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

• In terms of ‘gross efficiency’, it is <1 and usually around 7-14% (≈ 10%)

• However, the generalization of energy transfer can misleading… why?

Energy consumed by trophic level nEnergy consumed by trophic level n-1

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