Species Interactions and Community Structure

1. Species Interactions and Community Structure • Community Webs • Complexity and Structure • Indirect Interactions • Keystone Species • Effects on Diversity • Mutualistic Keystones

2. Figure 17_02

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. Winemiller food web Lowland stream in Venezuela; S > 88

5. 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.

6. Strong Interactions and Food Web Structure • Tscharntke studied food webs associated with wetland reeds (Phragmites australis). • Attacked by fly Giraudiella inclusa. • & 14 species of parasitoid wasps. • Blue tit

7. Figure 17_05

8. Indirect Interactions • Indirect interactions are the effects of one species on another through a third species • Examples: • Trophic cascades • Indirect commensalism • Apparent competition

9. Indirect Commensalism • One species indirectly benefits another species (through a third species) while it is neither helped or harmed • E.g., Martinsen et al. (1998) • Beavers cut cottonwood trees - trees produce stump sprouts • Beetles prefer sprout leaves • Beetles grow larger, faster and use defensive compounds in leaves

10. Figure 17_06

11. Apparent Competition • Negative effects between two competitors who share a predator or herbivore • One species may facilitate presence or increased abundance of a predator which suppresses the second species • Orrock et al. (2008) • Exotic plant Brassica nigra shelters mammals which increases herbivory on native bunchgrass Nassella pulchra

12. Indirect Interactions

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

14. Food Web Structure and Species Diversity • Paine found: increased number of species in intertidal food webs = proportion of predators also increased. • His hypothesis = higher proportion of predators produces higher predation pressure on prey populations, promoting higher diversity.

15. Keystone Species

16. Paine experiment: Remove top predator Monitor for 2 years

17. Consumer Effect on Local Diversity • Lubchenko proposed to resolve the effect herbivores have on plant diversity • Herbivore food preference. • Competitive relationships between plant species in the local community. • Variance in feeding preferences and competitive relationships across environments.

18. Consumers’ Effects on Local Diversity • Lubchenko: influence of intertidal snail (Littorina littorea) on algal community. • In Lab: snails prefer green (Enteromorpha spp.) over red (Chondrus crispus) algae. • In field, Enteromorpha out-competes Chondrus in tide pools.

19. Consumers’ Effects on Local Diversity

20. Consumers’ Effects on Local Diversity • When snails are present in high densities, Littorina grazes down Enteromorpha, releasing Chondrus from competition. • Green crabs, Carcinus, eat young snails, = no juveniles in tide pools. • Carcinus are controlled by seagulls.

21. 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.

22. Fish as River Keystone Species • Mary Power: do California roach Hesperoleucas symmetricus and steelhead trout Oncorhhyncus mykiss influence food web structure?

23. Fish as River Keystone Species

24. Mary Power experiment • Cages with or w/out predator fish, allow insects in • Fish present = low algae • No fish = high algae • Why?

26. Mary Power predictions: • Predatory fish decrease algal densities. • Low predator density increases midge production. • Increased feeding pressure on algal populations. • Thus, fish act as Keystone Species.

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

28. 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 displaced native ant species that disperse large seeds. • = reductions in seedling recruitment by plants producing large seeds.

29. = Exotic species is mutualistic keystone species

30. Keystone Nutrient Recycler Idea that a species that cycles large amount of nutrients can affect community

32. Upstream Sura43 fish species found hereFrugivores, insectivores, piscivores, herbivores

33. Alfaro Astyanax Terrestrial insects Archocentrus Astatheros Aquatic insects Insects Priapichthyes

34. Astyanax = tetra that excretes 10X P of entire community= Keystone Nutrient Recycler

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