BIOB50H3 Chapter Notes - Chapter L#21: Sea Lion, 18 Months, Invertebrate
LECTURE #21, pages: 471-488
CHAPTER 21: ENERGY FLOW AND FOOD WEBS
- the influence an organism has on the movement of energy and nutrients through an
ecosystem is determined by the type of food it consumes, as well as by what consumes
it
FEEDING RELATIONSHIPS (21.1)
Organisms can be grouped into trophic levels
-trophic level: feeding category is based on the # of feeding steps by which it is
separated by autotrophs
first trophic level generates the most dead organic matter in an ecosystem
In desert ecosystem: first trophic level = all the plants
In Lindeman’s lake ecosystems = primarly dead organic matter + autotrophs
o2nd trophc level = herbivores that consume autotroph biomass
o3rd = carnivores that consume the animals at the trophic level below them
- in trophic studies, heterotrophs that feed at multiple levels are called omnivores
All organisms are either consumed or end up as detritus
- all org. are either consumed by other org.
at higher trophic levels, or enter the pool of
dead organic matter (detritus) most of
the energy in terrestrial ecosystems flows
through as detritus
dead plant/microbial/animal patter are
consumed by detritivores in the
process of decomposition
- detritus is part of the first trophic level, and
detritivores are part of the 2nd
- detritus in terrestrial ecosystems comes
mainly from plants; in aquatic ecosystems, is mainly comes from terrestrial organic
matter (which is external to the aquatic ecosystem)
external energy inputs = allochthonous inputs (eg. Plant
leaves/stems/woods/dissolved organic matter)
internal energy inputs = autochthonous energy
ENERGY FLOW BETWEEN TROPHIC LEVELS (21.2)
- available energy decreases w/ each trophic levels as we move upward
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Energy flow between trophic levels can be depicted
using energy or biomass pyramids
-trophic pyramid: when assemble form lower to
higher trophic levels
see the rectangles decrease in size as we go
up the trophic levels
in terrestrial ecosystems, energy and biomass
pyramids are usually similar b/c biomass is
good proxy for energy
in aquatic, the high consumption rates and
relatively short lifespands onf the primary
producers (mainly phytoplankton) results in a
biomass pyramid that is inverted relative to
the energy of the pyramid means that the
biomass of heterotrophs is greater than
primary producers in aquatic systems, but the
autotrophs produce more energy
oinverted pyramids are greatest where
productivity is lowest (eg. Nutrient
poor regions of the open ocean)
phytoplankton in nutrient-poor
regions provide a greater energy
supply p/ unit time
Energy flow between trophic levels differs among ecosystem types
- very productive lakes can appear green, same as terrestrial ecosystem means:
bprimary productivity far exceeds rates of herbivory
herbivores on land consumer a much lower proportion of autotroph biomass than
do herbivores on my aquatic ecosystems
there is a positive relationship between NPP and the amount of biomass consumed
by herbivores
- Why is the proportion of autotroph biomass consumed in terrestrial ecosystems low,
especially when there is clearly an abundant food supply?
Hypothesis #1: Argue that population growth of herbivores is more constrained by
predation in aquatic ecosystems than in terrestrial ecosystems b/c the better
developed higher trophic-levels in aquatic ecosystems
Hypothesis #2: defense against herbivory, lower the amount of autotroph biomass
consumed i.e plants in tough env. Like deserts are more equipped for defense
against herbivory
oThis might explain why prop. Of plant biomass consumed is lower in
resource-poor terrestrial env
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Hypothesis #3: chemical composition of phytoplankton makes them more nutritious
for herbivores, than terrestrial plants are terrestrial plants have non nutritious
parts like the stem and woods
oFreshwater phytoplankton have ratios of carbon:phosphorus closer to
herbivores, than they do to terrestrial plants
All these factors lead to greater consumption of autotroph biomass in aquatic
ecosystems
The efficiency of energy transfer varies among consumers
-Trophic efficiency: the amount of
energy at the trophic level divided by
the amount of energy at the trophic
level immediately below it
-Consumption efficiency: the
proportion of the available biomass
that is ingested, is the consumption
efficiency
Is typically higher in aquatic
ecosystems than in terrestrial
- Once biomass is ingested by tehc
onsumer, it msut be assimilated by the
digestive system assimilation
efficiency: the proportion of the food
that is assimilated (determined by the
quality of te food and the physiology of the consumer)
Food quality is generally lower for herbivores, than it is for carnivores
How thoroughly food is digested depends on the consumer’s thermal physiology
i.e endotherms consume food more completely than ectotherms and thus have
higher assimilation efficiencies
-Production efficiency: the proportion of the assimilated food that is used to produce
new consumer biomass
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find more resources at oneclass.com
Document Summary
Chapter 21: energy flow and food webs the influence an organism has on the movement of energy and nutrients through an ecosystem is determined by the type of food it consumes, as well as by what consumes it. Organisms can be grouped into trophic levels trophic level: feeding category is based on the # of feeding steps by which it is separated by autotrophs. First trophic level generates the most dead organic matter in an ecosystem. In desert ecosystem: first trophic level = all the plants. External energy inputs = allochthonous inputs (eg. plant leaves/stems/woods/dissolved organic matter) Energy flow between trophic levels (21. 2) available energy decreases w/ each trophic levels as we move upward. Energy flow between trophic levels can be depicted using energy or biomass pyramids trophic pyramid: when assemble form lower to higher trophic levels. See the rectangles decrease in size as we go up the trophic levels.
