punctipennis can survive on the zooplankton in a stained, fishless bog lake, but is subject to heavy predation by 3rd and 4th instar C. americanus larvae. It is concluded that because of early recruitment by C. americanus, as well as its lack of extensive vertical migration, this species excludes C. punctipennis from stained, fishless bog lakes.

ISSN:0012-9658    

DOI:10.2307/1936474

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Data on the years in which fires burned, on fire frequency, and on intensity and areal extent of fires were gathered from 935 scars on 220 stumps of mixed conifer forest species in an 1800—ha study area in the Sierra Nevada, California, USA. Before 1875, fires scarred clusters of living trees every 9 yr on west—facing slopes at Redwood Mountain and every 16 yr on east—facing slopes. Mean fire—free intervals between 1700 and 1875 varied by habitat phase from 5 yr in ponderosa pine on a dry ridge to 15—18 yr in most moist sites with white fir. For most 1—ha sites, the maximum time without fire was 14—28 yr. From 1700 to 1875, fires on various sizes were found every 2—3 yr somewhere in a given drainage (not necessarily the same site) and every 5—9 yr in 3— to 16—ha sites. This compares with fires every 8—18 yr in 1—ha clusters and 11—39 yr on individual trees. Scar records of pre—1700 fires suggest intervals fairly comparable to those from 1700 to 1875. Evidence of fires diminished greatly after Indian burning was eliminated in the early 1870's, and such fire records became almost nonexistent after 1900, when fire suppression became more effective. Most of the pre—1875 fires were small and of low intensity. Even the larger fires were usually confined to 1 slope or 1 drainage area. The short mean intervals between fires suggest that pre—1875 mixed conifer forests did not usually have heavy accumulations of litter or dense thickets of understory trees. Instead, small—acreage, low—intensity surface first must have consumed accumulated litter at frequent intervals and at the same time killed most of the conifer regeneration which had become established since previous fires. Such frequent fires would have led to an intricate mosaic of age classes and vegetation subtypes which, in turn, insured that a subsequent fire would not burn large areas with great intensity. Intense fires which moved from crown to crown were absent in the study area for the past 400 to 2000 yr. If frequency of lightning ignition of fires over the past 50 yr is typical, ignitions by Indians must have augmented lightning—caused fires to yield the pre—1865 frequency of fires in the Sierra mixed conifer forest. Since 1900, the lack of frequent, low—intensity fires has resulted in a major increase in understory forest and fuels.

ISSN:0012-9658    

DOI:10.2307/1936475

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

Herbicide treatments (2, 4, 5—Trichlorophenoxyacetic acid) of 0.225 g/m2and 2.250 g/m2were applied in a Latin square design to the forest floor of an oak—hickory forest in Georgia. White oak (Quercus alba) leaf litter decomposition and litter and soil microarthropod populations were studied in control and treated areas. Weight loss of natural white oak leaf litter averaged 35% for the 1st yr and reached 67% after 2 yr. Litter decomposition rates were similar in control and treated areas. However, white oak leaves defoliated through stem injections lost 59% of their biomass in 1 yr. Decomposition was increased because of initially higher N levels and a lower C:N ratio in defoliated leaf tissue. Herbicide spray treatment increased microarthropod densities for up to 8 mo after application. The herbicide spray apparently influenced springtail and mite reproductive activity in treated areas. The increases in soil and litter microarthropod densities were not sufficient to affect leaf litter weight losses. Repeated defoliation of plant communities and the resulting increased decomposition rates could cause imbalances in litter dynamics and cycling processes. Changes in the timing of annual litterfall and the rates of nutrient release may thus lead to soil desiccation and the depletion of available nutrient pools.

ISSN:0012-9658    

DOI:10.2307/1936476

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

A conceptual model of Anolis—habitat interactions is proposed to explain the existence of the series of Anolis ecomorphs in the Greater Antilles. The use of perches, type of movement among perches, and the morphology of 7 species of Anolis lizards from 2 sites in Haiti were compared. Movements of anoles through the habitat are assumed to be constrained more by the dispersion of perches rather than the characters of the perches themselves. The habitat is viewed as consisting of three—dimensional structural matrices of perches differing primarily in the diameters, lengths, and distances apart of the perches. Species occupying divergent types of perch matrices are shown to differ in: (1) the rapidity of moves on surfaces, (2) the frequency of jumps between surfaces, and (3) the distances jumped between surfaces. These species can be separated into jumpers, runners, and crawlers based on movement type and various body and limb proportions. The horizontal and vertical distribution of each species within a habitat gradient coincides with the distribution of the particular perch matrices to which the species is adapted. The varying degree of restriction imposed by perch matrices may have important evolutionary implications, particularly for the dispersion of size classes within the habitat and hence for species' exploitation strategies. Applications of this approach to aboreal primates and foliage—gleaning birds are suggested.

ISSN:0012-9658    

DOI:10.2307/1936477

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

During 1976, midsummer reductions in densities of Daphnia pulicaria and Daphnia galeata mendotae populations occurred in Wintergreen Lake, Michigan. Previous attempts to determine the causes of such declines have indicated correlations between predator populations, the presence of unpalatable algae or high midsummer water temperatures, and the reduction or disappearance of the daphnid populations. In this study, in situ life table experiments were performed to assess the influence of natural food and temperature conditions on population growth. A weekly sampling program showed that D. pulicaria was dielly concentrated, after early summer, at depths between the anaerobic hypolimnion and the warmer epilimnion, consistent with previous information indicating it to be a cold—water species. However, in situ life table data showed that D. pulicaria was capable of surviving in epilimnetic water up to 27°C, although reproduction was greatly reduced for a 6—day period in mid—July. This reduction in reproduction appeared to be the result of an interaction of high temperatures, declining standing crops of small algae and increasing amounts of Anabaena, Ceratium and Volvox. Daphnia galeata mendotae did not show any adverse response to these mid—July algae—temperature conditions. Analysis of predator gut contents suggested that both daphnids were heavily preyed upon by bluegills (Lepomis macrochirus). Shifts in the size structure of the daphnid populations were also consistent with intense size—selective predation by bluegills, as was the tendency for the age distribution of parthenogenic eggs to become skewed to younger eggs as the midsummer decline proceeded. Differences in body size, ephippial production and habitat preferences between the 2 species are also consistent with the hypothesis that planktivory is a strong selective force in their evolution.

ISSN:0012-9658    

DOI:10.2307/1936478

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

We observed feeding assemblages of birds at a Costa Rican dry—forest population of Casearia corymbosa (Flacourtiaceae) in order to distinguish effective dispersers from a incidental visitors and to examine the relationship between fruit production and depletion by birds at individual plants. On the basis of 1956 recorded visits by birds, the Yellow—green Vireo (Vireo flavoviridis) was the most reliable disperser throughout the season, accounting for 65% of the arillate seeds removed. Other common visitors were the Streaked Flycatcher (Myiodynastes maculatus) 12%, Golden—fronted Woodpecker (Melanerpes aurifrons) 9%, Pale—throated Flycatcher (Myiarchus nuttingi) 8%. All stripped off the oily arils in the crop and regurgitated viable seeds from perches in surrounding forest. Sporadic visitation by 10 other trogons and passerines accounted for the remainder of fruit utilized by birds. Capsules opened early in the morning, and fruit accessible to birds was depleted by early afternoon. At 17 closely monitored plants, birds removed 91% of the accessible arillate seeds. Crop sizes ranged from 1—2700, with dispersal success highest among plants of intermediate fecundity. Variance was attributable to individuals with low fecundity, often missed by birds on all but the most productive days, and to an exceptionally fecund tree that satiated its visitors. The dispersal ecology of the small dry—forest tree offers a unique comparison to that of a more fecund overstory ecotype in wet forest. In secondary dry forest, a protocooperative relationship exists between several bird species and this plant, with an abundant and efficient vireo responsible for most dispersal.In the wet forest, dispersal occurs through a mutualistic relationship with a cotinga (Tityra semifasciata), while most common visitors are "fruit thieves" of little benefit to the plant. These differences are considered in light of comparative ecology of the 2 forms.

ISSN:0012-9658    

DOI:10.2307/1936479

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY