Unit B: Sustainable Ecosystems
4.4: Pests and Poisons
pg. 132
Key Concepts:
4. Pesticides are used to reduce yield losses from pests, but there are
ecological costs associated with them.
- In a natural ecosystem there is a large biodiversity.
- In an agroecosystem (artificial) the biodiversity is very limited.
- Using monocultures for the purpose of large scale food production has had
a dramatic impact ecologically.
- Has altered the water and biochemical cycles, altered food webs, and led to
habitat loss.
Table 1: Comparison of the Food Web in a Natural Grassland with That of a
Potato field
Natural Grassland
Potato Field
Producers
Herbivores
Carnivores
Food Web
Pests
- Monocultures are made up of single crop species, and other organisms are
eliminated.
- A pests are organisms (plant or animal) that may damage a monoculture
crop.
- The term pest is only used when humans are describing a negative impact
caused by an organism (mice, insects, or plant)
- With the control of a pest, the monoculture can grow with out natural
consumers and competitors.
- A monoculture can also become an excellent environment for some pests
to thrive and eventually harm the crop.
Pesticides
Pesticide: a substance used to kill a pest
- Pesticides are a chemical means to control pests, and are commonly used.
- Herbicides are used to control plants, insecticides control insects,
rodenticides control rodents, and fungicides deal with fungi.
Characteristics of Pesticides
Broad-spectrum pesticide: a pesticide that is effective against many types of
pest.
Narrow-spectrum pesticide: a pesticide that is effective against only a few
types of pest.
- Pests vary on how long they remain active in the environment.
- Long-lived pesticides can last for many years, while shot-lived pesticides
may last for only a few days.
-Natural pesticides are short lived. They are easy to break down.
-Synthetic pesticides are persistent, difficult to decompose.
- Broad-spectrum pesticides are toxic to a large number of species, while
Narrow-spectrum pesticides are toxic to only a few numbers of species.
How do Pesticides Work?
- Pesticides can cause physical or biological damage to organisms.
- Some pesticides can be delivered by contact, while others are indirectly
applied, such as spraying where the pest can pick it up over time.
Table 2: Characteristics of some Pesticides
Pesticide
Origin/Source
Synthetic
DDT
Rotenone
Natural toxin
extracted from
plant roots
Synthetic
Glyphosate
Bt
A protein
obtained from the
bacteria Bacillus
thuringiensis
Use
BroadSpectrum
insecticide
Insecticide
and pesticide
Broadspectrum
herbicide
Narrowspectrum
insecticide
Important
Characteristics
Evidence of Learning: Students can …
- identify pests and their effects on ecosystems.
- explain how human activity can alter the balance of an ecosystem.
- identify different kinds of pesticides and their effect on an environment.
Check Your Learning
Questions 1 – 8, page 134
Summary:
- Agroecosystems are often based on a single plant species and the
elimination of natural food webs.
- Synthetic pesticides tend to persist longer than natural pesticides.
- Monocultures create ideal conditions for certain pests, which are often
controlled using pesticides.
- Pesticides may have a physical effect or disrupt biological processes.
-Pesticides are used to control weeds, insects, rodents, fungi, and types of
pest organisms.