摘要:

Bumblebees of any one species in Maine forage for pollen and/or nectar from a large variety of morphologically diverse flowers, but individuals have limited foraging repertoires at any one time. Unspecialized individuals were sometimes unsuccessful in extracting nectar and/or pollen from highly rewarding flowers. In any one area with a variety of concurrently blooming plants, the bumblebees had apparent species preferences. Superimposed on the species preferences were individual preferences. Individuals had primary foraging specialties (their majors) and secondary specialities (their minors). Minors were often bridges to new majors. Queens in Maine necessarily have several successive majors during their lifetime since the blooming time of the plants they utilize are brief relative to their life—span as foragers. However, the blooming time of most plants available to Bombus fervidus workers are long relative to their lifetime. Switching was rarely observed in these bees, even in some individuals observed daily for up to 1 mo at the same foraging area containing other plant species in bloom that were highly attractive to other individuals of the same bumblebee species. On a per flower basis, those flowers producing the most food rewards generally had the largest number of bees majoring from them, and the food rewards available were roughly comparable between different kinds of flowers, regardless of their differences in rates of nectar production. Specializing was usually preceded by sampling a variety of rewarding as well as nonrewarding flowers. When the flowers from which bees were majoring in an area were experimentally removed many of the bees sampled flowers of other concurrently blooming plants, but they generally did not switch to flowers from which the food rewards were being depleted by specialists, unless these were experimentally fortified with syrup. Upon finding superior food rewards in enriched blossoms, they switched immediately. Flower—specificity is related to site—specificity. Many bees shared the same foraging area, but different individual bees at the same site utilized the flowers of different plant species. When the foraging area contained landmarks, the bees visited clumps of flowers in a sequence (foraging path) that was generally repeated several times on the same foraging trip when the foraging site was small. The foraging behavior of bumblebees is discussed from a comparative standpoint with other bees and in relation to food distribution and availability in the environment.

ISSN:0012-9615    

DOI:10.2307/1942246

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

We have developed a model which indicates that the pure strategy of "majoring"is always better than random foraging if the reward structure remains approximately constant through time. "Minoring" is a necessary compromise required to track resources changing through time.

ISSN:0012-9615    

DOI:10.2307/1942247

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Temperature and moisture stress of conifer saplings and needle nitrogen content of conifer saplings were measured at reference stands representing 16 forest communities in the central portion of the western Cascades province of Oregon. Most species occur over a wide range of temperature and moisture stress; many occupy a wider range of environments in the western Cascades than they do in the eastern Siskiyou Mountains of southwest Oregon. Differences between vegetation zones are reflected in a temperature index; within zones, communities are distinguished by moisture stress and, to a lesser extent, by temperature. In two cases vegetation differences appear to be related to low needle nitrogen contents. Use of complex gradients for vegetation ordination suggests certain environmental differences between communities which are contrary to the differences measured; therefore, we prefer the measured gradients over the complex gradients defined. Species diversity (the total number of vascular species) increases and dominance (Simpson's index) decreases away from moderate environmental conditions to warmer—drier and colder communities. Diversities of different strata are unrelated. Dominance is concentrated in fewer strata of the vegetation on the colder sites. However, discontinuities in the pattern of diversity with environment occur which are not related to major differences in our measured environmental indexes. Evergreenness of shrubs is highest in stands with the lowest foliar nitrogen levels.

ISSN:0012-9615    

DOI:10.2307/1942248

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

A heuristic model, developed from simple assumptions, generates testable hypotheses that predict the foraging patterns of predators which invest a significantly greater amount of energy in search for, as compared to pursuit and handling of, prey. Given: (1) the environmental distribution of potential food size, (2) the environmental distribution of potential food diversity within food size, and (3) the manner in which the limits of consumable food size vary with predator size; food and microhabitat niche breadths as a function of predator size can be predicted. The assumptions and derived hypotheses of the model are tested with data from vermivorous prosobranch gastropods of the genus Conus, ubiquitous associates of tropical Pacific Ocean and Indian Ocean coral reefs. Predictions are: (1) small Conus (25 mm shell length), trophic specialists. This pattern is supported by both within— and between—species comparisons of Conus populations of different mean sizes. Relationships of food niche breadth with predator size are interpreted in light of the marked behavioral and morphological stereotypy of Conus, probably evolved in response to evolutionary and proximate predictability of available prey.

ISSN:0012-9615    

DOI:10.2307/1942249

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

During a limnological investigation of the Holland River and Cook Bay of Lake Simcoe, seasonal changes in phytoplankton biomass and species composition in the lower Holland River (Holland Marsh) were evaluated with reference to data collected on nutrient concentrations and other environmental factors. Over the whole year, Chlorophyceae comprised about ½ of the average standing crop of phytoplankton, Bacillariophyceae, about ⅓, and Cyanophyceae,90% of the total phytoplankton biomass which was as high as 3.2 x 107μm3/ml in May of 1972. The high standing crops in summer (1.0—3.0 ? 107μm3/ml) were dominated by chlorococcalean forms (especially Scenedesmus quadricauda) with smaller volumes of diatoms (mainly Melosira granulata and M. ambigua) and blue—green algae (Oscillatoria articulata and O. tenuis) present. Despite summer inorganic—N concentrations of<10 mg N/1, phytoplankton species succession continued with several taxa developing maximum abundance during the period of low nitrogen concentrations. Although the winter phytoplankton showed more diversity, with diatoms, chrysomonads, dinoflagellates, cryptomonads, and euglenoids all prominent in the plankton, the biomass was considerably lower at 0.16—2.9 X 106μm3/ml. Maximum chlorophyll a concentrations in the Holland Marsh (as high as 358 mg/l) compare with data in the literature from highly eutrophic waters. Phaeopigment concentrations ranged as high as 81 mg/l and comprised up to 58%, but averaged °15% of total uncorrected chlorophyll a during the ice—free summer period. Over the year, chlorophyll a content of phytoplankton ranged from 0.14% to 1.65% of fresh weight. Despite almost identical species composition of the phytoplankton (°90% S. hantzschii by volume) in May and November, the chlorophyll a content as a percentage of fresh weight was almost 3 x greater in November than in May. According to the influence of total daily incident radiation and inorganic N concentrations as controlling factors of chlorophyll a content of phytoplankton, three conditions were recognized in the Holland Marsh: (1) chlorophyll a contents were intermediate in spring under highest daily incident radiation (favoring low chlorophyll contents) and ample supplies of inorganic—N (favoring high chlorophyll contents); (2) chlorophyll a contents were lowest in summer when inorganic N was in scarce supply and daily incident radiation was high; and (3) contents were highest in fall and winter when supplies of inorganic—N were ample and daily incident radiation was lowest. Inorganic—N most likely limited summer phytoplankton growth. The spring phytoplankton maximum (S. hantzschii) was likely limited by SiO2in 1971 and by P in 1972. The extent of depletion of soluble reactive P by diatoms in the autumn was controlled by silica concentration. Some evidence is presented which suggests that the initial effects of improved nutrient retention within intensively cultivated and urban areas on the Holland River watershed would be realized as a shift in the limiting nutrient for the spring phytoplankton bloom in the Holland Marsh and southern Cook Bay, from SiO2to P.

ISSN:0012-9615    

DOI:10.2307/1942250

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY

摘要:

Results of quantitative sampling of small mammal populations at different grassland sites for a 3—yr period are compared to evaluate the energy requirements and consumption, according to trophic levels, for total small mammal communities at all seasons in different years. The purpose was to search for patterns of food utilization by these consumers at the different sites. This picture of the bioenergetics of small mammal populations over a wide range of grassland sites was constructed from extensive US/IBP Grassland Biome diet and population data and from physiological information out of the literature. The work represents one step in the process of understanding the role of consumers in the ecosystem. No species occurs at all sites and different trophic strategies predominate among the small mammals at each site: Microtines (herbivores) dominate the tallgrass prairie, sciurids (omnivores) dominate the northern shortgrass prairie, and heteromyids (granivores) dominate the bunchgrass and desert grasslands. Other groups occur at these sites and vary in their importance. At the tallgrass and midgrass sites the small mammal populations are largely dependent on herbage consumption, while at the northern and southern shortgrass prairie sites the rodent fauna is largely dependent on invertebrates. The most uniform distribution of resource utilization by this component of the consumer community occurs at the desert grassland site, where herbage, seeds, and animal matter are all utilized. Relative to the total amount of consumable herbage available (that actually utilized by the small mammal population), this resource is only slightly utilized (from a fraction to a few percent of available). Animal matter, largely invertebrates, is highly utilized at most sites and may in fact be a limiting factor on small mammal populations. The energy consumed by the small mammal population was greatest at the tallgrass prairie site, where the average annual consumption was 172 x 103kcal/ha (= 720 MJ/ha). However, efficiency of biomass support was greatest at the northern shortgrass prairie site, where consumption of 32 x 103kcal/ha (= 134 MJ/ha) supported proportionately more biomass (277 g live wt./ha) of animals than at the tallgrass site (935 g/ha). The estimates of population energy requirements presented here compare well with other available estimates. These results emphasize the great between year and site variability, however. Daily population respiration in summer may be as little as 14 and as large as 1,038 kcal°ha—1°day—1. Highest small mammal biomass occurred at the tallgrass (3,075 g live wt./ha) and the desert (2,304 g/ha) sites, and lowest at the midgrass (14 g/ha) site. Within the period of study, small mammal biomass was most stable at the southern shortgrass site and most variable at the tallgrass site. The role of small mammal populations in grassland ecosystem remains incompletely defined. A broader view of the total consumer community in relation to resources is required.

ISSN:0012-9615    

DOI:10.2307/1942251

出版商:Ecological Society of America    

年代:1976

数据来源: WILEY