How to understand species’ niches and range dynamics: a demographic research agenda for biogeography
Abstract
Range dynamics causes mismatches between a species’ geographical distribution and the set of suitable environments in which population growth is positive (the Hutchinsonian niche). This is because source–sink population dynamics cause species to occupy unsuitable environments, and because environmental change creates non‐equilibrium situations in which species may be absent from suitable environments (due to migration limitation) or present in unsuitable environments that were previously suitable (due to time‐delayed extinction). Because correlative species distribution models do not account for these processes, they are likely to produce biased niche estimates and biased forecasts of future range dynamics. Recently developed dynamic range models (DRMs) overcome this problem: they statistically estimate both range dynamics and the underlying environmental response of demographic rates from species distribution data. This process‐based statistical approach qualitatively advances biogeographical analyses. Yet, the application of DRMs to a broad range of species and study systems requires substantial research efforts in statistical modelling, empirical data collection and ecological theory. Here we review current and potential contributions of these fields to a demographic understanding of niches and range dynamics. Our review serves to formulate a demographic research agenda that entails: (1) advances in incorporating process‐based models of demographic responses and range dynamics into a statistical framework, (2) systematic collection of data on temporal changes in distribution and abundance and on the response of demographic rates to environmental variation, and (3) improved theoretical understanding of the scaling of demographic rates and the dynamics of spatially coupled populations. This demographic research agenda is challenging but necessary for improved comprehension and quantification of niches and range dynamics. It also forms the basis for understanding how niches and range dynamics are shaped by evolutionary dynamics and biotic interactions. Ultimately, the demographic research agenda should lead to deeper integration of biogeography with empirical and theoretical ecology.
What the paper shows and why it matters
(AI-generated)
A species’ niche, in Hutchinson’s sense, is the set of environments where its population can grow when rare (its intrinsic growth rate is positive). The paper notes that niches and future range shifts are statistically estimated almost entirely with correlative species distribution models, which relate where a species is recorded to environmental conditions; direct demographic measurements and mechanistic niche models take too much effort for many species. The correlation is often taken to reflect the niche, yet the two can differ: individuals spill from good into poor habitat (Pulliam 2000), dispersal-limited species are missing from suitable places, and species linger where conditions have recently deteriorated. Unbiased niche estimates thus seem to require knowing range dynamics, but these in turn require knowing how demographic rates respond to the environment, which is the niche itself.
The paper argues that this circularity between niche and range dynamics can be undone by a demographic research agenda whose central step is to estimate niches and range dynamics together with dynamic range models (DRMs), which are fitted statistically to distribution and demographic data. A DRM links three parts: how birth, death and dispersal vary with the environment (birth and death determine the niche), how these rates shape the founding and loss of local populations, and how the populations are sampled in the data. As an illustration, the paper reuses a simulation by Pagel and Schurr (2012): a virtual species’ range was followed through 75 years of climate change, and models were fitted to data from the first 10 years. The DRM estimated the species’ true niche and its future range size much better than a logistic-regression distribution model fitted to the same data.
The paper sets out what is needed to apply DRMs to many species. Statistical work should extend them to structured populations and complex dynamics; for now they suit only species with simple life cycles and enough data. Data collection should add repeated distribution surveys, population time series, demographic measurements across environments and long-distance dispersal data. Theory should clarify how demographic rates scale up and how coupled populations behave. The paper reads this basis as a prerequisite for understanding how evolution and species interactions shape niches and ranges, and expects it to lead to closer integration of biogeography with empirical and theoretical ecology.
References
- Pulliam (2000). On the relationship between niche and distribution. Ecology Letters 3, 349–361. doi:10.1046/j.1461-0248.2000.00143.x
- Pagel & Schurr (2012). Forecasting species ranges by statistical estimation of ecological niches and spatial population dynamics. Global Ecology and Biogeography 21, 293–304. doi:10.1111/j.1466-8238.2011.00663.x