Understanding how rates of reproduction and survival respond to environmental variation across species’ geographical ranges is a key task for both basic and applied ecology. So far, however, environmental drivers of range-wide demographic variation have only been studied in a few plant species without considering the potentially confounding effects of population density on demographic rates. We present a large-scale demographic study of 26 shrub species (Proteaceae) from the Cape Floristic Region, South Africa. All study species have a fire-dependent life cycle and are serotinous: they exclusively form a canopy seed bank which contains the seeds produced since the last fire. Fire triggers seed release from the canopy so that recruitment is largely limited to a short period after fire.
Across the global geographical ranges of the study species, we collected 3454 population-level records of total fecundity since the last fire (size of individual canopy seed banks), per-capita recruitment (ratio between post-fire recruits and pre-fire adults) and adult fire survival. We used linear regressions to quantify how climate, population density, fire interval and soil nutrients affect demographic variation.
Results/Conclusions
A trade-off between survival and reproduction is evident throughout the geographical ranges of our study species: resprouting species with fire-protected buds had much higher fire survival than nonsprouters without fire-protected buds (97% vs. 2%) but they also had substantially lower fecundity and recruitment rates. We found little intraspecific variation in fire survival rates but considerable intraspecific variation in fecundity and recruitment. Range-wide variation in fecundity was dominated by fire interval whereas recruitment was mostly climate-driven. Population density and soil nutrients generally had smaller effects but were important for the fecundity and recruitment of several species. Effects of fire interval on fecundity were consistent across species, but other demography–environment relationships showed substantial interspecific differentiation.
This study extends demographic research beyond the population to cover the geographical ranges of multiple species. Such large-scale studies are a necessary first step of a research agenda that aims to understand how range dynamics emerge from first principles of demography, how they are shaped by functional traits and macroevolution and how they will be impacted by global change.