EN-SCI-LEC-6-POPULATION-ECOLOGY.docx

Population ecology is the study of how and why populations change **Population** Group of individuals of a single species that occupy the same general area. These individuals rely on the same resources, are influenced by the same environmental factors, and are likely to interact and breed w...

Population ecology is the study of how and why populations change **Population** Group of individuals of a single species that occupy the same general area. These individuals rely on the same resources, are influenced by the same environmental factors, and are likely to interact and breed with one another. **Population Ecology** Concerned with changes in population size and the factors that regulate populations over time. Population ecologist might use statistics (number and distribution of individuals) to describe a population. The interactions between biotic and abiotic factors that cause variation in population sizes. One important aspect of population dynamics is population growth. Births and immigration -- increases population Deaths and emigration -- decreases population Population ecologists might investigate how various environmental factors, such as availability of food, hunting by humans, or forest fires affect the size, distribution, or dynamics of the population. Data from population ecology are use to manage wildlife populations, develop sustainable fisheries, and gain insight into controlling the spread of pests and pathogens. Conservationists use these concepts to help identify and save endangered species. **Population Density** Is the number of individuals of a species per unit area or volume. The number of oak trees per square kilometer (km2) in a forest The number of earthworms per cubic meter (m3) in forest soil Ecologists use a variety of sampling techniques to estimate population densities. Total count method Sampling method= **Total Count Method** - Possible for few animals - Breeding colonies can be photographed then later counted **Examples:** - Human population census - Trees in a given area - Depends on the type of organism and its natural abundance and distribution. - **Two broad categories:** 1\. Plot-based (quadrat) methods 2\. Capture-based methods 1. **Quadrat Sampling method** Widely used in plant studies  2. **Mar-Recapture Method** Used for very mobile or elusive Species **Dispersion Pattern** - refers to the way individuals are spaced within their area. - Clumped dispersion pattern - Uniform dispersion pattern - Random dispersion pattern **Clumped Dispersion Pattern** - Individuals are grouped in patches, it is the most common in nature. - Clumping often results from an unequal distribution of resources in the environment. - Plants and fungi may be clumped in areas where soil conditions and other factors favor germination and growth. - Clumping of animals often results from uneven food distribution. - Sea stars are group together where food is abundant. Clumping may also reduce the risk of predation or be associated with social behavior.  **Uniform Dispersion Pattern** Results from interactions between the individuals of a population. Some plants secrete chemicals, inhibiting the germination and growth of nearby plants that could compete for resources. Animals may exhibit uniform dispersion as a result of territorial behavior. **Random Dispersion Pattern** Individuals in a population are spaced in an unpredictable way, without a pattern. Plants, such as dandelions, that grow from windblown seeds might be randomly dispersed. Varying habitat conditions and social interactions make random dispersion rare.  **Estimates of population density and** **dispersion patterns enable** **researchers to monitor changes in a** **population and to compare and** **contrast the growth and stability of** **populations in different areas.** **Life Tables** Track survivorship\[, the chance of an individual in a given population surviving to various ages. By identifying the most vulnerable stages of an organism's life, life table data help conservationists develop effective measures for maintaining a viable population. **Survivorship Curves** Plots of the number of survivors versus relative age. **Type I curves** -- apply to populations in which individuals are likely to live out their potential life span - Humans and many other large mammals usually produce few offspring but give them good care, increasing the likelihood that they will survive to maturity ** Type II curves** -- apply to populations in which mortality rates are constant throughout age classes - Individuals are no more or less vulnerable at one stage of the life cycle that at another - Observed in some invertebrates, lizards, and rodents **Type III curves** -- apply to populations in which mortality rates are the highest for the youngest cohorts - Species with this type of survivorship curve usually produce very large numbers of offspring but provide little or no care for them. - Fishes, many invertebrates such as clams **N = (B + I) -- (D + E)** **Where:** ** N** is the change in population ** B** is the number of births ** I** is the number of immigrants, or people who have moved into the area. ** D** is the number of deaths ** E** is the number of emigrants, or the number of people who have moved out the area. EXAMPLE: Let\'s take a look at an example population. Say you want to know the change in population of your town over the past two years. Your town currently has 20,000 people, but you don\'t know how many people it had two years ago. However, town records show the number of births, deaths, immigrations, and emigrations. Those numbers are listed below: 1\. There were 2,000 births. 2\. There were 700 new residents who immigrated. 3\. There were 1,500 deaths. 4\. There were 800 people who moved away. Taking this information and plugging it into the formula gives you this: N = (2,000 + 700) -- (1,500 + 800) Now that you have the information and the formula, all that\'s left is to solve the problem. N = 2,700 -- 2,300 N = 400 Over the last two years, the population of your town has increased by 400 people. That means that two years ago, the population was 19,600 20,000 -- 400 = 19, 600 The rate of population increase under ideal conditions - exponential growth. Can be calculated using the simple equation **G =rN** Where: G = growth rate of population (the number of new individuals added per time interval) N = is the population size (the number of individuals in the population at a particular time) r = per capita of increase (the average contribution of each individual to population growth for the time interval; per capita means "per person") **How do we estimate the per capita rate of increase?** Population Growth = number of births -- number of deaths The model assumes that immigration and emigration are equal A population of rabbits has 100 individuals, and there are 50 births and 20 deaths in one month. Population Growth = 50 -- 20 **G=rN\ ** 30 = r100 100 = 100 r= 30/100 =0.3 The per capita increase in the population is 0.3, for the month. In a population growing in an ideal environment with unlimited space and resources, r is the maximum capacity of members of that population to reproduce. Thus, the value of r depends on the kind of organism. Rabbits have a higher r than elephants Bacteria have a higher r than rabbits When a population is expanding without limits, r remains constant and the rate of population growth depends on the number of individuals already in the population (N).  Notice that the larger the population size, the more new individuals are added during each time interval. The exponential growth model gives an idealized picture of unlimited population growth. The population grows extremely rapid and at a constant rate; if a population has a constant birth rate through time and is never limited by food or disease Even elephants, the slowest breeders on the planet, would increase exponentially if enough resources were available. In nature, a population that is introduced to a new environment or is rebounding from a catastrophic decline in numbers may grow exponentially for a while. However, one or more environmental factors will limit its growth rate as the population reaches its maximum sustainable size. **LIMITING FACTOR** -- environmental factors that restrict population growth rate **LOGISTIC GROWTH** -- a description of idealized population growth that is slowed by limiting factor as the population size increases **CARRYING CAPACITY** -- the maximum population size that a particular environment can sustain To model logistic growth, the formula for exponential growth, rN, is multiplied by an expression that describes the effect of limiting factors on an increasing population size: **G= rN [(K-N)]** **K\ ** Where: G = growth rate of population (the number of new individuals added per time interval) N = is the population size (the number of individuals in the population at a particular time) r = per capita of increase (the average contribution of each individual to population growth for the time interval; per capita means "per person") K = carrying capacity The value of K varies, depending on the species and the resources available in the habitat. Even in one location, K is not a fixed number. Organisms interact with other organisms in their communities, including predators, parasites, and food resources, that may affect K. Changes in abiotic factors may also increase or decrease carrying capacity. The concept of carrying capacity expresses an essential fact of nature: Resources are finite!  **DENSITY-DEPENDENT FACTORS** Limiting factors whose intensity is related to population density. Appear to restrict growth in natural populations. **Intraspecific Competition** -- competition between individuals of the same species for limited resources As a limited food supply is divided among more and more individuals, birth rates may decline because individuals have less energy available for reproduction Plants that grow close together may experience increased mortality as competition for resources increases **Availability Of Space** The number of nesting sites on rocky islands may limit the population size of sea-birds The number of safe hiding places may limit a prey population by exposing some individuals to a greater risk of Predation **DENSITY-INDEPENDENT FACTORS** A population-limiting factor whose intensity is unrelated to population density. Affected by abiotic factors such as weather, climate, and disturbances.  **LIFE HISTORY** The traits that affect an organism's schedule of reproduction and death. Some key life history traits: Age of first reproduction Frequency of reproduction Number of offspring The amount of parental care given Life history pattern: r-selection K-selection - Natural selection cannot optimize all these traits simultaneously because an organism has limited time, energy, and nutrients Example: An organism that gives birth to a large number of offspring will not be able to provide great parental care **R-Selection** Typified by small-bodied, short-lived animals (insects and small rodents) that develop and reach sexual maturity rapidly, have a large number of offspring, and offer little or no parental care. Selection for this set of life history traits occur in environments where resources are abundant, permitting exponential growth. ** r-selection** -- r (the per capita rate of increase) is maximized Most r-selected species have an advantage in habitats that experience unpredictable disturbances (fire, floods, hurricanes, drought, or cold weather) which create new opportunities by suddenly reducing a population to low levels. Human activity is a major cause of disturbance, producing road cuffs, freshly cleared fields and woodlots, and poorly maintained lawns that are commonly colonized by r-selected plants and animals. **K-Selection** Large-bodied, long-lived animals (bears and elephants) develop slowly and produce few, but well-cared-for, offspring. Plants with comparable life history traits include coconut palms, which produce relatively few seeds that are well stocked with nutrient-rich material -- the plants version of parental care. Selection for this set of life history traits occurs in environments where the population size is near carrying capacity (K). Population growth in these situations is limited by density-dependent factors. Because competition for resources is keen, K-selected organisms gain an advantage by allocating energy to their own survival and to the survival of their descendants. K-selected organisms are adapted to environments that typically have stable climate and little opportunity for rapid population growth. **The human population continues to increase, but the growth rate is slowing** The human population grew rapidly during the 20th century and currently stands at more than 7.5 billion. **DEMOGRAPHIC TRANSITION** -- the shift from high birth rate and death rate to low birth and death rates, has lowered the rate of growth in developed countries In developing nations, death rates have dropped, but birth rates are still high. ** AGE STRUCTURE OF POPULATION** - the proportion of individuals in different age-groups, affects its future growth **POPULATION MOMENTUM** - the continued growth that occurs despite reduction of the fertility rate to replacement level and is a result of girls in the 0-14 age-group of a previously expanding population reaching their childbearing years. **Fertility Rate** - the average number of children produced by a woman over her lifetime---substantially exceeds the number of children needed to replace herself and her mate. **An ecological footprint is a measure of** **resource consumption** **ECOLOGICAL FOOTPRINT** Estimates the amount of land required by each person or country to produce all the resources it consumes (food, fuel, housing) and to absorb all its waste. **BIOCAPACITY** - the Earth's capacity to renew these resources, gives us a broad view of the sustainability of human activities **SUSTAINABILITY** - is the goal of developing, managing, and conserving Earth's resources in ways that meet the needs of people today without compromising the ability of future generations to meet theirs. The global ecological footprint already exceeds a sustainable level. There is disparity between resources consumption in more developed and less developed nations. Why it is important to know the population size or population density? Knowing population size is important in making environmental decisions that would affect the population. The changes in population can affect how the population interacts with its physical environment and with other species. By tracking populations over time, ecologists can see how these populations have changed and may be able to predict how they're likely to change in the future.