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This weeks schedule
Wednesday, Exam.
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Open book, take home. 70 questions.
Review of the material to date through Lesson 10.
70 short answer review questions (70 questions, 1.43 points
each, 100 points total.)
The exam will likely take 2 hours (possibly more) to
complete. You can use the lecture period on Wednesday if
you wish to complete the exam.
The exam is due Wed, Feb 11 at 9 am in class. I will
deduct 5 points if the exam is late that day, and I will not
accept it beyond 5 pm,, Feb 11.!
I will email the exam to everyone today and you can email
the answers back.
1
Field Trip
• Will only go if temperature is above -10û F.
• Equipment:
• Snow shoes or skis. Does everyone have these?
• Hat, gloves, long underwear, jacket, daypack, hand lens, notepad,
pencil, scotch tape, large garbage sack for collecting plants.
• Objectives:
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Learn some of the common local plants in their winter condition.
Examine a snow pit to examine characteristics of the snowpack.
Look at the plants beneath the snowpack.
Look at some wintertime plant-animal interactions.
Enjoy the boreal forest in winter.
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Resource competition: Effect of competition
between species for a single resource, R. Tilman
model (1982)
• Curves are the population
growth rates for species A and
B.
• ma and mb are the mortality rates
for species A and B.
• The intersection of the curves
with the m lines represent the
minimum amount of the
resource R needed to sustain the
population.
• Best competitor is the one with
the lower R* for the limiting
resource.
Recall from the last lesson the work of Tilman, who showed the consequences
of competition where more than one resource is limiting. Here, R* is the
minimum level of a resource required to sustain a population.
Under this assumption, R* represents the amount of a resource that is the
minimum required for an individual to maintain a positive net growth rate.
Competition between individuals can then be viewed as mediated by the R*’s
of different species. In this case, the better competitor is defined as the species
with the lower R* for a limiting resource because this species may continue to
thrive while drawing the resource below the minimum level for the other
species.
Above, curves A and B depict population growth curves dN/dt for two species
versus availability of resource, R. The two curves show the resourcedependent populations growth curves for species A and B. The dashed lines
(mA and mB) represent mortality rates for A and B. The intersection of the
curved solid and dashed lines represents the minimum resource level R* to
support and equilibrium population (represented by the vertical solid lines). In
this case, species B survives at lower resource levels than species A, and hence
is regarded as the superior resource competitor.
From Tilman, D. 1982. Resource competition and community structure.
Princeton University Press, Princeton, NJ.
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Tilman’s resource-ratio model (1982: How 2 species
can coexist competing for the same resource (1)
Lines A and B are Zero Net Growth
Isoclines (ZNGIs) of species A and B
for resources R1 and R2. These show
the availabilities of the two resources
for which the reproductive rate of the
species match the mortality rate.
CA and CB are resource consumption
vectors for each species. The slope of
each vector is the ratio of
consumption of resource R 2 divided
by the consumption of resource R 1.
Tilman’s resource-ratio model.
This model demonstrates how two or more species may differ in R* for the same resource and still coexist in nature.
The figure shows Zero Net Growth Isolines (ZNGIs) for species A and B along two resource gradients R1 and R2.
These lines show the minimum level of reource 1 and 2 required to support each species.
In (a) species A is superior competitor for both resources because it can exist at a lower level for both resources, and
will draw the resource down below the critical level required to support species B.
In (b) the opposite situation occurs, with B being the superior competitor.
In c, A is the superior competitor for resource 1, and B is the superior competitor for resource 2. The vectors CA and
CB are resource consuption vectors that show the relative consumption of resource 1 and 2 for each species. The
slopes of the lines are proportional to the relative amount each resource is limiting to each species. (For CA in c,
resource 2 is more limiting than resource 1 for species A, and resource 1 is more limiting for species B.)
The intersection of the ZNGIs is stable in this case because each species consumes relatively more of the resource that
is limiting to its growth at equilibrium. In zone 1, neither species can exist because resource levels are too low for
either species. In zone 2 and 3, species A will dominate, and in zones 5 and 6, species B will dominate (These are
situations where one resource is much more abundant than the other, and the other is more limiting, and one species
will prevail. Both species can coexist in zone 4 and will reduce the resources 1 and 2 to the equilibrium point.
In (d), the equilibrium point is unstable, because each species uses more of the resource that primarily limits the other
species. In this case, either species A or B can dominate, depending on the initial conditions.
From Tilman, D. 1982. Resource competition and community structure. Princeton University Press, Princeton, NJ.
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Tilman’s resource-ratio model (1982): How 2
species can coexist competing for the same resource
(2)
a)
b)
(a)
(b)
(c)
(d)
Think of R1 and R2 as light and water.
Species A has a lower ZNGI for both
resources, and is the superior competitor.
B has lower ZNGI for both resources and
is the superior competitor.
ZNGIs cross, and Species A is superior
competitor in areas 2 and 3, and species B
is superior competitor in areas 5 and 6.
Resource consumption vectors, CA and
CB, show that species A consumes more of
resource that is limiting to itself, and vice
versa for species B. Hence the species can
coexist in area 4.
Resource consumption vectors show that
each species consumes more of the
resource that is limiting to the other
species. Hence the species are unable to
coexist in area 4.
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Implications of Tilman’s Resource-ratio
hypothesis
Differences in the relative supply rates of limiting resources
should lead to differences in the composition of plant
communities.
• Species allocation patterns: Species with allocation patterns
focusing on shoots are assumed to be relatively effective competitors
for light, and those allocating more heavily to roots are assumed to be
good competitor for below-ground resources (water, nutrients).
• Succession implications: Resource supply ratios also vary
systematically through successional series to first favor root
specialists (because soil nutrition is more limiting than light in
primary succession) and then shoot specialists (because light is more
limiting in later stages of succession.
• Landscape implications: Various habitats within landscapes differ in
their level of key resources, and hence will favor either root or shoot
specialists depending on the local resource supply.
Resource-ratio hypothesis
Plants are unusual because they have organs for maintenance in two different
environments. The organs for acquiring water and nutrients are located
belowground (roots), and the organs for acquiring energy and CO2 for
photosynthesis are located above ground (leaves and stems).
Some plants are shoot specialists and are good competitors for light.
Some plants are root specialists and respond are competitors for water and
nutrients.
Although somewhat intuitive, this hypothesis is experimentally difficult to
demonstrate.
• Plants appear to be more flexible in the root-shoot allocation patterns
than suggested.
• There is wide variation in the spatial and temporal supply of
resources.
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Examples of situations where plants use
environmental tolerance to avoid competition
• Serpentine soils,
– Low in essential nutrients, extreme pH, high in toxic elements (e.g., Ni,
Cr)
– Support unusual plants, often highly endemic floras
– Experimental evidence (e.g., Kruckenberg 1954) indicate that although
serpentine plant species often can grow better in nonserpentine soils if
grown without other species, they are poor competitors when grown with
other species.
• Saline soils
– Halophytes can grow in soil with > 0.2-0.25% salt.
– Many have special structure whereby they secrete excess salts.
– Examples include mangroves, coastal salt marsh species, beach plants,
desert herbs.
Examples of situations where plants use environmental tolerance to avoid competition
Serpentine soils,
Low in essential nutrients, extreme pH, high in toxic elements (e.g., Ni, Cr)
Support unusual plants, often highly endemic floras
Experimental evidence (e.g., Kruckeberg 1954) indicate that although serpentine
plant species often can grow better in nonserpentine soils if grown without other
species, they are poor competitors when grown with other species.
Saline soils
Halophytes can grow in soil with > 0.2-0.25% salt.
Many have special structure whereby they secrete excess salts.
Examples include mangroves, coastal salt marsh species, beach plants, desert herbs.
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Amensalism
• Interaction which depresses one plant population while the
other species remains unaffected.
• A good example is the strongly negative effect that a large
species such as a tree might have on some small ground cover species.
Amensalism
Amensalism is an interaction which depresses one plant population while the
other species remains unaffected.
A good example is the strongly negative effect that a large species such as a
tree might have on some small ground -cover species.
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Allelopathy
• A negative biochemical influence of higher plants upon
another species (usually inhibition of germination or
growth) that is caused by the release of metabolic
substances under natural conditions.
Examples: several lichens, alders, Artemisia (sagebrush),
Larrea (creosote bush).
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Allelopathy: Salvia leucophylla-grassland interface,
Santa Barbara, CA (Muller 1966)
• Light bands around soft
chapperal (Salvia) are devoid of
plants.
• Salvia emits volatile oils
(cineole and camphor).
Allelopathy
Many plants release chemicals that inhibit the growth of other species. These
allelochemicals are selectively toxic to some species of plants.
In competitive situations plants are depriving other plants of a critical
resource. In allelopathic interactions a plant adds a substance to the
environment. This can be viewed as interference competition, i.e. direct
competition.
Studies of C.H. Muller
Some of the best documented examples of allelopathy come from the work of
C.H. Muller and his students.
The above photos show the nearly bare zones between soft chaparral and
annual grassland near Santa Barbara, California. (a) Aerial photograph of
Salvia leucophylla shrubs and adjoining grassland. The light bands beneath
and next to the shrubs are devoid of all but a few species of small herbs. (b)
Ground view of the same situation. A is Salvia; B is the bare zone, and C is the
grassland.
Muller showed that Salvia emits a number of volatile oils (e.g. cineole and
camphor) from their leaves that are toxic to the annuals.
The story, however, is more complex than it might appear. For example,
Kaminsky (1981) has shown that in chamise (Adenostoma fasciculata)
shrublands, the chemicals are released in a nontoxic form, and require soil
microbes to convert the chemicals to allelochemicals.
Some have argued that the effect could be due to animal seed predators that
keep the halo areas around the shrubs free of any seedlings.
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Effect of overstory and understory plants on
soil properties (Tappeiner and Alm 1975)
Effects of plants on soil properties
Plants can have a very inhibitory effect on growth of many organisms
including soil microbes. For example, pines create very acidic soils that are
toxic to many species of plants and soil organisms, including worms, and
many bacteria. Fungi tend to dominate the microflora in these soils, whereas
bacteria dominate the more neutral soils beneath deciduous forests.
The above table shows the difference in some key soil properties of pine and
birch forests. The pine forest have lower pH, lower bulk density, lower soil
nutrients, and slower turnover times. There is also some variation due to
understory species, but this effect is relatively minor.
From Tappeiner and A.A. Alm. Undergrowth vegetation effects on the nutrient
conten of litterfall and soils in red pine and birch stands in northern Minnesota.
Ecology 56: 1193-1200.
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Effect of canopy water throughfall on soil
chemistry
Carlisle et al. 1966
Effect of canopy throughfall
Difference in the chemistry of water reaching the soil surface can also greatly
modify the soil chemistry.
This table shows the dramatic difference in chemistry of rainfall versus the
chemistry of water that falls through an oak (Quercus petraea) forest overstory.
Note that N is slightly reduced beneath the trees because of direct
absorption of N into the tree leaves.
Other nutrients however, are leached out of the tree leaves.
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Summary
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Major types of competition: (1) interference competition (species directly interfere with each other,
e.g. allelopathy), (2) exploitation competition (mediated by exploitation for a shared resource, most
plant competition is of this type), (3) apparent competition (mediated through a third species such
as an herbivore).
Gause’s competitive exclusion principle for animals can be applied to plants in modeling situations,
but in the real world, plants do coexist because natural populations may not come into equilibrium
very often, or other interactions may limit the full competitive interaction between species.
Spatial and temporal variation in resource availability allows for the coexistence of several
species. This can be inferred using differences in dispersal abilities, or differences in above- and
below-ground allocation.
Tilman focused on resource competition as the basis for most competitive interactions. His
resource-ratio models are based on species’ relative abilities to compete for resources.
Grime’s models predict the strongest competition in high resource environments. Plants able to
convert resources to high growth rates are the best competitors in these situations.
Allelopathy is an example of an amensal (0,-) interaction (or interference competition). Many plants
release allelochemicals that are inhibitory to the growth of other species.
Summary:
Major types of competition: (1) interference competition (species directly interfere with each other, e.g. allelopathy), (2) exploitation
competition (mediated by exploitation for a shared resource, most plant competition is of this type), (3) apparent competition
(mediated through a third species such as an herbivore).
Gause’s competitive exclusion principle for animals can be applied to plants in modeling situations, but in the real world plants do
coexist because natural populations may not come into equilibrium very often, or other interactions may limit the full competitive
interaction between species.
Spatial and temporal variation in resource availability allows for the coexistence of several species. This can be inferred using
differences in dispersal abilities, or differences in above- and belowground allocation.
Tilman focused on resource competition as the basis for most competitive interactions. His resource-ratio models are based on species’
relative abilities to compete for resources.
Allelopathy is an example of an amensal (0,-) interaction (or interference competition). Many plants release allelochemicals that are
inhibitory to the growth of other species.
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Literature for Lesson 8
• Everyone please read the following paper:
• Grace, J.B. 1991. A clarification of the debate between Grime and
Tilman. Functional Ecology 5: 583-587.
• Becky, Esther, Brandon, Sandra also read: 1. Grime, J.P. 1977.
Evidence for the existence of three primary stategies in plants and its
relevance to ecological and evolutionary theory. The American
Naturalist,! 111:1169-1194. Brandon will present: it to the class.
• Darcy, Joe, Dan, Daniel, Brad will also read:
• 2. Tilman, D. 1985. The resource ratio hypothesis of plant succession.
The American Naturalist,! 125: 827-852. Brad will present it to the
class:
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Lesson 10: Species interactions:
Commensalism, mutualism, and herbivory
Commensalism
Examples: Epiphytes, Nurse plants,
Protocooperation
Examples: Root grafts, Transfer of nutrients through
mycorrhizal fungi
Mutualism
Examples: Mycorrhizae, Symbiotic N-fixation,
Pollination
Herbivory
Effect on plant communities
Limits to herbivory
Plant defenses against herbivory
Introduction
In the last lesson we examined the pairs of negative interactions of
amensalism, and competition. Here we will examine three generally positive
interactions (commensalism, protocooperation and mutualism) and
one more negative interaction (herbivory).
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Commensalism
• Definition: an interaction that stimulates
one organism but has no effect on the other.
• Examples:
– Epiphytes
– Nurse plants
Commensalism
Definition: an interaction that stimulates one organism but has no effect on the
other.
Examples:
Epiphytes
Nurse plants
Clear examples of true commensal relationships are difficult to come by. It is
not always clear that a host species is not affected negatively be the other.
And as we mentioned earlier, the relationship can change and grade into one of
parasitism or competition.
These two examples, are often given, but they are not always true
commensalism.
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Examples of epiphytes
Spanish Moss
(Tillandsia usneoides), a
Bromeliad
Spanish Moss, closeup
Arboreal lichen,
grandfather’s beard,
(Ramalina reticulata)
•23,000 vascykar-plant epiphyte species (not counting mosses, lichens, liverworts), in 879 plant groups.
Epiphytes
There are a wide variety of epiphytes that include vascular plants such as the
bromeliad, Tillandsia usneoides (a and b, Spanish Moss) or the arboreal lichen
Ramalina reticulata (grandfather’s beard).
Over 23,000 epiphyte species are distributed in 879 vascular plant groups.
Epiphytes have commensal relationships only as long as they do not harm the
host. Some are autotrophic and use the host only for support to gain access to
sunlight.
Others are parasites (e.g., mistletoe, Arceuthobium), so not all epiphytes
have a commensal relationship.
Sometimes a mutualistic relationship can occur if the lichen produces
nutrients that are leached to the tree roots. For example, Forman (1975)
found that most lichens in the upper canopy of a Columbian rain forest contain
a blue-green algae, Nostoc, that fixes carbon equivalent to the amount of
carbon provided by rainwater. This N is probably redistributed through
leaching and decomposition.
Some epiphytes have special leaf or root structures to trap water. Others
such as tree lichens can get all their water needs from the atmosphere.
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Microhabitats of epiphytes
• Zone 1: Small epiphytes. 86% of
these contain Nostoc, a blue-green
algae that fixes nitrogen (Forman
1975),
• Zone 2: Large epiphytes (e.g.
vines)
• Zone 3: Crustose lichens
• Zones 4 and 5: Bryophytes
Longman & Jenik 1974
Microhabitats of epiphytes
Competition for space in the tree canopy can be intense.
Censuses in tropical trees have shown that relatively distinct parts of the
canopy are associated with different epiphyte species (Longman and Jenik
1974, Janzen 1975).
The diagram shows an emergent tree in the tropical rain forest.
Small epiphytes are common in the Zone 1. 86% of these contain Nostoc, a
blue-green algae that fixes nitrogen in the amount of about 1.5-8.0 kg N
ha-1yr-1, about the equivalent of that fixed in rainwater by lightning.
(Forman 1975),
large epiphytes in zone 2,
crustos lichens in zone 3, and
bryophytes in zones 4 and 5.
From Longman, K.A. and J. Jenik. 1974. Tropical Rainforest and Its
Environment. Longman, London.
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Mutualistic epiphytes: Trees that produce canopy roots
The mass of epiphytes is also a great source of water and nutrients to the trees
themselves. Several tree species are thought to tap these nutrient sources by producing
adventitious roots in their canopies that penetrate the mass of humus associated with the
epiphytes. Thus the epiphytes can produce a positive effect for their host.
Nadkarni 1994
Trees that produce canopy roots to tap epiphyte nutrient resource
The mass of epiphytes is also a great source of water and nutrients to the trees
themselves. Several tree species are thought to tap these nutrient sources by
producing adventitious roots in their canopies that penetrate the mass of humus
associated with the epiphytes. Thus the epiphytes can produce a positive effect
for their host, (a mutualistic relationship, +, +).
Epiphytes, however, more commonly act as parasites (+,- interaction):
Several epiphytes’ roots (haustoria) penetrate the bark of the tree and tap the
phloem and xylem.
Hemiparasite: a species able to live facultatively as a parasite
or on its own (e.g., Phoradendron, a species of green mistletoe).
True parasite: a species that relies on the photosynthate and/or
other resources of its host; e.g. Arceuthobium).
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Parasitic epiphytes: Hemiparasite vs. true parasite
• Haustoria: Epiphyte roots that
penetrate the bark of the tree
and tap the phloem and xylem.
• Hemiparasite: a species able
to live facultatively as a
parasite or on its own (e.g.,
Phoradendron, a species of
green mistletoe).
• True parasite: a species that
relies on the photosynthate
and/or other resources of its
host; e.g. Arceuthobium).
Phoradendron californicum; Mistletoe.
Arceuthobium campylopodum;Western Dwarf Mistletoe,
Photos Alfred Brousseau, Saint Mary's College
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Parasitism for light: Strangler fig (Ficus leprieuri)
on palm (Elaeis quineensis)
Longman & Jenik 1974
Parasitism:
Perhaps the ultimate case of an epiphyte as a parasite is the strangler fig (Ficus
leprieuri).
(a) This plant begins its life as in typical epiphyte in the crown of a tree.
(b) As the strangler fig grows, aerial roots grow toward the soil.
(c ) Eventually these aerial roots reach the ground and and introduce a new
source of nutrients to the fig. At this point, the fig is no longer an epiphyte.
These roots thicken, engulfing the host trunk and preventing further growth of
the host tree.
(d) At the same time the canopy of the fig enlarge to overtop the host and
deprive it of light. And eventually the host dies, but the fig remains. In this
case the epiphyte parasitizes and competes with its host.
From Longman and Jenik. (1974) Tropical Rainforest and Its
Environment. London: Longman.
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Commensalism: Nurse plants
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Nurse plants are plants that afford seedlings protection from a
harsh environment while they grow large enough to establish.
Positive effects:
1. Reduce soils temperature and rate of soil drying.
2. Hide the young cactus from rodent herbivores.
3. Protection from frost.
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Examples:
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Palo verde (Cercidium floridum), for saguaro cactus (Cereus
gigantea). Dead palo verde plants are often found in close
association with mature saguaros, indicating that the relationship
may have shifted from a commensal one to a competitive one for
water (Vandermeer, 1980).
Desert annuals. For example, Malacothrix and Chaenactis are
positively associated with the canopies of burro bush and
turpentine broom. These plants have dense canopies that trap
debris that it is a better substrate for the annuals. The seeds are
also trapped in abundance (Went 1942, Muller 1953, Muller and
Muller 1956).
Desert shrubs such as bitterbrush, Purshia tridentata,
shadscale, Atriplex confertifolia, and winter fat, Eruotia lanata,
also require nurse plants. And many bunchgrasses require the
shade of mesquite, Prosopsis juliflora. (Yavit and Smith 1983).
Blue oak, Quercus douglassii, has a positive effect on
surrounding herbaceous plants if the tree has tapped its roots into
groundwater. However, if it hasn’t, it will deplete the soil surface
of soil moisture for herbaceous plant .
Malacothrix californica
Nurse plants:
Nurse plants are plants that afford seedlings protection from a harsh
environment while they grow large enough to withstand the travails of the
environment on their own (Muller 1953, Niering et al. 1963, Lowe 1969).
A good example is provided by the saguaro cactus (Cereus gigantea).
Sauguaro seedlings are nearly always found close to a shade producing object,
such as the palo verde tree in the above photo. Turmer et al. Studied these
relationships and found 14 other species that also act as nurse plants in
southern Arizona. The nurse plants have the following positive effects:
1. Reduce soils temperature and rate of soil drying.
2. Hide the young cactus from rodent herbivores.
3. Protection from frost.
Vandermeer (1980) showed that dead palo verde plants are often found in
close association with mature saguaros, indicating that the relationship may
have shifted from a commensal one to a competitive one for water.
Other examples of nurse plants are:
1. Desert annuals. For example, Malacothrix and Chaenactis are positively associated with
the canopies of burro bush and turpentine broom. These plants have dense canopies that trap
debris that it is a better substarte for the annuals. The seeds are also trapped in abundance
(Went 1942, Muller 1953, Muller and Muller 1956).
2. Many other desert shrubs such as bitterbrush, Purshia tridentata, shadscale, Atriplex
confertifolia, and winter fat, Eruotia lanata, also require nurse plants. And many
bunchgrasses require the shade of mesquite, Prosopsis juliflora. (Yavit and Smith 1983).
3, Blue oak, Quercus douglassii, has a positive effect on surrounding herbaceous plants if
the tree has tapped its roots into groundwater. However, if it hasn’t, it will deplete the
soil surface of soil moisture for herbaceous plants.
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A physiological perspective: Commensalism between maples (Acer
saccharum) and herb layer through nighttime hydraulic lift
Numbers on lines
are horizontal distance
from the tree.
X-axis numbers are dates at noon.
Emmerman & Dawson 1996
• Herbaceous species within 2 m of the base of the trees were larger and more vigorous because of the additional
water.
• Trees take up deep ground water and pass it out through the stomates during the day. At night, there is water
pressure gradient upward from the deep roots to the stem and back out through the near surface roots to upper soil
surface.
• The graph shows higher soil water potential during each night at the soil surface. The effect is diminshed at greater
distance from the tree. The effect is swamped after a rain event.
Another example of commensalism from a physiological perspective:
Emermon and Dawson observed that herbaceous species within 2 m of the
base of the trees were larger and more vigorous because of the additional
water.
Emermon and Dawson (1996) showed observed that sugar maples (Acer
saccharum) take up deep ground water and pass it out through the stomates
during the day during the day.
During the night, there is a water pressure gradient upward from the deep roots
to the stem and back out through the near surface roots to the upper soil
surface (hydraulic lift).
The above graph shows higher soil water potential occurring each night at the
soil surface. This was due to the hydraulic lift during the night, which was
most pronounced near the tree.
This effect is diminished at greater distances from the tree. Also after a rain
event.
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Protocooperation through root grafts
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Protocooperation: An interaction that stimulates both partners (+,+) but is not obligatory.
Growth and survivorship is possible in the absence of the interaction.
Example: two trees are connected by root grafts or unions between the same or different
species. (About 160 species of tree species can form grafts and 20% of these form
interspecific or intergeneric grafts.)
When one species is much smaller as in (b), then the relationship is one of parasitism.
Example of protocooperation through a root graft (left)
In this example, two trees are connected by root grafts or unions between the
same or different species. About 160 species of tree species can form grafts
and 20% of these form interspecific or intergeneric grafts.
When one species is much smaller as in (b), then the relationship is one of
parasitism.
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Protocooperation through soil mycorrhizae (Woods and
Brock 1964)
•Within 8 days 45% of the trees within a 7.3 radius of the stump showed radioactivity.
Woods and Brock’s concluded that the labeled nutrients had moved to the surrounding
plants through michorrhizal connections.
•They felt that the root mass of a forest often has such connections and can be viewed
as a single functional unit.
Protocooperation through mycorrhizal hyphae:
Woods and Brock (1964) put labeled 45Ca and 32P in bottle on top of a freshlycut maple stump and sealed it so it wouldn’t escape into the surrounding soil
or air.
Within 8 days 45% of the trees within a 7.3 radius of the stump showed
radioactivity. Woods and Brock’s concluded that the labeled nutrients had
moved to the surrounding plants through michorrhizal connections.
They felt that the root mass of a forest often has such connections and can be
viewed as a single functional unit.
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Mutualism
• A symbiotic relationship that is essential to the survival of
both species.
• Common examples:
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Lichen (algae for photosynthate and fungus for nutrients)
Mycorrhizal fungus
Symbiotic nitrogen-fixing bacteria
Pollinating insects, birds, mammals
Zoochory, animal dispersal of propagules
Mutualism;
A symbiotic relationship that is essential to the survival of both species.
Common examples:
Lichen (algae for photosynthate and fungus for nutrients, or structural
support). Ahmadjian and Jacobs (1982), however have shown that this
relationship is actually one of a controlled parasitism where the fungus
is actually an obligatory parasite of the alga.
Mycorrhizal fungus
Symbiotic nitrogen-fixing bacteria
Pollinating insects, birds, mammals
Zoochory, animal dispersal of propagules (for example, ants dispersing
seeds, while consuming nutritious appendage (eliasome) also frugivory
of birds)
Also example of ants in relation to epiphytes. Ants use the plants for
protection and nesting sites. The ants also pack feces around the
rhizomes and adventitious roots of the epiphytes. They also may reduce
herbivory and help in dispersal of seeds.
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Mutualism: Mycorrhizae: Ecto- vs. endomycorrhizae
Fungal hyphae
Fungal mantel (haustoria)
Root cells with
hyphae penetrating
between the cells
•Mycorrhizae are fungal
associations with the roots of
higher plants.
•Mycorrhizae transfer nutrients
and metabolites in both
directions between the vascular
plant and the fungus. They
exude nutrients, which are
absorbed by the fungus. And
the mycorrhizae help the plants,
which are somehow stimulated
to take up greater amounts of
nutrients (Ca, P, K).
•Endomycorrhizae are those
are those that penetrate the cell
walls.
• Ectomycorrhizae do not.
Mycorrhizae:
Mycorrhizae are fungal associations with the roots of higher plants.
Endomycorrhizae are those penetrate the cell walls.
Ectomycorrhizae do not.
Endomycorrhizal associations are the most common and appear to affect
nearly all higher plants with the exceptions of aquatic vascular plants, and
members of the Brassicacea, Cyperaceae, and Juncaceae. There are also few
members of the Poaceae that have mycorrhizae.
1. (a) of the above figure shows a root cross section with the long hair of a
fungus hypha. The area labled (2) is the fungal mantal (haustoria) covering
the root. (3) shows the fungus hyphae penetrating between the cortex cells of
the root.
(b) and (C) show some of the forms that mycorrhize can take. In both cases the
fungus covers short club-shaped lateral roots.
2. Mycorrhizae transfer nutrients and metabolites in both directions between
the vascular plant and the fungus. They exude nutrients, which are absorbed
by the fungus. And the mycorrhizae help the plants, which are somehow
stimulated to take up greater amounts of nutrients (Ca, P, K).
3.
27
Mutualism: Symbiotic N-fixation
• Nitrogen fixation is the conversion of atmospheric N into
organic ammonium NH3+.
• Usually a nitrogen fixing bacteria fixes N on a host in
return for carbon-based resources.
• Examples include:
– Rhizobium bacteria in root nodules of legumes
– Blue green algae Nostoc and Anabaena in association with
bryophyte gametophytes, root nodules of cycads, or the leaf tissues
of the fern Azolla.
– Soil actinomycetes (nodule forming filamentous bacteria)
Symbiotic N-fixation:
There are also numerous species of bacteria and algae that form symbiotic
relationships with plants provide nitrogen to the plant.
These organisms convert atmospheric N to organic ammonium NH3+.
4N2 + 6H20 = 4 NH3 + 3O2 (also require ATP and nitrogenase in a
reducing environment; see equation p. 163)
The plant in return, provides metabolites to the bacteria or algae.
Some examples include:
Rhizobium bacteria in root nodules of legumes
This association is well known and is the basis for interplanting
peas or other legumes with high nitrogen users such as corn. Also hay
fields are often planted with alfalfa. The alfalfa grows slower than the
grasses is harvested late in year after the grasses and adds to the
nitrogen content of the soil.
Blue green algae Nostoc and Anabaena in association with bryophyte
gametophytes, root nodules of cycads, or the leaf tissues of the fern
Azolla. Some lichens also have blue-green algal associations, such as
Peltigera aphthosa. 3/4 of the N requirements of rice can be met with
Azollai in rice paddies.
Soil actinomycetes (nodule forming filamentous bacteria)
These organism resemble the fungi micorrhizae associations.
They invade the roots causing elongate nodules, and can fix nitrogen in
rates comparable to legume nodules. Some 285 species of woody
plants possess Actinorhizal associations. Many are pioneering species
and occur on N-poor, acidic, saline, or sandy soils.
28
Mutualism between insects and plants: Pollination:
some characteristics of adapted plants
• Pollination is a special form of mutualism that is the key to
much evolution in flowering plants, and is responsible for
specialized morphology of many flowers of angiosperms.
• Provides a food source for the animals.
• Advantages to the plant:
– Increased pollination results in increased seed production in about
62% of species examined (Burd 1994).
– Possibility of accurate pollen dispersal far from the host anther,
allowing for outcrossing and genetic varibility.
29
Pollination (cont’)
• Plant adaptations to attract pollinators:
– Attractive petals, sepals, or inflorescences (either
visually or olfactorily).
– Sculpted or sticky pollen grains, sometimes massed
together.
– Nutritious nectar, pollen or starch bodies.
– Attractants that are available at pollinaton time.
Pollination:
1. Pollination is a special form of mutualism that is the key to much
evolution in flowering plants, and is responsible for specialized
morphology of many flowers of angiosperms.
2. It provides an obvious advantage for the possibility of accurate pollen
dispersal far from the host anther, allowing for outcrossing and genetic
varibility.
3. It provides a food source for the animals.
4. Increased pollination results in increased seed production in about 62% of
species examined (Burd 1994).
30
Adaptations of bee-pollinated flowers
and pollinators
Flowers:
–
–
–
–
–
–
Bilateral symmetry.
Mechanically strong flowers, often with sexual organs concealed.
Bright blue or yellow colors (bees can’t see red).
Nectar guides along a landing platform.
Moderate quantities of nectar that is sometimes concealed.
Many ovules per ovary, few stamens.
Pollinators:
-
Good color discrimination.
– High degree of intelligence, long memory.
– Long proboscis capable of probing for nectar.
Adaptations of bee-pollinated flowers:
Bilateral symmetry.
Mechanically strong flowers, often with sexual organs concealed.
Bright blue or yellow colors (bees can’t see red).
Nectar guides along a landing platform.
Moderate quantities of nectar that is sometimes concealed.
Many ovules per ovary, few stamens.
Adaptations of pollinators:
Good color discrimination.
High degree of intelligence, long memory.
Long proboscis capable of probing for nectar.
31
Herbivory
• The consumption of all or part of a living plant by a consumer.
• Includes:
–
–
–
–
Parasitic and phytophagous microbes (e.g., some fungi and algae)
Phytophagous invertebrates
Browzing and grazing vertebrates
Seed predators.
Herbivory:
The consumption of all or part of a living plant by a consumer.
Includes:
Parasitic and phytophagous microbes (e.g., some fungi and algae)
Phytophagous invertebrates
Browzing and grazing vertebrates
Seed predators
32
Herbivorous insects in 9 out of 29 orders
of insects
Numbers of species (
80% of macroscopic plants
and animals are plants,
herbivores, or species that
prey on herbivores.
Strong, Lawton & Southwood 1984
Although only 27 of 97 order of animals and 9 of 29 orders of insects
contain herbivores, these groups are very diverse. The shaded bars in the
above diagram are the orders that contain herbivores. Note the log scale
showing the large number of species in the order containing herbivores.
Strong et al. (1984) estimate that 80% of all macroscopic species of plants
and animals are plants, herbivores, or species that prey on herbivores.
33
Effects of herbivory
• Herbivores typically consume about 10% of net primary
production (NPP). (Deserts and tundra: 2-3%; Forests: 47%; Temperature grasslands: 10-15%; African grasslands
30-60%).
• Seedlings are most vulnerable
• Mature plants can withstand huge losses due to herbivory.
Typically, wood production is not affected until about 50%
of the leaf surface is consumed.
• Seed consumption is much higher than 10% and may reach
100%.
Effects of herbivory
Herbivores typically consume about 10% of net primary production (NPP).
(Deserts and tundra: 2-3%; Forests: 4-7%; Temperature grasslands: 10-15%;
African grasslands 30-60%).
Seedling are most vulnerable
Mature plants can withstand huge losses due to herbivory. Typically, wood
production is not affected until about 50% of the leaf surface is consumed.
Seed consumption is much higher than 10% and may reach 100%.
34
Escape hypothesis: Seed dispersal is mainly a
mechanism to escape from seed predators
• Escape from seed predators
may be the biggest factor
governing dispersal and plant
establishment, particularly in
tropical systems (Janzen 1970
and Connell 1971).
• Optimal dispersal distance
from parent plant is one where
survivorship from predators is
balanced by liklihood of
finding a favorable habitat.
Augsberger 1983
Escape hypothesis:
Janzen and Connel have hypothesized that the driving mechanism behind
much seed dispersal is escape from predators and pathogens. Seed predators
and pathogens associate parent plants with nearby food sources.
The closer the seed falls to the parent, the more likely it is to be consumed by
seed predators.
In the above model by Augsberger, the optimum survival distance for dispersal
is portrayed to be an intermediate distance from the parent where the density
of seeds is moderate and the probability of survivorship in a favorable
environment is also moderate.
35
Why is the world still green?
Limits to herbivory
• Top-down limits (predator control of herbivores)
• Bottom-up limits (poor nutritional quality of plants)
– Plant proteins are different from animal proteins and must be digested and
resynthesized by the herbivore.
– Protein content of plants is low.
– Carbohydrate content is high, but mostly in the form of poorly digestible
forms (lignin and cellulose).
– N is often bound in relatively inaccessible forms such as secondary
metabolites.
Limits to herbivory:
Although about 25% of the multicelled species are herbivores, only about 1020% of the aboveground green biomass is consumed annually by herbivores.
There are two primary reasons for the limited effect of herbivory:
1. Top-down limits (predator control of herbivores)
2. Bottom-up limits (poor nutritional quality of plants)
Plant proteins are different from animal proteins and must be digested and
resynthesized by the herbivore.
Protein content of plants is low.
Carbohydrate content is high, but mostly in the form of poorly digestible forms
(lignin and cellulose).
N is often bound in relatively inaccessible forms such as secondary metabolites.
36
Plant defenses
• Tolerate herbivory
– Cheap plant parts, rapid growth rates (typical in
resource-rich environments)
– Some plants may actually be stimulated to greater
production and reproduction through herbivory
(e.g., scarlet gilia, Ipomopsis aggregata, Paige
1992).
Ipompopsis aggregata
• Constitutive (physical) defenses
– Those that are a fixed part of plant allocation
– Generally more expensive for the plant
– Examples include hairy stems and leaves, spines, or
chemicals that are not induced.
• Inducible defenses
– Preformed inducible chemical defenses that are
stored in the plant but are transported and become
active under stimulation from attack (e.g.,
Furanocoumarin in cow parsnips, Pastinaca sativa).
– Induced chemical defenses that are produced after
stimulation (e.g., nicotine production in tobacco is
stimulated by early herbivory to the seedlings.)
Fouquieria splendens, Ocotillo, Photos St.
Marys of California
Heracleum lanatum, cow parsnip, Photos
Charles Webber, California Acad. Sci.
Plant defenses:
Plants have a wide variety ways of defending themselves from attack.
1. Tolerate herbivory
Cheap plant parts, rapid growth rates (typical in resource-rich environments)
2. Physical defenses (constitutive defenses)
Examples include hairy stems and leaves, spines
3. Chemical defenses (secondary plant compounds)
Preformed inducible chemical defenses that are stored in the plant but are
transported and become active under stimulation from attack (e.g., Furanocoumarin in
cow parsnips, Pastinaca sativa).
Induced chemical defenses that are produced after stimulation (e.g., nicotine
production in tobacco is stimulated by early herbivory to the seedlings.)
37
Major classes of secondary plant compounds
Ledum decumbens, an evergreeen shrub
with abundant phenols
Major classes of secondary plant compounds:
This table shows some of major classes of secondary plant compounds
involved in plant-animal interactions:
38
Hypothetical relationship between type of defense
and probability of attack (Bazzaz 1992)
• Optimal defense theory of Rhoades (1979)
states that a plant should neither overallocate
nor underallocate to its defenses.
• Plants that grow fast are usually poorly
defended.
• Predictability of attack should be correlated
with the allocaiton to constitutive and induced
defences, i.e., if plants are not likely to be
eaten they will preserve their resources for
defense until they are under attach (inducible
defences, see left, diagram).
• Apparency theory: long-lived plants are
apparent to herbivores and require more
heavy defenses, i. e., high levels of
Zangerl & Bazzaz 1992 constitutive defenses throughout their green
tissues including tannins, resins, and lignin,
also spines and tough leaves.
.
Numerous theories have developed that try to explain allocation of plant
resources to defense.
Optimal defense theory of Rhoades (1979) states that a plant should neither
overallocate nor underallocate to its defenses.
Plants that grow fast are generally poorly defended.
In the above figure, the predictability of attack should be correlated with the
allocation to constitutive and induced defenses (Zangerl and Bazzaz 1992) .
Apparency theory states that long-lived plants are apparent to herbivores and
require more heavy defenses. Easy to find perennials require high levels of
constitutive defenses throughout their green tissues because they are constantly
attacked by herbivores. This is why many tree leaves contain high levels of
tannins, resins, and lignin, also spines and tough leaves.
39
Summary
• Commensalism is an interaction that stimulates one organism but has
no effect on the other (+,0). Examples include epiphytes and nurse
plants.
• Protocooperation is an interaction that stimulates both partners (+,+)
but is not obligatory (e.g., root grafts in large trees).)
• Mutualism is a symbiotic relationship that is essential to the survival
of both species (e.g., lichens, mycorrhizae, symbiotic N-fixers,
pollination, zoochory)
• Herbivory is the consumption of all or part of a living plant by a
consumer (e.g. Parasitic and phytophagous microbes, phytophagous
invertebrates, browzing and grazing vertebrates, seed predators).
• Top-down limits to herbivory relate to predator control of herbivores
• Bottom-up limits are those associated with poor nutritional quality of
plant
• Secondary plant compounds are a primary method of defense against
herbivory in many plant species.
• Constitutive controls on herbivory are those that are produced without
stimulation from herbivores and are expensive. Induced controls are
activated or produced by stimulation from herbivores.
40
Literature for Lesson 10
Bertness, M.D. and S.M. Yeh. 1994. Cooperative and competitive interactions in
the recruitment of marsh elders. Ecology 75: 2416-2429.
**Bryant, J. P., F. D. Provenza, et al. 1991. Interactions between woody plants
and browsing mammals mediated by secondary metabolites. Annual Review of
Ecology and Systematics 22: 431-446.
Bryant, J. P., J. Tahvanainen, et al. 1989. Biogeographic evidence for the
evolution of chemical defense by boreal birch and willow against mammalian
browsing. American Naturalist 134: 20-34.
Kielland, K. and J.P. Bryant. 1998. Moose herbivory in taiga: Effects on
biogeochemistry and vegetation dynamics in primary succession . Oikos, 82:
377-383.
**Mulder, C.P.H. 1999. Vertebrate herbivores and plants in the Arctic and
subarctic: effects on individuals, populations, communities and ecosystems.
Perspectives in plant ecology, evolution, and systematics, 2: 29-55.
Ruess, R. W., R. L. Hendrick, and J. P. Bryant. 1998. Regulation of fine root
dynamics by mammalian browsers in early successional Alaskan taiga forests.
Ecology 79:2706-2720.
41
Economic model of mutualism
(Schwartz and Hoeksema 1998)
Economic model:
This example by Schwartz and Hoeksema demonstrates why it is adventageous
for two species to enter into a mutualistic relationship based on trade of
resources. We won’t go into the details of the trade, but it demonstrates why
both species can benefit.
For those who do want to go into the details:
Species A is equally efficient at obtaining R1 and R2. Species B is 3 times
more efficient at obtaining resource R2. Both species require both resources in
equal amounts.
Before the trade, Species A uses 24 units to consume 24 combined units of R1
and R2. Species B uses 9 units to consume 3 units of R1 and 3 units to consume
3 units of R2.
After the trade, A specializes in R1 and B specializes in R2.
A produces 24 units of R1 at a cost of 24 units, consuming 16 and trading 8 to
B.
B produces 12 units of R2 at a cost of 12 units, consuming 4, and trading 8.
For the same cost as before the trade, A gains 4 units of both resources (33%
gain) and B gains 1 unit of both resources (33% gain).
42
Theories to explain types of defensive
compounds used (Karban and Baldwin 1997)
• Supply side theories (secondary metabolites are waste
products of the plant)
– Carbon-nutrient balance theory.
– Substrate-enzyme imbalance theory
– Growth-differentiation balance theory
• Part of a multifaceted strategy of plant allocation
– Generalized stress-response theory
– Active defense reponse theory
Supply side theories (secondary metabolites are waste products of the plant)
Carbon-nutrient balance theory.
The types of defensive compounds will be determined by whether the plant is limited
by carbon or nitrogen.
Nitrogen limited plants will invest in carbon-based defenses (e.g., phenolics,
terpenoids, tannins, lignin).
Carbon limited plants will invest in nitrogen-based defenses (e.g., alkaloids)
Substrate-enzyme imbalance theory
Either C-based or N-based defensive compounds are produced as a result of excess
metabolic activity.
Growth-differentiation balance theory
Plant growth and differentiation of different allocation pathways: plants do not
differentiate tissues when maximizing growth and vice versa. If growth and
differentiation
processes are negatively correlated, then plants may produce secondary metabolites
for use as anti-herbivore defenses only when not maximally allocating toward
growth.
Part of a multifaceted strategy of plant allocation
Generalized stress-response theory (Chapin 1991)
Plants have a centralized mechanism that allows them to simultaneously and
interactively respond to diverse stresses.
Active defense reponse theory (Chesin and Zipf 1990)
The signaling system that plants use to induce the consturction of defensive
compounds is very specific.
43
Nutrient flux gradients (Huston and De Angelis
1994)
(a)
High nutrient flux: Plants can
coexist because each has
access to only a small portion
of the total available resource.
(b) Low nutrient flux: Plants
deplete nutrients over a much
broader area.
Effects of soil nutrient flux gradients
Tilman later modified his resource ratio model to incorporate allocation
patterns to roots and shoots (Tilman 1988).
And expanded the model to predict species life histories, diversity, and
competitive effects of communities at different successional stages (Tilman
1988, Tilman and Pacala 1993).
Huston and De Angelis examined the effects of local soil resource depletion
and soil nutrient transport rates on competition and coexistence. Because
nutrient transport rates in soil are low, plants have a very limited ability to
affect resource availability outside their root zones.
In the above figure, the lightly shaded regions represent portions of the root
zones where an individual plant has depleted local resources.
Species with similar resource requirements, but restricted rooting zones (as in
a) can coexist because each can access only a small portion of the of the total
resources available.
If soil resource depletion zones extend into the rooting zones of neighboring
individuals, then competitive effects become important.
Figure (a) represents the situation with high nutrient flux, where plant plants
deplete resources in a narrow region. Figure (b) represents the situation with
low nutrient flux, where plants deplete nutrients over a much broader area.
CR is the regional concentration of soil nutrient. Cp# is the soil nutrient
depletion zone created by each plant.
From M.A. Huston and D.L. DeAngelis. 1994 American Naturalist 140: 539572.
44
Productivity vs. species richness (Tilman
and Pacala 1993)
•
•
•
Habitats intermediate in
resources (and productivity)
tend to support the most
species.
Extremely poor soils are likely
to dominated by a few species
that can compete will for a
single limiting resource.
Extremely rich soils support
high biomass production and
are dominated by the few
species that compete the most
effectively for light.
Sites with intermediate resources tend to have the highest species
diversity.
Extreme habitats (too rich or too poor in resources) generally support
relatively low biological diversity.
Habitats intermediate in resources (and productivity) tend to support the most
species.
Extremely poor soils are likely to dominated by a few species that can
compete will for a single limiting resource.
Extremely rich soils support high biomass production and are dominated by
the few species that compete the most effectively for light.
From D. Tilman and S. Pacala. 1993. The maintenance of species richness in
plant communities. In: R.E. Ricklefs, and D. Schluter (eds.) Species diversity
in ecological communities. Historical and geographical perspective.
University of Chicago Press.
45
Salt marsh commensalism between Iva frutescens and Juncus
Bertness & Yeh 1994
• Marsh elder, Iva frutescens, is inhibited by dense perrenial turfs of plants, and
is typically found in disturbed bare patches.
• These sites, however, have hot soil conditions, high salinity, and have few
recruits.
• Iva that germinates under adults or in clumps of Juncus has higher success
rates.
Salt-marsh example
4. In the above example, Bertness and colleagues observed that in the Rhode
Island salt marshes, marsh elder, Iva frutescens, is inhibited by dense perrenial
turfs of plants, and is typically found in disturbed bare patches. These sites,
however, have hot soil conditions, high salinity, and have few recruits. Iva that
germinates under adults or in clumps of Juncus has higher success rates.
Caption for the above figure: Dry mass of surviving seedlings of Iva
frutescens, when grown solitarily or with other Iva seedlings or with
Juncus seedling neighbors.
Three treatments included watering to prevent saline buildup, shade to prevent
high evaporation, and other nurse plants. Dry mass of the surviving seedlings
are shown. In the control there is little surviving in any of the situations.
Plants that are grown with nurse plants, do as well as those in the watered or
shade experiments, and much better than the control. Each bar (and 1 standard
error) represents the mean dry mass of 4-100 seedlinigs. Means with same
letter are not significantly different at the p< 0.05 level.
From Bertness, M.D. and S.M. Yeh. 1994. Cooperative and competitive
interactions in the recruitment of marsh elders. Ecology 75: 2416-2429.
46
Bird Pollination
Faegri & van der Pijl 1971
Pollination:
In tropical regions, birds are much more important pollinators. Examples in
include sunbirds of Africa, honey creepers of Hawaii, and hummingbirds
of North and South America. There has also been a great deal of coevolution
between flowers and bird pollinators.
This topic of coevolution between birds and flowers has received a lot of
attention of plant evolutionary ecologists.
47