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Species Interactions and Community Structure. Community Webs Complexity and Structure Indirect Interactions Keystone Species Effects on Diversity Mutualistic Keystones. Figure 17_02. Community Webs. Winemiller described feeding relations among tropical freshwater fish.
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Species Interactions and Community Structure • Community Webs • Complexity and Structure • Indirect Interactions • Keystone Species • Effects on Diversity • Mutualistic Keystones
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.
Winemiller food web Lowland stream in Venezuela; S > 88
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.
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
Indirect Interactions • Indirect interactions are the effects of one species on another through a third species • Examples: • Trophic cascades • Indirect commensalism • Apparent competition
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
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
Keystone Species • If keystone species reduce likelihood of competitive exclusion, their activities increase the number of species that coexist in communities.
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.
Paine experiment: Remove top predator Monitor for 2 years
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.
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.
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.
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.
Fish as River Keystone Species • Mary Power: do California roach Hesperoleucas symmetricus and steelhead trout Oncorhhyncus mykiss influence food web structure?
Mary Power experiment • Cages with or w/out predator fish, allow insects in • Fish present = low algae • No fish = high algae • Why?
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.
Mutualistic Keystones • Power :Keystone species exert strong effects on community structure, despite low biomass.
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.
Keystone Nutrient Recycler Idea that a species that cycles large amount of nutrients can affect community
Upstream Sura43 fish species found hereFrugivores, insectivores, piscivores, herbivores
Alfaro Astyanax Terrestrial insects Archocentrus Astatheros Aquatic insects Insects Priapichthyes
Astyanax = tetra that excretes 10X P of entire community= Keystone Nutrient Recycler
Review • Community Webs • Complexity and Structure • Indirect Interactions • Keystone Species • Effects on Diversity • Mutualistic Keystones