摘要:

The pollination success of Agave palmeri is strongly dependent on nectar—feeding bats. In areas with very low or nonexistent bat populations, Agave seed set is<5% of its maximum potential. A 30 yr trend of declining seed set parallels a decline in bat numbers during that period. We speculate that habitat destruction and an increase in human use of Agave are causal factors. Over half of the measurable energy in the plant biomass is allocated to advertising and reproduction. We discuss the costs and risks involved in the semelparous strategy when pollinators decline. Where bats remain, agaves in dense patches show higher seed set than do outlying plants, suggesting that the pollinators favor grouped plants. This behavior may reinforce the plants' colonial tendencies.

ISSN:0012-9658    

DOI:10.2307/1936660

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

A nonparametric statistical test of the performance of ordinations is adapted and extended from the work of Feigin and Cohen (1978). Two measures of ordination performance are accuracy (the tendency of ordination results to mirror correctly the true community structure) and consistency (the ability of the ordination to produce concordant results). Statistical estimates of accuracy and consistency are given and tests are developed for choosing among ordination techniques.

ISSN:0012-9658    

DOI:10.2307/1936661

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Decomposition rates and amounts of calcium, magnesium, potassium, and phosphorus were measured for dead millipedes and crickets buried in forest litter in North Carolina and Georgia. An average of 30% of the original mass of millipedes and 14% of the original mass of crickets was recovered after 1 yr in the litter. Elemental losses generally followed the pattern: K @>P>Mg>Ca; however, elemental amounts occasionally stabilized and in one experiment calcium increased in amount over time. Decomposition of arthropod carcasses was described by a two—component, negative exponential decay model. Decay coefficients were used with literature estimates of arthropod standing crops to estimate standing crops of mass and elements of arthropod remains in forest litter and soil. Estimates of standing crops of mass, calcium, and magnesium of arthropod remains were greater than those of living forest floor arthropods.

ISSN:0012-9658    

DOI:10.2307/1936662

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The influence of variability in nectar reward per flower on the foraging behavior of individual bumblebees (Bombus sandersoni Fkln.) and paper wasps (Vespula vulgaris L.) was explored during late summer 1979. The area around each colony was enclosed in mosquito netting, and an artificial floral patch, yellow and blue cardboard squares randomly mixed and fixed beneath wells in a clear sheet of plexiglass, was established within the enclosure. The nectar reward per flower (i.e., the volume) was manipulated to generate different degrees of variability within a floral type. The first set of experiments had equal quantities of nectar in all yellow and blue flowers. Individual foraging bees showed a significant preference for yellow. However, when blue flowers were kept constant (2 μL in each) and yellow made variable (6 μL in every third flower and none in the remainder), individual foragers avoided the variable yellow even though the expected reward from yellow was equal to blue. As a control, the variable floral type was reversed with yellow now constant. Individual foragers then showed an equally strong avoidance of blue flowers. In order to insure that bees were avoiding uncertainty rather than merely avoiding empty flowers, a new set of experiments was run with some nectar in every flower. The constant floral type contained 2 μL of nectar in every flower while the variable type contained 5 μL in every third and .5 μL in the remainder. Again the foragers preferred the constant floral type; however, the preference was not as great as when there were empty flowers. Analysis of the sequence of visits made by individual forages seems to indicate that bees learned to distinguish the variable floral type during the foraging bout. Similar experiments were run on the wasps. When flowers were of equal quality, the wasps preferred yellow to blue. When one floral type was variable and the other constant, the wasps preferred the constant type. For the same magnitude of variability, however, the bumblebee's avoidance of the variable floral type was more pronounced than was the wasps' response. The results of these experiments are then analyzed within the context of a more general model of foraging behavior in variable environments.

ISSN:0012-9658    

DOI:10.2307/1936663

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Niche breadths in animal and plant populations are often measured without regard to the relative frequencies of the various resources available to the organisms. Recent papers have provided more accurate indices for use when available resources can be quantified. These indices may lack wide applicability or simple biological interpretations. We suggest that niche breadth be defined as the degree of similarity between the frequency distribution of resources used by members of a population and the frequency distribution of resources available to them. Similarity can be quantified with the familiar Proportional Similarity (PS) Index. This index, which measures objectively the similarity between two frequency distributions, reflects in satisfactory fashion the breadth of a population's niche.

ISSN:0012-9658    

DOI:10.2307/1936664

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The limiting effects of food and water on juvenile growth rates in the lizard Anolis aeneus were investigated in the field (Grenada, West Indies) and laboratory. Growth rates of lizards in the field were unrelated to their snout—vent lengths, but both prey biomass and rainfall had significant effects on juvenile growth rates. Laboratory experiments indicated that water had a primary limiting effect on growth; even when food supplies were superabundant, growth rates were low when drinking water was curtailed. Laboratory and field experiments suggest that limited water availability reduces growth rates for most (67%) of the dry season, whereas food levels are sufficiently low to limit growth during the weeks of the dry season when rainfall is sufficient for growth. During the wet season there is no evidence of water scarcity, food levels are high and average growth rates are 85% of the maximal rates observed under optimal conditions in the laboratory.

ISSN:0012-9658    

DOI:10.2307/1936665

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

A secondary succession sequence of 14 northern hardwoods stands was sampled for forest floor organic matter and nutrient content. During the first 15 yr following clear cutting, the forest floor decreased by 30.7 Mg/ha, a decline of over 50%. The decrease in the forest floor and slash (logging residue) may be greater than the increase in the living biomass. During the next 50 yr the forest floor increased by 28.0 Mg/ha and by year 64 was within 5% of an asymptote of 56.0 Mg/ha. Nutrients were analyzed in 6 of the 14 stands. Magnesium, potassium, and nitrogen concentrations showed no successional pattern. However, calcium concentrations were significantly higher in the stands in which forest floor mass was low. The initial decrease in forest floor mass is attributed to lower leaf and wood litter fall and to more rapid decay resulting from higher temperature, moisture content, and nutrient levels and to early successional litter being more easily decomposed. The recovery of the forest floor is explained primarily as resulting from the rapid increase in the quantity and diameter of wood litter fall. JABOWA, the northern hardwood forest growth simulator, predicts a maximum rate of increase in woody litter by years 10—20 with a leveling off by years 30—50. An apparent asynchrony in function of the forest floor and slash as nutrient sources may be important to the recovery process. During the first 15 yr the forest floor is a major source of nutrients, releasing a net amount of approximately 800 kg/ha of nitrogen. During this period nitrogen immobilization in the decay of slash may account for as much as one—half of the nitrogen released from the forest floor. After year 15 the forest floor is no longer a source but a sink for nutrients as nutrients and organic matter accumulate. By year 15 the slash probably shifts in function from a sink to a source, providing nitrogen for the continuing rapid nitrogen accumulation in vegetation beyond year 15.

ISSN:0012-9658    

DOI:10.2307/1936666

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The number of flowering stems for 82 species on a transect 2 x 400 m was counted twice weekly during 1978 at Tucker Prairie, Callaway County, Missouri, USA, a tall—grass prairie remnant. Phenological curves (number of flowering stems vs. day of the year) are narrower (smaller standard deviations) for wind—pollinated species than for insect—pollinated species. Symmetry (g1) does not differ significantly for wind— and insect—pollinated species. The curves are largely either symmetric or begin abruptly and end gradually (right—skewed). Overlap, measured as the fraction of the area under the curves occupied by two or more species, does not differ significantly for wind— and insect—pollinated species, among 20 groups of five randomly drawn species. Dates of peak flowering are distributed randomly over the season (May to November 1978) and their distributions do not differ for wind— and insect—pollinated species. Dates of peak flowering are also randomly distributed for grasses, legumes, and composites considered separately. Thus, although species with different pollination modes show different shapes for phenological curves, the species aggregated into the community do not have ensemble patterns of temporal dispersion or overlap and cannot be distinguished from a random collection.

ISSN:0012-9658    

DOI:10.2307/1936667

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Achievement of a critical body size provides the proximate stimulus for the metamorphic molt in Oncopeltus and other hemimetabolous and holometabolous insects. Effects of size—dependent stimuli on metamorphosis vary in detail among different insects, although the adaptive significance of this variation is as yet unknown. The phenomenon is important to life histories where growth rates are highly variable, since it allows larvae to delay eclosion until a prerequisite body size is attained. In Oncopeltus, variation in growth rate occurs where nymphs develop on individual host plants which differ greatly in nutritional quality, depending on the presence or absence of seeds. This variation in growth rates results in marked variation in adult size. The size at which nymphs eclose is determined by opposing selective forces in the adult and larval stages. When growth is slow, nymphs which must attain a larger size exhibit longer developmental times and lower survival to eclosion. However, bugs which eclose at a larger adult size survive longer under starvation conditions, and presumably could disperse for longer periods in search of food sources. This is particularly important for females eclosing on nonfruiting plants, since they must locate a supply of seeds before reproduction can commence.

ISSN:0012-9658    

DOI:10.2307/1936668

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The guild of plethodontid salamanders of the deciduous forest of the southern Appalachians shares the common resource of food, which consists of insects and other invertebrates inhabiting the forest floor and low vegetation. Since in practice, use of the guild concept implies competition for the shared resource, the validity of accepting a guild as delimited for the terrestrial Plethodontidae was tested by field experiments covering the 5 yr 1974—1978. The test consisted of regularly removing the most abundant species, Plethodon jordani, from one set of experimental plots, and the second most abundant species, P. glutinosus, from another set of plots. Each of these two species was favorably affected by the removal of the other P. glutinosus increased significantly in abundance where P. jordani was removed. Removal of P. glutinosus resulted in a significant increase in the proportion of young P. jordani. The remaining species, including the congeneric P. serratus, were not affected by the removal of either of the two most abundant species. The results call into question the common assumption that competition is the organizing force in all coexisting assemblages of species which share common resources. The data also imply that food is not the resource for which P. jordani and P. glutinosus compete. Nest sites may be the critical resource, but much information is needed before they can be implicated.

ISSN:0012-9658    

DOI:10.2307/1936669

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY