Eco-evo 2: Island syndrom Flashcards
Example: Dodo
The Dodo is a large, tame, flightless pigeon that went extinct from mauritius in the 1600s
It is a clear example of islands syndrome.
Key features
- Bread once a year
- Once egg
- Parental care
- Lives to 27 years
Its closest relative is a long-distance flight pigeon which colonised the islands 8 million years ago and evolved into the dodo.
Rodrigues is another species of flightless bird that has shown a similar pattern of evolutionary change.
Common features across islands which impact the biotic interactions.
Disharmonic
- There is a not an even spread of species compared to the surrounding mainland.
- Predators/ parasites are underrepresented.
Density inflated
- High population densities
Depauperate
- There are few species
Result
- Little interspecific competition, and higher interspecific competition
- Less parasitism and predation
The fundamental shifts in species interactions creates a consistntly different environment leading to patterns of evolutionary change.
Effects of reduced interspecific competition
Wider niche
-> Niche width increases do to less interspecific competition so more niches become available
-> Islands show significantly more diversity in foraging behaviours than their mainland counterparts.
-> It was initially proposed that a wider niche width leads to a larger size. Studies proved that increased size had more to do with the lack of predators.
Higher Densities
Effects of reduced parasitism and predators
Increased body size
Behavioral naivety/ tameness
-> Organsisms become non responsive to predators
-> Foraging time increases and species become less vigilant (group foraging becomes less common)
-> Example: A study on lizards found that flight initiation distance (a measure of response to risk) decreased with island isolation.
-> example: study on bull headed shrike found greater FID on islands -> but we’re there for breeding
-> makes at risk to introduced predator- 22 avian extinction result from introduced predator
Flightlessness/ reduced dispersal
-> Study comparing islands and land birds to the morphology of the pigeon and the dodo showed that island birds tended towards dodo morphology with shorter legs and reduced wing muscles.
increased survival
low gear locomotion
-> Mammals that start large evolve to have smaller/ stout limbs, leading to the development of slower, powerful locomotion.
Reduced BMR
-> This is due to the absence of predators and the lack of resources so energy expenditure is minimised.
-> tropical islands have reduced BMR due to heat stress and under nutrition
Change in pace of life history (Example: Birds)
-> Reduced clutch size, greater reproductive investment, longer developmental period, and tend towards slower development/ growth.
-> This change in life history is due to the limitation of resources and the lower risk of predation.
Increased colouration
- reported for populations of snails and lizards -> reduced risk from predators
- also found in some birds of paradise (e.g. Hawaii honey creeper)
- But conflicts with general trend towards duller birds
Effects of increased intra specific competition
Reduced territoriality
- elevated defender cost as high densities
Reduced agression
Change in colouration
- general trend for birds to become more dull -> but supported by few studies which rely on subjective human description of colour
- less sexual selections as reduced indirect benefits of mate choice-> lower parasite pressure and reduced genetic diversity
- Reduced need for recognition with less diversity- less signal complexity as fewer related sympathetic species
-> evidence: the most complex songs are found in the most specious families of birds
Effects of reduced need for diserpsal
Flightlessness
-> With less space to disperse, flight becomes less important.
-> metabolically expensive so is lost
-> Study comparing islands and land birds to the morphology of the pigeon and the dodo showed that island birds tended towards dodo morphology with shorter legs and reduced wing muscles.
-> evolved 150 independent times with 468/ 581 know flightless species occurring on islands
May just be reduced flight ability
- common snipe in Azores
May lose behaviour rather than ability
- loss of dispersal and migratory propensity
Rounded wings
-> Rounded wings evolve when there is less of a requirement for flight.
However literature is fragmented
Other factors as a result of island living
Increased disease susceptibility
Less difference between M and F plumage/ reduced brightness (less sexual selection)
-> A global analysis looking at 116 matched species between islands and mainlands found that island birds had reduced signal intensity and complexity of colouring
Change in colouration
- general trend for birds to become more dull -> but supported by few studies which rely on subjective human description of colour
- less sexual selections as reduced indirect benefits of mate choice-> lower predator pressure and reduced genetic diversity
- Reduced need for recognition with less diversity- less signal complexity as fewer related sympathetic species
-> evidence: the most complex songs are found in the most specious families of birds
Change in pace of life history (Example: Birds)
-> Reduced clutch size, greater reproductive investment, longer developmental period, and tend towards slower development/ growth.
-> This change in life history is due to the limitation of resources and the lower risk of predation.
The island rule
This is the idea that large individuals evolve to be smaller and small individuals evolve to be larger.
It is described as “Foster’s rule” for mammals.
This pattern can be seen in a range of organisms (mammals, birds, reptiles) but amphibians always tend towards a larger size.
Stronger size shift in more isolated islands
Testing traditional ideas: What causes the island rule?
it was initially hypothesised that small species evolve to be larger due to the wider niche width, so more available resources to exploit and the possibility of evolving to a larger size.
However, a recent study on Heron Island found that species had a narrow niche range (individual specialists rather than the predicted generalist) and that the shift in body size was due to the absence of predators.
Testing traditional ideas: What causes low- gear locomotion?
Traditional ideas suggested that low gear locomotion was due to the reduced risk of predation and therefore less of a need to be manourvarable.
However, recent studies have shown that the reduced locomotion is due to the number of competitors rather than the number of predators.
Island syndrome as an evolutionary trap
Once species have evolved island syndrome, they are unable to evolve back (e.g. loss of flight)
The evolved features can make them very vulnerable to change (e.g. anthropogenic change/ disease)
95% of mammal and avian extinction since the 1500s have been on islands.
Anthropogenic effect on island example: Hawaii
Pre-human Hawaii had a wide diversity of flightless and non-flightless avian species and endemic avian predators.
Two rounds of human arrival brought a range of rodents and also avian malaria. (e.g. the rat, dog, pig)
This resulted in many of the endemic species going extinct following both arrivals.
Evidence of the system evolving in turn.
1) Elepaio bird evolved to nest 50% higher to avoid predation by the rats.
2) Malaria was more prevalent in low land regions than high land regions due to the range of the mosquitos. A study showed that the birds in the low land regions had greater resistance to malaria due to higher levels of exposure.
86% of extinctions on islands due to invasive species
Lecture overview
Island forms exhibit a suite of common phenotypes that collectively comprise the island syndrome.
This is due to the unique biotic interactions that occur on islands, creating a standard environment leading to a pattern of evolutionary change.
Common characteristics of islands
-> disharmonic
-> density heavy
-> low diversity of species
Biotic results
-> Low interspecific competition
-> High intraspecific competition
-> Low levels of parasitism and predation.
Island syndrome
Changed in morphology
- tendency to medium body size from larger sizes
- less difference in male and female plumage
- less rounded wings
- shift body plan (short and squat)
Behaviour
- tameness
- Reduced aggression levels
- Increased sedentary behaviour
Ecology
- wider niche
- reduced dispersal snooty
- increased disease susceptibility
Change in life history -> shift towards slower K selected life histories
- longer life span
- smaller clutch size
- slower development (linked to more parental care)
- increase parental effort and investment in young (study found that extra pair maternity is lower on island promoting male parental care. Or more cooperative breeding due to increased population densities)
- reduced fecundity -> allocate more into maintenance to increase survival and maximise life span so can overall breed more (maximise lifetime fitness). Reduced fecundity due to competition for food due to high densities.
Islands and climate change
Islands are at extreme risk of climate change
- cannot disperse
- K selected so cannot track evolutionary change
- many have reached evolutionary dead ends and cannot evolve away
- at risk of sea level rises engulfing the island
Example: islands in Kiribati already lost
