Choosing Where to Live Chapter 8 or What side of the fence IS the grass greener on?
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Transcript of Choosing Where to Live Chapter 8 or What side of the fence IS the grass greener on?
Choosing Habitats
Fact: animals generally actively choose habitats, patches, etc.
This is shaped by natural selection
True or False Most animals have a instinctual ability to prefer proper habitat?
True
Choice of Where to Feed, etc.
no competition for food
no depletion of food
20 J/hr 10 J/hr
Choose A
Patch A Patch B
Ideal-Free Model of Habitat/Patch Choice
Ideal – animals know everything about food
Free – free to move anywhere
Key points:1. animals will have a preferred habitat
2. But will not necessarily choose that habitat (due to effects of competition).
All organisms are identical with no fighting over resources.
Testing IFD with Fish
Can vary food addition of feedersAssume maximizing fitness is maximizing rate of energy intake.At the IFD, all animals will have roughly equal energy intake rates.
Ideal Despotic Model of Habitat Choice
It’s all about monopolizing resources
Allow individuals to monopolize sites within habitats – ie. territories.
Some individuals can defend high quality patchesNo longer “free” to go where you will.
More realistic situation
Tested with Aphids on Cottonwood popular treesThese aphids can reproduce asexually as clones or
sexually.Sexually reproducing aphids are gall formersFemales fight for position on leafBest spot is at petiole of leaf
3
2
1
Ideal Despotic Model
2nd is worse than 1st, but may be better if it is a big leaf.
With 2 habitat patches – there are not always more on the best
BA
Chosen 1st
Moreindividuals
Questions
True or False The best habitats will always be entered first.
True or False Will the best habitat always have the most individuals
Depends on the size of the habitat and competition between individuals
True
Reading Assignment pp249-263
What are the sexual differences in dispersal
Contrast mammals vs. birds
Why Voluntarily Disperse?Inbreeding avoidanceLocal competition
How far to disperse?Out-breeding depressionAvoidance of competition
What factors might cause an animal to change breeding territories from one year to the next?
What are the costs and benefits of long-distance migration?Migration in Monarch butterfliesGeneral natural history of migrationWhy do monarchs migrate all the way to Mexico?
Dispersal Migration
More discussion on these topics to come later
Territoriality
Home rangeArea of activity
TerritoryExclusive area defended from intruders
Questions to ask?Whether to be territorial?
How to be territorial?
Reproduction
Remember the fighting aphidsRemoving others from leaf can allow a greater number of offspringDon’t fight due to cost in energy, time and injury.
BirdsNo territory = no breeding for most birdsDemonstrate many non-breeding floaters
No territory
Don’t fight for territory because they are young and can try to breed next year or the next
Florida Scrub-Jays pages 117 - 124
Energy
Territories outside of breeding seasonOnly defending food (energy)
Energy = fitness
Seen in some birds and some mammals
Ex. Nectar feeding birds (simple food source)Food source easily characterized and defendable
Golden-winged Sunbirds
High quality territories are very beneficial to ownersStable food source
Energy needs met in short time
But not always territorial
Territorial sunbirds
Flower Density - territories abandoned
Cost to defend larger area is too high
Flower Density - territories abandoned
No net benefit, plenty of food
Territoriality occurs at intermediate flower densities
Intruders Rate - territories abandoned
All time is spent defending
Territory Size
Cost / Benefit Approach
Minimal Territory Size
m = metabolic cost of owner
c = energetic cost of defending territory
f = food density (or energy) (energy/m2)
Territory size (minimal) = (m+c)/f
Territory Size to Maximize Energy Gain
Graphical model of optimal territory size.
cost
Benefit (can’t use it all)
T1T*
T2
Territory Size
Cos
t or
Ben
efit
T1 = min territory sizeT2 = max territory sizeT* = max energy gain (max = B-C) slopes are equal at this point - Break even when lines cross
Example
Hummingbirds increase territory size before migration to gain weight
In many cases, the environment may be saturated with territories that persist for long terms.
Effects on Territory Size
When food density declinesMay be very little change in sizeCost of expansion is too highRemoving owners from their own territory
When food density increasesCould see small territory sizeBut, re-expansion may be difficult
Often see no change at all in size - stay as you are!!
Unchanging Size in Pied Wagtails
Non-owners feed in nearby fieldsWhen food density in fields declines, they go
to the riverOwners cannot decrease or stop being territorial
Owner lets only 1 intruder onto territorySatelliteHelps with territory defense
Stay 180° apart on opposite sidesWhen intruder density decreases – satellite is
removed.
territory
Dispersal
CostsExposure to predators
Requires energy
Benefits1)Inbreeding depression
2)Reduced mate
competition
Inbreeding Depression
Less likely for recessive genes to be expressed
Females bear more of the energetic cost (avoid relatives)
Males disperse to have more mates
Female mammals hold territory and get help from another (ground squirrels), safe breeding den.
Mate competition
Males fight – loser moves on to find another mate
Not true for ground squirrels – happens before fighting
Winning males move on to avoid sexually mature daughters
Male lions leave (or forced out) when a new male comes into area
Bird Dispersal
Males return when previous year was successful
Disperse and move toward a more successful future if unsuccessful previous year
MigrationOccurred graduallyFirst with short range migrantsFollowed by long range migrants
(continents)
Costs of Migration
EnergeticPredators – group dilution
effectCross Mediterranean at Spain
– narrowest partCross Atlantic when possible,
if decreased body fat, go over to Mexico
Black poll warbler – across Atlantic – shorter, few predators, and tail winds
Benefits of Migration
Seasonality allows for bursts of food to support young
Fishbreed in freshwater
Tropics – more food in freshwater
Move to breeding ground (saltwater)
Monarch Butterflies
Central Mexico Oyamel fir forest
Less likely to have sudden freezesMoist and cool to maintain energy
reservesTree removal exposes them to rain and
cold night sky
Increased risk of freezing, exhaustion, and desiccation
Migrants and Non-migrants in Same Species
Exactly the same fitness in the long run
The 2 Strategy HypothesisDifferences are genetic
Don’t change behavior from year to year
Equal fitness on average
Actually, individuals switch behaviors
Territoriality and Dominance
Implies site-specific dominance
Owner is dominant individual only on its territory
Owner almost always wins in a dispute
Payoff Asymmetry Hypothesis
Owner places a higher value on
territory than an intruder will
Owner is more willing to fightIntruder immediately retreats from
Confrontation Intruder is looking
for free territory
ExampleCapture territory owner male bird
Hold it for a bit in an aviary
Territory taken over by another individual (floater)Let it hold the territory for a few
days
Release original owner back into territoryBIG FIGHT
Original owner almost always wins eventually
Not the case in butterflies.
Resource Holding Power HypothesisOwner wins because they have superior ability to
hold territoryTerritory owners are the best fightersEx. Damselflies
Fat reserves (not body size) dictate the winner in contest, more fit gets the territory.
Domains
An incompletely defended territory
Defend center stronger that edge/marginal territory
CoreTerritory
Te
nd
en
cy t
o
evi
ct
intr
ud
ers
Distance from center