Ecology Chapter 17: Species Interactions

• Chapter 17 discusses species interactions and community structure in Ecology

Food Webs and Feeding Relations

• Feeding relationships are commonly documented interactions in communities, often depicted as food webs
• Early community studies focused on describing who eats whom
• Food webs summarize feeding relations within a community
• Studying simplified communities allows characterization of complex feeding relationships, even in impoverished faunas
• Detailed food webs highlight a community's great complexity, focusing on common species and strongest trophic links to improve comprehension

Strong vs Weak Interactions in Food Webs

• Paine proposed only a few species’ feeding activities significantly influence community structure through strong interactions
• Species with strong interactions greatly influence community structure

Phragmites Food Web Example

• Tscharntke's food webs of wetland reeds (Phragmites australis) show reeds are attacked by the fly Giraudiella inclusa
• Giraudiella inclusa is then attacked by 14 parasitoid wasp species
• Predators specialize in eating fly larvae and parasitoid wasps
• Strong or weak interactions can be distinguished to determine the species with the most influence

Interaction Types

• Direct interactions involve immediate effects between two species
• Indirect interactions involve effects mediated through a third species, like trophic cascades, indirect commensalism, and apparent competition

Indirect Commensalism

• Indirect commensalism occurs when one species benefits from another without affecting the second species directly

Beaver - Beetle - Cottonwood Example

• Beavers that fell cottonwood trees cause stump sprouts to appear
• Herbivorous beetles (Chrysomela confluens) prefer the high-nutrition leaves of these sprouts
• Beetles grow larger and faster due to these leaves, which also provide defensive compounds
• Beavers indirectly benefit beetles

Apparent Competition

• Apparent competition is when two species negatively affect each other by sharing a predator or herbivore
• One species facilitates a predator population that then affects the other species

Exotic Plant Example

• Orrock et al. studied apparent competition between exotic Brassica nigra and native Nassella pulchra
• Brassica shelters mammals, increasing herbivory on the surrounding Nassella

Keystone Species

• Keystone species greatly influence community structure
• Their activities increase the number of species that can coexist by reducing competitive exclusion
• Predators that can increase species diversity are often keystone species

Food Web Structure Relative to Species Diversity

• Paine discovered that as the number of species in intertidal food webs increases, so does the predator proportion
• Higher predator proportions lead to more predation pressure, promoting greater diversity

Experimental Evidence of Keystone Species

• Removing the top predator Pisaster from an intertidal food web significantly reduced the diversity after monitoring for 2 years
• The diversity of the control plot remained at 15
• Diversity of removal plots dropped to 8
• Mussels and barnacles dominated removal plots
• As such, Pisaster is classified as a keystone species

Keystone Species - Snail Effects

• Understanding the influence of intertidal snails on algal diversity requires knowledge of herbivore food preference, plant competition, variance in feeding preferences, and competitive relationships across environments

Keystone Snail study (Littorina littorea)

• Lubchenco studied Littorina littorea, which prefers ephemeral algae like Enteromorpha spp. over tough species like Chondrus
• High Enteromorpha densities corresponded to low snail densities, while high Chondrus densities correlated to high snail densities
• Littorina prevents Chondrus from being competitively displaced
• High densities of Littorina graze down Enteromorpha, releasing Chondrus from competition
• Green crabs (Carcinus maenus) control snail populations by preying on juveniles in tide pools
• Seagulls then control Carcinus populations

Density Variations of Snails

• Low snail density results in Enteromorpha dominating tide pools
• Medium snail density eliminates competitive exclusion, increasing algal diversity
• High snail density causes snails to eat both preferred and less-preferred algae, decreasing algal diversity

Keystone Species - River Fish Example

• Power investigated California roach and steelhead trout's influence on food web structure
• Found that predatory fish decrease algal densities
• Low predator density increased midge production, which in turn increased midge feeding pressure on algal populations, thus fish act as keystone species

Mutualistic Keystone

• Mutualists can act as keystone species, having a high impact on community structure despite low biomass
• Species with low biomass can have large effects on community structure
• Keystone species have disproportionate influence relative to their biomass

Cleaner Fish as Keystone Species example

• Many fish on coral reefs engage in mutualism by cleaning ectoparasites from other fish
• Cleaner wrasse remove and eat 1,200 parasites per day.
• When wrasses disappear, fish species diversity decreases by a median of 24%
• When wrasses appear, fish species diversity increases by a median of 24%
• Cleaner wrasse are considered a keystone species on Red Sea coral reefs

Mutualistic Seed dispersal

• Christian found that native ants disperse 30% of shrubland seeds in South African fynbos
• Seed-dispersing ants bury these seeds in sites safe from predators and fire
• Invasive Argentine ants have displaced native ant species that disperse large seeds, reducing seedling recruitment for plants producing large seeds

Chapter 17 explores species interactions and community structure, with a focus on food webs and feeding relationships. Early studies described who eats whom, while modern approaches simplify communities to characterize complex feeding relationships. Strong interactions by a few species significantly influence community structure.