Migration vs. Nomadism

Transcription

1 Migration lecture overview What migration is and what it is NOT Who migrates? How do animals migrate? physiological, navigational, & other behavioral challenges How does a species become migratory? Why do some animals migrate (while others do not)? learned or innate? proximate reasons (or, knowing when it s the right time to go) ultimate reasons (the evolutionary answers) If we have time: a case study Migration vs. Nomadism Unpredictable seasonal movements Example: crossbills 1

2 Migration vs. Dispersal: Permanent movement away from a former home range to a new one Typically young animals Often sex-biased Benefits: Genetic benefits Reduce intra-specific competition Avoid extinction in unpredictable environments Costs: Energetic cost of moving Uncertainty & risk Migration: Definition depends on taxon Genes, rotifers, in population dynamics: = dispersal Insects (e.g., locusts): usually refers to one-way movements, return trip, no specific temporal pattern ( nomadism or dispersal) Birds, whales, fish, etc: Movements of all or most of the individuals in a population between spatially separated breeding and non-breeding home ranges occurring on predictable cyclical time scales 2

3 Who migrates? Marine mammals (whales) Terrestrial mammals (wildebeest) Aerial mammals (bats) Sea turtles Insects (monarchs) Birds of course Temporal patterns of migration Dial: e.g. pelagic zooplankton Annual: Same individual (birds) Different individuals Multiple generations in 1 year (many insect spp.) Each individual returns once in lifetime (> 1 yr), but some individuals migrate every year Milkweed Bug Lygaeus equestris Atlantic Salmon Salmo salar 3

4 Spatial patterns of migration: migratory distance huge variation between (& sometimes within) species Spatial patterns of migration: differences among sex & age groups in distance migrated Distance migrated (kms) Nebel et al,

5 Spatial patterns of migration: migratory routes sometime complex decision-making & navigation required How do animals migrate? Example of cognitive adaptations: Clocks and maps 5

6 How do animals migrate? Examples of physiological adaptations: Freshwater vs. saltwater Examples of physiological adaptations: Fat storage and metabolism Pre-migratory fattening hyperfagia. Involves switching to alternative metabolic pathways Being fat is dangerous and costly but necessary: migrants may increase normal body weight to 130% of normal or more with fat. Great knots reduce all body organs other than brains and lungs during sustained migratory flight. Reduction of BMR to 46% of normal! 6

7 How do animals migrate? Example of morphological adaptations: Wing length Migrants have pointier, longer wings that aid reduce maneuverability but reduce cost of long flights Innate or learned? Evidence for both Phylogenetic patterns Learned components even in innate migrations Anseriformes, Gruiformes Passeriformes 7

8 Evidence for genetic control of migratory behavior in birds: naïve birds, Emlen funnels Hand-raised chicks from egg Exposed to normal night sky At correct time, birds exhibited zugenruhe Oriented the correct direction for the right amount of time, then changed direction as if following migratory route Evidence for genetic control of migratory behavior in birds: breeding experiments 8

9 Evidence for learned migratory behavior in birds: loss of migration Endangered whooping cranes have lost cultural memory and have to be taught to migrate by humans How fast does migration evolve?: FAST phylogenetic evidence 9

10 ecological evidence: Eastern House Finches Native to Western North America Introduced to Long Island 1940 Source population 80% sedentary ecological evidence: European Blackcaps Black-caps have established novel nonbreeding grounds and new migratory pathways in response to increased use of bird feeders in England since 1940 s Both ecological and evolutionary evidence points to turning on and off of genetic program 10

11 How does migratory behavior originate? population dynamics Original population of resident individuals residents How does migratory behavior originate? population dynamics Original population of resident individuals Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource residents 11

12 How does migratory behavior originate? population dynamics Original population of resident individuals Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource They return to previous areas when resources run out partial migration of single species residents How does migratory behavior originate? population dynamics Original population of resident individuals Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource They return to previous areas when resources run out partial migration of single species Competition with residents eventually drives the 2 strategies apart, spatially & genetically 2 distinct populations +/or eventually species migrants residents 12

13 How does migratory behavior originate? population dynamics Original population of resident individuals Some individs move to adjacent habitat on boundary of their range to exploit some seasonally abundance resource They return to previous areas when resources run out partial migration of single species Competition with residents eventually drives the 2 strategies apart, spatially & genetically 2 distinct populations +/or eventually species Ranges of 2 groups evolve independently winter migrants summer residents How does migratory behavior originate? Pre-adaptations? Living in a very seasonal environment or relying on very seasonal resources? Year-round territoriality not hugely advantageous? Genetic predisposition? Having somewhere to go? others. 13

14 Proximate causes (cues) of migration Exogenous: Daylength/photoperiod Temperature Direct cues: e.g. changes in resource abundance Endogenous: Circannual rhythms Hormones Physiological status why go south? OR why go north? Reflects historical biases and ideas about ancestral range of migratory organisms Should attempt to explain both directions of movement Ultimate and proximate causes probably usually DIFFERENT for the two directions of migratory movements Goal: explain differences among individuals and species in terms of costs and benefits of different strategies 14

15 Ultimate causes of migration Seasonality of resource or seasonality of need Food-based hypotheses: Why leave the breeding area? High seasonality of habitat means that there are times when absolute food abundance is too low to survive Inter- or intra-specific competition drives resources to low levels. Migrants are the loosers. Why leave the non-breeding area? Seasonally high resource abundance means that birds that migrate to exploit peak can produce more offspring Year-round stability of tropical habitats means that migrants can take advantage of year-round food availability during non-breeding season Food and the costs of migration: Enormous energetic costs of migration mean migrants need much higher food intake rates 15

16 Weather and physiological tolerance Why leave the breeding area? Extreme cold or severe storms exceed the absolute range of conditions individuals can survive in Why leave the non-breeding area? Long daylength means that parents can forage for longer, feed more chicks, achieve higher growth rates and fledge bigger, healthier and more young. Moderate climates in non-breeding areas increase survival of adults Weather and the costs of migration: Bad storms can cause delays in migration or blow individuals off course May have to migrate over areas of climatic unpredictability without knowledge of area that residents have Predation risk Why leave the breeding area? Why leave the non-breeding area? High nest predation risk in tropical areas means that higher per-nest chance of success in temperate areas than tropical areas Predation and the costs of migration: Higher risk of adult predation when aggregated during migratory flights and in unknown areas 16

17 Others: Bugs (cariboo) Snow depth (moose) Water levels & water salinity Parasites? Pathogens? How do you test of hypotheses explaining evolution or migratory behavior? Some problems: Animals (by definition) don t stay put! Following them around is hard Very hard to invent effective manipulations Behaviors repeat on annual time scales FEW EMPIRICAL TESTS OF ANY HYPOTHESES REGARDING ULTIMATE CAUSES OF MIGRATION 17

18 How do you study the causes of migration? Comparative method Correlative predictions Case study: Why do some (but not all) birds migrate altitudinally in the tropics? Cloud forest Breeding Non-breeding Premontane forest Lowland forest Patterns of altitudinal migration 18

19 Why migrate? Food Predation Weather Migrants more frugivorous than their non-migrant counterparts but not more specialized on certain fruit types 19

20 Food Does the timing and location of white-ruffed manakin migration make sense if they are moving up and down hill to track food resources? Manakins not migrating because of protein needs and resources at breeding areas Fruit could still explain their migration 20

21 Why migrate? Food Predation Weather Most nest predation at the elevation where white-ruffed manakins breed 21