Individual variation in dispersal, and its sources, shape the fate of pushed vs. pulled range expansions

Ecological and evolutionary dynamics of range expansions are shaped by both dispersal and population growth. Accordingly, density-dependence in either dispersal or growth can determine whether expansions are pulled or pushed, i.e. whether expansion velocities and genetic diversity are mainly driven by recent, low-density edge populations, or by older populations closer to the core. Despite this and despite abundant evidence of dispersal evolution during expansions, the impact of density-dependent dispersal and its evolution on expansion dynamics remains understudied. Here, we used simulation models to examine the influence of individual trait variation in both dispersal capacity and dispersal density-dependence on expansions, and how it impacts the position of expansions on the pulled-pushed continuum. First, we found that knowing about the evolution of density-dependent dispersal at the range edge can greatly improve our ability to predict whether an expansion is (more) pushed or (more) pulled. Second, we found that both dispersal costs and the sources of variation in dispersal (genetic or non-genetic, in dispersal capacity versus in density-dependence) greatly influence how expansion dynamics evolve. Among other scenarios, pushed expansions tended to become more pulled with time only when density-dependence was highly heritable, dispersal costs were low and dispersal capacity could not evolve. When, on the other hand, variation in density-dependence had no genetic basis, but dispersal capacity could evolve, then pushed expansions tended to become more pushed with time, and pulled expansions more pulled. More generally, our results show that trying to predict expansion velocities and dynamics using trait information from non-expanding regions only may be problematic, that both dispersal variation and its sources play a key role in determining whether an expansion is and stays pushed, and that environmental context (here dispersal costs) cannot be neglected. Those simulations suggest new avenues of research to explore, both in terms of theoretical studies and regarding ways to empirically study pushed vs. pulled range expansions.

Trailing-edge zombie forests can increase population persistence in the face of climate change

Climate-driven habitat shifts pose challenges for dispersal-limited, late-maturing taxa such as trees. Older trees are often the most reproductive individuals in the population, but as habitats shift, these individuals can be left behind in the trailing range edge, generating "zombie forests" that may persist long after the suitable habitat has shifted. Are these zombie forests vestiges of ecosystems past or do they play an ecological role? To understand how zombie forests affect population persistence, we developed a spatially explicit, stage-structured model of tree populations occupying a shifting habitat. Our model shows that seed dispersal from zombie forests to the range core can considerably increase the maximum rate of climate change that a population can withstand. Moreover, the entire core population can ultimately descend from recruitment-limited zombie forests, highlighting their demographic value. Our results suggest that preserving trailing-edge zombie forests can greatly increase population persistence in the face of climate change.

Genetic variability and the ecology of geographic range: a test of the central-marginal hypothesis in Australian scincid lizards

For many species, both local abundance and regional occupancy are highest near the center of their geographic distributions. One hypothesis for this pattern is that niche suitability declines with increasing distance from a species geographic center, such that populations near range margins are characterized by reduced density and increased patchiness. In these smaller edge populations, genetic drift is more powerful, leading to the loss of genetic diversity. This simple verbal model has been formalized as the central-marginal hypothesis, which predicts that core populations should have greater genetic diversity than edge populations. However, demographic shifts over time can generate a similar pattern. For example, in species with expanding ranges, populations at the range edge experience serial founder effects, creating a gradient of declining genetic diversity from the range core to edge. Testing the central-marginal hypothesis properly thus requires us to consider the confounding role of historical demography. Here, we account for the role of history in testing the central-marginal hypothesis using a genomic dataset of 25 species-level taxa of Australian skink lizards (genus: Ctenotus and Lerista). We found support for the central-marginal hypothesis in 16 of our 25 taxa, of which eight taxa recovered significant support. Unexpectedly, species with the strongest evidence for range expansion were the least likely to follow predictions of the central-marginal hypothesis. The majority of these species had range expansions that originated at the range edge, which led to lower genetic diversity at the range edge compared to the core, contrary to the central-marginal hypothesis.

Increased rates of dispersal of free-ranging cane toads (Rhinella marina) during their global invasion

Invasions often accelerate through time, as dispersal-enhancing traits accumulate at the expanding range edge. How does the dispersal behaviour of individual organisms shift to increase rates of population spread? We collate data from 44 radio-tracking studies (in total, of 650 animals) of cane toads (Rhinella marina) to quantify distances moved per day, and the frequency of displacement in their native range (French Guiana) and two invaded areas (Hawai’i and Australia). We show that toads in their native-range, Hawai’i and eastern Australia are relatively sedentary, while toads dispersing across tropical Australia increased their daily distances travelled from 20 to 200 m per day. That increase reflects an increasing propensity to change diurnal retreat sites every day, as well as to move further during each nocturnal displacement. Daily changes in retreat site evolved earlier than did changes in distances moved per night, indicating a breakdown in philopatry before other movement behaviours were optimised to maximise dispersal.

Mapping nationally and globally at-risk species to identify hotspots for (and gaps in) conservation

Protecting habitat of species-at-risk is critical to their recovery, but can be contentious. For example, protecting species that are locally imperilled but globally common (e.g. species that only occur in a jurisdiction at the edge of their geographic range) is often thought to distract from protecting globally-imperilled species. However, such perceived trade-offs are based on the assumption that threatened groups have little spatial overlap, which is rarely quantified. Here, we compile range maps of terrestrial species-at-risk in Canada to assess the geographic overlap of nationally and globally at-risk species with each other, among taxonomic groups, and with protected areas. While many nationally-at-risk taxa only occurred in Canada at their northern range edge (median=4% of range in Canada), nationally-at-risk species were not significantly more peripheral in Canada than globally-at-risk species. Further, 56% of hotspots of nationally-at-risk taxa were also hotspots of globally-at-risk taxa in Canada, undercutting the perceived trade-off in their protection. Hotspots of nationally-at-risk taxa also strongly overlapped with hotspots of individual taxonomic groups, though less so for mammals. While strong spatial overlap across threat levels and taxa should facilitate efficient habitat protection, <7% of the area in Canada's at-risk hotspots is protected, and more than 70% of nationally and globally-at-risk species in Canada have <10% of their Canadian range protected. Our results counter the perception that protecting nationally vs. globally at-risk species are at odds, and identify critical areas to target as Canada strives to increase its protected areas and promote species-at-risk recovery.

New data on the status and ecology of a galliform at risk of extinction: the Pyrenean grey partridge (Perdix perdix hispaniensis) in the Iberian System (Soria, Spain)

A study was conducted in 2008–2010 to gain knowledge on the status and ecology of the endangered subspecies of grey partridge (Perdix perdix hispaniensis), at its southernmost range edge. From an historic breeding range of 28,300 ha, 15 different coveys (adults with juveniles) were observed in an area comprising 5,550 ha, with an estimated minimum autumn population size of 103–113 birds and a maximum of 163–181 birds. Spring pair density was estimated at 2.3 pairs/1,000 ha, and when considering only coveys, 6.8 partridges/1,000 ha. The majority of birds were located at an altitude above 1,690 m a.s.l., mainly in mountain shrubland (especially Calluna vulgaris and Erica spp.). Habitat loss was the most important threat for the species’ conservation. In conclusion, efforts should prioritize urgent habitat recovery and monitoring in order to change the fate of the species.

New data on the status and ecology of a galliform at risk of extinction: the Pyrenean grey partridge (Perdix perdix hispaniensis) in the Iberian System (Soria, Spain)

A study was conducted in 2008–2010 to gain knowledge on the status and ecology of the endangered subspecies of grey partridge (Perdix perdix hispaniensis), at its southernmost range edge. From an historic breeding range of 28,300 ha, 15 different coveys (adults with juveniles) were observed in an area comprising 5,550 ha, with an estimated minimum autumn population size of 103–113 birds and a maximum of 163–181 birds. Spring pair density was estimated at 2.3 pairs/1,000 ha, and when considering only coveys, 6.8 partridges/1,000 ha. The majority of birds were located at an altitude above 1,690 m a.s.l., mainly in mountain shrubland (especially Calluna vulgaris and Erica spp.). Habitat loss was the most important threat for the species’ conservation. In conclusion, efforts should prioritize urgent habitat recovery and monitoring in order to change the fate of the species.

Population level differences in overwintering survivorship of blue crabs (Callinectes sapidus): A caution on extrapolating climate sensitivities along latitudinal gradients

Winter mortality can strongly affect the population dynamics of blue crabs (Callinectes sapidus) near poleward range limits. We simulated winter in the lab to test the effects of temperature, salinity, and estuary of origin on blue crab winter mortality over three years using a broad range of crab sizes from both Great South Bay and Chesapeake Bay. We fit accelerated failure time models to our data and to data from prior blue crab winter mortality experiments, illustrating that, in a widely distributed, commercially valuable marine decapod, temperature, salinity, size, estuary of origin, and winter duration were important predictors of winter mortality. Furthermore, our results suggest that extrapolation of a Chesapeake Bay based survivorship model to crabs from New York estuaries yielded poor fits. As such, the severity and duration of winter can impact northern blue crab populations differently along latitudinal gradients. In the context of climate change, future warming could possibility confer a benefit to crab populations near the range edge that are currently limited by temperature-induced winter mortality by shifting their range edge poleward, but care must be taken in generalizing from models that are developed based on populations from one part of the range to populations near the edges, especially for species that occupy large geographical areas.