An invasive pathogen generally contracts species to their niche cores, not margins

Abstract

Quantifying how species' distributions contract in response to threats can reveal pathways of decline and the role of environmental conditions in moderating threat impacts. Two general patterns of niche contraction have been described: ecological marginalization, where species contract away from threat impacts to peripheral, sub-optimal areas of their niche, and; contraction to the core, where species contract toward their niche center where their fitness and capacity to withstand threat impacts is highest. Recent work has described widespread ecological marginalization in declining mammal species, for which land use change and overexploitation are key threats. Different threatening processes could result in contrasting patterns of niche contraction, although this has not been well-studied. Here, we examine patterns of realized niche contraction in Australian frog species impacted by the emergence of chytrid fungus Batrachochytrium dendrobatidis, a pathogen that has driven catastrophic amphibian declines globally. We quantified changes in species' environmental niche space following chytrid emergence and documented a pattern of contraction toward the niche core in declining species. We develop and apply a novel approach to show that these niche contractions are driven by losses in a subset of niche space, suggesting population extinctions due to chytrid are driven by factors shaping both pathogen fitness (threat impact) and host fitness (threat tolerance). Species declines have been concentrated in high elevation areas with cooler temperatures, which are more physiologically suitable for the pathogen and constrain the resilience of frog hosts at both individual and population levels. Given the contrast between our results and widespread ecological marginalization in mammals, we propose that while a given threat may result in common patterns of decline among affected species, patterns of decline may vary considerably between threatening processes and among taxa.