Population Dynamics

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 Density

To study populations, scientists measure population size (the number of individuals of a specific species occupying a given area/volume at a given time) and population density (the number of individuals of the same species that 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

Example

The population density of 480 moose living in a 600 hectare (ha) region of Algonquin Park:

D= N S

D= 480 moose 600 ha

D= 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 Sampling

A sampling technique used to estimate the population 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 Activity

1) 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 bag

5)Calculate the estimate of the total population size N= Mn m

6)Compare with other groups

Demography

Depending on the species and the environmental conditions, population numbers can undergo hourly, daily, seasonal and annual changes. Demography is the study of these changes. Ecologists use demographic analysis to predict the growth of a population

Birth rate, death rate, immigration and emigration can be used to determine the growth rate of the population in a given period of time. This data can also be used to develop 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 early/middle years, death rate increases in older age groups

Constant rate of mortality in all age groups

High death rate in early years, declines for the few individuals that survive

FecundityThe reproductive capacity of an individual or population.

A horseshoe crab can lay up to 90 000 eggs in a single spawning season

An elephant produces a average of only 4 offspring during a lifetime

An animal that has high fecundity normally does little to care for them.

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 Models

Population growth rate: the change in a population over a unit time period.

Population (births + immigration) – (deaths + emigration) = initial population X100

growth 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 Model

Associated with the name of Thomas Robert Malthus (1766-1834) who first realized that any species can potentially increase in numbers according to a geometric series.

• 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 equation

dN = B – Ddt

dN dt = the change in population size over 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 sizeK=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 Curve

Logistic growth can be seen in a population of fur seals on St. Paul Island, Alaska. In 1911, fur seal hunting was banned on St. Paul Island, since the population had become extremely low. Because their numbers were so severely depressed, the seals had many unused resources to support the recovering population. The population began to grow rapidly until it stabilized around its carrying capacity.

Activity

A white-tailed deer population in a small forest ecosystem exhibits logistic growth. The carrying capacity is 300 deer and 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 Growth

Individuals within a population do not live in isolation. They interact with members of their own species and members of other species. Coevolution occurs when one species evolves in response to the evolution of another species.

Relationships between pairs of organisms:

Herbivory: the interaction between herbivorous animals and plants they eat. Ex: White–tailed deer and foliage

Mutualism: an interaction in which both partners benefit. Ex: honey bee and flowering plant

Parasitism: awhile the tree is unaffected interaction in which one species benefits and the other is harmed. Ex: mistletoe, which attaches to a tree and takes water and nutrients from its host; usually stunts growth but can kill the tree with heavy infestation

Commensalism: an interaction in which one species benefits and the other is unaffected. Ex: moss, which grows on a tree, getting light and nutrient it needs

Defense MechanismsOrganisms have evolved mechanisms to avoid being caught and eaten.

Camouflage Chemical Defense Mimicry

Behavioural defense Spines and armour