Population and Community Dynamics

Population and Community Dynamics

Part 1. Genetic Diversity in Populations

Pages 676 to 701

Part 2. Population Growth and Interactions

Pages 702 to 745

Part 2. Population Growth and InteractionsI) Introduction

to understand how ecological communities change over time ecologists: take quantitative measurements using mathematical

models. observing growth and decline of populations

explore interactions among populations. they do this to create “snapshots” of communities that they

can compare to other communities and/or the same community at other periods of time.

Part 2. Population Growth and InteractionsII) Density and Distribution of Populations

populations can be described in terms of two characteristics: density distribution

density ecologists use a number of sampling methods to

estimate the density of a population plot/quadrant line-transect

Part 2. Population Growth and InteractionsII) Density and Distribution of Populations

Population Density(Dp) defined as the number of individuals organisms (N) in a given area

(A) or volume (V)

DP=N/A or Dp=N/V example

there are 12 gophers living in a 10.0 m2 area

DP=N/A

DP= (12 gophers)/(10.0m2)

DP=1.2 gophers/m2

if you know a field is 25.0 m2, then

(1.2 gophers/m2)(25.0 m2)

30 gophers

there are three theoretical distribution patterns for populations: uniform random clumped

Part 2. Population Growth and InteractionsII) Factors that Affect Distribution Patterns

distribution patterns are influenced by: the distribution of resources in a habitat interactions among members of a community.

random distribution: can occur when:

resources are very abundant population members do not have to compete with one

another. individuals are distributed throughout a suitable habitat

with no identifiable pattern. is generally rare in nature.

Part 2. Population Growth and InteractionsII) Factors that Affect Distribution Patterns

clumped distribution more common populations are found in close proximity. congregations occur in areas where food, water or

shelter is most abundant. uniform distribution

individuals are evenly spaced over a defined area. occurs in:

artificial populations (agriculture) animals that behave territorially.

Question

Most wild populations exhibit:

A) random distribution patterns

B) clumped distribution patterns

C) uniform distribution patterns

D) continuously changing distribution patterns

Part 2. Population Growth and InteractionsIII) Population Growth

there are four processes that can change the size of a population: the number of individuals (N) Births (b) Immigration (i)

movement into a population deaths (d) and emigration (e)

movement out of a population


the change in population size can be calculated using:

ΔN = (b + i) – (d +e)


the rate of population growth measures the speed at which a population changes size. a rapid increase in the size of a population is called a

population explosion. a rapid decrease in the size of a population is called a

population crash. a populations growth rate (gr) can be calculated using

the equation

gr= ΔN/ Δt

the calculation of growth rate does not take into account the initial size of the population.

if there are no limiting factors a large population can increase in size much faster than a smaller one.


to compare populations of the same species that are different sizes or live in different habitats you use per capita growth rate (cgr)

cgr= ΔN/N or cgr= (Nfinal –N)/Nexample:

in a town of 1000 there are 50 births and 30 deaths with no immigration or emigration

cgr= ΔN/Nremember ΔN = (b + i) – (d +e)

cgr = ((b + i) – (d +e) )/N cgr = ((50 + 0) – (30 + 0) )/1000 cgr = 20/1000 = 0.02

Example problem

Complete the following table

Initial Popluation

(N)

Time Period

( t)Δ

Births (n)

Deaths (m)

Immigrants

(i)Emigrants

(e)

Pop. Change

( N)Δ

Growth rate (gr)

Per capita

growth rate (cgr)

600 2 20 15 25 10

200 4 40 60 10 0

30000 1 450 350 100 150

10000 180 160 30 40 5

20 10 0.33

-10 -2.5 -0.05

580 150 0.005

2 10 0.01

Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth

both abiotic (non-living) and biotic (living) factors limit the growth of a population.

each species has an intrinsic rate of growth if they had unlimited resources and ideal conditions. the highest possible per capita growth rate (cgr) for a

species is called its biotic potential (r).


factors that affect biotic potential (r) include: the number of offspring per reproductive cycle. the number of offspring that survive long enough to

reproduce. the age of reproductive maturity and the number of

times an individual reproduces in a life span. the life span of the individual.


a species growing at its biotic potential would grow exponentially. an exponential growth pattern starts with a lag phase

followed by a steep increase in the growth curve (J-shaped curve)


in the beginning the growth rate is slow because there is only a few individuals to reproduce this is called the “lag phase”

a rapid growth rate follows because the birth rate is greater than the death rate.

under natural conditions this rapid growth cannot be sustained. when the death rate equals the birth rate the

population is in a stationary phase.


this pattern of growth rate is called S-shaped (sigmoidal) cure and represents a logistic growth pattern. the straight line running through the curve is the

carrying capacity (K) of the habitat the carrying capacity is the theoretical maximum

population a habitat can sustain over an extended period of time.


the factors that limit a habitats carrying capacity can be: density-dependent factors

biotic factors predators, food, water, disease

density-independent factors abiotic factors

weather, forest fires combined the factors are called environmental resistance

and it prevents a population from growing at is biotic potential.


organisms have adapted life styles that are suited to the availability of resources in their habitat. in an unstable environment organisms can reproduce

close to their biotic potential (r) this is called r-selected strategies

species have short life spans early reproductive ages reproduce large broods of offspring that have little

or no parental care. this strategy takes advantage of favourable but not

long lasting environmental conditions.

the other end of the spectrum is k-selected strategies. few offspring are produced per reproductive cycle. one or both parents take care of the offspring offspring take a relatively long time to mature and

reach reproductive age offspring have relatively long life spans and larger

bodies. k-selected strategies are useful for organisms that live

close to the carrying capacity of the habitat. long-term sustained availability of resources.


most populations have a combination of k-selected and r-selected strategies. populations can only be described as k- or r-selected by

comparing it to another population.

Part 2. Population Growth and InteractionsV) Population Interactions

the competition for limited resources among members of the same species is called intraspecific competition. is a density-dependent factor that limits the growth of a

population is very important in natural selection

the competition between two or more populations for limited resources is called interspecific competition.

due to interspecific competition no two species can share the same ecological niche.


is very important in natural selection the competition between two or more populations for limited resources is called

interspecific competition. due to interspecific competition no two species can share

the same ecological nicheProducer-Consumer Interactions not all interspecific interactions in a community are

competitive predators are organisms that kill and cosume other

organisms known as prey. predator-prey interactions are one factor in the boom or

bust cycles observed in populations.


Producer-Consumer Interactions not all interspecific interactions in a community are competitive

predators are organisms that kill and cosume other organisms known as prey.

predator-prey interactions are one factor in the boom or bust cycles observed in populations.


predators and prey typically co-evolve. prey will have adaptations for protection from predators

examples cryptic coloration or body colours act as natural defense

mechanisms. black, yellow and red a colours that predators usually

associate with dangerous plants. camouflage lets prey “blend in” into there environment. mimicry lets prey look like other animals or parts of the

environment. predators will have adaptations for protection against prey.


Symbiotic Relationships symbiosis is the direct or close relationship between

individuals of different species that live together. when both partners in relationship benefit it is called

mutualism when one partner benefits and other partners is

neither harmed or benefits it is called commensalism when one partner benefits and the other is harmed it

is called parasitism.


Succession succession is the sequence of invasion and replacement

of species in an ecosystem over time. driven by both abiotic and biotic factors.

primary succession begins when there is no soil present.

the first species to colonize an area and initiate succession form the pioneer community.




soil forms as the organisms of the pioneer community die. as soil builds up nutrients, pH and moisture change.

larger species move in. interspecific competition occurs.

the last species in succession form the climax community. the climax community remains relatively stable.


as soil builds up nutrients, pH and moisture change. larger species move in.

interspecific competition occurs. the last species in succession form the climax community.

the climax community remains relatively stable.


interspecific competition occurs. the last species in succession form the climax community.

the climax community remains relatively stable.

an event that changes the structure of a community (large scale destruction via forest fire) is called an ecological disturbance. secondary succession is the recolonization of an area

after an ecological distrubance. ie. forest fire, flood, urbanization

Part 2. Population Growth and InteractionsVI) Sharing the Biosphere

sustainability the concept of living in a manner that meets the needs of

the species while not compromising the health of future generations or the health of the planet.

example: Humans humans have many k-selected strategies

long life spans low reproductive rate heavy parental care of young

age pyramids tools that demographers use to help them assess a

population’s potential for growth. shows in the different ages the:

males (on the left) females (on the right)

used to the see the proportion of the population that is in: pre-reproductive stage of life (0 to 14 years) reproductive stage of life (14 to 44 years) post-reproductive stage of life (45 years and up)

an upright triangle-shaped age pyramid indicates more births than deaths in the population. the population is growing.

a rectangle-shaped age pyramid indicates a roughly equal number of births and deaths. the population is stable. roughly the same proportion of people are in each stage

of development. an inverted triangle-shaped age pyramid indicates more

deaths than births the population is declining.

Population and Community Dynamics

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