Population Level Variation in Dispersion Patterns

As noted above, the behavioral life styles of animals determine their distribution. While some animals, such as the wildebeest (Connochaetes taurinus), live in groups that migrate over large areas or restrict their dispersal to extended home ranges, many others are territorial, dividing the available habitat into separate areas which are defended against the intrusion of conspecifics.

In addition, different members of a population may exhibit widely different patterns of dispersion, resulting in highly complex overall spatial patterns. Territoriality, for instance, causes all reproducing individuals to adhere to a regular dispersion pattern - but for nonreproducing individuals the situation may be very different.

An example is provided by the tawny owl (Strix aluco) which is a long-lived, monogamous, territorial bird (Figure 5). Juveniles only get access to optimal resources when adults die and thereby leave vacant territories. Adults defend their territories fiercely against intruders, though they are more willing to accept juveniles within the territory boundaries during July and August, when the juveniles disperse from their natal territories.

This difference between the lifestyles of adults and juveniles means that only few individuals survive to reach adulthood. Adult birds within their territories have a relatively high survival; telemetry studies show that a main cause of death within the fragmented landscapes of Western Europe is traffic. The juveniles, on the other hand, are vulnerable to a range of factors. Since they have no territory, they are forced to move around and hunt in unknown areas and marginal habitats where survival chances are poorer (Figure 6). They are also more vulnerable to predators. Because only individuals possessing a territory can breed, the density of breeding animals

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Figure 5 Habitat map showing observations of juvenile individuals of tawny owl (Strix aluco) in a mosiac landscape of agricultural land, forest, and urban areas. The minimum convex polygon and the Kernel denote different methods of establishing the core area used by an individual or a group of individuals, and provide information on the habitat preferences, home range, and territory boundaries. Copyright: Peter Christiansen and Peter Sunde, unpublished data.

500 m

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Night locations

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Day locations

Deciduous forest

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Coniferous forest

Build-up areas

Open land

95% Minimum convex polygon

95% Kernel

Figure 5 Habitat map showing observations of juvenile individuals of tawny owl (Strix aluco) in a mosiac landscape of agricultural land, forest, and urban areas. The minimum convex polygon and the Kernel denote different methods of establishing the core area used by an individual or a group of individuals, and provide information on the habitat preferences, home range, and territory boundaries. Copyright: Peter Christiansen and Peter Sunde, unpublished data.

Figure 6 Juvenile tawny owls (Strix aluco) live a precarious life as vagrants while they are waiting to take over a vacant territory when an adult individual dies. Copyright: Peter Christiansen.

is relatively constant, even though the production of young varies significantly between years.

Heterogeneity in the spatial distribution of individuals is not limited to territorial species. Cods (Gadus morhua) have size-specific habitat selection - the smallest individuals stay in areas with dense vegetation. Here they are protected against predators, while the plants and their associated fauna provide an ample supply of food. When the cods grow larger, they move out into deeper waters to hunt.

Another example of variation in spatial distribution patterns is differences between the two sexes of the same species. The interests and the behavior of males and females can be widely different, and this is reflected in the distribution of individuals over space. In the African bush elephant (Loxodonta africana), for instance, the females are highly aggregated into small family groups spaced over the landscape, while the males wander alone over large areas, yielding a dispersion pattern which is probably best described by a random dispersion.

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