Linking Rates of Diffusion and Consumption in Relationto Resources

The functional response is a fundamental model of the relationship between consumer intake rate and resource abundance. The random walk is a fundamental model of animal movement and is well approximated by simple diffusion. Both models are central to our understanding of numerous ecological processes but are rarely linked in ecological theory. To derive a synthetic model, we draw on the common logical premise underlying these models and show how the diffusion and consumption rates of consumers depend on elementary attributes of naturally occurring consumer-resource interactions: the abundance, spatial aggregation, and traveling speed of resources as well as consumer handling time and directional persistence. We show that resource aggregation may lead to increased consumer diffusion and, in the case of mobile resources, reduced consumption rate. Resource-dependent movement patterns have traditionally been attributed to area-restricted search, reflecting adaptive decision making by the consumer. Our synthesis provides a simple alternative hypothesis that such patterns could also arise as a by-product of statistical movement mechanics

Seroprevalence of human T-Lymphotropic virus type I (HTLV-I) in Costa Rica

artículo -- Universidad de Costa Rica. Instituto de Investigaciones en Salud,1990Infection within the human T linphotropic virus type I (HTLV-I), a virus associated with two diseases, adult T-cell leukemia lyriphornal and tropical spastic paraparesis/HTLV-I associated myelopathy,2 is prevalent in Panama' and Colombia. To determine if HTLV-I is present in neighboring Costa Rica, we tested 436 sera from women who participated, between 1984 and 198.5, in a case-control study of cervical cancer in Costa Rica. These sera were all that were still available to us from the 765 collected from control participants in the study. Study participants had all been interviewed and tested for several serologic markers of sexually transmitted diseases (STDs).Universidad de Costa Rica. Instituto de Investigaciones en SaludUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias de la Salud::Instituto de Investigaciones en Salud (INISA

How complex do models need to be to predict dispersal of threatened species through matrix habitats?

Persistence of species in fragmented landscapes depends on dispersal among suitable breeding sites, and dispersal is often influenced by the "matrix" habitats that lie between breeding sites. However, measuring effects of different matrix habitats on movement and incorporating those differences into spatially explicit models to predict dispersal is costly in terms of time and financial resources. Hence a key question for conservation managers is: Do more costly, complex movement models yield more accurate dispersal predictions? We compared the abilities of a range of movement models, from simple to complex, to predict the dispersal of an endangered butterfly, the Saint Francis' satyr (Neonympha mitchellii francisci). The value of more complex models differed depending on how value was assessed. Although the most complex model, based on detailed movement behaviors, best predicted observed dispersal rates, it was only slightly better than the simplest model, which was based solely on distance between sites. Consequently, a parsimony approach using information criteria favors the simplest model we examined. However, when we applied the models to a larger landscape that included proposed habitat restoration sites, in which the composition of the matrix was different than the matrix surrounding extant breeding sites, the simplest model failed to identify a potentially important dispersal barrier, open habitat that butterflies rarely enter, which may completely isolate some of the proposed restoration sites from other breeding sites. Finally, we found that, although the gain in predicting dispersal with increasing model complexity was small, so was the increase in financial cost. Furthermore, a greater fit continued to accrue with greater financial cost, and more complex models made substantially different predictions than simple models when applied to a novel landscape in which butterflies are to be reintroduced to bolster their populations. This suggests that more complex models might be justifiable on financial grounds. Our results caution against a pure parsimony approach to deciding how complex movement models need to be to accurately predict dispersal through the matrix, especially if the models are to be applied to novel or modified landscapes

Breaking Functional Connectivity into Components: A Novel Approach Using an Individual-Based Model, and First Outcomes

Landscape connectivity is a key factor determining the viability of populations in fragmented landscapes. Predicting ‘functional connectivity’, namely whether a patch or a landscape functions as connected from the perspective of a focal species, poses various challenges. First, empirical data on the movement behaviour of species is often scarce. Second, animal-landscape interactions are bound to yield complex patterns. Lastly, functional connectivity involves various components that are rarely assessed separately. We introduce the spatially explicit, individual-based model FunCon as means to distinguish between components of functional connectivity and to assess how each of them affects the sensitivity of species and communities to landscape structures. We then present the results of exploratory simulations over six landscapes of different fragmentation levels and across a range of hypothetical bird species that differ in their response to habitat edges. i) Our results demonstrate that estimations of functional connectivity depend not only on the response of species to edges (avoidance versus penetration into the matrix), the movement mode investigated (home range movements versus dispersal), and the way in which the matrix is being crossed (random walk versus gap crossing), but also on the choice of connectivity measure (in this case, the model output examined). ii) We further show a strong effect of the mortality scenario applied, indicating that movement decisions that do not fully match the mortality risks are likely to reduce connectivity and enhance sensitivity to fragmentation. iii) Despite these complexities, some consistent patterns emerged. For instance, the ranking order of landscapes in terms of functional connectivity was mostly consistent across the entire range of hypothetical species, indicating that simple landscape indices can potentially serve as valuable surrogates for functional connectivity. Yet such simplifications must be carefully evaluated in terms of the components of functional connectivity they actually predict

Density- and Resource-Dependent Movement Characteristics in a Rotifer

Theory predicts that consumers can increase their energy intake by spending more time within resource‐rich areas and/or by avoiding areas where local competitor densities are high. A consumer whose movements are described by a simple random walk can achieve these objectives by adjusting its turn frequency and/or speed. We recorded movements, as series of steps, by 180 individual rotifers (Brachionus calyciflorus) in glass capillary tubes to test the influence of resource density, competitor density and their statistical interaction on movement parameters. Four treatments contrasted opposing levels of algae (resource) and conspecific density in a 2 × 2 factorial design. Our results indicate that density‐ and resource‐dependent behaviours act through different mechanisms to shape patterns of rotifer movement. Turn frequency increased up to twofold in resource‐rich treatments, depending on the presence or absence of competitors. In contrast, swimming speed was 50% greater in the presence of competitors under all treatments, but was only slightly depressed by the presence of resources alone. We show how these two different movement mechanisms may be integrated into predictions of consumer population spread as resource and competitor densities vary. We discuss implications of the contrasting and complementary nature of these different movement mechanisms and their possible adaptations to different environmental stimuli

Rotifer Population Spread in Relation to Food, Density and Predation Risk in an Experimental System

Despite the popular use of diffusion models to predict the spatial spread of populations over time, we currently know little about how diffusion rates change with the state of the environment or the internal condition of individuals. To address this gap in our understanding, we measured rates of spread for many populations of the rotifer Brachionus calyciflorus in a suite of well‐replicated experiments. 2. In one set of experiments, we manipulated food availability and population density along a continuous range of densities. In a second set, we manipulated the internal state of entire populations via food deprivation and exposure to predator kairomones. 3. Across replicate populations, diffusion rates were positively correlated with conspecific density. Diffusion rates were negatively correlated with food availability, especially when conspecific density was high. Diffusion rates of food‐deprived populations or those exposed to predation risk were lower than controls. 4. Our results provide direct experimental evidence that rates of population spread are conditional on population density, food availability, body condition and predation risk

Spatial Heterogeneity, Not Visitation Bias, Dominates Variation in Herbivory

Experiments in ecology can have unintended side effects. Recently, it has been suggested that the act of visiting a plant, inherent to studying herbivory, may alter plant performance and interactions. To evaluate the generality of this inference, we examined plant performance and herbivory on 14 plant species in three geographic regions. Visitation did not significantly affect any of the variables that we measured, including leaf damage, height, biomass, or survivorship, for any species. However, rates of herbivory varied significantly among sites and regions. Thus, our data do not support the generality of visitation impacting estimates of herbivory. We propose that future studies of herbivory will gain more by evaluating spatial heterogeneity in interaction outcomes than by quantifying possible experimenter-caused variation