Ecological marginalization is widespread and increases extinction risk in mammals

Abstract

Human land-use results in widespread range change across taxa. Anthropogenic pressures can result in species’ realized niches expanding, shifting, or contracting. Marginalization occurs when contraction constrains species to the geographic or ecological extremes of their historic niche. Using 4,785 terrestrial mammal species, we show that range contraction results in niche space and habitat diversity loss. Additionally, ecological marginalization is a common consequence of range contraction caused by human land use change. Remnant populations become located in the climatic and topographic extremes of their historic niche that are more likely to be at the periphery of their historic niche at greater distances from historic niche centroids. This ecological marginalization is associated with poor performance and increased extinction risk independent of geographic range loss. Range loss and marginalization may create a “double whammy” in vulnerable groups, such as large-bodied species and species with small geographical range size. Our results reveal a hitherto unrecognized conservation threat that is vital to incorporate into conservation assessment and management.

A grazer's niche edge is associated with increasing diet diversity and poor population performance

“…Recent studies have demonstrated that species performance trends and extinction risks are better explained by the distance to the species' ecological niche centroid rather than their geographic range centre (Britnell et al, 2023;Martínez-Meyer et al, 2013). Conditions at the range centre are rarely the same as those seen at niche centres (Britnell et al, 2023). The predictions of Caughley et al (1988) also assumed limiting factors, such as climate and resource use, acted independently and varied consistently across species ranges (Gaston, 2003).…”

“…However, an underappreciated factor is that PAs may be located within marginal habitats for the species they intend to protect (Kerley et al, 2012(Kerley et al, , 2020. Human pressures have pushed many mammal species towards their historic niche margins where they experience greater extinction risk (Britnell et al, 2023). In mild cases, there is large variation in PA effectiveness as species are protected as 'partial refugees' in both optimal and sub-optimal habitats (Lea et al, 2016).…”

“…A fundamental question in ecology is what limits species distributions and abundances. This question becomes crucial as human pressures cause widespread declines in species' ranges, niches and population sizes (Britnell et al, 2023;WWF, 2022). Ecological gradients result through variation in biotic and abiotic conditions (Holt, 2009).…”

“…With modern remote sensing and niche modelling techniques, we can now build an estimate of a species niche as a multidimensional volume (hyperspace) where axes correspond to environmental conditions that influence organismal fitness (Holt, 2009). Britnell et al (2023) recently developed an approach to quantify the position of a population relative to the species' historic niche centre.…”

Traits help explain species' performance away from their climate niche centre

“…As outlined above, species vary in the amount of decline or even whether or not performance declines at all away from the niche centre and towards presumably marginal climates (Pironon et al, 2017; Sporbert et al, 2020; but see Britnell et al, 2023; Fristoe et al, 2023). We propose that functional traits may help explain species' ability to increase performance under non‐optimal climate conditions.…”