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 4 o C and 25 o C – 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