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Callaway 1997
“The widespread acceptance of individualistic
species assemblages, but holistic ecosystems, in
academic ecological thought suggests a paradox
rather than simple semantic differences. The
definitions of communities and ecosystems are
irrelevant to the fundamental issue of
independence versus interdependence.”
The fact that conservation often uses the interconnectedness
of species in an ecosystem and keystone species to
demonstrate the importance of biodiversity indicates there
are either genuine artifacts of communities or that such
conservation practices are based on fallacy.
Positive interactions…?
• The importance or even presence of positive
interactions shift on environmental gradients…
– drought, low nitrogen availability, high soil salinity,
extremes of timberline/altitude, etc.
• Example given is P. albicaulis and A. lasiocarpa
– Often compete, but in some areas presence of former
dependent on presence of latter.
• Indirect interactions (parasitic plants, mycorrhizal
interactions, etc.) and mutualisms connect most
plants to some other members of community.
Bastow’s community
• Bastow’s loose definition of community may
allow flexibility to include the variations in
positive interactions…
• If we were to define communities based on
some level of positive interactions, perhaps we
would just end up with plant communities in
some places, and “competitive assemblages” in
others?
• To simply call any assemblage of plants in an
area a community, and to recognize that they
interact with each other in some way, seems to
agree with the author’s ideas.
Questions…
• How does Callaway’s emphasis on the
existence of some form of interaction
between plants make you feel about the
prevalence of positive interactions? Of
communities based on interaction?
• Is our whole emphasis on biodiversity in
conservation largely a sham, or do
ecosystem components really hold
together? If so, why not communities?
Talking Trees
Communication between
plants:
Induced resistance in wild tobacco
plants following clipping of
neighboring sagebrush
Karban, Baldwin, Baxter, Laue, Felton
Experiment
• Question: Do tobacco plants near clipped
sagebrush have increased resistance to
herbivores?
• Methyl Jasmonate: proposed signal
– 10.8 fold increase in air around clipped
branches
– PPO is a chemical that provides resistance to
several herbivores (including grasshoppers).
– PPO levels increased in plants near clipping
Signal Transfer
Plastic bags used to test signal transfer through
soil and air.
• Decreased shaded area after clipping was
one hypothetical explanation for decrease
in herbivore resistance.
Relation to Bastow
• Bastow says this experiment validates the
‘talking trees’ mechanism
• Bastow: “Plants can often increase their
content of defence compounds when
grazed”
– Do we have proof of the frequency he
suggests?
Bastow’s Remaining Question
• Bastow asks if ‘talking trees’ can be a
result of natural selection.
• Well?
Competition for Space
• Does it exist?
• Is it simply a proxy for other resources?
From the animal world
• Moksnes (2004)
• Shore crab instar growth and migration
– Crab growth (carapace width) is decreased under
crowded conditions, even when food availability is
held constant
– More migration away from food source occurs when
density of individuals is highest
• Underwood (2000)
– Space to settle needed by marine intertidal species
From the plant world
• Stafford and Bell (2006)
– Competition for space between invasive alga
(Caulerpa prolifera) and seagrass (Halodule
wrightii)
– In response to disturbance, alga was able to
preempt open space more quickly than
seagrass
• Kastendiek (1982)
– Competition between alga by overtopping (but
mechanism was denial of access to light)
From the plant world
• Schienk, Callaway, and Mahall (1999)
– Competition for rooting space
– Review of evidence for root segregation and
mechanisms causing it (e.g. allelopathy)
– But is this evidence for competition for space
per se, or simply a mechanism of competition
for other resources?
From the Cyber World
• Grist (1999)
– Cellular automata to model occupation of
space
– Nifty since models overlapping use of space
(as leaves overlap)
– Implies competition for light
Competition for Space
• Does it exist?
• Is it simply a proxy for other resources?
• Is it a useful concept for understanding
plant communities, or should we be more
explicit about space as a proxy?
Chapter 2 Summary
• Foundation is unclear
– Chapter conclusion to show how concepts are
building toward explanation of community
• Not enough detail
– Arguments left unexplored, statements made without
explanation
– Provide synthesis of studies rather than list of
references
• Audience
– Buffet style with tidbits of info – like or dislike?