Chapter 20 Trophic Structure
Automenis Io, The Io Moth
Harned Hall 301 (615) 963 - 5782

Email me

• Ecosystems
  • Ecosystem measurements
• Trophic Structure
  • Trophic Pyramids
• Food Webs
  • Generalizations
  • Objections
• Guilds
• Keystone Species
• Terms

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
  • standing crop is not the same as productivity, the new biomass added to a system in a given time, and large standing crops can be associated with either high or low productivity
• 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)

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

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 (see Lecture 17 on Diversity and Stability)

• 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 the same resource and do so all in a similar fashion

One plant may be attacked by:

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

Keystone Species

Some species alter the environment in such a way, either by their mere presence or by their activity, that other species can find niches in the environment only if they are there

• these are Keystone species
  • when keystone species are removed from a system, many other species also leave the system
• Beavers are a classic keystone species that act as ecosystem engineers
  • Alter the environment by building their homes by making ponds
  • hundreds of species in the ponds rely on the presence of the beavers
• Keystones species may have their effect by
  • being prey to many predators
  • predators that allow many species of prey to coexist because they eat the dominant competitor and prevent it from reaching a population size that crowds out the other competitors
  • keystone species may be parasties, predators, or herbivores as well as predators

A keystone species is not the same as a dominant species

• dominant species have an important effect on other species simply because they are so common
  • a keystone species may be a dominant species but its effect is not due to its population size
  • beavers are never that common in a pond (usually a single family) but are still a keystone species

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, Keystone species, Dominant species, ecosystem engineer

Last updated on September 26, 2006