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Dispersal evolution in response to global changes
As we previously mentioned, dispersal costs vary with the environment an organism is experiencing and on the organism itself. Thus, in a context of global changes, we expect that these costs will be modified, as well as the benefits (Bonte et al. 2012).
Increase in dispersal costs
Mankind is facing a major destruction and modification of natural habitats on the global scale. Because of anthropogenic activities and climate change, many species will lose their natural habitat, and thus, the associated resources they need, including food and shelter (IPCC 2014, Ripple et al. 2019). This loss or degradation of resources should have effects on all types of costs, either directly or indirectly (Bonte et al. 2012) (Fig. 8). First, lack of sufficient resources may have profound effects on body condition and available energy. For example, in common lizards (Lacerta vivipara), humidity and temperature play an important role on the abundance of food resources (Massot et al. 2002) and thus, drier environments due to global changes may lead dispersers to cross drier non-optimal environments, where they might incur supplementary costs (Massot et al. 2008). Because dispersal is a costly process, requiring individuals to be in good body condition, we expect that more individuals will choose philopatry over dispersal. This may have important consequences at the population level, because connectivity and gene flow may be reduced. Even if individuals still undertake a dispersal attempt, they may suffer more risks including higher exposure to predators or parasites, compared to if they were in good condition (Murray 2002, Krist et al. 2003, Tucker et al. 2016). Thus, the energy cost of dispersal for individuals in poor body condition may translate into an increased risk cost. A widespread loss of habitat also means that dispersers will have less settlement opportunities. Thus, they will have to search for a place to settle potentially for a long time, and may not be able to find an optimal habitat, leading to both time and opportunity costs (Stamps et al. 2005). Moreover, these two types of costs may in turn influence body condition and fitness, and thus, may lead to deferred energy and risk costs.
Fig. 8: Possible effects of the two main global changes studied in this PhD, climate change and anthropogenic activities, on the four types of dispersal costs. Anthropogenic activities may be responsible for increased fragmentation or habitat degradation and loss. Fragmentation should lead to more unsuitable habitats (in terms of resources, predators, lack of refuges), which may lead to increased risk, energy and time costs. Habitat loss and degradation should lead to decreased resource availability, which may impact energy, time and opportunity costs. On the other hand, climate change should have an impact on all aforementioned landscape changes (habitat loss, degradation and fragmentation), and consequently, on dispersal costs. It may also lead to phenological shifts in species, which may lead to mismatches between species phonologies, and potential opportunity costs.
Second, global changes are also responsible for a general loss of landscape connectivity (Pringle 2001, Crooks et al. 2011, Senior et al. 2019), which may, for dispersers, translate into increased mortality. Indeed, as suitable habitats are more and more fragmented, individuals will have to cross more inhospitable landscapes to be able to disperse and might incur more mortality risks through increased predation or road-kills (Aars et al. 1999, Bonte et al. 2003, Gardner and Gustafon 2004, Kramer-Schadt et al. 2004, Glista et al. 2007). Moreover, habitat boundary crossing increases mortality because of higher mortality in the matrix compared to within the habitat (Martin and Fahrig 2015). This higher mortality might increase the level of isolation between habitats. If individuals have to travel longer distances because habitats are not sufficiently connected anymore, they might incur either mortality or reproductive costs (Johnson et al. 2009, Coulon et al. 2010). Moreover, the energetic costs associated with dispersal increase when fragmentation increases, for example as a function of the number of roads crossed (Benoît et al. 2019). Lastly, if landscapes become more fragmented, individuals may have to move for a longer period of time before finding a suitable area where to settle, and thus, they may also incur time and energy costs (Benoît et al. 2019).
Thirdly, climate change may lead to phenological shifts in some species, but not is others (Parmesan 2006). Yet, species are included in a community in which they interact with each other through predation, mutualism, competition, etc. If a species moves from its natural range to track climate change, but the species it depends on does not disperse along with it, or not using the same dispersal decisions, this species may lack previous positive interactions it had in its previous range (i.e. incur opportunity costs) (Berg et al. 2010). For example, insects may disperse farther compared to their plant hosts, which may cause temporal or spatial mismatches and threaten their maintenance in their new habitat (Schweiger et al. 2008, Pelini et al. 2009, Berg et al. 2010, Schweiger et al. 2012). Mismatches may especially be important in the case of specialist species, which require specific conditions to settle and might not be able to adapt in their new range, in comparison to generalist species (Travis 2003, Berg et al. 2010).
Additionally, global changes may lead to shifts in dispersal tactics. For example, high temperature is usually associated with short-distance dispersal (i.e. rappelling) in spiders of the genus Erigone, while low temperature is rather associated with long-distance dispersal (i.e. ballooning). Until recently, spiders usually chose ballooning in spring, as temperatures are still rather low at this period, which allowed them to reach high-quality but far habitats for the summer period. However, with climate change, temperatures are rising much earlier in the year, and as a consequence, individuals may prefer rappelling, which may not allow them reaching habitats with the highest quality. Thus, there may be a mismatch between the strategy chose for dispersing and the quality of resources found in the settlement area (Bonte et al. 2008a) and these spiders may incur more opportunity costs with ongoing climate change.
How will dispersal evolve?
Because dispersal evolution is driven by the balance between dispersal costs and benefits and because global changes should exert novel selection pressures on dispersal (Travis et al. 2013, Clobert et al. 2001, Dytham 2009), we expect dispersal to evolve differently in the context of global changes, and that in some cases, dispersal may not be an optimal behaviour anymore (Le Galliard et al. 2012). As a consequence, if dispersal costs outweight the “costs” not to disperse (e.g. inbreeding, competition, etc.), we expect that selection should favour individuals that do not disperse. This should first happen through plasticity in dispersal behaviour, and then, through selection on some dispersal phenotypes.
Modifications in behaviours
First, plasticity may be an immediate response to modifications in dispersal costs, and individuals may express varying dispersal behaviours at each of the three dispersal phases. Several studies have already shown that increased dispersal costs and global changes may lead to reduced emigration propensity (Schtickzelle et al. 2006, Massot et al. 2008). Possible causes include increased mortality pressures (Schtickzelle et al. 2006). Individual response may also differ depending on its location within the population (at the core, leading or trailing margins). For exemple, dispersal rates can differ between leading and trailing range margins, which may imply differential connectivities and a global reduction in species ranges, with more global extinctions at the trailing range (Anderson et al. 2009, Philips et al. 2010). Drier environments predicted by climatologists may also lead to a decrease in floods in places, which may reduce dispersal propensity in riverine species adapted to frequent floods, such as the ground beetle (Bembidion atrocaeruleum), in addition to habitat fragmentation (Bates et al. 2006). Decline in dispersal events is a strong concern considering its importance for population dynamics, and might lead to evolutionary traps1 or suicides2, and potentially extinctions (Parvinen 2005, Massot et al. 2008, Robertson and Chalfoun 2016). Prior to dispersal, future dispersers may also modify their exploratory behaviour, as has been demonstrated in great tits (Parus major): when faced with higher mortality risks, they displayed a more risk-prone behaviour compared to individuals with a lower mortality risk, because it may provide them with potential future mates or resources (Nicolaus et al. 2012). Individuals may also respond plastically to environmental changes by modifying their dispersal trajectories. For example, habitat fragmentation can modify the distances travelled by dispersing individuals or the straightness of their trajectories (Schtickzelle et al. 2006, Van Houtan et al. 2007, Coulon et al. 2010). Lanscape structural modifications by man, such as large open habitats (for example fields), may also constrain the movement directionality (Selonen and Hanski 2004). Similarly, an increase in dessication risks for amphibian species due to climate change may also impact the directionality of dispersal movements, as a way to reach suitable habitats faster (Cosentino et al. 2011). Finally, increasing costs may also reduce selectivity of individuals, i.e. the period during which dispersers are only accepting to settle in a high-quality habitat, so that they settle fast after emigration (Stamps et al. 2005).
Evolution
Then, these plastic changes may be selected for depending on the environmental context, and thus can lead to changes in dispersal at the evolutionary level. We can expect that, in the aforementioned examples, some phenotypes leading to a reduction of dispersal propensity or distance may be selected for when fragmentation is high (Schtickzelle et al. 2006, Martin and Fahrig 2015). In other cases, dispersal might still occur, but the process itself may be modified, with the apparition of novel phenotypes or even dispersal syndromes (Cote et al. 2017). Dispersal evolution is highly dependent on landscape and phenotypic factors, such as patch size and initial dispersal strategy and species properties (Gros et al. 2006, Kokko and López-Sepulcre 2006), information cues (Bocedi et al. 2012), individual preferences (Bestion et al. 2015), and also interactions between factors, such as combination of landscape and climate changes (Travis 2003, Delattre et al. 2013). These modifications may lead to changes in dispersal strategies. For example, in many systems, more long-distance dispersal events have been shown to evolve in a context of global changes (Hanski et al. 2004, Trakhtenbrot et al. 2005, Moller et al. 2006, Kuparinen et al. 2009). Indeed increasing distance increases dispersal success, because dispersing small distances in a fragmented landscape may lead to settlement in the unsuitable matrix (Mathias et al. 2001). However, as we previously mentioned, increasing dispersal distance may lead to fitness costs (Moller et al. 2006, Coulon et al. 2010). Moreover, although the frequency of long-distance dispersal might increase, in general, the absolute dispersal distance will decrease (Trakhtenbrot et al. 2005). It is important to note that for some species, the effect of fragmentation will directly have a negative effect on dispersal distance: in the northern flying squirrels (Glaucomys sabrinus griseifrons), dispersal distance decreases with increased habitat fragmentation, linked with a decrease in the straightness of trajectories, which will also be detrimental for fitness, as inbreeding or competition may arise (Trapp et al. 2019). Thus, in the long term, we can expect selection to act against dispersal (Travis and Dytham 2012). Some species may develop specific alternative dispersal strategies. For example, the dwarf spider Erigone atra exhibits two types of dispersal behaviours, rappelling and ballooning (Bonte et al. 2008a). Both allowed individuals to disperse, but rappelling leads to shorter-distance dispersal compared to ballooning. Bonte et al. (2008a) have shown in a context of warming temperatures, rappelling is preferred over ballooning. There are two implications for this behavioural change with temperature. First, population connectivity and gene flow may be reduced, as spiders remain closer to their natal range when rappelling. Moreover, while ballooning allowed individuals to reach crop habitats distributed far from the natal ranges, rappelling may not allow enough individuals to reach crops, and thus, spiders will not benefit from these rich environments. Thus, if, because of climate change, dispersal evolves towards more rappelling events in this species, which will not be an optimal dispersal strategy for Erigone atra, we can expect more deleterious effects on population dynamics and genetics (Bonte et al. 2008a).
Table of contents :
Introduction
I- Era of global changes
1) Habitat destruction
2) Climate change
3) Overexploitation of species, pollution and species invasions
II- How species may cope with global changes
1) Species adapt…
2) Dispersal, another way to cope
III- Costs of dispersal
1) Four types of dispersal costs
2) Interactions between costs
3) Context and phenotype-dependence of costs
IV- Dispersal evolution in response to global changes
1) Increase in dispersal costs
2) How will dispersal evolve?
3) Aims of the PhD
Model species
Chapter I: Climate change increases survival costs of dispersal in roe deer
Chapter II: Is climate change increasing dispersal costs? A case study on roe deer reproduction and growth
Chapter III: Beyond dispersal versus philopatry? Alternative behavioural tactics of juvenile roe deer in a heterogeneous landscape
Chapter IV: Dispersing in risky environments: effects of increased costs of dispersal on the evolution of transfer traits
General discussion
I- Global changes increase dispersal costs
1) New insights on the impacts of dispersal in a changing world
2) Additive or interactive effect?
3) Impacts of dispersal costs on population dynamics: a matter of timing
4) Expected effects of global changes on dispersal costs
5) Sex-biased dispersal
6) Is dispersal still playing its role?
II – Alternative dispersal tactics: do they suffer the same costs?
1) Why dispersal tactics matter?
2) Two main implications of alternative tactics in a changing world
II- Dispersal evolution with increasing dispersal costs
1) Consequences on population dynamics
2) What about dispersal syndromes?
3) Interplay of numerous ecological factors and selection pressures
