#1 Population ecology

Genet

genetically distinct individuals 

• Every new genet is a new ramet, but not vice-versa

Ramet

physiologically independent but not genetically distinct individuals

Population

A group of individuals of the same species that interact in space and time

Community

A group of plant species that coexist in the same environment and interact with each other and their physical surroundings

Density-dependent factors

Factors where population density affects growth or other dynamics (e.g., competition).

Give an example of a density-dependent factor

Intraspecific competition — competition among individuals of the same species

How does increasing density affect density-dependent factors?

They become more powerful as density increases

Density-independent factors

Factors that impact population dynamics regardless of population density

• e.g., extreme weather events

Can fire be both density-independent and dependent? Explain

Yes — normally fire is independent, but if animals alter vegetation to influence fire frequency or intensity, it becomes density-dependent

What does λ (lambda) represent in population ecology?

The finite population growth rate

• showing how population size changes from one time step to the next

What does it mean if λ > 1.0? (greater than)

The population is growing

• more births/immigrants than deaths/emigrants

What does it mean if λ = 1.0?

The population remains stable in size

What does it mean if λ < 1.0?

The population is declining

• more deaths/emigrants than births/immigrants

What is the Population Growth Equation?

In the Population Growth Equation, what does N represent?

Population size at a given time

In the Population Growth Equation, what does ΔN/Δt or dN/dt represent?

Change in population size over time (growth rate)

In the Population Growth Equation, what does r_max represent?

The maximum per capita rate of increase

• intrinsic growth rate under ideal conditions

In the Population Growth Equation, what does K represent?

Carrying capacity — the maximum population size the environment can sustain

In the Population Growth Equation, what does (1 - N/K) represent?

The density-dependent term that slows growth as N approaches K

In the Population Growth Equation, what happens when N is small?

Growth is fast and nearly exponential

In the Population Growth Equation, what happens when N approaches K?

Growth slows and levels off

What does the Population Growth Equation look like when you are comparing two species?

In the Population Growth Equation→ comparing two species, what do r₁ and r₂ represent

The intrinsic (maximum) growth rates of species 1 and 2

In the Population Growth Equation→ comparing two species, what does α₁₂ represent?

The effect of species 2 on species 1 (in units of species 1)

In the Population Growth Equation→ comparing two species, what does α₂₁ represent?

The effect of species 1 on species 2 (in units of species 2)

In the Population Growth Equation→ comparing two species, what does

(K₁ − N₁ − α₁₂N₂)/K₁ show?

How intra- and interspecific competition reduce growth for species 1

• same for species 2 in the other equation

What was the purpose of the McGraw & Furedi (2005) – American Ginseng Study?

To determine whether deer browsing was causing American ginseng population decline in West Virginia

How many populations were monitored, and for how long in the McGraw & Furedi (2005) – American Ginseng Study?

Seven populations, from 2000–2004

What did researchers track for each plant in the McGraw & Furedi (2005) – American Ginseng Study?

• growth stage

• survival

• reproduction

• deer browsing evidence

What was the observed mean λ with deer browsing in the McGraw & Furedi (2005) – American Ginseng Study?

0.973 — indicating population decline.

What was the simulated mean λ with no browsing in the McGraw & Furedi (2005) – American Ginseng Study? (when they took out the browsed plants)

1.021 — indicating population growth

What does this graph represent from the McGraw & Furedi (2005) – American Ginseng Study?

population viability analysis (PVA)

What does a population viability analysis (PVA) estimate?

A population’s risk of extinction over time.

In the population viability analysis of the American Ginseng Study, what was the “viable” population threshold?

A 0.95 probability of survival over 100 years

In the American Ginseng Study, how did browsing affect viability?

Heavy browsing reduced viability and required larger populations to persist

• ~800 plants (to get to the viable population size)

In the American Ginseng Study, what happened when browsing was reduced by ≥ 75 %?

Populations remained viable at much smaller sizes

What does Population ecology focus on?

how individuals of the same species interact and how populations change

• done by modeling populations

Why do ecologists model populations?

To understand and predict population growth, decline, and structure

What are age-structured models used for?

Animals

• where chronological age determines survival and reproduction

How are populations divided in age-structured models?

Into age classes such as juveniles, adults, and seniors

What are stage-structured models used for?

Plants

• where size or developmental stage determines function and reproduction

Why are stage models more appropriate for plants?

size or developmental stage is a much better predictor of a plant's vital rates (survival, growth, and fecundity) than its chronological age

• Plants can grow, shrink, or stay the same size, enter dormancy, or revert to earlier stages

What do stage models use to describe transitions?

• Probabilities of growth

• survival

• reproduction between stages

Why do stage models matter?

They capture complex life cycles and predict population viability under factors like herbivory