Abrupt expansion of climate change risks for species globally

Climate change is already exposing species to dangerous temperatures driving widespread population and geographical contractions. However, little is known about how these risks of thermal exposure will expand across species' existing geographical ranges over time as climate change continues. Here, using geographical data for approximately 36,000 marine and terrestrial species and climate projections to 2100, we show that the area of each species' geographical range at risk of thermal exposure will expand abruptly. On average, more than 50% of the increase in exposure projected for a species will occur in a single decade. This abruptness is partly due to the rapid pace of future projected warming but also because the greater area available at the warm end of thermal gradients constrains species to disproportionately occupy sites close to their upper thermal limit. These geographical constraints on the structure of species ranges operate both on land and in the ocean and mean that, even in the absence of amplifying ecological feedbacks, thermally sensitive species may be inherently vulnerable to sudden warming-driven collapse. With higher levels of warming, the number of species passing these thermal thresholds, and at risk of abrupt and widespread thermal exposure, increases, doubling from less than 15% to more than 30% between 1.5 °C and 2.5 °C of global warming. These results indicate that climate threats to thousands of species are expected to expand abruptly in the coming decades, thereby highlighting the urgency of mitigation and adaptation actions.

Drivers of elevational richness peaks, evaluated for trees in the east Himalaya

Along elevational gradients, species richness often peaks at intermediate elevations and not the base. Here we refine and test eight hypotheses to evaluate causes of a richness peak in trees of the eastern Himalaya. In the field, we enumerated trees in 50 plots of size 0.1 ha each at eight zones along an elevational gradient and compared richness patterns with interpolation of elevational ranges of species from a thorough review of literature, including floras from the plains of India. The maximum number of species peaks at similar elevations in the two data sets (at 500 m in the field sampling and between 500 m and 1,000 m in range interpolation); concordance between the methods implies that statistical artefacts are unlikely to explain the peak in the data. We reject most hypotheses (e.g., area, speciation rate, mixing of distinct floras). We find support for a model in which climate (actual evapotranspiration [AET] or its correlates) sets both the number of species and each species optimum, coupled with a geometric constraint. We consider that AET declines with elevation, but an abrupt change in the association of AET with geographical distance into the plains means that the location of highest AET, at the base of the mountain, receives range overlaps from fewer species than the location just above the base. We formalize this explanation with a mathematical model to show how this can generate the observed low-elevation richness peak.

Marine biodiversity and the chessboard of life

Species richness is greater in places where the number of potential niches is high. Consequently, the niche may be fundamental for understanding the arrangement of life and especially, the establishment and maintenance of the well-known Latitudinal Biodiversity Gradient (LBG). However, not all potential niches may be occupied fully in a habitat, as measured by niche vacancy/saturation. Here, we theoretically reconstruct oceanic biodiversity and analyse modeled and observed data together to examine patterns in niche saturation (i.e. the ratio between observed and theoretical biodiversity of a given taxon) for several taxonomic groups. Our results led us to hypothesize that the arrangement of marine life is constrained by the distribution of the maximal number of species’ niches available, which represents a fundamental mathematical limit to the number of species that can co-exist locally. We liken this arrangement to a type of chessboard where each square on the board is a geographic area, itself comprising a distinct number of sub-squares (species’ niches). Each sub-square on the chessboard can accept a unique species of a given ecological guild, whose occurrence is determined by speciation/extinction. Because of the interaction between the thermal niche and changes in temperature, our study shows that the chessboard has more sub-squares at mid-latitudes and we suggest that many clades should exhibit a LBG because their probability of emergence should be higher in the tropics where more niches are available. Our work reveals that each taxonomic group has its own unique chessboard and that global niche saturation increases when organismal complexity decreases. As a result, the mathematical influence of the chessboard is likely to be more prominent for taxonomic groups with low (e.g. plankton) than great (e.g. mammals) biocomplexity. Our study therefore reveals the complex interplay between a fundamental mathematical constraint on biodiversity resulting from the interaction between the species’ ecological niche and fluctuations in the environmental regime (here, temperature), which has a predictable component and a stochastic-like biological influence (diversification rates, origination and clade age) that may alter or blur the former.