15 by linzhengnd


									15.8 FOOD WEBS
In Module 2.8 we summarised the way in which living organisms depend on one another for food
by writing simple food chains. Food chains follow a basic pattern like the one below.
                                                  A basic food chain
                 eaten by                           eaten by                      eaten by
      PLANT                    HERBIVORE                             CARNIVORE                 CARNIVORE
    (producer)             (primary consumer)                   (secondary consumer)       (tertiary consumer)
In a natural ecosystem, the criss-crossing food chains can be represented by a food web. A
simplified example is shown on the next page. Every food chain starts with a producer (a green
plant or an alga) and ends with a tertiary
consumer. Tertiary consumers (sometimes
called top carnivores) feed on secondary
consumers (carnivores) and may also eat
herbivores. Tertiary consumers include
predators that hunt and kill their prey (such as
sharks, crocodiles, eagles and the big cats),
and scavengers that eat animals, or the
remains of animals, that are already dead
(such as hyenas and vultures). The photo
shows hyenas and vultures feeding on an
antelope killed by a cheetah.
In a grassland ecosystem, the producers are
mainly grass and other flowering plants, and
                        the top predators are the big carnivores like tigers, wolves and birds-of-
                        prey. In a marine ecosystem, the main producer is the phytoplankton.
                        Water plants and algae contribute little because they grow only in shallow
                        water. The top predators are sharks, dolphins and killer whales.
                Krill   However, the biggest consumer is the blue whale which filters more than
                        three tonnes of krill through its mouth every day. Krill is a small (1 to 2
cm) crustacean like a shrimp that feeds on plankton.
In any ecosystem, the numbers of herbivores are limited by the numbers of producers available
for them to eat. In the same way, the numbers of carnivores are limited by the numbers of
herbivores. These balances tend to be self-regulating. If the number of carnivores increases, they
will eat more herbivores. The number of herbivores will decrease and eventually some of the
carnivores will starve. In time, a new balance will establish itself. If the number of carnivores goes
down, they will eat fewer herbivores. The numbers of herbivores will go up until there are not
enough producers for them to eat, then some of them will starve. In time, a new balance will
establish itself. These balances apply not only to the ecosystem community as a whole, but also
to the populations of each species within the community.
Natural food webs are complicated and we do not always understand them completely. If disease
                                               or human interference disturbs a food web, it may
                   Plague of mice in Australia be hard to predict what will happen. For example, if
                                               a modern grain crop replaces a natural grassland,
                                               the increase in the yield of the producers can lead
                                               to an increase in the population of the natural
                                               consumers. Especially the consumers that like
                                               eating grain, for example mice! And if the wrong
                                               chemicals are carelessly used to protect the new
                                               grain crop from insect pests, these chemicals can
                                               also kill the small birds that eat the poisoned
                                               insects, and the top predators that eat the
                                               poisoned birds. Without enough predators to keep
the mouse population in check, there may be a sudden plague of mice which destroys the crops
and spreads disease. Things like this have happened in many countries.
   1. What are (i) food webs, (ii) secondary consumers,             3. Many organisms are missing from the food web on
    (iii) tertiary consumers, (iv) predators, (v) scavengers?         the next page. Draw a new food web of your own that
                                                                      includes frogs and lizards.
   2. Research krill and write a few sentences about it.

                                                            15 - 8


                                                                                       LARGE MAMMALS

                                                                         CROCODILES                                  RATS
                                                   WATER BIRDS                                                                             DEAD ANIMALS
 PHYTOPLANKTON                                                                                                              INSECTS
                      LARGE FISH                                                          INSECTS                           carnivore
                                                 WATER PLANTS                             herbivores
                                                                         ANTELOPES                                                            SPIDERS
                          SMALL FISH                                                                   SMALL BIRDS            INSECTS
                           carnivores       MOLLUSCS                                                    carnivores            herbivores
                                                                 GRASS                   TREES
                   SMALL FISH             WORMS                             CATTLE                          WORMS                 FLOWERING PLANTS
 herbivores                                                      SMALL MAMMALS              LARGE BIRDS
                                   DEAD PLANTS                      carnivores
              WATER PLANTS                                                                                 FUNGI           DEAD PLANTS
ALGA                                                              SMALL BIRDS         SMALL BIRDS      MOLLUSCS
                                                                   herbivores          herbivores
                                        SMALL MAMMALS
                                           herbivores                                                                  KEY:

                                                                                      FLOWERING PLANTS                                TOP CARNIVORES

                                            Simplified food web for a lagoon and grassland ecosystem

                                                                      15 - 8

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