Download Ecological Succession

• Ecology 4
• Community Ecology
• What Is a Community?
• A biological community
– Is an assemblage of populations of various species living close enough
for potential interaction
• Populations are linked by interspecific interactions
– That affect the survival and reproduction of the species engaged in the
interaction
• A community’s interactions include
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competition,
predation,
herbivory,
symbiosis (parasitism,mutualism, commensalism)
• Interspecific interactions
Can have differing effects on the populations involved:
+/- positive or negative effects of the interaction
0
not affected by the interaction
Competition
• Interspecific competition
– Occurs when species compete for a particular resource that is in short
supply
• Strong competition can lead to competitive exclusion
– The local elimination of one of the two competing species
• In a pivital laboratory experiment, G. F. Gause (1934) demonstrated
resource competition between two species of one-celled animal,
Paramecium caudatum and Paramecium aurelia. They both feed on
a bacteria Bacillus pyocyaneus.
When each species is grown alone without competition from the other, each
species multiplies until it reaches the carrying capacity of the
environment.
When grown together they competed for the same food. P. caudatum
quickly declined. It was unable to successfully compete for the food.
Competition
Gause proposed the “The Competitive Exclusion Principle”:
• States that two species competing for the same limiting resources
cannot coexist in the same place (G.F. Gause).
– Paramecium caudatum vs. Paramecium aurelia
Competition
• Ecological niches
– The ecological niche is the total of an organism’s use of the biotic and
abiotic resources in its environment.
• Eugene Odum:
• organism’s habitat: address
• organisms’s niche: profession
Competition
• Ecological niches
– The niche concept allows restatement of the competitive exclusion
principle:
• Two species cannot coexist in a community if their niches are
identical.
• However, ecologically similar species can coexist if there are one or
more significant differences in their niches
Competition
• Ecological niches
– As a result of competition a species’ fundamental niche may be different
from its realized niche.
• Fundamental niche: potential niche in the absence of competition
• Realized niche: niche of a species when competition is present.
Joseph Connell
Balanus blanoides vs. Chthamalus stellatus. Removing Balanus allows
Chthamalus to expand its range to lower on the rocky intertidal.
Competition
• Resource partitioning
– The existence of slightly different niches enables similar species to
coexist in a community. They are indirect evidence of past interspecific
competition. Resource partitioning is one way species can avoid
competitive exclusion and extinction
 Dominican Republic Anolis lizards
Competition
• Character displacement
– Among closely related species
• there is a tendency for characteristics to be more divergent in sympatric
populations (geographically overlapping) of two species
• than in allopatric (geographically separate) populations of the same two species
Predation
• Predation refers to an interaction
– Where one species, the predator, kills and eats the other, the prey
– Feeding adaptations of predators include
• Claws, teeth, fangs, stingers, and poison
Predation
• Prey animals also display a great variety of defensive adaptations:
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Behavioral defenses
Alarm calls
Adaptive coloration
Mechanical or Chemical defenses
Mimicry
Predation
Adaptive coloration
• Cryptic coloration or camouflage
• Makes prey difficult to spot
Predation
•Mechanical defenses
– Spines.
– Porcupine, Erethizon dorsatum
•Chemical defenses
– Odors and toxins
– Aposematic or warning coloration
 Warns predators to stay away from prey that are toxic or smell bad.
Predation
• In some cases, one prey species
– May gain significant protection by mimicking the appearance of another
Predation
•In Batesian mimicry
– A palatable or harmless species mimics an unpalatable or harmful model
Predation
•In Müllerian mimicry
– Two or more unpalatable species resemble each other
Herbivory
• Herbivory, the process in which an herbivore eats parts of a plant
– Has led to the evolution of plant mechanical and chemical defenses and
consequent adaptations by herbivores
•Herbivores face substantial nutritional problems, because they must overcome the
physical and chemical defenses of plants.
– Physical defenses –
• Thorns and spines.
• Tissue with lots of cellulose, lignin, or silica that make it tough to chew.
• Increasing the amount of cellulose and lignin in the plant tissue also
lowers its food value to the herbivore. Cellulose and lignin have no
nitrogen, a necessary nutrient to make protein.
• Plants may also employ toxins to deter predators.
– Locoweed high in alkaloids.
Parasitism
• In parasitism
– one organism, the parasite derives its nourishment from another
organism, its host, which is harmed in the process
Parasitism
• Parasitism exerts substantial influence on populations
– And the structure of communities
• Endoparasites: live within host
• (i.e. tapeworms)
• Ectoparasites: live on the surface of the host (i.e. ticks and lice)
• Parasitoidism: A type of parasitism in which the parasite, usually
small wasps, lay eggs in living hosts, eventually killing it.
Parasitoidism
•A quarter-inch-long parasitic wasp, Peristenus digoneutis, prepares to lay an egg in
a tarnished plant bug nymph
Disease
• Pathogens are disease-causing agents
– typically bacteria, viruses, or protists
– Fungi, prions (protein bodies)
• The effects of disease on populations and communities
– Is similar to that of parasites
Mutualism
• Mutualistic symbiosis or mutualism
– Is an interspecific interaction that benefits both species
• Mycorrhizae
• Lichens
• Nitrogen fixation by cyanobacteria on root nodules of legumes
• Angiosperms and pollinators
Ant and Bullhorn Acacia mutualism
Commensalism
•In commensalism one species benefits and the other is not affected
• “hitchhiking” species, like the egret follow herds of grazing animals, feeding on
insects stirred up by their hooves.
Commensalism
• Commensal interactions have been difficult to document in nature
– Because any close association between species likely affects both
species
Interspecific Interactions and Adaptation
• Co-evolution
– reciprocal genetic change by interacting populations.
– The “arms race” between predators and their prey, or herbivores and
their plants.
Co-evolution between passion vine Passiflora pittieri, Heliconius sapho, and
the cucurbid vine Psiguria warscewiczii
• How the holstein butterfly has overcome this problem is an interesting
story which has become one of the text book examples of
co-evolution. The key in this story is the Cucurbid vine Psiguria
warscewiczii. This vine and the butterfly have virtually become
dependant on each other (mutualism).
Community Structure
• Species diversity
• Feeding relationships
Species Diversity
• The species diversity of a community
– Is the variety of different kinds of organisms that make up the
community
– Has two components
• 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
• Two different 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 rare
Which community is more diverse?
• Food chains
Terrestrial and aquatic food chains
Trophic Structure
• Each step in a food chain is called a trophic level.
– At the bottom are the primary producers, usually green plants.
– The tropic relationships of a community is a description of the various
feeding relationships in the community, such as the diagram of the
Antarctic marine food web…
Food Webs
•A food web
Limits on Food Chain Length
• Each food chain in a food web
– Is usually only a few steps. 3-5 steps.
– There are two hypotheses that attempt to explain food chain length
• The energetic hypothesis suggests that the length of a food chain
– Is limited by the inefficiency of energy transfer along the chain.
– Typically only 10% of the energy in one trophic level is passed to the
next one up.
– Therefore the higher on the food chain an organism feeds, the greater
area it must forage in to find enough to eat. Apex predators, such as
wolves, need large home ranges to feed themselves.
• The dynamic stability hypothesis
– Proposes that long food chains are less stable than short ones
• The longer the food chain, the slower the recovery for top predators,
if the environment is unpredictable and populations fluctuate in size
dramatically.
• Most of the available data
– Support the energetic hypothesis
Species with a Large Impact
• Certain species have an especially large impact on the structure of
entire communities
– Either because they are highly abundant or because they play a pivotal
role in community dynamics
Dominant Species
•Dominant species
– Are those species in a community that are most abundant or have the highest
biomass
– Exert powerful control over the occurrence and distribution of other species
• One hypothesis suggests that dominant species
– Are most competitive in exploiting limited resources
• Another hypothesis for dominant species success
– Is that they are most successful at avoiding predation or disease
Keystone Species
• Keystone species
– Are not necessarily abundant in a community
– Exert strong control on a community by their ecological roles, or niches,
not by their numerical might.
•Field studies of sea stars
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Exhibit their role as a keystone species in intertidal communities
• Pisaster ochraceous
• Observation of sea otter populations and their predation
Sea otters are a keystone in the north Pacific. They feed on sea urchins, that
feed on kelp. Where otters are present there are healthy kelp forests.
Where they are absent, urchin proliferate and overgraze kelp forests.
Ecosystem “Engineers” (Foundation Species)
• Some organisms exert their influence
– By causing physical changes in the environment that affect community
structure
•Beaver dams
– Can transform landscapes on a very large scale
• Some foundation species act as facilitators
– That have positive effects on the survival and reproduction of some of the
other species in the community
Disturbances and Community Structure
• Disturbance influences species diversity and composition
• Decades ago, most ecologists favored the traditional view
– That communities are in a state of equilibrium
• Stability
– The tendency of a community to reach and maintain a relatively
constant composition of species despite disturbance
• However, a recent emphasis on change has led to a nonequilibrium
model
– Which describes communities as constantly changing after being
buffeted by disturbances
What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability
• The intermediate disturbance hypothesis
– Suggests that moderate levels of disturbance can foster higher species
diversity than low levels of disturbance
• Fire
– Is a significant disturbance in most terrestrial ecosystems
– Is often a necessity in some communities
• The large-scale fire in Yellowstone National Park in 1988
– Demonstrated what can happen in the absence of smaller disturbances
– Demonstrated that communities can often respond very rapidly to a
massive disturbance
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
Ecological Succession
• Ecological succession
– Is the sequence of community and ecosystem changes after a
disturbance
• Primary succession
Begins where soil has not yet form
 Volcanic island
 Rubble left behind by a retreating glacier (moraine)
• Secondary succession
– Begins in an area where soil remains after a disturbance
• Early-arriving species
– May facilitate the appearance of later species by making the
environment more favorable
– May inhibit establishment of later species
– May tolerate later species but have no impact on their establishment
• Retreating glaciers
– Provide a valuable field-research opportunity on succession
• Succession on the moraines in Glacier Bay, Alaska
– Follows a predictable pattern of change in vegetation and soil characteristics
August 13, 1941
August 31, 2004