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
Commensalism
•
•
Stimulates one organism but has no effect on the other.
Examples:
– Epiphytes
– Nurse plants
Examples of
epiphytes
Tillandsia usneoides
(Spanish Moss)
Bromeliaceae, the Pineapple
Family!
www.csdl.tamu.edu/
FLORA/LCP/LCP94.JP
• 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.
• Over 23,000 vascular-plant
epiphyte species (not
counting mosses and
lichens).
• Others are parasites (e.g.,
mistletoe, Arceuthobium).
• Sometimes a mutualistic
relationship can occur if
the lichen produces
nutrients that are leached
to the tree roots.
Bryoria sp. (Black tree lichen)
Boreal forest arboreal lichen
host04.ipowerweb.com/.../ Botany/Bryoria_sp2.jpg
Ramalina reticulata (Fishnet lichen)
Chapparal arboreal lichen
http://ww1.clunet.edu/wf/chap/common/bjc-671.htm
• 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 redistributed through
leaching and decomposition.
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,
upper photo).
True parasite: a species that
relies on the photosynthate
and/or other resources of its
host; e.g. Arceuthobium (lower
photo).
Phoradendron californicum; Mistletoe.
Arceuthobium campylopodum;Western Dwarf Mistletoe,
Photos Alfred Brousseau, Saint Mary's College
Daintree Rainforest, Australia
Entada phaseoloides (matchbox bean)
Fabaceae
www.seabean.com/locations/ Australia/index2.htm
Platycerium superbum (staghorn fern)
Polypodiaceae
farrer.riv.csu.edu.au/ ASGAP/gall5.html
Lawyer vine, Australia
Calamus muelleri (Lawyer vine, Wait awhile, Rattan)
Palmae
the-i.org/pic_flora/04
• Grows in areas of disturbance in subtropical rainforests.
• Starts as a small inconspicuous palm that sprouts in the understory and remains small until opening
develops in the canopy.
• THEN its growth form changes! It develops long flagella like leaders (canes) with spines that reach up
into the plant canopy and grasp whatever allows it to gain the top of the canopy. These leaders are
stiff leafless stems of one to two metres in length with backward facing hooks which allow the plant to
be a successful canopy climber. The older smooth canes can be used for weaving, tying and
construction purposes (Rattan furniture).
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
Mutualistic epiphytes: Trees that produce canopy roots
Nadkarni 1994, cited in Barbour et al. 1999
• 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.
Parasitism for light: Strangler fig
(Ficus leprieuri)
Longman & Jenik 1974, cited in Barbour et al.
1999
The host tree has died
and is long gone!
www.cropsoil.uga.edu/.../ 2003/SanLuis/148.html
(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.
Commensalism: Nurse plants
•
•
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.
•
Examples:
–
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 hasnt, it will deplete the soil
surface of soil moisture for herbaceous plants.
Palo Verde and Saguaro
Malacothrix californica
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 cited in
Barbour et al. 1999
•
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.
Protocooperation through root grafts
•
•
•
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.
Protocooperation through soil mycorrhizae
(Woods and Brock 1964)
Woods and Brock 1964 cited in Barbour et al. 1999.
• Isotopes of Ca and P were introduced into a fresh stump of red maple.
• Within 8 days 45% of the trees within a 7.3 radius of the stump showed
radioactivity. Woods and Brocks concluded that the labeled nutrients had
moved to the surrounding plants through mycorrhizal connections.
•They felt that the root mass of a forest often has such extensive connections
and can be viewed as a single functional unit.
Mutualism
•
•
A symbiotic relationship that is essential to the survival of both species.
Common examples:
–
–
–
–
–
Lichen (algae for photosynthate and fungus for nutrients)
Mycorrhizal fungus
Symbiotic nitrogen-fixing bacteria
Pollinating insects, birds, mammals
Zoochory, animal dispersal of propagules
Mutualism: Mycorrhizae: Ecto- vs. endomycorrhizae
Fungal hyphae
Fungal mantel (haustoria)
on outer surface of root
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.
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 carbonbased resources.
Examples include:
–
–
–
Rhizobium bacteria in root nodules of legumes
Soil actinomycetes (nodule forming filamentous bacteria)
Blue green algae Nostoc and Anabaena in association with bryophyte
gametophytes, some lichens, root nodules of cycads, or the leaf tissues of the
fern Azolla.
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 variability.
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.
Adaptations of bee-pollinated flowers and pollinators
Flowers:
–
–
–
–
–
–
Bilateral symmetry.
Mechanically strong flowers, often with sexual organs concealed.
Bright blue or yellow colors (bees cant 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.
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.
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
Effects of herbivory
•
•
•
•
Herbivores typically consume about 10% of net primary production (NPP).
(Deserts and tundra: 2-3%; Forests: 4-7%; Temperature grasslands: 1015%; 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%.
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
likelihood of finding a
favorable habitat.
Augsberger 1983
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.
Plant defenses
•
Tolerate herbivory
–
–
•
Constitutive (physical) defenses
–
–
–
•
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).
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.
Ipompopsis aggregata
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.
Major classes of secondary plant compounds
Ledum decumbens, an evergreeen shrub
with abundant phenols
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 allocation to
constitutive and induced defenses,
i.e., if plants are not likely to be
eaten they will preserve their
resources for defense until they
are under attach (inducible
defenses, see left, diagram).
Zangerl & Bazzaz 1992
Apparency theory: long-lived plants are
apparent to herbivores and require more
heavy defenses, i. e., high levels of
constitutive defenses throughout their
green tissues including tannins, resins,
and lignin, also spines and tough leaves.
.
Coevolution of chemical defenses
in boreal birch and willow
(Bryant et al. 1989)
Study tested the hypothesis that winter
browsing by mammals has selected
boreal woody plants especially the
juvenile phase for chemical defense in
winter.
Assumed woody plants from regions with
comparatively low numbers of
browsing mammals would be more
palatable and have fewer chemical
defenses.
Also assumed that selective pressures
would be strongest in areas with 10year hare cycles.
Studied willows and birch from Iceland (no
hares), Finland (no 10-yr cycle),
Siuberia and Alaska (areas with 10-yr
cycles).
Snowshoe hare
Betula pendula and B. exilis
Mountain hare:
Chemical defenses: Bryant et al. 1989 (2)
Feeding trials on mountain hares
from Finland and snowshoe hares
from Alaska. Species of birch are
arranged according to their
concentrations of toxic resins and
terpenes.
Bryant et al. 1989 (3)
Salix alaxensis from Alaska
http://www.arctic.uoguelph.ca/cpl/sightssounds/
Org_stills/Organism/Stills/Terrestrial/B&W/bg/15mr.jpg
Salix caprea from Finland
http://popgen0146uns50.unimaas.nl/~jlindsey/commanster/Plants/Trees/Trees/Salix.caprea2.jpg
•
•
Finnish hares would eat only
the poorly defended species
of willows and birch from
Finland.
Alaska hares would eat both
but preferred the Finnish
shrubs.
Bryant et al. (4)
•
Resin and papyrific acid
concentrations were
especially high in juvenile
tree birches from Alaska, and
were not eaten by the Finnish
hares.
Bryant et al. 1989 (5)
•
•
•
Mountain hares much
preferred the poorly defended
Finnish willows.
Alaska hares would eat a
wider range of species, but
still preferred the Finnish
willows.
They would not eat Salix
arbusculoides, which was
particularly high in phenol
glycosides in the willow inner
bark.
Bryant et al. 1989 (6)
•
Conclusions
–
–
–
–
Winter-dormant boreal woody
plants, particularly in the juvenile
phase, contain secondary
metabolites that deter browzing
mammals such as hares.
Winter browzing by hares has
selected boreal woody plants for
chemical defense and particularly
in areas with a 10-yr hare cycle.
Alaska hares have evolved more
effective methods of
detoxification of birch and willow
chemical defenses.
Suggests that chemical
coevolution has occurred
between boreal woody plants and
boreal hares and this process has
been influenced by the 10-yr hare
cycle in Alaska.
Summer and winter snowshoe hare.
http://www.northpolealaska.com/BeaverSprings/Nature/Wildlife/Mammals/hares.jpg
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.
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.