Transcript Ecology

Chap.14
Mutualism and Commensalism
鄭先祐 (Ayo)
教授
國立台南大學 環境與生態學院
生態科學與技術學系
環境生態 + 生態旅遊 (碩士班)
14 Mutualism and Commensalism
Case Study: The First Farmers
1.Positive Interactions
2.Characteristics of Mutualism
3.Ecological Consequences
Case Study Revisited
Connections in Nature: From
Mandibles to Nutrient Cycling
2
Case Study: The First Farmers
The fungus-growing ants started
cultivating fungi for food at least 50
million years before the first human
farmers.
Figure 14.1 Collecting Food for Their Fungi
3
Case Study: The First Farmers
The ant farmers nourish, protect,
and eat the species they grow,
forming a relationship that benefits
both the farmer and the crop.
The ants cannot survive without their
fungi, many of the fungi cannot
survive without the ants.
4
Case Study: The First Farmers
When a queen leaves the nest to mate
and begin a new colony, she carries in
her mouth some of the fungi from her
birth colony.
The fungi are cultivated in subterranean
gardens.
A colony may contain hundreds of
gardens, each the size of a football;
they can provide enough food to
support 2–8 million ants.
5
Figure 14.2 The Fungal Garden of a Leaf-cutter Ant
the mound above ground is made up
of soil excavated by the ants.
the fungus is cultured
in garden chambers,
each about the size of
a football.
6
(B) This photo shows a cut-away
view of a garden chamber. Several
The dump chambers singed ants can be seen hiding from
contain refuse (渣滓) the disturbance created by excavating
from the fungal
the garden. They have placed their
gardens.
heads into crevices of the garden, and
they would be relatively motionless.
Case Study: The First Farmers
Leaf-cutter ants cut bits of leaves
from plants and feed them to the
fungi in their gardens.
Atta worker ants maintain trails
through the vegetation, and the
workers of a single colony can harvest
as much plant matter each day as it
would take to feed a cow.
7
Case Study: The First Farmers
The ants chew the leaves to a pulp,
fertilize them with their own droppings,
and “weed” the fungal gardens to help
control bacterial and fungal invaders.
The fungi produce specialized
structures called gongylidia, on which
the ants feed.
8
Case Study: The First Farmers
Both ants and fungi benefit from the
relationship.
The ants scrape a waxy covering from
the leaves that the fungi have
difficulty penetrating.
The fungus digests and renders
harmless the chemicals that plants
use to kill or deter insect herbivores.
9
Case Study: The First Farmers
Nonresident fungi, pathogens, and
parasites can sometimes invade the
colonies.
What prevents invaders from
destroying the gardens?
10
Introduction
Positive interactions between species
are those in which one or both species
benefit and neither is harmed.
Example: Most vascular plants form
associations with fungi in which the
fungus absorbs nutrients from the soil,
improving the plant’s growth and
survival.
11
Introduction
Fossil evidence indicates that the
earliest vascular plants formed these
associations with fungi more than 400
million years ago (Selosse and Le Tacon
1998).
Early vascular plants lacked true roots,
so their interactions with fungi may
have aided their colonization of land.
12
Positive Interactions
Concept 14.1: Positive interactions occur
when neither species is harmed and the
benefits of the interaction are greater than the
costs for at least one species.
Mutualism —mutually beneficial
interaction between individuals of two
species (+/+).
Commensalism —individuals of one
species benefit, while individuals of the
other species do not benefit and are
not harmed (+/0).
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Positive Interactions
Symbiosis —a relationship in which
the two species live in close
physiological contact with each other,
such as corals and algae.
Parasites can also form symbiotic
relationships.
Symbioses can include parasitism
(+/–), commensalism (+/0), and
mutualism (+/+).
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Positive Interactions
The benefits of positive interactions can
take many forms.
Sometimes there is a cost to one or
both partners, but the net effect of the
interaction is positive because for each
species, the benefits are greater than
the costs.
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Positive Interactions
Mutualistic associations are everywhere.
Most plants form mycorrhizae,
symbiotic associations between plant
roots and various types of fungi.
The fungi increase the surface area
over which the plants can extract soil
nutrients (over 3 m of fungal hyphae
may extend from 1 cm of plant root).
16
Positive Interactions
The fungi may also protect the plants
from pathogens and help them take up
water.
The plants supply the fungi with
carbohydrates.
Eight major types of mycorrhizal
associations correspond closely with the
major terrestrial biomes.
17
Figure 14.3 Mycorrhizal Associations Cover Earth’s Land Surface
18
Each color on the map represents the region in which one of
eight major types of mycorrhizal association is found.
Notice that the locations of the different types of mycorrhizal
associations correspond fairly closely to the locations of major
terrestrial biomes.
Positive Interactions
Two categories of mycorrhizae:
Ectomycorrhizae —the fungus grows
between root cells and forms a mantle
around the exterior of the root.
Arbuscular mycorrhizae —the fungus
grows into the soil, extending some
distance away from the root; and also
penetrates into some of the plant root
cells.
19
Figure 14.4 Two Major Types of Mycorrhizae (Part 1)
Some of the
hyphae grow
between root cells.
20
in ectomycorrhizae, fungal hyphae form a
mantle or sheath around the root.
Figure 14.4 Two Major Types of Mycorrhizae (Part 2)
in arbuscular
mycorrhizae, fungal
hyphae extend into
the soil.
Hyphae also grow
between some root
cells....
... and penetrate
others.
21
Positive Interactions
Corals form a mutualism with symbiotic
algae.
The coral provides the alga with a home,
nutrients (nitrogen and phosphorus),
and access to sunlight.
The alga provides the coral with
carbohydrates produced by
photosynthesis.
22
Positive Interactions
Mammalian herbivores such as cattle
and sheep depend on bacteria and
protists(原生生物) that live in their guts
and help metabolize cellulose.
Insects also have mutualisms with
plants, protists, and bacteria. Woodeating insects have gut protists that
can digest cellulose.
23
Figure 14.5 A Protist Gut Mutualist
this wood-eating
cockroach would starve
if gut mutualists such as
the protist (原生生物)
shown here (a
hypermastigote) did not
help it to digest wood.
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原生生物界 (Protista)
 至少包含5萬種生物，分為以下三類：
1. 藻類
2. 原生動物類
3. 原生菌類
 原生生物的特徵：
 簡單的真核生物，多為單細胞生物，亦有部份是
多細胞的，但不具組織分化。這個界別是真核生
物中最低等的。製造養分的方式，有的跟真菌一
樣，吸收外間的營養；有的能行光合作用，亦能
捕食，例如裸藻。所有原生生物都生存於水中。
 常見的原生生物包括纖毛蟲（ciliates）、變形蟲、瘧
原蟲、粘菌、浮游生物、海藻，也有光自營的單細胞
遊動微生物，如眼蟲等。
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維基百科
Positive Interactions
Commensalism is also everywhere.
Millions of species form +/0 relationships
with organisms that provide habitat.
 Examples: lichens that grow on trees, bacteria on
your skin.
 In kelp forests, many species depend on the kelp for
habitat, and do no harm to the kelp.
 Countless insect and understory plant species live in
tropical rainforests and depend on the forests for
habitat, yet many have little or no effect on the trees
that tower above them.
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Positive Interactions
Different types of ecological interactions
can evolve into commensalism or
mutualism.
Example: Lichens on tree leaves may
initially harm the tree by blocking
sunlight. The Australian palm has
adapted by increasing the
concentration of chlorophyll in parts of
leaves that are covered with lichens.
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Positive Interactions
Mutualism can arise from a host–
parasite interaction.
This was observed in a strain of
Amoeba proteus that was infected by a
bacterium.
Initially, the bacteria caused the hosts
to be smaller, grow slowly, and often
killed the hosts.
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Positive Interactions
But parasites and hosts can coevolve.
Five years later, the bacterium had
evolved to be harmless to the amoeba;
the amoeba had evolved to be
dependent on the bacterium for
metabolic functions.
Various tests showed that the two
species could no longer exist alone
(Jeon 1972).
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Positive Interactions
Some positive interactions are highly
species-specific, and obligate (not
optional for either species).
Example: The leaf cutter ants and
fungus cannot survive without each
other, and the interaction has led both
to evolve unique features that benefit
the other species.
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Positive Interactions
Tropical figs (無花果) are pollinated by
one or a few species of fig wasps.
Neither species can reproduce without
the other.
The wasps and the figs have
coevolved. The wasps have complex
reproductive behaviors in the fig
receptacle (花托); that ensures that the
fig flowers get pollinated, and the next
generation of wasps are hatched.
31
Figure 14.6 Fig Flowers and the Wasp That Pollinates Them
in this species, the
receptacle contains both
male and female flowers.
but the female flowers
mature 3-4 weeks earlier
than the male flowers.
32
Some of the female flowers
have short styles, while others
have long styles
Fig-fig wasp interactions
Monoecious figs – each tree has
separate male and female flowers, the
male and female flowers are located in
different parts of the receptacle, and
the male flowers mature after the
female flowers.
A female fig wasp enters the receptacle,
where she inserts her ovipositor
through the styles to lay eggs in the
ovaries (Fig. 14.6).
33
Fig-fig wasp interactions
 Perhaps because wasp ovipositors are not long
enough to reach the ovaries of long-styled
flowers, wasp larvae typically develop within
short-styled flowers and feed on some of their
seeds.
 When the young wasps complete their
development, they mate, the males burrow
through the receptacle, and the females exit
through this passageway.
 Before the females leave the receptacle,
however, they visit male flowers(which are
now mature), collect pollen from them, and
store it in a specialized sac for use when they
lay their eggs in another receptacle.
34
Positive Interactions
Many mutualisms and commensalisms
are facultative (not obligate) and
show few signs of coevolution.
In deserts, the shade of adult plants
creates cooler, moister conditions.
Seeds of many plants can only
germinate in this shade. The adult is
called a nurse plant.
35
Positive Interactions
One species of nurse plant may
protect the seedlings of many other
species.
Desert ironwood serves as a nurse
plant for 165 different species.
The nurse plant and the beneficiary
species may evolve little in response to
one another.
36
Positive Interactions
Large herbivores such as deer or moose
often consume seeds of herbaceous
plants.
Many of the seed pass through
unharmed, and are deposited with the
feces. Thus, it becomes a dispersal
mechanism.
Such interactions are sporadic (偶發的)
and facultative; there is little evidence
to suggest that the species have
coevolved.
37
Figure 14.7 Deer Can Move Plant Seeds Long Distances
This line marks the farthest (最遠的) distance that any ant is known
to have dispersed(被驅散的) a seed from a forest understory plant.
38
Positive Interactions
Interactions between two species can be
categorized by the outcome for each
species:
Positive (benefits > costs)
Negative (costs > benefits)
Neutral (benefits = costs)
But costs and benefits can vary in time
and space.
39
Positive Interactions
Soil temperature can determine
whether a pair of wetland plants are
commensals or competitors.
Wetland soils can be anoxic. Some
plants such as cattails can aerate soils
by passively transporting oxygen
through continuous air spaces in their
leaves, stems, and roots.
Some of this oxygen becomes available
to other plants.
40
Positive Interactions
An experiment with cattails (Typha
latifolia) and Myosotis laxa (smallflowered forget-me-not, a species that
lacks continuous air spaces) (勿忘草屬植
物):
The plants were grown at two different
temperatures.
 At low temperatures, soil oxygen increased
when Typha (香蒲)was present, but not at
the higher temperature.
41
Figure 14.8 A Wetland Plant Aerates the Soil under Some Conditions (Part 1)
at low soil temperatures, Typha (香蒲) increased the
dissolved oxygen content of soils.
42
Figure 14.8 A Wetland Plant Aerates the Soil under Some Conditions (Part 2)
no such effect of Typha was found at
high soil temperatures.
43
Positive Interactions
 At low temperatures, growth of Myosotis
increased when Typha was present.
 At the higher temperature, the presence of
Typha decreased growth of Myosotis.
At low temperatures Typha had a
positive effect on Myosotis, but a
negative effect at high temperatures.
44
Figure 14.9 From Benefactor to Competitor
at low soil temperatures,
Typha increased the
growth of Myosotis,
perhaps by aerating the
soil.
45
Positive Interactions
Many recent studies have shown that
positive interactions are important in
many communities.
Studies often compare performance of
a target species when neighbors are
present with its performance when
neighbors are removed.
46
Positive Interactions
An international groups of ecologists
looked at the effects of neighboring
plants on 115 target species.
Performance was measured as change
in biomass or leaf number.
The “relative neighbor effect” (RNE)
= target species’ performance with
neighbors present minus its
performance when neighbors were
removed.
47
Positive Interactions
RNE was generally positive at highelevation sites, indicating that
neighbors had a positive effect on the
target species.
RNE was generally negative at lowelevation sites.
48
Figure 14.10 Neighbors Increase Plant Performance at High-Elevation Sites (Part 1)
in most regions, neighbors decreased target species performance
at low-elevation site.....
... but
increased
it at highelevation
sites.
49
Figure 14.10 Neighbors Increase Plant Performance at High-Elevation Sites (Part 2)
50
Positive Interactions
At high-elevation sites, neighbors also
tended to increase the target species
survival and reproduction.
Neighbors had the opposite effect at
low-elevation sites.
51
Figure 14.11 Negative Effects at Low Elevations, Benefits at High Elevations
plants with neighbors survived less well, and produced fewer flowers and fruits, than
plants without neighbors at low elevations....
.... but they survived better, and produced more
flowers and fruits, than plants without neighbors
at high elevations.
52
Positive Interactions
Because environmental conditions tend
to be more extreme at high-elevation
sites, these results suggest that
positive interactions may be more
common in stressful environments.
Similar results have been found in
intertidal communities.
53
Characteristics of Mutualism
Concept 14.2: Each partner in a mutualism
acts to serve its own ecological and
evolutionary interests.
Mutualisms can be categorized by the
type of benefits that result.
Often, the two partners may receive
different types of benefits, and the
mutualism can be classified two ways.
54
Characteristics of Mutualism
Trophic mutualisms —a mutualist
receives energy or nutrients from its
partner.
 Example: Leaf-cutter ants and fungus.
 In mycorrhizae, the fungus gets energy in
the form of carbohydrates and the plant
gets help in taking up limiting nutrients,
such as phosphorus.
55
Characteristics of Mutualism
Habitat mutualisms —one partner
provides the other with shelter, a place
to live, or favorable habitat.
 Example: Alpheid (pistol) shrimp dig a
burrows that that they share with a goby
fish (刺鰭魚). The goby gets a refuge, and
in turn serves as a “seeing eye fish” for the
nearly blind shrimp.
56
Figure 14.12 A Seeing-Eye Fish
The shrimp digs a
burrow, which it shares
with a goby.
57
Outside of its burrow, the nearly
blind shrimp keeps an antenna on
the goby, whose movement warn it
of danger.
Characteristics of Mutualism
The grass Dichanthelium lanuginosum
grows next to hot springs in soils whose
temperatures can be as high as 60°C.
It has a fungal symbiont that grows
throughout the plant.
Experiments showed that grass plants
without their symbiont could not
survive at 60°C.
58
Characteristics of Mutualism
In field experiments, grass plants with
symbionts had greater root and leaf
mass than plants without symbionts in
soil temperatures up to 40°C.
In soils above 40°C, plants with
symbionts continued to grow well, but
all grass plants without symbionts died.
59
Characteristics of Mutualism
In another study, the symbiont was
transferred to watermelons, tomatoes,
and wheat.
These plants were then able to survive
high temperature soils.
60
病毒、真菌、植物三態共生
內生真菌(endophytic fungi)生長於植物組織
中，常與植物形成互利共生的關係，有些內生
真菌可幫助植物適應極端的環境。
美國黃石公園中有一種內生真菌Curvularia
protuberata生長在熱帶黍Dichanthelium
lanuginosum的根部與葉片中，這種共生關
係使得彼此可以生長在高達65℃的環境中。
在C. protuberata內分離到一株真菌病毒，命
名為Curvularia thermal tolerance virus,
CThTV。
61
若將此內生真菌中的病毒殺死，此內生真菌與
熱帶黍皆無法生存於高溫環境中；而再將此病
毒感染內生真菌後接種於熱帶黍上，則此內生
真菌與熱帶黍復具有耐高溫的能力。
另外，將具有病毒的內生真菌接種於蕃茄
(Solanum lycopersicon)上，也得到相同的
結果，證實CThTV病毒感染之C. protuberata
可使單子葉植物及雙子葉植物具有耐高溫的能
力。
A Virus in a Fungus in a Plant: Three-Way Symbiosis and
Thermal Tolerance
62
Characteristics of Mutualism
Service mutualisms —interactions in
which one partner performs an
ecological service for the other.
Ecological services include pollination,
dispersal, and defense against
herbivores, predators, or parasites.
63
Characteristics of Mutualism
Although both partners in a mutualism
benefit, there are also costs.
In the coral–alga mutualism, the
cost to the coral includes supplying
nutrients and space; the cost to the
alga is giving up some of the
carbohydrates it could use for itself.
64
Characteristics of Mutualism
Sometimes the cost is clear—a “reward”
for a service.
During flowering, milkweeds (馬利筋屬
植物) use up to 37% of the energy gain
from photosynthesis to produce nectar
that attracts insect pollinators.
65
Characteristics of Mutualism
In a mutualism, the net benefits must
exceed the net costs for both partners.
If environmental conditions change and
benefit is reduced or cost increased for
either partner, the outcome of the
interaction may change, particularly for
facultative interactions.
66
Characteristics of Mutualism
Some ants protect treehoppers (角蟬科昆
蟲) from predators, and the treehoppers
secrete “honeydew” (sugar solution),
which the ants feed on.
Treehoppers always secrete honeydew,
so ants always have this resource.
But when predators are few, the treehoppers may get no benefit from the
ants. The interaction may shift from +/+
to +/0.
67
Figure 14.13 A Green Weaver Ant Guards Its Treehopper Mutualist
to defend the treehopper larva that it is standing above, this Australian green
weaver ant has raised its abdomen, from which it can spray a toxic compound.
Treehoppers secrete "honeydew", a liquid rich in carbohydrates that is used by
the ants as a source of food.
68
Characteristics of Mutualism
A mutualist may withdraw the reward
that it usually provides.
In high-nutrient environments, plants
can easily get nutrients, and may
reduce the carbohydrate reward to
mycorrhizal fungi.
The costs of supporting the fungus are
greater than the benefits the fungus
can provide.
69
Characteristics of Mutualism
Cheaters are individuals that increase
offspring production by overexploiting
their mutualistic partner.
If this happens, the interaction
probably won’t persist.
Several factors contribute to the
persistence of mutualisms.
70
Characteristics of Mutualism
“Penalties” may be imposed on
cheaters.
Pellmyr and Huth (1994) documented
this in an obligate, coevolved
mutualism between a yucca (絲蘭) and
its exclusive pollinator, the yucca
moth.
The female moth collects pollen with
specialized mouthparts. She lays eggs
in another yucca, and then deliberately
deposits the pollen in this flower.
71
Figure 14.14 Yuccas and Yucca Moths
Yucca filamentosa has an obligate relationship with its
exclusive pollinator, the yucca moth.
(A) The yucca flower in specialized
mouthparts. She may carry a load of
up to 10,000 pollen grains, nearly
72 10% of her own weight.
(B) The moth at the lower right of this
photo is laying eggs in the ovary of a
yucca flower, the moth at the top is
placing pollen on the stigma.
Characteristics of Mutualism
The larvae complete development by
eating the seeds in the flower.
Exploitation can occur if moths lay too
many eggs and hence consume too
many seeds.
Yuccas are able to abort flowers with
too many eggs, before the moth larvae
hatch.
73
Figure 14.15 A Penalty for Cheating
Yucca plants retain an average of 62% of flowers
that contain 1-6 moth eggs.....
(維持，保留)
.... but only 11|% of
flowers that contain 9-12
moth eggs.
74
Characteristics of Mutualism
The partners in a mutualism are not
altruistic.
Both partners take actions that
promote their own best interests.
In general, a mutualism evolves and is
maintained because the net effect is
advantageous to both partners.
75
Ecological Consequences
Concept 14.3: Positive interactions affect the
distributions and abundances of organisms
as well as the composition of ecological
communities.
Mutualism can influence demographic
factors.
This is demonstrated by ants
(Pseudomyrmex) and acacia trees.
76
Ecological Consequences
The trees have large thorns, which
house ant colonies.
The tree produces Beltian bodies on
the leaf tips, which are high in protein
and fat. The ants gather these to feed
to the larvae.
Ant workers patrol the tree 24 hours a
day and aggressively attack insect and
even mammal herbivores.
The ants also use their mandibles to
maul other plants within 10–150 cm
the tree, providing the acacia with a
competitor-free zone.
77
Figure 14.16 An Ant–Plant Mutualism
78
Ecological Consequences
To determine the benefits the acacias
receive, Janzen (1966) removed ants
from some and compared them to trees
with their ants.
 Acacias with ant colonies weighed over 14
times as much as plants without ant
colonies.
 They also survived better and were
attacked by insect herbivores less
frequently.
79
Figure 14.17 Effects of a Mutualism with Ants on Swollenthorn Acacias
Acacias with ants
survived better than those
without ants.
80
Far fewer herbivorous
insects were found on
acacias with ants.
Ecological Consequences
Acacias without ant colonies are often
killed by herbivores in 6–12 months.
The ants also cannot survive without
the trees.
Both species have evolved unusual
characteristics that benefit the other
species.
81
Ecological Consequences
The ants are highly aggressive,
attacking both herbivores and other
plants. Other ants in this genus don’t
have this trait.
The acacias have enlarged thorns,
specialized nectaries, and Beltian
bodies, and produce leaves nearly yearround (providing a reliable food source
for the ants).
82
Ecological Consequences
When one species provides another
with favorable habitat, it influences the
distribution of that species.
 Examples: Corals and algal symbionts; the
grass Dichanthelium and its fungal
symbiont.
83
Ecological Consequences
It is very common for a group of
dominant species (such as trees in a
forest) to determine the distributions of
other species by physically providing
the habitat on which they depend.
Many plant and animal species are
found only in forests; they can’t
tolerate conditions (or competitors) in
other habitats.
84
Ecological Consequences
In rocky intertidal zones, many species
live under the strands of seaweed that
grow on the rocks. The seaweed
creates a moist, cool environment at
low tide.
Beach grasses stabilize the sand and
enable the formation of entire
communities of plants and animals.
85
Ecological Consequences
Positive interactions can also influence
community composition and
ecosystem properties.
Many coral reef fish have service
mutualisms with smaller organisms
(cleaners) that remove parasites from
the fish (clients).
The benefit the client receives is
greater than the energy benefit it could
gain by eating the cleaner.
86
Figure 14.18 A Ecological effects of the cleaner fish, Labroides dimidiatus
87
Ecological Consequences
Studies of a cleaner fish on the Great
Barrier Reef showed that individuals
were visited by an average of 2,297
clients each day, from which the
cleaner fish removed (and ate) an
average of 1,218 parasites per day.
88
Ecological Consequences
In an experiment, the cleaner fish were
removed from five small reefs.
After 12 days, there were 3.8 times
more parasites on one fish species than
in control reefs.
After 18 months, the abundance and
number of fish species on the reefs had
decreased.
89
Figure 14.18 B,C Ecological effects of the cleaner fish, Labroides dimidiatus
90
Ecological Consequences
In an experiment with two prairie
grasses, Hetrick et al. (1989) grew
them in a greenhouse with and without
mycorrhizal fungi.
When mycorrhizal fungi were present,
big bluestem grass dominated; when
absent, junegrass dominated.
91
Ecological Consequences
In a natural prairie, Hartnett and
Wilson (1999) suppressed mycorrhizal
fungi with a fungicide.
Big bluestem (which had been
dominant) decreased, while a variety of
other species increased.
The mycorrhizal fungi may have
given big bluestem a competitive
advantage.
92
Ecological Consequences
In a large-scale field experiment, the
species of mycorrhizal fungi were
manipulated.
Soils with different numbers of fungal
species were seeded with 15 plant
species.
In one growing season, plant root and
shoot biomass, and phosphorus
uptake increased as the number of
fungal species increased.
93
Figure 14.19 Mycorrhizal Fungi Affect Ecosystem Properties
Plant phosphorus content rose
steadily as the number of
mycorrhizal fungal species
increased.
shoot and root biomass increased initially
and then leveled off as the number of
mycorrhizal fungal species increased.
94
Ecological Consequences
Thus, mutualistic interactions can
influence key features of ecosystems,
such as net primary productivity and
the supply and cycling of nutrients
such as phosphorus.
95
Case Study Revisited: The First
Farmers
In 1999, a parasitic fungus
(Escovopsis) was discovered that
attacks the fungal gardens of leafcutter ants.
The parasite can be transmitted from
one garden to another, and rapidly
destroy the gardens, leading to death
of the ant colony.
96
Case Study Revisited: The
First Farmers
Ants respond to Escovopsis by
increasing the garden weeding rate.
They also appear to enlist the help of
other species. The ants carry a
bacterium that makes chemicals that
inhibit Escovopsis.
The bacteria also secrete compounds
that promote the growth of the
cultivated fungi.
97
Figure 14.20 A Specialized Parasite Stimulates Weeding by Ants
in response to Escovopsis, ants greatly increased the
rate at which they weeded (清除) their gardens.
98
Case Study Revisited: The
First Farmers
The bacteria also benefit: They get a
place to live (in specialized structures
called crypts on the ant’s exoskeleton
and a source of food (glandular
secretions) from the ants.
Thus, the bacterium is a third
mutualist.
99
Case Study Revisited: The
First Farmers
The ant colonies cultivate a single
clone of the fungus.
 This fungus actively rejects fungi
introduced from outside the colony.
 The more different the invading fungus is
genetically, the stronger the rejection.
 The fungus thus imposes single-crop
farming on the leaf-cutter ants.
100
Figure 14.21 Resident Fungi Inhibit Foreign Fungi (Part 1)
No incompatibility: the two clones
can grow into one another (no
demarcation zone劃界 develops)
Moderate
incompatibility: a
distinct
demarcation zone
develops.
101
Mild incompatibility: a slight but
visible demarcation zone develops.
Strong
incompatibility: a
broad
demarcation zone
with brown
coloration
develops.
Figure 14.21 Resident Fungi Inhibit Foreign Fungi (Part 2)
102
Connections in Nature: From
Mandibles to Nutrient Cycling
Leaf-cutter ants are potent (強有力的)
herbivores and can be a pest of human
agriculture.
These ants tend to increase in
abundance after a forest is cut.
 This may be one reason that farms in
some tropical regions are often abandoned
after just a few years.
103
Connections in Nature: From
Mandibles to Nutrient Cycling
Leaf-cutter ants also introduce large
amounts of organic matter into tropical
forest soils.
Thus, they affect nutrient supply and
cycling in the forest.
Ant refuse areas contain about 48 times
the nutrients found in leaf litter.
Plants increase their production of fine
roots in ant refuse areas.
104
Connections in Nature: From
Mandibles to Nutrient Cycling
Although leaf-cutter ants reduce net
primary productivity (NPP) by
harvesting leaves, some of the other
activities (tillage耕種, fertilization) may
increase NPP.
The net effect of the ants on NPP is
difficult to estimate.
105
Connections in Nature: From
Mandibles to Nutrient Cycling
Other intriguing questions remain.
Ecologists sometimes fall through the
soil, landing in what appear to be
empty ant chambers.
Are they abandoned ant chambers? If
so why were they abandoned? Why
don’t plant roots proliferate there?
As we learn more, new questions
always arise.
106
問題與討論
Ayo NUTN website:
http://myweb.nutn.edu.tw/~hycheng/