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• Feeding relationships are easily documented interactions within communities
• Early community studies focused on describing which organisms consume each other
• These feeding relationships are called food webs

Community Webs

• Initial food web research concentrated on simplified communities
• Feeding relations are complex, even in "impoverished faunas"
• Simplified webs are more manageable for study compared to those of diverse communities

Food Web Complexity

• Winemiller outlined feeding relationships of tropical freshwater fish across two sites
• Food webs were represented in various ways, including webs with only common species, and webs that excluded the weakest trophic links
• Complex food webs were produced through both
• The simplest food webs focused on the strongest trophic links

Strong Interactions and Food Web Structure

• Paine suggested that the feeding habits of certain species can greatly affect community structure; these are the strong interactions
• The degree of influence on community structure acts as a strong interaction criteria
• The strength of the interactions affects the depiction of a particular food web

Phragmites Food Web

• Tscharntke researched food webs linked to wetland reeds (Phragmites australis)
• The reeds are targeted by the fly, Giraudiella inclusa
• 14 parasitoid wasp species attack the fly
• A predator specializes in consuming fly larvae, and thus parasitoid wasps
• Weak and strong interactions were distinguished within the food web
• It allows for the determination of species that have the greatest influences

Indirect Interactions

• Direct interactions (predation or competition) between two species do not involve intermediary species
• Indirect interactions are the effects of species on one another via a third species
• Trophic cascades, apparent competition and indirect commensalism are all indirect interactions

Indirect Commensalism

• One species indirectly benefits another (through a third species), while they themselves are neither helped nor harmed
• Beavers fell cottonwood trees, which leads to stump sprouts
• Herbivorous beetles (Chrysomela confluens) prefer consuming high nutrition sprout leaves
• Beetles grow bigger and faster; beetles also utilize defensive compounds found in leaves
• Beavers have a positive effect on beetles through cottonwood trees

Apparent Competition

• Apparent competition happens when two competitors share a predator or herbivore
• One species may promote predator populations for a second species
• Orrock et al. studied apparent competition between Brassica nigra (exotic) and Nassella pulchra (native)
• Brassica shelters mammals that feed on surrounding vegetation
• It results in increased herbivory on Nassella

Keystone Species

• The feeding habits of a few keystone species greatly affect the structure of a community
• Keystone species can lower the likelihood of competitive exclusion; these activities raise the number of species able coexist in communities
• Certain predators can increase species diversity

Food Web Structure and Species Diversity

• Paine found that the proportion of the food web that predators represented increased with the number of species in intertidal food webs
• The higher proportion of predators results in more predation pressure on prey populations, which causes higher diversity

Sea Star Removals

• Paine got rid of Pisaster, a top predator, in an intertidal food web and monitored changes for over two years
• The control plot maintained a diversity of 15
• Diversity of removal plot dropped to 8
• After Pisaster was removed, gooseneck barnacles and mussels began to dominate the plot
• Pisaster is a keystone species and the community collapsed without it

Snail Effects on Algal Diversity

• Understanding the impact of intertidal snails on the diversity of intertidal algae requires knowing the herbivore food preference, competitive relationships between plant species in the local community, and variance in feeding preferences/competitive relationships across environments

Littorina littorea

• Lubchenco researched Littorina littorea
• Snails preferred small, ephemeral, tender algae (Enteromorpha spp.), rather than tough perennial species (like Chondrus)
• Low snail density tidepools had high Enteromorpha densities, while pools with higher snail densities had high Chondrus densities
• Chondrus is competitively displaced without Littorina

Snails and Crabs

• Littorina grazes down Enteromorpha when snails present in high densities, which frees Chondrus from competition
• Green crabs (Carcinus maenus) predate young snails, which stops juveniles from colonizing tide pools
• Seagulls control the populations of Carcinus

Variations in Snail Density

• Low snail density leads to Enteromorpha dominating tide pools
• Medium snail density eliminates competitive exclusion and increases algal diversity
• High snail density creates high enough feeding requirements that snails begin to eat all preferred and less-preferred algae
• Algal diversity decreases in this situation

River Keystone Species

• Power researched whether California roach and steelhead trout greatly impacted food web structure
• Predatory fish have been shown to decrease algal densities
• Low predator density increased midge production
• Increased midge feeding pressure on algal populations
• Fish thus act as keystone species

Mutualistic Keystones

• Mutualists can act as keystone species
• Despite low biomass, keystone species have a high impact on community structure

Cleaner Fish

• Numerous fish clean ectoparasites off of other fish on coral reefs (mutualism)
• Cleaner wrasse can eat and remove about 1,200 parasites per day
• Fish species diversity had a 24% median reduction when the wrasses disappeared (or were removed)
• Fish species diversity had a 24% median increase when the wrasses reappear (or are added)
• The cleaner wrasse acts as a keystone species on coral reefs in the Red Sea

Seed Dispersal

• Christian found that 30% of shrubland seeds in South Africa fynbos are dispersed by native ants
• Seed-dispersing ants bury seeds in locations that are safe from predators and fires
• Argentine ants have supplanted multiple native ant species known to disperse large seeds
• Plants that produce large seeds have experienced substantial reductions in seedling recruitment