We now make a jump shift into

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We now make a jump shift into Community
Ecology Community Ecology is the study of
ecological interactions among different
species. Those interactions vary in their effects
on the species One kind of interaction is
mutually beneficial mutualism Some kinds are
harmful to one species, but beneficial to the
other predation, parasitism, herbivory One kind
is mutually detrimental competition Others have
no effect on at least one participant
commensalism, detritivory, and theoretically
neutralism. Well consider these interactions
later, but first some more general principles
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Why is community ecology important? Community
ecology has important applications because of the
global loss of species in ecological
communities. To understand why species are lost,
we must understand how interactions among species
affect the members of a community. For example,
invasion of communities by exotic species can be
especially damaging. Consider the effects of the
zebra mussel, the lamprey, and the alewife in
the Great Lakes, or the effect of purple
loosestrife along the shorelines of the lakes.
Those are local examples. Are they unique?
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Other global pests youve probably heard
about Africanized honey bee The African
honey bee is far more aggressive than native
honeybee pollinators. It escaped from research
captivity in Brazil, and has moved northward, as
well as cross-breeding with native bees. The
cross breeds are still more aggressive than
natives, displace native hives, and depress
honey production. The aggressiveness can cause
them to attack humans en masse, with fatal
consequences. Here are recent maps of their
distribution in the U.S.
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Kudzu kudzu is an Asian vine that can grow as
much as a foot/day. In the southern U.S. it has
already destroyed 4 million acres of trees and
shrubs. Rare species in infested areas have been
driven at least locally extinct. Asian Tiger
Mosquito Was accidentally introduced into the
U.S. in 1985. It replaces native aquatic
invertebrate larvae, and thus the adults that
would mature from them. In addition to its
ecological consequences, unlike the native
mosquitoes it has replaced, it is a potential
disease carrier.
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Kudzu vines completely covering trees (and
everything else)
The Asian tiger mosquito, almost uniquely, is
active (and biting) throughout the day.
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• So, we study species interactions to learn how
  and why community changes can arise from them.
• Well begin with mutualism.
• Mutualism benefits both species participating in
  the interaction. It may occur in two ways
• obligate mutualism both species depend on the
  interaction for survival
• facultative mutualism the interaction
  is beneficial, but not necessary.

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A case study the swollen thorn Acacia and ants
that live only on Acacias
swollen thorn they have a soft pith inside that
ants excavate for nests.
The Acacia also has Beltian bodies at the tips of
each leaflet, very high in protein to help feed
the ants, and
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• Nectaries at the base of each leaf that provide
  nectar for the ants.
• The plants thus clearly benefit the ants
• They provide a home (the
• hollow inside of the thorns), and
• They provide food (protein from
• the Beltian bodies and nectar
• from the nectaries)

How do the ants benefit the Acacia?
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• Ants attack any herbivores (usually other
  insects) that
• attempt to eat the acacia. They not only attack
  leaf-eating
• insects, but also eggs and larvae of
  herbivorous insects.
• They even bite and sting browsing mammals that
  attempt
• to eat the acacia.
• They cut back the growing tips of other plants
  that grow
• near, or, in the case of vines, on the acacia.
• This defense of the plant has been shown
  experimentally to be important to the growth and
  survival of the acacia. Small acacias were cut
  down, then allowed to re-grow, some with ants
  having re-colonized, and others where that was
  prevented. Heres what the shoots looked like

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with ants without ants
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Dan Janzen quantified the importance of the ants
to Acacia survival Group Acacia
survival (10 months) Ants removed from 43
Acacia Ants present 72
. Ant-plant mutualisms have evolved many times
on different continents. 39 families of plants
have sugar-secreting nectaries
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The acacia-ant interaction is probably an
obligate mutualism. There are more obviously
obligate mutualisms, made obvious by the extent
of evolutionary design involved. That is
frequently the case with pollinator-plant
interactions. One of the classic cases is flowers
designed to have the shape of female wasps they
attract males that attempt to copulate with the
female, but only either gather or deliver pollen.
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Other mutualisms are facultative at least one
of the two species may have other possibilities
to achieve an end. Think of pollinators that may
have many flowers from which they can gather
pollen and nectar. Think of all the different
species that might choose to eat some attractive
fruit. For example, bright red fruits attract
many different birds
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There is another type of interaction
commensalism that benefits one species without
either benefiting or harming the other. How can
that happen? When the actions of one species are
of no importance to the other, but those actions
are beneficial to the first species, a
commensalism is occurring. There are numerous
examples The remora is a small fish that swims
with sharks. Sharks are messy eaters, and the
scraps they miss are the diet of the remora.
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This is a whale shark being accompanied by two
remoras.
In other cases the remora attaches itself to a
host by a sucker disc. By attaching itself and
potentially affecting the efficiency of movement
of the host, this may move the interaction beyond
the bounds of commensalism.
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Another example is the interaction between
cowbirds or cattle egrets and the large grazers
of grasslands. The animals walk though the
grassland and, in moving through the plant
community, flush insects from within it. Feeding
rates for the insect-eating birds clearly
increase, but there is no apparent effect on the
grazers.
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This interaction, too, may grade from
commensalism to mutualism. The birds may ride on
the backs of the grazers, picking off
ectoparasites. When they do, there is benefit to
the grazer, as well as the bird.
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• There is also an interaction called amensalism
  an interaction in which one species is harmed
  without effect on the other. Usually it is
  apparent that the negative effect was
  unintentional, but nevertheless important.
  Examples
• Mammals bare the spaces around waterholes by
  using the
• waterhole intensively enough to trample the
  vegetation
• around it. Feel for the trampled plants. The
  mammal didnt
• mean to kill plants, it just happened.
• Small, ground plants in a forest are damaged by
  falling
• parts of large, older trees. Herbs, shrubs and
  small trees in
• tropical forests are frequently damaged more by
  falling
• objects than by herbivores.

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Now, on to the interactions that dominate
community ecology Two types, one has positive
effect on one participant and negative on the
other predation, parasitism and herbivory. The
other has negative impact on both participants
competition.
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In parasitism, the parasite consumes a resource
the host but the host is not killed immediately
(and in theory not by the parasite directly) as
is the case in a predator prey interaction. In
the case of disease host interactions (a
parasitism) the duration of the interaction can
lead the host to develop immunity to the
parasite. Very often, human bacterial and viral
diseases have very high initial infection rates,
followed by increased immunity. Examples
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• A (relatively) recent outbreak of bubonic plague
  in India
• The source of infection was a bacterium living in
  fleas on rats. When the fleas moved onto humans
  and bit them, the infection was passed.
• Year Number of Cases
  Mortality rate ()
• 21,000 70
• 6,100 84
• 700 23
• 300 20
• 40 0
• 30 0
• 40 0

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• The transmission of the plaque bacterium from
  fleas to humans does not often occur. The fleas
  must move from rats, their preferred hosts, to
  humans. That only occurs when
• the fleas heavily infest rats and
• rats are in close contact with humans, for
  example in crowded, garbage-strewn conditions of
  medieval Europe or conditions in densely
  populated cities in India.
• We know how the great plague of the 14th century
  originated It began in 1346 during the siege of
  Caffa along the Crimean Straits. It spread to
  Italy and the south of France in 1347, then to
  all of Europe in 1348. It had disappeared
  entirely by 1357.

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• Why is the plague so feared?
• Its initial high mortality and
• Its reappearance at intervals.
• There were 3 more episodes in Europe during the
  14th century. They occurred at intervals of
  10-13 years. Enough immunity persisted to make
  each of those episodes have a lower incidence
  and mortality.
• However, there were 3 episodes in England
  during the 17th century. Those occurred at
  intervals of 22-40 years. Enough time had passed
  between episodes that both a loss of immunity in
  survivors had occurred and an immune population
  had largely been replaced with a susceptible
  one. Each episode killed 13-15 of the
  population of London.

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The recent time course of the West Nile virus is
probably a similar example. Here are dead bird
and human case data for Canada, Ontario, and
Windsor/Essex County Bird
data Year Canada Ontario Windsor/Essex 2001
127 127 20 2002 555 281 9 2003
1633 242 11 2005 447 300 12
Human data Year Canada Ontario Windsor/Esse
x 2002 340 319 35 2003 1388
5 (10) 2004 ??? 14
1 2005 159 89 (28)
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A (vaguely) personal example The human
schistosome (Schistosoma mansoni) causes
debilitating diseases called schistosomiasis or
bilharziasis by infesting the liver. Schistosomes
(blood flukes) excite an immune response when
they enter a human through pores, but the first
wave dont succumb, both because they face a
small initial immune response, and because they
rapidly coat themselves with host proteins. Later
attacks invoke a very large and rapid immune
response, and they succumb. There is also
cross-resistance. Infection by one species evokes
immune responses against later attacks by other
species.
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Now the personal part There are schistosomes
that infect ducks in mid-continental lakes. All
schistosomes have complex life histories that
involve a life stage that infests an intermediate
host, usually a mollusc. The first time you are
exposed to the duck schistosomes, you dont even
know it. Later exposures, for me when I helped a
field class sample for fish in an Iowa stream on
a warm day when the snails were shedding the
intermediate stage, evoke an extremely strong
reaction. The duck schistosomes never get past
the lower dermal layers, but the later exposure
causes what is called swimmers itch, a rash
far itchier than poison ivy.
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Herbivore Plant Interactions Some herbivores
act as predators, uprooting the plants they eat,
for example sheep. Most herbivores, though, do
not kill the plants they eat. They can, however,
strongly influence, or even control plant
populations. Many examples of control come from
conservation biology and biological
control Klammath weed is a European plant that
accidentally became established in California in
the early 1900s. By 1944 it spread over 2 million
acres of range land in 30 counties. Biological
control specialists brought in a beetle from
Australia, a Chrysolina, and within 10 years the
weed was controlled.
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Klammath weed was an important pest species
because it contains high concentrations of a
toxic alkaloid, hypericin, that is used in low
doses as a medicine. The concentrations in the
plant are dangerous for cattle. The plant and
the control agent
30
Year Status

1944 2 million acres covered 1945 Chrysolina
introduced 1954 99 reduction in Klammath
weed Australia has had its own control problems.
Opuntia cactuses were introduced to create cheap,
natural fencerows to manage cattle in pastures.
The cactuses did not grow only along the designed
fencerows. At one time, millions of acres of
grassland were unusable due to the cactuses.
Australian control specialists went to South
America, where the cactuses had come from, and
found a control agent, the moth Cactoblastis
cactorum. Within 10 years both the cactus and the
moth remained present, but at low abundance.
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Heres what cactus pad looks like after moth
attack. Eggs laid on a cactus pad hatch into
larvae that burrow into the pad and eat the
inside, hollowing it out and killing that stem.
moth larva
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Other examples of herbivory having significant
influence on the plant community Algonquin Park
spruce forest before and after spruce budworm
attack -
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Relative biomass in and outside vole exclosures.
The y-axis here is summed height of plants in 100
cm2 areas as an indication of plant biomass. This
was the difference after 2 years.
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• Finally, see Figure 17.18 in the textbook. That
  exclosure was for cattle in Hawaii.
• Even when there isnt physical protection, plants
  have evolved a host of defenses against being
  eaten
• Structural defenses a hooks, prickles, spines,
  

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2. Structural defenses b the design of the C4
leaf discourages herbivores
Most of the useful food is concentrated in the
cells surrounding the vein. These cells are
hardened with high levels of silica and lignin.
They are hard to chew and digest.
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3. Chemical defenses plants produce a huge
variety of chemicals that are harmful or
toxic to herbivores. They can be loosely
organized into 3 categories a. digestibility
reducers tannins and phenols bind proteins as
they are freed from organelles by the herbivore
chewing. They reduce the nutritional value of
the plant. b. toxins alkaloids, glycosides,
pyrethrins, non- protein amino acids some are
directly toxic, others affect functions and
result in toxic impact. A few examples
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A non-protein amino acid, that when incorporated
causes failure of critical enzymes during insect
larval development
replacement of a carbon in the side chain with O
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Daucus carota (Queen Annes lace, a common weed
around here) contains furanocoumarins that are
toxic to a wide variety of insects. The coumarins
are secondary chemicals produced by an extra step
in a normal pathway that produces lignin (see
Figure 20.15 in the text). The furanocoumarins
affect DNA replication and herbivore growth and
reproduction. Indian grass (Sorghastrum nutans)
produces and stores cyanogenic glycosides during
drought. The salivary enzymes of the herbivores
break complex sugars into simple ones. When an
herbivore eats Indian grass during dry
conditions, its digestive enzymes free cyanide,
which poisons the consumer.
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We have frequent experience with toxin
defenses a) alkaloids - flavors of vanilla,
chocolate (caffeine and ???) are toxins
effective against insect herbivores b) the
flavor of cinnamon comes from trans-cinnamic
acid, again effective against insects c) tobacco
is protected by nicotine in the leaves. We
can become addicted, but aphids are paralyzed
by it. Other examples morphine (and its
derivatives), strychnine, digitalis,
capsisic acid (the flavor in peppers,...
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4. Pharmaco-active compounds many of these are
mimics of animal hormones. You can imagine
how they might disrupt normal developmental
pathways in insects. Among the most
devious - plant-made functioning analogs of
animal hormones - examples insect
molting and juvenile hormones. If its a plant
that produces juvenile hormone, an insect
eating it never molts to adulthood and can never
reproduce. If its a molting hormone, and
insect eating this plant is fooled into
molting too early, and it matures as a very
small adult capable of producing far fewer
young than if it hadnt taken in the hormone.