High turn-over rates at the upper range limit and elevational source-sink dynamics in a widespread songbird

Martin U Grüebler,Johann Von Hirschheydt,Fränzi Korner-Nievergelt

doi:10.1038/s41598-021-98100-x

Abstract

The formation of an upper distributional range limit for species breeding along mountain slopes is often based on environmental gradients resulting in changing demographic rates towards high elevations. However, we still lack an empirical understanding of how the interplay of demographic parameters forms the upper range limit in highly mobile species. Here, we study apparent survival and within-study area dispersal over a 700 m elevational gradient in barn swallows (Hirundo rustica) by using 15 years of capture-mark-recapture data. Annual apparent survival of adult breeding birds decreased while breeding dispersal probability of adult females, but not males increased towards the upper range limit. Individuals at high elevations dispersed to farms situated at elevations lower than would be expected by random dispersal. These results suggest higher turn-over rates of breeding individuals at high elevations, an elevational increase in immigration and thus, within-population source-sink dynamics between low and high elevations. The formation of the upper range limit therefore is based on preference for low-elevation breeding sites and immigration to high elevations. Thus, shifts of the upper range limit are not only affected by changes in the quality of high-elevation habitats but also by factors affecting the number of immigrants produced at low elevations.

Highlights

The formation of an upper distributional range limit for species breeding along mountain slopes is often based on environmental gradients resulting in changing demographic rates towards high elevations
Upper range limits often occur within populations, in particular in highly mobile species such as birds where populations cover large extents of elevations associated with environmental g radients[19], connected by high rates of dispersal between high and low elevations
Apparent survival tended to be lower for females compared to males at low elevations and the stronger decrease in male apparent survival with elevation resulted in similar survival estimates between sexes at high elevations

Summary

Introduction

The formation of an upper distributional range limit for species breeding along mountain slopes is often based on environmental gradients resulting in changing demographic rates towards high elevations. Individuals at high elevations dispersed to farms situated at elevations lower than would be expected by random dispersal These results suggest higher turn-over rates of breeding individuals at high elevations, an elevational increase in immigration and within-population source-sink dynamics between low and high elevations. Theoretical c onsiderations[3,8,10] and transplant experiments[18] suggest that in species with high dispersal ability, range limits are shifted upwards beyond conditions supporting sustainable populations Such species establish sink populations at the upper range limit producing source-sink dynamics over the elevational g radient[3,10,11,12]. Knowledge of within-population elevational gradients of demographic rates rather than that of between-population differences in demography at varying elevations can help to understand the mechanisms underlying the formation of the upper limit of population distribution

Methods

Results

Conclusion