Population Distribution and Survey Methods Quiz

Reminders

• No seminars next week (Remembrance Day)
• Poster presentation — week of November 25th

Lecture 11: Population Distributions (BIOL 203)

• November 8, 2024

Learning Objectives

• Explain why population distribution is limited to suitable habitats
• List five important characteristics of population distributions
• Describe methods to estimate population distribution properties
• Explain the relationship between population abundance/density, geographic range, and adult body size
• Explain why dispersal is essential for colonization of new areas

Key Concept

• Population distribution is limited to ecologically suitable habitats

The distribution of populations is limited to ecologically suitable habitats

• A habitat is a place or physical setting where an organism lives
• Determined by the organism's niche (range of abiotic and biotic conditions it can tolerate)
• Abiotic factors include temperature, humidity, and pH
• Biotic factors include predators, parasites, and competitors

Two types of niche:

• Fundamental niche — range of abiotic conditions a species can live in
• Realized niche — includes biotic factors which may limit a species' distribution

Determining suitable habitats

• Patterns of distribution suggest only certain habitats are suitable
• Example: Monkeyflowers (Scarlet, Lewis')

Ecological niche modelling

• Generally, the more suitable the habitat, the larger a population can grow within that habitat.
• Allows prediction of species distributions (useful for invasive species, climate change).
• Use historical data/data from other populations to determine suitable habitat conditions

Modelling the spread of invasive species

• Example: Chinese bushclover (Lespedeza cuneata)
• Introduced in 1800s and displaced native plants
• Data from Asia used to predict potential distribution in North America

Difficulties with ecological niche modelling

• Assuming historical conditions accurately predict suitable habitat in new environments
• Example: Raccoon (Procyon lotor) — vastly different realized niches in different locations

Habitat suitability and climate change

• Current suitable habitats can predict species responses to climate change
• Example: Fish in the North Sea - climate change increased water temperature, leading to increased species richness

Concept check

• Define fundamental and realized niche
• Explain possible reasons for the difference between fundamental and realized niches

Population distributions have five important characteristics

• Geographic range
• Abundance
• Density
• Dispersion
• Dispersal

Geographic range

• Total area covered by a population
• Not every location in geographic range may be occupied
• Factors impacting range include: climate, topography, soil, vegetation, other factors
• Includes areas occupied through the organism's entire life history (e.g., migratory species)
• Endemic species — restricted range increase sensitivity to natural disasters

Abundance

• Total number of individuals within a defined area
• Example: Number of rainbow trout in a lake

Density

• Number of individuals in a unit of area or volume
• Ecologists use this to examine habitat capacity to sustain individuals.
• Too high a population density results in lower growth rates/survival

Dispersion

• Spacing of individuals within a geographic range
• Three patterns: clustered, evenly-spaced, random

Dispersion (Clustered)

• Individuals aggregate in discrete groups
• May be due to social groups (flocks of birds) or resource clustering (bugs under logs)
• Offspring often cluster around parents

Dispersion (Evenly spaced)

• Individuals maintain uniform distance from neighbors
• Territories common cause for even spacing (e.g., pika)
• Competition common cause for even spacing (e.g., plants)

Dispersion (Random)

• Individuals' position is independent of other individuals
• Not very common in nature

Dispersal

• Movement of individuals from one area to another
• Distinct from migration
• Mechanism for colonizing new suitable habitats and avoiding high competition/predation risk

Concept check

• Why aren't all locations in a species' geographic range occupied?
• What is the difference between dispersal and dispersion?

Estimating population size

• Direct method — census
• Indirect method — surveys (e.g., counting subset of population)
• Methods: area-based surveys, line-transect surveys, mark-recapture surveys

Area- and volume-based surveys

• Scientists define boundaries and count all individuals within
• Size depends on organism density
• Example: cm³ of soil for bacteria, m² for reef corals

Line-transect surveys

• Scientists count individuals as they move along a line
• Many variations (length, number of transects)

Mark-recapture surveys

• Collect, mark, and return individuals to a population
• Second collection happens later
• Proportion of marked individuals informs population size

Indirect counts

• Use methods like tracks or droppings to estimate population
• Relative measure of population size

Quantifying dispersal of individuals

• Need to know the source of individuals
• Mark-recapture methods useful (e.g., ear tags, radio transmitters)
• Fluorescent dye on pollen

Lifetime dispersal distance

• Average distance individuals move from birth to reproduction
• Used to estimate how fast a population can expand its range
• Examples: European starling's rapid spread across the US, birds

Concept check

• Why use surveys instead of censuses to measure animal populations?
• List and define the methods for quantifying population size

Population abundance and geographic range

• Populations with higher abundance may have larger geographic ranges
• This correlation observed for plants, mammals, birds, and protists
• Example: Birds in North America

Population abundance and geographic range (continued)

• Cause of correlation debated
• Possible causes include resource availability, change in habitat use, or reproduction/survival

Population density and adult body size

• Population density generally negatively correlated with body mass
• Related to space and resource availability

Concept check

• What is the relationship between population abundance and geographic range?
• Define the relationship between population density and adult body size

Dispersal is essential to colonize new areas

• Essential for population dispersion
• Barriers including time, substantial barriers, and inhospitable habitats can limit dispersal
• Humans facilitate dispersal (e.g., commensal species)
• Habitat corridors — favorable areas aiding dispersal between habitats

Testing the importance of habitat corridors

• Example: pine forests in South Carolina
• Researchers cleared forest patches and analyzed dispersal among plants and insects

Ideal free distribution among habitats

• Individuals ideally move to best-quality habitats
• However, high density in best habitats means fewer resources

Ideal free distribution (continued)

• Per-capita benefit often decreases with increasing population in quality habitats
• Individuals distribute based on per-capita benefit, ensuring equal quality among habitats

Ideal free distribution (continued)

• Individuals may not always know of other habitat options
• Fitness influenced by more than just resources
• Predators, occupied territories

The effects of habitat quality

• High-quality habitats often result in more reproductive success.
• Habitats may serve as dispersal sources to habitats of lower quality

The effects of habitat quality (continued)

• Example: Blue tit birds in downy and holm oak forests
• Population persistence due to migration between suitable habitats (rescue effect)

Concept check

• Why are some species absent from continents with suitable habitat?
• How do habitat corridors influence dispersal?
• What does an individual seek to optimize in ideal free distribution?

Next class

• Population growth and regulation (Chapter 11)
• No seminars next week (Remembrance Day)
• Poster presentation — week of November 25th

Test your knowledge on the distinctions between dispersal and dispersion, various population survey methods, and the principles behind mark-recapture surveys. This quiz covers key concepts in population ecology, sampling techniques, and survey methodologies.