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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.

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Presentation on theme: "Chapter 8: Choosing where to live  Habitat Selection: Choosing an appropriate place to breed or overwinter is an essential decision for most organisms."— Presentation transcript:

1 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.

2 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.

3  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.

4  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).

5 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.

6 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.

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8 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.


10 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.

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12 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.

13 Honey bee habitat evaluation  Scout bees visit a site only a few times and each time dance less vigorously than before.


15 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.

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20 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.

21 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.

22 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.

23 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.

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25 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.

26 Habitat preferences of territorial aphids. 8.7

27 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.

28 Costs and benefits of territoriality  Territoriality is widespread, but not all organisms defend territories.  Why do some do so, but not others?

29 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!

30 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 food- supplemented males, which suggests mortality was due to high energetic costs.



33 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.


35  Because territoriality is costly we expect it to be maintained only when it pays.  Thus, territories may be only intermittently defended.

36  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.


38 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.

39 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.

40 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. 

41 Overwintering redstarts  Birds using Black Mangrove forests maintain their weight over winter, but birds in second growth scrub generally lose weight.

42 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.

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45 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.

46 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.

47 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.

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49 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.

50 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).

51 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.

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53 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.

54 Residents have more to lose then intruders  For example, as eggfly butterflies age they become increasingly persistent in aggressive interactions and fight harder.

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56 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.

57 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.

58 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.

59 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.

60 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.

61 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.

62 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?

63 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.

64 Costs and benefits of dispersal  For example, inbred oldfield mice are 50% less likely to survive than outbred mice and reproduce later.

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66  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.

67 Migration  Migration is a widespread from of dispersal.  Some insects and mammals migrate, but the behavior is especially common among birds.

68 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!


70 Migration  Ruby-throated hummingbirds (weight about 3 grams) fly directly across the Gulf of Mexico. Store about 2g of fat for the trip.

71 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.

72 Short-distance migration of the Three-wattled Bellbird

73 Migration  Short-range migrants occur in nine Neotropical songbird families and seven of these families also include long-distance migrants. Co-occurrence suggests short- range migration preceded long-range migration.

74 Migration  The thrush genus Catharus includes 12 species, 7 of which are resident in Central or South America. Swainson’s Thrush


76 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.


78 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.

79 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.

80 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.

81 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%.


83 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.

84 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.

85 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.


87 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.


89 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.

90 Costs and benefits of migration  In the spring, blackpolls do not migrate over the long sea route, but mainly travel over land.  Why?

91 Costs and benefits of migration  Traveling over land makes more sense because the prevailing winds would be against them over the sea.

92 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.

93 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.

94 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.


96 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.


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