Chapter 16 Trophic Structure

Fall, 1998

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Ecosystems:

Include the abiotic factors affecting the community living in a definable environment

Notice that here community refers to all of the species in the environment, not just a subset of similar species (like the bird community or the grazing community)

In practice, boundaries of ecosystems are often hard to define

Some edges are difficult to detect

Some species move between habitats, and link ecosystems

Some ecosystem-level processes

Trophic structure

This is the hierarchy of levels that describes where species derive their nourishment

Note that some species may span trophic levels (especially when juveniles and adults feed on different things)

Energy transfer between species and trophic levels

Organisms all need a source of energy, and ecosystems all have patterns of energy flow between species and trophic levels

Material cycling (nutrient recycling) between species and trophic levels

Organisms all need material resources (usually inorganic compounds) and ecosystems have patterns of material flow between species and trophic levels

Often the flow is cyclic, so that materials spend lots of time in the system and do not simply flow in one side and out of the other

Ecosystem measurements

Biomass

Total standing crop of any species or trophic level

Notice that we can't count individuals, as different species have very differently sized individuals and so counts aren't comparable with one another

Energy flow

Describes how species and trophic levels are linked to one another

Does not always correlate with biomass, as small biomass can still produce large flows if individuals are productive

Nutrient flow

Some more important than others

If level of a nutrient controls the productivity of a trophic level, it is termed a Limiting Nutrient

Flow rates of limiting nutrients important to both biomass and energy flow

Turnover rate important

Turnover is the ratio of input/output to total amount present

High ratio means that nutrients do not reside in system or at the trophic level for long

Low ratio means that nutrients reside in system or at the trophic level for long periods

Trophic Structure:

Food Chain

Simply a diagram of who eats whom in an ecosystem

Trophic level

A step in the food chain

Producer - the level that uses energy from some source other than organic compounds

Producers are autotrophs

Consumer - any level that uses energy from organic compounds

Consumers are heterotrophs

Primary consumer - eats producers

Herbivores are primary consumers

Secondary consumer - eats primary consumers or other secondary consumers

Carnivores are secondary consumers

Omnivores take food from more than one trophic level

Tertiary consumers - top consumers -- fed on by decomposers and transformers only

Decomposers -- feed only on organic compounds in dead material

Transformers - feed on dead organic material and convert important nutrients between inorganic forms not useable by other organisms and forms that are useable

Notice that this level is not in the book

Trophic link - the relationship between a pair of species indicating that one eats the other (from the idea of the food chain)

Food Webs:

Most organisms do not feed on a single other species

Thus food chains are only chains when looking at aggregations of species or at trophic levels

When all of the links are put into a trophic diagram for all species in the ecosystem, the outcome is a food web, in which there are multiple links between species

Connectance

Connectance is the ratio of actual links to the total number of links possible for an ecosystem

Interest in this comes from the idea of stability

Some believe that interconnected systems are more stable

Others believe that as interconnectivity increases, instability increases

Connectance = number of links/total number of links possible

Total number of links possible = (n[n-1])/2 = with n

Low ratio means that species eat relatively few other species and the food web is simpler

High ratio means that species eat lots of other species and the food web is complex

Linkage density - average number of links per species

d = total number of links/number of species

a second way to look at food web complexity (only partially correlated with connectance)

as you add species, if d remains the same, then connectance will fall

if d does not change, actual links will change as a linear function of the increase in species number

total number of possible links increases faster than a linear increase as species number increases

this means that (if d does not change) the denominator is increasing faster than the numerator and the ratio will decrease

Generalizations about food webs:

• Omnivory is rare
• Cycles are rare
• Because this is so, we can still talk of food chain lengths in webs
  • Food Chain Length is the number of links (on average) between producer and tertiary consumer
• Food chains lengths tend to be short
• Food chain lengths do not correlate well with total productivity
• Food chains are shortened by:
  • Island size (smaller islands have shorter food chains)
  • Human disturbance (disturbed habitats have shorter chains)
  • Habitat complexity
  • Chains are shorter in two-dimensional habitats (grasslands, organisms attached to hard substrates) than three dimensions (forests)

There is no agreement about how connectance and linkage density change with an increase in the number of species

Objections to the generalizations:

• We almost always have incomplete information on food webs
• Often we aggregate (lump) lots of species that look similar without knowing if their diets are really similar
• The theory does not make any adjustment for the importance of a species in another species diet
• If most species feed on lots of others, connectance is high
• If most of the species in the diet are just occasional snacks, the food web is actually simpler than the food web indicates
• Limiting nutrients are not usually known
• Minor links can be critical if they supply limiting nutrients
• We don't have a way to account for species that cross ecosystem boundaries

Guilds:

Groups of species that feed on similar foods in a similar fashion

One plant may be attacked by:

• Sap-sucking guild
• Leaf-mining guild
• Stem-boring guild
• Leaf-chewing guild
• Grazing guild

Trophic Pyramids:

When one assigns species to a trophic level, one can:

Depict the levels as a hierarchy

Weight the size of a level by its:

• Biomass
• Productivity (energy uptake per unit time)

This produces a pyramid of numbers

Energy pyramid is always broadest at base

Outcome of second law of thermodynamics

Biomass pyramids usually wider at base, but can be inverted

When turnover is high, biomass can be small, but productivity is still high

Terms:

Ecosystems, Biomass, Standing crop, Energy flow, Nutrient flow, Turnover , Trophic Structure, Food Chain, Trophic level, Producer, autotrophs, Consumer, heterotrophs, Primary consumer, Herbivores, Secondary consumer, Carnivores, Omnivores, Tertiary consumers, Decomposers, Transformers, link, Webs, Connectance, Linkage density, Food Chain Length, Guilds, Sap-sucking guild, Leaf-mining guild, Stem-boring guild, Leaf-chewing guild, Grazing guild, Trophic Pyramids, inverted pyramid

Last updated on November 10, 1998