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

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

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

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 n Energy 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