Download Food Chain Length

Ecology (BIO C322)
Ecological Pyramids
Pyramid of Numbers – upright or inverted??
Pyramid of biomass - upright or inverted??
Food Chain
• Transfer of food energy from source to
consumers.
• Two types: grazing food chain & detritus
food chain.
• Food web: Interconnected food chains.
Practice Concept
• The proportion of net production energy that flows
down the two pathways (i.e. grazing & detritus food
chains) varies in different kinds of ecosystems.
• In heavily grazed pastures or grasslands, 50% or
more of net production may pass down the grazing
pathway.
• In contrast, marshes, oceans & forests operate as
detrital systems, in that 90% or more of the
autotrophic production is not consumed by
heterotrophs until the leaves, stems & other plant
parts die, & are decomposed.
Detritus Food Chains
• Detritus  Detritivores  Predators
• In some ecosystems, detritivory >>
herbivorous grazing.
• Such food chains may involve 90% or
more of energy flow.
Hubbard Brook Experimental Forest
(Units in kCal/m2/year)
Respiration
5720 (55%)
Detritus F.C.
3446 (74%)
PG
10,400
PN
4680 (45%)
Grazing F.C.
35 (1%)
Storage
1199 (25%)
• Review Article:
“The Long and Short of
Food Chain Length”
• By David M. Post,
Trends in Ecology &
Evolution,
Vol. 17, No. 6,
June 2002.
What is Food Chain Length (FCL)?
• FCL = Number of transfers of energy or
nutrients from the base to the top of food web.
• FCL influences community structure.
• Maximum trophic position = FCL + 1
• In nature, linear food chains are rare.
FCL = ?
FCL = 4
12
10
11
7
8
5
1
Energy Web:
Max FCL = ?
Min FCL = ?
2
9
Max FCL = 4
Min FCL = 2
6
3
4
What determines FCL in natural
ecological systems?
• History of Community Organization
• Resource Availability
• Ecosystem Size
• Predator-prey Interactions
• Disturbance
History of Community Organization
• Colonization history influences food-web
structure and FCL most strongly in ecologically
isolated or evolutionarily young systems.
• History limits which type of species can join
the local community & food web.
• Constraints on local community membership
might modify predator-prey interactions.
• If a top or key intermediate predator is unable
to invade a system or evolve in situ, FCL
might remain short.
Resource Availability
Energy/resource
availability might
limit FCL when
colonization
does not.
Energetic or Productivity Hypothesis
•There is a diminishing amount of energy available to
support each subsequent trophic level.
•FCL should increase as the energetic efficiency of
organisms within a food web increases.
•FCL should increase as the amount of resources
available at the base of the food web increases.
•FCL should increase as primary production or detrital
input increases.
Productive Space Hypothesis: A variant
of Energetic Hypothesis
• Energetic hypothesis considered only per-unitsize resource availability.
• Productive Space hypothesis includes a spatial
component also – ecosystem size.
• FCL should increase as a function of total
ecosystem productivity = product of ecosystem
size (area or volume) and some measure of
per-unit-size productivity or resource
availability (e.g. gram carbon per m2 per year).
• Larger ecosystems support food webs with
longer FCL.
• The observed relationship b/w FCL and
ecosystem size could derive from the
positive relationship b/w ecosystem size &
species richness.
Resource Availability – The Other Side
• Longest food chains are often found at intermediate, rather than at high resource availability.
• At high resource availability, top predators can
reach higher densities and can exterminate
intermediate predators, thereby shortening FCL.
• An efficient higher predator, with limited food supply,
might eliminate his prey and be forced either to drop
to a lower trophic level or to become extinct.
• Studies suggest a threshold (10 g C/m2/year) below
which resource availability constrains FCL, but
above which FCL is determined by other factors.
• Conclusion: Resources limit FCL only in small
systems with very low total resource availability.
Predator-Prey Interactions
• Where resource availability
and colonization are not
limiting, the size structure
of predator-prey
interactions influence FCL.
• When predators are similar in size to their
prey, FCL is around 3 trophic levels.
• There is little evolutionary advantage to feeding
on other carnivores when herbivores are
generally more abundant, offer similar energetic
and nutritional value & are less well-defended.
• But most food webs are dominated by sizestructured predator-prey interactions where body
size increases with trophic position.
• In these food webs, some other factor, such as
disturbance, might determine FCL.
Dynamical Constraints & Disturbance
(Dynamical Stability Hypothesis)
• Developed by Pimm and Lawton (1978).
• They found that longer food chains had
longer return times therefore less stable
than shorter food chains.
• Food chains with short FCL should
dominate natural systems, particularly
those subject to frequent disturbance.
Models to understand Food Web Structure
• Rank of Omnivory – Number of predator-prey
interactions in excess of those in the model
with no omnivory.
• Food webs with large number of omnivores
(high ranking omnivory) are rare in the real
world.
• It is less probable to find species that feed
simultaneously both high & low in a food web.
Human Omnivore
Any Species left !?
Trophic
Level
4
3
2
1
4 Trophic
Levels;
Rank = 0;
No Omnivory
4 Trophic
Levels;
Rank = 1;
Rare species
3 Trophic
Levels;
Rank = 2
2 Trophic
Levels;
Rank = 0;
No Omnivory
Human Influences
on FCL
• Humans overexploit top predators;
• Change ecosystem size through habitat
fragmentation;
• Change resource availability by translocating
large quantities of nutrients, water and CO2.
• Move species around the landscape.
In passing…
• Ecological Footprint (E.F.): Area of
productive ecosystems (crop/forest-lands,
water bodies) outside a city required to
support life in the city.
• e.g. U.S.A.  E.F. is
5.1 hectares/person;
• India  0.4 ha/person.
• E. P. Odum: If highly developed countries
would reduce their resource and energy
consumption, then international conflicts &
terrorist threats would be reduced.