More losers than winners: investigating Anthropocene defaunation through the diversity of population trends

Abstract

The global‐scale decline of animal biodiversity (‘defaunation’) represents one of the most alarming consequences of human impacts on the planet. The quantification of this extinction crisis has traditionally relied on the use of IUCN Red List conservation categories assigned to each assessed species. This approach reveals that a quarter of the world's animal species are currently threatened with extinction, and ~1% have been declared extinct. However, extinctions are preceded by progressive population declines through time that leave demographic ‘footprints’ that can alert us about the trajectories of species towards extinction. Therefore, an exclusive focus on IUCN conservation categories, without consideration of dynamic population trends, may underestimate the true extent of the processes of ongoing extinctions across nature. In fact, emerging evidence (e.g. the Living Planet Report), reveals a widespread tendency for sustained demographic declines (an average 69% decline in population abundances) of species globally. Yet, animal species are not only declining. Many species worldwide exhibit stable populations, while others are even thriving. Here, using population trend data for >71,000 animal species spanning all five groups of vertebrates (mammals, birds, reptiles, amphibians and fishes) and insects, we provide a comprehensive global‐scale assessment of the diversity of population trends across species undergoing not only declines, but also population stability and increases. We show a widespread global erosion of species, with 48% undergoing declines, while 49% and 3% of species currently remain stable or are increasing, respectively. Geographically, we reveal an intriguing pattern similar to that of threatened species, whereby declines tend to concentrate around tropical regions, whereas stability and increases show a tendency to expand towards temperate climates. Importantly, we find that for species currently classed by the IUCN Red List as ‘non‐threatened’, 33% are declining. Critically, in contrast with previous mass extinction events, our assessment shows that the Anthropocene extinction crisis is undergoing a rapid biodiversity imbalance, with levels of declines (a symptom of extinction) greatly exceeding levels of increases (a symptom of ecological expansion and potentially of evolution) for all groups. Our study contributes a further signal indicating that global biodiversity is entering a mass extinction, with ecosystem heterogeneity and functioning, biodiversity persistence, and human well‐being under increasing threat.

Bird species' tolerance to human pressures and associations with population change

“…Our results make it possible to identify those threatened species that are tolerant to human pressures, which supports the approach of conserving them also in the highly modified environments. The link between human pressure tolerance and population trend was stronger than the link between human pressure and Red List threat status, likely because more species are declining than are threatened (Finn et al, 2023).…”

“…org, downloaded 27.04.2023). Because global data on quantitative and continuous population trends of all bird species are not available, we followed the example set by Ceballos et al (2017) and Finn et al (2023) and used the categorical population trend measures of the IUCN Red List (i.e. declining, stable, or increasing).…”

Urban tolerance does not protect against population decline in North American birds

“…Climate change, invasive species, habitat loss and other human-caused challenges degrade ecosystems and threaten biodiversity on a global scale [1]. Ongoing losses of biodiversity are apparent across taxa and have been described as the planet's sixth mass extinction event [2][3][4][5][6]. For example, recent evidence indicates dramatic declines across many bird species [7], including a net loss of almost 3 billion individual birds since the 1970s in North America alone [8].…”

On being a forest soil scientist—Reflections at the 14th North American Forest Soils Conference

“…We also have increasing rates of species extinctions, which has the potential to impact us in ways that we perhaps cannot even imagine. Species extinction rates have increased markedly during the Anthropocene (Finn et al, 2023), which should alarm us as "canaries in the coal mine," but most go completely undetected while we focus our efforts on individual charismatic species. Dead zones in river deltas reflect the inefficiencies of agricultural production and the over application of N fertilizers to make up for the soil degradation described above.…”