Transcript Notes - Community Ecologyx

Big Idea 4 - Ecology
Ch. 53 – community ecology
Levels of Organization
•
•
To study relationships within the
biosphere, ecologists have organized it
into smaller pieces.
The levels are:
1. Organism (a single individual – one
fish, for example)
2. Population (when organisms of a
single species share the same
geographic location at the same
time)
3. Biological community (a group of
populations that interact and occupy
the same space at the same time)
4. Ecosystem (a biological community
plus all of the abiotic factors that
affect it)
ex – trees plus water and soil
5. Biome (a large group of ecosystems
that share the same climate and have
similar communities)
Ex – marine or desert biomes
6. Biosphere (all the biomes on Earth
combined)
Communities
• A community is a
group of organisms
of different species
that live in a
particular area
Overview
• Ecologists call
relationships between
species in a community
interspecific interactions
• Interspecific interactions
affect species survival and
reproduction
• Examples are
competition, predation,
herbivory, symbiosis
(parasitism, mutualism,
and commensalism), and
disease
Ecological Niches
• An organism’s niche is the specific
role it plays in its environment
– All of its uses of biotic and abiotic
resources in its environment
– Example: oak tree in a deciduous forest
•
•
•
•
•
Provides oxygen to plants, animals, etc.
Home for squirrels
Nesting ground for blue jays
Takes water out of the soil
Etc., etc.
• Ecologically similar species can
coexist in a community if there
are one or more significant
differences in their niches
Fundamental v. Realized
Niche
• Fundamental Niche
includes resources an
organism could
theoretically use (if no
competition)
• Realized Niche includes
resources it actually
does use given
competition from other
species.
Resource partitioning
• Resource partitioning is
differentiation of
ecological niches
– enables similar species
to coexist in a
community
• Similar species develop
ways to partition/divide
resources in order to
coexist.
Interspecific Competition
• Competition between organisms of different
species
• occurs when species compete for a resource in
short supply
• can lead to competitive exclusion (local
elimination of a competing species)
• The Competitive Exclusion Principle:
– Two species with similar needs for the same limiting
resources cannot coexist in the same place
– Niches may overlap but they may not be identical.
Interspecific Interactions
• There are 4 major
interspecific interactions
(symbiotic relationships):
– Parasitism (includes
predation and herbivory)
– Competition
– Commensalism
– Mutualism
Predation (and Parasitism)
• (+ -)
– The interaction is
beneficial to one
species and
detrimental to the
other
• Predation:
– When a predator eats
its prey
– Some feeding
adaptations of
predators are claws,
teeth, fangs, stingers,
and poison
– Example in picture:
Herbivory
• Herbivory refers to an
interaction in which an
herbivore eats parts of
a plant or alga
• It has led to evolution
of plant mechanical and
chemical defenses and
adaptations by
herbivores
Plant Defenses Against
Herbivores
• “Plants Fight Back!”
• Plants have 2 major
mechanisms by
which they defend
themselves against
being eaten
– Mechanical
Defenses
• Thorns, hooks, etc.
Plant Defenses Against
Herbivores
• Chemical Defenses
– Produce chemicals
that are distasteful
or harmful to an
herbivore
– Morphine (opium
poppy)
– Nicotine (tobacco)
Animal Defenses Against
Predators
• Animals defend
themselves against
predators passively
(hiding) or actively
(fleeing)
• Cryptic coloration
(camouflage) makes prey
difficult to spot
• Aposematic coloration
(warning
coloration)warns
predators not to each
animals that may be toxic
or may sting.
Animal Defenses Against
Predation
• Mimicry
– When one species “imitates” or “mimics” another
– Batesian mimicry
• When one edible or harmless species mimics an bad-tasting
(unpalatable) or harmful species
• Example: hawkmoth mimics a snake
LE 53-7
Green parrot snake
Hawkmoth larva
Animal Defenses Against
Predation
• Mimicry
– Mullerian mimicry
• Two species, both of which are unpalatable (taste bad) or
harmful, resemble each other
• Example: monarch butterfly (unpalatable) and queen
butterfly (unpalatable) resemble each other
Parasitism
• One organism (the
parasite) gets its
nourishment from
another organism (the
host), which is harmed in
the process
• Usually do not kill their
hosts
• Endoparasites:
– Live within host tissues
(tapeworms)
• Ectoparasites:
– Feed on external
surfaces (mosquitoes)
Mutualism
(++) relationship
Both partners benefit from the
relationship
“You scratch my back, I’ll scratch
yours”
Examples:
Ants & acacia tree
tree provides high protein
food in beltian bodies & habitat for
nests inside thorns; ant protects
against predators
Mycorrhizae-fungal extentions on
plant roots
Plant gets increased
water/nutrition, fungi gets
food
Hummingbirds & flowers
Hummingbirds get food, flowers
can reproduce
Commensalism
• Commensalism – one organism
benefits and the other is neither
helped nor harmed
– Ex – shark and remora fish;
sea anemones and clownfish
Dominant & Keystone Species
•
•
•
•
In general, a few species in a community exert
strong control on that community’s structure
Two fundamental features of community
structure are species diversity and feeding
relationships
Dominant Species:
– Species in a community that have the
highest abundance or highest biomass
– One hypothesis suggests that dominant
species are most competitive in exploiting
resources
– Another hypothesis is that they are most
successful at avoiding predators
– American Chestnut tree and the chestnut
blight
Keystone Species:
– Important to a community because of
their ecological roles (niches), not by
numbers
– In contrast to dominant species, keystone
species are not necessarily abundant in a
community
– Wolves control deer populations
– Bees help pollinate local plants
– Elephants eat small trees, which preserve
grasslands
– Prarie dogs eat grass which keeps water in
the soil and prevents evaporation
LE 53-17
Otter number
(% max. count)
100
80
60
40
20
0
Sea otter abundance
Number per
0.25 m2
Grams per
0.25 m2
400
300
200
100
0
Sea urchin biomass
Food chain before
killer whale
involvement in
chain
10
8
6
4
2
0
1972
1985
Total kelp density
1989
Year
1993 1997
Food chain after
killer whales started
preying on otters
Ecosystem “Engineers” (Foundation
Species)
• Some organisms exert
influence by causing
physical changes in the
environment that affect
community structure
• Some foundation species
act as facilitators that have
positive effects on survival
and reproduction of some
other species in the
community
• For example, beaver dams
can transform landscapes
on a very large scale
LE 53-19
Number of plant species
8
6
4
2
0
Salt marsh with Juncus
(foreground)
With
Juncus
Without
Juncus
Conditions
Ecological Succession
• Ecological succession is a
change in the species that
live in a given area over a
period of time
– One community replaces
another
• Primary succession = occurs
in places where soil is not
yet formed (bare bedrock)
• Secondary succession =
occurs in places where
there is soil, but where
some disturbance has
eliminated the previous
community
Ecological Succession
• The first organisms to
inhabit an area undergoing
succession are known as
pioneer organisms
– These are usually small
organisms (bacteria,
lichens, algae, etc.)
• The ecosystem goes
through a number of stages,
with each new stage usually
consisting of larger
organisms than the last one
• Once a community has
become stable and is not
changing much, it is known
as a climax community
Ecological Succession cont.
• Early-arriving species and
later-arriving species may
be linked in one of three
processes:
– Early arrivals may facilitate
appearance of later species
by making the
environment favorable
– They may inhibit
establishment of later
species
– They may tolerate later
species but have no impact
on their establishment
Causes of Ecological Succession
•
There are 3 major causes of ecological succession:
1. Human Activities
- logging, mining, development, etc.
2. Natural Disasters/Disturbances
- fires, volcanic eruptions, etc.
3. Natural Competition Among Species
- Fictitious example:
- turtles and frogs both eat crickets
- frogs are faster, turtles are slower
- frogs eat more crickets, turtles starve
- turtle population dies out, frog population
gets bigger
Species Diversity
• Species diversity of a community is the variety of
organisms that make up the community
• It has two components:
– species richness
– relative abundance
• Species richness is the total number of different
species in the community
• Relative abundance is the proportion each species
represents of the total individuals in the
community
Species diversity cont.
• Two communities can
have the same species
richness but a different
relative abundance
• A community with an
even species abundance
is more diverse than
one in which one or two
species are abundant
and the remainder are
rare
Trophic Structure
• Trophic structure is the
feeding relationships
between organisms in a
community
• It is a key factor in
community dynamics
• Food chains link trophic
levels from producers to
top carnivores
LE 53-12
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
A terrestrial food chain
Phytoplankton
A marine food chain
Food Webs
• A food web is a series of
connected food chains
with complex trophic
interactions
Limits on Food Chain Length
• Each food chain in a food web is usually only a few
links long
• Two hypotheses attempt to explain food chain
length:
– the energetic hypothesis and
– the dynamic stability hypothesis
Limits on food chain links cont.
• The energetic hypothesis
suggests that length is
limited by inefficient
energy transfer
– Most data support the
energetic hypothesis
• The dynamic stability
hypothesis proposes that
long food chains are less
stable than short ones
Bottom-Up and Top-Down Controls
• The bottom-up model of
community organization
proposes a unidirectional
influence from lower to
higher trophic levels
• In this case, presence or
absence of mineral
nutrients determines
community structure,
including abundance of
primary producers
• The top-down model
proposes that control
comes from the trophic
level above
• In this case, predators
control herbivores,
which in turn control
primary producers
• Long-term experimental studies have shown
that communities can shift periodically from
bottom-up to top-down controls
• Pollution can affect community dynamics
• Biomanipulation can help restore polluted
communities
What Is Disturbance?
• A disturbance is an
event that changes a
community, removes
organisms from it, and
alters resource
availability
• Fire is a significant
disturbance in most
terrestrial ecosystems
• It is often a necessity in
some communities
LE 53-21
Before a controlled burn.
A prairie that has not burned for
several years has a high propor-tion of
detritus (dead grass).
During the burn. The detritus serves as
fuel for fires.
After the burn. Approximately one
month after the controlled burn,
virtually all of the biomass in this
prairie is living.
• The intermediate disturbance hypothesis suggests
that moderate levels of disturbance can foster
higher diversity than low levels of disturbance
• The large-scale fire in Yellowstone National Park in
1988 demonstrated that communities can often
respond very rapidly to a massive disturbance
LE 53-22
Soon after fire. As this photo taken soon after the fire shows, the
burn left a patchy landscape. Note the unburned trees in the
distance.
One year after fire. This photo of the same general area taken the
following year indicates how rapidly the com-munity began to
recover. A variety of herbaceous plants, different from those in the
former forest, cover the ground.
Human Disturbance
• Humans are the most widespread agents of
disturbance
• Human disturbance to communities usually
reduces species diversity
• Humans also prevent some naturally occurring
disturbances, which can be important to
community structure
Biogeographic factors affect
community diversity
• Equatorial-Polar
Gradients
• Species richness generally
declines along an
equatorial-polar gradient
and is especially great in
the tropics
• The greater age, climate,
and amount of light found
in tropical environments
may account for the
greater species richness
Biogeographic factors affect
community diversity cont.
• Climate is likely the primary cause of the
latitudinal gradient in biodiversity
• Two main climatic factors correlated with
biodiversity are:
– solar energy and
– water availability
• They can be considered together by measuring a
community’s rate of evapotranspiration
• Evapotranspiration is evaporation of water from
soil plus transpiration of water from plants
Area Effects
• The species-area curve
quantifies the idea that,
all other factors being
equal, a larger geographic
area has more species
• Species richness on
islands depends on island
size, distance from the
mainland, immigration,
and extinction
Rivet and Redundancy Models
• The rivet model suggests that all species in a
community are linked in a tight web of
interactions
– It also states that loss of even a single species has
strong repercussions for the community
• The redundancy model proposes that if a
species is lost, other species will fill the gap
• Community hypotheses and models represent
extremes; most communities probably lie
somewhere in the middle