Overlap In Resource Use Research Articles

Resource partitioning between sympatric wild and domestic herbivores in the Tarangire region of Tanzania.

The effect of the introduction of an exotic species (cattle) into a native African herbivore assemblage was investigated by studying resource partitioning between zebu cattle, wildebeest and zebra. Resource partitioning was investigated by analysing grass sward characteristics (such as sward height and percentage nitrogen in leaves) of feeding sites selected by the different herbivore species. Linear discriminant analysis was used to determine whether a distinction could be made between feeding sites selected by the different animal species or whether the animal species showed overlap in resource use by selecting similar feeding sites. Wildebeest and zebra did not show overlap in resource use except in the wet season when resources were ample. Cattle showed overlap in resource use with zebra in the early wet season and with wildebeest in the early dry season, seasons when food limitation is likely. In the wet season, cattle showed overlap in resource use with both zebra and wildebeest.Implications of these results for competitive relationships between livestock and wildlife are discussed. We suggest that interpretation of overlap in resource use may be different for an assemblage of long-term coexisting native species as compared to an assemblage of native and exotic species. Among native herbivores, overlap in resource use is not expected based on evolutionary segregation. In a native assemblage to which an exotic species has been introduced, however, overlap in resource use can occur under food-limited conditions and consequently implies competition.

DIET SELECTION AND RESOURCE USE BY FLYING FOXES (GENUSPTEROPUS)

Flying foxes of the genus Pteropus (Pteropodidae: Chiroptera) play important roles as pollinators and seed dispersers in oceanic-island forest communities. This research examined general theories of diet breadth, diet selection, and the evolution of feeding strategies in bats in light of information from members of the genus Pteropus that inhabit oceanic islands. The feeding ecology of two species of flying fox, Pteropus samoensis and Pteropus tonganus on the Samoan archipelago, was examined in detail by direct observation and by examining feeding refuse. P. samoensis and P. tonganus fed on over 78 plant species from 39 families throughout their range and on over 69 plant species in Samoa alone. Flying foxes interacted with 59% of the forest tree species in Amalau Valley for fruit or flower resources. Twenty-eight percent of the forest trees were commonly used, and 79% of forest canopy trees were used. Broad overlap in resource use was noted between P. samoensis and P. tonganus with over 22 shared plant species. Seasonal variation in fruit resource use was quantified with a preference index. Flying foxes ate a taxonomically nonrandom subset of fruit species and generally preferred primary forest fruits to those in the secondary forest. Six plant families (Sapotaceae, Mrytaceae, Moraceae, Combretaceae, Fabaceae, and Sapindaceae) were particularly important to flying foxes in Samoa. However, a set of core plant taxa does not exist for Pteropus spp. in Samoa. Differences in the evolution of feeding strategies between mainland fruit-eating chiroptera and island species likely reflect differences in the spatio-temporal availability of resources in the two systems.

Life history traits and resource utilisation in an assemblage of north temperateAphodiusdung beetles (Coleoptera: Scarabaeidae)

To help understand and interpret the structure and function ofAphodiusdung beetle assemblages, life history traits and resource utilisation were studied for the ten species comprising the local assemblage of intensively grazed pastures in southern Ireland Most species were univoltine but one species (A fimetarius) was at least partly bivoltine However, temporal overlap m adult flight periods does not necessarily imply overlap in resource use Three different strategies of ovarian development were distinguished and were related to the preferred oviposition site and successional occurrence of the various species Evidence suggested that absence of mature eggs in a female's ovaries did not necessarily imply that a female was in a non‐reproductive state Two species (A prodromusandA sphacelatus) did not breed in dung, in the laboratory larvae were reared in decaying vegetation One species (A errattcus) developed m brood masses beneath the dung pat Larvae of all the other species developed within the dung pat There were consistent Interspecific differences in the larval development rates, with two species (A rufipesandA rufus) overwintering mainly as prepupae and the other species mainly as adults Previous studies have consideredAphodiusassemblages as single guilds but the detailed natural histories of these species may affect guild designation

Prey Selectivity and Size-Specific Diet Changes in Bufo cognatus and B. woodhousii during Early Postmetamorphic Ontogeny

Ontogenetic diet shifts have been documented in many anurans (Smith and Bragg, 1949; Oplinger, 1967; Labanick, 1976; Christian, 1982; Donnelly, 1991; Lima and Moreira, 1993). These studies compared the diets of juveniles as a whole to those of adults (e.g., Livezey, 1961; Clarke, 1974a; Gittins, 1987). Diet patterns among juvenile size classes, however, have yet to be investigated. Bufonids exhibit high postmetamorphic growth rates early in ontogeny, as do other anurans (Clarke, 1974b). These differences in body size may be a means of avoiding overlap in resource use (Schoener, 1974; Toft, 1985). Hence, diet shifts associated with rapid growth during this relatively short period may be important for survival to reproductive age and may affect interactions with heteroand conspecifics (Werner and Gilliam, 1984). Discrete, size-specific diet shifts may result in selectivity for different prey among toad size classes (Werner and Gilliam, 1984). Some authors have suggested that certain Bufo feed indiscriminately (e.g., Smith and Bragg, 1949; Clarke, 1974a; Duellman and Trueb, 1986; Larsen, 1992). However, these conclusions could be an artifact of pooling size classes consisting of selective feeders into a single class for dietary analysis. Indeed, recent studies indicate that toads are selective predators (e.g., Toft, 1980, 1981). We quantified ontogenetic differences in the diets of toads over a range of sizes that occur within the first month after metamorphosis, and compared prey consumption to prey availability for several prey types consumed by juvenile toads during early ontogeny. We collected postmetamorphic, juvenile toads by hand while walking a transect 1 m from the edge of Rose Lake, a prairie-pothole marsh located approximately 5 km NE of Vermillion, Clay Co., South Dakota. We collected 36 Bufo cognatus and 14 B. woodhousii on 15 July 1992, and 20 B. woodhousii on 27 July 1993. During 1992, all breeding occurred on 15-18 May and most tadpoles of Bufo transformed around 15 June (Graves et al., 1993). The average SVL of B. woodhousii at metamorphosis in South Dakota is 10 mm (Underhill, 1960). The largest toad collected had a SVL of

Faunal assembly in chironomids (Diptera): generic association and spread

Faunal assembly in chironomids was analysed using published data in the fauna of three rivers in southern England and of Great Britain as a whole. Two questions, both relating to the generic composition of local faunal assemblages, were asked: (i) to what extent are faunas from different sites within a single geographical area associated in terms of the occurrence of genera (‘generic association’); (u) given the total number of species observed, is the number of different genera represented (‘generic spread’) more than would be expected by chance. The latter question relates to the hypothesis that if interspecific competition is a strong organizing force of local faunal assemblages, species would be more likely to occur as widely spread out among different genera, thus decreasing overlap in resource use and hence the possibility of outright competition. Null models of species occurrence were constructed through Monte Carlo simulation to evaluate quantitatively these two questions. Generic association of faunal assemblages from three localities was statistically significant, though there was some variation among different subfamilies of Chironomidae. No evidence of generic diversification (more genera occurring in the fauna than expected by chance) was found. These results and the original questions were given critical considerations in the discussion.

Resource Partitioning and Competition Among Cervids in the Northern Rocky Mountains

(1) Patterns of spatial distribution, habitat preferences, forage-class selection, and niche overlap were investigated for sympatric white-tailed deer (Odocoileus virginianus ochrourus Rafinesque), Rocky Mountain wapiti (Cervus elaphus nelsoni Bailey) and Shiras moose (Alces alces shirasi Linnaeus) during consecutive severe and mild winters in northwestern Montana, U.S.A. The objectives were to determine mechanisms of resource partitioning, the role of interspecific competition in shaping niche relationships, and the current potential for interspecific competition. (2) Fourteen discrete plant communities were identified on the basis of previous community classifications. Additionally, two primary gradients of vegetative structure were described using principal components analysis: (i) a successional gradient of overstorey complexity, and (ii) a gradient of tall-shrub biomass. (3) White-tailed deer, wapiti, and moose were distributed unevenly among three segments of the study area that partitioned elevational and snow-depth gradients, and among vegetation communities. Habitat preferences were predictable on the basis of body size, energy expenditures, and nutritional requirements. Cervids were ranked white-tailed deer-wapiti-moose, in order of increasing body size and affinities for open canopy, deep snow, and dense shrubs. Wapiti also favoured grass-dominated communities when snow did not limit foraging options. (4) White-tailed deer and wapiti fed on dwarf evergreen shrubs and grasses, respectively, although deep snow restricted diet selection during the severe winter. Moose were browse specialists in both years and favoured deciduous shrubs more and coniferous browse less during a mild than a harsh winter. (5) Interspecific and annual comparisons of niche overlap provided little evidence that resource partitioning resulted from interspecific competition: (i) trophic and ecological overlap between species was greatest during nutritionally restrictive winters; (ii) overlap was greater between similar-sized cervids than between species of dissimilar body size; (iii) species pairs with the greatest trophic overlap did not exhibit compensatory spatial divergence. Large overlap in resource use, which increased during severe winter periods, suggested possible temporary interspecific competition between white-tailed deer and wapiti and between moose and wapiti.

Grass/Grazer Radiations: An Interpretation of Silica-Body Diversity

Ann. Rev. Ecol. Syst. 4: 53-74. 1980. Guild structure in desert lizards. Oikos 35: 194-201. , Huey, R. B., and Lawlor, L. R. 1979. Niche segregation in desert lizards. In: Horn, D. J., Stairs, G. R., and Mitchell, R. D. (eds), Analysis of ecological systems. Ohio State Univ. Press, Columbus, OH. Poysa, H. 1983. Resource utilization pattern and guild structure in a waterfowl community. Oikos 40: 295-307. Root, R. B. 1967. The niche exploitation pattern of the bluegray gnatcatcher. Ecol. Monogr. 37: 317-350. Sale, P. F 1974. Overlap in resource use, and interspecific competition. Oecologia (Berl.) 17: 245-256. Thompson, J. D. and Rusterholz, K. A. 1982. Overlap summary indices and the detection of community structure. Ecology 63: 274-277. Tukey, J. W. 1980. We need both exploratory and confirmatory. Am. Statistician 34: 23-25. Waite, R. K. 1984. Winter habitat selection and foraging behaviour in sympatric corvids. Ornis Scand. 15: 55-62.

Ecology of the dominant copepod species at a Low Arctic site

Four species of calanoid copepod regularly co-occur in tundra ponds near Churchill, Manitoba. Despite the physical uniformity of the habitats they occupy, twofold variation in body size and fivefold variation in brood size existed among conspecific populations of each species. Size shifts were concordant in the sexes, but the extent of sexual dimorphism was size dependent in each species. Size differences among populations persisted through two summers. The three herbivorous species showed parallel variation in body sizes and fecundities, while the predatory species did not. The size covariation in the herbivorous taxa may reflect significant overlap in resource use despite their size divergence.

The Ontogenetic Niche and Species Interactions in Size-Structured Populations

Body size is manifestly one of the most important attributes of an organism from an ecological and evolutionary point of view. Size has a predominant influence on an animal's energetic requirements, its potential for resource exploitation, and its susceptibility to natural enemies. A large literature now exists on how physiological, life history, and population parameters scale with body dimensions (24, 131). The ecological literature on species interactions and the structure of animal communities also stresses the importance of body size. Differences in body size are a major means by which species avoid direct overlap in resource use (153), and size-selective predation can be a primary organizing force in some communities (20, 70). Size thus imposes important constraints on the manner in which an organism interacts with its environment and influences the strength, type, and symmetry of interactions with other species (152, 207). Paradoxically, ecologists have virtually ignored the implications of these observations for interactions among species that exhibit size-distributed populations. For instance, it has been often suggested that competing species

Age Structure Component of Niche Width and Intraspecific Resource Partitioning: Can Age Groups Function as Ecological Species?

Many species pass through several distinct age classes (instars or year classes) as they grow. The variety of resources used by age classes effectively expands niche width. This "age-specific component" of niche width is analyzed for a desert scorpion, Paruroctonus mesaensis, whose populations exhibit both instars and discrete year classes. Another purpose of this paper is to evaluate if instars or year classes function as "ecological species" by showing the same differences in morphology and resource use as exists between biological species. The similarity of the growth ratio (Dyar's constant) to Hutchinson's Santa Rosalia ratio has promoted the speculation that the size divergence between age groups has the same ecological basis as that between competing species. Empirical differences between species for a variety of resource-related parameters were determined from a survey of the literature. These interspecific differences are compared to differences between age classes of the same population. Intraspecific differences in body size and resource use were analyzed statistically, and then quantified by using the following similarity indices: the Santa Rosalia ratio for predator length, overlap in prey taxa and prey size, and total overlap (including prey taxa, prey size, and phenology). Successive instars exhibited growth ratios greater than the Santa Rosalia ratio and were significantly different in the size of prey eaten. Differences in actual prey eaten, however, were less than the differences that exist between most species. The three year-classes are fairly distinct in patterns of prey size and use: differences are well within the range of interspecific differences. Young and adults are less similar in all parameters of size and resource use than are the majority of species pairs. The differences between young and intermediate year-classes are typical of the differences observed between many species. Although intermediates and adults show moderately high overlap in resource use, they still differ more than 16%-80% of all species for the various resource parameters. During certain periods of the year, intermediate-aged scorpions experience moderate to high overlap with large populations of both young and adult year-classes. By foraging primarily during colder periods of the year when the other two age classes are relatively inactive, intermediate animals tend to minimize co-occurrence on the surface and use different resources. The evolution and significance of the "age structure component" of niche width is discussed. Under conditions of reduced interspecific competition, there is selective pressure for age groups to diverge in resource use. This appears to be the case with P. mesaensis because there is little competition by other scorpion species, in part, as a result of frequent intraguild predation by adult P. mesaensis on smaller scorpion species. The age structure component of niche width may have significant interspecific influence on both competition and guild structure. Species that develop through a wide size range during their lifetime will use a broader range of resources and affect other species differently than those species that have an effectively more narrow size range during their lifetime of activity.

SEX RATIO, BODY SIZE AND SEASONALITY IN A SOLITARY BEE, OSMIA LIGNARIA PROPINQUA CRESSON (HYMENOPTERA: MEGACHILIDAE).

Although sex ratio and investment theory have been recently refined and made more mathematically rigorous (Hamilton, 1967; Leigh, 1970; Trivers, 1972; Trivers and Hare, 1976; Charnov, 1978; MacNair, 1978), Fisher's (1930) original arguments remain essentially unchanged. In a panmictic population we expect equal amounts of time and energy to be allocated to the production of each sex. A disproportionate investment in the offspring of one sex will be corrected in subsequent generations by selection for parents that produce the sex in short supply. Because investment must be divided equally between the sexes, the optimal sex ratio is expected to be unity only when the parental effort expended on an average individual of each sex is also equal. If production of a female requires twice the effort as production of a male, the sex ratio should be 2 d :1 9 rather than unity. This study is, in part, an empirical test of sex ratio theory using a solitary bee, Osmia lignaria propinqua Cresson. It was prompted by the paucity of field data currently available to test the accuracy of theoretical formulations. Additionally, observations by Torchio (unpubl.) suggest that the sex ratio of offspring of 0. lignaria varies seasonally. Indeed, seasonal variation in the sex ratio is not uncommon in bees (Michener, 1974), although the reasons for such variability require clarification. Here we document seasonal variability and offer an explanation for it. Until recently (Trivers and Hare, 1976), the interrelationship between the ratio of investment in an average male and female and the sex ratio was not clearly perceived. It is now apparent that sex ratio is related to investment ratio and any study of sex ratio naturally extends to questions of optimal allocation of investment within each sex. Investment expended upon an average individual of each sex in many species is unequal, although total investment in each sex must be equal. Such differences are most clearly manifest in those species that exhibit sexual dimorphism in size. In Osmia lignaria females are distinctly larger than males. Of the explanations proposed for sexual dimorphism, two are frequently cited. Trivers (1972), in elaborating on Darwin (1871), hypothesized that members of the sex that invest most in offspring are subjects of intrasexual competition between members of the other sex. As a result of intrasexual selection (probably sexual selection also), members of the competing sex are expected to be larger and fewer because of the usual positive relation between size and mating success. This explanation is not applicable to 0. lignaria because females are responsible for all but sperm investment in offspring. The second hypothesis (Amadon, 1959; Selander, 1972) argues that sexual dimorphism is frequently due to intersexual competition for resources; a divergence in size reduces overlap in resource use, thereby increasing the overall efficiency of each sex. This hypothesis is not appropriate to 0. lignaria because the sexes do not differ in distribution across the limited number of flower species used for nectar (P. F. T., pers. observ.). We will demonstrate that the asymmetry in investment between individual males and females is probably due to a complicated process involving seasonal changes in resource availability.

Resource division by perlid stoneflies (Plecoptera) in a lake outlet ecosystem

In a stream draining a productive lake, two abundant (250/m2) stoneflies Calineuria californica and Hesperoperla pacifica differed in population size structure, habitat use and food although overlap in resource use was extensive. Life cycles of two (Calineuria) and three years (Hesperoperla) were indicated. A multivariate analysis of habitat use demonstrated size dependent habitat selection in both species and between species habitat differences. Feeding habits differed although only the largest Hesperoperla had a food refuge. Diets of individual nymphs represented an interaction of species, size and habitat.

Species Packing and Niche Complementarity in Three Sunfishes

A fundamental problem in ecology is to identify and relate those factors which determine the number of species that can stably coexist in an area. Interspecific competition is one such factor, and in principle it limits the similarity of competing species in relation to the abundance and diversity of critical resources. MacArthur and Levins (1967) first explored this limit with a three-species Lotka-Volterra model. The success of a species invading between two residents on a resource axis was related to the differences in mean utilization on this axis. Resource abundance was considered uniform across types, and utilization functions were assumed to be normal, differing only in position on the axis. The analysis of this problem has subsequently been extended to include factors such as environmental variability, the relative abundance of resources, the shape of the utilization curve, and predation pressure (e.g., May and MacArthur 1972; MacArthur 1972a; Roughgarden 1974b; Roughgarden and Feldman 1975). The variety of resources used by a species (i.e., niche width) is a central issue in this theory and is usually treated as a given parameter. Both the niche width and location of a species are related to functional morphology (i.e., the tools used in obtaining resources). In fact, one of the first observations suggesting the direction of the theory was Hutchinson's (1959) remarks on the uniformity of character displacement ratios among a wide variety of sympatric species, particularly in trophic apparatus. Morphological measurements have been widely used since as relative indices of competition or niche width (e.g., Cody 1974a; Van Valen 1965; Soule and Stewart 1970). Very seldom, however, have the differences in foraging efficiency of morphologically similar species been quantified in relation to a set of resources that change in a recognizable (hopefully orderable) manner. Careful studies in this direction would facilitate the testing and development of theory by providing a link between morphology, the efficiency of resource utilization, and the parameters of concern, usually measures of overlap in resource use. In this study I quantitatively relate the body plan in fishes to foraging efficiency on the food size dimension of the niche. Using methods derived from optimal foraging theory, cost curves are constructed on this coordinate for

Overlap in resource use, and interspecific competition.

When several species co-exist, the amount by which they overlap in their use of resources is a measure of their similarity to one another. As such, resource overlap does not measure the amount of competition among them. When the resources are not limiting to population growth, patterns of resource use may overlap to any degree. However, when the species are frequently in competition for their resources, natural selection will favor the separation of their requirements, and the amount of resource overlap will be reduced.This paper presents a technique which permits comparison of the amount of resource overlap observed in a given case with that expected for a group of similar species co-existing in the absence of competitive interactions. From this comparison can be evaluated the likelihood of competitive processes being important in the situation under study.