The population is collection of individuals of same species which live together in region. Population ecology is study of populations (particularly population abundance) and how they change over time. Populations are limited by the resources in the capacity to grow; maximum population abundance (for given species) the environment can carry on is known as carrying capacity. Ecological models which focus on single species and appropriate carrying capacity are single species models. On the other hand, multi-species or community models focus on interactions of specific species. Discipline of population ecology holds great deal of philosophical interest.
Density, Distribution and Size:
Population density is number of individuals of the certain species per unit area or volume, and population distribution is pattern of dispersal of them within that area. They are crucial variables for ecologists to examine and discover spreading pattern of the certain species within the certain area and time. Thus, basing ecological population models only on density can be misleading. Density and distribution of a population changes with time, because of abiotic factors(inorganic factors) and biotic factors(organic factors). Abiotic factors which could have the influence on the population comprise temperature, rainfall, kind of soil and so forth; biotic factors are those which are associated to other living things.
Theoretically, there exist two distinct and simple growth patterns, or mathematical models for population growth. In first one, only one reproductive chance is given to members of population during their whole lifespan. Expressed in mathematical equations and graphs, two growth patterns can be referred to as exponential growth pattern and logistic growth pattern respectively.
Mortality Patterns and Survivorship Curves:
Population growth patterns need the assumption which members of the population are all identical individuals. Though, individuals are in their different stages of lifespan. In a given period of time, some are born and some die.
This is the number of individuals in different age classes; pre-reproductive, reproductive, and post-reproductive. Short-lived organisms increase rapidly, with short span between generations Long-lived organisms, increase slowly, and have long span between generations
i) Determining individual ages
Animals: marking young individuals through time and examining representative sample of carcasses of individuals wear and replacement of teeth in deer and other ungulates. Annual growth rings in horns of sheep, plumage changes and wear in birds and growth rings in scales of fishes
ii) Age Pyramids
Comparing percentages of population in different age groups, pyramids with broad base of young suggest growing populations. Pyramids with narrow base of young and even ratios, suggest declining or aging population and depict changing dynamics of population.
Population Growth Curve:
This illustrates net result of births, deaths, and dispersion. It generally illustrates 3 to 5 phases. Most organisms illustrate 3 phases: lag phase, exponential growth phase, and equilibrium phase
1) Lag phase - slow growth as the procedure of growth and reproduction of offspring takes time
2) Exponential growth phase - classified by more organisms undergoing reproduction, so that population starts to increase at very fast rate; birthrate exceeds death rate
3) Equilibrium phase - classified by birth rate and death rate which are equal to one another; population will stop growing and reach relatively stable population.
Population Growth Rate:
Net result of births, deaths, and dispersion can take numerous forms:
1) J-shaped or exponential growth form - density. Increases in the geometric fashion until population runs out of some resource or encounters some other limitation
2) S-shaped or Sigmoid growth - restrictive factors resulting from crowding give negative feedback which decreases rate of growth more and more as density increases. If limitation is linearly proportional to density, growth form will be symmetrical sigmoid curve with density leveling off as to reach carrying capacity; carrying capacity represents maximum sustainable density.
Basic Concepts of Rate:
A. Population dynamics - study of changes in relative number of organisms in populations and factor explaining these changes
B. Rate - obtained by dividing change in some quantity by period of time elapsed during change
1. Equation ΔN/Δt = average rate of change in number of organisms per time.
Instantaneous rate: rate at the particular time (rate of change when Δ t approaches 0, d= derivative dN/dt = i.e. rate of change in number of organisms per time at particular instant) Point of inflection - point where growth rate is maximum.
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