Population regulation and distribution
Density Dependence: Magnitude of effect depends on population density. For example, the effects
of a sickness willbe morepronounced in areas with higherdensity. Sicknesswould be a density
dependent factor. More effect in areas with more density. These factors usuallyused to regulate
populations through births and deaths.
Density Independence:Effect isthe same regardlessof population density. For example, an ice age
would wipeout every species regardlessof how dense their populations are. An examplewould be a
major freeze. This isdensity independent as it affects all populations equally, but it would intensify
density dependent factors like food supply as areas with higher density willbe more affected.
Spatial: Clumped,even or random.
• Clumpeddistribution: Some sort of causation for all the species staying in this area. Could be
food, water, shelter.
Possible mechanisms for species location
1) Endemism: Evolvedhere and is only found in this region. Example: Polar bears are endemic
to Arctic regions.
• Bio geographical exampleof biotic similaritieson different continents: Not explainedby
A. Rangeexpansion: Evolved elsewhereand then expanded its range to alsoinclude this area.
Natal: Movement of young from their birthplace prior to reproducing.
Breeding:Movement of reproductivelyactive adults
B. Range Shift:Evolved elsewhereand used to be found elsewhere,but its range shifted to
include ONLY the current distribution and NOT the former distribution
C. Long-RangeDispersal : Invasivespecies usually. Arrivedfrom somewherefar away.
Wind Assisted Dispersal:Gliders,parachutes, helicopters, cottony seeds, flutters are all
structures that cause wind dispersal of seeds
Animal-assisted:Sticky top (burrs for example)to stick to animals. Some seedsor berriesget
eaten and travel in the digestivetract to be releasedelsewhere.
3) ContinentalDrift and Vicariance: Continents used to all be joinedas Pangaea. Evolved
elsewherebut physical landscape changed. Explains geographical species distributions along
Biogeography,Vicariance and Dispersal
• Species are distributed in clear patterns geographicallyand not just by habitat
• Major patterns are now known to correspond to Earth history
• Similarbiotas on very distant continents: Connected by lineknown as Wallace'sLine.