Chapter 17
Species Interactions and Community
Structure
Objectives:
1. Determine the difference between a food
chain and a food web and describe each
2. Describe the attributes of a Keystone
Species and what it does for the community
3. Describe what changes occur in a
community when an exotic invader alters
the food web
Who eats Whom?
Food chains:
Show simple the feeding relationships in a
community. Chain infers one connected to
one other link (food item).
Food Chains - Antarctic
Killer whales
Ross seals
Crabeater seals
Krill
Food Chains - Antarctic
Killer whales
Ross seals
Blue Whale
Crabeater seals
Krill
Flying birds
Fig 17.2
Fig 17.3
Food Web Facts
1. Can be very complex and hard to interpret
2. Elimination of “weak” interactions creates
easier picture to follow
3. Identify several trophic levels
4. Identify top predators, intermediate
predators, down to base of food web
Food Web Facts
5. Do not indicate the strength of a food
relationship
6. Identify the direction of energy flowing
through the system
7. Does not identify the amount (strength) of
energy flowing between trophic levels
Food Webs
Strong/Weak interactions:
Not necessarily the amount of energy flowing
between the two, but the degree of influence
on community structure.
Fig. 17.5
Food Webs
Fig. 17.5
Food Web Structure
Degree of influence varies for each
association in the food web
Those species that are most influential may be
called:
What do we know so far?
1.
2.
3.
4.
Food webs are complex with many species
Some interactions are weak
Some interactions are strong
Keystone species may be the most
influential in the community--- What does
this mean?
Keystone Species
Keystone
Fig. 17.14
Keystone Species
Paine’s keystone species hypothesis:
1. Predators may keep prey below K
2. Potential for competitive exclusion low
3. Number of species in a community would
increase
OR------------
Keystone Species
Paine’s keystone species hypothesis:
Some predators may increase species
diversity!
Fig. 17.6
Fig. 17.6
Keystone Species
Paine’s experiment, page 328 book:
1. Removed the Keystone Predator (starfish)
2. Followed community for two years
3. 3 months - barnacle Balanus occupied
from 60-80 % of space
4. One year - Balanus crowded out by
mussels and gooseneck barnacles
Keystone Species
Paine’s experiment, page 328 book:
5. Benthic algae declined - lack of attachment
space (taken up by mussels and gooseneck
barncles)
6. Herbivorous chitons and limpets left - no
food
7. Sponges crowded out
Keystone Species
Paine’s experiment, page 328 book:
8. After 5 years - community had only two
species: mussel and goose-neck barnacle
Fig. 14.6
Exotic species
Exotic predators can collapse and simplify the
structure of the food web.
• Fewer #’s species
• Simplified food web, loss of species
• Loss of one or more trophic layers
• Change in ecosystem dynamics
Exotic Species Invasion
Lake Victoria: a case study, pages 335-337
• 26,000 square miles
• Tropical environment
• 400 species of fish, 300 endemic
• Last three decades, half species extinct
• Major changes in water quality AND
• Introduced Fish The Nile Perch
African Cichlids
Nile Perch
Nile Perch
• 1950’s introduced
• Low level until ~ 1980
• Populations exploded, from 20% to 80% of
biomass
• Loss of 200 species of fish
• Complete change in water quality (due to
fish change and/or other environmental
factors)
Fig. 17.15
Question for the Day????
Can the introduction of a species that will
reduce the number of species and the
number of individuals in the community a
good thing? Defend your answer, cite an
example if possible.
Done for Today
1. Food Chains vs. Food Webs
2. Food webs can be very complex
3. Keystone species and importance to food
web
4. Exotic species can collapse a food web
5. Communities infer a complex interaction
of species, to think otherwise would be folly