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The term ‘biodiversity,’ while casually used in practice, is a complicated subject to measure, interpret, contextualize, and compare. Yet the possible advent of deep-sea mining in the mid-2020’s compels us to compare potential impacts of biodiversity loss across ecologically distant realms, a formidable task. Supplying the world’s green infrastructure is expected to lead to shortages of nickel, cobalt and other metals; meanwhile polymetallic nodules sitting atop the abyssal plains of the Clarion-Clipperton Zone (CCZ) of the Pacific Ocean contain billions of tons of nickel, cobalt, copper and manganese, enough to solve the supply issues. Implicit in society’s decision of whether to exploit this resource is a tradeoff of harm to biodiversity in the CCZ’s abyssal seafloor and its overlying water column, versus intensification of harm to rainforests and other terrestrial mining habitats. Here we frame the challenges of comparing biodiversity impacts across such different realms, spanning the gamut from normative to fundamental: ambiguities in definitions, lack of protocol standardization, physical challenges in measurement, difficulties to integrate measures among different taxonomic groups, profound differences between ecologically distant realms, contextual necessity to attribute value to mathematical index results, and constraints of current knowledge about species, ecosystems and system level impacts of biodiversity change. Quantitative biodiversity measures alone cannot rank one system above the other; measures must be supplemented with qualitative judgements of the tangible and intangible values of species and habitats to natural systems and to humans, along with consideration of other threats that they and we face.

Recent long-term studies in protected areas have revealed the loss of biodiversity, yet the ramifications for ecosystem health and resilience remain unknown. Here, we investigate how the loss of understory birds, in the lowest stratum of the forest, affects avian biomass and functional diversity in the Amazon rainforest. Across approximately 30 years in the Biological Dynamics of Forest Fragments Project, we used a historical baseline of avian communities to contrast the avian communities in today's primary forest with those in modern disturbed habitat. We found that in primary rainforest, the reduced abundance of insectivorous species led to reduced functional diversity, but no reduction of biomass, indicating that species with similar functional traits are less likely to coexist in modern primary forests. Because today's forests contain fewer functionally redundant species—those with similar traits—we argue that avian communities in modern primary Amazonian rainforests are less resilient, which may ultimately disrupt the ecosystem in dynamic and unforeseen ways.

*Center for Applied Biodiversity Science, Conservation International, Washington, D.C, 20037, U.S.A.†Climate Change Research Group, Ecology and Conservation, National Botanical Institute, Cape Town, South Africa‡The World Bank, Washington, D.C. 20433, U.S.A.§Botany Department, University of Cape Town, Cape Town, South Africa**Department of Biological Sciences, College of Science and Liberal Arts, Florida Institute of Technology, Melbourne,FL 32901–6975, U.S.A.††Environment Department, University of York, York, Y010 5DD, United Kingdom‡‡Adaptation and Impacts Research Group, Environment Canada at the Faculty of Environmental Studies, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada§§Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, United Kingdom

The Amazon is hypothesized to reach an irreversible ‘tipping point’ when deforestation slows the hydrological cycle sufficiently that tropical forest ecosystems cannot be sustained. However, inception of such a tipping point has not been supported by observations and the relevant links between deforestation and atmospheric moisture recycling are poorly understood. Here we show that reduction in evapotranspiration from 20 years of deforestation dried the atmosphere persistently and caused moisture decoupling, i.e. an opposite sign of moisture change between the lower and middle troposphere. Increased deforestation exacerbated the lower troposphere drying and caused it to penetrate deeper into the middle troposphere in the dry and transition seasons over monsoon forests and savannas. Deforestation induced warming-enhanced buoyant updrafts, elevated hot and dry air and thereby reduced downward mixing of water supplies from the tropical Atlantic that normally moisten the Amazon forests. The severe atmospheric desiccation in the southern and eastern Amazon cannot be compensated by enhanced water supplies from the Atlantic Ocean, demonstrating an irreversible transition in Amazon hydrological cycle exacerbated by rapid deforestation. The more recent drying through the seasons over rainforests and during the wet season over the transition zones from rainforests to monsoon forests and savannas, however, suggests a window of opportunity for preventing ecosystem collapse with forest conservation.

Description, Since 1980, 30 to 69% of species have shown reductions in mass, increases in wing length, and decreases in mass:wing ratio., Warming from climate change is expected to reduce body size of endotherms, but studies from temperate systems have produced equivocal results. Over four decades, we collected morphometric data on a nonmigratory understory bird community within Amazonian primary rainforest that is experiencing increasingly extreme climate. All 77 species showed lower mean mass since the early 1980s—nearly half with 95% confidence. A third of species concomitantly increased wing length, driving a decrease in mass:wing ratio for 69% of species. Seasonal precipitation patterns were generally better than temperature at explaining morphological variation. Short-term climatic conditions affected all metrics, but time trends in wing and mass:wing remained robust even after controlling for annual seasonal conditions. We attribute these results to pressures to increase resource economy under warming. Both seasonal and long-term morphological shifts suggest response to climate change and highlight its pervasive consequences, even in the heart of the world’s largest rainforest.