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.
Figure 53.15 Sea otters as keystone predators in the North Pacific
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.