Tuesday 18th September - Habitats and Territories

Question 1

Which three ways do animals find suitable habitat?

Answer 1

Regular passive dispersal – high risk of loss, so viable only in small species with huge productivity (rstrategists) – ballooning spiders, insects, plankton.

Accidental – many birds and bats carried to NZ from Australia by wind, e.g. during storms. – colonisation depends on finding mates on arriving (or travelling in groups), and an empty niche.

Active choice – orientation along gradients of light, scent, moisture, salinity, current – presence of critical resources; e.g. food, nesting sites

Question 2

How do animals select habitat?

The Ideal Free Distribution model (IFD)

• Animals act in an “ideal” manner – choose habitats that maximize survival and reproduction AND are free to enter any habitat.

Assumptions of the IFD:

1. Individuals try to maximize fitness when settling in a habitat.
2. Habitat locations differ in their resources.

‘3. Fitness of individuals decreases with increasing density due to increased competition – fitness is negatively density-dependent.

4. Individuals have equal competitive ability & can accurately assess fitness payoffs of habitats.
5. Individuals are free to move between habitats at no cost.

Question 3

How do animals select habitat?

Question 4

Model prediction: derived from density ~ fitness relationship for each habitat

How do northern pike select habitat?

Answer 4

Question 5

Conspecific attraction

IFD predicts a decline in fitness with increasing individual density in a habitat due to competition…

However, fitness may sometimes increase with density due to:

• Avoidance of Allee effects
• Attraction to others to help decide on high-quality habitat

– Conspecific queuing

Question 6

Consequences of habitat choice

Habitat choice is something carried out by individuals.

This raises two important questions:

• Is habitat choice heritable?
• What are the consequences of habitat choice for species?

Question 7

Habitat choice in deer mice Very widespread species, occupies both woods and fields in N. America

• Forest clearance has reduced woodland cover

Do mice actively choose which habitat to live in?

Answer 7

Habitat choice in deer mice

Wecker (1964) constructed experimental pen 100 ft long, with 5 compartments each of woodland & grassland habitats connected by runways with recorders.

• He tested 132 mice from different populations, varied by early experience

Conclusions

1. Preferences depended on both heredity and learning (preference inherited, but reinforced by own experience)
2. Rearing in other habitats doesn’t over-rule preference until after many generations, and never removes capacity to re-learn it

Question 8

Consequences for speciation

Habitat preferences create local breeding groups

– eventually leads to full speciation

– Peromyscus has >50 species & many subspecies

• Two subspecies of P. maniculatus have different habitat preferences

– prairie subspecies extended into woodland subspecies habitat following forest clearing

• They don’t interbreed in overlap zone (though reproductively compatible)

– Gene pools mostly isolated by habitat, but not reproductive incompatibility

– Isolation enhances differences; possibly derived from different habitats (e.g., resource phenology)

Question 9

What are home ranges?

• Areas of repeated use (habitat) which are undefended

– Not all used equally

– Home range includes overlaps with neighbours

Question 10

What are territories?

Area of exclusive use, which is actively defended

– Core area defended even if the rest of home range is not

– Definition by active defense easier to document than by exclusive use

• Size of territory depends on the costs and benefits of excluding others

– Vigilance takes time & energy that may be better spent on e.g., foraging or breeding; resource must be defensible

• X & X’ denote optimum territory area for a given habitat quality.

Question 11

Economics of territory defense

Golden-winged sunbirds decide whether to defend territory based on nectar load of flowers in undefended territory

– Defense costs energy, BUT sunbirds can greatly reduce time spent foraging by monopolizing higher-reward flowers.

• If nectar in territory is high, relative to undefended habitat, then it is worth defending

– can save energy by resting.

– This must outweigh the extra cost of defense.

– If not, sunbirds abandon their territory.

• Gill & Wolf (1975) calculated threshold for territoriality based on caloric net gains

Question 12

Limits to territory defense behaviour

Lower threshold

(does extra food exceed costs of defence?)

– For example, territory defense for golden sunbird is worth cost if it increases nectar supply from 2 to 3 µl/flower (Gill & Wolf 1975)

Upper threshold

1. If too many intruders, raises cost of defense

• Larger territory attracts more intruders, so best to keep it at smallest economic size
• Sunbird territory area varied by 300x but all included ~1600 flowers

2. If resource is too abundant, owner can’t use additional benefits

• Rate of intake limited by handling time
• if nectar increases from 4 to 6 µl/flower, territory defense only minimally decreases foraging time – not enough to pay for defense
• Therefore, at very high levels of nectar (in the landscape) territories are simply abandoned

Question 13

Limits to territory defense behaviour

Territoriality is also dependent on the body condition of the defender.

• The rubyspot damselfly exhibits strong territoriality, but only in males with high fat reserves.
• When forced to interact with intruders, ‘experimental’ males suffered significant reductions in fat reserves.

This demonstrates a clear cost to territoriality!

Question 14

Habitat quality and residential status in migrant colonies

First arrivals choose best habitat & defend small territories until full (at point A)

• Later arrivals excluded & move to poorer habitat, defending larger territories until full (at point B)
• Last arrivals cant settle – they keep on moving & are temporary or permanent non-residents
• If territory in best habitat falls vacant, poor-habitat residents or vagrants move in.

Question 15

Interspecific territoriality

Rare because:

• Principle of competitive exclusion: Sympatric species ignore each other in same habitats by using different resources/times

– Niche partitioning

– E.g., Zebras eat 100% grass, small antelopes 60% herbs Observed only where a needed new resource accelerates competition

– zebras can exclude other game from waterholes

Question 16

Interspecific variation in territorial behaviour

Example: Field voles

• 10 species: territorial behaviour variable among species and sexes.
• Food is a key resource for females, which are a key resource for males.
• Abundance/distribution/renewal rate of food determines spacing pattern of females, which determines spacing pattern of males
• – If diet = seeds + fruits (patchy, slow renewal), then females are evenly spaced and territorial – but males are wide-ranging and non-territorial.*
• – If diet = grass (widespread, renewed rapidly), then females are clumped – then males are territorial.*

Question 17

Effect of territory quality on family formation in Acorn woodpeckers

Yearlings stay home more often when territories scarce, to form multigenerational group Emlen 1997:232

• Most of the yearlings that choose to stay home come from high/mid quality territories.
• Demonstrates cooperative territoriality within families.

Answer 17

Question 18

Summary: what drives territoriality?

• Food

– Abundance: worth defending only at intermediate levels.

• too scarce = required home range too large, cant repay costs.
• too abundant = too many intruders, not enough additional benefit.

– Distribution: defensible if patchy, not if too widespread.

– Renewability: defensible if rapidly renewed & predictable.

• Mates – Think back to lectures on sexual selection and mating systems: territories will depend on distribution of females in space and time.

Can drive high variation in male territoriality!