Lecture 6: Mutualism and large-scale influences on community structure

. For each of the mutualisms we discussed in class (pollination, coralsalgae, seed dispersal, plants-fungi), what is the benefit that each partner receives? 

Pollination:

• Plants get pollination.
  
  

• Pollinators get food (nectar/pollen).

Coral–algae:

• Corals get sugars/energy.
  
  

• Algae get shelter and nutrients.

Seed dispersal:

• Plants get their seeds spread.
  
  

• Animals get food (fruit).

Plants–fungi (mycorrhizae):

• Plants get better nutrient/water uptake.
  
  

• Fungi get sugars from the plant.

 How can mutualism expand the realized niche beyond the fundamental niche? 

It can provide resources that the species couldn’t otherwise access (e.g., plants getting nutrients from mycorrhizal fungi). It can offer protection from stress or predators, allowing the species to survive in harsher environments (e.g., corals with algae thriving in nutrient-poor waters). It can facilitate reproduction or dispersal, letting species occupy new areas (e.g., seed dispersal by animals).

What is the “The Paradox of the Plankton”?

 the paradox highlights that ecosystems are dynamic, and species can coexist even when theory predicts they shouldn’t.

How did Hutchinson suggest we can “solve” this paradox? In other words, how might environmental variability influence competitive relationships?

 Environmental variability continually shifts the competitive balance, allowing many plankton species to coexist instead of one outcompeting all the others.

Latitudinal biodiversity gradient – At which latitude does diversity peak for most species?

For most groups of organisms, diversity peaks at the equator (0° latitude) and decreases toward the poles.

What is a potential explanation for this pattern? 

Higher solar energy and productivity: More sunlight → more photosynthesis → more energy to support larger and more complex food webs.

Stable climate: Less seasonal variation allows species with narrow niches to persist.

Longer evolutionary time: Tropics have been climatically stable for millions of years, allowing more time for speciation.
Habitat heterogeneity: Complex environments (forests, mountains) create more niches.
Lower extinction rates: Fewer extreme events (like freezing temperatures) reduce species loss.

What’s the most common relationship between species richness and area?

Larger areas usually have more habitats and resources, support larger populations, and reduce extinction risk, all of which increase species richness.

What is the expected relationship between immigration and the number of species on an island? And why does this relationship occur?

The expected relationship is negative: As the number of species on an island increases, the immigration rate decreases.

What is the expected relationship between the extinction rate and the number of species on an island? And why does this relationship occur?

Extinction rates rise as species richness increases because of competition and smaller population sizes, which is another key part of the MacArthur–Wilson island biogeography model. 

Know the key predictions from Island Biogeography theory: Which island type should have the highest species richness? Which should have the lowest?

Highest species richness: Large, near islands (low extinction, high immigration).

Lowest species richness: Small, distant islands (high extinction, low immigration)