Population and Community Ecology

New England Forests Come Full Circle Deforestation peaked in the 1800s due to agriculture. Plants started to return such as goldenrod. Eventually they were replaced with tree species. Species interactions can alter species abundance. Populations can increase and decrease dramatically over time. Human activity can alter the distribution and diversity of species within an ecosystem

Levels of Complexity in Nature Species: most specific. Population: same species found in the same area at the same time (can evolve). Community: All the populations of organisms in the same area. Ecosystem: all living and non-living components within an area. Biosphere: where live can exisit.

Ecosystem: all living and non-living components within an area. Biosphere: where live can exist.

Population Ecology Population Ecology: the study of factors that cause populations to increase or decrease.

Population Characteristics Population Size (N): total number of individuals within a defined area in a given time. Population Density: number of individuals per unit area.

Population Distribution: how individuals are distributed with respect to one another.

Population Sex Ratio: the ratio between males and females. Population Age Structure: how many individuals fit into particular age categories.

Factors That Influence Population Size Density –Dependent Factors: influence an individual’s probability of survival and reproduction in a manner that depends on the size of the population. Example: food available

Limiting Resource: a resource that a population cannot live without and which occurs in quantities lower than the population would require to increase in size. Carrying Capacity (K): the limit of how many organisms an environment can contain.

Density-Independent Factors: Have the same effect on an individual’s probability of survival and amount of reproduction at any population size. Examples: hurricanes, floods, fires, etc.

Growth Models Represent Population Changes Exponential Growth Model When populations are not limited by resources, their growth can be very rapid. Creates a “J” shape curve. Illustrates density independent growth pattern.

The Logistic Growth Model Initial growth is exponential but slows as the population approaches the carrying capacity of the environment (K). Creates an “S” shaped curve. Density dependent type of growth.

Variations Overshoots Die offs

Hare and Lynx Cyclic pattern The lynx population peaks 1 or 2 years after the hare population peaks. Isle Royale, Michigan Wolves and moose Wolf population declined drastically due to a virus. Overpopulation of moose then they ran out of food and their population declined.

Reproductive Strategies and Survivorship Curves K-Selected Species (large mammals, most birds) Few number of offspring Long life span Long time to reproductive age Size of offspring is large Parental care present Slow population growth rate Density dependent Stable, near carrying capacity

r -Selected Species (mice, fishes, insects, weeds) Short life span Short time to reproductive maturity Many reproductive events Many offspring Small size of offspring No parental care Fast population growth rate Density independent growth Variable population dynamics

Survivorship Curves Type 1: K-selected species. High survival rates throughout most of their lives. Die off as they get old (humans, whales, elephants). Type 3: R-selected species. Low survivorship at younger age, few reach adulthood (mice, weeds, frogs, fungi). Type 2: Constant decline in survivorship throughout their lives (squirrels, corals).

Metapopulations A group of spatially distinct populations that are connected by occasional movements of individuals between them. Corridors: strip of habitat in which organisms can travel to one population to another. Growing due to human intervention. Maintain corridors is an important part to preserving biodiversity

Example: Cougars (mountain lions, pumas) Once lived throughout North America. Population decreased due to habitat destruction and overhunting. Now found in remote mountain ranges. Each area is considered a population. Corridors provide “connectedness”. Important part of each population’s persistence. Small populations subject to extinction and density independent factors (winters, hurricanes).

Community Ecology: Interactions Between Species Competition Predation Symbiotic Relationships

Competition The struggle of individuals to obtain a limiting resource. Competitive Exclusion Principal Two species competing for the same limiting resource cannot coexist. Resource Partitioning Two species divide a resource based on differences in the species’ behavior or morphology.

3 Types of Resource Partitioning Temporal Spatial Morphological

Predation The use of one species as a resource of another species.

True Predators Kill their prey and consume it.

Herbivores Consume plants as prey. Typically do not kill the plant.

Symbiotic Relationship A close relationship between two species.

1. Parasites Live on or in the organism they consume, the host. Rarely kill the host. Parasites that cause disease are called pathogens. Pathogens include viruses, bacteria, fungi, protists and wormlike organisms called helminths.

Parasitoids Lay eggs inside other organisms. Wasps and flies.

2. Mutualism Both species benefit. Examples: Plants and pollinators. Acacia trees and ants. Algae and fungus (lichen). Algae and coral.

3. Commensalism One species benefits and the other is neither harmed nor helped. Examples: Barnacles on whales. Birds perching on trees. Fish using coral reefs in order to hide from predators.

Keystone Species Plays a role in its community that is far more important than its relative abundance might suggest. The most abundant species or the major energy producers are NOT keystone species. They typically exist in LOW numbers. Example: Sea stars and mussels. Example of predator-mediated competition.

Ecosystem Engineers Create or maintain a habitat for other species. Example: Beaver

Changes in an Ecosystem Ecological Succession: the predictable replacement of one group of species by another group of species over time.

Primary Succession Occurs on surfaces devoid of soil. Results in trees and shrubs. Bare rock is colonized by lichens and mosses-- They break down the rock with acids--- The rock turns into soil---Grasses and shrubs colonize. Usually occurs on lava!

Secondary Succession Happens in disturbed areas that have not lost their soil. Happens after an event that removes vegetation but not the soil. Pioneer Species: able to colonize new areas rapidly and grow well in full sunshine. Ends with a climax stage or climax forest.

Aquatic Succession Happens near the ocean shore or in ponds or small lakes.

Species Richness of a Community is Influenced by Many Factors 1. Latitude Declines as we move from the equator toward the North or South Pole. 2. Time The longer it has been around the more opportunity for speciation.

Habitat Size and Distance: The Theory of Island Biogeography 3. Larger habitats typically contain more species. 4. The closer “islands” are to other natural areas the greater the species richness. Islands can be considered on land masses as well such as National Parks!