POPULATIONS RESPOND TO PRESSURES Population Growth is Limited: Birth, Death, Immigration & Emigration Limiting Factors Effects of Population Density Populations have Distinct Reproductive Survival Strategies: Reproduction is necessary for the survival of a species. Scientists observe patterns in the reproductive strategies used among species Opportunist strategies & Competitor strategies

Birth/Death/Immigration/Emmigration Population Change = (birth + immigration) – (death + emigration) Immigration= movement of individuals into a population (winds blowing plant seeds in) Emigration= movement of individuals out of a population (some leave if resources get scarce) If a storm comes, a flock of seagulls may fly into a city dump and add to the existing population. Raccoons have been eating the seagull eggs there, so the immigrating birds could help stabilize the population or increase it (especially if some of the raccoons moved away)

Limiting Factors When birth rate is much higher than death rate, population grows rapidly; eventually, population will stop growing due to lack of resources in habitat. Limiting Factor= prevents the continuing growth of a population in an ecosystem. Abiotic (non-living) factors = air, light, water. Biotic (living) factors = other organisms in same habitat or same species in same area. Competition may occur between different populations or same populations in a habitat. (Deer growth in our area is making resources scarce for entire population)

Population Density Effects Density-dependent Factor= limiting factor affecting a population when density is high (competition for food, disease-spreads easier in crowds). These have a greater effect as population grows. Density-independent Factor= limiting factor having same effect on population whether high or low density. (Freezing temp., natural disasters, famine-no food available)

Opportunist Strategies Opportunists: species that reproduce rapidly if their population falls below carrying capacity. Have: Short life span usually & ability to reproduce large quantities of offspring. Population size changes often & live across many areas. Include: algae, dandelions, bacteria, & insects. All reproduce quickly & move across area quickly. Can adapt quickly to environmental changes.

Competitor Strategies Competitor: species with adaptations that allow them to remain at or near their carrying capacity for long period of time (different from other competitor who struggles with another to get resources). Often live longer & have fewer offspring (elephants & saguaro cacti). Offspring usually take longer to develop into adults so are cared for by parent longer. Not spread out over wide area as much, but more of offspring survive to reproductive age.