| Summary: | Although species replacements along environmental gradients are commonly attributed to interspecific competition, predictions of what species distributions would be in the absence of competition are usually lacking. I tested alternative hypotheses that might account for species distributions including: food, predation, and habitat structure. These factors were unable to account for the distributions of sparrows wintering along an elevational gradient in the Sonoran Desert of southern California, USA. Sage sparrows (Amphispiza belli), black-throated sparrows (A. bilineata) and dark-eyed juncos (Junco hyemalis) inhabit different vegetation types. Under the food hypothesis, species occupy different habitats because they eat foods that occur indifferent habitats. Species ate seeds of the same species of plants, and the profitability of seeds (seed mass ingested / handling time) ranked similarly among sparrow species suggesting that species should have similar distributions. Also, species can apparently forage profitably outside of their customary habitats. Several measures of food availability suggested that two species are missing from habitats in which food is at least as available as in occupied habitats. The measures included: food standing crop; food intake rate estimated from seed abundance, seed size, and handling time; and food intake rates observed in an "introduction" experiment in which individuals of each species were placed in an aviary and allowed to forage in each habitat. Observed differences in food intake rate between habitats were small suggesting that species would be more broadly distributed if food shaped their distributions.
Predation could be responsible for habitat distributions if species are safest from predation in different habitats and predation risk is severe. Alternatively, predation can not be responsible for habitat partitioning if all species rank habitats similarly by safety. Data support the latter alternative. All species escape predators by fleeing to woody cover and feed near cover. Hence, they are likely to be safest in the habitat with the greatest amount of cover. Also, I recorded the rate at which birds scanned the environment while they foraged in an aviary to test two predictions: (1) if species experience similar changes in risk between habitats and if the same level of vigilance yields the same probability of detecting predators, species should exhibit similar changes in vigilance level between habitats; (2) if some species experience increased risk when moved from one habitat to another whereas other species experience decreased risk, species should exhibit dissimilar changes in vigilance levels. Two species for which comparisons were possible exhibited similar changes in vigilance between habitats. I also considered structural features of habitat that might affect foraging ability or the risk of predation. Foraging microhabitats used by individual species were more widely distributed than the species themselves, suggesting that species' distributions are not limited by habitat structure.
I addressed the competition hypothesis by testing two conditions necessary for competition: species must share limiting resources and they must deplete the amount of food available to one another. Species overlapped in the kinds of foods that they ate and in the types of microhabitats where they foraged. To test the food limitation hypothesis, I carried out a short-term food addition experiment. Bird densities increased as the result of the food addition, supporting the hypothesis of food limitation in the short term. The chronology and magnitude of recruitment to experimental plots illuminated the existence and dynamics of depletion. Initially less common, yet mobile species (mourning doves and house finches) recruited fastest and in the greatest numbers to experimental plots, removing up to 72 percent of the total amount of seeds available to birds. The initially more common yet least mobile species (white-crowned sparrows, black-throated sparrows) recruited more slowly and in lower numbers. Much less food was available to these later species than would have been in the absence of the earlier species. === Science, Faculty of === Zoology, Department of === Graduate
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