#3 Interspecific interactions and trophic cascades

What are the first 2 key ideas behind the Gleasonian view of plant communities?

• Communities result from interactions between individual species and the environment.

• Species occurrence depends on each one’s environmental tolerance.

What are the last 2 key ideas behind the Gleasonian view of plant communities?

• Communities are not fixed entities but arbitrary human distinctions.

• Species presence is influenced by chance events if abiotic factors are within their tolerance ranges

What are the key ideas behind the Clementsian view of plant communities?

• Communities are highly organized and consist of interdependent species.

• A community functions like a “superorganism.”

• Species interactions are the main determinants of community structure

How do the Gleasonian and Clementsian views differ?

Gleasonian:

• Communities are loose assemblages of species shaped mainly by environment and chance.

Clementsian:

• Communities are tightly integrated units shaped by biotic interactions and succession

Allelopathy

A chemical interaction where one species releases biochemicals (allelochemicals) that inhibit the germination or growth of another.

• (+/-)

Interspecies competition

Competition between individuals of different species for shared, limited resources

Intraspecies competition

Competition among individuals of the same species

Symbiosis

Any close, long-term biological relationship between two or more species

Parasitism

A relationship where one species (the parasite) benefits, and the other (the host) is harmed

• (+/-)

Commensalism

A relationship where one species benefits and the other is neither helped nor harmed

• (+/0)

Predation

When one species (the predator) kills and consumes another (the prey)

• (+/-)

Guilds

A group of species that use the same resources in similar ways and live in the same geographic area, regardless of taxonomy

Assemblage

A group of species that are related and live in the same area

• community + taxa

Ensemble

A group of related species in the same area that also use the same resources

• community + taxa + guild

Niche partitioning

The physical or temporal division of resources that enables species to coexist without directly outcompeting each other

Why is niche partitioning important?

It reduces competition and promotes biodiversity by allowing species to occupy different niches

What happened when Argentine ants invaded the Fynbos in the 1980s?

• displaced native ants → disrupted native seed-dispersal mutualisms

• built shallower nests→ leaving seeds unprotected from fire

• Plant seeds were no longer buried deep enough to survive heat

What were the experimental results of the Argentine ants study?

The mutualism between ants and plants collapsed

• With Argentine ants: 0.67% seedling emergence (1 seedling)

• Without Argentine ants: 35% seedling emergence (53 seedlings)

What type of mutualisms were represented in the Argentine ants study?

• Trophic Mutualism

• Service Mutualism

What was the Trophic Mutualism seen in the Argentine ants study?

• Ants receive food (elaiosomes) from seeds.

• Plants benefit from seed burial in nutrient-rich refuse piles

What was the Service Mutualism seen in the Argentine ants study?

• Ants provide a service by dispersing and protecting seeds from fire

• Plants provide a food resource in return

What was the overall ecological effect of Argentine ant invasion?

Disruption of trophic and service mutualisms, leading to reduced seed survival and plant recruitment

Trophic cascades

A process where predators suppress or alter the behavior of their prey, which then indirectly affects the next trophic level

• (often plants or primary producers)

What type of effect does a trophic cascade describe?

An indirect effect

• changes in one species influence of another species through a chain of interactions

How can removing producers influence trophic dynamics?

It causes bottom-up effects that ripple upward through the food web

What does this trophic cascade describe?

indirect impacts on one species from eating something else

What does this trophic cascade describe?

The removal of producer influences trophic web from the bottom up

How do sea otters influence kelp forests?

• Sea otters eat sea urchins.

• Urchins eat kelp.

• When otters are present, urchins are controlled → kelp flourishes

What happens when orcas move into the area with sea otters?

• Orcas prey on otters → otter populations decline.

• Urchin populations increase → kelp abundance decreases

What is the otter→ urchin→ orca an example of?

A top-down trophic cascade

How do introduced foxes affect seabird populations in AK?

They reduce seabird presence through predation

How does reduced seabird activity affect the ecosystem with the introduction of the foxes?

• Decreases guano (nutrient) input

  • seabirds are the main nutrient source

• Leads to nutrient-poor soils and shifts in plant community structure

What kind of plant community shift occurs with the introduction of the foxes?

From tall, guano-fertilized grasses → to short grasses, forbs, lichens, and dwarf shrubs

What is the long-term effect of the bird and ecosystem shifts regarding the introduction of foxes?

It makes it harder for seabirds to return, perpetuating the altered community and reducing food availability for foxes

What ecological concept does the fox problem in AK illustrate?

A trophic cascade driven by predator introduction and nutrient cycle disruption