Species distribution models are used for a range of ecological and evolutionary questions, but often are constructed from few and/or biased species occurrence records. Recent work has shown that the presence‐only model Maxent performs well with small sample sizes. While the apparent accuracy of such models with small samples has been studied, less emphasis has been placed on the effect of small or biased species records on the secondary modeling steps, specifically accuracy assessment and threshold selection, particularly with profile (presence‐only) modeling techniques. When testing the effects of small sample sizes on distribution models, accuracy assessment has generally been conducted with complete species occurrence data, rather than similarly limited (e.g. few or biased) test data. Likewise, selection of a probability threshold – a selection of probability that classifies a model into discrete areas of presences and absences – has also generally been conducted with complete data. In this study we subsampled distribution data for an endangered rodent across multiple years to assess the effects of different sample sizes and types of bias on threshold selection, and examine the differences between apparent and actual accuracy of the models. Although some previously recommended threshold selection techniques showed little difference in threshold selection, the most commonly used methods performed poorly. Apparent model accuracy calculated from limited data was much higher than true model accuracy, but the true model accuracy was lower than it could have been with a more optimal threshold. That is, models with thresholds and accuracy calculated from biased and limited data had inflated reported accuracy, but were less accurate than they could have been if better data on species distribution were available and an optimal threshold were used.
An unprecedented era of climatic volatility is altering ecosystems across our planet 1. The potential scale, pace and consequences of this global change have been modelled extensively 2 , yet little empirical research has quantified the impacts of extreme climate events on the composition of contemporary ecological communities. Here, we quantified the responses of 423 sympatric species of plants, arthropods, birds, reptiles and mammals to California's drought of 2012-2015-the driest period in the past 1,200 years 3 for this global biodiversity hotspot. Plants were most responsive to one-year water deficits, whereas vertebrates responded to longer-term deficits, and extended drought had the greatest impact on carnivorous animals. Locally rare species were more likely to increase in numbers and abundant species were more likely to decline in response to drought, and this negative density dependence was remarkably consistent across taxa and drought durations. Our system-wide analysis reveals that droughts indirectly promote the long-term persistence of rare species by stressing dominant species throughout the food web. These findings highlight processes that shape community structure in highly variable environments and provide insights into whole-community responses to modern climate volatility. The frequency, severity and duration of droughts is increasing due to global warming 4-6. High socioeconomic costs of severe droughts are among the most worrisome of climate change impacts, and effects on natural ecosystems may likewise be substantial 7. Predicting the ecological impacts of drought is complicated by the fact that species can be impacted through multiple pathways. Drought affects communities directly through physiological impacts on species' survival and growth rates, and indirectly by altering species interactions such as competition 8,9. Some theoretical models show that droughts can increase coexistence probabilities through selective mortality on dominant species 8,10 , while others predict that drought can increase dominance through increased competitive intensity 9. Here, we tested these conflicting predictions regarding the pathways through which drought impacts ecosystems, while broadly characterizing the response of a community to a once-in-amillennium climate-induced disturbance. Theoretical predictions of drought effects are rooted in plant ecology 11 , and it is not known whether they apply to animal populations that may primarily be indirectly affected by soil moisture deficits. Drought may affect all trophic levels similarly via generalized processes of disturbance or competition. Alternatively, drought effects may move up the food web with time lags or opposing effects. For example, droughts in sub-Saharan east Africa have led to ungulate die-offs 12 , resulting in a short-term resource pulse for scavengers 13. Thus, the effects of drought on resource availability differ among trophic levels and over time. Emerging studies indicate that droughts can strongly affect the dynamics of animal popula...
Summary1. The high cost of directly measuring habitat quality has led ecologists to test alternate methods for estimating and predicting this critically important ecological variable. In particular, it is frequently assumed but rarely tested that models of habitat suitability ('species distribution models', SDMs) may provide useful indices of habitat quality, either from an individual animal or manager's perspective. Critically, SDMs are increasingly used to estimate species' ranges, with an implicit assumption that areas of high suitability will result in higher probability of persistence. This assumption underlies efforts to use SDMs to design protected areas, assess the status of cryptic species or manage responses to climate change. Recent tests of this relationship have provided mixed results, suggesting SDMs may predict abundance but not other measures of high-quality habitat (e.g. survival, persistence). 2. In this study, we created a suite of SDMs for the endangered giant kangaroo rat Dipodomys ingens at three distinct scales using the machine-learning method Maxent. We compared these models with three measures of habitat quality: survival, abundance and body condition. 3. Species distribution models were not correlated with survival, while models at all scales were positively correlated with abundance. Finer-scale models were more closely correlated with abundance than the largest scale. Body condition was not correlated with habitat suitability at any scale. The inability of models to predict survival may be due to a lack of information in environmental covariates; unmeasured community processes or stochastic events; or the inadequacy of using models that predict species presence to also predict demography. 4. Synthesis and applications. Species distribution models (SDMs), especially fine scale ones, may be useful for longer-term management goals, such as identifying high-quality habitat for protection. However, short-term management decisions should be based only on models that use covariates appropriate for the necessary temporal and spatial scales. Assumptions about the relationship between habitat suitability and habitat quality must be made explicit. Even then, care should be taken in inferring multiple types of habitat quality from SDMs.
The ultraviolet (UV) photochemistry and photobiology of spores and vegetative cells of Bacillus megaterium have been studied. The response of vegetative cells of B. megaterium appears qualitatively similar to those of Escherichia coli, Micrococcus radiodurans, and Bacillus subtilis with respect to photoproduct formation and repair mechanisms. UV irradiation, however, does not produce cyclobutane-type thymine dimers in the DNA of spores, although other thymine photo-products are produced. The photoproducts do not disappear after photoreactivation, but they are eliminated from the DNA by a dark-repair mechanism different from that found for dimers in vegetative cells. Irradiations performed at three wavelengths produce the same amounts of spore photoproduct and give the same survival curves. Variation of the sporulation medium before irradiation results in comparable alterations in the rate of spore photoproduct production and in survival.
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