Home range

A home range is the area an individual animal regularly uses during its activities over a specified period, for example finding food, resting and reproducing. It describes space use; a territory adds the behavioural question of which area the animal defends.

Animal radiotelemetry can supply a sequence of locations from a transmitter carried by the animal. A 2011 study combined location tracking and collar activity sensors to compare owned and unowned free-roaming domestic cats. Measuring location and activity together linked the spatial pattern to behaviour rather than treating every recorded point as equivalent.^[ref]

A boundary depends on the observation period and the method used to summarize positions. In an invented example, $95$ of $100$ recorded positions fall in one region and five are distant excursions. That region contains $95\%$ of these observations; this alone does not prove that the animal spends $95\%$ of all future time there. Unequal sampling and serially related locations matter. A utilization distribution instead describes how use varies across space, connecting a range outline with the frequency of use within it.

Home-range overlap describes how the estimated space used by two animals is shared during specified observation periods. It requires an explicit measure: the common area, the fraction of each home range shared, and overlap between utilization distributions quantify different aspects of space use. The result depends on how the home ranges were estimated from locations.^[ref]

Let $A$ and $B$ be planar home range regions. Their shared region is the intersection $A\cap B$. Two directional fractions areFor an invented example with areas2 and8square kilometres and shared area1square kilometre, half of $A$ is shared but only one eighth of $B$ is shared. A symmetric alternative uses the union:Reporting only a percentage without the denominator leaves these interpretations ambiguous.

A density-based comparison can retain how intensely different places are used. For normalized probability density functions $u_A$ and $u_B$, the shared probability masslies between0 and1. It is not generally equal to the area fraction above. In the same example, if use is uniform within each region, this coefficient is $1\times \min (1/2,1/8)=1/8$.

Radio tracking can supply locations from which such regions and densities are estimated. A2011 free-roaming cat study combined radio telemetry with collar activity sensors, illustrating the distinction between where an animal ranges and its activity. Spatial overlap without matching time information does not prove simultaneous presence, contact or interaction.^[ref]

A utilization distribution is a probability distribution describing an animal's location during a specified period and sampling regime. When represented by a probability density function $u(x,y)$, it obeysThe probability assigned to a region is the integral of the density over that region. A home range boundary can summarize this pattern, but the utilization distribution also distinguishes frequently used patches from rarely visited ones.

Consider an invented animal that spends probability $0.8$ uniformly in a feeding patch of $1{\mathrm{km}}^2$ and probability $0.2$ uniformly in a disjoint resting patch of $4{\mathrm{km}}^2$. Their densities are $0.8{\mathrm{km}}^{-2}$ and $0.05{\mathrm{km}}^{-2}$, respectively. Multiplying each density by its patch area recovers $0.8+0.2=1$. The larger patch has lower total use as well as lower density: occupied area and intensity of use answer different questions.

B. J. Worton's 1989 methodological paper used kernel density estimation to estimate a utilization distribution from animal locations, comparing fixed and adaptive approaches and choices of smoothing parameter.^[ref] A contour containing $95\%$ of estimated probability is defined by an integral, not by taking $95\%$ of the maximum density. Unequal observation intervals and serial dependence between positions also matter when using animal radiotelemetry to infer space use.