Species Interactions

1
Species Interactions Community Structure

• Chapter 17

2
Outline

• Community Webs
• Complexity and Structure
• Keystone Species
• Effects on Diversity
• Exotic Predators
• Mutualistic Keystones

3
Introduction

• A community can be defined as an association of
  interacting species.
• One type of interaction would be who eats whom.
• Food web map of the feeding interactions within
  a community.

4
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.

5
Strong Interactions and Food Web Structure

• Paine suggested feeding activities of a few
  species may have a dominant influence on
  community structure.
• Strong interactions
• Criterion for strong interaction is the degree of
  influence on community structure.

6
Strong Interactions and Food Web Structure

• Tscharntke studied food webs associated with
  wetland reeds (Phragmites australis).
• Attacked by fly Giraudiella inclusa.
• Attacked by 14 species of parasitoid wasps.
• Distinguished weak and strong interactions.

7
Keystone Species

• The feeding activities of a few keystone species
  may control the structure of communities.
• If keystone species reduce likelihood of
  competitive exclusion, their activities would
  increase the number of species that could coexist
  in communities.

8
Keystone Species

• Paine studied the rocky intertidal communities in
  Washington and Gulf of California.
• As the number of species increased, the
  proportion of the web represented by predators
  increased.

9
Food Web Structure and Species Diversity

• According to Paines 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
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.

11
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
  Littorina prefers Enteromorpha.
• In the absence of snails, Chondrus is
  competitively displaced.

12
Consumers Effects on Local Diversity
13
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.

14
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.

15
Fish as River Keystone Species

• Power investigated whether California roach
  Hesperoleucas symmetricus and steelhead trout
  Oncorhhyncus mykiss significantly influence food
  web structure.
• Caging studies to exclude large fishes.

16
Fish as River Keystone Species

• Predatory fish decreased algal densities.

17
Fish as River Keystone Species

• Enclosing fish led to increased midge production.
• Increased feeding pressure on algal populations.
• Decreased numbers of predatory insects and young
  roach sticklebacks.
• Thus, fish act as Keystone Species.

18
Keystone Species Summation

• Keystone species exert strong effects on their
  community structure, despite low biomass.
• Dominant species are those that have substantial
  influence on the community due to high biomass.

19
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.
• Formerly over 400 species, now 3 species dominate
  catches (only 1 native).

20
Exotic Predators
21
Exotic Predators
22
Exotic Predators

• Kaufman pointed out changes in Lake Victoria fish
  community coincide with other ecosystem changes.
• Dissolved oxygen concentrations significantly
  decreased.
• Nile perch may be impacting the community
  directly through predation and indirectly through
  its influences on the lake ecosystem.

23
Mutualistic Keystones

• Mutualists can act as keystone species.
• Low biomass
• High impact on community structure

24
Cleaner Fish as a Keystone Species

• Many fishes clean other fishes of ectoparasites.
• Labroides dimidiatus widely distributed cleaner
  wrasse.
• Fish without cleaner wrasses available have 4x
  more parasites.
• Acts as a keystone species.

25
Seed Dispersal Mutualists as Keystone Species

• Christian observed that native ants disperse 30
  of shrubland seeds in fynbos of South Africa.
• Seed-dispersing ants bury seeds in sites safe
  from predators and fire.

26
Seed Dispersal Mutualists as Keystone Species

• Argentine ants have displaced many native ant
  species that disperse large seeds.
• Substantial reductions in seedling recruitment by
  plants producing large seeds.