Download Ch. 6 Population and Community Ecology

Chapter 6
Population and Community Ecology
Nature exists at several levels of
complexity
• Individual (the unit of natural selection)
Population (the unit of evolution, composed
of all individuals that belong to the same
species and lives in a given area)
Community (interactions amongst species
within a given area) Ecosystem (flow of
energy and matter, both biotic and abiotic
factors in a particular location) Biosphere
(global processes that incorporates all of the
Earth’s ecosystems.
The factors that regulate population
abundance and distribution
• The study of factors that cause populations to
increase or decrease is the science of
population ecology.
• Inputs that increase population size=
immigration and births. Outputs that decrease
population size= emigration and deaths
• Population characteristics include: size,
density, distribution, sex ratio and age
structure.
Population Size
• N is the total number of individuals within a
defined area at a given time
• Density dependent factors influence an
individual’s probability of survival and
reproduction in a manner that depend on the size
of the population. Ex: amount of available food
(is a limiting resource)
• K is the carrying capacity of the environment or
the limit that the supply can sustain.
• Density-independent factors is the individuals
probability of survival and reproduction at any
population size. Ex: tornadoes, hurricanes, fires,
floods.
• Population Density-Is the number of
individuals per unit of area at any given time.
• Important to wildlife managers who set up
hunting and fishing limits on species
• Population Distribution- is a description of
how individuals are distributed with respect to
one another. Ex: Random (no pattern),
uniform (evenly spaced out), or clumped
(living in large groups enhance feeding
opportunities or protection from predators
Distribution patterns-populations in
nature distribute themselves in three
ways:
• Population Sex Ratio- ratio of males to
females (knowing this helps to figure out the
number of offspring a population will produce
in the next generation.
• Population Age structure-is the description of
how many individuals fit into particular age
categories. If they are all old, they will not
reproduce.
Growth models and population
changes
• Population Growth Models are mathematical
equations that an be used to predict population
size at any moment in time.
• Growth Rate=the number of offspring an
individual can produce in a given time period,
minus the death rates of the individual or its
offspring during the same period of time.
• Intrinsic growth rate= a populations particular
maximum potential for growth (r) under ideal
conditions
Exponential Growth Model
As time goes by the population increases rapidly, creating a
J-shaped curve
Exponential Growth and Mathematics
Nt= Noert
Nt= The populations future size
No= The current size of the population
r= the intrinsic rate of the population
t= the amount of time the population grows
The value grows by the same percentage every year, but
cannot grow like this indefinitely. Eventually the carrying
capacity levels off the population
The Logistic Growth Model
• Describes a population whose growth is
initially exponential, but slows as the
population approaches the carrying capacity
of the environment (K).
This model is used to
predict
population growth that is
subject to densitydependent constraints
This S- shaped curve
shows that as populations
reach carrying capacity
the populations stop
growing and remain at a
constant rate.
Population Oscillations
When the population is larger than the carrying capacity
there is an overshoot. Usually this leads to a die off, or
population crash (below carrying capacity) and then
oscillates again . Over time, the population will reach
carrying capacity
Population Oscillations in Lynx and
Hares
Predation is another factor in limiting population growth. This graph
indicates that populations of both species cycle over time.
Reproductive Strategies and
Survivorship curves
• K- selected species(abundance of specie is
determined by carrying capacity and have
small fluctuations)have certain traits in
common: large organisms, produce few large
offspring, provide substantial parental care
(large mammals and birds). Their populations
grow slowly and cannot respond quickly to
efforts to save them from extinction.
• r-Selected species= have a high intrinsic
growth rate, reproduce often, have large
numbers of offspring, and show little or no
parental care. These populations do not
remain near their carrying capacity but exhibit
offshoots and die-offs. Ex: fish, insects, mice,
dandelions and other weeds.
• Most species fall somewhere between these 2
extremes of reproductive strategies.
Survivorship Curves
Type I (man, whales, elephants) have high survival rates throughout
most of their life span. As they age, they die out in large numbers.
Type II (squirrels, coral) experience a constant decline throughout
their life span
Type III (mosquitoes, weeds) low early survivorship, few reach
adulthood
Metapopulations contribute to the
preservation of biodiversity
• Metapopulations = separate and distinct
populations that have been connected by
occasional movements of individuals between
them.
• This provides s species with some protection
against threats such as disease.
• Increases the little genetic variation that
occurs in smaller populations
Community Ecology
• For a specie to be distributed in an area it must :
be able to live in a range of abiotic factors, it must
be able to disperse to that area, and it must be
able to interact with other species that live there.
4 categories of interactions: competition,
predation, mutualism, commensalism
Community Ecology studies these interactions
which determine the survival of a species in a
habitat.
Competition-the struggle of individuals to
obtain a limiting resource
• Competitive Exclusion Principle-states that two
species that compete for the exact same resources
cannot stably coexist.
• In his experiments, Gause found that three species
• of Paramecium grew well alone in culture tubes.
• But Paramecium caudatum would decline to
extinction when grown with P. aurelia because they
shared the same realized niche, and P. aurelia
outcompeted P.caudatum for food resources.
• However, P. caudatum and P. bursaria were able to
coexist because the two have different realized
niches and thus avoided competition.
G.F. Gause’s Experiment
• Russian biologist, in lab
• Both Paramecia feed on
bacteria
– P. aurelia consistantly
outcompetes P. caudatum
Resource Partitioning
• Two species divide a limited resource based
on differences in the specie’s behavior or
morphology.
• Hunt at different times of day or flowering at
different times a year ((temporal resource
partitioning).
• Use different habitats (spatial resource
partitioning)
• Evolution of differences in body size or
shape(morphological resource partitioning)
Predation –the use of one species as a
resource by another species
• True predators: kill their prey and consume most of
what they kill. Ex: Lions
• Herbivores: consume plant as prey, typically eating
only a small part of a plant w/o killing it. Ex: deer
• Parasites: live on or in a host and consume a small
portion of a host and rarely does a single individual
cause the death of host. Ex: tapeworms
• Parasitoids: organisms that lay eggs inside other
organisms. When they hatch, they consume host
from inside to out causing death of host. Ex: wasps
and flies
Parasitism (tick on dog)
Mutualism- benefits 2 interacting
species (by each assisting the other to
benefit itself
Ex: The acacia tree and ants.
Tree supplies food and
shelter for ants, ants supply
protection for trees by
stinging other insects that
might eat it.
Ex: Lichen- Fungus provides
nutrients for the alga and the alga
provides carbohydrates to fungus
via photosynthesis
Commensalism-one specie benefits
but the other is neither harmed or
helped
• A symbiotic relationship is a relationship of
two species that live in close association with
each other. Commensalism is one example.
Keystone Species
• Is a species that plays a role in its community that
is far more important than its relative abundance
might suggest. They usually exist in low numbers.
• They may be predators, sources of food,
mutualistic species, or providers of some other
essential service.
• Keystone predator-mediated competition
• Is an example of how a keystone specie plays
such an important role.
Predator-mediated competition
Sea Stars (in the intertidal community) although not very numerous,
prey on mussels which normally take over rock space.
This cleared space allows other species to attach to rock. Thus, the
predatory sea star reduced the abundance of a superior competitor
and allowed inferior competitors to persist.
Ecosystem Engineers
• A keystone species may create or maintain a
habitat for other species.
Beavers have a vital role
in the forest community.
They build dams that
convert narrow streams
into large ponds,
creating new habitats
for plants and animals.
Ecological Succession- the predictable
replacement of one group of species
by another over time.
• Primary Succession-occurs on surfaces that
are devoid of soil. Ex: rocks, cooled lava etc.
• Rockslichens and mossesannual weeds
• perennial weeds and
grassesShrubsAspen , cherry, young pine
• Forest(pioneering species)Beech and maple
broadleaf forest (climax forest)
Primary Succession- starts with a
surface devoid of soil
Secondary Succession
• Occurs in areas that have been disturbed but
have not lost their soil. It usually follows an
event like fire or hurricanes.
Aquatic Succession
Species richness of a community is
influenced by 4 factors
• Latitude= As you move from the equator to the
poles, the number of species decline. This pattern
is observed in plants, birds, reptiles and
amphibians and insects.
• Time= The longer a habitat exists, the more
colonization, speciation and extinction can occur
there.
• Habitat Size and Distance= Specie richness
increases as habitat size increases. Distance
matters because many species can disperse only
short distances.