A stage-structured, Aedes albopictus population model

Aedes albopictus has been the fastest spreading invasive animal species in the world from the mid-1980s until the mid-2000s. In areas it infests, it disrupts native mosquito ecology and can potentially vector up to 21 viruses. To better understand the population dynamics of this species, we created a temperature dependent population model. A stage-structured model was chosen to allow each life-stage to have different temperature dependent mortality and development rates, and each stage was modeled with an ordinary differential equation. Model parameters and distributions were based upon literature values. Initially, a basic model was constructed. This model then had parameters that were forced based upon daily average temperatures. Several criteria were used to evaluate the model, including a comparison to field data from Lubbock, TX. In a stochastic version of the model, a 95% confidence limit contained 70.7% of the field data points. Based upon these results, we feel reasonably confident that we have captured the role of temperature in driving the population dynamics of Ae. albopictus.     

Continuous versus binary representations of landscape heterogeneity in spatially-explicit models of mobile populations

How a landscape is represented is an important structural assumption in spatially-explicit simulation models. Simple models tend to specify just habitat and non-habitat (binary), while more complex models may use multiple levels or a continuum of habitat quality (continuous). How these different representations influence model projections is unclear. To assess the influence of landscape representation on population models, I developed a general, individual-based model with local dispersal and examined population persistence across binary and continuous landscapes varying in the amount and fragmentation of habitat. In binary and continuous landscapes habitat and non-habitat were assigned a unique mean suitability. In continuous landscapes, suitability of each individual site was then drawn from a normal distribution with fixed variance. Populations went extinct less often and abundances were higher in continuous landscapes. Production in habitat and non-habitat was higher in continuous landscapes, because the range of habitat suitability sampled by randomly dispersing individuals was higher than the overall mean habitat suitability. Increasing mortality, dispersal distance, and spatial heterogeneity all increased the discrepancy between continuous and binary landscapes. The effect of spatial structure on the probability of extinction was greater in binary landscapes. These results show that, under certain circumstances, model projections are influenced by how variation in suitability within a landscape is represented. Care should be taken to assess how a given species actually perceives the landscape when conducting population viability analyses or empirical validation of theory.     

Field evidence for pervasive indirect effects of fishing on prey foraging behavior

The indirect, ecosystem-level consequences of ocean fishing, and particularly the mechanisms driving them, are poorly understood. Most studies focus on density-mediated trophic cascades, where removal of predators alternately causes increases and decreases in abundances of lower trophic levels. However, cascades could also be driven by where and when prey forage rather than solely by prey abundance. Over a large gradient of fishing intensity in the central Pacific's remote northern Line Islands, including a nearly pristine, baseline coral reef system, we found that changes in predation risk elicit strong behavioral responses in foraging patterns across multiple prey fish species. These responses were observed as a function of both short-term ("acute") risk and longer-term ("chronic") risk, as well as when prey were exposed to model predators to isolate the effect of perceived predation risk from other potentially confounding factors. Compared to numerical prey responses, antipredator behavioral responses such as these can potentially have far greater net impacts (by occurring over entire assemblages) and operate over shorter temporal scales (with potentially instantaneous response times) in transmitting top-down effects. A rich body of literature exists on both the direct effects of human removal of predators from ecosystems and predators' effects on prey behavior. Our results draw together these lines of research and provide the first empirical evidence that large-scale human removal of predators from a natural ecosystem indirectly alters prey behavior. These behavioral changes may, in turn, drive previously unsuspected alterations in reef food webs.     

Species diversity reduces parasite infection through cross-generational effects on host abundance

With growing interest in the effects of biodiversity on disease, there is a critical need for studies that empirically identify the mechanisms underlying the diversity-disease relationship. Here, we combined wetland surveys of host community structure with mechanistic experiments involving a multi-host parasite to evaluate competing explanations for the dilution effect. Sampling of 320 wetlands in California indicated that snail host communities were strongly nested, with competent hosts for the trematode Ribeiroia ondatrae predominating in low-richness assemblages and unsuitable hosts increasingly present in more diverse communities. Moreover, competent host density was negatively associated with increases in snail species richness. These patterns in host community assembly support a key prerequisite underlying the dilution effect. Results of multigenerational mesocosm experiments designed to mimic field-observed community assemblages allowed us to evaluate the relative importance of host density and diversity in influencing parasite infection success. Increases in snail species richness (from one to four species) had sharply negative effects on the density of infected hosts (-90% reduction). However, this effect was indirect; competition associated with non-host species led to a 95% reduction in host density (susceptible host regulation), owing primarily to a reduction in host reproduction. Among susceptible hosts, there were no differences in infection prevalence as a function of community structure, indicating a lack of support for a direct effect of diversity on infection (encounter reduction). In monospecific conditions, higher initial host densities increased infection among adult hosts; however, compensatory reproduction in the low-density treatments equalized the final number of infected hosts by the next generation, underscoring the relevance of multigenerational studies in understanding the dilution effect. These findings highlight the role of interspecific competition in mediating the relationship between species richness and parasite infection and emphasize the importance of field-informed experimental research in understanding mechanisms underlying the diversity-disease relationship.     

Community ecology of invasions: direct and indirect effects of multiple invasive species on aquatic communities

With many ecosystems now supporting multiple nonnative species from different trophic levels, it can be challenging to disentangle the net effects of invaders within a community context. Here, we combined wetland surveys with a mesocosm experiment to examine the individual and combined effects of nonnative fish predators and nonnative bullfrogs on aquatic communities. Among 139 wetlands, nonnative fish (bass, sunfish, and mosquitofish) negatively influenced the probability of occupancy of Pacific treefrogs (Pseudacris regilla), but neither invader correlated strongly with occupancy by California newts (Taricha torosa), western toads (Anaxyrus boreas), or California red-legged frogs (Rana draytonii). In mesocosms, mosquitofish dramatically reduced the abundance of zooplankton and palatable amphibian larvae (P. regilla and T. torosa), leading to increases in nutrient concentrations and phytoplankton (through loss of zooplankton), and rapid growth of unpalatable toad larvae (through competitive release). Bullfrog larvae reduced the growth of native anurans but had no effect on survival. Despite strong effects on natives, invaders did not negatively influence one another, and their combined effects were additive. Our results highlight how the net effects of multiple nonnative species depend on the trophic level of each invader, the form and magnitude of invader interactions, and the traits of native community members.     

Decoupling of component and ensemble density feedbacks in birds and mammals

A component density feedback represents the effect of change in population size on single demographic rates, whereas an ensemble density feedback captures that effect on the overall growth rate of a population. Given that a population's growth rate is a synthesis of the interplay of all demographic rates operating in a population, we test the hypothesis that the strength of ensemble density feedback must augment with increasing strength of component density feedback, using long-term censuses of population size, fertility, and survival rates of 109 bird and mammal populations (97 species). We found that compensatory and depensatory component feedbacks were common (each detected in approximately 50% of the demographic rates). However, component feedback strength only explained <10% of the variation in ensemble feedback strength. To explain why, we illustrate the different sources of decoupling between component and ensemble feedbacks. We argue that the management of anthropogenic impacts on populations using component feedbacks alone is ill-advised, just as managing on the basis of ensemble feedbacks without a mechanistic understanding of the contributions made by its components and environmental variability can lead to suboptimal decisions.     

Phenological tracking enables positive species responses to climate change

Earlier spring phenology observed in many plant species in recent decades provides compelling evidence that species are already responding to the rising global temperatures associated with anthropogenic climate change. There is great variability among species, however, in their phenological sensitivity to temperature. Species that do not phenologically "track" climate change may be at a disadvantage if their growth becomes limited by missed interactions with mutualists, or a shorter growing season relative to earlier-active competitors. Here, we set out to test the hypothesis that phenological sensitivity could be used to predict species performance in a warming climate, by synthesizing results across terrestrial warming experiments. We assembled data for 57 species across 24 studies where flowering or vegetative phenology was matched with a measure of species performance. Performance metrics included biomass, percent cover, number of flowers, or individual growth. We found that species that advanced their phenology with warming also increased their performance, whereas those that did not advance tended to decline in performance with warming. This indicates that species that cannot phenologically "track" climate may be at increased risk with future climate change, and it suggests that phenological monitoring may provide an important tool for setting future conservation priorities.     

Telomere dynamics in the Sydney rock oyster (<Emphasis Type="Italic">Saccostrea glomerata</Emphasis>): an investigation into the effects of age,tissue type,location and time of sampling

Telomere length has been purported as a biomarker for age and could offer a non-lethal method for determining the age of wild-caught individuals. Molluscs, including oysters and abalone, are the basis of important fisheries globally and have been problematic to accurately age. To determine whether telomere length could provide an alternative means of ageing molluscs, we evaluated the relationship between telomere length and age using the commercially important Sydney rock oyster (Saccostrea glomerata). Telomere lengths were estimated from tissues of known age individuals from different age classes, locations and at different sampling times. Telomere length tended to decrease with age only in young oysters less than 18 months old, but no decrease was observed in older oysters aged 2–4 years. Regional and temporal differences in telomere attrition rates were also observed. The relationship between telomere length and age was weak, however, with individuals of identical age varying significantly in their telomere length making it an imprecise age biomarker in oysters.     

Assessing the implications of a 1.5 °C temperature limit for the Jamaican agriculture sector

Despite recent calls to limit future increases in the global average temperature to well below 2 °C, little is known about how different climatic thresholds will impact human society. Future warming trends have significant global food security implications, particularly for small island developing states (SIDS) that are recognized as being among the most vulnerable to global climate change. In the case of the Caribbean, any significant change in the region’s climate is likely to have significant adverse effects on the agriculture sector. This paper explores the potential biophysical impacts of a +?1.5 °C warming scenario on several economically important crops grown in the Caribbean island of Jamaica. Also, it explores differences to a >?2.0 °C warming scenario, which is more likely, if the current policy agreements cannot be complied with by the international community. We use the ECOCROP niche model to estimate how predicted changes in future climate could affect the growing conditions of several commonly cultivated crops from both future scenarios. We then discuss some key policy considerations for Jamaica’s agriculture sector, specifically related to the challenges posed to future adaptation pathways amidst growing climate uncertainty and complexity. Our model results show that even an increase less than +?1.5 °C is expected to have an overall negative impact on crop suitability and a general reduction in the range of crops available to Jamaican farmers. This observation is instructive as increases above the +?1.5 °C threshold would likely lead to even more irreversible and potentially catastrophic changes to the sustainability of Jamaica’s agriculture sector. The paper concludes by outlining some key considerations for future action, paying keen attention to the policy relevance of a +?1.5 °C temperature limit. Given little room for optimism with respect to the imminent changes that SIDS will need to confront in the near future, broad-based policy engagement by stakeholders in these geographies is paramount, irrespective of the climate warming scenario.     

A method for determining the analytical form of a radionuclide depth distribution using multiple gamma spectrometry measurements

When characterizing environmental radioactivity, whether in the soil or within concrete building structures undergoing remediation or decommissioning, it is highly desirable to know the radionuclide depth distribution. This is typically modeled using continuous analytical expressions, whose forms are believed to best represent the true source distributions. In situ gamma ray spectroscopic measurements are combined with these models to fully describe the source. Currently, the choice of analytical expressions is based upon prior experimental core sampling results at similar locations, any known site history, or radionuclide transport models. This paper presents a method, employing multiple in situ measurements at a single site, for determining the analytical form that best represents the true depth distribution present. The measurements can be made using a variety of geometries, each of which has a different sensitivity variation with source spatial distribution. Using non-linear least squares numerical optimization methods, the results can be fit to a collection of analytical models and the parameters of each model determined. The analytical expression that results in the fit with the lowest residual is selected as the most accurate representation. A cursory examination is made of the effects of measurement errors on the method.