Food Webs
Chapter 17
11
Outline

Community Webs


Keystone Species



Complexity and Structure
Effects on Diversity
Exotic Predators
Mutualistic Keystones
22
Fig. 17.2
33
Food Web Complexity

Winemiller described feeding relations among
tropical freshwater fish.

Represented food webs in various ways:



Only included common species.
Top-predator sink.
Excluded weakest trophic links.
44
Fig. 17.3b
55
Strong Interactions and Food Web
Structure

Paine suggested feeding activities of a few
species may have a dominant influence on
community structure.

Suggested criterion for strong interaction is
degree of influence on community structure.
66
Strong Interactions and Food Web
Structure

Tscharntke studied food webs
associated with wetland reeds
(Phragmites australis) NE
Germany.

Attacked by fly Giraudiella inclusa.

Attacked by 14 species of
parasitoid wasps.


Predator specialization
Distinguished weak and strong
interactions.

Determination of keystone species.
77
88
Keystone Species

If keystone species reduce likelihood of
competitive exclusion, their activities would
increase the number of species that could
coexist in communities.
99
Food Web Structure and Species
Diversity

Paine found as number of species in intertidal
food webs increased, proportion of the web
represented by predators also increased.

According to his hypothesis, higher proportion of
predators produces higher predation pressure on
prey populations, in turn promoting higher
diversity.

Removal of starfish (top predator) caused decline in
diversity from 15 to 8 species.
10 10
Fig. 17.6
11 11
Keystone Species
12 12
Fig. 17.7
13 13
14 14
Consumers’ Effects on Local Diversity

Lubchenko proposed to resolve the effect
herbivores have on plant diversity, you need
to know:



Herbivore food preference.
Competitive relationships between plant species
in the local community.
Variance in feeding preferences and competitive
relationships across environments.
15 15
Consumers’ Effects on Local Diversity

Lubchenko studied influence of intertidal snail
(Littorina littorea) on structure of an algal
community.

Snails fed on green (Enteromorpha spp.) and red
(Chondrus crispus) algae.

Under normal conditions, Enteromorpha out-competes
Chondrus in tide pools, and Littornia prefers
Enteromorpha.

In the absence of snails, Chondrus is competitively
displaced.
16 16
Consumers’ Effects on Local Diversity
17 17
Consumers’ Effects on Local Diversity

When snails are present in high densities,
Littorina grazes down Enteromorpha,
releasing Chondrus from competition.


Green crabs (Carcinus maenus) prey on young
snails, preventing juveniles from colonizing tide
pools.
Populations of Carcinus are controlled by
seagulls.
18 18
Consumers’ Effects on Local Diversity



Low snail density - Enteromorpha dominates tide
pool.
Medium snail density - Competitive exclusion
eliminated, and algal diversity increased.
High snail density - Feeding requirements are
high enough that snails eat preferred algae and
less-preferred algae.

Algal diversity decreased.
19 19
Fig. 17.9
20 20
Fish as River Keystone Species

Power investigated whether California roach
Hsperoleucas symmetricus and steelhead
trout Oncorhhyncus mykiss significantly
influence food web structure.

Predatory fish decrease algal densities.

Low predator density increased midge production.

Increased feeding pressure on algal populations.
 Thus, fish act as Keystone Species.
21 21
Fig. 17.10a
22 22
Fig. 17.10b
23 23
Fish as River Keystone Species
24 24
Keystone Species: Summation

Power : Keystone species exert strong
effects on their community structure, despite
low biomass.
25 25
Exotic Predators

Exotic species have dramatic impacts on
communities because they were outside the
evolutionary experience of local prey
populations.

Nile Perch (Lates nilotica) exotic fish predator in
Lake Victoria.

Fish fauna dramatically reduced.
26 26
Exotic Predators
27 27
Exotic Predators
28 28
Exotic Predators

Kaufman pointed out changes in Lake
Victoria fish community coincide with other
ecosystem changes.


Dissolved oxygen concentrations significantly
decreased.
Cultural eutrophication.
29 29
Seed Dispersal Mutualists as Keystone
Species

Christian observed native ants disperse 30% of
shrubland seeds in fynbos of South Africa.

Seed-dispersing ants bury seeds in sites safe from
predators and fire.

Argentine ants have displaced many native ant species that
disperse large seeds.

Substantial reductions in seedling recruitment by plants
producing large seeds.
linepithema humile
30 30
31 31
Fig. 17.21
32 32
33 33
34 34