We connect evidence that 20 oC is the most stable temperature for cellular processes with macroecological observations. Examples show that temperatures warmer than ~20 oC result in decreases in: aquatic species’ tolerance to low oxygen; marine pelagic and benthic algal productivity; pelagic and benthic predation rates; global species richness in pelagic fishes, plankton and benthic invertebrates; and genetic diversity; but increased extinctions in the fossil record. The realised thermal niche of reef fishes and invertebrates globally is narrowest among species with distributions centred on 20 oC, as also seen in microbes. While many species have evolved to live at warmer and colder temperatures, most species live at, and extinctions in the fossil record across seven phyla were lower at, 20 oC. The mathematical “Corkrey” model, which predicts that thermal breadth should be minimized and species richness maximised at 20 oC across all Domains of life, provides an explanation for this “20 oC effect”. A literature search found highest species richness at ~20 oC across life in air and water, including animals, plants and microbes. That life seems centred around ~20 oC implies fundamental constraints that compromise the ability of extant tropical species to adapt to higher temperatures.

The global lockdown to mitigate COVID-19 pandemic health risks has altered human interactions with nature. Here, we report immediate impacts of changes in human activities on wildlife and environmental threats during the early lockdown months of 2020, based on 877 qualitative reports and 332 quantitative assessments from 89 different studies. Hundreds of reports of unusual species observations from around the world suggest that animals quickly responded to the reductions in human presence. However, negative effects of lockdown on conservation also emerged, as confinement resulted in some park officials being unable to perform conservation, restoration and enforcement tasks, resulting in local increases in illegal activities such as hunting. Overall, there is a complex mixture of positive and negative effects of the pandemic lockdown on nature, all of which have the potential to lead to cascading responses which in turn impact wildlife and nature conservation. While the net effect of the lockdown will need to be assessed over years as data becomes available and persistent effects emerge, immediate responses were detected across the world. Thus, initial qualitative and quantitative data arising from this serendipitous global quasi-experimental perturbation highlights the dual role that humans play in threatening and protecting species and ecosystems. Pathways to favorably tilt this delicate balance include reducing impacts and increasing conservation effectiveness.