Ecology Presentation 

By Ryan, Alex and Chandler

Populations

Population: A local group of organisms of one species

Objectives

• To explain how ecologists define a population for study.

• To explain how population density is calculated.

• To describe several methods scientists use to measure population density.

Defining Populations

Populations can change dramatically in size over time• However, some are more stable.

Size is affected by:• Availability of food and space• Breeding patterns• Weather conditions

In order to study populations, ecologists determine geographic boundaries.• ie. the edges of a lake• ie. the boundaries of state

Population Density 

Population density: the amount of individuals of a particular species per unit in a specified area or volume.• Helpful measurement for comparing populations in different

locations.

Population Density = Total # of Individuals = Reduced # of Individuals                                          Total Area                        Per Unit Area

• However, every single individual member of a population must be counted, and therefore it is often ineffective.o Other methods are used to approximate the density.

Sampling Techniques Quadrats: mark off a particular area and count the number of a species with in the boundaries. • This procedure is repeated multiple times, then the average is taken

to estimate the population density.• more quadrats = more accurate.

Indirect Counting: counting the tracks, nests, burrows or other traces of the particular species, rather than counting the organisms themselves.• Good for nomadic organisms or organisms that are hard to see. 

Mark-Recapture:the process of capturing animals, marking them with dye or other means, then releasing them and capturing new animals. Total Population= # of First Capture x # of Second Capture                                     # of Marked Animals Recaptured 

Limits to Accuracy

All the methods make assumptions.• If the initial assumptions are wrong, then the final number

will be incorrect.

Some populations may not be evenly dispersed.• Quadrat Method will produce invalid results.

Previously marked animals will be more wary of traps and will avoid traps.• The Mark-Recapture Method will produce invalid results

Limits to Population Growth

Objectives 

• To describe the pattern of exponential growth.

• To relate limiting factors and carrying capacity.

• To compare and contrast density-dependent and density-independent factors.

• To identify hypothesis for the causes of population growth cycles.

Exponential Growth

Exponential Growth: When a population multiplies by a constant factor at constant time intervals.• If the population of bacteria doubles every 20 minutes, it is

undergoing exponential growth.

Carrying Capacity 

Carrying Capacity: the number of organisms in a population that the environment can maintain, or carry, with no net increase or decrease.• when a population approaches capacity, birth-rates and death-rates

approach equilibrium.Limiting Factors: a condition or factor that can cause the growth of a population to plateau. • Space• Disease• Availability of Food• Competition 

Factors Affecting Population Growth

Density-Dependent Factor: A factor that limits a population more as population density increases.• ie. Availability of food• ie. Disease

Density-Independent Factor: A factor that limits populations but is unrelated to population density.• ie. Extreme weather events• ie. Seasonal climate change

Population Growth Cycles

"Boom-and-Bust" Cycle: when a population develops a trend of growth (Boom), which is then followed by a rapid decline (Bust).Causes for These Cycles:• Changes in food supply• Environmental conditions • Hormonal Balances• Predator-prey relationship