Transcript Populations
Population Dynamics UNIT II Characteristics of Populations Three important characteristics of a population are: Its geographic distribution (range) Population Density Describes the area inhabited by a population – can vary in size The number of individuals per unit area Growth rate The number of births minus the number of deaths Patterns of Dispersion Environmental and social factors influence the spacing of individuals in a population. 3 Patterns: Clumped Uniform random Survivorship Curves A life table is an age-specific summary of the survival pattern of a population Data in a life table can be represented graphically by a survival curve. Curve usually based on a standardized population of 1000 individuals and the X-axis scale is logarithmic. Type 1 Type 2 Type 3 Survivorship Curves Survivorship curves can be classified into three general types Type I, Type II, and Type III Number of survivors (log scale) 1,000 I TYPE 1: high survivorship until old age. 100 II TYPE 2: constant proportion of individuals die at each age. 10 III 1 0 100 50 Percentage of maximum life span TYPE 3: experience high mortality as larvae but decreased mortality later in life. Type I curve Type I curve typical of animals that produce few young but care for them well (e.g. humans, elephants) Death rate low until late in life where rate increases sharply as a result of old age (wear and tear, accumulation of cellular damage, cancer). Type II curve Type II curve has fairly steady death rate throughout life (e.g. rodents). Death is usually a result of chance processes over which the organism has little control (e.g. predation) Type III curve Type III curve typical of species that produce large numbers of young which receive little or no care (e.g. Oyster). Survival of young is dependent on luck. Larvae released into sea have only a small chance of settling on a suitable substrate. Once settled however, prospects of survival are much better and a long life is possible. Population Growth Three factors that can affect population size: Number of births Number of deaths Number of individuals that enter or leave the population (immigration & emigration) A population will increase or decrease in size depending on how many individuals are added to it or removed from it Exponential Population Growth (EPG) Describes population growth in an idealized, unlimited environment. During EPG the rate of reproduction is at its maximum. Exponential Growth Exponential Growth Trends If a population has abundant space and food, and is protected from predators and disease, then organisms in that population will multiply and the population size will increase. The J shaped curve indicates that the population is undergoing exponential growth. This occurs when the individuals in a population reproduce at a constant rate. Under ideal conditions with unlimited resources, a population will grow exponentially and reach BIOTIC POTENTIAL. Carrying Capacity Exponential growth cannot be sustained for long in any population. A more realistic population model limits growth by incorporating carrying capacity. Carrying Capacity (K) is the maximum population size the environment can support. The Logistic Growth Model In the logistic population growth model the rate of increase declines as carrying capacity is approached. Logistic Growth Trends As resources become less available, the growth of a population slows or stops. The general S shaped curve of this growth pattern is called logistic growth. Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth. Can slow when the birthrate decreases, the death rate increases, or when both occur at the same rate Can slow when the rate of immigration decreases, the rate of emigration increases, or both Can slow as the population encounters a limiting factor The point at which carrying capacity line intercepts the y axis tells you the size of the population when the average growth rate is zero. That number represents the largest number of individuals that a given environment can support (carrying capacity). Phases of Logistic Growth Curve 3 2 1 Logistic model produces a sigmoid (S-shaped) population growth curve. K vs. R Reproduction Strategies K-selection, or density-dependent selection Selects for life history traits that are sensitive to population density. Produce relative FEW offspring that have a GOOD chance of survival. r-selection, or density-independent selection Selects for life history traits that maximize reproduction. High reproductive rate is the chief determinant of life history. The concepts of K-selection and r-selection have been criticized by ecologists as oversimplifications. Most organisms exhibit intermediate traits or can adjust their behavior to different conditions. Limits to Growth Limiting Factors- any factor that causes population growth to decrease Limits to Growth In the context of populations, a limiting factor is a factor that causes population growth to decrease Competition Predation Parasitism Drought & other climate extremes Human disturbances Density & Growth Regulation Density Dependent factors – include disease, competition, parasites and food. These have an increasing effect as the population increases. Density Independent factors – affect all populations regardless of their density (numbers) Most are abiotic factors such as temperature, storms, floods, droughts and habitat destruction. Population Regulation Populations are regulated by a complex interaction of biotic and abiotic influences: In density-independent populations birth rate and death rate do not change with population density. For example, in dune fescue grass environmental conditions kill a similar proportion of individuals regardless of density. In contrast in density-dependent populations birth rates fall and death rates rise with population density. Density-dependent population regulation much more common than density- independent Density-Dependent Factors A limiting factor that depends on population size is called a density-dependent limiting factor These factors become limiting only when the number of organisms per unit area reaches a certain level Competition Predation Parasitism Disease Competition When populations become crowded, organisms compete, or struggle, with one another for food, water, space, sunlight, and other essential elements of life The more individuals that live in an area, the sooner they will use up an available resource Interspecific competition occurs between members of two different species. Intraspecific competition occurs between members of the same species. Predation Populations in nature are often controlled by predation The regulation of a population by predation takes place within the predator-prey relationship (one of the best known mechanisms of population control) Predation Can Affect Population Sizes Parasitism & Disease Parasites can limit the growth of a population because they take nourishment at the expense of their hosts This often weakens the host and can lead to disease or death Density-Independent Factors Density-independent limiting factors affect all populations in similar ways, regardless of the population size Weather Natural disasters Seasonal cycles Human activities Damming rivers, cutting forests Human Population Growth Like the populations of many other living organisms, the size of the human population tends to increase with time Began growing more rapidly ~500 years ago because agriculture and industry made life easier Improved sanitation, medicine and health care But…earth’s resources are limited! Human Population Growth Patterns of Population Growth The scientific study of human populations is called demography Examines the characteristics of human populations and attempts to explain how those populations will change over time Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly The Demographic Transition The Demographic Transition Over the past century, population growth in the US, Japan, and much of Europe has slowed dramatically. One hypothesis as to why is that these countries have completed the demographic transition (a dramatic change in birth and death rates). As countries modernize, advances in nutrition, medicine, and sanitation result in more children surviving into adulthood and more adults living to old age. These changes lower the death rate and begin the demographic transition. Age Structure Population growth depends in large part on how many people of different ages make up a given population We can predict future growth using models called age-structure diagrams Graph the numbers of people in different age groups in the population Age Structure Diagrams Age structure diagram- a population profile, graphs the numbers of people in different age groups in the population