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.

Keywords: dynamics; marginality; performance; range loss; threats.

Fig. 1.

Dynamics of species response curves and niche spaces following shifts from anthropogenic pressures. (Top) possible outcomes of niche shifts where green to yellow gradients represent high-quality to low-quality conditions, respectively. (Centre) changes in niche space in relation to these shifts where crosses represent niche centroids, blue—contemporary and orange—historic. Ellipses depict niche space, blue—contemporary, orange—historic. (Bottom) predicted changes in average distance to historic niche center with increasing range contraction.

Fig. 2.

Examples of species demonstrating contraction to niche centroid, ecological marginalization and stable niches under range contraction in three-dimensional environmental. (Top) Feathertail glider (Acrobates pygmaeus) as an example of a species with a stable niche under very low levels of range contraction. (Middle) Grevy’s zebra (Equus grevyi) as an example of contraction to niche centroid under very high range contraction (~90%). (Bottom) Red wolf (Canis rufus) as an example of ecological marginalization. All three-dimensional niche spaces are constructed from the first three principal components of contemporary and historic topographic and climatic variables separately. “Historic” niche points (modeled with Mid-Holocene period climate, ~6,000 y ago). Orange dots and ellipse are PCA coordinate points of each species’ contemporary niche and niche space respectively. Blue: historic niche points and niche space.

Fig. 3.

Geographic range loss has severe consequences for species niches—restricting them to smaller, less diverse, ecologically extreme niche spaces. (A) Geographic range loss results in niche shrinkage, (B) Small ranged species are more vulnerable to displacement to less diverse ranges- Range size is in km², Habitat diversity loss is estimated as proportion of ecoregions lost, Middle): Geographic range contraction restricts species to climatic and topographic extremes, (C) precipitation, (D) elevation, Bottom row) Geographic range loss leads to ecological marginalization. (E) Effect size of Mahalanobis distance to historic niche center, (F) Small ranged species are most vulnerable to ecological marginalization. R and P values presented were calculated from Pearson’s correlation coefficient. Yellow outlines represent 95% confidence interval of Pearson’s CC. Species correspond to range contracted species only. Ecological marginalization against geographic range loss with species names is available SI Appendix, Fig. S4.