Accumulation of heavy metals in food web components across a gradient of lakes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109770/1/lno20004571525.pd

Population Regulation of Frogs and Lizards in a Lowland Wet Neotropical Forest: Integrating Alternative Hypotheses

Populations can be regulated in space and time from the “bottom-up” by resources (e.g., food), from the “top-down” by predation, or from parallel factors, such as interactions with conspecifics or heterospecifics. Amphibians and reptiles are species rich and abundant in lowland wet forests of Central America, and are important components of trophic communities as low or mid-level consumers of arthropods, and as prey to a diverse assemblage of vertebrate and invertebrate species. Theory describing population and community structure of Neotropical amphibians and reptiles has been generated largely from studies of a species-rich assemblage of frogs (Craugastoridae, Dendrobatidae) and anole lizards (Dactyloidae) that occur terrestrially on the forest floor and have generally supported hypotheses of bottom-up regulation mediated by resources, such as leaf-litter and prey abundance. Because trees regulate the input of leaf litter into terrestrial systems, a conceptual model has been suggested where leaf-abscission phenology of trees regulates patch dynamics of terrestrial frogs and lizards through forests in ways consistent with bottom-up regulation (the litter-mosaic hypothesis). However, a comparative review of anole life history in mainland Central American and Caribbean island habitats has described an alternative, top-down hypothesis of population regulation, where anoles in Central America are more greatly influenced by predation than anoles in Caribbean islands (the mainland-island model). Additionally, studies of the island anole Dactyloa aenea have demonstrated that juveniles are attracted to conspecifics when settling habitat, and conspecific attraction may be an additional parallel factor influencing forest dynamics of anoles and frogs in Central America. In an effort to generate a more integrative conceptual model describing population regulation and forest dynamics of frogs and lizards assemblage in Central America, I tested three hypotheses describing bottom-up, top-down, and parallel population regulation of a model assemblage at La Selva Biological Station, Costa Rica. First, I tested the litter-mosaic hypothesis that leaf-drop phenology of trees regulates population cycles and patch dynamics of terrestrial arthropods, frogs (Craugastor bransfordii, Oophaga pumilio), and lizards (Norops humilis). I used a comparative experimental approach, where I (1) compared seasonal abundance cycles of terrestrial litter and fauna beneath two tree species differing in phenology of leaf abscission (Castilla elastica, Dipteryx panamensis), and (2) experimentally manipulated leaf abscission beneath C. elastica with litter supplementation. I observed seasonal patterns of leaf litter, arthropods, and vertebrate abundance, but these patterns did not vary between tree species. Experimental supplementation caused elevated abundance of N. humilis in manipulated C. elastica plots relative to controls, as a result of demographic shifts in either apparent survival, immigration, or recruitment. My results suggest the litter-mosaic hypothesis can be refined and restricted to describing spatiotemporal variance of terrestrial anoles, while excluding frogs. Second, to better understand how predators influence prey frog and lizard ecology, I tested the mainland-island model prediction that predation exerts stronger effects than food resources for anoles and, by extension, frogs in Central America. I modeled the relative contribution of food, microhabitat, and predatory spiders (Ctenidae) to frog and anole occupancy. Frog occupancy was most strongly influenced by predators, an effect which increased at reduced biomass of leaf litter. Anoles occupied sites independent of predators, an observation inconsistent with the mainland-island model. All species were positively associated with leaf-litter depth and had elevated detection when predators were present. Third and last, I used eighteen months of mark-recapture observations of O. pumilio in a homogenous, fallow cacao plantation to test predictions of the conspecific attraction hypothesis. I found that juveniles settled habitat and used space in significant association with adults with previous history in plots, in ways consistent with conspecific attraction of juveniles and strong male territoriality. Together, my results contribute to our understanding of patch dynamics for terrestrial frogs and lizards in Neotropical wet forests. My experimental results suggest that Norops humilis populations respond to pulses of terrestrial litter in ways consistent with the litter-mosaic hypothesis, but the O. pumilio and C. bransfordii do not. Dispersal of O. pumilio and other frogs through the landscape may be driven in part by conspecific attraction. Because predatory spiders exert stronger effects on the occupancy of terrestrial frogs at lower abundance of terrestrial litter, seasonal declines in frog abundance during the wet season may be driven in part by elevated predation pressure when leaf litter is scarce. Predator-prey models reject a large literature invoking predation as the dominant force shaping ecology of diverse mainland anoles, and suggest similar mechanisms may regulate the ecology and evolution of mainland and island anoles

New observations of frog and lizard predation by wandering and orb-weaver spiders in Costa Rica

Estudos sugeriram que a predação por aranhas pode ser uma força importante na regulação da historia de vida de anuros e largartos Neotropicais, mas descrições detalhadas das relações predador-presa são escassas. Decrevemos aqui novas observações em que aranhas contribuem para a mortalidade de anuros e lagartos no nordeste da Costa Rica e corrigimos ou esclarecemos três erros de identificação de aranhas da literatura. Os predadores mais frequentemente observados foram aranhas-andarilhas (Ctenidae), que parecem ser predadoras generalistas de anuros e lagartos. Uma aranha-de-teia-orbicular (Araneidae) também contribuiu para a mortalidade de anuros, provavelmente depois que o animal ficou preso na teia. Estudos mais detalhados são necessários para elucidar o papel que a predação por aranhas exerce sobre a demografía de anuros e lagartos em florestas Neotropicais.Studies have suggested that predation by spiders may be an important force regulating life history in neotropical frogs and lizards, but detailed descriptions of predator-prey relationships are few. Here we describe novel observations where spiders contributed to the mortality of frogs and lizards in northeastern Costa Rica, and we corrected or clarified three identification errors of spiders from the literature. The most frequently observed predators were wandering spiders (Ctenidae), which seem to be generalist predators on frogs and lizards. An orb-weaver spider (Araneidae) also contributed to frog mortality, likely after the frog became entangled in the spider’s web. More detailed studies are needed to elucidate the role that spider predation contributes to frog and lizard demography in neotropical forests

Plankton lattices and the role of chaos in plankton patchiness

Spatiotemporal and interspecies irregularities in planktonic populations have been widely observed. Much research into the drivers of such plankton patches has been initiated over the past few decades but only recently have the dynamics of the interacting patches themselves been considered. We take a coupled lattice approach to model continuous-in-time plankton patch dynamics, as opposed to the more common continuum type reaction-diffusion-advection model, because it potentially offers a broader scope of application and numerical study with relative ease. We show that nonsynchronous plankton patch dynamics (the discrete analog of spatiotemporal irregularity) arise quite naturally for patches whose underlying dynamics are chaotic. However, we also observe that for parameters in a neighborhood of the chaotic regime, smooth generalized synchronization of nonidentical patches is more readily supported which reduces the incidence of distinct patchiness. We demonstrate that simply associating the coupling strength with measurements of (effective) turbulent diffusivity results in a realistic critical length of the order of 100 km, above which one would expect to observe unsynchronized behavior. It is likely that this estimate of critical length may be reduced by a more exact interpretation of coupling in turbulent flows

Habitat-Mediated Foraging Limitations Drive Survival Bottlenecks for Juvenile Salmon

Realistic population models and effective conservation strategies require a thorough understanding of mechanisms driving stage-specific mortality. Mortality bottlenecks for many species occur in the juvenile stage and are thought to result from limitation on food or foraging habitat during a critical period for growth and survival. Without a way to account for maternal effects or to measure integrated consumption rates in the field, it has been virtually impossible to test these relationships directly. Hence uncertainties about mechanisms underlying such bottlenecks remain. In this study we randomize maternal effects across sites and apply a new method for measuring consumption integrated over weeks to months to test the hypothesis that food limitation drives early-season juvenile mortality bottlenecks in Atlantic salmon (Salmo salar). Using natural signatures of geologically derived cesium (133Cs), we estimated consumption rates of \u3e400 fry stocked into six streams. Two to four weeks after stocking, consumption was extremely low across sites (0.005 g x g(-1) x d(-1)) and was predicted to be below maintenance rations (i.e., yielding negative energy balances) for the majority of individuals from five of six sites. However, consumption during this time was positively correlated with growth rates and survival (measured at the end of the growing season). In contrast, consumption rates increased in mid- (0.030 g x g(-1) x d(-1)) and late (0.035 g x g(-1) x d(-1)) seasons, but juvenile survival and consumption were not correlated, and correlations between growth and consumption were weak. These findings are consistent with predictions of a habitat-based bioenergetic model constructed using the actual stream positions of the individual fish in the present study, which indicates that habitat-based models capture important environmental determinants of juvenile growth and survival. Hence, by combining approaches, reducing maternal effects and controlling initial conditions, we offer a general framework for linking foraging with juvenile survival and present the first direct consumption-based evidence for the early season bottleneck hypothesis

Multi-Objective Modeling as a Decision-Support Tool for Free-Roaming Horse Management

Decisions related to controversial problems in natural resource management receive the greatest support when they account for multiple objectives of stakeholders in a structured and transparent fashion. In the United States, management of free-roaming horses (Equus caballus; horses) is a controversial multiple-objective problem because disparate stakeholder groups have varying objectives and opinions about how to manage fast-growing horse populations in ways that sustain both natural ecosystems and healthy horses. Despite much decision-support research on management alternatives that prevent excessive population size or cost, horse management decisions still receive resistance from a variety of stakeholder groups, potentially because decisions fail to explicitly or transparently account for multiple objectives of diverse stakeholders. Here, we used a predictive model for horse populations to evaluate the degree to which alternative management strategies involving removals and fertility control treatment with the immunocontraceptive vaccine PZP-22 maximize 4 objectives in horse management: maximize ecosystem health, maximize horse health, minimize effects on horse behavior, and minimize management cost. We simulated scenarios varying in management action, frequency, magnitude, and starting population size over a 10-year interval and evaluated scenario performance with a weighted multiple-objective utility reward function. Management involving high-magnitude removals along with PZP-22 treatment generally outperformed other alternatives by achieving higher reward relative to alternatives in 2 scenario analyses. Simulation of 1,372 scenarios at 5 starting population sizes generally found that management with biannual removals and 2 doses of PZP-22 treatment for half of eligible females during years 1 and 5 generated the most rewarding outcomes. However, a removal scenario with more frequent PZP-22 application generated the greatest reward when starting population size was already within target population size range. Our paper demonstrates how values and objectives of diverse stakeholders can be used to support management decisions in ways that might lead to greater acceptance of decisions by a broad array of stakeholder groups

Comparing nearshore benthic and pelagic prey as mercury sources to lake fish: the importance of prey quality and mercury content

Mercury (Hg) bioaccumulation in fish poses well-known health risks to wildlife and humans through fish consumption. Yet fish Hg concentrations are highly variable, and key factors driving this variability remain unclear. One little studied source of variation is the influence of habitat-specific feeding on Hg accumulation in lake fish. However, this is likely important because most lake fish feed in multiple habitats during their lives, and the Hg and caloric content of prey from different habitats can differ. This study used a three-pronged approach to investigate the extent to which habitat-specific prey determine differences in Hg bioaccumulation in fish. This study first compared Hg concentrations in common nearshore benthic invertebrates and pelagic zooplankton across five lakes and over the summer season in one lake, and found that pelagic zooplankton generally had higher Hg concentrations than most benthic taxa across lakes, and over a season in one lake. Second, using a bioenergetics model, the effects of prey caloric content from habitat-specific diets on fish growth and Hg accumulation were calculated. This model predicted that the consumption of benthic prey results in lower fish Hg concentrations due to higher prey caloric content and growth dilution (high weight gain relative to Hg from food), in addition to lower prey Hg levels. Third, using data from the literature, links between fish Hg content and the degree of benthivory, were examined,

Do Low-Mercury Terrestrial Resources Subsidize Low-Mercury Growth of Stream Fish? Differences between Species along a Productivity Gradient

Low productivity in aquatic ecosystems is associated with reduced individual growth of fish and increased concentrations of methylmercury (MeHg) in fish and their prey. However, many stream-dwelling fish species can use terrestrially-derived food resources, potentially subsidizing growth at low-productivity sites, and, because terrestrial resources have lower MeHg concentrations than aquatic resources, preventing an increase in diet-borne MeHg accumulation. We used a large-scale field study to evaluate relationships among terrestrial subsidy use, growth, and MeHg concentrations in two stream-dwelling fish species across an in-stream productivity gradient. We sampled young-of-the-year brook trout ( Salvelinus fontinalis ) and Atlantic salmon ( Salmo salar ), potential competitors with similar foraging habits, from 20 study sites in streams in New Hampshire and Massachusetts that encompassed a wide range of aquatic prey biomass. Stable isotope analysis showed that brook trout used more terrestrial resources than Atlantic salmon. Over their first growing season, Atlantic salmon tended to grow larger than brook trout at sites with high aquatic prey biomass, but brook grew two-fold larger than Atlantic salmon at sites with low aquatic prey biomass. The MeHg concentrations of brook trout and Atlantic salmon were similar at sites with high aquatic prey biomass and the MeHg concentrations of both species increased at sites with low prey biomass and high MeHg in aquatic prey. However, brook trout had three-fold lower MeHg concentrations than Atlantic salmon at low- productivity, high-MeHg sites. These results suggest that differential use of terrestrial resource subsidies reversed the growth asymmetry between potential competitors across a productivity gradient and, for one species, moderated the effect of low in-stream productivity on MeHg accumulation

Association of Rice and Rice-Product Consumption With Arsenic Exposure Early in Life

Rice—a typical first food and major ingredient in various infant foods—contains inorganic arsenic (As), but the extent of As exposure from these foods has not been well characterized in early childhood