Process-based estimation of niches and range dynamics of South African Proteaceae from demographic data and range-wide abundance variation

Background/Question/Methods

The geographical distributions of species are determined by the dynamic interplay of reproduction, mortality and dispersal in a spatially and temporally heterogeneous environment. Widely applied species distribution models (SDMs) ignore these dynamics by taking a phenomenological and static approach to the estimation of species’ ranges and ecological niches.

Here, we propose a process-based approach for the statistical estimation of Dynamic Range Models (DRMs) from biogeographic and demographic data: DRMs integrate Hutchinson’s niche concept with spatial population dynamics in a hierarchical Bayesian state–space model to estimate the environmental response of demographic rates, local population dynamics and dispersal rates conditionally upon each other while accounting for various sources of uncertainty. The approach thereby jointly infers species’ niches and spatio-temporal population dynamics from data and provides probabilistic forecasts of range dynamics under environmental change. Parameters of the model are related to demographic rates that can be measured in the field or in experiments. DRMs thus enable the quantification of niches and range dynamics from a combination of demographic measurements and biogeographic distribution data.

Results/Conclusions

We present results from a case study where we investigate niches and range dynamics of serotinous shrubs (Proteaceae) in the Cape Floristic Region, South African. For the quantification of species’ niches in terms of demographic response functions we assembled a dataset that describes how demographic rates (mortality, fecundity, recruitment) vary among more than 3,000 populations across the ranges of 26 Proteaceae species. We demonstrate how these field data are combined with information on long-distance seed dispersal and data on range-wide variation in population size from the Protea Atlas citizen-science project. The integration of these different data sources within the DRM framework provides estimates of species-specific demographic response functions and parameters of spatial population dynamics. Investigating inter-specific correlations of the inferred characteristics of niches and range dynamics provides novel insights into large-scale strategies of our study species, e.g. by testing for a generalist-specialist trade-off between overall niche width and maximum population growth rates. The integrative modeling approach furthermore helps to understand how interactions between niches and spatial population dynamics drive the formation of different species´ ranges in variable environments and can be expected to enhance the forecasting of biodiversity change in the Cape Floristic Region.