Perturbation Model

The Perturbation Model predicts whether individuals will exhibit spatial or socio-spatial perturbation based on whether the population has an Ideal Free Distribution, Ideal Dominance Distribution or other type of population distribution (Figure A.1). Central to this model is the assumption that disturbance events will generally affect resource availability, either directly (e.g., fire reducing food availability) or indirectly (e.g., changes in population density subsequently adjusting resource availability). In the Perturbation Model, populations with an Ideal Free Distribution or Ideal Dominance Distribution are predicted to display perturbation following a disturbance event, due to resource levels being affected. If a population follows an Ideal Free Distribution, this perturbation is likely to be spatial only. In contrast, if populations follow an Ideal Dominance Distribution pattern, they are predicted to exhibit socio-spatial perturbation, due to not only changes in resource availability, but potentially also the re-shuffling of dominance hierarchies (e.g., due to the removal of dominant individuals). Localised culling of Ideal Free Distribution and Ideal Dominance Distribution populations will likely create a population sink that is filled as individuals reoccupy patches of reduced competition (Coulson, 2009).

Populations with other types of distributions (Rose Petal Distribution, Den Site Distribution, Fixed Home Range Distribution or Predation Risk Distribution) are not predicted to exhibit immediate perturbation following a disturbance event, as home ranges are not primarily dictated by resource availability. In Rose Petal Distribution populations, home ranges are determined by philopatry; in Den Site Distribution or Fixed Home Range Distribution populations, the cost of relocating a den site or home range/territory is too high for settled individuals; and in the Predation Risk Distribution populations, changes in predation risk primarily determine home ranges. For example, female white- tailed deer (Odocoileus virginianus) in Huntington Wildlife Forest in New York are very philopatric to their summer home range and this is postulated to be the reason why female deer do not move into new areas where family groups are removed (McNulty et al., 1997; Oyer and Porter, 2004). Culling of these four populations types will consequently result in gaps in the distribution of a population, which could persist for several generations (Coulson, 2009), or at least until juveniles disperse and re- populate these cleared areas. Populations with these types of distribution patterns are therefore considered to be the most stable to disturbance. However, these populations may still alter their home ranges if the intensity of the disturbance is great enough. For example, if a fire removes all resources from an area, it is likely that these populations will be forced to move to new areas to survive.

The intensity and permanence of the disturbance may in turn influence the intensity and permanence of the perturbation. For example, perturbation may not be long-term if the disturbance is temporary or low in intensity. Seasonality may also influence the occurrence of perturbation. If a disturbance event affects the availability of mates, for example, individuals may only change their

home ranges if the disturbance occurs during the breeding season. Moreover, territorial behaviour is generally more pronounced during the breeding season (Sutherland, 1996), therefore populations following an Ideal Dominance Distribution pattern may respond to a greater degree during this time. The occurrence and magnitude of perturbation may also vary between sexes. Females tend to compete for food and nest sites, as these resources generally limit their reproductive success (Davies et al., 2012). In contrast, mates generally limit reproductive success of males, so they will likely compete against each other for access to females, either directly or by controlling the resources that females require (Davies et al., 2012). This may result in males and females responding to disturbance in varying ways, depending on which resources are affected. Furthermore, if males and females display different distribution patterns, such as females displaying a Rose Petal Distribution and males displaying an Ideal Dominance Distribution, then according to the Perturbation Model this will result in gender-related differences in the occurrence of perturbation. Males also often have larger territories than females, and can be more aggressive and territorial (Carpenter, 1987; Whitworth and Southwick, 1984). In these cases, males may exhibit socio-spatial perturbation to a greater degree than females.

Figure A.1 Perturbation Model predicting whether a disturbance event will result in spatial perturbation, socio-spatial perturbation or no immediate perturbation, depending on the distribution pattern and territoriality of the population. IFD = Ideal Free Distribution, IDD = Ideal Dominance Distribution, RPD = Rose Petal Distribution, DSD = Den Site Distribution, FHRD = Fixed Home Range Distribution and PRD = Predation Risk Distribution.

SOCIO-SPATIAL

PERTURBATION PERTURBATION SPATIAL PERTURBATION NO

DISTURBANCE EVENT AFFECTING RESOURCE AVAILABILITY

(e.g., culling, natural disaster)

DEPENDING ON:

• Intensity of disturbance (i.e., minor or major event) • Permanence of disturbance

(i.e., short or long term) • Season

• Sex of individual

NON-IFD/IDD POPULATIONS (RPD, DSD, FHRD AND PRD POPULATIONS) IFD AND IDD POPULATIONS

PERTURBATION VARYING IN:

• Intensity

(i.e., minor or major change) • Permanence

(i.e., short or long term change) TERRITORIAL POPULATIONS (IDD POPULATIONS) NON-TERRITORIAL POPULATIONS (IFD POPULATIONS)

A.3.1

Application of the Perturbation Model to possums

Possums exhibit a dominance hierarchy (Biggins and Overstreet, 1978; Green, 1984; Jolly and Spurr, 1996; Wehi et al., 2006). Some studies have recorded males as being dominant (e.g., Biggins and Overstreet, 1978) and others, females (e.g., Jolly and Spurr, 1996; Spurr and Jolly, 1999). This social behaviour may regulate possum density, with dominant individuals preventing subordinates from accessing limiting resources, such as food and dens (Green, 1984). In support of this, male and female possums have been recorded defending their dens and feeding sources if challenged by other possums (Biggins and Overstreet, 1978; Day et al., 2000a; Hickling and Sun, unpublished data; Spurr and Jolly, 1999). In addition, although there is not strong evidence of complete territoriality in possums, it is likely that limited parts of home ranges, such as core areas, are defended (Day et al., 2000a). This suggests that both male and female possums might follow an Ideal Dominance Distribution pattern. According to the Perturbation Model, this would mean that both sexes would exhibit socio-spatial perturbation following a disturbance event.

Female possums, however, generally show natal philopatry, inheriting home ranges that overlap with their mothers; in contrast, males disperse from natal areas to establish new home ranges (Clinchy, 1999; Clout and Efford, 1984; Crawley, 1973; Ji et al., 2001; Stow et al., 2006). This may mean that female possums follow a Rose Petal Distribution pattern and males follow an Ideal Dominance Distribution pattern. Under the Perturbation Model, this would mean that females would not exhibit perturbation following a disturbance event, whereas males would exhibit socio-spatial perturbation.

Possums are not expected to follow a Den Site Distribution pattern, as they move freely between dens in the landscape. They use approximately 11 – 15 dens per year and change these every two nights in three on average (Cowan, 1989). It is also predicted that they do not follow a Fixed Home Range Distribution pattern, as individuals likely disperse and re-aggregate after density reduction (Sweetapple and Nugent, 2009). As possums in New Zealand have no known predators, they are also not expected to follow a Predation Risk Distribution pattern.