Population Dynamics: Dispersal, Metapopulations, and Patterns of Survival

Population Dynamics

Introduction to Population Dynamics

Population dynamics is the study of how and why populations change in size and structure over time. Populations are influenced by several key processes:

• Birth – Addition of new individuals through reproduction.

• Death – Loss of individuals from the population.

• Migration – Movement of individuals into (immigration) or out of (emigration) a population.

These processes collectively determine the dynamics of population size and composition.

Dispersal and Metapopulation

Dispersal

Dispersal refers to the movement of individuals from their place of birth to new locations, which can increase or decrease local population density.

• Immigration: Movement of individuals into a population.

• Emigration: Movement of individuals out of a population.

Dispersal can occur in various life stages and is crucial for colonization, gene flow, and population persistence.

Examples:

• Taraxacum officinale (Dandelion): Windblown seeds disperse long distances.

• Chthamalus sp. (Barnacle): Waterborne larvae disperse before settling.

• Araneus sp. (Garden Spider): Juvenile spiders disperse by ballooning on silk threads.

Case Study: Africanized Honeybees

Apis mellifera (honeybees) originated in Africa and Europe, evolving into many locally adapted subspecies. Africanized honeybees disperse much faster than European honeybees. Within 30 years, they spread throughout South America, Mexico, and Central America, demonstrating rapid population expansion through dispersal.

Other Factors Influencing Population Growth

• Climate changes: Can alter habitat suitability and resource availability.

• Changes in food supply: Affect birth and death rates, influencing population size.

Metapopulations

A metapopulation consists of a group of spatially separated subpopulations connected by the movement (dispersal) of individuals among them. This structure allows for local extinctions and recolonizations, contributing to the persistence of the species as a whole.

• Example: Alpine Butterfly (Roland et al.), Lesser Kestrels (Serrano and Tella).

Metapopulation dynamics are important for conservation biology and understanding species persistence in fragmented habitats.

Estimating Patterns of Survival

Life Tables

Life tables are demographic tools that summarize the survival and reproductive rates of individuals at each age or stage of life. They are essential for understanding population dynamics and predicting future changes.

• Cohort life table: Follows a group of individuals born at the same time (a cohort) from birth to death. Data collection is often difficult in natural populations.

• Static life table: Records the ages at death of individuals in a population at a single time, regardless of their birth cohort. Ages can be estimated using various techniques (e.g., growth rings in trees, carapaces of turtles, coral stems, horns in sheep).

• Age distribution: Calculates the proportion of individuals in each age class within a population. The difference in numbers between age classes is assumed to result from mortality.

Example: Dall Sheep Study

Researchers studied 608 skulls of Dall sheep to determine the age at which each sheep died. The major assumption is that the proportion of skulls in each age class represents the typical proportion of individuals dying at that age.

Age (years)	Number of survivors at beginning of year	Number of deaths during year
0–1	608	54
1–2	554	12
2–3	542	13
3–4	529	12
4–5	517	30
5–6	487	46
6–7	441	48
7–8	393	69
8–9	324	132
9–10	192	51
10–11	141	24
11–12	117	3

Additional info: Table values inferred from slide image; actual numbers may vary slightly.

Survivorship Curves

Definition and Purpose

Survivorship curves graphically represent the number of individuals surviving at each age for a given species or group. They help visualize patterns of survival and mortality within populations.

Types of Survivorship Curves

• Type I: High juvenile survival; most individuals live to old age, then mortality increases rapidly. Examples: Dall Sheep, Humans.

• Type II: Uniform survival; individuals have a constant probability of dying throughout life. Examples: Many birds (e.g., American Robins, white-crowned sparrow).

• Type III: High juvenile mortality; most individuals die young, but survivors live much longer. Examples: Desert plants, many marine invertebrates.

Graphical Representation

Survivorship curves are typically plotted with age on the x-axis and number of survivors (often on a logarithmic scale) on the y-axis. The three types can be summarized as follows:

Type	Pattern	Example Organisms
I	High survival in early/mid life, steep decline in old age	Humans, Dall Sheep
II	Constant mortality rate at all ages	Birds, some reptiles
III	High mortality in early life, survivors live long	Oysters, many plants

Applications

• Understanding survivorship curves helps ecologists predict population growth and decline.

• They are used in conservation biology to identify vulnerable life stages and inform management strategies.

Summary Table: Methods for Estimating Survival Patterns

Method	Description	Advantages	Limitations
Cohort Life Table	Follows a group born at the same time from birth to death	Direct, detailed data	Difficult to collect in wild populations
Static Life Table	Records ages at death of individuals at one time	Easier data collection	Assumes stable age distribution
Age Distribution	Compares proportions in each age class	Quick estimation	Assumes mortality is the only factor affecting age class size

Additional info: This summary table is inferred for clarity and completeness.