No abstract

As anthropogenic climate change continues, the risks to biodiversity will increase over time, with future projections indicating a potentially catastrophic loss of global biodiversity on the horizon 1-3. However, our understanding of how this climatedriven disruption of biodiversity will unfold over time is limited because biodiversity forecasts typically focus on individual snapshots of the future. Here, we use annual projections (1850-2100) of temperature and precipitation across >30,000 marine and terrestrial species' ranges to estimate the timing of species exposure to potentially dangerous climate conditions. We project that future disruption of ecological assemblages from climate change will be abrupt, because within any given ecological assemblage the exposure of most species to climate conditions beyond their realised niche limits occurs near simultaneously. Under a high emissions scenario (RCP8.5), such abrupt exposure events begin before 2030 in tropical oceans and spread to tropical forests and higher latitudes by 2050. Below 2°C global warming, <2% of assemblages globally are projected to undergo abrupt exposure events of >20% of their constituent species, but the risk accelerates with the magnitude of warming, threatening 15% of assemblages at 4°C, with similar levels of risk in protected and unprotected areas. These results highlight the impending risk of sudden and severe biodiversity losses from climate change and provide a framework for predicting both when and where these events may occur.

Variation in species richness across environmental gradients may be associated with an expanded volume or increased packing of ecological niche space. However, the relative importance of these alternative scenarios remains unknown, largely because standardized information on functional traits and their ecological relevance is lacking for major diversity gradients. Here, we combine data on morphological and ecological traits for 523 species of passerine birds distributed across an Andes-to-Amazon elevation gradient. We show that morphological traits capture substantial variation in species dietary (75%) and foraging niches (60%) when multiple independent trait dimensions are considered. Having established these relationships, we show that the 14-fold increase in species richness towards the lowlands is associated with both an increased volume and density of functional trait space. However, we find that increases in volume contribute little to changes in richness, with most (78%) lowland species occurring within the range of trait space occupied at high elevations. Taken together, our results suggest that high species richness is mainly associated with a denser occupation of functional trait space, implying an increased specialization or overlap of ecological niches, and supporting the view that niche packing is the dominant trend underlying gradients of increasing biodiversity towards the lowland tropics.