Population- an interbreeding group ofindividuals of a single species that occupy thesame general areaCommunity-the assemblage of interactingpopulations that inhabit the same area.Ecosystem- comprised of 1 or morecommunities and the abiotic environment withinan area.
MEASURING DENSITYDensity – Number of individuals per unit of area. •Determination of Density •Counting Individuals •Estimates By Counting Individuals •Estimates By Indirect Indicators •Mark-recapture Method N = (Number Marked) X (Catch Second Time) Number Of Marked Recaptures
Population Dynamics •Characteristics of Dynamics •Size •Density •Dispersal •Immigration •Emigration •Births/ natality •Deaths/ mortality •Survivorship
Parameters that effect size or density of a population: Immigration Birth Population (N) Death Emigration Figure 1. The size of a population is determined by a balance between births, immigration, deaths and emigration
Age Structure: the proportion of individuals in eachage class of a population Age Pyramid Female Male Age Interval ~ y 8-9 6-7 4-5 2-3 0-1 -10.0 -5.0 0.0 5.0 10.0 Percent of Population Figure 2. Age pyramid. Notice that it is split into two halves for male and female members of the population.
–A graphic way of representing the data is a survivorship curve. • This is a plot of the number of individuals in a cohort still alive at each age. –A Type I curve shows a low death rate early in life (humans). –The Type II curve shows constant mortality (squirrels). –Type III curve shows a high death rate early in life (oysters).
Life histories are very diverse, but they exhibit patterns in their variability• Life histories are a result of natural selection, and often parallel environmental factors.• Some organisms, such as the agave plant,exhibit what is known as big-bang reproduction, where large numbers of offspring are produced in each reproduction, after which the individual often dies. Agaves
• Variations also occur in seed crop size in plants. – The number of offspring produced at each reproductive episode exhibits a trade-off between number and quality of offspring. dandelion Coconut palm
– We can simplify the equation and use r to represent the difference in per capita birth and death rates. ∀ ∆N/∆t = rN OR dN/dt = rN– If B = D then there is zero population growth (ZPG).– Under ideal conditions, a population grows rapidly. • Exponential population growth is said to be happening • Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth • dN/dt = rmaxN
Example of Exponential Growth Kruger National Park, South Africa
POPULATION GROWTH RATELOGISTIC GROWTH RATE Assumes that the rate of population growth slows as the population size approaches carrying capacity, leveling to a constant level. S-shaped curveCARRYING CAPACITY The maximum sustainable population a particular environment can support over a long period of time.
Figure 52.11 Population growth predicted by the logistic model
• How well does the logistic model fit the growth of real populations? – The growth of laboratory populations of some animals fits the S-shaped curves fairly well. Stable population Seasonal increase
– Some of the assumptions built into the logistic model do not apply to all populations. • It is a model which provides a basis from which we can compare real populations. Severe Environmental Impact
Negative feedback prevents unlimited population growth• A variety of factors can cause negative feedback. – Resource limitation in crowded populations can stop population growth by reducing reproduction.
• Intraspecific competition for food can also cause density-dependent behavior of populations. – Territoriality. – Predation.
• Other populations have regular boom-and- bust cycles. – There are populations that fluctuate greatly. – A good example involves the lynx and snowshoe hare that cycle on a ten year basis.
Models to study population growth & interaction• A population model is a type of mathematical model that is applied to the study of population dynamics.• Models allow a better understanding of how complex interactions and processes work. Modelling of dynamic interactions in nature can provide a manageable way of understanding how numbers change over time or in relation to each other.
Logistic growth equation:Lotka-Volterra equation:Island biogeography: Species area:
The human population has beengrowing almost exponentially for threecenturies but cannot do so indefinitely• The human population increased relatively slowly until about 1650 when the Plague took an untold number of lives. – Ever since, human population numbers have doubled twice • How might this population increase stop?
POPULATION CYCLESHUMAN POPULATION 1650 - 500,000,000 1850 - ONE BILLION 1930 - TWO BILLION 1975 - FOUR BILLION 2010 – SIX BILLION 2017 - EIGHT BILLION
Changes in populations• ΔN = +B +I –D –E – B = births (birth rate) – I = immigrants (immigration rate) – D = deaths (death rate) – E = emigrants (emigration rate) – (for many [most] natural populations I and E are minimal) Populations.ppt 60
• Wide range of estimates for carrying capacity. – What is the carrying capacity of Earth for humans? – This question is difficult to answer. • Estimates are usually based on food, but human agriculture limits assumptions on available amounts.• Ecological footprint. – Humans have multiple constraints besides food. – The concept an of ecological footprint uses the idea of multiple constraints.