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Population Characteristics

OVERVIEW Popuation And IndividualsDefining EcologyDistributionAbundance Population CharacteristicsAge Structure Sex Ratio Dispersal

POPULATIONS AND INDIVIDUALSPopulations are local groups of organisms, all of the same species, in which organisms interact.

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Modular Growthgrowth by vegetative budding of new individuals genetically identical to the original individual.Modular growth leads to two different sorts of individuals: Genet and RametIn animals, genetically identical individuals are often called Clones.In such cases, the total weight of the species (its Biomass) is assessed.Figure 8.6 Plants and Animals That Form Clones

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DEFINING ECOLOGYOperational definition of ecology is the study of the distribution and abundance of organismsThe first complication is the problem of Spatial Scale (the size of area examined)Ex : Dispersal of the plant may involve longer distances if birds eat the fruit and fly from field to field depositing seeds

Defining ecological communities

DistributionDistribution is the spatial location of organisms in a population. There are two areas to consider in describing distribution: the boundary and patterns within the boundary.

1. Geographic Range The outer limit of the location of members of the population, the boundary beyond which no members are located..The boundary may be set by Edaphic Conditions physical factors like climate, soil type, temperature, salinity, etc. geography (a coast, a river, a mountain chain)biological factors (competitors, presence of disease, predators, etc.)

Spatial Pattern the manner in which organisms are distributed within the boundary of the populationPopulation Density is the average number of organisms per unit areaThe Average (or Mean) is calculated by totaling all observations and dividing by the number of observations.Variance is calculated by subtracting the mean from each observation to get the difference ,squaring each deviation and taking the total of the deviations.

Organisms can be distributed within an area in one of three ways:Uniform - when organisms are distributed so that there the distance between organisms does not vary much so that all organisms are about the average distance apart.Random - neither clumped or uniform. In this situation, there is no relationship between the locations of individuals.Clumped - ramets or genets occur as clumps surrounded by areas where fewer than expected organisms occur, like islands in a lake.Dispersion -How organisms are distributed within their habitat; affects measures of density:

AbundanceAbundance of organisms in a population can be considered from two perspectivesTotal abundance is simply the size of the population, whether you measure it as organisms, ramets, genets, or even biomass.The book separates the two ways of looking at abundance into Crude Density and Ecological Density

AbundanceIt is rarely possible to count or weigh all members of a population, and so one needs to take Samples in order to predict population size.The method of assessing population size depends on the organism being studiedSessile organisms are often sampled by using Quadrat SamplingAn example of a statistical approach to abundance of motile organisms is the use of the Lincoln-Peterson Index to estimate population size.

POPULATION CHARACTERISTICSWe have seen that abundance is a more complicated phenomenon when examined in depth. There are other characteristics of populations to consider that are related to abundance.

1. Age Structure Age structure is the distribution of members of the population according to their age.Different age structures can lead to very different predictions about the fate of a population.One means of depicting age structure is to make a horizontal histogram so that the ages stack up from youngest to oldest. Age Structure

2. Sex RatioThe ratio of males to females in a population has great consequence.Optimizing the sex ratio is a complicated business and has lead to an entire body of mathematical theory, Sex Ratio Theory.

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