9 Population Distribution and Abundance Chapter 9 Population

9 Population Distribution and Abundance

Chapter 9 Population Distribution and Abundance CONCEPT 9. 1 Populations are dynamic entities that vary in size over time and space. CONCEPT 9. 2 The distributions and abundances of organisms are limited by habitat suitability, historical factors, and dispersal. CONCEPT 9. 3 Many species have a patchy distribution of populations across their geographic range.

Chapter 9 Population Distribution and Abundance CONCEPT 9. 4 The dispersion of individuals within a populations depends on the location of essential resources, competition, dispersal, and behavioral interactions. CONCEPT 9. 5 Population abundances and distributions can be estimated with areabased counts, distance methods, mark– recapture studies, and niche models.

From Kelp Forest to Urchin Barren: A Case Study Waters around the Aleutian Islands have abundant marine life, including the sea otter and kelp forests.

From Kelp Forest to Urchin Barren: A Case Study Kelp are brown algae that form dense forest-like stands and harbor a diverse community of organisms. Some islands are surrounded by “urchin barrens”—many sea urchins, very little kelp, low species diversity. Grazing of kelp by sea urchins could explain the differences.

From Kelp Forest to Urchin Barren: A Case Study Field studies show a negative correlation between urchin abundance and kelp cover. Experimental studies: Urchins were removed from plots with no kelp. Kelp abundance increased dramatically. But what determines urchin abundance?

Figure 9. 2 Do Sea Urchins Limit the Distribution of Kelp Forests?

Introduction Distribution: Geographic area where individuals of a species occur. Abundance: Number of individuals in a given area. Ecologists try to understand what factors determine the distribution and abundance of species.

Introduction Populations are dynamic—distribution and abundance can change over time and space. Understanding the factors that influence these dynamics helps us manage populations for harvest or conservation.

CONCEPT 9. 1 Populations are dynamic entities that vary in size over time and space.

Concept 9. 1 Populations Population: Group of interacting individuals of the same species living in a particular area. Interactions include sexual reproduction and competition.

Concept 9. 1 Populations Abundance can be reported as population size (# of individuals), or density (# of individuals per unit area). Example: On a 20 -hectare island there are 2, 500 lizards. Population size = 2, 500 Population density = 125/hectare

Concept 9. 1 Populations Sometimes the total area occupied by a population is not known. It is often difficult to know how far organisms or their gametes can travel. When the area isn’t fully known, an area is delimited based on best available knowledge of the species.

Concept 9. 1 Populations Individuals can be defined as products of a single fertilization: The aspen grove would then be a single genetic individual, or genet. If members of a genet are independent physiologically, each member is called a ramet, e. g. strawberries.

CONCEPT 9. 2 The distributions and abundances of organisms are limited by habitat suitability, historical factors, and dispersal.

Concept 9. 2 Distribution and Abundance Habitat Suitability Abiotic features: Moisture, temperature, p. H, sunlight, nutrients, etc. Some species can tolerate broad ranges of physical conditions, others have narrow ranges.

Concept 9. 2 Distribution and Abundance Creosote bush is very tolerant of dry conditions, as well as cold, and occurs widely in North American deserts. Saguaro cactus can tolerate dry conditions, but not cold temperatures and has a much more limited distribution.

Figure 9. 7 The Distributions of Two Drought-Tolerant Plants (Part 1)

Figure 9. 7 The Distributions of Two Drought-Tolerant Plants (Part 2)

Concept 9. 2 Distribution and Abundance Some species distributions depend on disturbance: Events that kill or damage some individuals, creating opportunities for other individuals to grow and reproduce. Example: Some species persist only where there are periodic fires.

Concept 9. 2 Distribution and Abundance Historical Factors Evolutionary history and geologic events affect modern distribution of species. Example: Polar bears evolved from brown bears in the Arctic. They are not found in Antarctica because of an inability to disperse through tropical regions.

Concept 9. 2 Distribution and Abundance Dispersal limitation can prevent species from reaching areas of suitable habitat. Example: The Hawaiian Islands have only one native mammal, the hoary bat, which was able to fly there.

Concept 9. 2 Distribution and Abundance Dispersal limitation has also been shown in plant species: When seeds of the annual plant Impatiens capensis were dispersed by hand, the distribution of the plants expanded.

Figure 9. 10 Populations Can Expand after Experimental Dispersal

Concept 9. 2 Distribution and Abundance 27 populations of English bluebells were established experimentally in suitable habitat in 1960. After 45 years, 11 populations persisted, each with many individuals. This suggests that dispersal limitation prevented the bluebells from reaching habitat where they could thrive.

Concept 9. 2 Distribution and Abundance Dispersal can also affect population density, and vice versa. Many species of aphids produce winged forms (capable of dispersing) in response to crowding. The percentage of offspring that develop wings increases as the density of aphids increases.

CONCEPT 9. 3 Many species have a patchy distribution of populations across their geographic range.

Concept 9. 3 Geographic Range Geographic range—the entire geographic region over which a species is found.

Concept 9. 3 Geographic Range There is great variation in species geographic ranges: Devil’s Hole pupfish live in a single small desert pool. Many tropical plants have small ranges. In 1978, 90 new species were discovered, restricted to a single mountain ridge in Ecuador.

Concept 9. 3 Geographic Range Geographic range includes areas occupied during all life stages. Some species, such as monarch butterflies, migrate long distances between summer and winter habitats. For some species, it is difficult to find all the life stages and the ranges they inhabit.

Figure 9. 13 Monarch Migrations

Concept 9. 3 Geographic Range Not all habitat within a range is suitable, resulting in patchy distributions. This can operate at different spatial scales. At large scales, climate may dictate locations of populations. At small scales, soils, topography, other species, etc. , can determine patchiness.

Concept 9. 3 Geographic Range Patchiness at different scales is illustrated by the shrub Clematis fremontii. It is restricted to areas of dry, rocky soil with few trees, called barrens or glades. The glades occur on outcrops of limestone on south- or west-facing slopes.

Figure 9. 14 Many Populations Have a Patchy Distribution

CONCEPT 9. 4 The dispersion of individuals within a population depends on the location of essential resources, competition, dispersal, and behavioral interactions.

Concept 9. 4 Dispersion within Populations Dispersion: Spatial arrangement of individuals within a population: Regular—individuals are evenly spaced Random—individuals scattered randomly Clumped—the most common pattern

Figure 9. 16 Dispersion of Individuals within Populations

Concept 9. 4 Dispersion within Populations Dispersion patterns often result from the distribution of resources. Random or clumped dispersion can also result from short dispersal distances. Competition appears to result in the regular dispersion of some species.

CONCEPT 9. 5 Population abundances and distributions can be estimated with area-based counts, distance methods, mark–recapture studies, and niche modeling.

Concept 9. 5 Estimating Abundances and Distributions Complete counts of individual organisms in a population are often difficult or impossible. Several methods are used to estimate the actual abundance or absolute population size.

Concept 9. 5 Estimating Abundances and Distributions Relative population size: Number of individuals in one time period or place relative to the number in another. Estimates are based on data presumed to be related to absolute population size. Examples: Number of cougar tracks in a given area, or number of fish caught per unit of effort.

Concept 9. 5 Estimating Abundances and Distributions Interpretation of relative population size can be tricky. Example: Number of cougar tracks is related to population density, but also activity levels of individuals.

Concept 9. 5 Estimating Abundances and Distributions Area-based counts: individuals in a given area or volume are counted. Used most often to estimate abundance of immobile organisms.

Concept 9. 5 Estimating Abundances and Distributions Quadrats: Sampling areas of specific size, such as 1 m 2. Individuals are counted in several quadrats; the counts are averaged to estimate population size.

Ecological Toolkit 9. 1, Figure A An Underwater Quadrat

Concept 9. 5 Estimating Abundances and Distributions Example: 40, 10, 70, 80, and 50 chinch bugs are counted in five 10 cm x 10 cm (0. 01 m 2) quadrats.

Concept 9. 5 Estimating Abundances and Distributions Distance methods Distances of individuals from a line or point are converted into estimates of abundance.

Concept 9. 5 Estimating Abundances and Distributions Line transects: Observer travels along line and counts individuals and their distance from the line.

Ecological Toolkit 9. 1, Figure B Counting Trees from a Line Transect

Concept 9. 5 Estimating Abundances and Distributions Mark–recapture studies: Used for mobile organisms. A subset of individuals is captured and marked or tagged, then released. At a later date, individuals are captured again, and the ratio of marked to unmarked individuals is used to estimate population size.

Ecological Toolkit 9. 1, Figure C Release of Marked Salmon

Concept 9. 5 Estimating Abundances and Distributions Niche Modeling Ecological niche: Abiotic and biotic conditions that a species needs to grow, survive, and reproduce. A niche model predicts a species’ distribution based on conditions at locations the species is known to occupy.

Concept 9. 5 Estimating Abundances and Distributions Niche model for chameleons in Madagascar: Environmental data were recorded for 1 x 1 km 2 “grid cells. ” “Habitat rules” for each species described environmental conditions where it was most likely to be found.

Concept 9. 5 Estimating Abundances and Distributions A computer program (GARP) compared grid cell data with habitat rules for each species. Predictions of species occurrences were correct 75– 85% of the time. In areas where predictions were not correct, researchers found 7 previously unknown chameleon species.

Figure 9. 19 Predicted Distributions of Madagascar Chameleons

A Case Study Revisited: From Kelp Forest to Urchin Barren Sea urchin barrens: Can persist for years after kelp is gone. Urchins can survive on other foods—other algae, benthic diatoms, detritus. When food is very scarce, they can reduce their metabolic rate, reabsorb sex organs, and absorb dissolved nutrients directly from seawater.

A Case Study Revisited: From Kelp Forest to Urchin Barren But urchins are vulnerable to predation by sea otters. Aleutian Islands that have sea otters have few sea urchins, leading to the hypothesis that sea otters control population size of urchins, and thus the distribution of kelp forests.

A Case Study Revisited: From Kelp Forest to Urchin Barren Estes and Duggins (1995) compared sites with and without otters to confirm the hypothesis. In southern Alaska sites newly colonized by sea otters, urchins disappeared within two years, and kelp densities increased dramatically.

Figure 9. 20 The Effect of Otters on Urchins and Kelp (Part 1)

A Case Study Revisited: From Kelp Forest to Urchin Barren In Aleutian Islands sites, otters reduced large urchin biomass by about 50%, but new urchin larvae arrived via ocean currents to continuously recolonize. Kelp forest recovery was slower at these sites.

Figure 9. 20 The Effect of Otters on Urchins and Kelp (Part 2)

A Case Study Revisited: From Kelp Forest to Urchin Barren Historically, sea otters were abundant throughout the North Pacific, but were hunted to near extinction for furs. By 1911, only scattered colonies survived. By the 1970 s, otter populations had begun to recover, but declined again in the 1990 s, and urchins made a comeback.

Figure 9. 21 Killer Whale Predation on Otters May Have Led to Kelp Declines (Part 1)

A Case Study Revisited: From Kelp Forest to Urchin Barren Recent sea otter declines may be due to predation by killer whales, but it is unclear why killer whales began to eat more sea otters. Whaling may have reduced killer whale preferred prey, causing them to switch to seals, sea lions, and then otters.

A Case Study Revisited: From Kelp Forest to Urchin Barren Or, reduced fish populations and lack of food reduced seal and sea lion populations and killer whales turned to sea otters.

Figure 9. 21 Killer Whale Predation on Otters May Have Led to Kelp Declines (Part 2)