Factors Limiting Distribution: Dispersal – Chapter 4

Dispersal The transport of animals to geographical areas not currently inhabited by that species. –Simplest explanation as to why a particular species may not be located in a geographical are. If colonization is successful, dispersal will result in gene flow and thus affect the genetic structure of a population. –Also result in a founder effect

Example: Zebra Mussel Probably introduced from ship ballast water. Spread throughout the majority of the Mississippi drainage within 10 years.

Example: Gypsy Moth Accidentally introduced by a French astronomer in Control programs ceased around Began to spread again, and accidentally transported to Michigan. Spread 21 km per year between 1966 and 1989.

Example: Chestnut Blight Fungus that is lethal to Chestnuts. First noticed in 1900, and apparently introduced on nursery stocks from Asia. Search began in 1927 to find blight resistant trees. Oak-chestnut forests have been replaced with oak or oak- hickory forests.

Example: California Sea Otter Thought to be extinct in Small population found in 1914 – was now protected. Northern range spread about 1.4 km/year and southern range spread about 3.1 km/year. Southern otters move more as individuals and northern suffering higher mortality?

Three Modes of Dispersal: Diffusion – Gradual movement of a population across a hospitable terrain for a period of several generations. Jump Dispersal – Movement of individual organisms across large distances of inhospitable habitat followed by the successful establishment of a population in the new area. Secular Dispersal – Diffusion occurring in evolutionary time.

Jump and Diffusion vs. Secular Most colonization's involve Jump dispersal first, followed by Diffusion. Secular diffusion occurs over geologic time. Although the geographic range is expanding, natural selection is causing migrants to diverge from the ancestral population. –Not of immediate interest for ecologists working in ecological time

Simple Diffusion Mathematically: Distance moved = Dn  log e R 0 Where:D = average dispersal distance n = number of generations R 0 = reproductive rate per generation Can this explain the spread of oak trees since the last ice age?

Oak Tree Dispersal Following Last Ice Age: Distance moved = Dn  log e R 0 If a tree produces 10 7 (R 0 )seeds per generation for 300 (n) generations and seeds disperse 30 m(D) with each generation then: Dispersal distance = (30m)(300yr)(  log e (10 7 seeds/yr) = 36 km However, actual migration distance since the last ice age is about 1,000 km – known as Reid’s paradox.

Measuring Tree Seed Dispersal Seed traps can be use to determine seed dispersal Distances are too small to account for dispersal of trees after the ice age. Answer to Reid’s paradox seems to lie in haphazard, long range dispersal of seeds. Colonization not driven by mean seed dispersal, but extreme dispersal events. Wind, animals, Johnny Appleseed!

Dispersal Can Be Affected by Barriers Freshwater organisms are prevented from dispersing by land and saltwater –Local populations strongly affected by jump dispersal Water can be a barrier to some terrestrial animals –Ruffed Grouse found only on three Michigan islands of the great lakes, all within 800 m of the mainland –Palmer (1962) showed that these birds could not fly for more than 800 m – can’t colonize far islands by jump dispersal –Artificial stockings have been successful –Isn’t it ironic that immobile trees colonized offshore islands that flight capable birds didn’t!

Is Dispersal The Only Limit To Distribution? Humans have moved many species around the globe – often with disastrous consequences (think locally: nutria, hydrilla and water hyacinth). –Humans have allowed several species to bypass traditional geographic barriers However, many times it is not just inaccessibility that determines whether or not a species is found in a particular habitat.

Terms for Introduced Nonnative Species StatesTransitionDefinition Imported Escaping Brought into the Country Transition from imported to introduced Introduced Establishing Found in the wild; feral Transition from introduced to established Established Becoming a Pest Has a self-sustaining population Transition from established to pest PestHas a negative economic (ecological?) impact

How Successful Are Introductions? -actually, they are usually failures Overall, continental bird introductions are successful about 10 – 30% of the time.

Statistical Generalizations: Tens Rule Williams and Fitter (1996) predicted that –1 species in 10 imported to a country becomes introduced –1 in 10 of introduced species becomes established –1 in 10 of the established species becomes a pest Of course exceptions occur –Much of Hawaii has been cleared, so habitats have become unsuitable to native birds but not introduced ones

Local Scale Dispersal Transport is rarely a limiting factor in plant dispersal –Seeds/spores carried primarily by wind or animals –Rumex crispus var. littoreus not limited by dispersion or seed-germination –Wind dispersed seeds and spores usually colonize disturbed areas first Small animal are often dispersed by wind –Spiders –Mosquitos Salt marsh mosquitoes from LA found km offshore!

Colonization and Extinction: Krakatau August 26, 1883 – Krakatau exploded (25 km 3 ) Two islands a few kilometers away were completely covered in ash The nearest island not destroyed by the explosion was 40 kilometers away Time Since Explosion Species found 9 monthsSingle Spider 3 yearsBlue-green algae; 11 fern species; 15 flowering plant species 10 yearsCoconut trees growing 25 years263 species of animals; dense forest

Groups Colonizing Krakatau Birds dependent on plant colonization Most plants and animals probably colonized by wind Large vertebrates may have arrived by floating on driftwood rafts or possibly swam.

Continental Drift – Disjunct Distributions Dispersal before or after barriers were formed? Continental drift takes some continents farther apart, while bringing others closer together. Antarctic beech modern distribution

Explanation Of Disjunct Distributions Dispersal explanations – assumes organism dispersed across preexisting barriers (e.g., mountains and rivers). Vicariance explanantions – assume that a species was present on the entire area and subsequently was fragmented by the formation of barriers.

Why Disperse? Pro: Natural selection will favor those that disperse from a crowded area to an ‘empty’ area. Con: Most individuals that disperse die. Two choices: Stay at home and produce a few descendants or take a chance to colonize a new area and leave many descendants.

Examples of Dispersal Abandonment Flightless birds on islands Insect Species –Subantarctic islands ~ 76% of the insects are flightless –Ecologogical islands (alpine zone of tropical mountains)

Fugitive Species Devote most of their effort to dispersal ‘Weeds’ of plant (dandelion) and animal (water boatman) kingdoms –Colonize temporary habitats –Grow almost predominately on disturbed areas Species of water boatman No. immigrants to temp. pond % of total numbers collected in temporary habitats Corixa nigrolineata Pond sp Corixa falleni Large lake species 11 23

Summary Three methods of dispersal: diffusion, jump, secular Transplant experiment – inaccessibility Ten’s rule (most introduced sp. die out) Dispersal rarely limits local distribution of plants and animals Dispersal is adaptive if it allows successful colonization –Fugitive species