Flashcards in Unit 4 Deck (48)
defined as all the organisms of a particular species in a particular area at a given time.
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
A random distribution will occur if organisms in a population act completely independently of one another. (a very scattered graph)
when the animals are very social or attached to parents (very clustered but very empty in other parts graph)
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
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
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)
are usually well adapted to their niche, not specific for resources or can grow faster and reproduce more
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
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)
lots of individuals, then some rare ones
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
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
Birth rate might decline at high populations
malnutrition reduced fecundity or the ability to
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.
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)
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)
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.
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.
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
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
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.
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
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
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 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.