The demography of range boundaries versus range cores in eastern US tree species

Regional species–climate correlations are well documented, but little is known about the ecological processes responsible for generating these patterns. Using the data from over 690 000 individual trees I estimated five demographic rates—canopy growth, understorey growth, canopy lifespan, understorey lifespan and per capita reproduction—for 19 common eastern US tree species, within the core and the northern and southern boundaries, of the species range. Most species showed statistically significant boundary versus core differences in most rates at both boundary types. Differences in canopy and understorey growth were relatively small in magnitude but consistent among species, being lower at the northern (average −17%) and higher at the southern (average +12%) boundaries. Differences in lifespan were larger in magnitude but highly variable among species, except for a marked trend for reduced canopy lifespan at the northern boundary (average −49%). Differences in per capita reproduction were large and statistically significant for some species, but highly variable among species. The rate estimates were combined to calculate two performance indices: R₀ (a measure of lifetime fitness in the absence of competition) was consistently lower at the northern boundary (average −86%) whereas Z^* (a measure of competitive ability in closed forest) showed no sign of a consistent boundary–core difference at either boundary.     

The data void in modeling current and future distributions of tropical species

Conserving biodiversity in the face of climate change requires a predictive ecology of species distributions. Nowhere is this need more acute than in the tropics, which harbor the majority of Earth's species and face rapid and large climate and land‐use changes. However, the study of species distributions and their responses to climate change in high diversity tropical regions is potentially crippled by a lack of basic data. We analyzed a database representing more than 800 000 unique geo‐referenced natural history collections to determine what fraction of tropical plant species has sufficient numbers of available collections for use in the habitat or niche models commonly used to predict species responses to climate change. We found that more than nine out of 10 species from the three principle tropical realms are so poorly collected (n < 20 records) that they are essentially invisible to modern modeling and conservation tools. In order to predict the impact of climate change on tropical species, efforts must be made to increase the amount of data available from tropical countries through a combination of collecting new specimens and digitizing existing records.     

Phylogenetic patterns in zopherine beetles are related to ecological niche width and dispersal limitation

Niche conservatism has been proposed as the mechanism driving speciation in temperate montane clades through range fragmentation during climatic oscillations. Thus, a negative relationship between speciation rates and niche width is expected. Here, we test this prediction using American zopherine beetles. Our phylogenetic analyses recovered two clades in addition to that of the genus Zopherus: the genera Verodes and Phloeodes, which originated most likely in the Eocene, and diversified during the Miocene and the Pliocene. The assessment of clade niche width in relation to clade diversity supported the proposition of narrow niches leading to a higher probability of range fragmentation during climatic oscillations, thus increasing speciation. Additionally, almost all current populations of Phloeodes and Verodes are located within regions that retained favourable climatic conditions across warm and cold Pleistocene periods, suggesting that dispersal limitation is a strong factor controlling clade distribution. In sum, our results suggest that (i) niche width is a major determinant of the probability of speciation in temperate montane clades, by controlling the probability of potential range fragmentation and (ii) dispersal limitation is also a major determinant of the speciation process, by increasing the fragmentation of realized ranges even when potential distributions are cyclically fused during climatic oscillations. When dispersal limitation is extreme, as in zopherine beetles, populations persist just in those areas that have retained suitable conditions during extremes of past climatic oscillations. Paradoxically, this relict condition confers zopherine beetles great resilience for facing future climate change.     

Land‐use and climate change effects on population size and extinction risk of Andean plants

Andean plant species are predicted to shift their distributions, or ‘migrate,’ upslope in response to future warming. The impacts of these shifts on species' population sizes and their abilities to persist in the face of climate change will depend on many factors including the distribution of individuals within species' ranges, the ability of species to migrate and remain at equilibrium with climate, and patterns of human land‐use. Human land‐use may be especially important in the Andes where anthropogenic activities above tree line may create a hard barrier to upward migrations, imperiling high‐elevation Andean biodiversity. In order to better understand how climate change may impact the Andean biodiversity hotspot, we predict the distributional responses of hundreds of plant species to changes in temperature incorporating population density distributions, migration rates, and patterns of human land‐use. We show that plant species from high Andean forests may increase their population sizes if able to migrate onto the expansive land areas above current tree line. However, if the pace of climate change exceeds species' abilities to migrate, all species will experience large population losses and consequently may face high risk of extinction. Using intermediate migration rates consistent with those observed for the region, most species are still predicted to experience population declines. Under a business‐as‐usual land‐use scenario, we find that all species will experience large population losses regardless of migration rate. The effect of human land‐use is most pronounced for high‐elevation species that switch from predicted increases in population sizes to predicted decreases. The overriding influence of land‐use on the predicted responses of Andean species to climate change can be viewed as encouraging since there is still time to initiate conservation programs that limit disturbances and/or facilitate the upward migration and persistence of Andean plant species.