Recent global changes have decoupled species richness from specialization patterns in North American birds

Abstract

Aim Theory suggests that increasing productivity and climate stability towards the tropics favours specialization, thus contributing to the latitudinal richness gradient. A positive relationship between species richness and specialization should therefore emerge as a fundamental biogeographical pattern. However, land‐use and climate changes disproportionally increase the local extirpation risk for specialists, potentially weakening the relationship between richness and specialization. Here, we quantify empirically the richness–specialization prediction and test how 50 years of climate and land‐use change has affected the richness–specialization relationship. Location USA. Time period 1966–2015. Major taxa studied Birds. Methods We used the North American Breeding Bird Survey to quantify bird community richness and specialization to habitat and climate. We (a) quantify temporal change in the slope of the richness–specialization relationship, using a generalized mixed model; (b) assess how this change translates spatially, using generalized additive models; and (c) attribute spatio‐temporal change in the richness–specialization relationship to land use, climate and topographic drivers. Results We found evidence for a positive but weak richness–specialization relationship in bird communities that greatly weakened over time. Given that specialization was not the main driver of richness, this relationship did not translate spatially into a linear spatial covariation between richness and specialization. Instead, the spatial covariation in richness and specialization followed a unimodal pattern, the peak of which shifted towards less specialized communities over time. These temporal changes were associated with precipitation change, decreasing temperature stability and land use. Main conclusions Recent climate and land‐use changes have induced two contrasting types of community responses. In human‐dominated areas, the decoupling of richness and specialization drove a general trend for biotic homogenization. In areas of low human impact experiencing increasing climate harshness, specialization increased, whereas richness decreased. Our results offer new support for specialization as a key driver of macroecological diversity patterns and show that global changes are weakening this fundamental macroecological pattern.

Scale‐dependent effects of niche specialisation: The disconnect between individual and species ranges

“…The positive association between geographical range size and niche breadth has received much attention partly because it was proposed as one of the mechanisms for the most pervasive biodiversity pattern, the latitudinal diversity gradient (Jocque et al, 2010; Stevens, 1989). Tropical species are often expected to have smaller ranges and narrower niches (reviewed in Mimet et al, 2019). However, our analyses showed that this apparent association only applies to habitat but not dietary niche breadth, complementing previous community network analyses that showed reduced rather than increasing animal dietary specialisation on flowering and fruiting plants towards lower latitudes (Schleuning et al, 2012; but see contrasting patterns at the assemblage level in Dalsgaard et al, 2017).…”

Variable colour patterns indicate multidimensional, intraspecific trait variation and ecological generalization in moths

“…Latitudinal gradients and positive species richness‐specialization relations are apparent in in large‐scale analysis of other taxa (e.g. Mimet et al and references therein). The incidence of generalist species may be higher at higher latitudes, where abiotic stressors are dominant, than in tropical and other species‐rich regions, where the relative importance of biotic interactions is stronger.…”

Phylogenetic Endemism Hotspots of North American Birds Are Associated With Warm Temperatures and Long- and Short-Term Climate Stability

“…At a global scale, high PE is associated with high temperatures and precipitation (Rosauer and Jetz, 2015;Sandel et al, 2020), suggesting that anthropogenic climate change may lead to changes in the distribution of PE. Changing landscape patterns in anthropogenic environments may also favor generalist over specialist species and threaten small-ranged endemics, tending to reduce PE (Ferrer-Sánchez and Rodríguez-Estrella, 2015;González-Orozco et al, 2015;Mimet et al, 2019). However, most studies of PE patterns lack the temporal or spatial resolution to assess changes through time in local assemblages.…”