I would like to introduce you to a few technicalities that are often applied and discussed in wildlife research when scientists try to determine why some animals live in some places and not in other places, what habitats would suit them best, how they use their resources and, accordingly, how to protect those sites or habitat features that are the most important ones to a species.
In order to achieve this, scientists look at where the species chooses to live and where it chooses to carry out its daily activities.
These choices are called habitat selectivity.
This lengthy article (which you can read in sections) looks at:
- What resources an animal is looking for in its environment;
- How selectivity is determined (scientific methods) in order to avoid confusion related to some resources being far more abundant (or safe) than others;
- How habitat selection is different when analyzed on different scales (e.g., planet, region, home range etc.);
- Why habitat selectivity is important in nature conservation.
What is selectivity?
Looking for resources
Any individual needs to select a place for living which would ensure the provision of essential resources.
For an animal, these fundamental resources would ordinarily be comprised of:
- Food;
- Shelter;
- Breeding opportunities.
Food resources are both the actual foraging items and their accessibility as well as safe use.
It is not enough for the food to simply be there.
The individual also needs to be able to acquire the food resources in a safe manner (e.g., avoiding predation risk in deer species or avoiding human disturbance in large predator species) and in a manner physically compatible with the organism (e.g., foraging in canopies vs. foraging over open water in different bat species).
Food acquisition is weighed out against energy expenditure (only the most nutritionally valuable food items are consumed if they demand a high energy input) and survival (animals will avoid using certain resources if the use can result in death, for example, through predation).
Foraging cannot be achieved if the habitat cannot be travelled by the individual – some species need tree lines (bats) or waterways (otters) or sufficient tall and dense vegetation cover (water voles) to move from one foraging place (or resting site) to another.
There are species that have evolved to adapt their habitats to their needs.
For example, beavers build dams, ponds and canals to ensure safer access to food resources and many animals, e.g., moles, engage in underground burrowing activities thus creating their own travel passages, shelters and even food storage ‘cellars’.
However, such species then need riparian tree resources and suitable soil conditions for building and burrowing, respectively.
Shelter comes in form of resting sites, natal dens, dens for raising the young and as far as species that undergo hibernation or torpor (winter inactivity) are concerned, also – wintering dens.
Natal dens are shelters (nests, burrows, cavities, caves etc.) where the young are born.
Sometimes the animal babies grow up in the natal den until they begin moving around on their own or with their mother (parents), or until they reach independence.
Many species, however, move their young while the babies are still incapable of following the parents because staying in one place for a long time can result in accumulation of smells (predator attractants) or accumulation of parasites (e.g., fleas).
This means that for many species – several breeding den sites are needed in the same home range.
Also, parents might choose a new den site if the previous year was proven unsuccessful regarding the den selection or if the old den has become damaged (flood, cave-ins etc.).
Often as the young are developing, new needs appear.
The natal dens are frequently highly concealed and provide excellent thermoregulation.
As the young are growing up, they learn to hide and to flee and they are no longer as dependent on external temperatures.
They rather begin to explore and to play and to try gathering some food on their own.
When this happens, natal dens are exchanged for more spacious and physically/cognitively challenging sites to ensure the current developmental needs of the young animals.
Breeding dens constitute one aspect of breeding resources.
Upon choosing a place where to settle down, the animal will also be looking for mates (an existing local population of the same species) and in some species – the animal will be looking for mating sites (e.g., some bat species have mating roosts).
These are some of the examples of the elements that an individual will be considering upon selecting its place of living (home range).
Selectivity vs. availability
In science, it is very important to distinguish selectivity from random choice.
What does that mean?
It means that it is important to find out what the animal especially likes and needs, namely, what the individual (group, population) is selecting for (rather than simply using because it is the most abundant resource available).
For example, if you have 30 hazelnuts and 3 blackberries and you eat 15 hazelnuts and 3 blackberries, does it necessarily follow that you like hazelnuts more than you like blackberries?
You have eaten more hazelnuts than you have eaten blackberries but if we consider the abundance of these resources for you, you ate only half of the hazelnuts while you ate all of your blackberries.
This might imply that you actually enjoy blackberries more than you enjoy hazelnuts but the blackberries are a scarce resource for you.
Furthermore, if you had the hazelnuts in your pocket already but you had to walk over to pluck 2 of the 3 the blackberries, this adds specific weight to your selectivity because you had to pay an effort in order to acquire the blackberries while the hazelnuts were readily there (degree of availability).
You had 30 hazelnuts available and you did not ate all of them; you did not use the hazelnuts in accordance with their availability.
Instead you used them at a rate which was lower than their availability – you selected against them.
If you had consumed all 30 of them, you would have used them according to their availability.
Meanwhile, you had 1 blackberry readily available but you demonstrated the effort to acquire 2 more.
You used the resource more than expected from its availability, i.e., you strongly selected for the blackberries.
In order to understand these choices, in scientific studies, resource or habitat use (how often an individual, group or population uses a resource of habitat) is is divided by the resource or habitat availability (how many of these resources or habitats are there within the area of interest).
In order to determine these rates, the animals are tracked (e.g., with GPS collars) to see where they prefer to live, forage, rest etc.
Then the availability (abundance) of these resources or sites is estimated (per a specific area, for example, per individual’s home range).
Later the two values are compared to see if the animal, for example, has spent more / less time in some habitat than it would be expected from the availability of this habitat or if the habitat has been used according to its availability.
For example, if there is only one vegetarian restaurant in your town and there are 20 other types of restaurants but you frequent the vegetarian restaurant every day choosing to ignore the others, you select for this restaurant among all town restaurants.
These estimates are very important in understanding the needs and the preferences by a species.
Many animals nowadays are not truly free to choose what they like and what is necessary for them.
Instead they are often forced to accept what has been left in the environment after the Great Destructivity by the Human.
If we look simply at where the animal chooses to settle, where it hunts, where it raises its young, we might make erroneous assumptions that these choices reflect the animal’s preferences or needs.
Sometimes the animal might prefer or need something else entirely but the animal just gets by with that is available (also taking into account that the resources are limited and that there is competition).
For example, I should believe most bears would prefer to hibernate in dens on remote forest-covered mountain slopes without human disturbance.
Yet some bears den under some folk’s winter cottage deck because, where they live, it might be the only available option for them.
Meanwhile, a wolf might choose to stay close to the forested lands despite high deer abundances near human settlements.
Surely, the wolf would like to hunt in areas where there are more deer on the landscape but this is not possible if the wolf wishes to avoid human infrastructure and human disturbance.
If suboptimal (not ideal) resources are the most plentiful ones, it can sometimes happen that these resources are used the most often although they are not the best resources, nor they are truly preferred by the animal.
European roe deer do not particularly like grains because their organism is not well suited to digesting cereals.
But if they live in a habitat where there are only vast crop-fields with perhaps tiny patches of woodland, of course, they will be foraging on the cereals, as well, to survive.
In some cases, the enforced consumption rate of the cereals might be high enough to mask the actual incompatibility of the food resource with the roe deer’s physiology.
If we only considered the rates of consumption and not the availability of the resources, we might assume that roe deer are quite readily grazers (grassy plant eaters) rather than browsers (woody plant eaters).
This is why selectivity is measured with respects to actual use divided by availability.
It is determined to what extent an animal exploits (uses) some habitat or resource and then it is determined how available that habitat or resource is to that animal (e.g. percentage of land cover).
As explained before, if the animal selects the habitat or resource more intensely than it is available (for example, if a badger has just this one nice, earthworm-rich deciduous woodland on its home range and it strikes for it whenever possible, avoiding the vastly available conifers which have lower earthworm abundances), the animal has demonstrated positive selection (the badger selects for deciduous woodland).
If the animal avoids some habitat or resource despite its availability (for example, if a hedgehog does not forage on pastures or fields because there is nowhere to rest securely nearby, i.e., there are no hedgerows, shrubs, nor grassy verges etc.), it is considered negative selection (the hedgehog selects against pastures or fields even if such habitats perhaps extend over 50% of the accessible land).
Negative selectivity can also sometimes go undetected if both use and availability have not been taken into account because the very abundant resources might be used relatively often and perhaps even more often than preferred resources that are very scarce.
The high use of some resources vs. the low use of other resources might confuse us and, without cracking some numbers, we might be unable to see that the highly used resources are actually avoided while the low-use resources are being sought for.
However, quite often, if the resources are too scarce (even though highly valuable), the animals abandon trying to find them or to reach them and they are no longer selected for.
If their abundance, once again, exceeds a certain threshold, they are selected for as before.
For example, if you really enjoy hanging out in libraries but two libraries in your town are permanently closed for summer and the only open library is very far from your home, you might stop hanging out in libraries during summer and you might renew your activity when all libraries are open in autumn.
Only when we compare the use with availability, we can see that the individual actually avoids or prefers using some resource, or uses it to its availability.
Habitat selectivity and nature conservation
How does this relate to environmental issues (e.g., conservation) and habitat selectivity?
We should determine which habitats the animal actually prefers and needs (rather than which are simply used because the animal has little other choice) and we should safeguard or restore those types of habitats or elements of habitats.
First- to fourth-order selection
Habitat preferences are usually researched on four scopes which are called ‘orders’ and they go from ‘big’ (vast – e.g., continent) to ‘small’ (a specific patch within a home range).
First-order selection refers to the entire range of the species.
First-order selection is determined by processes and qualities of great magnitude, for example, by climate, latitude, altitude etc.
First-order selection entitles the so-called ‘species distribution range’.
For example, wolverine needs deep enough snow for denning (natal dens) and for caching food which is why wolverine species distribution range (first-order habitat selection) has everything to do with winter and spring snow accumulation.
This is why, with climate change (loss of snow cover) and persecution by humans (which have led to local extinctions), nowadays, the European wolverine populations are only found in Fennoscandinavia (Finland, Norway, Sweden) – the most remote and northerly regions of Europe.
Wolverine is thereby a circumpolar species – a species living close to one of the Earth’s poles (in this case, the North pole) and inhabiting arctic tundra, boreal forests and also alpine habitats because elevations higher above sea level tend to be colder and accumulate more snow.
While we tend to think of distribution on a horizontal level, altitude (vertical level) can place some limitations on first-order selection.
For example, just like tree species have a distribution range reaching only as far toward the poles (beyond which forests are replaced by tundra vegetation), trees can also be found up to only a certain altitude (timberline).
Deciduous tree species (trees with leaves that fall in autumn) tend to grow in lower altitudes than conifers and, similarly, conifers also comprise most of boreal forests while deciduous trees tend to be more temperate / tropical.
Altitude and latitude determine many climatic conditions (e.g., temperature, snow accumulation).
Which is why, for example, the mountain hare (a ‘cold-loving’ species) will be found further north and in higher altitudes compared to the brown (European) hare.
Latitude and altitude also interact (e.g., species might be able to survive in higher altitudes closer to equator rather than closer to the poles).
However, there are differences between altitude and latitude because, e.g., the amount of daylight during the seasons decreases with latitude but increases with altitude.
Now what about the second-order selection?
Second-order selection determines where the individual animal or its social group will choose to live within their geographic range.
First-order selection demonstrated the species’ geographic range of distribution while second-order selection already pinpoints choices within this range.
Second-order selection advises us on where the animal (or a group of animals) picks its home (home range) within the vaster landscape (e.g., a continent, a region, a country, a district, a mountain range, a river basin etc.).
Imagine that you are one of the first wolves to recolonize Ireland 🙂. (Currently, there are no wolves in Ireland.)
When it comes to the climate (latitude, altitude), the entire country is pretty much available to you.
But where will you settle?
(Granted that, for example, you will find other wolves there, as well, because an important consideration for any wolf in choosing a home range is to find potential mates and associates which is why it can be difficult to be the first ones colonizing islands or other isolated areas.)
Usually, when choosing a home range, wolves would be looking for a vaster area of forest – further away from human settlements and agricultural lands.
In the general landscape, the wolf would prefer a ‘wilder’ home (as opposed to landscapes altered by humans) with plenty of trees to provide cover (e.g., national parks or larger forested ranges) while avoiding urban areas and extensive agricultural open fields.
In Ireland, where much of the forest cover has been lost, it might be difficult for the wolf to pick out the optimal home range within the larger landscape and the wolf might have to settle for forest patches that are interspersed with pastures.
Due to low tolerance for human presence and persecution, the wolf might also opt for more mountainous regions that tend to have sparser human and road densities (although wolves are not known to prefer very high altitudes and rough terrains).
Some species are limited to very specific habitats and these species are often called – habitat specialists (they use a narrow range of habitat types or only one type of habitat).
For example, pygmy owls really like old growth spruce forests.
Within the bigger landscape, the pygmy owl would perhaps pass by younger conifer groves as well as mixed or deciduous forests opting to settle in ancient spruce woodlands.
This preference also results in a potentially fragmented pygmy owl metapopulation because ancient spruce woodlands are rare and they are not connected with one another (spatially scattered and segregated).
On the other hand, generalist (non-specialist) species (e.g., roe deer) form a more contingent, continuous population which stretches over different types of forests, riparian meadows, agricultural fields etc. because they can use a vaster array of resources.
Meanwhile, pygmy owls and other habitat specialists can only be found in habitat ‘pockets’ and between these ‘islands’ – there would be wide gaps with no pygmy owls at all.
For such species, it would be extremely important to maintain connectivity between their smaller, fragmented populations that are, hopefully, interconnected through migration as the individuals are leaving their natal range and seeking a new home.
Second-order (level) of selection is related to rather particular sets of considerations.
If you compare it with your own life, this is the level where you try to figure out ‘what kind of lifestyle you enjoy’ and not just ‘where you can survive at all’.
For example, you might decide that, as a human who can live on most of the terrestrial systems of this planet, you are, more specifically, a city person or a countryside person or that without a lake, you will languish.
At this level, you essentially go for a still rather general but also somewhat personalized category, e.g., ‘city’, ‘village’, ‘in the middle of the woods’, ‘lakeside’, ‘seaside’ etc.
Third-order selectivity shows how you organize your life within the home range that you have chosen.
Let us assume you are the aforementioned wolf and you have found a vast forest in Ireland which the Irish people have restored specifically for you.
Now, the curious thing is that the selectivity on second- and third-order level can differ between themselves.
Namely, what an individual has selected for on the second-order level, might not always be selected-for (as intensely) on the third-order level.
For example, while you, as a wolf, would go for forests within the larger landscape, when it comes to the actual use of various parts of the home range that you have chosen, you might not necessarily at all times keep to the densest groves.
For example, deer species prefer browsing in forest glades, along forest edges and forest roads or on fields adjacent to forests (such transitional zones are called ecotones).
Ecotones offer the best browse (young trees, shrubs, brambles, ivy etc.) and lush herbaceous and grassy species because various types of forest edges allow for more sunlight to reach the vegetation.
Meanwhile, proximity to the forest and the dense shrubbery provides decent cover for the deer to avoid predators.
Therefore, as a wolf (who selected for forests while choosing your home range) searching for prey within your home range, you might actually demonstrate selectivity not for ‘the middle of the forest’ but for ‘forest edge’.
‘Edge’ might also be a place where it is easier to capture prey because the snow depth outside the forest can be greater giving advantage to the lighter wolf (compared to the bigger, heavier red deer).
In the forest – under the tree canopies that intercept part of the precipitation- snow tends to be more shallow.
Also, outside of forest, there could be ditches that further aggravate the act of fleeing by the prey.
Additionally, while, as a wolf, you would be opting to settle farther from human settlements and highways – within your home range you might actually use dirt roads or smaller country / forest roads because they can ease your travel and you can run faster covering greater distances.
Sometimes wolves might be forced to select for roads within their home range if the prey species are not abundant and thus, the wolves have to travel further in order to encounter more prey.
Thus, in some regions (where roads do not pose excessive mortality risk due to traffic or access by hunters/poachers), wolves might select against proximity to roads when choosing a home within the landscape but they might select for roads when choosing how to use different areas within their home range.
However, while in thick old-growth forests hunting is not always optimal, woodlands are necessary for safely raising pups, resting and altogether finding refuge from weather, human disturbance and other threats.
These discrepancies can be confusing for scientists because animals can select for some attributes on some level and select against those very same attributes on another level.
Context is essential and without estimating selectivity on different scopes, false assumptions about the species might be made.
Selectivity within the home range is more subtle and indicative of the needs of the animal during its different activities as well as annual and/or life stages.
For example, a fox will not attempt burrowing right by the riverside because there is a greater risk of the den becoming too wet or flooded.
But the riverside is where the fox might come down to hunt because much of its preferred prey can be found there.
Thus, the fox might select for the riverside when foraging is concerned but select against it when the choice of the natal den is the issue.
Different activities call for different resources.
So do different seasonal or life stages (growing up, breeding, hibernating etc.).
Finally, fourth-order selection is the ultimate ‘zoom in’ on the animal’s preferences.
It looks at very specific sites and even microsites that the organism is using.
To compare, the third-order selection might suggest whether you do your shopping in supermarkets, farmer’s markets or online.
The fourth-order selection already shows which particular supermarkets, farmers or online shops you make your purchases at and it can inform the scientists studying you that perhaps you, personally, would be severely affected if a particular shop was closed (e.g., just a few decades ago there were extremely few options for people with gluten-intolerance and the closing of the sometimes only place to access gluten-free resources would have seriously compromised individual’s diet).
There can be sites of vital importance when it comes to a species’ thriving.
Those could be salmon catching rapids where bears gather to feed during salmon runs.
Of all the segments of all the rivers within the bear’s home range, there may be few and specific (one, two, three etc.) spots that are crucial for the bear and that the entire local bear population might depend on.
Alternatively, such fourth-order selection could be directed toward a veteran tree which has several available nesting cavities.
If there are 100 younger trees in the forest and 1 veteran tree, the veteran tree might be the only one hosting certain species or it might be the individual tree hosting a disproportionate number of species (e.g., 100 vs. 40) and thereby it will be the most essential habitat feature to consider with respect to conservation efforts.
It could also be a wintering yard of deer, a beaver pond etc.
It is usually a very specific place, object or a very specific plant community etc.
Well, there – now you are entirely educated on what habitat selection is and how habitats are selected in accordance with the four selection orders.
Taking good and well-advised care of a species
In order to understand a species and to develop appropriate conservation measures, it would be recommendable to determine habitat selectivity in all four orders of selectivity.
However, usually, it is financially or practically impossible (it is not easy, nor it is cheap to track individuals to the point of precision that allows to study their microhabitat selectivity but in some difficult-to-access or politically unstable regions it is even difficult to collect data that could inform us on the first-order selection of the species’ range).
Yet it is most useful to gather information on all of these aspects.
First-order selection tells us where the species can live and what factors limit its distribution (for some species it is temperature or vegetation cover, or precipitation, or closeness to the sea or other aquatic features etc.).
The species’ distribution range is not as fixed as we might assume.
As the climate changes, as glaciers melt around the poles but also on mountaintops and as sea levels fluctuate, the respective species also expand or retract their ranges and in many places species can colonize entirely new areas or they can go extinct in their historical areas of occupancy.
Second-order selection demonstrates what type of predominant habitats are needed to be maintained or even added / expanded in order to ensure the species’ prolonged presence or (re)colonization.
Second-order selection also frequently permits us to evaluate human impact on a species (how tolerant a species is of human disturbance / presence).
Third-order selection already allows to study the different types of resources needed for the species’ varied activities throughout seasons and throughout different life stages.
It indicates which are the most important areas within the individual’s or group’s home range and how they are related to various crucial needs.
For example, foraging sites might be dissimilar to resting sites or natal den sites, and all of these resources must be made available and some might call for special protection or management.
Additionally, third-order selection can help us determine which specific activities of a species are most impacted by anthropogenic influence.
Perhaps the species can forage quite easily despite human proximity but its denning sites are subject to threat by human disturbance.
Fourth-order selection is invaluable to conserve and to appreciate specific landmarks (areas, objects, communities etc.) that are most meaningful for a species.
Usually, such landmarks tend to be important for an array of wildlife and they can serve as a biodiversity hub.
As mentioned before, it is rarely possible to study all of the selection orders for all of the species of concern and interest.
Which is why scientists attempt to identify keystone, umbrella and flagstone species.
I hope to discuss these terms in another article but, briefly, those are species that:
– indicate the general state of an ecosystem – whether it is improving, remaining stable or deteriorating (e.g., certain butterfly species that inform us on the state of a grassland);
– provide for the needs for many other species (e.g. beavers that construct wetlands for birds, insects, amphibians etc.);
– represent many other species – by protecting these representative species, we protect other species that depend on them or we ensure optimal habitat conditions for many other species that are indirectly related to them (e.g., wolves that bear impact on their prey species and on plant communities but also, indirectly, on a miriad of other species like plants, songbirds, scavengers, beavers, amphibians, even fish etc.).
When we cannot dedicate enough time and funding to study all of the species of the world with the scrutiny they deserve, sometimes these representative species are prioritized and researched on all orders of habitat selectivity – in hopes that by doing so, we aim to protect entire habitats or a greater multitude of species.
Notes by Ieva 