Ecological limits

Question 1

How does niche occupation limit adaptive radiations?

Answer 1

• As the number of occupied niches increased, the number of evolved morphotypes decreased.
• Lack of ecological opportunity (unoccupied niches) constrains divergence.
• Consistent with the theory of diversity dependence, where limited ecological opportunity reduces potential for diversification.

Question 2

How does clade growth in modern birds show evidence for ecological limits?

Answer 2

→ far fewer species exist than we would expect mathematically
→ suggesting there are ecological limits to diversity

Question 3

What is asymptotic diversity dependence?

Answer 3

• Where the rate of diversification slows down as species richness (diversity) approaches a maximum limit (number of niches)
• Diversification slows over time and eventually levels off, with no significant new species emerging as ecological niches are filled.

Example: In a saturated ecosystem, the introduction of new species becomes less likely as more niches are occupied, leading to a plateau in species richness.

Question 4

What is Darwinian diversity dependence?

Answer 4

• The concept that as species diversity increases, the rate of diversification slows due to natural selection and ecological constraints.
• Selection Pressure: Increased competition for limited niches and resources.
• Limited Niches: Fewer unfilled niches make it harder for new species to emerge.
• Slowdown in Diversification: As diversity grows, new species evolve more slowly due to stronger selection pressures and tighter ecological limits.

Question 5

What are the main assertions of the Ecological Limits Hypothesis (ELH)?

Answer 5

• Dynamic Equilibrium: Species richness at biogeographic scales exists in a state of dynamic equilibrium.
• Diversity Dependence: Speciation and extinction rates depend on biodiversity levels.
• Resource Constraints: Evolutionary rates are limited by total resource availability, constraining overall diversity.

Question 6

What is diversity dependence?

Answer 6

-Idea that the rate of evolutionary change (speciation, competition etc.) is influenced by the no. of sp already present in a community or clade
- Positive: low diversity = many niches = increase speciation
- Negative: higher diversity = increase competition = niche saturation = decrease speciation rate

Question 7

What does the equilibrial carrying capacity model predict?

Answer 7

That species diversity approaches an upper limit due to ecological constraints.

• Finite Carrying Capacity: Ecosystems can only support a limited number of species.
• Slowdown in Diversification: As species richness increases, competition, resource limits, and niche saturation reduce the rate of new species formation.
• Equilibrium: Over time, speciation and extinction rates balance, leading to a relatively stable number of species.

Question 8

What are waiting times?

Answer 8

• The time between successive speciation events is modeled as the waiting time.
• Model assumes that diversification is a continuous process, and the number of lineages increases exponentially as long as speciation events continue at the same rate

Question 9

How does diversity dependence manifest in phylogenies?

Answer 9

Species richness shows three distinct phases:
- Exponential Rise: Rapid diversification initially.
- Deceleration: Slower diversification as niches fill.
- Saturation: Species richness plateaus at equilibrium.
Example: North American wood warblers follow this pattern

Question 10

What do Anolis lizards reveal about diversity dependence?

Answer 10

• Observation: Decline in diversification rates correlates with island size (ecological opportunity).
• Larger islands support more species but show slower declines in speciation rates.
• Supports the idea of ecological limits on species richness.

Question 11

What are the arguments for ecological limits in macroevolution?

Answer 11

For Ecological Limits:
- Rabosky (2013): Studied ray-finned fishes, showing strong influence of ecological limits on macroevolutionary patterns.
- Rabosky & Hurlbert (2015): Focused on global patterns of avian diversity, concluding species richness is constrained by ecological factors.

Question 12

Describe the Diversity-Dependence Model (DDM)

Answer 12

• Rate of evolutionary change (such as speciation) is expected to slow as diversity increases
• Because the community becomes more saturated with species, leading to stronger competition.

Question 13

How is Darwinian diversity dependence different from asymptotic diversity dependence?

Answer 13

Darwinian Diversity Dependence: Slowdown in diversification due to increased competition and natural selection pressures as more niches are filled.

Asymptotic Diversity Dependence: Diversification slows and eventually levels off as species richness approaches a maximum limit (e.g niche, resources) , regardless of evolutionary constraints.

Question 14

Asymptotic diversity dependence can correspond to several limiting factors:

Answer 14

• Number of ecological niches.
• Resource availability (food, space, etc.).
• Habitat space.
• Predator-prey interactions.
• Environmental stability.
• Genetic constraints.
• Competition.
• Climate/environmental changes.

Question 15

What are the arguments against ecological limits in macroevolution?

Answer 15

Diversity is Dynamic & Unbounded
- Wiens (2011): Studied amphibians, found no strict cap on diversity.
- Harmon & Harrison (2015): Examined lizards and mammals, argued diversity is not ecologically limited.

Key Reasons:
- Niche Expansion & Innovation: Evolution creates new niches
- Changing habitats allow species persistence.
- Competition can drive speciation instead of limiting diversity.
- Speciation Outpaces Extinction: Continents allow dispersal and diversification.
- Scale-Dependent Limits: Local constraints may exist, but at larger scales, diversity continues to grow.

Question 16

Describe how competition can limit speciation

Answer 16

• Competitive Exclusion: The weaker species may go extinct before divergence occurs.
• Resource Limitation: Scarce resources prevent populations from growing and specializing.
• Stabilizing Selection: Strong competition reinforces existing traits, preventing divergence.

E.g Invasive species outcompeting natives, reducing diversification opportunities.

Question 17

Describe how competition can drive speciation

Answer 17

• ECD: Species evolve differences to reduce niche overlap
• Adaptive Radiation: High competition in new environments leads to rapid diversification
• Disruptive Selection: Extreme traits are favored, splitting populations into distinct groups.

Example: Beak size divergence in Darwin’s finches to specialize on different seed types.