CHANGING POPULATION SIZES Four Rates Determine Population SizeFour Rates Determine Population Size   Size of a population changes through time o o There are 4 factors that determine population  Natality = births  Mortality = deaths  Immigration = moving IN  Emigration = moving OUT

If a biologist counts 700 ponderosa pines in a hill in Colorado in 1990, but counts 500 in 2000, there was a decrease in the population Change can be expressed as a rate (rate = amount divided by certain amount of time) Ex: trees  10 years = - 20 trees per year Decrease may be due to fire, cutting, or other means Either way, decrease represents death rate (mortality rate) of pine tree population Mortality rate: death in population per unit of time Mortality rate: death in population per unit of time

Population of Ponderosa Pines (200 less)

Even though some trees die, some are starting to grow from a seed, so it will increase the population Increase represents reproduction increase called birthrate or natality Natality rate: birth in population per unit of time Natality rate: birth in population per unit of time

Death or birth rate is not only way population can change Immigration – organisms moving INTO ecosystem Emigration – organisms EXITING ecosystem If no immigration or emigration is allowed, it’s called a closed population  open population If immigration and emigration are allowed, it’s called an open population

Open System ex: Pacific Ocean (organisms are free to immigrate and emigrate to other oceans) Closed System ex: aquarium (organisms are not allowed to immigrate or emigrate [unless owner intervenes])

if add all population limiting factors together, get growth rate  mortality rate + natality rate + immigration + emigration = growth rate  ex: studying number of frogs in Balboa Lake over 1 year’s time  mortality rate = - 17  natality rate = + 26  immigration = + 7 (from people dumping them off)  emigration = - 6 (from people catching them)

growth rate = (- 17) + (+26) + (+7) + (- 6) = +10   Means we get 10 more frogs in Balboa Lake each year, which means over 10 years, 100 more frogs will be there!

Balboa Lake

Population Density May Fluctuate  In all populations, only certain amount of organisms that it can hold o Balboa Lake can’t hold 10,000,000 frogs! There wouldn’t be enough food or water to sustain them all How many frogs can it hold? This is called carrying capacity, or amount of individuals an ecosystem can hold before it crashes Like carrying books in your backpack, it can only carry so many before it breaks

o Sometimes population fluctuations occur regularly with little knowledge of why they fluctuate Lemmings (like big hamsters) increase 1,000 fold every years During big spikes in population, lemmings frequently emigrate to other areas where they can find food and shelter for survival

Succession  If certain ecosystem or environment gets destroyed by fire or flood or something else, will it ever get back to its original form? o Yes, but it takes many years and it happens in specific order Process of re-growth that follows environmental change is called succession

Two types of succession: 1. Primary succession: occurs in area where no communities ever existed before After volcano spreads lava, the area is barren and has no growth After glacier recedes (melts away), land also is barren and has never had growth  Both have NO SOIL, so must make soil before land can grow anything. It can take years to make soil BEFORE anything can grow

Causes of Primary Succession < Glacier retreats (melts) ^ Volcanic eruption

2. Secondary succession: occurs following destruction of a community After fire, is still soil to support growth, so re-growth doesn’t take very long

Communities that grow after succession happens in specific order 1. pioneer community  First plants to grow which are small shrubs and ferns that can tolerate lots of sun (no shade around) high temperatures 2. seral stages  In between communities which are medium shrubs, some trees 3. climax community  Final stage which are lots of trees, shrubs, ferns, animals

Primary Succession a.No soil, barren land b.Making soil c.Lichens d.Some shrubs e.Some trees f.Lots of trees Pictures show growth after glacier has retreated

Lichens (white) with Moss (green) Fern Shrub

Primary Succession

Secondary Succession Notice that there is soil on the ground for seeds to take hold and grow

Secondary Succession