Populations And Communities

Populations Definition: A group of individuals of the same species, living in a shared space at a specific point in time. The Gaspésie Caribou Herd

EXAMPLE - 2

Population Size Definition: It refers to the number of individuals in a population. FACTORS 1.Births 2.Deaths 3.Immigration (arrivals) 4.Emigrations (departures)

How does the populations size change?

And what if we get rid of predators?

Methods for measuring population size 1) It is sometimes possible to simply count all individuals in a populations. DIFFICULT!!!!

Methods for measuring population size 2) Counting by sample area - Counting individuals in randomly selected areas and using the following proportion. Average number of individuals per section = Population size Area of a section Total study area

quadrats

Methods for measuring population size 3) Mark and Recapture -good to study species that are very mobile and widely scattered within their habitat # of marked individuals recaptured = Number of marked individuals Total number of individuals captured Population size the 2 nd time

5 = x

Population Density Definition: The number of individuals per unit area or volume

Population Density EXAMPLE Population Density = Number of individuals Space occupied Since you were in grade 1, you have fish at home! In an aquarium, you have 5 goldfish in a 100L of water Pop density = 5 individuals = 0.05 fish per liter of water 100 L of water

Population Distribution Definition: the way in which individuals are dispersed within their habitat.

Population Distribution Individuals form groups, where conditions are better. Individuals are dispersed equally throughout the habitat because of competition. Rarely found in nature. Individuals are randomly dispersed.

ECOLOGICAL FACTORS Definition: An aspect of a habitat that can affect the organisms living there. ABIOTIC (non-living) – Physical or chemical factors affecting organisms, such as pH, or terrain. BIOTIC (living) – Biological factors affecting organisms, such as the presence of predators or disease.

LIMITING FACTORS Definition: an ecological factor that causes the density of a population to decrease. EXAMPLE: If there is too much water in the ground, cactus roots rot and the cactus populations decreases. The limiting factor is the amount of water.

BIOLOGICAL CYCLES IN POPULATIONS It is composed of alternating periods of rise and fall in population size. These periods are of fixed duration and are repeated continually. For example: Lynx and hare populations cycle over 10 year periods.

BIODIVERSITY Definition: It describes the variety of species living in a community. Ex: Sri Lanka: hot spot!

How do we measure biodiversity? 2 FACTORS 1)The number of species 2)Relative abundance: the number of individuals of a particular species in relation to the total number of individuals in the community.

Let’s compare both of these forests! 1) Both forest have 4 species of trees. Species richness is the same! BUT! 2) In community 2, most trees are the same (tree A). In community 1, the relative abundance of each species is the same, 25%.

WHEN IS BIODIVERSITY HIGH? 1)The number of species is high 2)The relative abundance of each species is similar

Interactions Between Individuals in a Community Individuals establish relationships. For example: Lions eat antelopes!

COMPETITION Definition: An interaction between living organisms that seek access to the same resource in their habitat. (such as food) Intraspecific (same species) Interspecific (different species)

Example – Intraspecific Competition What are cactus species competing for?

PREDATION Definition: Interaction between two living organisms in which one feeds on the other. MOUSE GREAT GREY OWL

MUTUALISM Definition: An interaction between two living organisms that benefits both organisms. The clown fish is protected by the anemone, but also serve as bait for the anemone! (anemones eat fish)

EXAMPLE OF MUTUALISM: HONEY BEES How do these two species benefit from one another?

COMMENSALISM Definition: The interaction between two living organisms in which one organism benefits from the relationship, while the other remains unaffected. This warbler’s nest does not harm the pine tree. The birds benefit from the tree, but the tree gains nothing.

EXAMPLE of COMMENSALISM Barnacles grow on whales, but whale don’t gain anything from these small molluscs.