#1 Interspecific interactions and trophic cascades pt 2

Competition

occurs when two organisms use the same limited resource

• (e.g., light, water, nutrients, space, pollinators)

Why is competition considered beneficial for plant populations?

Competition selects for individuals with traits that improve resource acquisition, thereby increasing overall population fitness and adaptive capacity

Competitive Exclusion Principle (Gause’s Law)

Two species with identical resource needs cannot coexist indefinitely

• one will eventually outcompete and exclude the other

Why is the Competitive Exclusion Principle often not reflective of natural ecosystems?

Because natural environments are heterogeneous and dynamic, species can shift their resource use or habitat (niche partitioning), reducing direct competition and avoiding exclusion

Niche partitioning

the physical or temporal division of resources among species

How does niche partitioning promote species coexistence?

By reducing niche overlap, interspecific competition decreases, allowing coexistence

Why is intraspecific competition often a stronger force than interspecific competition?

Individuals of the same species require identical resources

• leading to higher overlap and stronger competitive effects

What is an example that explains how species interactions can be both competitive and facilitative?

Alders compete with spruce for light and soil resources (–/–) but also facilitate spruce growth by fixing nitrogen and enriching soil (+/?)

• Net outcomes depend on environmental context

What did Risch & Boucher (1976) find when surveying ecology textbooks?

found textbooks emphasized competition and predation as the dominant forces structuring communities, while underrepresenting mutualisms

Why might competition and predation be overemphasized in ecology instruction?

• They may be easier to observe experimentally

• They are perceived as more common or important

• They influence human culture and economics

  • (e.g., resource competition)

• Ecology as a field has historically focused on them

Trophic cascade

occurs when predators suppress prey abundance or behavior, indirectly affecting the next trophic level down (top-down control

Summarize the Knight et al. (2005) trophic cascade involving fish, dragonflies, pollinators, and flowers

• Fish eat dragonfly larvae → fewer dragonflies

• Fewer dragonflies → more pollinators

• More pollinators → increased plant pollination and flower production

demonstrates cross-ecosystem interactions

What methods were used in Knight et al. (2005) to study this cascade?

• Ponds selected with and without fish

• Sampling dragonfly larvae + adults

• Counting pollinator visits

• Predation experiments and behavioral observations

• Pollen supplementation to test pollen limitation

How can shifts in dragonfly abundance influence plant community composition?

• Dragonfly decline → more pollinators → insect-pollinated species favored

• Dragonfly increase → fewer pollinators → insect-pollinated species suppressed, wind-pollinated species may dominate

Who first described trophic cascades, and what was the observation?

• Aldo Leopold

• observed that removing wolves increased deer abundance, which overgrazed vegetation

  • an early top-down cascade

What ecological process does the logistic growth model represent?

Intraspecific competition

• population growth slows as size approaches carrying capacity (K)

What major contribution did Vito Volterra make to ecology?

• Co-development of the Lotka-Volterra models

• including competition equations describing interactions between species

What does a competition coefficient (α) represent in Lotka-Volterra models?

The per capita effect of one species on the population growth of another

• expressed as a reduction in effective carrying capacity

Under what conditions do Lotka-Volterra models predict coexistence?

When interspecific competition is weaker than intraspecific competition (α₁₂ < 1 and α₂₁ < 1)

What does it mean if α₁₂ > 1?

A single individual of species 2 has a greater competitive impact on species 1 than an individual of species 1 has on itself

Green World Hypothesis (GWH)

Herbivores consume little plant biomass because predators, parasites, and diseases keep herbivore populations in check

• (top-down forces)

What evidence supports GWH?

• Herbivore populations collapse without predators

• Decomposers rapidly consume organic matter → food limits decomposers (bottom-up control)

• Plants compete for space more than herbivores suppress them

What are some criticisms of the Green World Hypothesis?

• Plants have defenses

  • (toxins, growth changes, signaling compounds)

• Herbivores are also limited by non-antagonistic factors

  • (e.g., habitat structure)

• Food webs are complex; simple top-down control rarely explains everything