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Introduction• What is population dynamics? • The changes in the populations of organisms over time • Population ecology is the study of populations. Their size, density, distribution and changes over time • By doing this ecologists are able to gather data that can help them predict growth trends, health, manage sizes
Population Size & Population DensityTo study populations, scientists measure population size(the number of individuals of a specific species occupyinga given area/volume at a given time) and populationdensity (the number of individuals of the same speciesthat occur per unit area or volume)• Knowing the population size and density provides more information about the population’s relationship to resources it uses D=N/SN=Total number of individuals in the populationS=Space occupied by the population
ExampleThe population density of 480 moose living in a 600hectare (ha) region of Algonquin Park:D= N SD= 480 moose 600 haD= 0.8 moose/ha
Population Dispersionthe pattern of distribution in which a population exists
Clumped Dispersion Pattern A pattern in which individuals in a population are more concentrated in certain parts of the habitat. It occurs in these 3 situations: 1) When suitable living conditions are distributed in patches 2) Mates are easier to locate in groups 3) Limited seed dispersal or asexual reproduction
Random Dispersion Occurs when environmental conditions do not vary greatly within a habitat and when individuals are neither attracted to nor repelled by others of their species. Organisms are distributed unpredictably.
Uniform Dispersion Equally spaced throughout a habitat.
Mark-Recapture SamplingA sampling technique used to estimate thepopulation size of a species.• A specific number of animals in the natural population is captured, marked or tagged in some way and then released back into the population• After a period of time when the marked animals have mixed in with the unmarked animals, another sample is captured• Biologists use the proportion of marked to unmarked animals in the second sample to estimate the size of the entire population
Accuracy is based on 5 assumptions• the chances for each individual in the population to be caught are equal and constant for the initial capture and the recapture• No new individuals can enter the population through birth or immigration and no individuals can leave through death or emigration• Enough time is given between the release and recapture so individuals can disperse• The animals are not affected by their markings• The marked animals do not lose their markings The estimated population size can be calculated as follows: Total number marked (M) = number of recaptures (m) Total population (N) size of second sample (n) OR N= Mn m
Mark-Recapture Sampling Activity1) Pick a species2) Randomly pick out 20 marshmallows from your paper bag3) Mark each captured marshmallows with your marker and return them all to the bag. Shake and stir the bag to mix the marshmallows up.4) Without looking in the bag, capture a similar amount of marshmallows from the bag5)Calculate the estimate of the total population size N= Mn m6)Compare with other groups
DemographyDepending on the species and the environmentalconditions, population numbers can undergohourly, daily, seasonal and annual changes.Demography is the study of these changes. Ecologistsuse demographic analysis to predict the growth of apopulationBirth rate, death rate, immigration and emigration can beused to determine the growth rate of the population in agiven period of time. This data can also be used todevelop plans to protect endangered species
Survivorship Curves Data about survival can be depicted graphically in a survivorship curve. This displays the survival of individuals over the lifespan of the species. Most organisms exhibit survivorship patterns that fall somewhere between these general patterns:Low death rate in Constant rate of High death rate in earlyearly/middle years, death mortality in all age years, declines for therate increases in older age groups few individuals thatgroups survive
Fecundity The reproductive capacity of an individual or population.A horseshoe crab can lay An elephant produces aup to 90 000 eggs in a average of only 4 offspringsingle spawning season during a lifetime
An animal that has high fecundity normally does little to care forthem. An animal that has only 1 or 2 offspring per year tend to be very protective of them. This is called extensive parental care.
Population Growth ModelsPopulation growth rate: the change in a population over a unit timeperiod.Population (births + immigration) – (deaths + emigration) = initial population X100growth rate• A positive growth rate indicates that the population is increasing• A negative growth rate indicates that the population is decreasing• A growth rate of zero indicates there was no difference between birth rates and death rates• Some species reproduce continuously so the sizes of these populations have the potential to increase exponentially by a contrast ratio per unit of time. Ex, bacteria, virus
Exponential Growth ModelAssociated with the name of Thomas Robert Malthus(1766-1834) who first realized that any species canpotentially increase in numbers according to a geometricseries.
• Populations that grow exponentially increase in numbers rapidly, resulting in a J-shaped growth curve. As the population gets larger, it grows faster and becomes steeper.• We can determine the change in population size over time by using the equationdN = B – DdtdNdt = the change in population sizeover time.B=Birth rateD=death rate• This equation can be used for ANY population if we know the exact number of births and deaths
Limitations• Environmental limitations prevent populations from experiencing continuous growth• Per capita birth rate decrease and the per capita death rate increases as competition for resources such as food and shelter occurs• Large sudden birth rates in Canada due to breeding periods (frogs reproduce in fixes intervals) not continuously
Logistic Model• Environmental limitations create this model. It describes limited population growth, often due to limited resources or predation.• An environment only provides enough resources to sustain only a limited amount of any species.• The maximum number of individuals that an environment can support indefinitely is its carrying capacity. N=population size K=carrying capacity• If a population is very small then plenty of resources are available and therefore the value of (K-N)/K is close to 1.• Vice versa, if the population is large, few resources are available and the value of (K-N)/K is small and the per capita growth rate is very low.• Lastly, if the size of the population exceeds the carrying capacity of the habitat then the population would decrease from lack of food and other resources. If this is graphed it produces an S shape curve.
Sigmoid CurveLogistic growth can be seen in a population of fur seals on St. Paul Island, Alaska. In1911, fur seal hunting was banned on St. Paul Island, since the population hadbecome extremely low. Because their numbers were so severely depressed, the sealshad many unused resources to support the recovering population. The populationbegan to grow rapidly until it stabilized around its carrying capacity.
ActivityA white-tailed deer population in a small forest ecosystemexhibits logistic growth. The carrying capacity is 300 deerand rmax =0.23.a) Determine the population growth rates of 100, 200 and 300 deer.Equation: dN=rmax N(K-N) dt K
Limitations• The logistic growth model assumes that all individuals reproduce, die and use resources at an identical rate.• It assumes that the carrying capacity remains constant and there are no environmental variations that might cause the carrying capacity to fluctuate.• Assumption that there is no migration and that populations do not interact with each other.
Factors that Affect Population GrowthIndividuals within a population do not live in isolation. They interact with membersof their own species and members of other species. Coevolution occurs when onespecies evolves in response to the evolution of another species.Relationships between pairs of organisms:Herbivory: the interaction between herbivorous animalsand plants they eat. Ex: White–tailed deer and foliageMutualism: an interaction in which both partners benefit.Ex: honey bee and flowering plantParasitism: awhile the tree is unaffected interaction in which onespecies benefits and the other is harmed. Ex: mistletoe, whichattaches to a tree and takes water and nutrients from its host;usually stunts growth but can kill the tree with heavy infestationCommensalism: an interaction in which one species benefits andthe other is unaffected. Ex: moss, which grows on a tree, gettinglight and nutrient it needs
Defense Mechanisms Organisms have evolved mechanisms to avoid being caught and eaten.Camouflage Chemical Defense MimicryBehavioural defense Spines and armour