Population Ecology PDF

Population Ecology  Characteristics of Populations POPULATION: A group of individuals that belong to the same species that live in the same area at the same time. These are the 1. Population Size important...

Population Ecology  Characteristics of Populations POPULATION: A group of individuals that belong to the same species that live in the same area at the same time. These are the 1. Population Size important 2. Geographic distribution 3. Density characteristics of 4. Dispersion a population. 5. Growth rate 6. Age structure  Population Size A population’s size is the number of individuals in the population Size is a very important property of a population but can be very difficult to measure A population of daisies that live in a field is easy to measure. The plants are immobile and are confined to a relatively small area. A scientist would simply count the individual plants in the field. More often, the individuals in a population are very hard to count because they are too widespread, too abundant, or too mobile to be easily counted.  For example: How big is the population of bullfrogs in Everglades National Park? (Park size is 1.5 million acres or 6,110 km2.) To determine the size of this population, scientists would have to use various methods of estimation, which have the potential for error.  Population Density Population density is: the number of individuals per unit area. Density is one of the main characteristics that describes a population. There is tremendous variation in density depending on the species and the ecosystem. Some populations have low densities, while other populations have high densities.  Density is the number of individuals in a given area When studying population density, some questions to be answered are: 1. Why is the birth rate unusually high (or low)? 2. Why are more individuals dying than normal? 3. Is there a reason for an unusually high immigration or emigration? Geographic distribution is the range of the population. This term describes the area that is inhabited by the population. The range can vary in size. - a few centimeters, ex: mold on bread - huge, ex: migration area of whales  Clumped Dispersion Dispersion is the spatial distribution of individuals within the population. The three types of dispersion are: Random Dispersion Uniform or Even Dispersion  Clumped Dispersion a) In clumped dispersion the individuals are clustered together. b) Clumped distributions often occur when resources such as food, water, or living spaces are clumped together. c) Clumped distributions may also occur because a species has: a certain social behavior, such as herding animals, flocks of birds, schools of fish or hives of bees.  Uniform or Even Dispersion These penguins exhibit even dispersion because of aggressive interactions that occur between neighbors. a) In an even dispersion, b) Even distributions are usually the individuals are separated result of social interactions c) but the interaction results in the by a fairly even distance. individuals trying to get as far away from each other as possible.  Random Dispersion a) In random dispersion, the location of one individual is independent of the location of the other individuals. b)Random dispersal may result from seed dispersal by the wind or animals. c) A forest or a field of wildflowers results from the random dispersal of seeds.  Population Dynamics All populations are dynamic, meaning that they change in size and composition over time. Natality: Increases to population size through reproduction (births) Immigration: Increases to population size from external populations Mortality: Decreases to population size as a result of death Emigration: Decreases to population size due to loss to external populations Life expectancy: the length of time an individual is expected to live Growth Rate: The amount by which a population’s size changes in a given time  Population Dynamics To understand the changes that are taking place in a population, all of the above, along with life expectancy, must be considered A population may remain very stable over time, or there can be fluctuations in population size.  Migration is the movement of individuals into and out of a population. Immigration - the movement of individuals into an area. Emigration - the movement of individuals out of an area. Immigration would increase the Video: Wildebeest Migration - Two Million size of the population while Move Across the Serengeti emigration would cause the population to decrease in size. Types of Population Growth  For example: A single Exponential bacterium can reproduce by dividing into two cells every 20 growth minutes. describes a At the end of the first 20 minutes, there would be two population that bacterial cells. is increasing At the end of 40 minutes, four cells will have been produced, rapidly. and by the end of the first hour, The larger the there would be eight cells. population gets, This growth doesn’t seem too impressive at first. the faster it But, if the number of cells doubles every 20 minutes, then at the end of one day the colony would contain 4.72 x 1021 grows. cells (4,720,000,000,000,000,000,000 cells!)  Exponential Growth Under ideal conditions, with unlimited resources, a population will grow exponentially. At first, the population seems to be growing slowly, but over time, the power of exponential growth can produce a population of extremely large size. Exponential Growth occurs when the members of the population are reproducing at a constant rate.  Exponential Growth Exponential population growth will occur in an ideal environment where resources are unlimited In this environment there is no competition to place limits rate of growth Initially population growth will be slow as there is a shortage of reproducing individuals that may be widely dispersed As population increases, the number of reproducing individuals increases, so growth rate increases, resulting in an exponential (J-shaped) curve This maximal growth rate for a given population is known as its biotic potential Exponential growth can be seen in populations that are very small or in regions that are newly colonised by a species Logistic Growth A model where population growth slows or stops following a period of exponential growth A graph of logistic growth looks like a stretched out letter “S” Logistic Growth - Carrying Capacity The level where population growth slows and stops is When the population size is small, called carrying capacity birth rates are high and death rates are low, causing the population to grow nearly exponentially. When necessary environmental resources for the population become limited the birth rate equals the death rate, and growth stops.  carry:to hold or contain Logistic Growth - Carrying Capacity capacity: fullness, limit Carrying capacity - the average population size of a species that a particular habitat can support. The species population size is limited by environmental factors like adequate food, shelter, water, and mates. Logistic Growth - Carrying Capacity In reality, populations will overshoot” the environmental carrying capacity, and then “dieback” below carrying capacity. The size of the population will fluctuate above and below the carrying capacity of the environment.  Logistical Growth Logistic population growth will occur when population numbers begin to approach carrying capacity The carrying capacity is the maximum number of a species that can be sustainably supported by the environment As a population approaches the carrying capacity, environmental resistance occurs, slowing the rate of growth This results in a sigmoidal (S-shaped) growth curve that plateaus at the carrying capacity (the variable that represents carrying capacity is the letter K κ) Logistic growth will eventually be seen in any stable population occupying a fixed geographic space Assumptions of the Exponential Model and the Logistic Model One assumption made by both models is that the carrying capacity is constant and does not fluctuate. In reality, the carrying capacity changes with the environment. Ex: during a drought in a prairie, there would be less vegetation. Fewer prairie The exponential model and the dogs would survive since there is less logistic model are not accurate plant matter to eat. The carrying representations of real populations, capacity is lower than normal but they are useful tools that scientists can use to study populations.  Limits to Growth No population can undergo exponential growth forever. There are limits to how fast and how big a population can grow. A limiting factor is anything that constrains (limits) a population's size and slows or stops it from Examples of limiting factors include: 1. Competition for food, mates, space growing. 2. Predation 3. Parasitism and disease Limiting population factors can 4. Drought and other climate extremes 5. Human disturbances exert their influence via either top 6. Humidity, precipitation,temperature, down or bottom up control altitude, sunlight Top Down Control Top down factors are pressures applied by a higher trophic level to control the population dynamics of the ecosystem The top predator either suppresses the abundance of its prey or alters its behaviour to limit its rate of population growth Top down control results in an oscillating trophic cascade (suppression at one level increases numbers at the next level) Keystone species commonly exert top down control by preventing lower trophic levels from monopolising essential resources Bottom Up Control Bottom up factors are pressures that limit the availability of resources to lower trophic levels (e.g. producers) A lack of resources at lower trophic levels suppresses the abundance of organisms at higher trophic levels Population growth will be reduced for all higher levels as the suppression of the 'bottom’ restricts energy supply to the ‘top' Human activity can often limit resource availability and hence inadvertantly exert bottom up pressure on an ecosystem  A few examples of limiting factors a) Food will become more and f) There will be increased competition more scarce between the members of the group b) There may be water shortages g) The accumulation of wastes could lead to an increase in diseases and c) A disease might be introduced poor health into the population d) The population will run out of space e) Additional predators may be attracted to the rising prey population In reality, populations cannot continue to grow exponentially for very long Resources will become scarce and wastes will accumulate, limiting growth. At the same time, competition for the limited resources will intensify as the population grows. Density Independent & Dependent Factors  Density-Independent Factors The density-independent factors affect all populations in similar ways, regardless of the size of the population. These factors (fires, floods, etc.) would affect any population of any size. Examples of density-independent factors include: a) Unusual weather or natural disasters such as floods and fires. b) Certain human activities, such as the damning of a river or clear-cutting a forest. In response to density-independent factors, populations may experience a “crash ” where the population is nearly eliminated. If one population is nearly eliminated, it may also affect another population if the second population depends on the first as a food source. Ecosystems are a network of interrelated and interdependent factors.  Density-Dependent Factors A limiting factor that depends on population size is called a density -dependent limiting factor. This means that the limiting factor only becomes limiting when the population density reaches a certain level. Population density refers to the number of organisms per unit area. Density-dependent limiting factors include: ✔competition ✔predation ✔parasitism and disease ✔shortages of food ✔shortage of nesting sites  As a population becomes more Competition and more crowded, organisms will have to compete with one another for food, water, sunlight, space to grow, and nesting sites. Competition is a density-dependent factor. The more individuals that live in the population the sooner they will use up the available resources. Competition may also occur The greater the population size between populations consisting of becomes, the more intense the separate species. competition for resources.  Predation The interaction between two different organisms in which one captures and feeds on the other. Predator: In a predator-prey relationship, the predator is the organism that feeds upon the other. Prey: In a predator-prey relationship, the prey is the organism that is the food source for the other. The predator-prey relationship is one of the best known methods of controlling the size of a population. Study the graph to the right. Notice that an increase in prey population is followed by an increase in predator population. As the predator population increases, As the prey population drops, the they kill more prey, and predators have less food to eat, so the predator population declines. This the prey population relationship controls the size of each drops population.  Parasitism and Disease 1. Parasite - an organism that feeds upon another living organism. 2. Host - the organism that the parasite feeds upon. 3. This is very similar to a predator-prey relationship, and can also control the size of the population. 4. The parasite takes nourishment and nutrition from the host. The host is weakened and may eventually die. TICK  Territoriality A territory is a space that an animal defends against encroachment by other individuals, usually of its own species. The territory becomes a resource for which individuals must compete The benefit of a territory is that the “owner” of the territory has: unlimited use of the resources found there without competition from others.  Population Growth At any given time a population may experience tremendous growth or rapid decline. Can you think of some examples? Video: Elephants - Selection for Tusklessness  Population Ecology Resource Videos for this lesson: ▪ Population Ecology – Bozeman Science (12:09) ▪ R and K Selected Populations (7:07) ▪ Human Population Dynamics (11:13) ▪ Ecological Relationships & Carrying Capacity - Ameba Sisters (6:49) https://www.youtube.com/watch?v=rNjPI84sApQ ▪ Population Ecology: The Texas Mosquito Mystery - Crash Course Ecology #2 (11:52) https://www.youtube.com/watch?v=RBOsqmBQBQk ▪ Ecological Carrying Capacity https://www.youtube.com/watch?v=ozW7y-y6Ymw&t=4s