Why are animals where they are?

Guest post by Endre Grüner Ofstad. Norwegian version available here.

Use, selection, and home range properties: complex patterns of individual habitat utilization (2019) Endre Ofstad et al., Ecosphere, 10(4), https://doi.org/10.1002/ecs2.2695

The Crux

The areas in which we find an animal is often called its ‘habitat’. Yet it’s a fairly ambiguous term. Where animals are found is usually the outcome of a range of considerations, primarily foraging, predator avoidance and mating opportunities. Animals trade-off among these in order to maximise their contribution to future generations (i.e. ‘fitness’).

When considering which habitats we most likely find animals one often works with habitat selection. Habitat selection is how much a certain habitat type is used compared to its availability, i.e. what is the relative probability that an animal will use a given habitat upon encounter. The amount of time an individual spends (or density of individuals) in a habitat is usually a good proxy for the importance the habitat to the animals. Therefore we often use this to evaluate which areas to target for management and conservation efforts.

Some habitat types are very important and animals will cover large distances in order to access them. Consequently, the benefits of using a habitat type may drop if the area the animals need to cover on a regular basis (ie. the home range) to use it is too large. The availability of habitat types may also vary based on the environmental features of the home range – a certain habitat type might be very close by, but it might be on the other side of a mountain.

As such, habitat selection depends a great deal on a home range’s size and environmental features. Yet individuals may change their habitat selection while remaining in the same home range. This might not matter if our only concern is to predict where animals are to be found. However, it may be important if we want understand why animals are where they.

What We Did

In this paper we wanted to look at a population of moose (Alces alces, aka the King of the forest), where individuals differ in traits that influence both the home range size and the net benefits of using certain habitat types. We used a well-monitored population on the island of Vega, just off the coast of Central Norway. For each individual we estimated the summer and winter home range size. We estimated what proportion of a certain habitat type was present in each home range to calculate habitat availability, and estimated habitat use by using GPS tracking to see what proportion of time the moose spent in that habitat type. We used the ratio between use and availability (‘selection ratio’) as a measure of habitat selection. We then looked at how availability was related to home range size and sex (and age and season), and then how all these were related to habitat selection.

Did You Know: Habitat Analysis

Methods used to analyse habitat selection are the same as the methods used to analyse species distributions. Often, species distribution models incorporate data on a species’ traits, while habitat selection incorporates traits of an individual animal or plant. As such both have the same main challenges: how to link presence to absolute numbers such as population density and abundance, which are more useful metrics to management and conservation interests.

What We Found

As expected from differences in body mass we saw that males tended to have larger home ranges than females. Among males an increase in home range meant a larger change in the home range composition, often with an increase in the availability of poorer-quality forage and open (lack of forest) habitat types. Among females, there was little change in home range composition with increasing home range size.

Despite this, we found very few differences in habitat selection which were directly attributed to sex, season or age. As such males and females could at face-value from statistics appear similar in habitat selection. However, sex influenced characteristics such as home range size and habitat availability – and habitat use. Males were more prone to change habitat availability whereas females were more prone to change habitat use across with changes in home range size, with the result of seeming quite similar in habitat selection.

Problems

mooseboi

A male moose foraging birch and rowan. Although males prefer deciduous tree leaves as food, they were more inclined than females to include more alternative forage with increasing home range size. This can result from males, being larger than females, are more able to use poorer quality food (Image Credit: Endre Ofstad, CC BY-SA 2.0)

The relationships among habitat use, habitat availability and home range size are highly dynamic in both space and time. In our study we only looked at the outcome of these dynamics across the season, meaning we disregarded how these characteristics may change over time. Few studies have looked at how these characteristics may change over the course of a summer. Will the home range composition and size stabilise over time as the moose achieves the perfect home range composition? Or will it constantly be perturbed by conspecifics, and forage and predator dynamics? My money is on the latter.

So What?

A big challenge when studying habitat selection is to transfer the results from one area to another. These studies are often very site specific, making it uncertain how much our knowledge about habitat selection of moose in the Midwest tells us about the habitat selection of moose in Canada.

A big part of problem of transferability stems from not being able to account for local variation in habitat availability and individual variation in the net benefits of habitat use. Even though males and females seem to show similar habitat selection, it comes about via different pathways – where males appear to be more flexible in changing their home range composition. Males and females may therefore have different net benefits of a given level of habitat selection. Generally, this suggests that management and conservation efforts towards specific habitat types will not benefit all individuals equally – as one might assume from habitat selection alone.

 

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