Dispersal is usually understood as the process (and movement) during which the individual leaves their natal home (birth place) and finds a new home.
Dispersal frequently occurs once the individual has reached independence (had learned to provide for themselves, to defend themselves and to acquire other resources, e.g., dens etc.).
Sometimes individuals disperse before reaching the average age of independence in the species.
This often happens due to food scarcity, parent mortality or other pressures.
Independence is not always equal to sexual maturity and individuals may be weaned and they may disperse before they are on the lookout for a mate.
Commonly, nevertheless, dispersal is associated with sexual maturity – the individual leaves to establish their own home and to find a mate and to raise offspring.
The association between dispersal and sexual maturity is related to the high energy demands during the period of raising the young.
If these demands cannot be met for several breeding individuals in the area, only the dominant individual breeds and the rest have to abide as subordinate non-breeders or they have to leave to have a chance at their own reproductive effort.
The individual. however, may not necessarily disperse right after having reached independence and/or sexual maturity, either, and the individual may stay in its natal range for as long as they are tolerated by the dominating animal(s) who might be the individual’s parent(s).
Such strategies are more characteristic of social species where subadult and adult offspring become helpers or breed collectively with the parent(s).
Dispersal upon independence is common in solitary species.
Not all dispersal cases adhere to the aforementioned scenario.
Dispersal can occur in adult individuals who have already held the breeder’s status outside of their natal range but who, for some reason, need to leave their home once again.
This might happen, for example, during breeder displacement (a new breeding individual outcompetes the former breeder and the former breeder is not integrated in the social group dominated by the new breeder) or after loss of mate/territory (e.g., due to mortality, territorial conflict, natural disaster etc.).
In such cases, the adult individual might disperse once more – later in life.
While dispersal involves the act of migration (immigration, emigration, migratory movement) and, for example, migratory corridors (paths connecting available habitats for settlement) are referred to in dispersal context, in the scientific literature, migration is sometimes contrasted with dispersal where dispersal has a more specific meaning than migration (the meaning being that of permanently leaving the old home and finding a new home).
Migration can be understood as a less irreversible act of changing locations.
It does not necessarily involve resettling in a new home (finding a mate, breeding, producing offspring) on a lasting basis.
A popular example – think of the migratory bird species that spend summer breeding (nesting) season in one region and undertake autumnal migrations in order to overwinter in another (warmer, seasonally more productive) region only to return in the following spring to the former summer breeding grounds (frequently to the very same nesting site/area).
However, some migration and dispersal aspects might not be as strictly separable.
For example, some Eurasian crane pairs can be territorial on their winter grounds.
Meanwhile, many bat species (e.g., common noctules) copulate during the migration or in the wintering range but the reproductive roosting occurs after their return to the summer breeding region.
It is important to remember that dispersal is also observed in plant species and fungi whereupon propagules (seeds, spores, fruit etc.) leave the parent plant and establish elsewhere as seedlings or become stored within the soil seed bank.
In plants and fungi, vectors of dispersal ensure the movement of propagules.
Vectors of dispersal are the living or the non-living agents that aid the dispersal of the propagules.
Abiotic (non-living agents) are, for example, wind and water.
Biotic (living) agents are birds, small mammals, large herbivores etc.
In plants, dispersal sometimes occurs in two stages where the seed is first dispersed by one vector reaching its first destination.
There the seed is relocated by the second vector to its second destination.
This can be repeated several times.
For example, some seeds can be initially dispersed by wind and once they drop on the soil, they are gathered by small rodents that bury them in their food caches whence the seed can sprout if it survives (if the rodent forgets or otherwise fails to consume it).
Nuts buried by squirrels or larger seed-eating birds can be found, dug up again and relocated by a secondary herbivore.
In this article, mostly animal dispersal will be discussed.
The following aspects of dispersal will be reviewed:
- How is dispersal studied?
- When to disperse?
- Populations and meta-populations;
- The significance of dispersal.
How is dispersal studied?
Dispersal can be researched considering many factors but it is common to analyze the following parameters:
- Dispersal age (juvenile, subadult, adult);
- Dispersal distance (in some species individuals or certain age / sex groups might not disperse at all while in other species dispersal can be short (moving out of natal home but not much further) or it can cover distances as great as 1000 km crossing difficult terrain or even open seas);
- Dispersal dimorphism (as mentioned before, in many species either females or males disperse or one sex disperses further / more frequently than the other);
- Dispersal causes and onset factors (discussed below);
- Dispersal potential (some species, e.g., flying birds, wolves etc. are much more mobile than others, e.g., non-flying insects or strictly burrowing mammals);
- Dispersal corridors and dispersal connectivity (dispersal is vital in ensuring population connectivity therefore the ease of dispersal is studied to determine if the animal can effectively reach new ranges – e.g., in species that only move along tree lines and that are unable to cross large open fields; or in species that travel in water environment and that are impeded by dams, HES etc.).
Other aspects of dispersal are studied, as well.
For example, species’ phylogenetic evolution is strongly linked to dispersal because the ability or inability to disperse in space determines the limits of population and subpopulation distribution.
Genetically distinct populations are formed where dispersal is bound by, e.g., mountain ranges, seas, intensely urbanized areas that pose high mortality risk etc.
Individuals cannot leave this ‘gene pool’ and, with local adaptations, they eventually form a population that is different from other populations in the same species.
Sometimes this gives rise to subspecies and sometimes even a new species can diverge from the former ancestry.
If dispersal limitations are too great and the available range is not overly vast, the population can suffer such effects as inbreeding risk, genetic drift (loss of beneficial genetic variation) etc.
Dispersal is crucial in population conservation.
It ensures persistence, resilience and genetic diversity.
When to disperse?
The onset of dispersal is dependent on several factors and some of them will be discussed below although, of course, the causality is not limited to these considerations.
Resource availability
One of such factors is resource availability (how many animals the natal range can sustain – especially, once the parent(s) has (have) mated again and the energetic needs during gestation, lactation and the raising of the young become higher than during the non-reproductive season).
Another determinant is social organization.
Some species are more social than others and form larger groups into which the individual can be recruited, that is to say, accepted as a functioning, participating adult.
Sometimes these individuals who stay in their natal groups, get to breed, as well, but sometimes they simply help their parent(s) raise the new generations.
In the latter case, the individual still has to disperse if he or she wants to breed (unless one of the parents or dominant breeders die and open up the vacancy on the natal range).
Other species form small groups with no room for older offspring or they lead a solitary lifestyle.
Social organization is usually related to life strategy, for example, how the species forage, defend themselves, raise their young etc.
Red deer females raise their young in herds because the habitat can usually sustain several mother-calf units and the collective grazing/browsing can be beneficial in lowering the risk of predation.
All adults can watch out for predators increasing the amount of time an individual can dedicate to foraging.
Also, upon an incident of predation, the risk of having a specific individual captures is lower in a larger group (risk diffusion).
Meanwhile, pine marten mothers take care of their kits alone (without the help of the father or older siblings / other relations), and they have to hunt very frequently in order to provide for their offspring.
Usually, pine marten’s home range can only sustain one female (with kits).
She cannot travel too far because she needs to return to the den in order to nurse/feed her kits and the kits are too small to keep up with their mum on her travels.
It would be difficult for the pine marten female to share the already limited resources with the children of the previous years and if the older siblings lingered around, their resource acquisition might negatively impact the survival of the current year’s kits.
Therefore, (after the winter period when it would have been difficult to establish a new range and to explore its resources during the harsh conditions) in spring pine marten subadults disperse from their mother’s home.
Population density
Dispersal also depends on the population density.
Interestingly, dispersal is higher at low population density (not many individuals and/or groups per area) and at high population density (many individuals and/or groups per area).
Low population density is typical to newly colonized areas when there is still plenty of habitat to choose from and the individual / group range is not tucked tightly between neighbouring ranges.
(Low population density can also result from heavy mortality and great turnover (replacement of individuals or groups by new individuals or groups), e.g., due to hunting by humans.)
Under such conditions, it is easy to disperse because the individual does not necessarily have to travel far, nor they are subject to intense competition and territory/resource defense.
Many dispersers die as they trespass on territories established by adults.
While crossing unfamiliar and frequently less sheltered landscape, dispersers are also subject to predation risk by birds or prey or mammalian predators.
Starvation can be an issue on ‘strange lands’.
Similarly, during dispersal, individuals often have to cross roads and other human infrastructure features that pose their own threat.
In small rodent species, dispersal can be one of the critical times that result in the highest mortality rates.
At low population densities, at least some of these threats are reduced.
At low population densities, individuals can also be pickier selecting the best sites (optimal habitat) while saturated populations offer little choice and the newly settled areas might be suboptimal (not as good) compared to other habitats that have already been taken during the initial phases of colonization.
On the other hand, at low population densities (recently colonized areas) it might be difficult to find mates that are not related to the dispersing individual.
As the population density increases, the availability of habitats drops and risk factors rise (competition, territorial conflicts etc.) because the individual has to compete with other residents and, during their travels, they have to cross established territories (or court potential mates that already have other suitors).
This is one of the reasons why in many species, prior to making the final decision, individuals engage in the so-called pre-dispersal forays.
Forays are excursions outside of natal range in order to explore and to determine the population density and the resource availability outside of the natal range.
The individual does not leave permanently but she or he travels outside of parent(s)’ range to inspect such signs as scent-marks (occupancy or mating opportunities) and resource distribution (available territories that have suitable food resources, den sites etc.).
If the individual deems the fortunes yet unfavourable, they might return and spend another year in their parent(s)’ / group’s range before making another attempt.
Such strategy is common in, e.g., beavers and wolves.
Sometimes it is difficult to strictly distinguish between dispersal and foray.
For example, an in-depth wolf population report from Minnesota (Fritts, S.H. & Mech, L.D., 1981) tells the story of yearling female wolf 5173 who left her natal range in autumn, in year 1976.
Autumn is a typical time of dispersal in subordinate wolves (non-breeding older siblings) due to the high energy demands of pups who have reached an almost-adult size.
The female travelled at least 138 km (straight-line distance) but she failed to settle as she had come across poor quality habitat (farmland).
5173 did not abandon her intent, however, and kept roaming until she found herself in an area surrounded by extensive wildfire.
The authors of the report assume it was the traumatic experience of the natural disaster that prompted 5173’s return to her natal group in October of the same year.
However, she presumably undertook dispersal venture once more in December, 1976 (her further fates are not known).
It is not easy to establish whether her first dispersal attempt was a foray or an actual dispersal.
Forays are normally shorter with respect to distance and, initially, the female did not appear set on returning (it is unlikely that the female herself perceived her journey as an exploratory excursion) but rather she was forced to return by ill fortunes.
Sometimes wolves can return to their natal or former packs even after months or years of absence.
Wolves, badgers and other species can also spend considerable time associating with more than one social group during the same period in more than one range, and such cases are not unambiguous to define (which group and which range do they belong to?).
It is customary to view dispersal as a one-time event while, in these cases, the individual can ‘disperse’ between groups several times during a relatively short period.
It is not a foray, either, because the individual appears to establish themselves as a part of the social group and the feeding ecology of both ranges.
The female wolf’s long distance dispersal, coincidentally, serves as a good example of dispersal behaviour under higher population densities (the Minnesota population had been recently granted legal protection which resulted in a population increase).
At high densities, the tension between groups and individuals might be too severe (and territory size might but, thereby, resource availability might become reduced) causing individuals to disperse at a younger age or under circumstances that would not encourage dispersal if the population densities had been low.
As mentioned before, high population densities can alter dispersal behaviour – the individual might have to travel far in order to find unoccupied habitats and available resources.
At low population densities, an individual can settle right outside of their natal range.
At high population densities (unless a vacancy opens in some group or in some range), the individual might even have to traverse the entire population range before they discover any available sites that are not defended by residents.
On the other hand, high population densities can ensure greater mating prospects.
At moderate population densities, individuals appear to weigh out their options because they are not pressured by food limitations and the dispersal poses a high risk enough risk not to be undertaken lightly.
Mating opportunities become greater closer to the natal range as the population is dense enough to provide other, unrelated individuals of the same age.
In many species, at moderate population densities, subadults do not disperse as readily.
In the Minnesota wolf population (wolf 5173’s home area), the densities were not yet as high as to entirely limit contractions or expansions of the ranges of specific wolf packs.
As a result, it was quite common for young wolves to solicit mates from nearby packs and to establish their new territories on the margins of their parental home ranges or in ranges vacated due to unexpected circumstance (pack disruption, death of breeders etc.).
Far-ranging dispersal (as in the case of female 5173) was not as common as soliciting mates from neighbouring packs and taking over the margins of natal ranges.
In such populations, young wolves can also quickly replace lost breeders in the nearby packs.
Character and phenotype
Scientific studies have shown that character (individual’s personality) can be significant in predicting the individual’s dispersal behaviour.
Character is formed by:
- genetic traits (e.g., phenotype – see below);
- the psychological state of the local population (stressful vs. relaxed environment);
- personal experience (including traumatic or encouraging experience);
- parental / group experience (knowledge and skills passed on by parent(s) or group);
- environmental factors (e.g., animals that are forced to live close to humans can acquire boldness which later can impact dispersal decisions on human-altered landscape);
- other factors.
Character may also change over time and during different stages in the life history (e.g., juveniles vs. adults).
For example, boldness is sometimes measured as willingness to approach human settlements and human infrastructure (roads), including making use of them (roads as travel corridors facilitating speed; bridges as means to cross rivers etc.).
Barry, T. et al., 2020 demonstrated that wolves (a naturally timid species that prefers to establish home far from human influence) acquired greater boldness during dispersal when (due to habitat fragmentation) they likely had no other choice but leave forests and other wilderness areas and to cross roads, to tolerate human presence etc.
Wolf use of habitat after dispersal demonstrated greater tolerance toward human presence (sometimes termed ‘boldness’) than wolf behaviour before dispersal (while back in their natal range) even though wolves still settled as far from urban areas as possible.
Thus, the character trait was altered through (inescapable) experience.
The authors indicated that the boldness in post-dispersal wolves could possibly become reversed because dispersing individuals who have recently established their own territory might be more ‘reckless’ than individuals who have become parents and pack leaders.
With the new responsibilities, wolves might, once again, revert to less bold behaviour.
Wolves are an excellent species to discuss dispersal patterns because their dispersal is highly varied.
While some individuals do not disperse at all, there are also wolves who travel extraordinary distances crossing four-lane high-ways and mountain ranges of extremely rugged terrain (such as the Alps).
Not all of these individuals have been coerced into making these decisions through the lack of mates, lack of food, high population density and other contributing factors.
Some individuals simply appear to have a heart for adventure.
For example, a wolf dispersing from Switzerland chose to swim across the river Danube which can be up to 1.5 km wide.
There was no true need for the wolf to make this plunge because the wolf could have altered his course to avoid the river or he could have attempted to find a bridge or other type of easier pass (which wolves sometimes use).
It appears that the wolf wished to challenge himself as he faced the river and as he resolved, ‘I can conquer it – I want to test my might.’
This wolf altogether was tracked for 1900 km (1181 miles) during his dispersal which was concluded in Hungary.
Dispersal is, after all, a time of a certain level of ‘irresponsibility’ (one only has to take care of oneself and nobody other with no other concerned adults monitoring one’s decisions) before the adult life stage when the individual will be responsible for offspring, mate and social group.
The ability by wolves to disperse over such great distances enables them to colonize and recolonize new ranges at a very quick pace.
The genetic potential by a species to disperse determines the history if its distribution.
For example, scientists have attempted to reconstruct the geological changes on our planet during different climatic periods (such as glacials and interglacials) by quantifying the annual dispersal potential by a species (including plant species) and then evaluating how many years it could have taken for the species to have arrived in a certain place.
This is how scientists can also detect alternative routes of (re)colonization – when a fossil record has been found that has been dated to a period when the species could not yet possibly have reached the area from the southern interglacial refugias.
It means that there must have been nearer refugias for these species to have survived the glacials and travelled to the spot where their remains have been found.
Caution must be exerted, however, because there are genetic phenotypes affecting dispersal potential by different individuals of the same species.
For example, roe deer are thought to be highly philopatric only dispersing very short distances (in many populations the average dispersal is as low as 1 – 1.5 km) and sometimes not dispersing at all (if they can stay on their natal range).
However, scientists in Scandinavia (Wahlström, L.K. & Liberg, O., 1995) studied the distribution of roe deer on the peninsula.
Roe deer were almost extirpated in Scandinavia in the early 18th century and by 1830 the population had shrunk to approximately 100 individuals in the southernmost part of Sweden.
After introduction of strict hunting laws, the population began recovering – recolonizing its former range.
In 150 years the roe deer population re-expanded over 1300 km with an average recolonization speed of 9 km per year.
However, the annual dispersal distance of the southern population in Sweden (which never went extinct) averaged 4 km per year with only two individuals dispersing further than 15 km.
At 4 km per year, the roe deer population could not have recolonized the 1300 km range that it apparently did during 150 years.
Meanwhile, the population in the more northern region (close to the current distribution edge of the newly recolonized population) demonstrated far greater dispersal distances averaging 120 km (with only one disperser settling closer than 39 km from its natal range).
Such dispersal distances that appear ‘abnormal’ to an average roe deer in most regions, could easily have ensured the observed recolonization rate (9 km per year).
The number of individuals demonstrating the great dispersal distances in the northern population is too large to imply mere inter-individual differences.
Namely, those were not just a few individuals who dispersed far in the northern population but the majority of the individuals in the northern population dispersed over greater distances than could be considered average for roe deer (including in the southern population).
The contrast between dispersal rates in the long-established (southern) and the newly recolonized, ‘edge’ (northern) populations (4 km vs. 120 km per year) gives rise to assumptions that two genetic phenotypes might exist.
One roe deer phenotype might be sedentary and more characteristic to stable, old populations that do not expand much over years.
The other roe deer phenotype might be responsible for population expansion and recolonization of the historic range because they disperse much further and occupy vacant habitats where sedentary individuals can later settle and multiply while the ‘exploratory phenotype’ keeps pushing the range limits.
For this reason, the population on the edge of the newly achieved distribution range consists of a greater number of far-dispersing individuals because the edge habitats are first reached by the respective phenotypes.
As the edge population settles and stabilizes, it is possible that the average dispersal distance also decreases to more typical values.
Dispersal phenotypes (genetically predisposed dispersers who travel far vs. who prefer to stay put) are thought to exist in other species, as well.
Populations and meta-populations
Dispersal ensures the establishment of populations and subpopulations (colonization, recolonization, expansion etc.) as well as the maintenance of the established populations and subpopulations and the connection between them.
Sometimes meta-populations rather than populations are formed and meta-populations are even more dependent on successful dispersal than populations.
Populations are different from meta-populations in that they are contiguous and, mostly, they can also sustain themselves (even with limited immigration).
Meanwhile, meta-populations consist of several subpopulations that are scattered and inter-isolated to a greater or lesser degree (usually, isolation is observed in space although temporal fragmentation can also occur).
For example, rivers where beavers live usually constitute populations and not meta-populations because the entire river stretch is occupied by beaver families.
Where one family’s territory ends, another family’s territory begins.
The population is demarcated by the river basin and perhaps connected to another population which lives on another river if one of the rivers flows into the other and if there are no barriers to dispersal and settlement (e.g., dams, HES and other artificial obstructions).
Meanwhile, water voles, another riparian species, tend to form subpopulations.
They inhabit available and suitable habitats that provide vegetation for food and cover.
Such habitats are not always found evenly along the riverside.
For example, water voles will settle where the riparian vegetation is denser (where their favourite grasses can be found) and where it is possible to create underground burrows.
Water voles will avoid areas where the vegetation is very scarce or absent.
Often riverbanks are not uniform in their vegetation cover – more open stretches are exchanged by more densely overgrown stretches etc.
Thus, a patchy settlement by water voles is observed and some river stretches have water voles while they are absent from others.
The inhabited areas are too small to sustain an entire population (i.e., the water voles living there need immigration and emigration in order to persist, otherwise, they would go extinct).
The water vole-settled patches form subpopulations that are interconnected through the process of dispersal.
These subpopulations rely on dispersal as well as on the existence of other subpopulations nearby enough and, therefore, these sub-populationscan be regarded as a meta-population where dispersal is crucial to maintain any single subpopulation.
Interestingly, subpopulations in water voles also fluctuate in time.
Some burrows are abandoned and some are recolonized resulting in a dynamic picture of where water voles live and where they do not live during any given year.
This temporal-settlement dynamic is achieved through mortality (water voles like many fossorial rodents exhibit a cyclic density pattern with peak density years followed by decrease and population crash after which the population grows again to reach another peak density).
It is also achieved through dispersal.
One subpopulation may disperse to live elsewhere (e.g., due to depletion of food resources) while another moves into a previously abandoned patch (once the vegetation has regenerated).
Dispersal maintains the meta-population in space and in time.
Meta-populations are especially characteristic of species that have very particular habitat requirements (habitat specialists) – if these habitats are not overly available throughout the landscape.
Many butterfly species form meta-populations because butterfly reproduction is reliant on host plants.
Butterfly larvae feed on host plants that can sometimes be as particular as a single species or a single genus.
If these plants are distributed sparsely (only in bogs, only in deciduous forests, only in wet meadows etc.), the butterfly meta-population will also be fragmented and subpopulations will be found in optimal habitats with the host plant availability.
If the habitat is large enough (which is rarely the case), it might support a population (no immigrants are needed to ensure reproduction and genetic diversity).
However, usually the habitats are small and interconnectivity is vital in order to establish a meta-population through the process of dispersal among subpopulations.
The term ‘meta-population’ is not always used in the ‘threatened and fragmented’ context.
‘Meta-populations’ can also be comprised of large, healthy, sustainable and self-sufficient populations that are connected through dispersal.
For example, geographic objects such as mountain ranges or lakes can separate populations.
Similarly, large and sustainable populations can be separated by areas that are not suitable for occupancy of the species.
These populations can persist on their own but sometimes dispersal occurs between them ensuring gene exchange.
From this perspective, there can be even continental meta-populations if all the populations of a species on the continent allow for connectivity through dispersal.
The significance of dispersal
While dispersal often constitutes the appearingly ‘simple choice’ of leaving one’s family and making an independent living, it is, in truth, the wondrous mechanism through which species have colonized and recolonized their distribution ranges and through which species have acquired their unique traits and adaptations persisting through the geological history.
On a global level, this is how the Earth is inhabited and how its dynamic harmony is attained and maintained.
It is why our planet looks and functions the way it does.
On a personal level, this is how homes are made, how partners meet one another, how families are created.
Dispersal entitles an independent exploration and an adventurous discovery as well as individual aspirations to lead a good life in a place where one’s children can be safe and healthy.
Dispersal is determined both by the character of the individual and many external factors.
It is, ultimately, what connects our inner needs and desires to the vast world and its rules of play.
It is also how individuals can bear our impact on these rules of play.
References
Barry, T. et al., Does dispersal make the heart grow bolder? Avoidance of anthropogenic habitat elements across wolf life history, Animal Behaviour, Volume 166, 2020, Pages 219-231, ISSN 0003-3472, https://doi.org/10.1016/j.anbehav.2020.06.015
Fritts, S.H. & Mech, L.D. Dynamics, movements and feeding ecology of a newly protected wolf population in northwestern Minnesota. Wildlife Monographs (ISSN: 0084-0173), A Publication of The Wildlife Society, OCTOBER 1981, No. 80, https://digitalcommons.unl.edu/usgsnpwrc/379/
Wahlström, L.K., & Liberg, O. (1995). Contrasting dispersal patterns in two Scandinavian roe deer Capreolus capreolus populations. Wildlife Biology, 1, 159 – 164. https://www.semanticscholar.org/paper/Contrasting-dispersal-patterns-in-two-Scandinavian-Wahlstr%C3%B6m-Liberg/98736d892f1bc62ef4ceb76f83ff583fa256dd39
Notes by Ieva 