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defined as all the organisms of a particular species in a 
particular area at a given time.


Harsh Environments

Organisms function in the context of an environment and can threaten stability of life.

Harsh environments (e.g. deserts, tundra, caves, sand dunes, volcanic fields) are likely to have sparse 

life, because of limits on the availability of nutrients, energy & water, or because of conditions beyond 

the tolerance range of the organism (low or high temperature, salinity, pH, toxic heavy metals, etc.)


Favourable environments are

very populated but very competitive


how can a species exist?

A population of any given species will exist wherever it can stay alive physically, out-compete 

other species for the available resources and reproduce fast enough to sustain numbers(provided 

it can get there in the first place).


Ranges of species

Within its range, a population may be distributed in a regular, random or clustered way

More often there are factors that tend to keep 

organisms close to each other - short distance 

dispersal from parents, social attraction, 

dependence on a localized resource - and 

opposing forces that tend to push individuals apart 

- competition for distributed resources, social 

repulsion such as territoriality


random distribution

A random distribution will occur if organisms 

in a population act completely 

independently of one another. (a very scattered graph)


clustered distribution

when the animals are very social or attached to parents (very clustered but very empty in other parts graph)


regular distribution

when animals are territorial (a very evenly spread graph with one dot in each "area")


patchiness of habitats

Of course, the distribution pattern 

may be a complex mixture of 

geographic patterns.

Some animals, like wolves, monkeys 

and some birds, live in groups 

(clustered) but as a unit each group 

defends a territory (regular).

Many organisms depend on other 

species (parasites, specialist feeders). 

The host species may be randomly or 

regularly distributed, but this will 

result in a clustered distribution of the 

dependent species (on or around the 


Ecologists refer to this as 

"patchiness" of the habitat.


coarse grained patchiness

A coarse grained, patchy habitat - where the size 

of patches is bigger than the normal movement 

range of the species - will often have a greater 

effect on distribution than social interactions. 

Members of a species will be found only in the 

scattered suitable patches


fine grained patchy habitat

A fine grained patchy habitat - where individuals 

of the species regularly move distances greater 

than the size of the patches - may encourage 

clumped distributions (flocks, packs) if groups find 

useful patches more easily than individuals do.


how is the geographic spread of a species determined?

where the physical conditions are within the species tolerance 

range (physical adaptation).

where there are the biotic and abiotic resources required by the 

species (competitive adaptation).

where there is a sufficient area of suitable habitat to support a self-

sustaining population (habitat availability).

where the species can reach geographically (dispersal ability).



- the individual local populations 

are not sustainable, but dispersal between 

habitat patches is sufficient to allow the 

establishment of new populations as fast 

as others are extirpated.

In environments where 

human activity has fragmented the natural 

environment, there may not be enough 

contiguous habitat to guarantee the 

survival of many species, but it may be 

possible to connect small habitat 

fragments to produce a metapopulation, 

where local extinctions are balanced by 

local invasions.


limits the growth of a population

- when one species is very successful


graph of number of species to number of individuals

at one end, there is the high number of species but low individuals and the other end it is high number of individuals but low number of species (looks exponential)


common species

are usually well adapted to their niche, not specific for resources or can grow faster and reproduce more


rare species

usually are top carnivores that require a lot of energy, are adapted to a specific niche that is disappearing, very rare to find or is outcompeted by other species


how does a population grow

4 factors:
The number of births (B)
The number of deaths (D)
The number of immigrants (I)
The number of emigrants (E)

 Nt+1 = Nt + B - D assuming that immigration equals emigration (it would be I-e)


birth and death rate and growth rate

is the number of births/deaths over the number of individuals in population.

rate of growth is measured by B-D (more births is growth)


 ri (or rmax)

"intrinsic rate of natural increase" or 
biotic potential - rate at which a population 
would grow if it were not constrained by the 

Rarely do species reach ri in the real world, 
because the environment always imposes 
some limits or obstacles. if it would, the graph would be exponential


an unrestrained population equation

The actual size of an unrestrained population at 

a future time will thus be a function of the 

current size and the natural rate of increase:

 Nt+1 = Nt + riNt

The instantaneous rate of change in the 
population (slope of the growth curve) is then 

the derivative of this equation:

 dN = riN

this shows the difference between birth and death rate a higher value of a derivative means greater difference between births and deaths (higher growth rate)


typical distribution

lots of individuals, then some rare ones


density-independent regulation

In some harsh environments, periodic or random events may 

wipe out much of a population, thus resetting growth. this event will wipe everything out regardless of the individuals (tsunami will kill both 5 and 500)



carrying capacity (curve increases then flattens out at a certain level)


density dependent regulation

Density-dependent regulation of population size 

means that at higher population densities either 

the death rate must increase or the birth rate 
must decrease spontaneously.

It is clear that death rates will increase if
numbers exceed the available food supply
crowding causes exclusion from access to 

some other vital resource
crowding causes a build-up of waste that 

affects survival
crowding increases aggressive behaviour
crowding increases disease transmission
higher density populations attract the attention 

of predators which then take proportionally 

more of that kind of prey
higher numbers allow predator populations to 

grow rapidly


Birth rate might decline at high populations

malnutrition reduced fecundity or the ability to 

raise young.
stress hormones affected fertility or fecundity.
limited access to breeding territories or suitable 

lairs or nest sites meant that some individuals 

failed to breed.
deferring reproduction increased a female's 

chances of surviving to reproduce later.


sustainable yeild

graph is exponential to the maximum yield. the best time to take the crop is at the part where it is increasing the fastest (in the middle)


the logistic model suggests that

a population should increase rapidly at low numbers and slow as it reaches k
- decrease if it exceeds k

 dN = rN(K-N)
 dt K


life history

the array of stages, strategies and behaviours an organism 

uses over the course of its life to enhance survival, fertility and fecundity.

It includes things such as life cycle stages, 

growth, maturation, breeding timing and 

strategies, parental care and investment.

Natural selection will favour any strategy that 

maximizes long term fitness, provided of course 

that the behaviours are under genetic control.


best for genetic selection (alleles)

Generally, selection 

will favour shortening the stages that are most 


The life history of any population should be the 

compromise that maximizes inclusive fitness of the 

average individual. Any allele that influences a 

strategic action that produces more copies of itself 

should become common in the gene pool.


rapid reproduction vs mortality

rapid reproduction: make lots of offspring but each time, mortality rate increases.

slow production: less offspring but less mortality

these curves are both downward and pretty much identical

In fact, the advantage for the fast reproducers might be even higher, as they would also have 

higher ri, by virtue of getting more of their offspring to reproductive age earlier.


life tables


number alive at start/mortality rate/survival rate/survivorship/ potential fecundity/average fecundity


calculate mortality rate

Mortality rate is the 
proportion of the individuals starting a time period who die during the period.

1-survival rate


survival rate calculation

2nd gen over 1st gen



Survivorship is the proportion of the original cohort still 
alive at the beginning of the time period.

number of individuals of x generation over ORIGINAL GENERATION


r selection

If a population is actually below the carrying capacity of the environment, and is 

growing exponentially, maximizing r may be the best possible way to raise 
individual fitness.

Since available resources are not being exploited, there are opportunities for 

offspring, even those that may not be highly competitive. Natural selection thus 

favours quantity of offspring over quality. This is referred to as r-selection.

this is when a graph is exponential but increases slowly. at the increasing range, the population is below K and selection favours maximizing r


r selected population life history

this may include:
earlier maturity and reproduction
 Shorter generations mean faster exponential growth.

larger numbers of smaller eggs
 Large eggs require more energy, thus lowering birth rate.

 Saving energy for future reproduction lengthens average 

 generation time. It is also practical if the chances of 

 surviving long enough to reproduce again are low.

short life expectancy
 Using energy to increase survival diverts energy from 


little or no parental care
 Investment in current offspring comes at the expense of 

 future offspring. In semelparous species, any energy 

 used for care could have been used to produce more eggs.


k selection

If a population is typically at or near its carrying capacity most of the time, the 

level of competition will always be high, and few new recruits will find a place in 

the population

There is no advantage to producing lots of offspring. Instead, the best way to 

increase one's alleles in the next generation is to produce offspring that can out-

compete others when an opportunity exists. The optimum strategy is to produce 

quality offspring, not quantity.

This is referred to as K-selection, and produces a different set of life history traits.

this is when the graph is flat, it favours higher quality offspring


k selected population life history

This may include:
long growth periods and late maturity
 Maximizing "readiness" is better than starting early.

small clutches or litters
 Large eggs or young give a better head start and reduce offspring 

 mortality rates.

 Having multiple rounds of reproduction increases the chances of 

having young at the times when spaces become  available.

long life expectancy
 Long life span maximizes total lifetime reproduction, and 

 allows for more care.

extensive parental investment
 Protecting, nurturing, socializing and even teaching young 

 increases their chances of competing when spaces open up.


actual fecundity rate

potential fecundity times the number of individuals in that generation


viable seeds per plant is

sum of the fecundity of the plant



single reproductive event, r selected



many small reproductive events, k selected


List four key environmental variables that determine where an organism can live. (4K)



low survival rates early in life mean

more reproduction!


more constant resources

is iteroparity. Due to the constant conditions, they would likely live a high percentage of their lifespan and be able to reproduce offspring that would need to be strong competitors.


exponential growth can occur when

The growth only tends to happen when the environmental resources are plentiful or the population is below its carrying capacity.


population pyramid growth rates

the one that has the most slow growth is the one that is only slightly larger than the current reproducing population.