Territories 2 Flashcards
Economic defendability and territoriality
- If animals weigh up costs & benefits
– can we measure costs & benefits (trade-offs)? - use optimality theory to answer ‘when defence is worthwhile or how big a defended area should be?’ (Introduced by Jerram Brown, 1964)
- To be favoured by natural selection: benefits > costs
BUT
conditions under which this happens depend on how
costs & benefits vary with territory ownership
Economic defendability and territoriality: graph model
see diagrams in notes
Shifts in benefit (B → B1) curve alter range of viable territorysizes & alter optimal size (X → X1).
- e.g: increase in resource quality will shift benefit curve up so that range increases & optimal territory size decreases:
Shifts in cost (C → C1) curve alter range of viable territory sizes& shift optimal size (X → X1).
* e.g: increased levels of competition → increased cost →reduces optimal territory size
But, if shape of cost/benefit curves change, increased resource quality can lead to an increase in optimal size.
Optimal territory size is likely to be fluid, increasing & decreasing as cost/benefit functions vary
BUT – how to apply numbers to these graphs?
- ‘currency’ for costs & benefits?– ideally true measure of fitness.
- Typically measured indirectly:– e.g: feeding rate, number of visiting females etc.
BUT
- can be complicated
e.g: male great tits - winter territories are not critical for food or mates in winter– payoff comes following summer
Optimal territory size example:
Hawaiian honeycreepers (Vestiariacoccinea)
(Carpenter & MacMillen 1976)
- Nectar feeding bird species
- defend territories to maintain access to sufficient flowers
- calculated upper threshold (defence should begin) to be c. 207flowers
- lower threshold (defence should cease) c. 60 flowers.
- 9/10 birds agreed with prediction
Habitat quality and effects on demography
e.g. Red grouse (Mougeotet al., 2003)
- Variable quality of habitats (Ideal Free Distribution?)
- Intensive competition for good habitat may result in those without habitat – floaters –do not hold territories & unlikely to breed
E.g. Red grouse Lagopus lagopus scoticus
- 90% of food consumed by adults is the growing tips of heather. Moorland territories.
- High mortality of grouse non-territory holders even though only 2% of heather eaten. Territory holders have high testosterone and are aggressive (Mougeotet al., 2003)
Variable quality of habitats – effects of spatial distribution
e.g. in Oystercatchers Netherlands (Ens et al., 1992)
- In the Netherlands, oyster catcher (Haematopus ostralegus) have territories including saltmarsh habitat for nesting and mudflat for feeding.
- Resident (one spatial unit) territory or Leapfrog (two units, separated)
- ‘Resident’ – more young fledged; more food delivered plus young can follow adults to mudflats earlier, whereas leapfrog adults have to deliver food to ‘trapped’ chicks
(Ens et al., 1992)
Causes of asymmetry
- Several explanations
- Resident always wins
- Morphological and physiological asymmetry
- Payoff asymmetry - Why is there not a continual escalation for more aggressive, dominant individuals?e.g. Testosterone implants and supplementary feeding in mountain spiny lizards Sceloporus jarrovi (Marler & Moore,1989, 1991)
Sharing a territory
- Territories need not be defended exclusively.
- Sometimes it may pay to share a territory with another individual that at other times would be regarded as an intruder
Sharing a territory example: Pied Wagtails
pied wagtails (Motacilla alba yarrellii)
(Davies & Houston 1981)
In winter - feed along river banks.
– Some defend territories, others in nonterritorial flocks.
– insects = renewing food supply → feeding sites replenished
– when food availability high owners often allow another bird(usually a juvenile or female) from flock to share
– walk round half a circuit behind each other
Cost: return time to any feeding site for owner was halved.
Benefit: additional defence carried out by the satellite
Owners predicted to share their territory when abundance & rate of renewal of food were above a threshold curve.
field test: most territory owners conformed to prediction
Threshold payment may differ between satellite & owner.
* owner - try to get max. payment from satellite (PS)
* Satellite – try for minimum payment (Po)
Complications:– e.g: competition among potential satellites might drive up going rate for payment
Allowing satellite to remain on territory as tradeoff between feeding & defence can lead to evolution of group territoriality - One element important in social, pack hunters?
Dispersal
Dispersal definition:
spreading out from home environment to find new resources; usually radially (e.g. terrestrial) but direction may be biased by environment
e.g. river, air/water currents and by prior experience and habitat selection.
E.g. Dispersal of fledged birds from parental territory
Dispersal of voles from highly populated areas of habitat
Dispersal of insect larvae downstream in rivers by
allowing themselves to enter ‘the drift’
Why disperse? Costs and potential benefits
Costs: Substantial costs of dispersing
e.g. energy, development
- Risk of not finding suitable habitat
- Risk of not finding mates
- Increased risk of predation?
Potential benefits
- reduce competition, especially with closely related individuals
- Reduce inbreeding – hybrid vigour
- Colonisation of variable environments e.g. ephemeral
Evidence for mechanisms regulating dispersal?
Winged and flightless cricket forms – increased rate of gonadal development in wingless forms, greater potential fecundity
Manipulation of densities, food etc e.g. pupfish (McMahon &Tash, 1988) voles, tilapia – increased densities, reduced food increases ‘dispersal’ (one way movement from home habitat).
Dominance hierarchy, aggression, lack of resources –hormone responses?
When to disperse?
- Females often oust young from their territory when they have reached the stage of independence – especially common in iteroparous species e.g. tawny owl, fox
- Particular dispersal forms e.g. alate aphids, responding to food stress
- Increase in dispersal / emigration in rodents in response to food stress, crowding and aggression e.g. lemmings
