Chapter 10

Dispersal

A relatively short-distance, one-way movement away from a site.

The competition hypothesis

States that dispersal reduces competition for resources

The 2 predictions for the competition hypothesis

Prediction 1: If two sites have identical amounts of food but differ in their density of individuals, we can expect higher levels of dispersal behavior in the high-density sites.

Prediction 2: If two sites have the same density of individuals but differ in their quantity of food, we can expect higher levels of dispersal behavior in the site with less food.

Research question for density-dependent dispersal in earthworms

Does conspecific density affect earthworm (Aporrectodea icterica) dispersal?

Hypothesis for density-dependent dispersal in earthworms

Competition for resources will result in increased dispersal

Prediction for density-dependent dispersal in earthworms

Greater dispersal will occur with higher population density

Methods for density-dependent dispersal in earthworms

• Created 1.0 m x 0.18 m long arena divided into 3 regions

• Ends contained “suitable” soil for earthworms

• Center contained “unsuitable” soil that they tend to avoid

• 3 Density treatments of 10, 20, and 30 earthworms

• Recorded percent dispersed from one to the other over a week

Results for density-dependent dispersal in earthworms

• For two low density treatments < 50% of individuals dispersed

• High density treatment > 67% dispersed

• Competitor density affects dispersal

• Supports prediction 1 of competition hypothesis

Observation for food-related dispersal in water boatmen

Water boatmen are freshwater aquatic invertebrates that live in pools and feed on herbivorous aquatic invertebrates. Adults can disperse from one pool to another

Research question for food-related dispersal in water boatmen

How is water boatmen dispersal affected by prey density level?

Methods for food-related dispersal in water boatmen

• Created 40 artificial pools (20L each)

• Density of prey in pools differed from 0 to 100 water fleas (Moina macrocarpa) per liter

• 5 pools each day separated by 5 m

• 10 water boatmen per pool with different densities of prey

• Measured water boatmen dispersal over 24 hours

Results for food-related dispersal in water boatmen

• Proportion dispersing ranged from 0% to 60%

• Water boatmen more likely to disperse from low density prey pools

• Competition for food facilitates predator dispersal

• Supports prediction 2 of competition hypothesis

Inbreeding depression

A reduction in fitness as a result of mating with close relatives.

Natal dispersal

A one-time movement away from an individual’s place of birth.

Inbreeding avoidance hypothesis

The hypothesis that natal dispersal behavior minimizes the likelihood of inbreeding.

Prediction for inbreeding avoidance in great tits

Individuals that dispersed greater distances were less likely to be mated with a relative

Methods for inbreeding avoidance in great tits

• Used Wytham Woods population

• Great tit nest box

• All individuals uniquely marked for decades

• Each spring 50% of breeders immigrate into population

• Characterized degree of inbreeding in population and distance each member of breeding pair dispersed before breeding

Results for inbreeding avoidance in great tits

• Individuals that mated with close relative had dispersed shortest from natal site

• Dispersal behavior can influence the likelihood of inbreeding

• Supports inbreeding avoidance hypothesis

Breeding dispersal

The abandonment of one breeding site to move to another

Site fidelity

Individuals that remain at or return to a previous location to breed.

Win-stay-lose-shift dispersal

A common behavioral pattern in which individuals return to a previous breeding location after reproductive success there, but disperse to a different location after a reproductive failure.

Research questions for breeding dispersal in dragonflies

What factors affect breeding dispersal in dragonflies?

Hypothesis for breeding dispersal in dragonflies

Dragonfly dispersal behaviour is affected by their reproductive success

Prediction for breeding dispersal in dragonflies

Dragonflies will remain in the same territory if they have high mating success and will disperse to a new territory if they have low mating success

Methods for breeding dispersal in dragonflies

• Created 20 breeding territories on a pond using sticks

• Reduced mating success in ten territories (treatment) and kept it high in others (controls)

• Recorded the number of matings for each male and whether he returned to the same territory or dispersed to a different territory on Day 2

Results for breeding dispersal in dragonflies

• Control males had higher mating success than treatment males.

• All control males returned to their oviposition site, whereas only 50% of treatment males returned to their oviposition site.

• Mating success affects breeding dispersal: individuals use a win-stay lose-shift strategy when making dispersal decisions

Research question for public information and breeding dispersal in kittiwakes

What affects breeding dispersal in kittiwakes?

Hypothesis for public information and breeding dispersal in kittiwakes

Patch-wide reproductive success in subcolonies (public information) affects breeding dispersal

Prediction for public information and breeding dispersal in kittiwakes

• If kittiwakes use only personal information, then failed breeders should abandon the breeding site

• If kittiwakes use public information and a failed breeder is surrounded by successful breeders, they should not abandon the breeding site

Methods for public information and breeding dispersal in kittiwakes

• Black-legged kittiwake nest

• Uniquely marked focal birds

• Treatment patches:

  • removed eggs from nests of all birds (all birds had reproductive failure)

• Control patches:

  • removed eggs only from focal bird (only focal bird had reproductive failure)

• Observed where birds bred the next year

Results for public information and breeding dispersal in kittiwakes

• Many (70%) of the control patch birds returned to the same breeding sites the next year

• Fewer of the treatment patch birds returned to the same breeding sites the next year (less than 50%)

• Information from conspecifics affects breeding dispersal decisions

Migration

Relatively long-distance, two-way movement.

Research question for resource variation and migration in neotropical birds

What factors are important in determining whether a species migrates or not?

Hypothesis for resource variation and migration in neotropical birds

Migration evolved to allow individuals to: (1) take advantage of spatial variation in conditions and (2) avoid seasonal resource depression at different locations

Prediction for resource variation and migration in neotropical birds

Species exposed to high degree of fluctuation in environmental conditions and resources will be more likely to migrate than species that live in more stable resource environments

Methods for resource variation and migration in neotropical birds

• Collected previously published data on 300 species in Tyranni

• For each species, determined:

  • Migrant or sedentary

  • Insectivore or frugivore

  • Habitat use (tropical forest habitats more stable than nonforest habitats)

Results for resource variation and migration in neotropical birds

• Insectivorous species in forest habitats were less likely to be migratory than nonforest species

• No relationship between habitat type and migration in frugivorous species

• Fluctuations in resource levels and environmental conditions can affect migratory behaviour

Research question for heritability of migration behaviour in Eurasian blackcaps

What is the heritability of migratory behaviour in Eurasian blackcaps?

Methods for heritability of migration behaviour in Eurasian blackcaps

• Collected nestlings and allowed them to breed

• Exposed to natural declining autumnal photoperiod

• Migrate at night

• Measured the date of onset of migratory restlessness in adults and their offspring by recording the first night a bird was active during at least five 30-minute observation periods

• Conducted parent-offspring regression

Results for heritability of migration behaviour in Eurasian blackcaps

• A positive correlation in the parent-offspring regression

• Conducted artificial selection experiment to confirm findings

• Onset of migratory behaviour is a heritable trait

Research question for competition and migratory behaviour of newts

How do environmental conditions affect migratory behaviour?

Hypothesis for competition and migratory behaviour of newts

Competition for resources will result in more individuals migrating

Prediction for competition and migratory behaviour of newts

As density increases, more individuals should migrate

Methods for competition and migratory behaviour of newts

• Red-spotted newt (Notophthalmus viridescens)

• Small enclosures around large pond

• Treatments:

  • Density of newts (low, medium, or high)

  • Sex ratio (male or female biased, 2:1)

• Measured migration out of enclosures daily

Results for competition and migratory behaviour of newts

• More newts migrated out of high-density enclosures

• Migrated out of high-density enclosures 8 days earlier than low-density enclosures

• Sex-ratio did not affect migratory behaviour

• Females migrated more often than males

• Population density does affect the migratory behaviour of newts

Observation for alternative migratory behaviours in dippers

Many populations exhibit partial migration

Research question for alternative migratory behaviours in dippers

What maintains resident and migratory behaviours within a population of dippers?

Hypothesis and Prediction 1 for alternative migratory behaviours in dippers

• Migration vs residency is a fixed genetic trait, and both behaviours are maintained by frequency-dependent selection

• Migrants and residents have equal fitness; individuals keep the same strategy across year

Hypothesis and Prediction 2 for alternative migratory behaviours in dippers

• All individuals have a genetic disposition to migrate, but expression depends on condition (best of a bad job)

• Migrants and residents differ in fitness; poor-condition individuals are the ones that migrate

Methods for alternative migratory behaviours in dippers

• American dippers (Cinclus mexicanus)

• Uniquely banded individuals at several sites over 7 years

• Conducted 5 censuses each year

• Residents – occupied single territory year round

• Migrants – did not occupy a single territory year round

• Observed migration behaviour over seven years

Results for alternative migratory behaviours in dippers

Residents:

• began nesting 2 weeks earlier than migrants

• more likely to initiate 2nd brood

• fledged 2.6 more offspring than migrants 50 banded nestlings became breeders but adopted migratory behaviour of parents less than 50% of time

The fitness of residents is higher than that of migrants. Migratory behaviour is not fixed genetic dimorphism. Behaviours have different fitness.

Orientation

The process of determining and maintaining a proper direction.

Navigation

The process of determining a particular location and moving toward it.

Different cues animals use for directional information

• Landmarks

• Sun compass

• Star compass

• Geomagnetic compass

Sun compass

The use of the sun for orientation.

Star compass

The use of stars or constellations for orientation.

Geomagnetic compass

The ability to orient using the earth’s magnetic field.

Observation for antennae and the sun compass system in monarchs

Monarch butterflies display intergenerational migration (each generation moves farther along the migration route). Many animals have internal clock that is regulated by daily light and dark cycles. Position of sun provides directional information

Research question for antennae and the sun compass system in monarchs

Do monarch butterflies use a sun compass?

Methods for antennae and the sun compass system in monarchs

• Conducted “clock-shift” experiment in Kansas

• Two groups of monarchs in lab – both had 12 hours light/ 12 hours dark

• One group clock-shifted six hours over several weeks

• Released monarchs outdoors in September just after their lights turned on

• Observed direction they flew

Results for antennae and the sun compass system in monarchs

• Controls migrated in correct direction (S/SW)

• Clock-shifted group moved in westward direction Conclusion:

• Clock-shifted animals had reset internal clock so that their “daybreak” occurred at noon.

• At noon, the sun was in the southern sky, but they interpreted this position as east (the normal direction of the sun at daybreak) and so migrated in the wrong direction

• Monarchs appear to use a sun compass

Intergenerational migration

Migration that occurs over more than one generation.

Research question for the role of antennae in the monarch butterfly sun compass

How do monarch butterflies use a sun compass?

Hypothesis and Prediction 1 for the role of antennae in the monarch butterfly sun compass

• The sun compass is associated with the antennae

• Individuals without antennae will not be able to orient properly.

Methods 1 for the role of antennae in the monarch butterfly sun compass

• Removed antennae from ten butterflies (antennae-less treatment) and left the antennae of ten control butterflies intact

• Tethered individuals and recorded flight behavior direction

Results 1 for the role of antennae in the monarch butterfly sun compass

Control butterflies oriented south/southwest, while antennae-less butterflies did not orient in any particular direction.

Hypothesis and Prediction 2 for the role of antennae in the monarch butterfly sun compass

• Antennae contain photoreceptors needed for proper orientation.

• Butterflies with clear paint or no paint on their antennae will be able to orient properly, whereas butterflies with black paint on their antennae will not be able to orient properly.

Methods 2 for the role of antennae in the monarch butterfly sun compass

• Painted butterflies’ antennae with opaque black paint, clear paint (Control 1), or no paint (Control 2)

• Tethered individuals and recorded flight behavior direction

Results 2 for the role of antennae in the monarch butterfly sun compass

Butterflies with no paint or clear paint on their antennae oriented south/southeast. Butterflies with black-painted antennae oriented north. Antennae and functioning photoreceptors are required for sun compass orientation in monarch butterflies.

Research question for the magnetic compass in sea turtles

Do sea turtles use a magnetic compass to orient properly during migration?

Methods for the magnetic compass in sea turtles

• Juvenile loggerhead sea turtles (Caretta caretta)

• Treatment – altered magnetic fields

• Control – normal magnetic fields

• Measured movement and orientation

Results for the magnetic compass in sea turtles

Only controls moved in correct direction. Loggerhead turtles do use a geomagnetic compass

Bicoordinate navigation

The ability to identify a geographic location using two varying environmental gradients.

Observation for multimodial orientation in pigeons

• Pigeons use a sun compass but can also orient on cloudy days

• Do pigeons use another/ backup compass?

• Pigeons with altered magnetic fields failed to orient properly

• Pigeons lacking chemoreception also fail to orient properly

• Evidence of multimodal orientation

Observation for bicoordinate navigation in sea turtles

Angle of inclination of earth’s magnetic field changes with latitude. Intensity of magnetic field varies around the globe

Research question for bicoordinate navigation in sea turtles

Do sea turtles use bicoordinate navigation?

Methods for bicoordinate navigation in sea turtles

• Juvenile loggerhead sea turtles (Caretta caretta)

• Conducted in eastern Florida

• Exposed to one of three different magnetic fields characteristic of three different geographic locations in North Atlantic gyre

• Measured orientation and movement

Results for bicoordinate navigation in sea turtles

• All turtles oriented to appropriate direction

• Sea turtles can use the earth’s magnetic field to orient in different directions, and this ability is inherited, not learned

Observation for bicoordinate navigation in green sea turtles

Juveniles remains in shallow coastal feeding ground near natal beach for extended period

Research question for bicoordinate navigation in green sea turtles

Do green sea turtles display bicoordinate navigation?

Methods for bicoordinate navigation in green sea turtles

• Exposed turtles to magnetic field simulating location 227 km north and south of capture location

• Used apparatus and measured direction of swimming

Results for bicoordinate navigation in green sea turtles

• Turtles displaced 337 km north oriented and tried to move south

• Turtles displaced 337 km south oriented and tried to move north

• Sea turtles can use variation in earth’s magnetic field as map for orientation

Research question for bicoordinate navigation in birds

Do Eurasian reed warblers display bicoordinate navigation during their migration

Methods for bicoordinate navigation in birds

• Eurasian reed warblers (Acrocephalus scirpaceus)

• Migrate between sub- Saharan Africa and Eurasia

• Captured birds during spring migration

• Displaced half 1,000 km east

• Controls left in Rybachy

• Placed birds in view of sky in Emlen funnel to measure orientation

Results for bicoordinate navigation in birds

• Control oriented to northeast as expected

• Displaced birds oriented to northwest to compensate for being displaced

• Eurasian reed warblers display bicoordinate navigation

Observation for homing migration in salmon

Salmonids return to natal stream for spawning

Research question for homing migration in salmon

• How do they find their natal stream

• May use olfaction imprinting and geomagnetic reception

• Both rely on imprinting during smoltification

Methods for homing migration in salmon

• Olfaction imprinting -

  • Captured salmon migrating upstream from two forks of a Y-shaped stream near Seattle

  • Uniquely tagged each fish to identify the fork in which it was captured

  • Blocked olfactory pits of half the fish and the other half (controls) were left untreated

  • Released fish 1.6 km below fork so they could resume their migration upstream

• Geomagnetic imprinting –

  • Exposed juvenile salmon to different magnetic field intensities

  • Observed direction of swimming

Results for homing migration in salmon

Results olfaction imprinting:

• Only 40% of treatment fish recaptured in original stream

• 89% of controls recaptured in original stream

Results geomagnetic imprinting:

• Juvenile salmon changed orientation in tanks in response to changes in magnetic field intensities

Salmon may use geomagnetic imprinting to get close to natal stream and olfaction imprinting to determine specific natal stream