Download Food Webs and Ecological Pyramids

Feeding Relationships in Ecosystems
All living things need a continuous supply of energy in order to
survive, and they need extra energy in order to grow and
reproduce.
They use whatever abilities they have to get the energy they need
to survive and thrive (thrive – grow and reproduce!)
Some organisms have the ability to produce their own food by
harnessing the energy from the sun. They are called producers
because they PRODUCE their own food rather than
CONSUMING it by eating something else (consumers).
As stated previously, energy from the sun flows into the biotic
components of an ecosystem (living things) through the process
of photosynthesis.
Producers are responsible for this unique and essential process,
where the energy from sunlight and abiotic components
(nutrients) are combined to produce sugar (oxygen is a byproduct
of this process).
The producers use the sugar to survive just as all other living
things do, but the extra sugar is stored as starch or is used to
grow or to reproduce. The excess sugar production, whether it is
used to grow or reproduce forms a net biomass (mass of living
tissue).
Ecologists study how different living things obtain their food and
the relationships that organisms have between each other as a
result of this primal need for energy.
The way in which an organism interacts with other species well as
with their environment is called a species’ niche in an ecosystem.
This is sort of the role or job that species plays in the very
complex set of interactions that exist in an ecosystem. We’ll start
by focussing on the interaction that organisms have with other
species by focussing on how they obtain their food.
Food Chains and Food Webs
Feeding relationships often form a succession of steps or levels
where one organism is consumed by another, and that organism
is consumed by another … and so on forming a long chain called
a food chain.
The feeding ALWAYS begins with the producers, organisms that
produce food by using the sun’s energy and the nutrients in the
soil and the air to produce their own food. They feed themselves,
and as such they are called autotrophs (‘self feeders’).
Other organisms that cannot produce their own food (but still
need nutrients) get their food by feeding on producers, called
herbivores. These organisms are called heterotrophs (‘other
feeders’).
Next in the chain are a group of heterotrophs that cannot easily
digest the bodies of the producers and therefore need to get their
energy by consuming the herbivores (or any other animal), called
carnivores.
There are also animals that
perform both options, where
they eat both producers and
other consumers (still
heterotrophs) called
omnivores.
Each feeding level in a food chain is called a trophic level. The
first trophic level is ALWAYS occupied by the producers. The
second trophic level can be occupied by either herbivores or
omnivores (as they consume the producers) and the later trophic
levels are occupied by the carnivores and the omnivores
(herbivores cannot occupy theses levels as they would be eating
other animals rather than plants). The diagram below illustrates
these levels:
Fifth Trophic Level
Fourth Trophic Level
Third Trophic Level
Second Trophic Level
First Trophic Level
We also label these organisms as primary, secondary, tertiary
consumers (suggesting a levels or ordering in the food chain) with
the producers always called the primary producers.
Feeding relationships in ecosystems are not always so simple. A
consumer may have multiple food sources and as such feed on
many different organisms. In addition, many consumers may feed
on one specific species.
This reality causes the many food chains to cross over and
intertwine, forming a food web. A food web is the sum total of all
the feeding relationships between all species in an ecosystem.
A salt marsh
food web
In a food web, the arrows point toward the organism that is
consuming or eating the other … arrows away means they are
eaten, arrows toward mean that organism does the eating.
eg – the arrow from the rat to the snake means that the snake
eats the rat (fairly simple!)
An aquatic
food web
A food web is very versatile and adaptive. If certain species
decline in number due to disease or over-consumption, the web
adjusts and flexes to accommodate the change.
For example … if the food web above if the crab population
declines due to overfishing by humans, everything it feeds on will
increase in number (no predator), everything it eats will decrease
in number (less available food).
The food web then adapts … the predators find an alternate food
source, the prey are eaten by other predators, and the food web
survives and thrives.
Some food webs have very important species called indicator
species. These species are essential to normally functioning food
webs and must be present. If they disappear or change in
number then the food web is definitely at risk of collapse.
In this aquatic food web … the indicator species is the Krill. The
food web will have great difficulty adjusting with the loss of this
species and can in fact collapse as a result of this loss.
Food Pyramids
In an ecosystem, feeding relationships exist so that organisms
can get energy to live, grow, and reproduce. Unfortunately, the
organisms do not keep all of the energy they obtain from the food
and add it to their bodies as biomass.
Most of the energy is lost as heat, to maintain life, and to collect
and hunt food. As a result approximately 10% of what an animal
consumes is actually added to its biomass for the next animal in
the food chain to obtain.
The diagram below shows the energy transfer in a simple food
chain.
As a result of this relationship, those at the top of a food chain
require a great deal or organisms to support it. We can first look
at this idea using a pyramid of numbers …
In this figure we see that due to the
energy loss at each level 1,500,000
producers are necessary to support the
200,000 herbivores that support the
90,000 primary carnivores that support a
single top carnivore (btw - this is for a
carnivore’s entire lifetime)
These pyramids aren’t always direct measures of the nature of
feeding relationships. If one oak tree is given the same value as
a small weed the numbers are an inaccurate way of analyzing
feeding relationships. This is due to the fact that an oak tree
could support a much larger number of organisms than a single
dandilion.
To see feeding relationships in an ecosystem more clearly, we
focus on the biomass (in kg) of the prey and how it supports a
given biomass of a predator in a food chain. This pyramid of
biomass shows a more accurate version of energy transfer:
This shows a comparison of the
benefits of a pyramid of numbers
and a pyramid of biomass when
observing the energy transfer in
feeding relationships
The most accurate vision of energy transfer (in J) is a pyramid of
energy. This pyramid shows directly the energy necessary to
support a top consumer:
These pyramids are evidence of a few important factors. An
ecosystem requires a great deal of numbers/biomass/energy to
support a single top consumer. This is simply why top consumers
are almost always small in number in comparison to primary
carnivores or herbivores in any ecosystem.
Most food chains cannot be sustained if they are to contain more
than three to four members.
THIS … is a compelling advertisement for a vegetarian diet for the
human population of the future … think about the pure energetics
of it!!!!
Homework
Pg 47 #1 – 8 + Food Web Assignment