Trophic Structure
• Many early researchers quickly realized
communities and systems were much to
complex to analyze easily
• Consequently many adopted a
trophodynamic approach
Trophic Structure
• Feeding relationships were identified
and communities were divided
accordingly
• Any problems with this approach?
Trophic Structure
• Problems associated with trophic
position (can be variable in both time &
space), omnivory, and their relative
position in the food web can change in
time & space (how?)
Trophic Structure
• However, energy does flow through a
system and certainly offers the
potential to classify organisms
• Producers: produce organic compounds
from inorganic compounds
• Consumers: primary consumers feed
directly upon producers or each other
while secondary consumers feed on
primary consumers…
Trophic Structure
• Can you think of an example of a
relatively simple food chain involving 3+
levels?
• We can add another important
component to our trophic… decomposers
•
Trophic Structure
Trophic Structure
• Why is it a pyramid?
• Energy is lost at each trophic level
through metabolic and respiratory
activity
• How big do you think a pyramid could
get?
• Why is it limited to that length?
Trophic Structure
• This representation is too simplistic
(why?) and many organisms don’t fit
neatly into a single level
Trophic Structure
• While exact relationships within a
trophic community may be difficult to
discern, it is frequently more
interesting to compare how different
communities are structured
• E.g. number of levels, relative
importance (top-down vs. bottom up)
Trophic Structure
Trophic Structure
• Such differences in trophic emphasis
reflect different methods of opertation
in the different communities and can be
used to understand their functional
roles
• If true, it may suggests the trophic
organization of a community shows
rather precise adaptation to that
community’s function
Trophic Structure
• This was demonstrated by Heatwole and
Levins (1972) reanalyzing the data of
Simberloff and Wilson (1969)
• S&W had ‘cleansed’ small mangrove
islands in the FL Keys and monitored the
subsequent colonization by arthropods
• What do you think they found?
Trophic Structure
• However, H&L re-analyzed the original
data in terms of trophic organization
• Counted number of species before and
after defaunation in each trophic
category (herbivores, scavengers,
detritus feeders, wood borers, ants,
predators and parasites)
Trophic Structure
Trophic Structure
• However, don’t get too excited…
• What might be a test of ‘random’
Trophic Structure:
energy flow
• Examining trophic relationships occurs
when they are ‘static’
• Many studies have come to the
conclusion that trophic systems are
inherently unstable
• However, given enough energy passes
through a system, almost all can become
stablized
Trophic Structure:
energy flow
• Consider 4 aspects of energy flow
–
–
–
–
1) amounts of energy
2) efficiency of energy transfer
3) rate (or speed) of flow
4) nature of the associated nutrient cycle
(e.g. open or closed; sedimentary or non-)
Trophic Structure:
energy flow
• 1) Amount of energy…
Trophic Structure:
energy flow
• The total amount of energy captured
within a system by the producers is
known as gross primary productivity
• (GPP-respiratory cost) = Net PP
Trophic Structure:
energy flow
• 2) Efficiency of energy transfer
• We already discussed losses
Energy consumed by trophic level n
Energy consumed by trophic level n-1
• In terms of ‘gross efficiency’, it is <1
and usually around 7-14% (≈ 10%)
• However, the generalization of energy
transfer can misleading… why?
Trophic Structure:
energy flow
• Again, it can be the differences (e.g.
between levels or among similar levels)
than can provide insight into different
mechanics of systems
Trophic Structure:
energy flow
• Table 1.2
Trophic Structure:
energy flow
• 3) the rate at which energy moves
through a system can also be very
enlightening (structure)
• It may stabilize unstable systems
• May support a much larger and complex
system than otherwise could persist
• Consistent vs. pulses
Trophic Structure:
energy flow
• 4) Some communities may be enhanced
(e.g. freshwater streams) or completely
dependent upon external inputs of
energy (e.g. detritus)
• The ra
Trophic Structure:
energy flow
• Variation in
GPP across
different
terrestrial
ecosystems
Trophic Structure:
energy flow
• Despite the insight trophodynamic
perspectives have provided for
community ecologists, they provide
little insight in how communities are
actually structured
Trophic Structure:
energy flow
• However, the idea of utilizing trophic
levels permeates several analytical
aspects of community ecology
• E.g. simple predator-prey dynamics
• E.g. in guild analysis, trophic guilds (or a
variation thereof) are used
• E.g. food webs are a more biologically
realistic approach to trophic dynamics