A pesticide is a chemical
used to kill pests they include
insecticides: used to kill
insect pests that damage
a crop and Pesticides:
used to kill plants that
compete with a crop (e.g.
weed)
Insecticides
Plants provide food for a wide
range of animals and insects
that will eat crops if not
protected. These animals (pests)
damage plants- reducing their
ability to make food and
produce new tissues, resulting in
smaller crop yields.
Insecticides are useful: they are sprayed onto plants
and eaten by pests, killing/harming them. This helps to
increase crop and growth yield.
Problems:
Killing lots of different types of insect
in an area will reduce the amount of
food available for any animals that
specialise in eating insects (e.g.
insectivorous birds). This will affect
other organisms in the food web,
because they depend on each other/
If the other species that are killed are predators of the
pest then, once the insecticide has been washed away
by rain, it is possible for pest species to return and
increase in number even more rapidly, causing even
more damage to crop.
Pest species evolving resistance to chemicals: individuals
that survive use of insecticide more resistant than those
killed. The individuals that reproduce have offspring that
carry genes for resistance. Farmers responded by using
greater amounts of insecticide- increasing damaging effects
on environment.
Herbicides
Usually sprayed on field where they selectively kill weeds, leaving
crop plants unaffected. Clears any plants that might compete with
crop for nutrients, water and light. So crop can grow more rapidly
and produce greater yield.
Problems:
Many weeds important food plants for wide range of insect species. Removing all
weeds reduces food and shelter available for them. Some insects may be pests of
crop, however some insect species may be predators of insect pest species. Clearing
weeds may make it easier for insect pest species to increase in number more
quickly.
Some herbicides quickly break
down in soil to simple
substances that are no danger
to environment. Some include
highly dangerous substances
that can poison soil organisms
and anything that eats them.
Nuclear fall-out
Contains radioactive
particles that get into
environment from accidental
leakage of radioactive
materials from a nuclear
power station or processing
plant, or as result of an
explosion involving nuclear
material.
If particles thrown high into
atmosphere by explosion, they may
be transported by winds over long
distances before they fall to the
ground. Larger particles fall out more
quickly- nearer to explosion site than
smaller particles (may travel
hundreds of miles)
Problems:
Some radioactive particles stay radioactive for
long time and may cause damage if touch living
material (e.g. burning tissue, cause cancer)
Radioactive particles of elements that are
normally stored in tissue (e.g. iodine) can cause
great problems as can be passed along food
chain in animal tissue. Results in increasing
amounts of radioactivity in each trophic level of
food chain, leading to greater risks of damage in
animals higher up chain.
Discarded plastics
Many plastics non-biodegradable:
cannot be decayed by action of
decomposers. If left in environment,
remain unchanged for many years.
Problems:
Landfills: once covered with plastic refuse, land
cannot be used for growing crops or grass for herd
animals, due to risk of leakage of poisonous
chemicals. Only used for building decades after
burial, when ground settles/no decay gases
produced.
Plastics that get into water systems may end up at
sea, where it collects in huge areas: "garbage
patches", where the oceans circulate
Plastic bags may be swallowed by animals: turtles
that mistake them for prey, jellyfish.
Plastic nets/ropes entangle organisms: die of starvation
Plastic broken down to release
toxins that affect organisms in
area.
Plastic may break
down into smaller
particles "nurdles" that
are swallowed by
animals that mistake
them for food.
Sewage
Human waste, faeces
and urine. Contain high
concentrations of
nitrogen-containing
substances, and so are
good sources of
nutrients for plants and
microorganisms.
Problems:
When untreated sewage added to water,
nutrients dissolve. This leads to eutrophication,
increasing plant and algal growth, resulting in
increase in bacterial growth, a fall in oxygen
concentration in water and death of aquatic
organisms.
Human waste contains many bacteria: can cause
infection, leading to vomiting, diarrhoea, fever and
death.
Sewage may contain concentrations of substances
that can cause harm to organisms. Female sex
hormones used in contraceptive pills excreted in
urine: tiny amounts not broken down by sewage
treatment left in water and is returned to natural
water systems e.g. rivers. Male fishes may have
reduced sperm count and produce eggs instead of
sperm (feminization of males). Drinking water from
these rivers with sufficient concentrations of
hormones may affect men and reduce sperm count.
Liquid waste from
industries:
Problems:
Some substances, like sewage
can lead to eutrophication.
Metals copper, mercury and
lead and some organic
chemicals are toxic. These must
be cleaned completely from
waste water that drains into
water systems. Prevents toxins
from being absorbed by plants
and animals and passing into
food chains where they can
cause damage.
Fertilisers and
eutrophication
Fertilisers are chemicals that farmers
use on fields to add nutrients (nitrates) to
help crops grow better and produce
greater yields.
Problems:
If too much fertiliser added to field than crops
can absorb, remaining nutrients soak away in
ground water to nearby streams and rivers. Also, If
there is heavy rainfall soon after fertiliser
spread, nutrients will dissolve in rain water and
run off the surface of the field into streams and
rivers.
Eutrophication: adding nutrients to water.
Eutrophication increases rate of growth of photosynthesising
organisms (producers) e.g. algae on surface of water. Producers grow so
much that they block light to plants growing deeper in water, which die
as they cannot photosynthesise. This provides more food for
decomposers, which increase in numbers rapidly. Decomposers respire
more rapidly to make new materials for growth and reproduction.
Respiration reduces oxygen concentration in water. Other aquatic
animals find it difficult to get oxygen for respiration- many organisms
e.g. fish die. Decay of dead organisms provides more nutrients, so more
bacteria grow, respire and take more oxygen from water. Eventually,
most large aquatic plants and animals in water die.