1 November 20071

NRES 310: Wildlife Ecology and Management

Defn: Dispersal – movement of an individual animals makes from its place of birth to the place where it reproduces.

Migration – Movement back and forth between winter and summer range.

Local movement – movement within the home range.

Dispersion is the pattern of spatial distribution taken up by the animals of an area. May be fixed (sessile animals) or change with time under the influence of changing dispersion of resources. Over time it may be changed by dispersal, local movement or both.

Distribution of a population or species is the area occupied by that population or species. Depicted

Dispersal

Action performed by an individual. An animal disperses or remains within its maternal home range.

  1. when dispersing an animal may move to the nearest unoccupied suitable habitat
  2. or may move long distance, crossing many possibilities before setting down.

Mechanisms vary.

  1. may be pushed out of the maternal home range by a parent.
  2. May move without any prompting, other than that supplied by its genes.
  3. Some young never even meet their parents. Frogs, reptiles

2 recognized forms:

Presaturation dispersal: seen in some small mammal species where juveniles leave their natal range even when the density of the population is low.

  1. either juveniles leave voluntarily, behavior innately determined by their genes (seen in Belding’s ground squirrels.)
  1. Adults forcible exclude juveniles

Saturation dispersal: seen in large mammals. Dispersal occurs when a population reaches a threshold density determined by food limitation. So it is density dependent. Population density remains the same in the initial area. (Himilayan Thar) and wood bison

Patterns of dispersal are related to mating system!

Mammals: females are concerned with obtaining resources while males compete for mates.

  1. Promiscuous and polygynous species
  2. Males disperse becaue they are more likely to find mates
  1. Females are philopatric (remain in their birth site) because they are more likely to find food in areas that they know well.
  1. Monogamous species – both sexes disperse.

In higher vertebrates one sex is more prone to dispersal than the other.

Generally Mammals – males disperse

Birds – birds disperse

Exceptions to both.

Mammals – African wild dogs – matricharcial society – females disperse

Fishers and wolves – both sexes disperse equally.

Causes of dispersal:

  1. competition for mates
  1. avoidance of inbreeding
  1. competition for resources.

Polygynous species

  1. Females invest more in each offspring than males so reproductive success is determined by resource competition.
  1. Male reproductive success is limited by number of mates they can find so competition for mates is important.

Inbreeding avoidance is often cited on theoretical grounds. Some species inbreeding depression has been observed in captive situations, but not in wild. Has been observed in some bird species.

Dispersers tend to have lower survival than those that remain in their natal area.

  • Arctic ground squirrels survival of dispersers 25-40% and survival of philopatric individuals was 73%.
  • Survival also declines with distance of dispersal due to the increasing probability of being predated. Always are exceptions San Juaquin kit foxes dispersers had higher survival.

HOME RANGE – first defined by Burt in 1943 as that area traversed by the individual in its normal activities of food gathering, mating and caring for young.

Resting, foraging and watering areas including the travel routes to each together form the home range.

  1. Portions may be shared with other individuals (such as watering holes) of the same species.
  2. Home ranges may overlap and generally in this situation there is some social hierarchy, in which
  3. certain individuals are dominant and exert some control over resources in areas of overlap.
  4. Subdominant individuals tend to avoid contact with dominant individuals either spatially or temporally (or both).
  5. Portions may be defended such as nest or den sites (go into more detail in a moment)
  1. Generally home ranges do not have fixed boundaries.
  2. may be compact, continuous, or broken into 2 or more discontinuous parts reached by trails and runways.
  3. Irregularities in distribution of food and cover produce corresponding irregularities in home range and in frequency of animal visitation.
  4. Basically the animal does not visit every portion of its home range daily.
  5. Movements may be restricted to trails and most of its activities may be concentrated in a smaller core area used more intensively than other parts.
  1. Home range sizes also relate to body size. Large mammals have larger home ranges than smaller ones.
  2. Carnivores have larger home ranges than herbivores and omnivores of similar size.
  3. Males and adults have larger home ranges than females and subadults.
  4. Differences in body size and diet are enough to explain differences in home range size without invoking competitive interactions.
  5. Home range of herbivores and omnivores increase at a nearly constant rate as body size increases.
  6. Carnivores the home range increases at a greater rate as body size increases.
  1. Possession of a home range conveys certain advantages,
  2. animal is familiar with the local area,
  3. knowing where to find food and water, shelter, and cover from enemies with a minimum expenditure of energy.
  4. Series of escape routes to cover and travel routes to food sources throughout the year is known or defined.

TERRITORY – defined as a defended portion of the home range.

Frequently misused term when describing home range. Territories are defended areas and usually see dominance reversals across a common boundary.

Sometimes, but not always, resource based

Sometimes not, but kind of a symbolism in which males express social dominance over other males.

Home ranges may overlap, territories do not.

Territoriality is best understood in birds, more conspicuous and readily observed than mammals or other vertebrates.

Territories are proclaimed with some combination of visual displays, threats sounds and scent.

Territorial individuals tend to occur in more or less regular patterns of spatial distribution.

Breeding territories, common among songbirds, all activities from feeding to mating and rearing young, late in breeding season territoriality breaks down, less need to expend the energy to defend it. Resource based.

Mating and Nesting territory – common among hawks – feeding is done elsewhere defend area around nest site.

LEKSMating territories that are not based on resources (or defending females), basically a display ground males compete for dominance, female choice is involved., sage grouse, Uganda Kob, pronghorn,

Why defend territory ? Always increased probability of survival and improved reproductive success i.e. increased fitness!

  1. Territorial animal that claims the “best” territory, has the highest reproductive success.
  2. reproductive cost of losing is lower fecundity or don’t reproduce at all.
  3. Territorial defense has costs, especially if optimal resources are limited. Defense uses energy, consumes time, and interferes with feeding, courtship, mating and rearing of young.

Territory size:

  1. As size increases the cost of defense also increases. Minimum size, particularly if resource based that they need to obtain resources that they need.
  2. influenced by resource availability, fewer resources usually result in larger territories.

How do you estimate home range size?

I’m not going to go into huge detail about how to estimate home range, because some of those techniques become very complex. There are 3 primary methods to estimate home range size.

Minimum Convex polygon –MCP

Joins the peripheral locations of an animal to determine its home range. That area within the polygon is calculated to determine the area of the home range.

  • Simple and easy to draw
  • Not constrained by any underlying statistical distribution of the animals movements
  • Extremely sensitive to outliers
  • Provides only an outline of the home range
  • Assumes animal uses all parts of the home range with equal intensity
  • Very sensitive to sample size.

Kernal Estimators –

First need to convert the definition of home range to a statistical definition.

Utilization Distribution – density function which describes the probability of an animal being in any part of its home range. In other words it’s a probabilistic model that describes the relative amount of time that an animal spends in any particular place.

Within this framework a home range is defined as the smallest sub-region which accounts for a specific portion of its total utilization. Usually reported as a specific percentage (go into more detail in a minute).

Kernal methods –

  • Advantageous for several reasons
  • Highlight areas of concentrated activity by an animal
  • Nonparametric (no assumption of any underlying distribution of animal movements) Some of the older estimators that are no longer used (bivariate normal, harmonic mean) did assume an underlying normal distribution.
  • Provide information on concentrated activity within a home range in addition to providing the extent of the range (where MCP only gave you the perimeter, gives you some information about how the animal uses the home range).

Take points of animal locations put a kernel over it. Then the more kernels in a given area get hills and valleys, use a smoothing parameter to smooth the surface.

Fixed kernel – smoothing parameter is fixed so it’s the same for all points (animal locations) in the distribution.

Adaptive kernel – smoothing parameter varies in value so that areas with fewer locations or lower concentrations of points are smoothed more.

In other words points with close neighbors have greater weight in estimating the utilization distribution.

Common to see home range analysis using subsets of total dataset to portray the utilization distribution.

60%, 70%, 80%, and 95%

  • The definition of home range specifically omits occasional exploratory movements outside the area of normal activities of the animal (why most use 95%).
  • Using 100% of home range would include those movements and likely includes areas that the animal doesn’t use again.
  • Small percentages may cause home range to break up into fragments.
  • Depending upon hypothesis that you are testing and specific information that is needed, using lower proportions may be most useful.

Some uses for smaller UDs

  • Identify areas that are most frequently used.
  • Reveals information about internal structure and may identify areas of greatest importance
  • Important for planning management strategies.