Search

The selection of bedding sites is important for the ecology of ruminants, but has mainly been described for temperate species. Here we assessed the bed site selection of two Southeast Asian tropical deer, red muntjac and sambar, in Khao Yai National Park, Thailand. We surveyed transects weekly for 10 weeks each in 2003 and 2004 to locate bed sites, and compared the slope, aspect, and forest canopy cover of bed site locations between the two species and with available habitat. As with most temperate deer, muntjac and sambar both avoided sites with low levels of cover for their bed site locations; this could be for concealment or thermoregulation. Sambar also selected flatter sites than would be expected by the availability of topographic slopes; this could be to reduce the energy associated with getting to and from bed sites, or to increase long-range visibility from sites. Muntjac and sambar differed in their choice of aspects for bed sites; muntjac disproportionately chose west-facing areas, while sambar chose east-facing locations. This could represent a strategy by which one species avoids the other, or else differential resource requirements between the two species. [ABSTRACT FROM AUTHOR]

Many plants interact with groups of mutualist pollinators and seed dispersers. A key issue for both basic ecology and conservation is whether the different species within these guilds of mutualist animals are functionally equivalent. Comparing the relative effects of sympatric mutualists is important for understanding the evolution of multispecies mutualisms and for predicting mutualism stability in the face of anthropogenic change. However, empirical comparisons of the population-level impacts of mutualist animals on their host plant are rare, particularly for seed dispersal mutualisms in species-rich ecosystems. We compared the influence of three seed-dispersing tropical mammals, lar gibbons (Hylobates lar), sambar deer (Rusa unicolor), and red muntjac deer (Muntiacus muntjak), on the demography of a shared host tree in Thailand, Choerospondias axillaris (Anacardiaceae). Sambar and muntjac dispersed far more C. axillaris seeds than did gibbons. While sambar deposited many seeds under female tree canopies, muntjac were the only disperser to move seeds to open microhabitats, where C. axillaris seed germination, seedling survival, and initial growth are enhanced. Using stage-based population models, we assessed how disperser-specific seed dispersal, variation in the frequency of canopy gap formation, and their interaction influenced the potential population growth of C. axillaris. Large differences in dispersal quantity and small differences in dispersal quality among sambar and gibbons resulted in similar and negligible impacts on the tree's population dynamics. Muntjac, by taking some of the seeds to open microhabitats, are projected to have a greater positive impact on C. axillaris demography than either sambar or gibbons. Model comparisons of population-level species impacts may allow us to predict which ecological interactions are at risk from loss of critical species. [ABSTRACT FROM AUTHOR]

Indices or density estimators derived from counts of scat can be used as indirect measures of animal population density. An important issue in these studies is that scat decay rates vary across habitats and seasons. This is an often overlooked source of bias when using scat-based indices to make comparisons of relative abundance among sites or over time. I propose a simple method by which experimentally determined scat-persistence rates can be used as a correction factor in such indices. This measured persistence rate also serves as an important parameter in models that convert scat count data into population estimators. I test this method in a relative abundance survey of sambar deer (Cervus unicolor) for 6 sites across 2 national parks in northern Thailand. Using the correction factor changed the qualitative predictions of the abundance survey by altering the rank of the sites ordered by estimated sambar density. The ratio of corrected:uncorrected index values, which would stay constant across sites if there was no habitat-based variability in scat-persistence rates, changed by a factor of 2.7 when measured across sites and by 1.6 when measured across parks. This suggests that the application of persistence-rate correction factors could reduce the bias of scat-based index or estimator surveys by specifically accounting for decay rate variability. [ABSTRACT FROM AUTHOR]

Myriad tropical vertebrates are threatened by overharvest. Whether this harvest has indirect effects on nonhunted organisms that interact with the game species is a critical question. Many tropical birds and mammals disperse seeds. Their overhunting in forests can cause zoochorous trees to suffer from reduced seed dispersal. Yet how these reductions in seed dispersal influence tree abundance and population dynamics remains unclear. Reproductive parameters in long-lived organisms often have very low elasticities; indeed the demographic importance of seed dispersal is an open question. We asked how variation in hunting pressure across four national parks with seasonal forest in northern Thailand influenced the relative abundance of gibbons, muntjac deer, and sambar deer, the sole dispersers of seeds of the canopy tree Choerospondias axillaris. We quantified how variation in disperser numbers affected C. axillaris seed dispersal and seedling abundance across the four parks. We then used these data in a structured population model based on vital rates measured in Khao Yai National Park (where poaching pressure is minimal) to explore how variation in illegal hunting pressure might influence C. axillaris population growth and persistence. Densities of the mammals varied strongly across the parks, from relatively high in Khao Yai to essentially zero in Doi Suthep-Pui. Levels of C. axillaris seed dispersal and seedling abundance positively tracked mammal density. If hunting in Khao Yai were to increase to the levels seen in the other parks, C. axillaris population growth rate would decline, but only slightly. Extinction of C. axillaris is a real possibility, but may take many decades. Recent and ongoing extirpations of vertebrates in many tropical forests could be creating an extinction debt for zoochorous trees whose vulnerability is belied by their current abundance.