Population Dynamics Lecture 13

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Questions and Answers

What factor contributes to the faster growth of small populations compared to large populations?

Density-dependent factors (correct)

Random environmental changes

Limited resource availability

Density-independent factors

What is a primary reason small populations are more vulnerable to extinction?

They experience less competition for resources.

They are more affected by density-independent factors. (correct)

Their growth rates are consistently higher.

They have more available habitat space.

Which phenomenon is highlighted as contributing to the greater extinction rates seen on smaller islands?

Limited food resources

Increased genetic diversity

Higher breeding success

Greater vulnerability to chance events (correct)

In what way do small populations of plants in Germany demonstrate vulnerability to extinction?

Smaller populations are more likely to go extinct. Signup and view all the answers

How does density-dependent population growth differ between smaller and larger populations?

Smaller populations have the ability to recover quickly. Signup and view all the answers

What is a characteristic of deterministic models in population growth?

They predict outcomes based solely on fixed rates of birth and death. Signup and view all the answers

What does demographic stochasticity refer to?

Random variation in birth and death rates among individuals. Signup and view all the answers

In a stochastic model, what is the outcome of many consecutive years of below-average growth?

Accelerated rate of extinction Signup and view all the answers

How does population size affect the probability of extinction?

Larger populations have a lower probability of extinction. Signup and view all the answers

What is the primary difference between demographic and environmental stochasticity?

Demographic stochasticity arises from individual differences, while environmental is due to external conditions. Signup and view all the answers

What is the effect of patch size on extinction rates in metapopulations?

Larger patches typically support more individuals, leading to lower extinction rates. Signup and view all the answers

How does isolation affect patch occupancy in metapopulations?

Proximity to other patches increases the likelihood of being occupied. Signup and view all the answers

What is the 'rescue effect' in the context of metapopulations?

A mechanism where nearby dispersers help prevent extinction in less isolated patches. Signup and view all the answers

Which statement accurately reflects the occupation likelihood of skipper butterflies in relation to patch size and distance?

Largest and least isolated patches are the most likely to be occupied. Signup and view all the answers

What percentage of patches were monitored for colonization and extinction in the study of the Glanville fritillary butterfly?

12-39% Signup and view all the answers

What affects population dynamics according to their body size?

Larger organisms are less sensitive to environmental changes. Signup and view all the answers

Which factor can lead to a cyclic population size?

Density dependence with time delays Signup and view all the answers

What is a consequence of small population sizes in terms of extinction?

Chance events can lead to the extinction of small populations. Signup and view all the answers

What typically happens to populations during an overshoot?

There is a significant decline in population density. Signup and view all the answers

How can fluctuations in age structure indicate past population dynamics?

High/low numbers in age groups indicate past high/low birth or death rates. Signup and view all the answers

What allows small organisms to quickly respond to environmental conditions?

Rapid reproduction rates allow quick responses. Signup and view all the answers

What typically causes a die-off in a population?

Carrying capacity decreases over time. Signup and view all the answers

What is a metapopulation characterized by?

Independent population dynamics among subpopulations. Signup and view all the answers

What is the likely consequence of a population overshooting its carrying capacity?

Massive die-off due to resource depletion Signup and view all the answers

How does delayed density dependence influence population cycling?

It allows populations to overshoot and undershoot carrying capacity Signup and view all the answers

Which mathematical relationship indicates no oscillations in population size?

rτ < 0.37 Signup and view all the answers

What is a characteristic of populations with damped oscillations?

Decreasing magnitude of fluctuations over time Signup and view all the answers

Which factor can influence the time delay in density dependence?

Breeding seasons Signup and view all the answers

What is likely to happen to a population with a high intrinsic growth rate and a long time delay?

Greater susceptibility to cycling Signup and view all the answers

How does resource storage in Daphnia galeata affect their population dynamics?

It buffers against food depletion for future generations Signup and view all the answers

What defines a stable limit cycle in population cycling?

Population experiences large continuous oscillations Signup and view all the answers

Which of the following best describes the concept of carrying capacity?

The maximum population size that the environment can sustain Signup and view all the answers

What characterizes metapopulations?

They are made up of subpopulations with independent dynamics. Signup and view all the answers

What is a feature of the basic metapopulation model?

Suitable patches exist within unsuitably matrix habitats. Signup and view all the answers

How can metapopulations occur due to human activities?

Via habitat fragmentation into smaller patches. Signup and view all the answers

What do source subpopulations provide in the source-sink metapopulation model?

High-quality habitats for dispersal. Signup and view all the answers

Which statement describes how the landscape metapopulation model differs from the source-sink model?

It considers the variation in unsuitable habitat surrounding patches. Signup and view all the answers

What does an increase in colonization rates in a metapopulation indicate?

Improvement of habitat corridors. Signup and view all the answers

Which factor influences the proportion of occupied patches at equilibrium in metapopulations?

The fixed probability of extinction and colonization. Signup and view all the answers

In a metapopulation, what effect does decreasing extinction have?

It increases the number of occupied habitats. Signup and view all the answers

What can be inferred about the relationship between colonization and extinction rates in nature?

A balance between them is often observed. Signup and view all the answers

Study Notes

Lecture 13: Population Dynamics Over Time and Space

Population size fluctuates naturally over time.
Population dynamics describe variation in population size over time or space.
Population dynamics can be relatively stable or widely fluctuating.
Differences in body size, population response time, and sensitivity to environmental change affect population dynamics.
Small organisms (e.g., algae) reproduce quickly (hours), allowing quick responses to environmental changes.
Small organisms have a high surface-area-to-volume ratio, making them sensitive to environmental changes.
Larger, longer-lived species have longer generation times and include many age classes.
Fluctuations in birth rates have less impact on the growth of larger, longer-lived species.
Populations maintain homeostasis (less sensitive to environmental change).
Age structure can fluctuate.
If a specific age group has unusually high or low numbers, the population experienced unusually high or low birth or death rates in the past.
Example: whitefish (Coregonus clupeaformis).

Overshoots and Die-offs

Populations can temporarily exceed carrying capacity (overshoot).
Overshoots can occur when carrying capacity decreases.
A die-off is a substantial decline in population density.
Populations typically go well below carrying capacity after a die-off.
Example: Reindeer on St. Paul Island (25 individuals introduced in 1911, exponential growth, massive die-off in 1938, now maintained at ~400 individuals).

Density Dependence with Time Delays

Density dependence with time delays can cause population size to be inherently cyclic.
Some populations experience regular patterns of fluctuations (population cycles).
Example: Gyrfalcon exports match natural population cycles.
Populations fluctuate above and below carrying capacity.
At carrying capacity, populations are stable.
When population decreases (predation, disease), growth starts.

Delayed Density Dependence

Delayed density dependence occurs when density dependence occurs based on a past population density.
It can be caused by factors related to carrying capacity, such as long gestation periods or predator response times.
Example: Moose breed in fall but don't give birth in spring; available resources may be different by then.

Modeling Delayed Density Dependence

Delayed density dependence can be modeled with a modified logistic growth model equation.
Whether a population cycles depends on the time delay (τ) and the intrinsic growth rate (r).
As time delay increases, density dependence is further delayed, making the population more prone to overshooting/undershooting carrying capacity.
A high intrinsic growth rate increases the chance of overshoot.

Modeling Delayed Density Dependence: Cycling

The amount of cycling depends on the product of r and τ.
When rt is low (rt < 0.37), no oscillations occur. Example: a graph showing a gradually increasing growth.
When rt is intermediate (0.37 < rt < 1.57), damped oscillations occur. There is ongoing fluctuation. Growth then reduces, and starts to grow again.
When rt is high (rt > 1.57), stable limit cycle occurs. Large oscillations persist over time (a cyclical graph pattern).

Population Sizes Cycle in Laboratory Populations

Example: Daphnia galeata; Bosmina longirostris.
When population is low and food abundant, individuals store excess food droplets.
Although growing, populations deplete resources but still reproduce because of stored food droplets.
Causes the populations to overshoot, then die off.
Bosmina longirostris do not store many lipid droplets, cannot reproduce beyond carrying capacity.

Extinction in Small Populations

Small populations are more vulnerable to extinction than large ones.
Small populations are vulnerable to both density-independent factors (e.g., natural disasters) and density-dependent factors (Allee effects).
Relationship between population size and probability of extinction.
Example: Birds in the Channel Islands, extinction rates were greater on smaller islands.
Example: Plant populations in Germany, sampled over 10 years, smaller populations are likely to go extinct.

Extinction Due to Variation in Population Growth Rates

Density-dependent population models show that small populations initially grow more rapidly than large ones.
But, small populations are more likely to go extinct due to random environmental variation in birth/death rates.
Stochastic models incorporate random variations in population growth rates.
Stochasticity varies between demographic (differences among individuals) and environmental (changes in environmental conditions like weather).

Metapopulations

Population dynamics vary over space.
Populations are often divided into subpopulations.
Subpopulations are smaller groups of conspecifics in isolated patches.
Dispersal among subpopulations is infrequent; dynamics are independent.
A collection of subpopulations forms a metapopulation.

The Fragmented Nature of Habitats

Metapopulations can occur when habitat is naturally patchy.
Example: Wetlands in North America; terrestrial habitat often inhospitable, minimal dispersal between water bodies.
Metapopulations can also occur due to human activities.
Example: Habitat fragmentation; breaking large habitats into smaller ones.

Conceptual Models of Spatial Structure

Three models for spatial structure of subpopulations: Basic metapopulation model, Source-sink metapopulation model, Landscape metapopulation model.

The Basic Metapopulation Model

Describes a scenario with patches of suitable habitat embedded in unsuitable habitat.
All suitable patches are equally good.
Some patches are occupied, others not.
Unoccupied patches can be colonized by dispersers.
The model emphasizes how colonization and extinction events affect the proportion of suitable habitats.

The Source-Sink Metapopulation Model

Builds on the basic model, including variation in patch quality.
Source subpopulations are high-quality habitats with dispersers.
Sink subpopulations are low-quality habitats; outside dispersers maintain the subpopulation.

The Landscape Metapopulation Model

Builds on the source-sink model by including variation in unsuitable habitat around patches.

The Basic Dynamics of Metapopulations

Assume patches are equal quality. The same subpopulation size and the same number of dispersers.
Some proportion of the habitats will be occupied (p).
Fixed probability that a patch becomes unoccupied through extinction (e).
Fixed probability of colonization (c).

Observing Metapopulation Dynamics in Nature

Do we see a balance of colonization/extinction rates in nature?
Example: Glanville fritillary butterfly (Melitaea cinxia); lives in isolated patches of dry meadows.
Monitored extinction/colonization of 1600; 12-39% occupied over a 9-year period; ~100 colonized, ~100 extinct.

Patch Size and Isolation

Habitat patches rarely have equal quality.
Larger patches typically support more individuals.
Example: California spotted owl (Strix occidentalis occidentalis).
Smaller patches experience higher extinction rates.
Distance between patches affects the rate of patch colonization.
Less isolated patches could be supported by nearby dispersers = rescue effect.
Example: skipper butterflies (Hesperia comma).

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Description

This quiz focuses on the concepts of population dynamics over time and space, highlighting natural fluctuations in population size. It explores the factors influencing these dynamics, including body size, reproductive rates, and environmental sensitivity. Test your understanding of how different species respond to changes in their environment and maintain homeostasis.