Botkin Environmental Science Earth as Living Planet 8th txtbk

5.2 Ecological Communities and Food Chains 87
Food Web
Food Chain
Top predators
Large sharks
Large sharks
Quaternary
consumer
Smaller sharks
Marlin
Smaller sharks
Tertiary
consumer
Tuna
Lancet fish
Predators
Filterers
Squid
Mackerel
Ocean sunfish
Secondary
consumer
Zooplankton
Lantern fish
Amphipods
Ocean sunfish
Shrimp
Phytoplankton
Copepods
Pteropods
Copepods
Primary
consumer
Dinoflagellates
Producer
Dinoflagellates
Diatoms
Energy
from Sun
Energy
from Sun
FIGURE 5.6 An oceanic food web. (Source: NOAA)
is that many creatures feed on several trophic levels. For
example, consider the food web of the harp seal (Figure
5.7). This is a species of special interest because large
numbers of the pups are harvested each year in Canada
for their fur, giving rise to widespread controversy over
the humane treatment of animals even though the species
is not endangered (there are more than 5 million
harp seals.) 10 This controversy is one reason that the
harp seal has been well studied, so we can show its complex
food web.
The harp seal is shown at the fifth level. 11 It feeds on
flatfish (fourth level), which feed on sand launces (third level),
which feed on euphausiids (second level), which feed on
phytoplankton (first level). But the harp seal actually feeds
at several trophic levels, from the second through the fourth.
Thus, it feeds on predators of some of its prey and therefore
competes with some of its own prey. 12 A species that feeds
on several trophic levels is typically classified as belonging to
the trophic level above the highest level from which it feeds.
Thus, we place the harp seal on the fifth trophic level.