Population ecology for epidemiologists


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Population ecology for epidemiologists

  1. 1. Population Ecology Population ecology, study of the processes that affect the distribution and abundance of animal and plant population. Dr. Bhoj R singh, Principal Scientist (VM) I/C Epidemiology; Centre for Animal Disease Research and Diagnosis Indian Veterinary Research Institute, Izatnagar-243122, Bareilly, UP, India. TeleFax +91-581-2302188
  2. 2. Population • A population is a subset of individuals of one species that occupies a particular geographic area and, in sexually reproducing species, interbreeds. – Closed: Defined territories, geographically isolated from, and lack exchange with, other populations of the same species. The number of individuals is governed by the rates of birth (natality), growth, reproduction, and death (mortality). – Open: show varying degrees of connectedness with other populations of the same species. Besides natality and mortality, immigration or emigration affects size and density. – Metapopulation: Regional groups of interconnected populations are called metapopulations. These metapopulations are, in turn, connected to one another over broa As local populations within a metapopulation fluctuate in size, they become vulnerable to extinction during periods when their numbers are low. der geographic ranges.
  3. 3. Types of populations • r-selected: Populations are considered opportunistic because their reproductive behaviour involves a high intrinsic rate of growth (r)—individuals give birth once at an early age to many offspring. Populations that exhibit this strategy often have been shaped by an extremely variable and uncertain environment. Because mortality occurs randomly in this setting, quantity of progeny rather than quality of care serves the species better. • K-selected, populations tend to remain near the carrying capacity (K), the maximum number of individuals that the environment can sustain. Individuals in a K-selected population give birth at a later age to fewer offspring. This equilibrial life history is exhibited in more stable environments where reproductive success depends more on the fitness of the offspring than on their numbers.
  4. 4. Determination of Population size • Direct method: Counting each individual – Can be used where study area/ population size is small and there is no movement of individuals or individuals are large. – Drawback: labour intensive, not applicable when very large area/ large population, lot of emigration/ immigration, organism size is small. • Indirect methods: using sampling and mathematical calculations. It is also applicable when it is difficult to observe the organism. Common methods used are observations of tracks, droppings, nests or other signs of inhabitation. • Mark and recapture methods.
  5. 5. Sampling for population size • Sample size is based on population density, demography of the area and size of population and area to be covered. • Useful when population is very large, area to be mapped is large. • When it is difficult to remember what and which has been count and which is not. • Useful for wildlife count.
  6. 6. Factors affecting populations • Biotic factors: The biotic factors are the population density and its relation with the food and pathogens.. It is ability to achieve the maximum birth rate at ideal conditions, potential natality and realized natality (actual number of births that take place in a given environment). • Vital index= Natality/ mortality • Immigration/ emigration. • Carrying capacity: The maximum number of the resources which can support the maximum number of individuals in given area is known as a carrying capacity. The population size at which growth stops is generally called the carrying capacity (K). • The natural calamities destabilize the ecosystem and population. The most common calamities include earthquake, land, flood and fire. • Abiotic factors: Sunlight, , temperature, humidity, rainfall, minerals, substratum availability. The hibernation, aestivation are controlled by the changes in weather. The hibernation, aestivation are controlled by the changes in weather.
  7. 7. Biotic factors affecting the population size • Factors limiting population may vary from one population to other population leading to ‘Population change over’. • Common biotic factors: – Food- both the quantity and the quality of food are important. Snails cannot reproduce successfully in an environment low in calcium. – Predators- as a prey population becomes larger, predators may also. If the number of predators suddenly falls, there may be erruptive increase in prey population. – Competitors- other organisms may require the same resources from the environment, and so reduce growth of a population. – Parasites- These may cause disease, and slow down the growth and reproductive rate of organisms within a population.
  8. 8. Human intervention and Population size • Humans can greatly influence the size of animal populations they directly interact with. Some common methods are: • Spaying - removing the ovaries and uterus of a female animal - medical term = ovariohysterectomy. • Neutering - removing the testes of a male animal - medical term = orchiectomy. • Various humans activities (e.g. hunting, farming, fishing, industrialization and urbanization) all impact various animal populations. • Culling, translocation, Animal population control is the practice of intentionally altering the size of any animal population besides humans. It may involve manipulation of the reproductive capability. • Animal euthanasia is often used as a final resort to control animal populations. It may be for control of diseases (depopulation at some farm or in a territory.
  9. 9. Population fluctuations • Irruptive: There is a quick increase in population which occurs in a short time and increases the population density. It is also followed by the reduction in the size of population. • Cyclic: There is a slow increase in the population which occurs in a long time and increases the population density. It occurs due to the seasonal changes. • Geometric (pulsed/ seasonal reproduction) and Exponential rates (no seasonality, like in humans). • Exponential growth: Low density, favourable environment. Exponential growth may apply to populations establishing new environments, during transient, favourable conditions, and by populations with low initial population density. • Logistic growth : As resources are depleted, population growth rate slows and eventually stops.
  10. 10. Limitation of population • Density dependent: Often biotic. Density-dependent factors include disease, competition, and predation. Density-dependant factors can have either a positive or a negative correlation to population size. Density- dependant factors may influence the size of the population by changes in reproduction or survival. Density dependant factors may also affect population mortality and migration. – Exploitation competition: a density dependent, often intraspecies or with related interspecies, for common needs with limited resources. – Interference competition: Often interspecies, certain individuals obtain an adequate supply of the limited resource at the expense of other individuals in the population. – Disease: • Density independent: Often abiotic. Many sources of environmental stress affect population growth, irrespective of the density of the population. Density-independent factors, such as environmental stressors and catastrophe, are not influenced by population density change. Viz., food or nutrient limitation, pollutants in the environment, and climate extremes, including seasonal cycles such as monsoons. In addition, catastrophic factors can also impact population growth, such as earthquakes, volcanoes, floods, heavy snow, blizzards, fires and hurricanes. – Inbreeding: It an important density independent biotic factor which affect survival due to limited genetic diversity required for adaptation under stress of change. This inbreeding depression may make inbred individuals more susceptible to disease, less able to find food, or less likely to breed successfully.
  11. 11. Natural selection/ Stress factors • Genetic conservation: The individuals in the environment that are best adapted to survive will reproduce and pass the genes for adaptation along to their off-springs. • The resilience is defined as the ability of a population to bear the changes caused by the changes in temperature, biotic factors and the changes in humidity. • Evolution: Over the time natural selection may lead to change in bodies of the organisms and new strains/ varieties/ species may arise. • Genetic variation is more easily sustained in large and open populations than in small and closed populations. Through the effects of random genetic drift, a genetic trait can be lost from a small population relatively quickly.
  12. 12. Geometric and exponential growth rates • Exponential growth: Populations, in which individuals live and reproduce for many years and in which reproduction is distributed throughout the year, grow exponentially. – Exponential population growth can be determined by dividing the change in population size (ΔN) by the time interval (Δt) for a certain population size (N). – The growth curve of these populations is smooth and becomes increasingly steeper over time. • Geometric growth: Insects and plants that live for a single year/ short duration and reproduce once before dying are examples of organisms whose growth is geometric. In these species a population grows as a series of increasingly steep steps rather than as a smooth curve. • Logistic population growth: If growth is limited by resources such as food, the exponential growth of the population begins to slow as competition for those resources increases. The growth of the population eventually slows nearly to zero as the population reaches the carrying capacity (K) for the environment. The result is an S-shaped curve of population growth known as the logistic curve. It is determined by the equation
  13. 13. Species interactions and population growth • Community-level interactions are made up of the combined interactions between species within the biological community where the species coexist. The effects of one species upon another that derive from these interactions may take one of three forms: positive (+), negative (–), and neutral (0). Hence, interactions between any two species in any given biological community can take any of six forms: • Mutualism (+, +), in which both species benefit from the interaction. • Exploitation (+, –), in which one species benefits at the expense of the other. • Commensalism (+, 0), in which one species benefits from the interaction while the other species neither benefits nor suffers. • Interspecific competition (–, –), in which both species incur a cost of the interaction between them. • Amensalism (–, 0), in which one species suffers while the other incurs no measurable cost of the interaction. • Neutrality (0, 0), in which both species neither benefit nor suffer from the interaction.
  14. 14. LOTKA-VOLTERRA EQUATIONS • The effects of species interactions on the population dynamics of the species involved can be predicted by a pair of linked equations that were developed independently during the 1920s by American mathematician and physical scientist Alfred J. Lotka and Italian physicist Vito Volterra. • Lotka-Volterra equations are often used to assess the potential benefits or demise of one species involved in competition with another species. dN1/dt = r1N1(1 – N1/K1 – α1,2N2/K2)dN2/dt = r2N2(1 – N2/K2 – α2,1N1/K1) Here r = rate of increase, N = population size, and K = carrying capacity of any given species. In the first equation, the change in population size of species 1 over a specific period of time (dN1/dt) is determined by its own population dynamics in the absence of species 2 (r1N1[1 – N1/K1]) as well as by its interaction with species 2 (α1,2N2/K2). As the formula implies, the effect of species 2 on species 1 (α1,2) in turn is determined by the population size and carrying capacity of species 2 (N2 and K2).
  15. 15. Calculation of population growth rates • Survivor curves • Life tables • Reproductive rates
  16. 16. Survivor curves and life history • Type I survivorship curve : In K-selected species which have fewer numbers of offspring but invest much time and energy in caring for their young. This relatively flat curve reflects low juvenile mortality, with most individuals living to old age. • Type II survivorship curve: In r-selected species. A constant probability of dying at any age, is evident as a straight line with a constant slope that decreases over time toward zero. The species produces many offspring but provide little care for them, mortality is greatest among the youngest individuals. • Type III survivorship curve: Life history is initially very steep, reflectiing very high mortality among the young, but flattens out as those individuals who reach maturity survive for a relatively longer time; it is exhibited by animals such as many insects or shellfish. • Complex Type survivorship curve: Species commonly suffer high mortality during the first year of life and a lower, more constant rate of death in subsequent years.
  17. 17. Life tables • Life tables were originally developed by insurance companies to provide a means of determining how long a person of a particular age could be expected to live. • They are used by plant, animal, and microbial ecologists to make projections about the life expectancies of populations, as well as the effects of variation on demography and population growth. • Life tables are designed to evaluate how rates of birth and death influence the overall growth rate of a population. • Life tables also are used to study population growth. The average number of offspring left by a female at each age together with the proportion of individuals surviving to each age can be used to evaluate the rate at which the size of the population changes over time.
  18. 18. Reproductive rate • The average number of offspring that a female produces during her lifetime is called the net reproductive rate (R0). • If all females survived to the oldest possible age for that population, the net reproductive rate would simply be the sum of the average number of offspring produced by females at each age. • The net reproductive rate for a set cohort is obtained by multiplying the proportion of females surviving to each age (lx) by the average number of offspring produced at each age (mx) and then adding the products from all the age groups: R0 = Σlxmx. • A net reproductive rate of 1.0 indicates that a population is neither increasing nor decreasing but replacing its numbers exactly. This rate indicates population stability. Any number below 1.0 indicates a decrease in population, while any number above indicates an increase. • Mean generation time = T = (Σxlxmx)/(R0) = 6.08 years, Generation time is the average interval between the birth of an individual and the birth of its offspring. To determine the mean generation time of a population, the age of the individuals (x) is multiplied by the proportion of females surviving to that age (lx) and the average number of offspring left by females at that age (mx). • Intrinsic rate of natural increase of the population = r = approximately 1nR0 / T = 2.101/6.08 = 0.346, n=number of females. Intrinsic rate of natural increase (r), or the Malthusian parameter. It is the number of births minus the number of deaths per generation time—in other words, the reproduction rate less the death rate. To derive this value using a life table, the natural logarithm of the net reproductive rate is divided by the mean generation time. Values above zero indicate that the population is increasing; the higher the value, the faster the growth rate.