Biotic & Abiotic Influences on Ecosystems

Population Change and Stability

Populations• Recall that a population is the amount of species in an

ecosystem• What would cause the human population on Earth to

change?– Births and Deaths

• What would causes the human population of Milton to change?– Births, Deaths, Immigration, Emigration

• Earth is a closed population since organisms cannot enter or leave

• Milton is an open population since they can

Limits on Populations

• There are natural limits to the size of populations in an ecosystem

• For example, Milpond might have a population of a few hundred fish, but Lake Ontario would have many thousands - Why?

• Limiting Factors!– A factor that puts an upper limit on the size of a

population

– Amount of food, space, access to water, …

Abiotic Factors

• Every species can survive within a certain range of abiotic factors – tolerance range– Example: A pike needs water between 4oC and 25oC –

outside of that range, they cannot survive

• Within a tolerance range is an optimal range – the range in which the species is best adapted to survive

• Each species has a tolerance and optimal range for each abiotic factor

• Species with a large tolerance range tend to be widespread

Tolerance and Optimum Range

• See Also Figure 1, Pg 52

Key Abiotic Factors in Terrestrial Ecosystems

• Light availability

• Water availability

• Nutrient availability

• Temperature

Key Abiotic Factors in Aquatic Ecosystems

• Light availability• Nutrient availability• Acidity

– Some species prefer acidic environments, others basic

• Temperature• Salinity

– The amount of salt in the water

Biotic Factors

• While abiotic factors determine where an organism can live, the biotic factors determine how well it can survive (population size)

• Organisms in the same ecosystem affect each other greatly

• They affect each other in 5 main ways

Competition

• Two species compete for access to the same resource

• Foxes and coyotes feed on the same prey

• Humans and wolves feed on the same livestock

Predation

• One species feeds on another

• In general,– If prey goes up, predator will go up soon after– If predator goes up, prey will soon go down

• If no or few predators exist, the population will grow out of control (Deer in New York State)

Mutualism

• Two species working together and both benefiting

• Nitrogen-Fixing bacteria (recall yesterday!)

• Studying with a friend for a test

Parasitism

• One species lives on (or in) another and feeds on it (the host)

• Tapeworms live in the digestive tract of animals and absorb much of the nutrients

• Ticks burrow under the skin of animals

Commensalism

• One species benefits while the other is neither helped nor harmed

• Barnacles live on large whales but don’t hurt them

• Birds nest it trees

Carrying Capacity

• The population of a species naturally varies over time– If the weather is unseasonably warm, the population of

mice may increase, but as their food supply is used up, the population will decline again

• But, communities tend towards stability– When they are stable, they are said to be in equilibrium

• The maximum number of individuals of a species that can be supported indefinitely by an ecosystem is called its carrying capacity

Carrying Capacity Con’t

• The carrying capacity is determined by the limiting factors in an ecosystem

• An organism can exceed its carrying capacity temporarily, but will eventually return to it

Homework

• Study for Friday’s Quiz– Everything up to and including tomorrow– (Chapter 2)

• Pg 55 #1 – 4, 8