Food Webs Chapter 17
Community Webs • Summerhayes and Elton studied feeding relations on Bear Island in High Arctic. • Primary producers were terrestrial plants and aquatic algae. • Fed on by several kinds of terrestrial and aquatic invertebrates. • Consumed by birds. • Attacked by arctic foxes.
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.
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. • Attacked by 14 species of parasitoid wasps. • Predator specialization • Distinguished weak and strong interactions. • Determination of keystone species.
Keystone Species • Keystone species – species that, despite low biomass, exert strong effects on the structure of the community they inhabit • http://www.prairiedogs.org/keystone.html • If keystone species reduce likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities.
Food Web Structure and Species Diversity • Paine found as number of species in intertidal food webs increased, proportion of the foodweb 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.
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.
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.
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 • 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.
Effects of Predation by Birds on Herbivory • Atlegrim studied influence of birds on herbivorous insects and insect-induced plant damage. • Insectivorous birds may act as keystone species via effects on herbivorous insects. • Larval insect densities peak when many insectivorous birds are feeding their young.
Effects of Predation by Birds on Herbivory • Birds reduce densities of insect larvae feeding on dominant understory shrub. • Higher densities of insect larvae translated directly into higher levels of damage to shrubs. • Marquis and Whelan found higher densities of herbivorous insects on trees from which predatory insects were excluded.
Keystone Species: Summation • Power:Keystone species exert strong effects on their community structure, despite low biomass.
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.
Exotic Predators • Kaufman pointed out changes in Lake Victoria fish community coincide with other ecosystem changes. • Dissolved oxygen concentrations significantly decreased. • Cultural eutrophication.
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.