Download Community Ecology

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.
We’ll consider these interactions later, but first some more
general principles…
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?
Other global pests you’ve 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….
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.
Kudzu vines completely
covering trees (and
everything else)
The Asian tiger mosquito,
almost uniquely, is active
(and biting) throughout the
day.
So, we study species interactions to learn how and why
community changes can arise from them.
We’ll 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.
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…
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?
• 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. Here’s what the shoots looked like…
with ants
without ants
Dan Janzen quantified the importance of the ants to Acacia
survival:
Group
Acacia survival (10 months)
Ants removed from
Acacia
43%
Ants present
72%
Ant-plant mutualisms have evolved many times on
different continents.
39 families of plants have sugar-secreting nectaries
.
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.
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…
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.
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.
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.
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.
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 didn’t
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.
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.
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…
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 (%)
1953
21,000
70
1954
6,100
84
1955
700
23
1956
300
20
1957
40
0
1958
30
0
1959
40
0
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:
1) the fleas heavily infest rats and
2) 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.
Why is the plague so feared?
1. Its initial high mortality and
2. 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.
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:
Year
2001
2002
2003
2005
Year
2002
2003
2004
2005
Bird data
Canada
Ontario
127
127
555
281
1633
242
447
300
Human data
Canada
Ontario
340
319
1388
5
???
14
159
89
Windsor/Essex
20
9
11
12
Windsor/Essex
35
(10)
1
(28)
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 don’t
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.
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 don’t 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 “swimmer’s itch”, a ‘rash’ far itchier than poison ivy.
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.
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:
Year
1944
1945
1954
Status
2 million acres covered
Chrysolina introduced
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.
Here’s 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
Other examples of herbivory having significant influence on
the plant community:
Algonquin Park spruce forest before and after spruce
budworm attack -
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.
Finally, see Figure 17.18 in the textbook. That exclosure
was for cattle in Hawaii.
Even when there isn’t physical protection, plants have
evolved a host of defenses against being eaten:
1. Structural defenses a – hooks, prickles, spines, …
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.
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, nonprotein amino acids – some are directly toxic, others
affect functions and result in toxic impact. A few
examples:
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
Daucus carota (Queen Anne’s 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.
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,...
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
it’s a plant that produces juvenile hormone, an insect
eating it never molts to adulthood and can never
reproduce. If it’s 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 hadn’t taken in
the hormone.