Transcript Communities and Ecosystems

Communities & Ecosystems
Communities
• A biological community is an
assemblage of the populations
of all the organisms that live
close enough together to
interact.
• South Florida communities
include: pinewoods (upper
right) and tropical hardwoods
hammocks (lower right)
• The characteristics of
communities include:
– Species diversity
– Dominant species
– Trophic structure
– Response to disturbance
Competition
• In a community there may be interspecific competition for
resources, such as food, light, living space, and nutrients.
• A resource that is in short supply may act as a limiting factor.
• The competitive exclusion principle states that if two species
are competing for the same limited resource, any difference in the
ability to efficiently use this resources, will lead to a reproductive
advantage for one species with the eventually elimination of the
other species.
• Each species has a niche or specific role in the community.
• If two species have exactly the same niche, then either one will
become extinct or it will evolve to use a different set of resources.
• The latter is known as resource partitioning.
Predators and Prey
• Predation is an interaction between species in which one species,
the predator, kills and eats another species, the prey.
• Natural selection has lead to many adaptations to both predator
and prey species. For predators This may include speed and
agility, poison injecting fangs and stingers, sharp claws and talons,
ir keen vision to spot the prey. Likewise, prey species may be
swift, may have keen senses of smell, hearing, or vision to detect
the presence of predators.
• Beyond these obvious adaptations, there are some others that
merit special attention. On the next several pages, we will look at
four of these: cryptic coloration, warning coloration, Bastesian
mimicry and Mullerian mimicry.
Batesian Mimicry
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Batesian mimicry can be best seen
in insects. It occurs when an
edible species closely resembles a
distasteful, unpalatable species.
Predators that learn to avoid the
distasteful form also avoid the
edible mimic. This presumes that
the model is usually more
abundant than the mimic.
The Queen butterfly (upper right),
whose larva feed on poisonous
milkweed plants has an edible
mimic, the Viceroy (lower right).
The Queen butterfly is closely
related to the unpalatable
Monarch butterfly.
Mullerian Mimicry
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The larva of all of the Heliconid
butterflies (seen to the right) feed
on passion flower vines and store
the toxic compounds found in their
leaves. Mullerian mimicry occurs
when a number of distasteful
species, found in the same area,
have a similar color pattern. A
predator becoming imprinted on
one heliconid, tends to avoid other
butterflies with similar patterns.
Thus all receive protection from
predation by their mutual
resemblance.
Warning Coloration
• Bright colors, such as reds
and oranges, can be used as
warning colors. Predators
quickly learn to recognize and
avoid unpalatable or toxic
brightly colored animals.
• The colors of this poison
arrow frog are one example.
• In South Florida the larva of
the Atala butterfly are easily
seen, due to their red colored
body and yellow spots. They
also feed on the toxic leaves
of the coontie (a native
cycad).
Cryptic Coloration
• Cryptic coloration
(camouflage) allows a
predator to wait, unseen by its
prey.
• It also allows animals to hide
from predators by blending
into the background.
• Normally this Sphinx moth
would have its wings closed,
hiding the abdomen and
lighter colored hind wings.
• Can you find the stone fly
larvae on the rock to the right?
Predation – Keystone species
• A keystone species is a predator that maintains strong control over
community structure. A keystone species of the everglades
ecosystem and an indicator of its health is the American alligator.
This ancient reptile builds "alligator holes" that provide an important
food and water source for many other animals in times of drought.
Coevolution of plants and herbivores
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Plants have evolved many
physical and chemical defenses
against herbivores. Sharp spines,
thorns, bad tasting chemical are
but a few.
Another interesting example is the
coevolution of ant and bull-horn
acacia trees. The ants live in the
hollow horn (thorns) and eat sugarrich nectar secreted by the plants.
In turn they quickly attack any
herbivorous creature that tries to
dine on the acacia.
Studies have shown that the
removal of ant colonies from the
acacia trees, negatively affects the
overall health of the trees.
Symbiotic relationships
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Symbiotic relationships are
interactions between two or more
species that live in direct contact
with each other. These
relationships can take many forms,
though the commonest are
commensalism, mutualism and
parasitism.
The example to the right is
commensalism. The Cape Buffalo
stir up insects upon which cattle
egrets feed. Small tick birds have
a mutualistic relationship with the
buffalo. They feed upon ticks and
other pests of the large herbivore.
They get food, while the parasites
are removed from the buffalo.
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In South Florida, cattle egrets can
often be seen following tractors
and lawn mowers. We are now
part of the symbiotic pathways!
Symbiotic relationships 2
• In the next slide there is an
example of both parasitism
and mutualism. Aphids are a
major plant pest. They such
the sap out of the plant and
their sweet secretions may
cause a heavy growth of sooty
mildew on the leaf surfaces.
These same aphids are
‘farmed’ by ants. The ants
feed on the sugar rich
secretions of the aphids,
move them to better sites, and
protect them from predators.
• Ants and aphids can be seen
in the right illustration.
Parasitism and Mutualism
Disturbance and Succession
• Communities are constantly
changing in response to
disturbances.
• This leads to ecological
succession, where species
are gradually replaced by a
succession of other species.
• If this begins in a bare, lifeless
area, it is called primary
succession. Secondary
succession occurs when an
existing community is
destroyed or disrupted.
• In South Florida, disturbances
may be fire, a hurricane,
lowered water table, or even
agriculture.
• The final stage of succession
is called the climax. The
climax community in South
Florida is the tropical hardwood hammock, however
periodic fire kept much of the
area in pinewoods. This is
called a fire climax.
• Old abandoned farmlands in
North Carolina will undergo
secondary succession and
slowly return to temperate
forest.
Succession (in Alaska)
Trophic levels
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Feeding levels are called trophic levels. At the base of every food chain and
food web are the primary producers. These are photosynthetic organisms
that fix solar energy into sugar. Primary consumers feed on the primary
producers. In turn primary consumers fall prey to secondary consumers.
Tertiary and quaternary consumers are found at successively higher trophic
levels.
Trophic levels continued
• Another important trophic level is that of the decomposers or
detritivores. These are the organisms that consume dead and
decaying organic matter. They are critical to the recycling of
nutrients and minerals. The materials that they release are
available
• Animals, such as, crayfish, vultures, earthworms and mice often
play this role.
• However, the most important recyclers are fungi and bacteria. If
you turn over a pile of leaves that have been on the ground for a
long time, many of the leaves will be held together by white threadlike fungal filaments. Did you know that when you see a
mushroom in a field you are only seeing the reproductive structure
of a fungus? The rest of it is spread out through the dead organic
matter in the soil.
Food Chains
• Food chains show the steps in
the energy flow pathway.
• While it is a useful concept,
the feeding relationships
within a real community are
rarely that simple.
• Think of your supper. Unless
you are a Vegan (pure
vegetarian) your meal
probable represents several
trophic levels. That doesn’t fit
neatly into a food chain like
the one you see at the right.
Food Webs
• Food webs can be used to
study the complexity of energy
flow through a community or
ecosystem.
• The web at the right could be
even more complex. What
kinds of fishes? What is their
relative abundance? Would it
matter if a particular species
of copepod were eliminated by
disease? Does the loss of
one species of birds affect the
web or do the others increase
in abundance once it is
eliminated?
Energy Flow and Chemical Cycling
Primary Production
• Three terms used in ecology are biomass, standing crop and
productivity. Don’t confuse them!
• Biomass is a measure of the amount of biological material.
• Standing crop is the amount of biomass at a point in time.
• Productivity is a measure of the rate at which biomass is
produced.
• Imagine two one acre fields of grass. You cut the grass in both
fields one inch from the ground, and then you weight the grass.
Field A has 1000 lbs of biomass, while Field B has 800 lbs of
biomass.
• Is field A more productive? You can’t say unless you know the rate
of production of grass. It is July 20th. Field A hasn’t been mowed
since May 5th . Field B was mowed May 20th, June 15th, and July
1st. Even though the standing crop in Field A was greater, the
productivity in Field B was much greater.
Energy Pyramids and Energy Flow
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Pyramids are a way to show
energy relationships.
These may take the form of
a pyramid of biomass or a
pyramid of numbers of
organisms. The general
rule is that there is about a
10% transfer of energy from
trophic level to the next.
In the example to the right,
the upper pyramid is typical
of most energy pyramids.
How could you explain the
lower pyramid? It is very
atypical.
Water Cycle
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Freshwater is critical for plants and
animal communities. The water
cycle can be see at the right.
There is a net evaporation of water
over the oceans. Water vapor
moves over land, driven by
atmospheric patterns of circulation.
Rainfall on the plants is taken up
by plants, but also sinks into the
soil and/or forms lakes and rivers,
eventually returning to the oceans.
A small amount of available water
is trapped in polar and high
elevation ice caps.
Carbon Cycle
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Life on earth is based on carbon
compounds. All of the energy-rich
organic molecules of living
systems are constructed using
carbon.
The carbon cycle can be seen at
the right. The oceans can serve
as a carbon sink, however it is a
limited sink. The huge amounts of
carbon dumped into the
atmosphere by the burning of
fossil fuels threaten to upset the
atmospheric patterns of
temperature and rainfall. More
about that in our last lesson.
Nitrogen cycle
• Nitrogen is an essential
nutrient for plants.
• Although over 78% of the
atmosphere is comprised of
nitrogen, it is not available to
plants in the form of N2.
• As seen in the diagram at the
right, nitrogen fixation and
cycling depends upon the
action of species of bacteria.
They fix atmospheric nitrogen,
decompose the amino groups
of proteins to ammonia,
convert ammonia to nitrite and
nitrite to nitrates.
Phosphorus cycle
• Phosphorus is needed By
both plants and animals. It is
an essential part of ATP and
the nucleic acid molecules
that carry genetic information.
• Mosat phosphorus comes
from rocks, and is released
through the process of
weathering.
• Phosphorus can be a limiting
factor, due to the difficulty in
releasing from rocks and
sediments.
• The lack of phosphorus in
lakes often limits the growth of
algae and other plant life.
Eutrophication
• Runoff from man’s activities,
such as agriculture or lawn
fertilizing, can remove
phosphorus as a limiting factor
and lead to tremendous algal
blooms.
• Eutrophication occurs when
nutrients accumulate in a lake
and the growth of vegetation
is accelerated. This can be a
natural aging process, but
more frequently is caused by
the addition of nutrients
directly or indirectly.
Deforestation
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When land is cleared and the soil is denuded of vegetation, there is nothing
to hold the soil in place when rain falls. As a result, silt and nutrients are
washed into streams and rivers, and alter the aquatic communities found
there
Deforestation
The loss of forests in Haiti, often
leads to massive flooding and
erosion during the heavy rains
associated with tropical
storms.
Massive scale land clearing for
oil palm plantations in
Indonesia threatens
biodiversity and leads to
polluted waterways.
Scenes like the one to the right
can be found throughout the
tropical regions of the world.
In our next and last lesson, man’s
impact on the environment will
be examined.