Projects

BEHAVIORAL ECOLOGY OF TROPICAL BIRDS

We have studied neotropical migratory birds on their tropical wintering grounds and in their breeding habitats in forests of Indiana and the Great Smoky Mountains. These are really particularly adventurous tropical birds that exploit the short but intense productive season in the "temperate" zone, and we have asked how they fit into communities throughout the year. Pete Fauth's PhD work showed how habitat fragmentation affects the reproductive success and habitat selection of migratory thrushes during the breeding season and during fall migration through the Midwest. Several undergraduate honors students have also studied the behavioral ecology of short-distance migrants that winter in Indiana. In this work, we made good use of our field station at the Ross Biological Reserve.

BirdOur investigation of cooperative breeding in tropical birds has spanned 25 years, focusing on three species in the cactus-wren genus (Campylorhynchus) in Venezuela and Peru, and on Costa Rican jays. After showing that a family approach to raising young substantially enhances reproductive success, acting through communal predator defense, and that monogamy and age-dependent dominance determine breeding status, we have focused on the process of dispersal. Many collaborators have been involved in this work, most recently Maria Carolina Yaber for her PhD. Dispersal from the natal group marks a critical transition in these birds from helping to rear siblings to the direct approach of breeding with the help of others. Such movement, and that of breeders changing venues, is important for gene flow within and between populations. We have found that there are strong competitive limits to such movement and that males and birdfemales encounter very different sets of limiting factors. Suitable habitat in the savanna mosaic is naturally patchy (and teeming with predators), so that dispersal is both dangerous and important for population viability that is probably determined at the "metapopulation" level. Differences between the sexes in dispersive behavior, and in the factors favoring or discouraging it, in turn vary strikingly among social species. Natal dispersal is male-biased in Costa Rican brown jays (reported in recent theses by Dean Williams and Amanda Hale), and their social system is more complex in part because polygamy is more common within and between groups. The following is the summary for a paper published in the Journal of Animal Ecology, entitled "Effects of sociality on short-distance, female-biased dispersal in tropical wrens," coauthored with Maria Carolina Yaber:

Dispersal from the natal area, or between breeding sites, marks a critical transition for individual animals, and a fundamental demographic and genetic process. Limitations on dispersal in natural populations are important in predicting population dynamics and structure. Tropical habitats that can be naturally patchy are also being rapidly fragmented, and these habitats harbor many social species. We studied dispersal in cooperatively breeding stripe-backed wrens (Campylorhynchus nuchalis) in the Venezuelan savanna, using 20 years of data from marked populations over 40km2. We test the hypotheses that dispersal is limited by competition for social position and that such limitation is sex-specific. Dispersal is strongly female-biased, and mainly results in attainment of breeding status in neighboring groups. Reproductive success is determined by the structure of the breeding group, and is concentrated in a small minority of individuals. Natal dispersal is delayed, and represents a dramatic shift from indirect contribution to kin production (helping) to the direct approach of breeding. Monogamy limits breeding opportunities, compared to other social species, and both natal and breeding dispersal are constrained to short distances, particularly when compared to non-cooperative breeders. Males are more philopatric, compared to females for which more frequent and distant dispersal leads to greater reproductive success. Competition for breeding status in an established social order impedes dispersal, in part because experience and previous familiarity are important, particularly in males that rarely disperse to productive large groups. Diverse patterns of sex bias in dispersal among cooperatively breeding birds can be explained by the comparative diversity of reproductive alternatives available to males and females. A "Breeding Diversity" hypothesis is illustrated by comparison with male-biased dispersal in brown jays. Social resistance to movement establishes a brake on gene flow and demographic interaction among populations that has important implications for population viability.

DIVERSITY AND STABILITY OF NATURAL COMMUNITIES

Highlands of Costa RicaThe highlands of Costa Rica and Panama are recognized internationally as a high-priority region for conservation, since many species occur only there and because specialization to narrow altitudinal zones creates patchy ecological distributions and small vulnerable populations. Montane rainforest and cloudforest are distributed in a high-elevation "archipelago" through Central America, and geographical variation in species composition makes each mountain range unique. We have focused on indicator species that are characteristic of cloudforest and sensitive to habitat modification, and we have targeted sets of related species to ask whether species with the most limited geographical distributions are also the most locally specialized and threatened. We find that communities demonstrate integrity within habitats and between reserves, that communities change birdstrikingly across very narrow zones of subtle environmental change, and that many endemic species (like some nightingale-thrushes closely related to latitudinal migrants) are sufficiently specialized to intact high-elevation habitats to put them at risk with respect to deforestation and climate change. Our team, including current graduate students Matt Gasner, Keiller Kyle, and Meghan Lout, plans to continue these studies to better understand local distributions and resulting between-habitat diversity ("beta" diversity). The following is an abstract for a talk presented at the 2002 meeting of the Ecological Society of America, entitled "Spatial organization of diversity in tropical montane forest avifaunas: Implications for persistence of endemic species."

We investigate the rate of change in species composition of bird communities across a variety of scales in the mountain forests of Costa Rica. Understanding how high regional biodiversity of areas like Central America is partitioned between local ("alpha") diversity of small areas (10-100ha) within particular habitat types, and landscape ("beta") diversity over larger areas encompassing multiple habitats, is a longstanding conceptual goal in ecology and an important guide for conservation planning. If species are sufficiently specialized for particular forest types along an altitudinal gradient, beta diversity will be high and regionally endemic species may be further limited spatially on a local scale. We ask whether habitat specialization is associated with rarity within habitat types, resulting in a syndrome of vulnerability for regional endemics that are rare at all scales. We find strong similarity of species composition at a particular altitude (lower montane rainforest), both within the Monteverde Cloudforest Reserve, and between neighboring reserves in the Tilarán range. Using comparisons with other studies, we find differences increasing with distance among mountain ranges. Species composition changes rapidly with altitude on tropical mountains, especially on the Pacific slope of Costa Rica, and our data show surprisingly rapid change even within recognized life zones. At the highest elevations, where the impact of climate change is greatest, species with geographically limited ranges tend to also have narrow ecological tolerances and to be rare within their habitats. This accumulation of risk factors makes these endemics particularly vulnerable to extinction.

Winter in the mountains We have just renewed a long-term study of bird communities in Great Smoky Mountains National Park that is related to the work in the Costa Rican mountains. Temperate mountains also support a vertically organized array of community types, but both species richness and beta diversity, the rate of change in species composition with altitude, is much reduced. To better understand this, and to test the stability of bird community composition, we conducted intensive breeding-bird censuses in diverse old-growth cove-hardwoods forests and in high-elevation spruce-fir forests. The former has been studied for a decade in collaboration with ecologists at North Carolina State University, and the latter has been monitored for 30 years by Rabenold and coworkers. Spruce-fir forests, evocative of Canadian coniferous forests, are scattered on the highest ridges of the southern Appalachians, and harbor many endemic species. They have been impacted substantially by a combination of logging, invasions by exotic insects, and chemical pollution. We have tested the idea that large blocks of forest, like those in the Smokies, should be more resilient to such impacts than smaller areas in the smaller ranges of the southern Appalachians. Work of a decade ago upheld this expectation (published in the journal Conservation Biology), and our current results reinforce it as well. While the spruce-fir forests have changed substantially in the last few decades, the more diverse cove forests have been more stable, although their future is uncertain because of the recent arrival of a new exotic insect threat.

DogwoodAaron Pierce completed his thesis work on the dynamics of Midwestern hardwoods forests, focused at our Ross Biological Reserve. His work built upon studies of Bill Bromer 20 years ago, and work of students of Alton Lindsey who founded the reserve 50 years ago. The forests have changed substantially in this time, including ecological succession on previously deforested land and in the mature forests as well. A striking 50% decline in flowering dogwood (Cornus florida) appears to be caused not by fungal pathogens as in other parts of the US, but by increasing density of the canopy associated with an increase in the density of sugar maple (Acer saccharum). This change in the canopy species is probably a return to the more humid forests characteristic of the Midwest before the human use of fire favored the drier oak/hickory community.

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