Download Dispersal and persistence

Dispersal and
persistence
Disjunction – discontinuous range of a taxon in
which at least two closely related populations
are separated by a wide geographic distance
2 possible explanations
1. Dispersal
2. Geological events
Dispersalists
Darwin
vs Extensionists
Lyell
Theory of Continental Drift
Final evidence against
extentionist arguments, but
provided new theory critical
to understanding species’
distributions
Alfred Wegener
Vicariance – geographical range of a taxa is split into parts by
the formation of a barrier
•Events such as plate
tectonics, orogeny , climate
change, sea level rise or
glacial movement split
populations
Southern Beech
Nothofagus
Patagonia
New Zealand
• After vicariance events,
evolutionary divergence can
give rise to different (but
related) species in the
disjunctions
Debate: Vicariance vs Dispersal
What is the relative importance of long-distance dispersal events
and vicariance (population-splitting events)?
Eclecticist - dispersal and vicariance events are both important
Definitions
Ecological dispersal – movement of individuals
away from an existing population or parent
organism
Reduce intra-specific competition
But, habitat similarity decreases with distance
Biogeographic dispersal – species shift their
ranges by moving over long distances across large
barriers (broader spatiotemporal scales)
Infrequent but very important
Mostly historic examples
Why disperse?
• Reduce intraspecific competition
• Find more suitable habitats
Successful range expansion
1. Travel to new area
2. Tolerate conditions of new habitat
3. Reproduce in new habitat
Mechanisms of dispersal
Active: movement of an organism from one location
to another by its own means
Passive: movement of an organism from one place
to another by means of a stronger force, such as
water flow, wind or another organism
Monarch Butterfly
Golden Plover
Active dispersal – Air
Some species have extensive
migratory routes and thus show
great potential for dispersal
Ruby-throated hummingbird
Active dispersal - water
Himalayan Balsam
Squirting cucumber
Active dispersal - plants
Passive dispersal - water
Passive dispersal - water
Passive dispersal – floating islands
Passive dispersal – wind
Diaspore – plant
dispersal unit
(consists of seed
plus additional
tissues adapted for
dispersal)
Pappus
Wind dispersal of large propagules
Wings
Exozoochory – seeds dispersed on outside of animal
Endozoochory – seeds dispersed on inside of animal
Types of dispersal events
1. Jump dispersal
2. Diffusion
3. Secular migration
Jump dispersal
Long distance dispersal accomplished during a relatively short
period
– Occurs infrequently, large effect
– Some species are better at jump dispersal than others (water, wind, wings)
e.g. Hawaii and Galapagos
Consequences:
1.
2.
Explains wide and often discontinuous distributions of many taxa
Accounts for similarities/ differences among biotas inhabiting similar
environment in different geographic areas (some species good at distance
dispersal, but chance is still important)
Human-aided jump dispersal increasing
Diffusion
Slow dispersal of individuals spreading out from the margins of
the species’ range (accomplished over generations)
Often follows jump dispersal
Cattle egret
Jump dispersal + diffusion
Africanized honey bees
Brought from Africa
Escaped from Sao Paolo
Dispersed ~ 100 mi/yr
Secular migration
Very slow dispersal (e.g. hundreds of generations)
that commonly involves evolutionary changes in the
dispersing populations
Secular migration - camels
Guanaco
Vicuña
Barriers and Dispersal Routes
Barriers – abiotic or biotic feature that restricts movement of genes or
individuals from one place to another
• Species-specific
• Generally, organisms that inhabit fluctuating environments are
more tolerant of extreme/unusual conditions than species in
stable habitats
Janzen (1967)
Types of Barriers
Physiological
Ecological
Behavioral
Physiological Barriers
Physical environments outside
the range an organism
normally encounters so it
cannot survive and disperse
across the barrier
Common physiological barriers
– Land-water
– Salinity
– Temperature
Ecological Barriers
Dispersing organisms must
be able to withstand
ecological hazards
– Predation
– Competition
Behavioral Barriers
Ability of some organisms to select suitable habitat can
limit ability to disperse across barriers
– Example – bird species capable of flying long distances will
not cross open areas
Dispersal Routes
Corridors
Filters
Sweepstake routes
Increasing resistance
to biotic exchange
Corridor – dispersal route that permits movement of
many related species from one region to another
– Similar environment to that of the two regions
– Balanced taxonomic composition
Beringia - Pleistocene
Proposed I-70 corridor (unfunded)
Banff Corridor
Filters - selectively impede the movement of
certain taxa while allowing others to disperse
freely (biotic or abiotic)
– Colonist-biased subset
– Often form transition zones
Filters
Two-way filter for reptiles between Australia and Asia
Sweepstake Routes – severe barrier that permits rare
dispersal events (jump dispersal)
• Rare on ecological timescale, likely and important on
geological timescale
• Can be followed by dispersal and adaptive radiation
1995, Hurricane Luis
Vegetation mats
Green iguanas (Iguana iguana)
sweepstakes route from
Guadeloupe to Anguilla
Corridors
Favorable habitat
connecting larger
ones
Filters
Blocks or slows
passage of some
organisms
Sweepstake Routes
Severe barrier that
permits rare
dispersals
Successful range expansion
1. Travel to new area
2. Tolerate conditions of new habitat
3. Reproduce in new habitat
Once dispersed to new habitat, species may select
appropriate habitat
Mobile organisms – search for favorable habitats
Immobile organisms – some do not settle or germinate unless perceive
certain sensory cues
Which organisms often have good survival in new
ecosystems?
From highly fluctuating/unpredictable environment
From large, ecologically-diverse habitats
Successful range expansion
1. Travel to new area
2. Tolerate conditions of new habitat
3. Reproduce in new habitat
Reproductive traits of “good colonizers”
Asexual reproduction (need only one)
Vegetative reproduction (ramets, bulbs)
Apomictic (seeds from unfertilized ovules)
Parthenogenesis (embryos from unfertilized eggs)
Sexual reproduction
Hermaphroditic and self-compatible
High reproductive rate, short gestation