Habitat loss and disturbance due to human activity are major causes of global biodiversity decline. Beyond outright species loss, one potential outcome is modification of species interactions that are integral to ecosystem functioning. To investigate this possibility, I asked whether human activity influences avian frugivory and seed dispersal, bird-fruit interactions that facilitate plant reproduction. On Tobago (West Indies), I compared patterns of frugivory in three adjacent rainforest habitats along a gradient of increasing disturbance: primary forest in a reserve, unprotected intermediate forest outside the reserve, and nearby forest that was moderately disturbed by subsistence resource use. I assessed plant and bird community composition, seedling species, fruit removal, and bird fecal samples, to estimate human effects on seed dispersal and plant recruitment in this ecosystem. Disturbed forest had different species assemblages than primary forest, characterized by more light-demanding plants, more birds, and a shift in the relative abundance of avian feeding guilds: insectivores and frugivores declined, while nectarivores and omnivores increased. Canopy cover declined with disturbance; along with plant abundance, this explained much of the variation in bird species composition. The rate of avian fruit consumption in removal experiments varied considerably but tended to be highest in primary forest. Fecal samples showed that fruit composition of avian diets also varied with disturbance; birds captured in disturbed forest consumed more seeds from light-demanding plants. Seeds in the samples provided evidence of some seed transfer between habitats—from disturbed forest into the reserve and vice versa. Seedling composition was consistent with plant species fruiting in the same study plots, and illustrated some successful recruitment of light-demanding plants in primary forest and shade-tolerant plants in disturbed forest. Notably, the plant community of intermediate forest was more similar to disturbed than primary forest. This suggests that habitat adjacent to areas of human activity can be susceptible to ecological change, even though it does not experience the same direct disturbance. In summary, the unprotected portion of Tobago’s rainforest has a markedly different plant and bird community than the forest reserve, and my results indicate that avian frugivory and seed dispersal can be influenced by moderate human activity.

Tropical freshwater ecosystems are understudied and not well understood relative to temperate systems; however, they are becoming increasingly imperiled by escalating anthropogenic impacts. The aim of this thesis was to investigate how tropical freshwater macroinvertebrate communities changed both structurally and functionally over a pristine-rural-urban land-use gradient, in relation to different spatial and temporal scales, as well as to the availability of potential food sources. Fifteen streams in the Panama Canal Watershed were sampled during the dry and wet seasons of 2007 and 2008, for macroinvertebrate communities (benthic and leaf litter), environmental variables, and potential food sources. Along the land-use gradient, in both habitat types, taxon richness, diversity, and evenness all decreased significantly; whereas, abundance increased significantly. For the benthic macroinvertebrate community, unique variation was explained equally well by local (water chemistry and sediment type) and landscape (riparian vegetation and watershed land use) characteristics in the dry season, and landscape characteristics explained slightly more variation in the wet season. Leaf-litter macroinvertebrate community unique variation was better explained by local variables than by landscape variables in both seasons. In terms of potential food resources, fine detritus and inorganic material were the most common across all streams (increased quantities in urban streams) and seasons; whereas, the availability of diatoms and leaf material increased in the dry season. Using gut content analyses, we found that collectors (gatherers and filterers) were by far the most common functional feeding group, increasing in abundance along the land-use gradient. Predators, shredders, and scrapers were all most abundant in pristine streams and decreased along the land-use gradient. Finally, using seven community metrices, a potential biomonitoring tool was developed, the Neotropical Low-land Stream Multimetric Index (NLSMI), which distinguished well among the different levels of stream impairment. This study demonstrated that tropical communities were negatively affected by human land alteration, but that community responses depended on the habitat sampled, the influence of different spatial scales varied between the seasons, and the effect of food resources appeared to be complex. These aspects must be taken into consideration for management decisions and restoration strategies to be effective.

Tropical freshwater ecosystems are understudied and not well understood relative to temperate systems; however, they are becoming increasingly imperiled by escalating anthropogenic impacts. The aim of this thesis was to investigate how tropical freshwater macroinvertebrate communities changed both structurally and functionally over a pristine-rural-urban land-use gradient, in relation to different spatial and temporal scales, as well as to the availability of potential food sources. Fifteen streams in the Panama Canal Watershed were sampled during the dry and wet seasons of 2007 and 2008, for macroinvertebrate communities (benthic and leaf litter), environmental variables, and potential food sources. Along the land-use gradient, in both habitat types, taxon richness, diversity, and evenness all decreased significantly; whereas, abundance increased significantly. For the benthic macroinvertebrate community, unique variation was explained equally well by local (water chemistry and sediment type) and landscape (riparian vegetation and watershed land use) characteristics in the dry season, and landscape characteristics explained slightly more variation in the wet season. Leaf-litter macroinvertebrate community unique variation was better explained by local variables than by landscape variables in both seasons. In terms of potential food resources, fine detritus and inorganic material were the most common across all streams (increased quantities in urban streams) and seasons; whereas, the availability of diatoms and leaf material increased in the dry season. Using gut content analyses, we found that collectors (gatherers and filterers) were by far the most common functional feeding group, increasing in abundance along the land-use gradient. Predators, shredders, and scrapers were all most abundant in pristine streams and decreased along the land-use gradient. Finally, using seven community metrices, a potential biomonitoring tool was developed, the Neotropical Low-land Stream Multimetric Index (NLSMI), which distinguished well among the different levels of stream impairment. This study demonstrated that tropical communities were negatively affected by human land alteration, but that community responses depended on the habitat sampled, the influence of different spatial scales varied between the seasons, and the effect of food resources appeared to be complex. These aspects must be taken into consideration for management decisions and restoration strategies to be effective.

No description available.

No description available.

No description available.

The thesis consists of three projects. Each of these projects is a diagnostic study of the interaction between strong convective events and the background atmosphere. In all projects, we use a satellite rainfall dataset to identify strong rain events. We then use radiosonde soundings to generate composite anomaly patterns of meteorological variables about the strong rain events. In Project 1, we examine temperature, relative humidity, and divergence anomalies about strong convective events in the Western Tropical Pacific. A low-level convergence coupled to a midlevel divergence develops prior to peak rainfall. A midlevel convergence coupled to a low-level divergence develops after peak rainfall. Strong surface cold pools develop in response to high rainfall. Observations were compared to models and reanalyses. In general, models and reanalyses do not fully represent the timing, strength, and altitude of the mid-level convergence and divergence features. The surface cold anomaly is also underestimated in models. These discrepancies suggest that the mesoscale downward transport of mid-level air into the boundary layer in models may be too weak. In Project 2, we investigate the impact of convection on the background distribution of a chemical tracer (ozone). Negative ozone anomalies and higher frequency of midlevel cloud tops occur in a layer between 3 and 8 km prior to peak rainfall. Negative ozone anomalies in the upper troposphere develop in response to high rainfall. Chemistry transport model simulations also exhibit negative ozone anomalies at upper and midlevels. However, the ozone anomalies in the model are symmetric about peak rainfall and are more persistent than observations. In Project 3, we identify regional variations in the interaction between convection and the background atmosphere. In all four regions, deep convection imposes cooling in the lower and warming in the upper troposphere. In mid-latitudes, convection is associated with stronger anomalies in surface pressure, geopotential height, and CAPE. Over land, a low-level warm anomaly develops prior to peak rainfall and the surface cold pool that develops during peak rainfall is more persistent. The PV generated prior to peak rainfall, is advected towards the surface after peak rainfall and may play a role in hurricane genesis.

No description available.

Weeds severely limit agricultural production in many Third World tropical countries where productivity is already marginal due to unfavourable environmental, biological, economic and social factors. Lack of both ecological and distributional information at the scale of the weed assemblage, and lack of detailed data on the life histories and responses of the constituent weedy species hinders development of cheap and effective weed management strategies not contingent on intensive use of herbicides. / Plant geographers can contribute to the solution of tropical weed problems by investigating factors which mediate the dynamics of species: regional and local ranges. The research in this thesis departs from other plant geographic studies on tropical week assemblages (e.g. Kellman, 1973) by undertaking experimental field studies which are designed to uncover aspects of the differential abilities of species to make the transition from localized immigrant population to widespread and abundant weed. / The research focusses on six alien species invading agricultural and non-agricultural land in a lowland tropical region: coastal Guyana. Experiments are directed towards propositions concerning the basis for differential colonizing success of two common (Echinochloa colonum (L) Link, Macroptilium lathyroides L.) and four uncommon (Asclepias curassavica L., Emilia fosbergii D. H. Nicholson, Malachra alceifolia Jacq., Paspalum virgatum L.) species of cultivated fields in an area of prime agricultural land. Experimental colonizing trials, which equalize the opportunities of each species to colonize, provide a reference point against which to assess inter- and intra-species differences. / Results of colonizing trials suggest that the species represent four ecological groups. These are Macroptilium and Emilia (Group 1), Echinochloa (Group 2), Malachra and Asclepias (Group 3), and Paspalum (Group 4). / Species of Group 1 have the highest potential for establishment in cultivated fields within a single growing season. Emilia, though currently not a prominent species in the study area, should be controlled early. Heavy insect predation and consequent slow maturity of Asclepias reduce the likelihood of its rapid establishment in the area. Malachra and Paspalum, though not predated, have slow generation times, and hence are also unlikely to rapidly invade frequently cultivated fields. However, the high incidence of interruption of cultivation in the coastal farmlands is likely to promote their range expansion.

Habitat loss and disturbance due to human activity are major causes of global biodiversity decline. Beyond outright species loss, one potential outcome is modification of species interactions that are integral to ecosystem functioning. To investigate this possibility, I asked whether human activity influences avian frugivory and seed dispersal, bird-fruit interactions that facilitate plant reproduction. On Tobago (West Indies), I compared patterns of frugivory in three adjacent rainforest habitats along a gradient of increasing disturbance: primary forest in a reserve, unprotected intermediate forest outside the reserve, and nearby forest that was moderately disturbed by subsistence resource use. I assessed plant and bird community composition, seedling species, fruit removal, and bird fecal samples, to estimate human effects on seed dispersal and plant recruitment in this ecosystem. Disturbed forest had different species assemblages than primary forest, characterized by more light-demanding plants, more birds, and a shift in the relative abundance of avian feeding guilds: insectivores and frugivores declined, while nectarivores and omnivores increased. Canopy cover declined with disturbance; along with plant abundance, this explained much of the variation in bird species composition. The rate of avian fruit consumption in removal experiments varied considerably but tended to be highest in primary forest. Fecal samples showed that fruit composition of avian diets also varied with disturbance; birds captured in disturbed forest consumed more seeds from light-demanding plants. Seeds in the samples provided evidence of some seed transfer between habitats—from disturbed forest into the reserve and vice versa. Seedling composition was consistent with plant species fruiting in the same study plots, and illustrated some successful recruitment of light-demanding plants in primary forest and shade-tolerant plants in disturbed forest. Notably, the plant community of intermediate forest was more similar to disturbed than primary forest. This suggests that habitat adjacent to areas of human activity can be susceptible to ecological change, even though it does not experience the same direct disturbance. In summary, the unprotected portion of Tobago’s rainforest has a markedly different plant and bird community than the forest reserve, and my results indicate that avian frugivory and seed dispersal can be influenced by moderate human activity.