Temperate grassland songbird species accumulate incrementally along a gradient of primary productivity.

William L Harrower,Roy Turkington,Cindy Mccallum,Diane S Srivastava,Lauchlan H Fraser

doi:10.1371/journal.pone.0186809

William L Harrower, Roy Turkington + Show 3 more

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https://doi.org/10.1371/journal.pone.0186809

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Abstract

Global analyses of bird communities along elevation gradients suggest that bird diversity on arid mountains is primarily limited by water availability, not temperature or altitude. However, the mechanism by which water availability, and subsequently primary productivity, increases bird diversity is still unclear. Here we evaluate two possible mechanisms from species-energy theory. The more individuals hypothesis proposes that a higher availability of resources increases the total number of individuals that can be supported, and therefore the greater number of species that will be sampled. By contrast, the more specialization hypothesis proposes that increasing resource availability will permit specialists to exploit otherwise rare resources, thus increasing total diversity. We used 5 years of surveys of grassland songbird communities along an elevational gradient in British Columbia, Canada, to distinguish between these hypotheses. Vegetation changed markedly in composition along the gradient and contrary to the expectations of the more specialization hypothesis, bird community composition was remarkably constant. However, both total abundance and species richness of birds increased with increasing water availability to plants. When we used rarefaction to correct species richness for differences in total abundance, much of the increase in bird diversity was lost, consistent with the expectations of the more individuals hypothesis. Furthermore, high species richness was associated with reductions in territory size of common bird species, rather than the fine-scale spatial partitioning of the landscape. This suggests that bird diversity increases when greater resource availability allows higher densities rather than greater habitat specialization. These results help explain a pervasive global pattern in bird diversity on arid mountains, and suggest that in such landscapes conservation of grassland birds is strongly linked to climate and hydrology.

Highlights

Across North America, grassland songbirds have been declining in abundance for almost 40 years [1,2,3]
We examined the distribution of temperate grassland songbirds along an moisture gradient to determine if and how species number accumulates with plant productivity
Experiments manipulating grazing frequency and intensity on rotations of 4 to 5 years are essential in determining how rest-rotation grazing systems influence plants and how they can promote songbirds. These findings are important because they point to different mechanisms that underlie general increases in the number of songbird species with ecosystem productivity that are dependent on resources availability

Summary

Introduction

Across North America, grassland songbirds have been declining in abundance for almost 40 years [1,2,3] Recovery of these species will depend on maintaining and restoring high-quality breeding grounds. Effects of resources on birds is often thought to be mediated by vegetation change [15] Plant characteristics such as live plant biomass, the mass or cover of plant detritus, and shrub abundance have often emerged as important predictors of grassland songbird abundance and species number [14]. Examining patterns of diversity on local gradients can provide insight into how diversity changes along broad latitudinal or continental-scale variation in water and nutrient abundance [8,18,19] and provide local tests of species-energy relationships

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