For more than 50 y ecologists have believed that spatial heterogeneity in habitat conditions promotes species richness by increasing opportunities for niche partitioning. However, a recent stochastic model combining the main elements of niche theory and island biogeography theory suggests that environmental heterogeneity has a general unimodal rather than a positive effect on species richness. This result was explained by an inherent tradeoff between environmental heterogeneity and the amount of suitable area available for individual species: for a given area, as heterogeneity increases, the amount of effective area available for individual species decreases, thereby reducing population sizes and increasing the likelihood of stochastic extinctions. Here we provide a comprehensive evaluation of this hypothesis. First we analyze an extensive database of breeding bird distribution in Catalonia and show that patterns of species richness, species abundance, and extinction rates are consistent with the predictions of the area-heterogeneity tradeoff and its proposed mechanisms. We then perform a metaanalysis of heterogeneity-diversity relationships in 54 published datasets and show that empirical data better fit the unimodal pattern predicted by the area-heterogeneity tradeoff than the positive pattern predicted by classic niche theory. Simulations in which species may have variable niche widths along a continuous environmental gradient are consistent with all empirical findings. The area-heterogeneity tradeoff brings a unique perspective to current theories of species diversity and has important implications for biodiversity conservation.habitat heterogeneity | neutral theory | stochastic model of community dynamics | conservation planning
Protected areas (PAs) are intended to provide native biodiversity and habitats with a refuge against the impacts of global change, particularly acting as natural filters against biological invasions. In practice, however, it is unknown how effective PAs will be in shielding native species from invasions under projected climate change. Here, we investigate the current and future potential distributions of 100 of the most invasive terrestrial, freshwater, and marine species in Europe. We use this information to evaluate the combined threat posed by climate change and invasions to existing PAs and the most susceptible species they shelter. We found that only a quarter of Europe's marine and terrestrial areas protected over the last 100 years have been colonized by any of the invaders investigated, despite offering climatically suitable conditions for invasion. In addition, hotspots of invasive species and the most susceptible native species to their establishment do not match at large continental scales. Furthermore, the predicted richness of invaders is 11%-18% significantly lower inside PAs than outside them. Invasive species are rare in long-established national parks and nature reserves, which are actively protected and often located in remote and pristine regions with very low human density. In contrast, the richness of invasive species is high in the more recently designated Natura 2000 sites, which are subject to high human accessibility. This situation may change in the future, since our models anticipate important shifts in species ranges toward the north and east of Europe at unprecedented rates of 14-55 km/decade, depending on taxonomic group and scenario. This may seriously compromise the conservation of biodiversity and ecosystem services. This study is the first comprehensive assessment of the resistance that PAs provide against biological invasions and climate change on a continental scale and illustrates their strategic value in safeguarding native biodiversity.
It is predicted that climate change will drive extinctions of some reptiles and that the number of these extinctions will depend on whether reptiles are able to change their distribution. Whether the latitudinal distribution of reptiles may change in response to increases in temperature is unknown. We used data on reptile distributions collected during the 20th century to analyze whether changes in the distributions of reptiles in Spain are associated with increases in temperature. We controlled for biases in sampling effort and found a mean, statistically significant, northward shift of the northern extent of reptile distributions of about 15.2 km from 1940-1975 to 1991-2005. The southern extent of the distributions did not change significantly. Thus, our results suggest that the latitudinal distributions of reptiles may be changing in response to climate change.
Many factors affect the distribution of species richness. This study examines the relative influence of habitat heterogeneity, climate, human disturbance, and spatial structure on the species-richness distribution of terrestrial vertebrates (amphibians, reptiles, birds and mammals) in mainland Spain. The results indicate that spatial structure and environment exert similar influences on species richness. For all four taxa, species richness increases southward and northward, being lower in the center of the country, when controlled for other variables. This may be the result of a peninsular effect, as found in other studies, and reflect the importance of historical events on species richness in the Iberian Peninsula. Climate is more important than habitat heterogeneity in determining species richness. Temperature is positively correlated with amphibian, reptile, and bird species richness, while mammalian species richness is highest at intermediate temperatures. This effect is stronger in ectotherms than among endotherms, perhaps reflecting physiological differences. Precipitation positively correlates with bird and mammalian species richness, but has no effect on ectotherm species richness. Amphibian species richness increases with altitudinal range, and bird species richness with habitat diversity. Human population density is positively correlated with bird and mammalian species richness, but does not affect ectotherm species richness, while amphibian and bird species richness is highest at moderate levels of human land alteration (farmland). However, unexplained variance remains, and we discuss that the effects of environmental variables on species richness may vary geographically, causing different effects to be obscured on a national scale, diminishing the explanatory power of environmental variables.
Deforestation, climate change and invasive species constitute three global threats to biodiversity that act synergistically. However, drivers and rates of loss of freshwater biodiversity now and in the future are poorly understood. Here we focus on the potential impacts of global change on freshwater mussels (Order Unionida) in Sundaland (SE Asia), a vulnerable group facing global declines and recognized indicators of overall freshwater biodiversity. We used an ensemble of distribution models to identify habitats potentially suitable for freshwater mussels and their change under a range of climate, deforestation and invasion scenarios. Our data and models revealed that, at present, Sundaland features 47 and 32 Mha of habitat that can be considered environmentally suitable for native and invasive freshwater mussels, respectively. We anticipate that by 2050, the area suitable for palm oil cultivation may expand between 8 and 44 Mha, representing an annual increase of 2-11%. This is expected to result in a 20% decrease in suitable habitat for native mussels, a drop that reaches 30% by 2050 when considering concomitant climate change. In contrast, the habitat potentially suitable for invasive mussels may increase by 44-56% under 2050 future scenarios. Consequently, native mussels may compete for habitat, food resources and fish hosts with invasive mussels across approximately 60% of their suitable range. Our projections can be used to guide future expeditions to monitor the conservation status of freshwater biodiversity, and potentially reveal populations of endemic species on the brink of extinction. Future conservation measures-most importantly the designation of nature reserves-should take into account trends in freshwater biodiversity generally, and particularly species such as freshwater mussels, vital to safeguard fundamental ecosystem services.
In the present scenario of climatic change, climatic refugia will be of paramount importance for species persistence. Topography can generate a considerable climatic heterogeneity over short distances, which is often disregarded in macroclimatic predictive models. Here we investigate the role of rocky habitats as microclimatic refugia by combining two different analyses: exploring a thermal mechanism whereby rocky habitats might serve as refugia, and examining if the biogeographic pattern shows a high abundance of relict, endemic and peripheral species.The thermal profile of two populations of relict and endemic plant species occurring in Pyrenean cliffs was investigated by infrared images and in situ temperature data-loggers. Despite occurring in crevices of a south oriented slope, Androsace cylindrica showed a narrower daily range of temperature than the surrounding matrix, thereby avoiding extreme high temperatures. Borderea chouardii, of tropical ancestors, also occurred in patches where temperatures were buffered during the growth season, experiencing lower mean temperatures than the surrounding matrix and nearby areas during the warmer part of the day, and similar temperatures during the colder. The rocky habitats of both species, therefore, reduced temperature ranges and exposition to extreme climatic events. Compared to other habitats, the rocky ones also harboured a high fraction of both endemics and peripheral plant populations according to the largest vegetation dataset available in the Pyrenees (18,800 plant inventories and 400,000 records). Our results suggest an association between the habitats of relicts, endemics and species at their distribution limit, driven by a stabilizing effect of rocky habitats on extreme temperatures. Given the important role of rocky habitats as hotspots of singular and unique plants, their characterization seems a sensible first step to identify potential refugia in the context of climate change.
SummaryBonelli's Eagle Hieraaetus fasciatus and Golden Eagle Aquila chrysaetos are two declining species, in which floaters tend to be located outside of breeding territories during the dispersal period, in so-called settlement areas. We studied settlement areas for both these long-lived raptors in the southern Iberian Peninsula, to gain a better understanding of the ecological requirements of the eagles during their long pre-adult stage, a period accounting for around 80% of the species' mortality. Eagle abundance was calculated by road censuses, and habitat characteristics of settlement and non-settlement areas compared by General Discriminant Analysis (GDA) and Logistic Regression (LR). The best model of GDA and LR incorporated the abundance of main prey for eagles (rabbits, partridges) and orchard surface area, and explained 100% of eagle presence; the best model selected by GDA also included habitat heterogeneity. Both eagles tended to share settlement areas in the southern Iberian Peninsula and, when they did not, the mean annual temperature and slope appeared to explain the segregation between the two species. Management measures for the conservation of both threatened species during the dispersal period should be focused on identifying settlement areas, maintaining high prey densities and maximum habitat heterogeneity. ResumenEl Á guila Perdicera Hieraaetus fasciatus y el Á guila Real Aquila chrysaetos son dos especies en declive, cuyos inmaduros tienden a localizarse fuera de los territorios de los reproductores durante el período de dispersió n, en zonas conocidas como áreas de asentamiento. Hemos estudiado estas áreas para ambas rapaces en el sur de la Península Ibérica, con el fin de comprender mejor las necesidades ecoló gicas durante su prolongada etapa preadulta, un periodo en el que en torno al 80% de los individuos mueren. Mediante censos en carriles se calculó la abundancia de las dos águilas, y se midieron las características del hábitat en áreas de asentamiento y no asentamiento comparándose mediante un Análisis Discriminante General (ADG) y una Regresió n Logística (RL). El mejor modelo del ADG y RL incluye la abundancia de las presas principales de las águilas (conejos, perdices) y el porcentaje de cultivos arbó reos, explicando el 100% de la presencia de las águilas, y el mejor modelo seleccionado por el ADG también incluyó la heterogeneidad de hábitats. Ambas águilas tendieron a compartir las zonas de asentamiento en el sur de la Península Ibérica y, cuando esto no ocurrió , la temperatura media anual y la pendiente parecían explicar la segregació n entre las dos especies. Las medidas de gestión para la conservación de ambas especies amenazadas durante el periodo de dispersión deben centrarse en la identificació n de las áreas de asentamiento, el mantenimiento de altas densidades de presas y de alta heterogeneidad de hábitats en estas áreas.
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