1. 1 Species Interactions and Community Structure Chapter 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 2 Outline Community Webs  Complexity and Structure Indirect Interactions Keystone Species  Effects on Diversity Mutualistic Keystones

3. 3 Community Webs 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.

4. 4 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.

5. 5 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.  Predator specialization  Distinguished weak and strong interactions.  Determination of keystone species.

6. 6 Indirect Interactions Indirect interactions are the effects of one species on another through a third species  Examples:  Trophic cascades (Chapter 18)  Indirect commensalism  Apparent competition

7. 7 Indirect Commensalism One species indirectly benefits another species (through a third species) while it is neither helped or harmed Example: Martinsen, Driebe, Whitham (1998)  Beavers fell cottonwood trees which then produce stump sprouts  Beetles prefer consuming high nutrition sprout leaves  Beetles grow larger, faster and utilize defensive compounds found in leaves

8. 8 Apparent Competition Negative effects between two competitors who share a predator or herbivore  One species may facilitate the presence or increase the abundance of the predator which suppresses the second species Orrock, Witter, Reichman (2008)  Exotic plant Brassica nigra sheltered mammals which increased herbivory on native bunchgrass Nassella pulchra

9. 9 Indirect Interactions

10. 10 Keystone Species If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.

11. 11 Keystone Species

12. 12 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.

13. 13 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.

14. 14 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.

15. 15 Consumers’ Effects on Local Diversity

16. 16 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.

17. 17 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.

18. 18 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.

19. 19 Fish as River Keystone Species

20. 20 Mutualistic Keystones Power : Keystone species exert strong effects on their community structure, despite low biomass.

21. 21 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.

22. 22 Review Community Webs  Complexity and Structure Indirect Interactions Keystone Species  Effects on Diversity Mutualistic Keystones