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Chapter 8: Choosing where to live
 Habitat
Selection: Choosing an
appropriate place to breed or overwinter is
an essential decision for most organisms.
 Although
many organisms can succeed in
a variety of habitats, there is usually a
preferred habitat where the organism does
best.
Habitat Choice
 For
example, Great Tits nest in both mixed
woodlands and hedgerows but will move
into empty territories in mixed woodland if
they become available. This suggests that
mixed woodland is the preferred habitat.
 In fact, Great Tits do reproduce more
successfully in mixed woodlands.

Competition between individuals for territories
often results in the better competitors settling in
source habitats and the remainder in sink
habitats.
 Source habitats: reproductive success produces
an excess of offspring.
 Sink habitats: reproductive success is not
enough to sustain the population and immigrants
are necessary to maintain population size.
 Understanding
source-sink dynamics has
been a major focus of conservation
biology in recent years as has the whole
topic of habitat loss because many habitat
specialists depend completely on certain
limited and/or high demand sites (e.g.
wetlands and sandy beaches).
Habitat evaluation in side-blotched
lizards
 In
some cases of territorial competition
evenly matched individuals evaluate
habitat quality and adjust territorial
boundaries accordingly.
 Male
side-blotched lizards defend
territories that contain rocks females use
for basking and cover.
Habitat evaluation in side-blotched
lizards
 Males
defend territories that contain rocks
to attract females.
 When
researchers removed rocks from
some territories and added them to others
the lizards responded by adjusting territory
boundaries.
8.2
Habitat evaluation in side-blotched
lizards
 Males
who had lost rocks expanded their
territories, males who gained rocks lost
ground.
 End
result was that territories that each
attracted one female were established.
Honey bee habitat evaluation
 When
honeybee colonies split half of the
bees and the old queen depart and settle
in a tree for several days.
 During
this period workers scout for new
potential hive chambers in hollow trees,
cliffs and holes.
8.3
Honey bee habitat evaluation
 Sites
30-60 liters in volume cause scouts
to dance back at the swarm.
 Dancing
encourages other bees to visit the
potential sites.
 These
bees, if they find the site attractive,
may also dance when they return to the
swarm.
Honey bee habitat evaluation
 Scout
bees visit a site only a few times
and each time dance less vigorously than
before.
Honey bee habitat evaluation
 Consequently,
if a site is to remain in
contention it must be attractive enough
that the rate of recruitment of new scouts
exceeds the drop out rate of old scouts.
 Sites
that fail to attract new scouts drop
out of contention.
8.5
Honey bee habitat evaluation
 At
some point many or all scouts are
advertising and visiting the same location.
 As a result of encountering many other
scouts at a potential hive site some bees
may begin “piping” giving a vibrational
signal to the swarm.
 If enough scouts give this signal all bees in
the swarm begin to shiver to warm up and
then fly to the new site.
Honey bee habitat evaluation
 Interestingly,
when offered a choice of
equally suitable sites, but differing in their
distance from the source hive, bees
generally prefer the more distant site even
though the queen is a poor flier.
Honey bee habitat evaluation
 Choosing
the more distant site reduces
competition with the original hive.
 Consistent
with this competition
hypothesis, northern European colonies
(which are larger than those in the south)
move further when establishing new
colonies.
Habitat preferences of territorial
aphids.
 Female
aphids on cottonwood trees hatch
in spring and attempt to secure a leaf on
which to form a gall in which to reproduce.
 Females
choose large leaves and induce
gall formation on the midrib.
8.6
Habitat preferences of territorial
aphids.
 Females
often encounter competitors and
fight for as long as two days (and often
die) trying to secure a territory.
Habitat preferences of territorial
aphids.
8.7
Habitat preferences of territorial
aphids.
 Defeated
or small females are forced to
accept inferior habitats and choose either
small unoccupied leaves or settle on large
leaves that already have a gall, but further
along the midrib, where there are fewer
nutrients.
 As expected, solo occupancy of a small
leaf and sharing a larger leaf have equal
payoffs.
Costs and benefits of territoriality
 Territoriality
is widespread, but not all
organisms defend territories.
 Why
do some do so, but not others?
Costs and benefits of territoriality

Benefit of territoriality is exclusive control of its
contents and the females that these may attract.

However, costs of territorial defense can be
considerable.

Defense requires time and effort. E.g. territorial
surgeonfish on coral reefs may have to chase a
rival off its territory on average more than once
every minute!
Costs and benefits of territoriality

Energetic costs of territorial defense may be
enough to reduce lifespan.

Male Yarrow’s spiny lizards given testosterone
implants spent more time defending their
territories than non-implanted males.

Experimental males not given food supplements
died at higher rates than control and foodsupplemented males, which suggests mortality
was due to high energetic costs.
Costs and benefits of territoriality

As well as risks of injury and exhaustion, there
are indirect costs of elevated testosterone levels.
High levels may reduce immune function and
also make the organisms more vulnerable to
predators.

Different forms of the lizard Uta stansburiana
have different levels of circulating testosterone
and the form with the highest levels has the
lowest annual survival rates.
 Because
territoriality is costly we expect it
to be maintained only when it pays.
 Thus,
territories may be only intermittently
defended.
 E.g.
pseudoscorpions colonize dead or
dying trees dispersing once every several
generations by hitching a ride under the
wing covers of large beetles!
 Only
when riding on beetles are territories
small enough that the costs of defense are
low enough to make it worthwhile for
males to defend them.
Patterns of distribution
 Depending
on whether there are
differences between individuals in their
ability to defend resources we may get
one of two possible distributions of
foragers among sites.
Ideal free distribution
 Ideal
free distribution: in an ideal free
distribution organisms distribute
themselves among sites so that average
payoffs are the same at each site.
 I.e.
more valuable sites attract more
settlers so that the average payoff is
reduced to equal that of a less inherently
valuable site.
Ideal despotic distribution

Ideal despotic distribution: in this case some
individuals are competitively superior and can
exclude other individuals from an area.

For, example among wintering American
redstarts in Jamaica older males occupy
territories in Black Mangrove forest, the best
habitat, and most females and younger males
are compelled to use second growth scrub,
which is of lower quality.

Overwintering redstarts
 Birds
using Black Mangrove forests
maintain their weight over winter, but birds
in second growth scrub generally lose
weight.
Overwintering redstarts

The extra bodyweight of older males allows
them to depart to their breeding grounds earlier
in the spring, which is a big advantage because
early arriving males obtain the best breeding
territories.

Females that can secure high quality winter
territories also benefit by being able to depart
early as they have a longer period in which to
raise young.
8.27
Territorial contests
 Studies
of territorial defense show that the
resident almost always wins in an
interaction with an intruder.
 A variety
of potential explanations have
been advanced to explain this pattern.
ESS explanation
 One
explanation put forward is a game
theory explanation that animals may play
an evolutionarily stable strategy in which
they apply an arbitrary rule “resident
always wins.”
 Not
been possible to show that such a rule
applies in any species.
Speckled wood butterflies
 One
species in which rule was believed to
apply was speckled wood butterfly.
 Males
defend sunspots against other
males. Resident males trapped in a
butterfly net and then released from the
net back at their territory where a new
male had established itself almost always
retreated from the new resident male.
8.29
Speckled wood butterflies
 However,
when work was repeated without
the stress of holding males in a butterfly
net, the original resident evicted the
intruder in 50 of 52 instances.
 Appears
that in the first study the original
resident males were so affected by the net
that they simply tried to escape.
Resource holding power
 An
obvious alternative explanation for why
residents usually win is that they have
more resource holding power (they are
e.g. larger, stronger, more aggressive than
intruders).
Contests among dragonflies
 For
example male damselflies engage in
long chases with other males when
fighting over territories.
 In
these interactions size fat reserves are
the factor that determines who wins.
Males with larger fat reserves afford to
contest a war of attrition for longer.
8.31
Residents have more to lose then
intruders
 Differences
in resource holding power do
not always explain who wins in a dispute.
 In
many cases older individuals fight
harder and are more willing to persist in a
dispute than younger individuals.
Residents have more to lose then
intruders
 For
example, as eggfly butterflies age they
become increasingly persistent in
aggressive interactions and fight harder.
8.32
Residents have more to lose then
intruders
 Third
hypothesis for why residents usually
win is that residents have more to fight for
than intruders.
 Fitness
payoffs for residents on a territory
often increase with time and their chances
of securing another territory decline.
Thus, residents may be willing to fight
much more to keep what they have.
Residents have more to lose then
intruders

One way in which fitness benefits can increase
for a territory holder over time is that after an
initial period of intense interaction with neighbors
to establish boundaries, costs of defense usually
decline.

For example, in one African lizard, residents
allow familiar neighbors to approach closer and
chase them shorter distances than they chase
strangers.
Residents have more to lose then
intruders
 The
“dear enemy” hypothesis suggests
that residents may have an interest in
maintaining a stable set of neighbors
because of the reduced defense costs.
 Consistent
with this sometimes pairs of
neighbors collaborate to drive off an
intruder.
Residents have more to lose then
intruders
 Territorial
fiddler crabs occasionally assist
a neighbor in defending their territory
against an intruder. Usually, the neighbor
helps when the intruder is larger than the
neighbor but smaller than him.
 Assistance
helps and pairs win 88% of
interactions while unassisted residents win
only 71% of the time.
Residents have more to lose then
intruders

Dear enemy effect contributes to asymmetry in
what resident has to lose and what intruder has
to gain.

This asymmetry suggests that in replacement
experiments the longer the new resident has
been in place before the original resident is
released to challenge him the more likely the
contest will be prolonged.
Residents have more to lose then
intruders
 This
hypothesis supported in numerous
studies.
 E.g.
in tarantula wasps a resident removed
and returned only a few minutes after a
new resident is established evicts the new
resident in about 3 minutes. If the old
resident is held for an hour, fights last on
average 25 minutes.
Costs and benefits of dispersal
 Dispersing
to seek a new territory involves
a lot of costs including risk of predation
and significant energy costs.
 Why
then do organisms disperse?
Costs and benefits of dispersal
 One
major reason is to avoid inbreeding.
 Mating
with relatives increases the chance
of expressing deleterious recessive
alleles.
 Inbreeding
thus is likely to reduce fitness
of offspring.
Costs and benefits of dispersal
 For
example, inbred oldfield mice are 50%
less likely to survive than outbred mice
and reproduce later.
8.11

In general, males disperse further from their
natal area than do females. In some cases
females remain close to home because they
gain assistance in rearing young from their
relatives.

Males are often forced out by other males. For
example, in lions young males are often chased
out by dominant when a pride takeover occurs.
Migration
 Migration
is a widespread from of
dispersal.
 Some
insects and mammals migrate, but
the behavior is especially common among
birds.
Migration
 About
50% of all North American breeding
birds are summer visitors from Central and
South America.
 Distances
traveled in migration are
sometimes amazing. Arctic terns migrate
from high in the northern hemisphere to
Antarctica and back!
Migration
 Ruby-throated
hummingbirds (weight
about 3 grams) fly directly across the Gulf
of Mexico. Store about 2g of fat for the
trip.
Migration
 Migratory
species appear to have evolved
from sedentary species, but how?
 Many
species in the tropics engage in
short-range migrations where they move
from breeding grounds to wintering
grounds that are quite close.
Short-distance
migration of the
Three-wattled
Bellbird
Migration
 Short-range
migrants occur in nine
Neotropical songbird families and seven of
these families also include long-distance
migrants. Co-occurrence suggests shortrange migration preceded long-range
migration.
Migration
 The
thrush genus Catharus includes 12
species, 7 of which are resident in Central
or South America.
Swainson’s Thrush
Migration
 Long-distance
thrush migrants appear to
be derived from Central American and
Mexican species.
 Phylogenetic
analysis suggests that
migration has evolved three times in the
genus with subtropical or tropical resident
species giving rise to migratory lineages
each time.
Costs and benefits of migration
 Migration
has significant costs in risk of
mortality and energy.
 Long
over water flights may exhaust
migrants who then drown. Predators also
kill large numbers of exhausted migrants.
Costs and benefits of migration
 To
maximize their chances of completing a
flight birds store large quantities of fat as
fuel and then use a variety of energy
saving strategies.
 For
example, they wait to migrate until
there are tailwinds, which increase their
groundspeed and reduce costs of travel.
Costs and benefits of migration
 Many
birds (such as raptors, cranes and
storks) make use of soaring and gliding
flight, which is energy efficient, but these
birds cannot cross large bodies of water
because there is no rising warm air, which
provides lift, over water.
Costs and benefits of migration
 Birds
also save energy by flying in
formation because doing so enables birds
to benefit from lift provided by nearby
birds.
 Pelicans
flying in formation reduce their
energy expenditure by 11-14%.
Costs and benefits of migration
 Many
birds also minimize risk by avoiding
long-distance water crossings. Thus, most
European migrants cross the
Mediterranean Sea at its narrowest points
either at Gibralter or the Bosphorus in
Turkey.
Costs and benefits of migration

Individual birds also take into account their
condition when deciding whether to make a
crossing or not.

Red-eyed Vireos captured and weighed at
stopover sites on the Alabama coast en route to
wintering grounds in the Yucatan Peninsula and
Central America differ in their migratory
orientation depending on their condition.
Costs and benefits of migration
 Birds
with low fat reserves orient
westwards avoiding the shorter but riskier
oversea crossing. In contrast birds in
good condition orient south.
Costs and benefits of migration
 One
of the most striking migratory flights is
that of the Blackpoll Warbler, which flies
directly from the coasts of the northeastern
U.S. and Canada out over the Atlantic
directly to northern South America a
3,000km flight.
Costs and benefits of migration

Blackpoll’s trip requires 50-90 hours of
continuous flight, but many birds make it in good
condition.

The warblers make good use of winds to help
them on their way. They depart only when a
west to east cold front can push them out over
the Atlantic so that they can later pick up the
prevailing westerly winds over the Atlantic to
push them towards their destination.
Costs and benefits of migration
 In
the spring, blackpolls do not migrate
over the long sea route, but mainly travel
over land.
 Why?
Costs and benefits of migration
 Traveling
over land makes more sense
because the prevailing winds would be
against them over the sea.
Costs and benefits of migration
 The
main benefit of migration for most
species is that it allows organisms to
exploit seasonally abundant food supplies.
 By
moving north to breed during the long
days of the northern summer, birds have
more time to gather food for their young
and food is also abundant.
Costs and benefits of migration
 Resources
other than food are also
important in other cases.
 For
example, the mass migration of
wildebeest on the Serengeti Plain of East
Africa appears to be driven largely by
diminishing supplies of water.
Costs and benefits of migration
 For
Monarch butterflies which migrate to
Mexico to winter in a handful of high
valleys the attraction is sheltered roosting
spots.
 Killing
freezes are rare in the wooded
mountains where Monarchs winter. Moist
cool conditions on mountains suit
Monarchs.
Costs and benefits of migration
 Logging
of sites is having a severe effect
on the quality of the habitat as a roosting
site for Monarchs.
 Opening
of canopy even with only partial
cutting increases likelihood of Monarchs
becoming wet and then freezing,
especially when there is increased
exposure to open sky.