Lesson 6: Properties of Population PDF

PROPERTIES OF POPULATION Mark Joseph P. Jaylo, MSc Science Department Mindanao State University – General Santos As an individual, how do you perceive the world? We rarely consider individuals as part of a larger unit – a population. Population is a group of individuals of the s...

PROPERTIES OF POPULATION Mark Joseph P. Jaylo, MSc Science Department Mindanao State University – General Santos As an individual, how do you perceive the world? We rarely consider individuals as part of a larger unit – a population. Population is a group of individuals of the same species that inhabit a given area. 1 A population should have individuals if the same species (which has a potential of interbreeding among members of the population). The population is a genetic unit (the gene pool) and the focus of evolution. 2 The population is a spatial concept, requiring a defined spatial boundary. For example, the population of ground finch inhabiting the Island of Daphne Major in the Galapagos Islands. Populations have structure, which relates to characteristics of the collective, such as density, proportions of individuals in various age classes, and spacing of individuals relative to each other. Population also exhibit dynamics – a pattern of continuous change through time that results from the birth, death, and movement of individuals. Organisms May Be Unitary or Modular The Distribution of a Population Defines Its Spatial Location Distribution is based on the presence and absence of individuals in a specific location. The blue line define the population distribution – a spatial boundary within which all individuals in the population reside. The distribution describes the population’s geographic range when the defined area encompasses all the individuals of species. Habitat suitability is the primary factor influencing the distribution of a population. Each species has a range of abiotic environmental and resource conditions under which it can survive, grow, and reproduce. For example, red maple (Acer rubrum) is the most widespread of all deciduous trees of eastern North America. A species with a geographically widespread distribution, such as red maple, is referred to as ubiquitous. A species with a distribution that is restricted to a particular locality or localized habitat is referred to as endemic. Geographic barriers is another factor influencing the distribution of a population. The red maple is restricted in its ability to colonize areas in main land Florida, Gulf of Mexico, and Atlantic coastline. Mountain ranges or extensive areas of unsuitable habitat, may also restrict the spread and geographic range of species. We can describe distribution of a population at various spatial scales. Most populations are also divided intro subpopulations, due to the result of environmental heterogeneity. Ecologists refer to the collective of local subpopulations as a metapopulation. Abundance Reflects Population Density and Distribution Abundance is the size – the numbers of individuals in the population. Abundance is a function of two factors: Population density – number of individuals per unit area or per unit volume. The density can be calculated by counting the number of red dots that fall within its boundary. Density measured simply as the number of individuals per unit area is referred to as crude density. However, crude density can be misleading, why? ❑ Not all individuals are uniformly distributed across the landscape. ❑ Individuals can be clumped around resources ❑ It can also be randomly scattered by other forces Individuals of population may be distributed randomly, uniformly, or in clumps (aggregated). It can be randomly distributed if each individual’s position is independent of those of the others. Individuals distributed uniformly are more or less evenly spaced. Uniform distribution usually results from some form of negative interaction among individuals, such as competition, to maintain minimum distance among members of the population. Clumped is the most common spatial distribution, in which individuals occur in groups. The clumping is a result of variety of factors, such as suitable habitat, resources, species social groups, geography, and others. As with geographic distribution, the spatial distribution of individuals within the population can also be described at multiple scales. To account patchiness, ecologists often refer to ecological density, which is the number of individuals per unit of suitable habitat. Determining Density Requires Sampling Population size (abundance) is a function of population density and the area that is occupied (geographic distribution). Population size = density x area. How is density determined? When both the distribution and abundance are small (in the case of rare and endangered species), a complete count may be possible. In some habitats that are unusually open, density may be determined by a direct count of individuals. ❑ Quadrat Sampling Create a grid or squares on a landscape Count individuals in each square Mean density can be determined Multiplying the mean value by the total area provides an estimate of population size (abundance) It can be affected by distribution pattern Results of quadrat sampling should always include a confidence factor based on distribution pattern. ❑ Mark and Recapture It is based on trapping, marking, and releasing of individuals Capture and mark individuals Release marked individuals back into the population Re-capture some and count marked individuals Use ratios to determine population size Most widely used technique to estimate animal populations To compute an estimate of the population, the following relationship can be used. where: M - marked animals N – population n – population of captured individual R - recaptured The population can be solve using the this formula: This simplest method, the single mark–single recapture, is known as the Lincoln–Petersen index of relative population size. The accuracy of this methods depends on a number of assumptions. (1) The sampling is random (2) Marked individuals must be randomly distributed in the population (3) Ration of marked and unmarked must not changes between sampling periods. ❑ Incidence of Abundance Locate signs, sounds, evidence of individuals Estimate number of individuals based on evidence Frog calls, bird calls, animal tracks, scat trail cameras, etc. Measure of Population Structure Include Age, Developmental Stage, and Size The abundance describes the number of individuals in the population but provides no information on their characteristics – how individuals within the population may differ form one another. Population can be divided into three ecologically important age classes or stages: Prereproductive - individuals who are not yet able to reproduce Reproductive - individuals who are able to reproduce Postreporductive - individuals who are no longer able to reproduce Age Structure – number of proportion of individuals in different age classes in a population. Age data for wild animals can be obtained in several ways, and the method varies with the species. Age pyramid – graphical snapshot of the age structure of a population at some point of time. In many plant populations, the distribution of age classes in often highly skewed. For example: Dominant overstory trees can inhibit the establishment of seedlings and growth and survival of juvenile trees. Sex Ratios in Population May Shift with Age Population of sexually reproducing organisms in theory then toward a 1:1 ration (male to female). In most mammalian population, the secondary sex ratio (the ratio at birth) is often weighted toward males, but the population shifts toward females in the older age groups. Generally, male have shorter life span than females do. The shorter life span of male can be a result of both physiological and behavioral factors. Individuals Move within the Population ❑ Dispersal – movement of individuals in space ❑ Immigration – individuals enter a population ❑ Emigration – individuals leave a population Many plants depend on passive means of dispersal involving gravity, wind, water, and animals. ❑ Migration – round-trip that occurs at a regular time interval Zooplankton in the oceans, for example, move down to lower depths by day and move up to the surface by night. Bats leave their daytime roosting places in caves and trees, travel to their feeding grounds, and return by daybreak. Invasive Species: Threats to Native Wildlife An invasive species can be any kind of living organism—an amphibian (like the cane toad), plant, insect, fish, fungus, bacteria, or even an organism’s seeds or eggs—that is not native to an ecosystem and causes harm. They are among the leading threats to native wildlife. Approximately 42 percent of threatened or endangered species are at risk due to invasive species. It can be introduced accidentally or intentionally by humans. They can spread through ship ballast water, accidental release, or when people travel and import or export supplies. The introduction of IAS has the following adverse impact on biodiversity: decline or elimination of native species – through competition, predation, or transmission of pathogens; disruption of local ecosystems and ecosystem functions.