archives@tulane.edu / Although interspecific competition and relevant coexistence mechanisms are core concepts in ecology and evolution, most such research has focused on two-species interactions. Lacking is important information on how and when interspecific competition operates in more complex species assemblages. This dissertation examined interspecific competition and coexistence mechanisms between a focal species, the American redstart (Setophaga ruticilla, Parulidae) and its potential competitors in multiple communities. Using a theoretical framework that builds on the well documented population ecology of redstarts, combined with interspecific dietary overlaps, I demonstrate ongoing interspecific competition. I found that species generally both overlap highly in diet and exhibit small, but consistent, differences consistent with their foraging behavioral differences. Although some competitive interactions were mediated by direct aggressive interactions, this research also highlighted diffuse competition as more important than previously recognized. My findings also differed from systems that have shown similar patterns insofar as high dietary overlaps persisted during times of resource scarcity, were driven by overlaps on low value prey taxa, and did not involve exclusive use of any resources by different competitors. All of these factors increase the potential negative demographic effects of interspecific competition, some of which have been documented in these bird species. A long history of competition studies in these warblers has shown large differences in foraging behavior among species, assumed to be what allows coexistence. However, it is not clear how and if differences in foraging behavior correspond to differences in resource use. I confirmed that locally coexisting study species differed almost completely in foraging behavior, particularly in microhabitat use. Likewise, using a variety of arthropod sampling methods, I found that microhabitats differed greatly in their available prey. Combining the large differences in foraging behavior with available prey significantly predicted the small dietary differences, although it did a poor job of explaining the already limited variation in diet. Overall, I found mixed support for behavioral niche partitioning, leading me to examine alternative mechanisms of coexistence. Contrary to past work focused on interspecific interactions, I found that population level forces dominate in determining local distributions, with birds of the same species being more evenly distributed due to intraspecific territoriality. This pattern led to higher local (point count scale) species richness, and greater community covariance than predicted. This intraspecific territoriality and repulsion is proposed as as a novel coexistence mechanism. / 1 / Cody M. Kent

Diverse fish species consume fruits and seeds in the Neotropics, in particular in the lowland reaches of large rivers, such as the Amazon, Orinoco, and Parana in South America. Floodplains of the Amazon River and its lowland tributaries are characterized by marked hydrological seasonality and diverse assemblages of frugivorous fishes, including closely related and morphologically similar species of several characiform families. Here, I investigated whether or not these fishes are capable of detecting fluctuations in food availability and if they are, how they adjust their feeding strategies. I tested predictions of optimal foraging, limiting similarity and resource partitioning theories with regard to expansion or compression of niche breadth and reduction in trophic niche overlap among species in relation with fluctuations in the availability of alternative food resources. I monitored fruiting phenology patterns to assess food availability and conducted intensive fishing during the high-, falling-, and low-water seasons in an oligotrophic river and an adjacent oxbow lake in the Colombian Amazon. I combined analysis of stomach contents and stable isotope ratios to evaluate dietary patterns, niche breadth, and niche overlap. Diets of six characiform fish species (Brycon falcatus, B. melanopterus, Myloplus asterias, M. rubripinnis, and M. torquatus) changed in a manner that indicated responses to fluctuations in food availability. Feeding strategies during the peak of the flood pulse were consistent with predictions of optimal foraging theory. During times of high fruit abundance, fish preferentially consumed items to which their phenotype is best adapted, maximizing net energy gain and enhancing fitness. As the flood pulse subsided and the availability of forest food resources was reduced in aquatic habitats, there was not a consistent pattern of diet breadth expansion or compression, even though diet shifts occurred, suggesting interspecific differences in foraging efficiencies. Analyses of diets and isotopic ratios revealed a general pattern of increased dietary segregation as the water level receded. Although there never was complete niche segregation among these fishes, these dietary changes effectively reduced interspecific niche overlap. Implications of these results and contribution of allochthonous food resources to diversity maintenance of floodplain fishes are discussed.

No description available.

Climate change can affect habitat availability and species interactions at several spatial and temporal scales. I explored niche partitioning and spatial variation of Rock (Lagopus muta) and White-tailed Ptarmigan (L. leucura) in southwest Yukon. I examined habitat selection of foraging areas within a population and patches within foraging areas in a sympatric population of Rock and White-tailed Ptarmigan. At the larger foraging area scale, Rock Ptarmigan used areas with greater shrub cover compared to White-tailed Ptarmigan. At the smaller patch scale, both species selected patches with greater rock cover, but differed in other patch features. Second, I examined spatial variation in abundance of both ptarmigan species between the Ruby and Kluane Ranges using pellet count and transect surveys. Relative abundance was lower in the Kluane Range based on pellet counts, but transect surveys proved inadequate as a measure of population density. The Kluane Range also had fewer positive degree days above 0 C and a greater mean standard deviation of NDVI, and was composed of finer textured colluvium compared to the Ruby Range, which could influence relative abundance of ptarmigan. / Ecology

Magister Scientiae (Biodiversity and Conservation Biology) - MSc (Biodiv and Cons Biol) / Studies on snake competitive interactions have relatively been well documented globally, however, those examples tend to be dominated by non-African examples. Africa has a large and spectacular reptile diversity and yet robust and empirical studies on snake population ecology remain poorly understood or documented. Given the close phylogenetic relationship between the two species, as well as the remarkable similarities in overall appearance, morphology, reproductive biology, and most importantly geographic distribution, Psammophis crucifer and Psammophylax rhombeatus offered an ideal study system in which to ask questions related to interspecific competition and niche partitioning.

Saber-toothed remains have been found worldwide throughout the Cenozoic, until the end of the Pleistocene. One site from Alachua County, Florida preserves a diverse Miocene fauna, including the machairodontine Nimravides galiani (Felidae) and the saber-toothed Barbourofelis loveorum (Barbourofelidae). Both taxa roamed what would become the Love Bone Bed site during the Late Miocene (Late Clarendonian NALMA), ~ 9.5 Mya. Previous descriptions focused on crania; yet the large sample of postcrania remained undescribed. Hence, this project includes a detailed postcranial description of both taxa. Results show that N. galiani resembles extant felids, whereas B. loveorum resembles Smilodon fatalis, as well as ursids. Additionally, locomotion and hunting behavior (prey capture) was examined quantitatively to assess ecologic overlap (niche partitioning vs direct competition). N. galiani probably displayed terrestrial locomotion in open habitats, whereas B. loveorum likely inhabited the deciduous forest as an ambulatory opportunistic/ambush predator. Results support niche partitioning, rather than direct competition.

Masters of Science / Studies on snake competitive interactions have relatively been well documented globally, however, those examples tend to be dominated by non-African examples. Africa has a large and spectacular reptile diversity and yet robust and empirical studies on snake population ecology remain poorly understood or documented. Given the close phylogenetic relationship between the two species, as well as the remarkable similarities in overall appearance, morphology, reproductive biology, and most importantly geographic distribution, Psammophis crucifer and Psammophylax rhombeatus offered an ideal study system in which to ask questions related to interspecific competition and niche partitioning. Specifically I asked (1) whether broad scale geographic sympatry is facilitated by fine-scale allopatry through separation of space-use, and (2) whether the diets of the two species provided evidence of partitioning along the dietary niche. To answer these questions, I first quantified relative abundance of the two species at a fine scale, and secondly used existing dietary data to quantify dietary niche overlap. Even though P. rhombeatus was always more abundant in my sample, I found no evidence of space-use partitioning in this study, instead it showed a positive correlation in their abundance, and therefore suggesting space was not a limiting resource. Pianka niche overlap analysis showed significant differences in their feeding habits whereby P. rhombeatus had a broader diet which included mammals and birds, whereas P. crucifer predominantly fed on lizards and other snakes. In conclusion, my study suggests that across multiple geographic scales these two snakes use the same spatial resources and are able to co-exist by partitioning food resources. Lastly, my study serves to provoke more African studies of this nature with suitable candidate snake species.

Niche partitioning is a major component in understanding community ecology and how ecologically similar species coexist. Temporal and spatial partitioning and differences in foraging strategy, including sensitivity to risk (variance), likely contribute to partitioning as well. Here, we approach this partitioning with fine resolution to investigate differences in overall strategy between two species of diurnal, orb-weaving spiders, Verrucosa arenata and Micrathena gracilis (Araneae: Araneidae), that share similar spatial positioning, temporal foraging window, and prey. Through field observation, we found that V. arenata individuals appear to increase spatial and temporal sampling to compensate for an overall risk-prone strategy that depends on the interception and active capture of rare, large prey. Conversely, M. gracilis individuals employ a risk-averse strategy relying on passive capture of small but abundant prey consumed alongside the orb. We have thus identified how differing risk-sensitive foraging strategies may contribute to niche partitioning between otherwise similar species.

Guilds of species that share ecological similarities or recent ancestors are often the basis of studies concerning habitat partitioning. An interesting result of many of these studies is that means of partitioning in one area of sympatry may be different from those in disjunct areas of overlap. Here, we quantified the differences in habitat preference of two forest thrushes in the southern Appalachian Mountains. The Hermit Thrush (Catharus guttatus) has recently expanded its breeding range south along the spine of the Appalachians into the spruce-fir forests of Virginia, North Carolina, and Tennessee. There, it shares breeding habitat with the congeneric Veery (C. fuscescens), previously the only forest thrush breeding in this area. We used several multivariate analyses of habitat variables within both species' territories to understand how these species partition the available habitat and to test if these means of partitioning are similar to those found by other studies. We calculated the niche overlap to be 0.248, indicating that the habitat structures preferred by these two species are highly distinct. Hermit Thrushes generally preferred forest with a high degree of canopy closure, high percent coverage of leaf litter, and low percent cover below 3.0 m. Veery territories generally had a high percent of cover between 0.5 and 3.0 m (i.e., the shrub layer) and less canopy closure than the Hermit Thrush plots. Our data suggest that the Hermit Thrush is expanding south because it is attracted to the maturing of the spruce forests in this region.

Guilds of species that share ecological similarities or recent ancestors are often the basis of studies concerning habitat partitioning. An interesting result of many of these studies is that means of partitioning in one area of sympatry may be different from those in disjunct areas of overlap. Here, we quantified the differences in habitat preference of two forest thrushes in the southern Appalachian Mountains. The Hermit Thrush (Catharus guttatus) has recently expanded its breeding range south along the spine of the Appalachians into the spruce-fir forests of Virginia, North Carolina, and Tennessee. There, it shares breeding habitat with the congeneric Veery (C. fuscescens), previously the only forest thrush breeding in this area. We used several multivariate analyses of habitat variables within both species' territories to understand how these species partition the available habitat and to test if these means of partitioning are similar to those found by other studies. We calculated the niche overlap to be 0.248, indicating that the habitat structures preferred by these two species are highly distinct. Hermit Thrushes generally preferred forest with a high degree of canopy closure, high percent coverage of leaf litter, and low percent cover below 3.0 m. Veery territories generally had a high percent of cover between 0.5 and 3.0 m (i.e., the shrub layer) and less canopy closure than the Hermit Thrush plots. Our data suggest that the Hermit Thrush is expanding south because it is attracted to the maturing of the spruce forests in this region.