Food Webs Chapter 17

Outline Community Webs Keystone Species Exotic Predators Complexity and Structure Keystone Species Effects on Diversity Exotic Predators Mutualistic Keystones

Fig. 17.2

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.

Fig. 17.3b

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) NE Germany. 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 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 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.

Fig. 17.6

Keystone Species

Fig. 17.7

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.

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

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.

Fig. 17.9

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.

Fig. 17.10a

Fig. 17.10b

Fish as River Keystone Species

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

Exotic Predators

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. linepithema humile

Winged adult and worker of Solenopsis geminata fire ants.

Fig. 17.21