Ecological Dispersal: Patterns and Impacts

CHAPTER 3Factors Affecting Distribution and Abundance :1)DISPERSAL

Learning Outcome • To define and describe the concepts of dispersal • To describe the colonization process • To acquire knowledge on fugitive species • To explain the Lidicker’s Hypothesize - pre-saturation and saturation dispersal

Dispersal • Dispersal = Movement of individuals away from their place of birth • Ecological process that affects distribution • Disperse and colonize new area • Dispersal into or out of some area • Increase/decrease population density • Expanding population/response to climate change/changing in food supply

If dispersal is successful it results in gene flow (Darwinian fitness) • If colonization is successful, dispersal will result in gene flow and thus affect the genetic structure of a population.

Example of dispersal: Gypsy Moth (Lymantria dispar) • an introduced sp. - silk industry (purposely introduced) • Purpose: to make a caterpillar hybrid, that could resist diseases. • Escaped - found suitable habitat - multiply • Negative impact: initially in Europe only but now in US – destroy timber trees. • Overcome the spread: use of pheromone traps

Example of dispersal : Chestnut blight fungus (Cryphonectria parasitica) • Attack American chestnut tree (Castanea dentate) • Eliminated chestnut in 40 years only. The fungus kills only the above ground biomass - roots of trunks killed after 60 years still send out shoots. • Resulted in replacement by codominant trees such as oak, beech, red maple

Example of Dispersal: Sea Otter (Enhydra lutris ) - Hunted to near extinction by fur traders. • Protective laws since 1914: Range of the otters widened • Other example of Dispersal : African honey bee & zebra mussel

Modes of Dispersal • Diffusion • Jump Dispersal • Secular Dispersal

Modes of Dispersal 1) Diffusion • gradual/slow spread or movement of populations across hospitable terrains for a period of several generations • Sea otter, Gypsy moth & Chestnut blight

Modes of Dispersal 2) Jump Dispersal • quick movement of individual organisms across large distances - successful establishment in the new area • occurs in a short time during the life span • usually across unsuitable terrain • E.g. island colonization – Bird Bonasa umbellus, African Honey Bees and Fire Ant

Modes of Dispersal 3) Secular Dispersal • Occurs over geologic time. • Although the geographic range is expanding, natural selection is causing migrants to diverge from the ancestral population.

All forms of dispersal are affected by barriers such as……..

Water as barrier • Example of barriers in jump dispersal – water as a barrier for birds • Islands closer to mainland are colonized but further islands are not. • The ruffed grouse (Bonasa umbellus) are found on islands that are 800 m or closer to the mainland shore of the great lakes in US because this birds cannot fly for more than 800m.

Impact on native species : • dispersal may not limit distribution because introduced sp may not be able to survive local environments • Only 4 out of 50 introduced bird species have survived in US • Only 13 out of 85 introduced bird sp were successful in Europe • Only 15 out of 50 bird sp. introduced in Sydney survived • Generally the survival of introduced sp. is about 10 – 30% • 1 in 10 imported species to a country becomes introduced, 1 in 10 of introduced species becomes established, 1 in 10 of the established species becomes a pest

Dispersal : Who? • Juvenile – to find or colonize new areas • Adult - Escape from deteriorating situation Why? • Natural selection will favor individuals that leave a relatively crowded habitat and colonize an empty one in which it can leave many descendents – to maximize fitness • Dispersers increase chances of encountering new individuals to mate, thus reduces in-breeding, and increases fitness of offspring due to heterogeneity

When do they disperse? • the pre-reproductive period • when population increases How far? • Depends on density of surrounding population and availability of suitable unoccupied habitats (new area) The rule: • move to the first uncontested (open) available site • disperse in a straight line from their natal area or make exploratory surveys • settle in the first empty site

Monk parakeets • move to the first uncontested (open) available site • disperse in a straight line from their natal area or make exploratory surveys • settle in the first empty site Red squirrels

Nearly all studies show very local dispersal • Most tree seeds fall within a few meters of parent. • Only a few seeds make it to further distance.

Deermouse Expected dispersal Observed dispersal White crowned sparrow Expected dispersal Observed dispersal Proportion of the dispersing

What are the motivation to disperse? • Intra-specific competition – population increase, aggression increase, resources are less, breeding partners decreases • Aggressive individual force subdominants to move • Dispersal usually occurs before peak density of populations/ before reproductive period • Asocial members disperse – have no ties.

2 options : • Stay at home and be happy – living in crowded conditions (very dense, scarce food stock, low quality of habitat, less individual to mate - leave few offsprings. OR • Disperse & take a chance – find a new place – colonize it – and leave many descendents (good habitat, ample food resources, meeting new individuals to mate, reduces inbreeding - increases fitness of offspring due to heterogeneity.

Colonization • Colonization occurs whenever any one or more species populate an area

Colonization E.g Krakatau Explosion • 1883 Krakatau erupted and caused total extinction of all species on the island and the remaining land became sterile. • 2 nearby islands were completely covered by ash and this caused total extinction of species. • Nearest island not destroyed was 40 km away.

….one or more species populate an area Colonization process…

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.

Colonization milestone • Where there is vacant space organisms will find it – mostly by chance (exploratory survey) and adaptive ability

Fugitive Species • Devote most of their effort to dispersal (extreme disperser) • ‘Weeds’ of plant (dandelion) and animal (water boatman) kingdoms • Colonize temporary habitats. • Grow almost predominately on disturbed areas.

Characteristics of fugitive species: - Colonize temporary habitats. • Reproduce and leave before the habitat disappear or competition with other organisms increases. • These fugitive sp usually found in disturbed habitats. • They produce large number of seeds. • Adapted for long distance dispersal by wind or animals – Taraxacum officinale a fugitive weedy sp.

Dispersal is advantageous when habitats are patchy and unstable. • Organisms living in stable or uniform habitats also disperse as long as there are potential sites away from parents • Species that live in more permanent habitats generally disperse less.

Lidicker’s (1975)* Hypothesis • Presaturation Dispersal • During the increase phase of popbefore reaching peak/maximum or before resources get depleted • Dispersers are in good condition and are of any sex or age group- enough energy/fit • Have a good chance of survival and settling into a new area *Lidicker, W.Z (1975). The role of dispersal in the demography of small mammals. In: F.B. Golley, E. Petrusewicz & L. Ryszkowski (eds.). Small Mammals, Their Productivity and Population Dynamics. Cam bridge Univ. Press, Cambridge, England. p. 103-128.

Lidicker’s Hypothesis • Saturation dispersal • When population reach/exceed its carrying capacity • Mostly juveniles • Have 2 choices • Stay, perish and do not breed • Or leave • Survival is low (less energy/ unfit) • Not in good condition.

Conclusion • Dispersal ability has been molded by natural selection to maximize the chance of colonizing vacant areas • Dispersal requires: • a source • a sink (empty and unfilled, marginal and unsuitable habitat)

Ecological Dispersal: Patterns and Impacts