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How many species are there on Earth and to what groups do these species belong? These fundamental questions span systematics, ecology, and evolutionary biology. Yet, recent estimates of overall global biodiversity have ranged wildly, from the low millions to the trillions. Insects are a pivotal group for these estimates. Insects make up roughly half of currently described extant species (across all groups), with ~1 million described species. Insect diversity is also crucial because many other taxa have species that may be unique to each insect host species, including bacteria, apicomplexan protists, microsporidian fungi, nematodes, and mites. Several projections of total insect diversity (described and undescribed) have converged on ~6 million species. However, these projections have not incorporated the morphologically cryptic species revealed by molecular data. Here, we estimate the extent of cryptic insect diversity. We perform a systematic review of studies that used explicit species-delimitation methods with multilocus data. We estimate that each morphology-based insect species contains (on average) 3.1 cryptic species. We then use these estimates to project the overall number of species on Earth and their distribution among major groups. Our estimates suggest that overall global biodiversity may range from 563 million to 2.2 billion species. [Biodiversity; cryptic species; insects; species delimitation; species richness.] [ABSTRACT FROM AUTHOR]

Climate change may be a major threat to global biodiversity, especially to tropical species. Yet, why tropical species are more vulnerable to climate change remains unclear. Tropical species are thought to have narrower physiological tolerances to temperature, and they have already experienced a higher estimated frequency of climate‐related local extinctions. These two patterns suggest that tropical species are more vulnerable to climate change because they have narrower thermal niche widths. However, no studies have tested whether species with narrower climatic niche widths for temperature have experienced more local extinctions, and if these narrower niche widths can explain the higher frequency of tropical local extinctions. Here, we test these ideas using resurvey data from 538 plant and animal species from 10 studies. We found that mean niche widths among species and the extent of climate change (increase in maximum annual temperatures) together explained most variation (>75%) in the frequency of local extinction among studies. Surprisingly, neither latitude nor occurrence in the tropics alone significantly predicted local extinction among studies, but latitude and niche widths were strongly inversely related. Niche width also significantly predicted local extinction among species, as well as among and (sometimes) within studies. Overall, niche width may offer a relatively simple and accessible predictor of the vulnerability of populations to climate change. Intriguingly, niche width has the best predictive power to explain extinction from global warming when it incorporates coldest yearly temperatures. [ABSTRACT FROM AUTHOR]