The relative importance of density dependence regulation in natural population fluctuations has long been debated. The concept of density dependence implies that current abundance is determined by ...historical abundance. We have developed four models-two density dependent and two density independent-to predict population size one year beyond the training set and used predictive performance on more than 16,000 populations from 14 datasets to compare the understanding captured by those models. For 4 of 14 datasets the density dependent models make better predictions (i.e., density dependent regulated) than either of the density independent models. However, neither of the density dependent models is statistically significantly superior to density independent models for any of the 14 datasets. We conclude that the evidence for widespread density dependent population regulation in the forms represented by these two simple density-dependent models is weak. However, the density dependent models used here-the Logistic and Gompertz models-are simple representations of how population density might regulate natural populations and only examine density-dependent effects on population size. A comprehensive assessment of the relative importance of density-dependent population regulation will require testing the predictive ability of a wider range of density-dependent models including models examining effects on population characteristics other than population size.
The objective of science is to understand the natural world; we argue that prediction is the only way to demonstrate scientific understanding, implying that prediction should be a fundamental aspect ...of all scientific disciplines. Reproducibility is an essential requirement of good science and arises from the ability to develop models that make accurate predictions on new data. Ecology, however, with a few exceptions, has abandoned prediction as a central focus and faces its own crisis of reproducibility. Models are where ecological understanding is stored and they are the source of all predictions – no prediction is possible without a model of the world. Models can be improved in three ways: model variables, functional relationships among dependent and independent variables, and in parameter estimates. Ecologists rarely test to assess whether new models have made advances by identifying new and important variables, elucidating functional relationships, or improving parameter estimates. Without these tests it is difficult to know if we understand more today than we did yesterday. A new commitment to prediction in ecology would lead to, among other things, more mature (i.e. quantitative) hypotheses, prioritization of modeling techniques that are more appropriate for prediction (e.g. using continuous independent variables rather than categorical) and, ultimately, advancement towards a more general understanding of the natural world.
Synthesis
Ecology, with a few exceptions, has abandoned prediction and therefore the ability to demonstrate understanding. Here we address how this has inhibited progress in ecology and explore how a renewed focus on prediction would benefit ecologists. The lack of emphasis on prediction has resulted in a discipline that tests qualitative, imprecise hypotheses with little concern for whether the results are generalizable beyond where and when the data were collected. A renewed commitment to prediction would allow ecologists to address critical questions about the generalizability of our results and the progress we are making towards understanding the natural world.
Null hypothesis significance testing has been under attack in recent years, partly owing to the arbitrary nature of setting α (the decision-making threshold and probability of Type I error) at a ...constant value, usually 0.05. If the goal of null hypothesis testing is to present conclusions in which we have the highest possible confidence, then the only logical decision-making threshold is the value that minimizes the probability (or occasionally, cost) of making errors. Setting α to minimize the combination of Type I and Type II error at a critical effect size can easily be accomplished for traditional statistical tests by calculating the α associated with the minimum average of α and β at the critical effect size. This technique also has the flexibility to incorporate prior probabilities of null and alternate hypotheses and/or relative costs of Type I and Type II errors, if known. Using an optimal α results in stronger scientific inferences because it estimates and minimizes both Type I errors and relevant Type II errors for a test. It also results in greater transparency concerning assumptions about relevant effect size(s) and the relative costs of Type I and II errors. By contrast, the use of α = 0.05 results in arbitrary decisions about what effect sizes will likely be considered significant, if real, and results in arbitrary amounts of Type II error for meaningful potential effect sizes. We cannot identify a rationale for continuing to arbitrarily use α = 0.05 for null hypothesis significance tests in any field, when it is possible to determine an optimal α.
Although there is growing concern that amphibian populations are declining
globally, much of the supporting evidence is either anecdotal or derived from short-term studies at small geographical ...scales. This raises questions not only about the difficulty of detecting
temporal trends in populations which are notoriously variable,
but also about the validity of inferring global trends from local or regional
studies. Here we use data from 936 populations to assess
large-scale temporal and spatial variations in amphibian population trends.
On a global scale, our results indicate relatively rapid declines from the
late 1950s/early 1960s to the late 1960s, followed by a reduced rate of decline
to the present. Amphibian population trends during the 1960s were negative
in western Europe (including the United Kingdom) and North America, but only
the latter populations showed declines from the 1970s to the late 1990s. These
results suggest that while large-scale trends show considerable geographical
and temporal variability, amphibian populations are in fact declining-and
that this decline has been happening for several decades.
Road traffic kills hundreds of millions of animals every year, posing a critical threat to the populations of many species. To address this problem there are more than forty types of road mitigation ...measures available that aim to reduce wildlife mortality on roads (road-kill). For road planners, deciding on what mitigation method to use has been problematic because there is little good information about the relative effectiveness of these measures in reducing road-kill, and the costs of these measures vary greatly. We conducted a meta-analysis using data from 50 studies that quantified the relationship between road-kill and a mitigation measure designed to reduce road-kill. Overall, mitigation measures reduce road-kill by 40% compared to controls. Fences, with or without crossing structures, reduce road-kill by 54%. We found no detectable effect on road-kill of crossing structures without fencing. We found that comparatively expensive mitigation measures reduce large mammal road-kill much more than inexpensive measures. For example, the combination of fencing and crossing structures led to an 83% reduction in road-kill of large mammals, compared to a 57% reduction for animal detection systems, and only a 1% for wildlife reflectors. We suggest that inexpensive measures such as reflectors should not be used until and unless their effectiveness is tested using a high-quality experimental approach. Our meta-analysis also highlights the fact that there are insufficient data to answer many of the most pressing questions that road planners ask about the effectiveness of road mitigation measures, such as whether other less common mitigation measures (e.g., measures to reduce traffic volume and/or speed) reduce road mortality, or to what extent the attributes of crossing structures and fences influence their effectiveness. To improve evaluations of mitigation effectiveness, studies should incorporate data collection before the mitigation is applied, and we recommend a minimum study duration of four years for Before-After, and a minimum of either four years or four sites for Before-After-Control-Impact designs.
Chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), is widespread among amphibians in northeastern North America. It is unknown, however, whether Bd has the potential to cause extensive ...amphibian mortalities in northeastern North America as have occurred elsewhere. In the laboratory, we exposed seven common northeastern North American amphibian species to Bd to assess the likelihood of population-level effects from the disease. We exposed larval wood frogs (Lithobates sylvaticus.) and postmetamorphic frogs of six other species to two different strains of Bd, a northeastern strain (JEL404) and a strain that caused die-offs of amphibians in Panama QEL423), under ideal in vitro growth conditions for Bd Exposed American toads OVnaxyrus americanus) all died; thus, this species may be the most likely to die from Bd-caused disease in the wild. Both Bd strains were associated with mortalities of wood frogs, although half the metamorphs survived. The Bd strain from Panama killed metamorphic green frogs (L · clamitans), whereas the northeastern strain did not, which means novel strains of Bd may lead to death even when local strains may not. No mortality was observed in f our species (bullfrogs L · catesbeianus, northern leopard frogs L. pipiens, spring peepers Pseudacris crucifer, and blue-spotted salamanders Ambystoma laterale) and in some individuals of green frogs and wood frogs that we exposed. This finding suggests these six species may be Bd vectors. Our results show that systematic exposures of amphibian species to Bd in the laboratory may be a good first step in the identification of species susceptible to Bd-caused declines and in directing regional conservation efforts aimed at susceptible species. La quitidriomicosis, causada por Batrachochytrium dendrobatidis (Bd), está ampliamente extendida entre anfibios de Norteamérica. Sin embargo, se desconoce si Bd tiene el potencial para provocar mortalidades extensivas de anfibios en el noreste de Norteamérica como ha ocurrido en otros lugares. En el laboratorio, expusimos a siete especies comunes de anfibios de Norteamérica a Bd para evaluar la probabilidad de efectos de la enfermedad a nivel población. Expusimos larvas de Lithobates sylvaticus y ranas posmetamórficas de otras seis especies a dos cepas de Bd diferentes, una cepa nororiental JEL404) y una cepa que causó muerte de anfibios en Panamá JEL423), bajo condiciones de crecimiento in vitro de Bd ideales. Todos los individuos de Anaxyrus americanus murieron; por lo tanto esta especie tiene la mayor probabilidad de morir en su medio natural por enfermedad causada por Bd Ambas cepas de Bd se asociaron con mortalidad de L sylvaticus, aunque la mitad de las larvas sobrevivieron. La cepa de Bd de Panamá mató a individuos metamórficos de L. clamitans, mientras que la cepa nororiental no, lo cual significa que cepas nuevas de Bd pueden causar la muerte aun cuando las cepas locales no. No se observó mortalidad en cuatro de las especies (L · catesbeianus, L. pipiens, Pseudacris crucifera Ambystoma laterale) y de algunos individuos de L sylvaticusy L. clamitans. Este hallazgo sugiere que estas seis especies pueden ser vectores de Bd Nuestros resultados muestran que la exposición sistemática de especies de anfibios a Bd en el laboratorio puede ser un buen primer paso para la identificación de especies susceptibles a declinaciones causadas por Bd y para orientar los esfuerzos de conservación regionales enfocados en especies susceptibles.
The last 20 years have seen a dramatic increase in efforts to mitigate the negative effects of roads and traffic on wildlife, including fencing to prevent wildlife-vehicle collisions and wildlife ...crossing structures to facilitate landscape connectivity. While not necessarily explicitly articulated, the fundamental drivers behind road mitigation are human safety, animal welfare, and/or wildlife conservation. Concomitant with the increased effort to mitigate has been a focus on evaluating road mitigation. So far, research has mainly focussed on assessing the use of wildlife crossing structures, demonstrating that a broad range of species use them. However, this research has done little to address the question of the effectiveness of crossing structures, because use of a wildlife crossing structure does not necessarily equate to its effectiveness. The paucity of studies directly examining the effectiveness of crossing structures is exacerbated by the fact that such studies are often poorly designed, which limits the level of inference that can be made. Without well performed evaluations of the effectiveness of road mitigation measures, we may endanger the viability of wildlife populations and inefficiently use financial resources by installing structures that are not as effective as we think they are. In this paper we outline the essential elements of a good experimental design for such assessments and prioritize the parameters to be measured. The framework we propose will facilitate collaboration between road agencies and scientists to undertake research programs that fully evaluate effectiveness of road mitigation measures. We discuss the added value of road mitigation evaluations for policy makers and transportation agencies and provide recommendations on how to incorporate such evaluations in road planning practices.
Many environments are undergoing rapid environmental change and there is a need to understand the mechanisms by which species can persist in altered environments. Model systems, such as amphibian ...metamorphosis, which can be generalized across many types of environmental change and across many species, are a powerful tool for understanding mechanisms that facilitate persistence in altered and disturbed environments. Amphibian larvae respond to environmental change by varying age at metamorphosis, or size at metamorphosis. Differential selection pressures on age or size at metamorphosis may result in a differential response among taxa to environmental change. Using a meta-analysis, we investigated whether age at metamorphosis, size at metamorphosis, and larval growth rate vary within and among taxonomic families of amphibians in experiments that modified the environmental temperature, density of individuals, food, hydroperiod and the presence of predators. For all environmental factors except predators, the direction of the response was consistent across most of the studied taxa. However, there was considerable variation in effect size both within and among families. Results demonstrate that amphibian metamorphosis is a valuable model system for studying the effects of environmental change. Yet, we stress the need for caution in making generalizations about how individuals respond to environmental factors that have an indirect effect on physiology and require the perception of an environmental cue, such as the presence of predators.
Synthesis
As the current conditions of the environment are rapidly changing there is a need to understand how organisms respond to environmental change, and whether response of one species can be generalized to other species. Using a meta-analyses, we tested whether the phenotypic response of amphibian larvae to five types of environmental change is consistent among and within taxonomic families. The phenotypic response to changes in environmental factors was consistent when the environmental factor has a direct effect on physiology, but varies among and within family if the environmental factor has an indirect effect on physiology or requires the perception of an environmental cue.
Anthropogenic-derived stressors in the environment, such as contaminants, are increasingly considered important cofactors that may decrease the immune response of amphibians to pathogens. Few ...studies, however, have integrated amphibian disease and contaminants to test this multiple-stressor hypothesis for amphibian declines. We examined whether exposure to sublethal concentrations of a glyphosate-based herbicide and two strains of the pathogenic chytrid fungus,
Batrachochytrium dendrobatidis
(
Bd
) could: (1) sublethally affect wood frogs (
Lithobates sylvaticus
) by altering the time to and size at metamorphosis, and (2) directly affect survivability of wood frogs after metamorphosis. Neither
Bd
strain nor herbicide exposure alone significantly altered growth or time to metamorphosis. The two
Bd
strains did not differ in their pathogenicity, and both caused mortality in post-metamorphic wood frogs. There was no evidence of an interaction between treatments, indicating a lack of herbicide-induced susceptibility to
Bd
. However, the trends in our data suggest that exposure of wood frogs to a high concentration of glyphosate-based herbicide may reduce
Bd
-caused mortality compared to animals exposed to
Bd
alone. These results exemplify the complexities inherent when populations are coping with multiple stressors. In this case, the perceived stressor, glyphosate-based herbicide, appeared to affect the pathogen more than the host's immune system, relieving the host from disease-caused effects. This suggests caution when invoking multiple stressors as a cause for increased disease susceptibility and indicates that the effects of multiple stressors on disease outcome depend on the interrelationships of stressors to both the pathogen and the host.
Habitat destruction and fragmentation have been identified as possible causes of large-scale amphibian declines. Here, we examine the effects of adjacent land use and water quality on wetland ...amphibian species richness, abundance, and community composition in 74 Ontario wetlands. Species richness was positively correlated with wetland area, forest cover, and the amount of wetlands on adjacent lands and negatively correlated with road density and nitrogen levels. The land-use effects peak at 20003000 m. Amphibian abundance was positively correlated with forest cover, distance to wetlands >20 ha, and amount of marsh habitat and negatively correlated with road density. The effects of adjacent land use were strongest at around 200 m. Land-use and water quality effects varied widely across species, although most species are positively correlated with forest cover and amount of wetlands on adjacent lands and negatively correlated with road density and water quality. These results suggest that the effects of adjacent land use on amphibian communities can extend over comparatively large distances. As such, effective wetland conservation will not be achieved merely through the creation of narrow buffer zones between wetlands and intensive land uses, but rather will require maintaining a heterogeneous regional landscape containing relatively large areas of natural forest and wetlands.
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