Availability Of Prey Resources Research Articles

Colony-based foraging segregation by Antarctic fur seals at the Kerguelen Archipelago

The foraging behaviour of conspecific female Antarctic fur seals (AFS) was compared simultaneously at 2 breeding colonies at Iles Kerguelen (S Indian Ocean). A remnant colony at IIes Nuageuses (IN) thought to have escaped sealing is hypothesized to be the source of increasing fur seal numbers at Cap Noir (CN) on the Kerguelen mainland. Inter-annual variability in foraging areas is known to occur in response to local oceanographic changes at CN. Given the distance between the 2 sites (~160 km), we hypothesize that seals from the 2 colonies may show spatial segregation in foraging due to variability in local prey resource availability, although the transfer of foraging knowledge between sites via emigration may override such behaviour. The foraging zones, diving activity, diet and foraging success of seals were compared between sites using satellite telemetry, dive recorders and faecal analysis. No evidence of spatial foraging overlap was observed, with seals from IN conducting longer foraging trips, typified by a longer initial transit phase, than CN seals, which spent less time diving at night and dived more deeply. Pups nevertheless received higher absolute and daily energy delivery rates at IN. Diet was superficially similar at ~98% myctophid consumption; however, IN seals favoured the high-energy Gymnoscopelus nicholsi, indicating that local heterogeneity in marine resources likely influences the foraging zone choice of AFS. Finally, distribution patterns of 54 female AFS tracked during summer months from 1998 to 2006 reveal the importance of both on-shelf (<500 m) and shelf-break regions as foraging habitat. The core foraging area for CN in all years (10 400 km2) was small (~10% of total foraging space); however, time spent in this region alone totaled 38%. The likelihood of spatial overlap in foraging range is higher on the east coast of Kerguelen.

Reproductive Responses of Northern Goshawks to Variable Prey Populations

ABSTRACT Developing comprehensive conservation strategies requires knowledge of factors influencing population growth and persistence. Although variable prey resources are often associated with fluctuations in raptor demographic parameters, the mechanisms of food limitation are poorly understood, especially for a generalist predator like the northern goshawk (Accipiter gentilis). To determine the reproductive responses of goshawks to variable prey populations, we evaluated 823 goshawk breeding opportunities on the Kaibab Plateau, Arizona, USA, during 1994–2002. Concurrently, density was estimated for 4 prey species (2 avian, 2 mammalian). We explored the relationship between goshawk reproduction and prey density at one temporal scale (year) and 2 spatial scales (study area, forest type). Prey density for all 4 species combined accounted for 89% of the variation in goshawk reproduction within the entire study area (P &lt; 0.001), 74% in mixed conifer forest (P = 0.003) and 85% in ponderosa pine (Pinus ponderosa) forest (P &lt; 0.001). We found that an incremental increase in prey density resulted in a greater increase in goshawk reproduction in ponderosa pine forest than in mixed conifer forest, suggesting that the denser structural conditions of mixed conifer forest may have reduced prey availability. Red squirrel (Tamiasciurus hudsonicus) density explained more annual variation in goshawk reproduction within the study area (r2 = 0.87, P &lt; 0.001), mixed conifer forest (r2 = 0.80, P = 0.001), and ponderosa pine forest (r2 = 0.85, P &lt; 0.001) than did any other individual species. Although certain prey species were more strongly correlated with fluctuations in goshawk reproduction than were others, the high model selection uncertainty and the strong relationship between total prey density and number of goshawk fledglings produced indicated that alternate prey species were readily substituted for one another. Therefore, conservation strategies concerned with the status of goshawk populations should incorporate forest management practices that increase the abundance, diversity, and availability of prey resources.

Within‐season sequential density dependence regulates breeding success in mallards Anas platyrhynchos

Density dependence in vital rates is a key issue in population ecology but remains largely unexplored experimentally. We studied breeding success, lake use, and prey availability in wild mallards Anas platyrhynchos on small nemoral lakes in a replicated, two‐year cross‐over experiment in which pair density was increased. The number of wild mallards that settled on lakes prior to introductions of extra pairs did not differ between control and introduction years. Introductions led to a lake‐level reduction in the number of broods observed. However, the number of stage 2+ (almost fledged) ducklings did not differ between treatments, nor did lake utilization by nonbreeding adults, broods and ducklings. Prey resource availability differed greatly among lakes, but it did not correlate with breeding success. Partialling out the possible effect of food competition from wild adult nonbreeding mallards did not change this conclusion. Our study demonstrates sequential density dependence in breeding success; introductions caused a decrease in brood number, but despite fewer broods a similar number of nearly fledged ducklings were produced. We suggest that predation and/or lake change of broods soon after hatching created these patterns. We conclude that using a single and late measure of breeding success such as fledged birds can mask regulatory processes. Implications of density dependence and its relation to individual reproductive success are understood better if breeding success is decomposed into nest success, duckling survival and fledgling survival.

Effects of eutrophication on bottom habitat and prey resources of demersal fishes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 302:233-243 (2005) - doi:10.3354/meps302233 Effects of eutrophication on bottom habitat and prey resources of demersal fishes Sean P. Powers1,*, Charles H. Peterson2, Robert R. Christian3, Elizabeth Sullivan2, Monica J. Powers2, Melanie J. Bishop2, Christopher P. Buzzelli2 1Department of Marine Sciences, University of South Alabama and the Dauphin Island Sea Laboratory, 101 BienvilleBoulevard, Dauphin Island, Alabama 36528, USA2University of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, North Carolina 28557, USA3 Department of Biology, East Carolina University, Greenville, North Carolina 27858, USA *Email: spowers@disl.org ABSTRACT: Estuarine benthic habitats serve as essential feeding grounds for demersal finfishes and shellfishes and provide a link between the water column and demersal fisheries in coastal foodwebs. We hypothesize that the cascading linkages of water-column conditions to benthic invertebrates and from benthic invertebrates (as prey) to demersal fishes are a primary mechanism by which water-quality degradation affects sustainable production of demersal fisheries in coastal ecosystems. To evaluate these linkages, we related changes in water quality to changes in bottom-habitat quality for fishes (defined by the availability of prey resources) and assessed how changes in habitat quality affect fish diet. We examined the first link (water-column conditions to benthos) by intensively sampling the benthic community in the Neuse River Estuary, North Carolina, during 2 summers (1997 and 1998) in which depletion of bottom-water oxygen occurred. Dramatic decreases in the abundance of benthic macroinvertebrates were evident after hypoxia/anoxia in both years. Abundances of the clam Macoma balthica, a key prey item for fishes and crabs and the biomass dominant in the benthos, decreased by over 90% in deep and mid-depth areas (>2 m) throughout the estuary after hypoxia/anoxia in 1997. Although summer decreases in benthic macroinvertebrates were also exhibited in 1998, a year of less frequent and less severe hypoxia, the magnitude of decline was less than that of 1997. To evaluate how these changes in prey abundance may affect demersal fishes, we analyzed the diet of Atlantic croaker Micropagonias undulatus, the most abundant demersal fish in the system, prior to and after summer hypoxia of 1998. This analysis demonstrated a shift in croaker diet away from clams, an abundant item before hypoxia, to less nutritional items such as plant and detrital material afterwards. This dietary shift reflects the decreased abundance of clams as a result of hypoxia/anoxia. We conclude from our study of the Neuse River Estuary that hypoxia/anoxia events degrade essential fish habitat and that this degradation has the potential to reduce the capacity of the system to support production of demersal fisheries. KEY WORDS: Benthic invertebrates · Essential fish habitat · Habitat degradation · Hypoxia · Secondary production · Trophic linkages · Neuse River Estuary Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 302. Online publication date: November 04, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Demography of a recovering wolf population in the Yukon

We studied the dynamics of a wolf (Canis lupus) population recovering from intensive reduction in the Finlayson Lake area, Yukon, Canada. Within 6 years, numbers increased from 29 wolves, then stabilized at 245. The colonization of vacant territories by young wolf pairs was the primary mechanism of early population recovery. Reproduction and a low dispersal rate increased pack size in later years, and pack splitting allowed dispersing wolves to remain near natal packs. The rate of increase in the wolf population was density-dependent and related to wolf density, but was also related to the dispersal rate. The dispersal rate was density-independent and related to mean pack size and prey biomass : wolf index. The survival rate was age-dependent and not related to wolf density. In the early years of recovery, the rate of increase was supported by high survival rates and low dispersal rates. In later years, dispersal rates increased, stabilizing mean pack size and wolf density. Wolf density stabilized at levels predicted by the prey supply, but whether the wolf population is regulated by the availability of prey resources remains unresolved. Wolf density, pack density, and mean pack size were similar in 1983 and 1996, despite a 2- to 3-fold difference in prey biomass. We suggest that the interaction of wolf density and mean pack size in stable prey systems needs to be studied to determine the roles played by food supply and wolf social behavior in regulating wolf abundance.