A Natural Balance
Types of Interactions
• Competition happens when two or
more species depend on the same food
source or any limited resource.
• Animals that feed on other animals are
• Sharks in Sable Island’s offshore waters
are known to eat seals.
• The sharks are called predators and the
seals are prey.
• In symbiosis, at least one member
benefits from the partnership.
• A remora is a small fish that follows
sharks around and eats their scraps.
On Sable Island near Nova Scotia, gray seals and
harbor seals both feed on sand lances.
• A population is a group of
individuals of the same species
living in a given area.
• Populations change over time.
• The change in size of a population over time is called its
• Recently, European green
crabs have been found in the
• The arrival of the green crab is
cause for concern.
• Green crabs eat many types of
organisms including clams,
mussels, and the young of
other crab species.
How might European
green crabs be harmful
to U.S. ecosystems?
• A pollutant is
something that causes
harm to a living thing.
• Sulfur dioxide is a
chemical that is a good
example of a pollutant.
• Sulfur dioxide can make
breathing difficult even for
• It reacts with water in the
atmosphere to make acid
• Acid rain kills trees and
harm life in lakes, ponds,
• Three things often determine how harmful a
1. the pollutant’s ability to cause harm;
2. the amount of pollutant in air, water, or soil;
3. how long the pollutant stays in air, water, or soil.
Toxins in the food chain
• Human activities
create toxic pollutants
• Food chains
toxins into the tissues
Toxins in the food chain
• When carnivores eat many herbivores, they accumulate
toxins in their tissues.
• We protect the health
ecosystems by testing
• Common tests include:
– temperature, turbidity, dissolved oxygen, pH
• To learn about the water quality of a pond, first
make careful observations.
– What does the pond water look like or smell like?
– What animals and plants are living in the pond?
– Where is the pond located?
– Are there houses or farms nearby?
– Is the pond near a factory?
• The turbidity test measures
the cloudiness of water.
• If the water is too
cloudy, sunlight is
blocked, and pond plants do
not grow well.
The secchi disk is lowered into the water until the black and
white panels are no longer visible.
• The water temperature of a pond is
measured three or more inches below the
surface of the water.
• The higher the water temperature, the less
oxygen there may be in the water for living
• The pH scale ranges from 0
• Pure water is pH 7
• Most organisms in ponds
function best when the
water pH is near 7.
• Water quality is better
when dissolved oxygen
levels are high.
• Oxygen is needed by most
organisms living in the
• Nitrates and phosphates are
chemicals that can enter
ponds from farms, fertilized
lawns, or septic tanks.
• Excess nitrates or
phosphates endangers the
health of the pond
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