C. FACTORS AFFECTING POPULATION CHANGE 1. Density Dependent Factors – factors that have a greater effect when the pop density is greater

a. Intraspecific competition - individuals compete for nest space, food, light, etc, with others of the same species - eg. high density trees – lots of shade, little water, few nutrients per tree

b. Predation from a predator viewpoint: - more prey / predator -- predator has more food less prey / predator -- predator has more intraspecific competition Lots of prey, so you get to eat what you kill. Prey is scarce so lots of fights and less time to hunt.

from a prey view point: - more prey / predator – prey has less chance of being eaten - less prey / predator – prey has more chance of being eaten With lots to choose from, the predator is unlikely to pick ‘you’! With no choice, you are ‘lunch’!

c. Allee effect reproduction fails to equal mortality due to low density eg. can not find a mate eg. species that rely on ‘overwhelming predator’ can’t do so when numbers low Boat engine noises prevent whales from finding mates by call Passenger pigeons were so numerous that millions of deaths were not a problem, now extinct!

d. Minimum viable populations size - enough individuals so that the pop can cope with variations in births & deaths eg with human help – 23 whooping cranes was enough eg in wild – 200 elephant seals was enough for many species – are needed for this

2. Density Independent Factors - the number or density of individuals does not affect the outcome a. Limiting factor – one part of the life cycle that is limited and can not be compensated for by surpluses elsewhere

eg. if seeds need 3 days of frost to scarify them and we have a warm winter with no frost, none grow even if it is a good Spring! The rows missing were not scarified.

eg. DDT makes hawk egg shells thinner, regardless of how many hawks are in an area or how many good nest sites & how much food! DDT tainted eggs crushed when parent attempted to incubate

do Questions on handout Part A pg 8