Local adaptation primes cold‐edge populations for range expansion but not warming‐induced range shifts

Abstract

According to theory, edge populations may be poised to expand species’ ranges if they are locally adapted to extreme conditions, or ill‐suited to colonise beyond‐range habitat if their offspring are genetically and competitively inferior. We tested these contrasting predictions by transplanting low‐, mid‐, and high‐elevation (edge) populations of an annual plant throughout and above its elevational distribution. Seed from poor‐quality edge habitat (one of two transects) had inferior emergence, but edge seeds also had adaptive phenology (both transects). High‐elevation plants flowered earlier, required less heat accumulation to mature seed, and so achieved higher lifetime fitness at and above the range edge. Experimental warming improved fitness above the range, but eliminated the advantage of local cold‐edge populations, supporting recent models in which cold‐adapted edge populations do not facilitate warming‐induced range shifts. The highest above‐range fitness was achieved by a ‘super edge phenotype’ from a neighbouring mountain, suggesting key adaptations exist regionally even if absent from local edge populations.

Latitudinal biodiversity gradients at three levels: Linking species richness, population richness and genetic diversity

“…A species can then only broaden its niche through niche evolution and dispersal (Sandel et al, ; Wiens & Donoghue, ), which would influence population differentiation across a species’ range. For example, niche evolution may result in an increase in genetic differentiation among populations at range edges due to strong directional selection, similar to expectations from the central‐marginal hypothesis (Eckert, Samis, & Lougheed, ; Guo, ; Hargreaves & Eckert, ; Willi, Fracassetti, Zoller, & Buskirk, ). With more differentiated populations at range edges, we might expect high latitudes to have higher PopPerSpp, as species that have expanded their ranges outwards from the tropics would likely have more populations in these high latitude areas.…”

“…Thus, according to Rapoport’s rule, we would expect populations at or near the edge of a species’ range to experience lower gene flow. Range‐edge populations are more likely to be geographically isolated and more differentiated from neighbouring populations, particularly for large‐ranged species (Eckert et al, ; Hargreaves & Eckert, ; Pelletier & Carstens, ; Stevens, ; Willi et al, ). A likely consequence across a species’ range would be more distinct populations at or near range edges, and fewer within the ‘core’ due to increased gene flow (Pelletier & Carstens, ).…”

“…Additionally, adaptation to colder or new environments (i.e. niche evolution, Wiens & Donoghue, 2004) tends to elicit strong directional selection, often leading to losses in GenPerSpp (Eckert et al, 2008;Hargreaves & Eckert, 2019;Pierce, Gutierrez, Rice, & Pfennig, 2017).…”

Climate change disrupts local adaptation and favours upslope migration

“…We examined fitness in three situations relevant to climate change: (1) populations transplanted into lower elevation sites, which reflect future climatic conditions, but differ in non-climatic factors; (2) local populations experiencing simulated climate change in their home sites and (3) populations transplanted into higher elevation sites in climatic manipulations, which indicates how upslope migrants fare in novel locales. Field manipulations of abiotic factors have rarely been applied in an evolutionary context (De Frenne et al 2011;Pfeifer-Meister et al 2013;Hargreaves & Eckert 2019) despite their potential for illuminating the eco-evolutionary consequences of climate change.…”

Niche models do not predict experimental demography but both suggest dispersal limitation across the northern range limit of the scarlet monkeyflower (Erythranthe cardinalis)

“…Such an approach enabled us to estimate population mean fitness (λ) and potential population dynamic trajectories (λ > or<1) in naturally unoccupied sites. Despite a large number of studies that have transplanted species beyond their range limits (synthesized in Hargreaves et al, 2014;Lee-Yaw et al, 2016), very few have actually assessed lifetime fitness with demographic models (but see Hargreaves & Eckert, 2019;Latimer, Silander, Rebelo, & Midgley, 2009;Moore, 2009). Absolute estimates of λ are ecologically meaningful and necessary to infer dispersal limitation; relative comparisons of single vital rates cannot predict whether populations would be above or below the threshold for persistence.…”