EcoRaMa

Ecology and Evolution of Species Range Margins

The processes shaping species range margins have interested ecologists and evolutionary biologists for a long time and have received increasing attention as ongoing environmental changes are shifting species’ ranges. In the absence of dispersal limitation, range margins occur when species can maintain persistent populations within their ranges but fail to do so beyond range margins (Panel A in the diagram below). Various ecological factors, such as climate and species interactions, can limit population growth rates beyond range margins, leading niche limits to range limits. Over the evolutionary time scale, range margins occur when species can overcome environmental constraints by local adaptation within their ranges but fail to do so beyond range margins, leading adaptation limits to range limits (Panel B).

Two main population-genetic hypotheses have been advanced to explain the failure of local adaptation. First, local adaptation in marginal populations may be hindered by ongoing gene flow from the core of the species distribution, such that maladaptive immigrant alleles ‘swamp’ any responses to local selection (the ‘swamping’ hypothesis). Alternatively, marginal populations may be typically small and relatively inbred, with low genetic diversity due to repeated bottlenecks, with the consequent greater expression of deleterious recessive mutations and thus poor demographic performance; in this case, migration from other populations should ‘rescue’ marginal populations from the ill-effects of genetic drift and inbreeding (the ‘rescue’ hypothesis).

Aims

EcoRaMa aims to uncover the ecological and evolutionary processes leading to the range margins of Mercurialis annua (see Study system) . Combining a continental-scale transplant experiment (see Experimental design) with demographic modelling, quantitative genetics, and population genetics (see Data collection), EcoRaMa has three main aims:

To distinguish between the swamping and rescue explanations for species' range margins

To identify environmental factors, including abiotic and biotic factors, and demographic processes that limit population growth towards range margins

To uncover the phenotypic and genetic processes limiting local adaptation beyond range margins

Study system

EcoRaMa uses a wind-pollinated dioecious plant, and thus fully outcrossing, Mercurialis annua, as a model system. M. annua is an annual plant and has recently proved ideal for experimental evolution under semi-natural conditions. It has a range across Europe and the Middle East (the shaded area), resulting from northern- and westward range expansion out of an eastern Mediterranean refugium after the Pleistocene glaciation ended.

Experimental design

In 2023, we sampled seeds from 21 populations at seven localities across the range of M. annua: range edge (orange, n = 3 localities), core (green, n = 3 localities), and refugium (blue, n = 1 locality). These populations were grown in Lausanne in the summer of 2023 to produce maternal effects-free seeds. In spring 2024, we established experimental populations subject to four different ‘admixture’ (i.e. migration) treatments, ranging from zero (no admixture) to local (local admixture) to distant (core-edge admixture and refugium-edge admixture). The figure on the right below shows a typical design at an edge site. The same experimental design is repeated in 11 gardens across the entire range of M. annua and beyond its range margins: one garden in the refugium range (blue), three gardens in the range core (blue), three gardens at the range margins (orange) and four gardens beyond range margins (red). Therefore, the experimental populations are exposed to range-wide variation in abiotic and biotic conditions.