The combined effects of temperature and fragment area on the demographic rates of an Afrotropical bird community over 34 years.

ABSTRACT

Habitat fragmentation and climate change are two of the greatest threats to biodiversity, yet their combined impacts and potential interactions are poorly understood, particularly in the context of demographic rates. The Usambara Mountains, Tanzania, comprise a highly fragmented landscape where temperatures have increased by 0.58 °C over the last three decades. Here, we used a 34-year bird banding dataset from 14 forest fragments (0.2-908 ha) to examine the combined effects of fragment area and mean annual temperature on the demographic rates of 24 understory bird species. The population growth rates of two-thirds of species were negatively associated with at least one of the stressors, and, overall, population growth rates were 12-20 % lower in the warmest year compared to the coolest year, depending on fragment size. When temperature and fragment area were combined in models of recruitment, survival, and population growth, their effects were more frequently interactive than additive, however these interactions were rarely synergistic. Independently, temperature also received more model support than fragment area and tended to have a greater impact on demographic rates across species. Despite the complexity of the interactions between fragment area and temperature, their total effects on tropical bird demographic rates were largely detrimental. The development of effective conservation strategies for montane tropical bird species needs to account for these combined impacts.