Temperature‐associated decreases in demographic rates of Afrotropical bird species over 30 years

Abstract

Tropical mountains harbor globally significant levels of biodiversity and endemism. Climate change threatens many tropical montane species, yet little research has assessed the effects of climate change on the demographic rates of tropical species, particularly in the Afrotropics. Here, we report on the demographic rates of 21 Afrotropical bird species over 30 years in montane forests in Tanzania. We used mark–recapture analyses to model rates of population growth, recruitment, and apparent survival as functions of annual mean temperature and annual precipitation. For over one‐half of focal species, decreasing population growth rates were associated with increasing temperature. Due to the trend in temperature over time, we substituted a time covariate for the temperature covariate in top‐ranked population growth rate models. Temperature was a better explanatory covariate than time for 6 of the 12 species, or 29% of all focal species. Population growth rates were also lower for species found further below their elevational midpoint and for smaller‐bodied species. Changes in population growth rates were more closely tied to changes in recruitment than to changes in apparent survival. There were no consistent associations between demographic rates and precipitation. This study demonstrates temperature‐associated demographic impacts for 6 (29%) of 21 focal species in an Afrotropical understory bird community and highlights the need to incorporate the impacts of climate change on demographic rates into conservation planning across the tropics.

Afrotropical montane birds experience upslope shifts and range contractions along a fragmented elevational gradient in response to global warming

“…Furthermore, long-term monitoring of understory bird species in the East and West Usambara Mountains has indicated very limited dispersal ability for most species [21,63]. At the 1020 and 1310 m study sites, understory bird species have been surveyed annually over 34 years across a network of 14 forest fragments [21,51]. Over this time period and based on >31,500 captures and 4.43 × 10 6 m-mistnet-hrs, 21% of all understory bird species have never been recorded within a study fragment other than where they were originally banded [17].…”

The El Niño – Southern Oscillation dramatically influences the probability of reproduction and reproductive rate of a tropical forest bird

“…The proximate physiological mechanisms linking tropical climate variation to avian reproductive activity remain unclear. Given that arthropod abundance in tropical forests is seasonally correlated with rainfall (Wolda 1978, Boinski and Fowler 1989, Valtonen et al 2013), and may have been negatively affected by increasingly frequent high temperatures over recent decades (Lister and Garcia 2018), it is possible that prey scarcity is the primary factor limiting reproduction in dry years (Dowsett‐Lemaire 1985, Frith and Frith 2001, Neate‐Clegg et al 2021). Adults experiencing nutritional stress might lack the energetic resources to initiate reproduction in such years, or drought could affect reproductive physiology indirectly through endocrine or metabolic pathways (Goutte et al 2010, Martin and Mouton 2020).…”

“…However, the ecological pathways by which climate affects the demography of tropical forest birds are currently unresolved. Reports of ongoing enigmatic declines in tropical bird populations, even in relatively undisturbed areas of intact habitat, highlight the need to understand the mechanisms responsible for demographic variation (Blake and Loiselle 2015, Stouffer et al 2021, Neate‐Clegg et al 2021).…”

“…While some tropical studies have reported effects of El Niño conditions on adult survival, the magnitude and direction of putative effects differ widely, occasionally even within the same study area (negative: Stiles 1992, Woodworth et al 2018; positive: Ryder and Sillett 2016; inconclusive: Greenberg and Gradwohl 1986; opposing: Shogren et al 2019). Brawn et al (2017) and Neate‐Clegg et al (2021) found that apparent survival rates of adult birds were neither highly variable nor strongly correlated with climatic variation (but see Martin and Mouton 2020). Rather, both studies found that annual variation in population growth rates was driven primarily by strong reductions in recruitment (associated with longer dry seasons in Panama; Brawn et al 2017, and with higher mean temperatures in Tanzania; Neate‐Clegg et al 2021).…”

“…Brawn et al (2017) and Neate‐Clegg et al (2021) found that apparent survival rates of adult birds were neither highly variable nor strongly correlated with climatic variation (but see Martin and Mouton 2020). Rather, both studies found that annual variation in population growth rates was driven primarily by strong reductions in recruitment (associated with longer dry seasons in Panama; Brawn et al 2017, and with higher mean temperatures in Tanzania; Neate‐Clegg et al 2021). However, since neither study followed the fate of individual reproductive attempts, the mechanisms underlying reduced recruitment are not known.…”