General Biology 101
Chapter Outlines
Starr & Taggert
10th edition
Chapter 47:
Section 7
Forces Contributing to Community Stability:
A Succession ModelEcological succession – the developmental sequence for building a community from the
point of pioneer species that colonize an area to the climax community that is a “mixture”
of species in relative balance for a given area (and at a relative point in time).
Pioneer species – opportunistic colonizers of vacant or vacated habitats. They usually
disperse quickly, grow and mature very fast and have the ability to make many offspring.
Typically improve the conditions of the habitat for later colonization by other plants.
Note: pioneer plant species are often annuals that are fast growing and fast flowering, or
they may even be lichens and fungi forms.
Climax community: the complex set of organisms that make up the community towards
the end of the successional process. Self perpetuating array of species in equilibrium.
Types of succession:
I.
II.
Primary – original succession progression in an area that never before had a
community there e.g. a new volcanic island that emerges.
Secondary – a disturbed area within a community recovers and moves
towards a climax state e.g. after a fire, hurricane, or tsunami.
Different models
1) Climax-Pattern Model – A community is adapted to many environmental
factors that vary in their influence in a given region. For example - factors
of climate, soil, wind, and species interactions can influence this pattern.
2) Cyclic changes – such as felled trees or small scale episodic fires can
contribute to a greater mixture of heterogeneous communities. Note:
generally intermediate and small scale disturbances create the greatest
mixture of climax communities.
Restoration Ecology – the area of applied ecology that promotes “assisting” the
successional process via human intervention.
Examples of methods used in restoration ecology:
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Planting trees in wildfire areas.
Building artificial reefs
Repairing wetlands and grasslands i.e. reintroductions of lost species
Controlled burns of prairie lands.
Section 8
Forces Contributing to Community Instability
Resources are sustained as long as populations do not come to close to carrying capacity.
Disturbances can hurt the growth of some populations.
Long term changes in climate or some other environmental variable can have
destabilizing effects to a community.
Possible consequences:
1) Species can become extinct if they can’t compete or when disturbances
happen so fast that they cannot adapt.
Keystone species – a dominant species in a community that plays a “key” role in the
structure of that community.
Examples provided in the text:
1) Periwinkle (snails) affect the diversity of inter-tidal algae species, increasing
diversity in tide pools, but reducing it/algae on rocks that are exposed at low
tide.
2) Sea stars, a keystone species on the W. coast are important for maintaining
higher diversity, in their absence mussels take over by competitive exclusion
and crowd out their competitors. Key to this is that the sea stars are predators
on mussles.
Geographic dispersal – when organisms that are successful in their home community
branch out and successfully take up residence elsewhere.
Manner in which this may happen includes:
1) Over many generations – expansion into outlying areas.
2) Rapid transport – jump dispersal e.g. travel by ship or raft.
3) Imperceptible slowness – over long periods of time e.g. continental drift.
Examples of introduced species that changed community, ecological balance (mentioned
in the text).
1) Gypsy moths in the U.S. North east – defoliate forestes.
2) Zebra Mussels – block water pipes
3) Cryphonectaria (fungus) – killed American Chestnut trees
4) Fire Ants – U.S. Southeast – accidentally imported from S. America, likely by
ship.
5) African bees – migration assisted by humans but has gotten out of hand. A
deadly insect that has been on the march north from Latin America.
Other examples given in table 47.2
Section 9
Exotic and Endangered Species
Exotic species – a resident of an established community that was deliberately or
accidentally moved from its home range and became established elsewhere i.e. foreign,
imported species introduced to a new area. Usually proliferates in the absence of
predators.
Examples include:
1) Water hyacinth
2) Argentine fire ants
3) House sparrow
4) Japanese beetle
Close to 70% of the listed endangered species (or those that became recently extinct) owe
part of their demise to displacement by exotic species.
Case Study Examples:
I. Rabbits in Australia
In 1859 a settler in Australia imported 2 dozen wild European rabbits, for food and sport
hunting. The rabbit’s population boomed and took off because there were no natural
predators. Humans tried to control the increasing population by various means including
hunting and poisoning and fencing but nothing worked. In 1951 the government of
Australia introduced a rabbit virus via a different variety of rabbits from a different
region of the world i.e. S. America. The European rabbits that caught the virus died, but
some had a built in resistance that allowed them to survive even this very innovative and
crafty manner of control, thus a natural selection event occurred. In 1991 a new strain of
virus has been tested on a set of rabbits on an uninhabited island, but there has been fear
that this virus might spread to other non-rabbit animals.
II. Kudzu
Imported from Japan to Georgia it is a vine that was envisioned to help with erosion
control, but because it has no natural predators it spread quickly and has become a
menace. Kudzu can be used to process starch and possibly as an alternative source of
paper.
III. Caulerpa taxifolia (green algae)
A feathery exotic developed by an aquarium in Germany, which was accidentally
released into the Mediterranean and has caused havoc there. Wherever this exotic variant
is found it disrupts the local ecology and is able to exist even in cold waters and polluted
areas. It has been discovered off the West coast of the U.S. even though it is banned as
an aquarium plant here. An alternative method of control via sea slugs has been proposed
but there are risks with importing these animals as well to control the exotic algae
species.
Section 10
Biodiversity – refers to the variety of organisms, habitats and ecosystems on the Earth or
in a particular location. Differs significantly between geographic regions.
In general the highest diversity is in the tropics and systematically decreases from lower
to higher latitudes i.e. towards the poles.
Factors affecting this distribution:
1) Climate – there is more sunlight and higher rainfall in the tropics.
2) Species diversity may be self-reinforcing i.e. interactions between species i.e.
predators/prey and pollinators/plants exist when there are already pre-existing
higher levels of diversity.
3) Over geologic time, speciation rates in the tropics exceeded rates of extinction.
Island biogeography:
Colonization is at first slow, but quickly accelerates, though not indefinitely. Why not?
1) Distance effect – the farther the island is from the mainland the fewer
colonizing species that will arrive. E.g. bird species on nearby islands around
New Guinea have higher diversity than those further away.
2) Area effect – larger islands tend to support more diversity due to more varied
habitats available.
Note: Extinctions suppress biodiversity on small islands i.e. they are more susceptible to
changes that occur on small islands e.g. due to storms.