JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. In a fragmented, dry subtropical forest in northwestern Argentina, we compared pollination levels, fruit set, and seed set among small (< 1 ha) forest fragments, large (>2 ha) fragments, and continuous forest in 16 plant species representing a wide range of pollination systems, breeding systems, and growth forms. For three species, Prosopis nigra (Mimosoideae), Cercidium australe (Caesalpinoideae), and Atamisquea emarginata (Capparaceae), the three treatments were replicated across four sites; we achieved less replication for other species. Because comparisons between forest and fragment populations for different species took place in different sites, however, by treating all 16 species as a unit we lessened the potential bias of confounding site effects and could evaluate the overall impact of fragmentation. Significant or marginal (P < .10) fragmentation-related declines in number of pollen tubes per style, fruit set, and seed set occurred in 9 of 16, 5 of 15, and 3 of 14 species, respectively. Overall, significant or nonsignificant declines occurred in 8 1% (pollen tubes), 73% (fruit set), and 79% (seed set) of the species. In all cases these proportions were greater (P ' .06) than the null binomial expectation of a 1: 1 ratio of increases to decreases. Breeding system did not explain sensitivity to fragmentation: the magnitudes of declines in pollen tubes, fruit set, and seed set were virtually indistinguishable between self-compatible and self-incompatible species. At least 4 of the 10 self-incompatible species, however, were heavily visited in small fragments by Africanized honey bees, which may have compensated for a decline in visits by native pollinators.The exact nature of responses varied among plant species. In some, the absolute quantity of pollen grains transferred to stigmas decreased with fragmentation, and sometimes this was reflected in reduced fruit or seed set. In Cercidium, Prosopis, and Atamisquea, the quality of the grains transferred apparently changed: number of pollen tubes produced per pollen grain on the stigma declined with increasing fragmentation, and at least in the latter two species seed production declined as well.Overall, levels of pollination and seed production undoubtedly integrated many idiosyncratic effects of fragmentation on particular plant and animal populations, and indicated that "community health" of fragments suffered in comparison with that of continuous forest. Median decreases in pollination levels and seed output from forest to fragments approached 20%. The impact of these declines on plant recruitment is less ...

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. 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.

Fourteen months' observations on hummingbird foraging patterns in successional habitats at Monteverde, Costa Rica, showed that one territorial species (Amazilia saucerottei) dominated rich resource clumps, modified all other species' patterns, and thus organized the nectarivorous bird guild. The principal nonterritorial species (Chlorostilbon canivetii), which Amazilia usually excluded from rich resources, traplined dispersed flowers and interfered with foraging patterns of other nonterritorial species. The 12 additional hummingbird species that foraged in the study habitats included species important in nearby communities, specialists on particular resources, and highly migratory opportunists. Foraging pattern diverged along several dimensions, including (1) the species or flower density of the individual plant; (2) the strata of flowers within the plant; and, since nectar was renewed at variable rates, (3) the time of day. Analysis of foraging patterns along these dimensions required that a hierarchy of niche breadth and overlap measures be defined and contrasted. Patterns compared over an entire year showed that the two principal species exploited the broadest niches, but overlapped only 17%. In no case did overall foraging patterns overlap more than 21%, and overlaps between many approached zero. These values, however, did not reveal the degree of exclusion from resources that were potentially exploitable, or the intensity of competition. Statistical correlations showed that most month—to—month changes in niche breadth, niche overlap, and population size could be attributed to shifts in the resource base. Of the 16 plant species exploited by hummingbirds, the forb Lobelia laxiflora and the tree Inga brenesii were responsible for most fluctuations in resource levels. Flowering peaks of both species attracted large numbers of hummingbirds; in general, numbers and diversity of hummingbirds rose with increased flower abundance rather than increased flower diversity. Lobelia nectar was effectively superabundant, and niche overlap during Lobelia peaks was high. Birds foraging in Inga trees, however, could diverge along spatial and diurnal dimensions, and overlap during Inga peaks was much lower. In between Lobelia and Inga peaks, the two principal hummingbird species usually dominated the guild, and expanded their niches but overlapped little. Each hummingbird species responded to that particular patterns of resource states it could best exploit, and the guild as a whole tracked the entire resource base. The cycle of resource flushes entrained these patterns to an annual rhythm.

In subtropical dry forest (Chaco Serrano) of Tucuman province, northwestern Argentina, we assessed responses of the flower—visitor assemblage to habitat fragmentation by monitoring insect visits to two spring—flowering tree species and by sampling insects with yellow pan traps. Each of four study sites contained a tract of continuous forest, one large (>2.2 ha) forest fragment, and one small (<1 ha) fragment, with fragments isolated for 5—20 yr preceding the study. During its respective flowering peak each tree species examined, Prosopis nigra (Mimosoideae) and Cercidium australe (Caesalpinoideae), dominated the sites' entomophilous flora. Results indicate that flower—visitor assemblages respond to landscape features on the scale of hectares; specifically, forest fragmentation in the Chaco Serrano leads to an insect flower—visitor fauna increasingly dominated by the exotic honey bee (Apis mellifera). Bees as a group made >90% of observed flower visits to both plant species. The honey bee alone made 82% of all visits to the early—flowering tree species P. nigra and 44% to the later flowering C. australe. Frequency and taxon richness of native flower—visitors at both plant species declined with decreasing forest—fragment size, but frequency of honey bee visits tended to increase in complementary fashion, such that the total frequency of insect visits to flowers of either plant species varied little with fragment size. Frequencies of visits by honey bees and those by native insects were also negatively correlated across individual trees. In both plants, visits by native insects were most consistent (varied the least among plants or over time) in large forest tracts, whereas honey bee visits to C. australe were most consistent in small fragments. In pan trap samples the relative frequency of honey bees increased with decreasing fragment size. Native flower—visitors sampled by pan traps increased in numbers and taxon richness both with increasing patch size and as spring progressed. Thus, fragmentation of the Chaco Serrano appears to (a) affect native flower—visitors adversely and to (b) facilitate honey bees' access to floral resources. It is not clear that these two effects are directly related to each other, however.