READING NOTES - Chapter 9
Population Distribution and Abundance
From Kelp Forest to Urchin Barren: A Case Study
Stretching over 1,600 km of the Pacific Ocean to the west of Alaska, the mountainous Aleutian Islands are often shrouded in fog and battered by violent storms
The islands have few large trees, and except for the eastern islands that once were connected to the mainland, they lack mainland terrestrial mammals such as brown bears, caribou, and lemmings
Although there are few trees on land, the nearshore waters of some Aleutian islands harbor fascinating marine communities known as kelp forests, made up of brown algae such as Laminaria and Nereocystis
Dense clusters of kelp rise from their holdfasts on the sea bottom toward the surface, producing what feels like an underwater forest
Other nearby islands do not have kelp forests
Instead, the bottoms of their nearshore waters are carpeted with sea urchins and support few kelp or other large algae
Areas with large numbers of urchins are called “urchin barrens” because they lack kelp forests
One possibility is that islands with kelp forests differ from islands without kelp forests in terms of climate, ocean currents, tidal patterns, or physical features such as underwater rock surfaces
But no such differences have been found, leaving us to look for other reasons why some islands have kelp forests while others do not
Introduction
The distribution of a species is simply the geographic area where individuals of the species are present, while its abundance refers to the number of individuals of a species or population
These two measures are highly related because the distribution of a species can be viewed as a map of all areas where the abundance of the species is greater than zero
Determining the distributions and abundances of species, and the factors important to these patterns, can be challenging given that groups of individuals (or populations) often vary dramatically over space and time
Our ability to document this variability and predict these changes can serve as a “measuring stick” for how well we understand events in nature
Populations and Individuals
A population is a group of individuals of the same species that live in the same area at the same time and interact with one another
Our definition of a population also incorporates the area over which members of a species interact
If that area is known, as in a population of lizards that live on and move throughout a small island, we can report population abundance either as population size (the number of individuals in the population) or as population density (the number of individuals per unit of area)
What Are Individuals?
It can be a challenge to determine the size or density of a population, because it is necessary to know how many individuals are present within the population
Like many plant species, an individual aspen can produce genetically identical copies of itself, or clones
To cope with the complications that result from the formation of clones, biologists who study such organisms define individuals in several different ways
For example, an individual can be defined as the product of a single fertilization event
Under this definition, a grove of genetically identical aspen trees is a single genetic individual, or genet
However, members of a genet are often physiologically independent of one another, and they may in fact compete for resources
Such actually or potentially independent members of a genet are called ramets
The most direct way to determine how many individuals live in a population is to count all of them
This sounds simple enough, and it is possible in some cases, as for the lizards on one island, and other organisms that are confined to small areas, are easy to see, or do not move
Ecologists Estimate Abundance Using a Variety of Methods
Many ecological studies require an estimate of a population’s actual abundance, or absolute population size
For example, with species that are threatened or endangered, it is ideal to estimate their absolute population sizes in order to keep track of possible further declines
In other cases, it may be sufficient to estimate the relative population size, the number of individuals in one time interval or place relative to the number in another
Relative population size estimates are usually easier and less expensive to obtain than are absolute estimates
While useful, estimates of relative population size must be interpreted carefully
Methods for estimating abundance fall into three general categories:
Area-based counts in which the number of individuals in a given area or volume are counted
Distance methods in which the distances of individuals from a line or a point are measured and then converted into estimates of the number of individuals per unit of area
Mark–recapture studies, which involve releasing marked individuals and then recapturing them at a later time to estimate the total population size
Distribution and Abundance Patterns
Dispersal is simply the movement of individuals into (immigration) or out of (emigration) an existing population
A group of geographically isolated populations linked together by dispersal is known as a metapopulation
For example, a cluster of meadows might be considered a metapopulation if Clematis seeds from one meadow had the potential to disperse to another meadow
At larger spatial scales, the entire geographic range, or distribution, of a species might consist of one or multiple metapopulations, depending on the extent of the area occupied by a species
The aggregates of C. fremontii individuals found in the meadow populations provide an example of the dispersion, or spatial arrangement, of individuals within a population
In some cases, the members of a population have a regular dispersion, in which individuals are relatively evenly spaced throughout their habitat
In other cases, individuals show a random dispersion, similar to what occurs if individuals are positioned at locations selected at random
Finally, as in C. fremontii, individuals may be grouped together to form a clumped dispersion
The Geographic Ranges of Species Vary in Size
Although there are no species that are found everywhere, there is considerable variation in the sizes of their geographic ranges
This latter point was illustrated dramatically in 1978, when 90 new plant species were discovered on a single mountain ridge in Ecuador, each with a geographic range that was restricted to that ridge
We call such species endemic because they occur in one particular location and nowhere else on Earth
In some cases, we understand an organism’s range poorly because it has life stages that are hard to find or study; this is true of many fungi, plants, and insects
We may know under what conditions the adult organism lives, yet have no idea where or how other life stages live
The Geographic Ranges of Species Vary in Patchiness
Even within the geographic range of a species, much of the habitat is not suitable for the species
As a result, populations tend to have a patchy distribution
This observation holds at both large and small spatial scales
Finally, it is important to recognize that a population may exist in a series of habitat patches or fragments that are spatially isolated from one another but are linked by dispersal
Such a “patchy” population structure can result from features of the abiotic environment, as we saw with Clematis, but can also result from human actions
Species Distribution Models Can Be Used to Predict a Species’ Geographic Range
There are many species whose geographic ranges are not yet known
When such species are rare or in need of protection, it can be difficult to plan how best to protect them
Furthermore, ecologists often want to predict future distributions of species
One way to predict the current or future distribution of a species is to characterize how both abiotic and biotic conditions influence their occurrence or abundance
Such information can be used to construct a species distribution model, a tool that predicts the geographic distribution of a species based on the environmental conditions at locations the species is known to occupy
Processes Important to Distribution and Abundance
Many factors can influence the distributions and abundances of organisms
We’ll survey these factors by grouping them into three categories:
Habitat suitability
Historical factors (such as evolutionary history and continental drift)
Dispersal
Habitat Suitability Determines Distribution and Abundance
Good and poor places to live exist for all species
A desert species is not likely to perform well in the Arctic, or vice versa
Even small differences among environments in survival and/or reproduction of individuals can cause differences in abundance
Thus, the distribution and abundance of a species are influenced strongly by the presence of appropriate habitat
The Abiotic Environment
Some species can tolerate a broad range of physical conditions, while others have more narrow requirements
Creosote bush is very tolerant of arid conditions:
It uses water rapidly when it is available, then shuts down its metabolic processes during periods of extended drought
Creosote bush also tolerates cold, so its populations thrive in high-elevation deserts where winter temperatures can remain below freezing for several days
The saguaro cactus uses a unique photosynthetic pathway called crassulacean acid metabolism, which allows it to reduce water loss
Furthermore, during wet periods, the saguaro stores water in its massive trunk and arms, saving it for use during times of drought
Saguaro cannot tolerate cold, however; it is killed when temperatures remain below freezing for 36 hours or more
The Biotic Environment
The biotic environment also has important effects on distributions and abundances of species
Obviously, species that depend on one another for their growth, reproduction, and/or survival have to live together at the same location
Seychelles warblers are territorial:
A breeding pair defends its territory against other birds of the species
But not all territories are equal:
Some are of higher quality than others because they provide more food
Organisms can also be excluded from an area by herbivores, predators, competitors, parasites, or pathogens, any of which can greatly reduce the survival or reproduction of members of a population
Interactions Between Abiotic and Biotic Environment
On the Pacific coast of North America, temperatures are such that S. balanoides could be found 1,600 km farther south than it currently is
To the north, as temperatures become increasingly colder, a point is reached where S. balanoides outcompetes the other barnacles and maintains healthy populations
Thus, the abiotic and biotic environments interact to determine where populations of this barnacle are found
Disturbance
A disturbance is an abiotic event that kills or damages some individuals and thereby creates opportunities for other individuals to grow and reproduce
Many plant species, for example, persist in an area only if there are periodic fires
If humans prevent fires, such species are replaced by other species that are not as tolerant of fires but are superior competitors in the absence of fires
Thus, a change in the frequency of fires can change the composition of ecological communities, as you can explore in
Distribution and Abundance Reflect Evolutionary and Geologic History
Events in the evolutionary and geologic history of Earth have had a profound effect on where organisms live today
Polar bears hunt on ice packs and eat seals, both of which abound in Antarctica
Part of the answer to our question can be found in the evolutionary history of these bears
Fossils and genetic evidence indicate that polar bears evolved from brown bears (Ursus arctos) in the Arctic (Lindqvist et al. 2010); hence U. maritimus is found in the Arctic because the species originated there
Dispersal is a Process that Distributes Organisms Across the Landscape
Organisms differ greatly in their capacity for movement
In plants, for example, dispersal occurs when seeds move away from the parent plant
Although events such as storms can transport seeds long distances (hundreds of meters to many kilometers; see Cain et al. 2000), dispersal distances in plants are usually small (one to a few tens of meters)
Whales also migrate, which is a specific type of dispersal in response to seasonal variation in resources
Migration involves round-trip movement and usually includes the entire population
As demonstrated by the polar bear’s absence from Antarctica, a species’ limited capacity for dispersal can prevent it from reaching areas of suitable habitat—a phenomenon known as dispersal limitation
Dispersal limitation can also occur on smaller spatial scales, preventing populations from expanding to nearby areas of apparently suitable habitat
Metapopulations
The patchy nature of the landscape ensures that for many species, areas of suitable habitat do not cover large, continuous regions, but rather exist as a series of favorable sites that are spatially isolated from one another
As a result, the populations of a species are often scattered across the landscape, each in an area of favorable habitat but separated from one another by hundreds of meters or more
These seemingly isolated populations can be classified as a metapopulation when individuals (or gametes) occasionally disperse from one population to another
Literally, the term “metapopulation” refers to a population of populations, but it is usually defined in a more particular sense as a set of spatially isolated populations linked to one another by dispersal
Metapopulations are Characterized by Repeated Extinctions and Colonizations
As ecologists have long recognized, populations of some species are prone to extinction for two reasons:
The patchiness of their habitat makes dispersal between populations difficult
Environmental conditions can change in a rapid and unpredictable manner
Building on this idea of random extinctions and colonizations, Richard Levins (1969, 1970) represented metapopulation dynamics in terms of the extinction and colonization of habitat patches:
where p represents the proportion of habitat patches that are occupied at time t, while c and e are the patch colonization and patch extinction rates, respectively
In deriving Equation 9.4, Levins made a few assumptions, including the following:
There is a very large (infinite) number of identical habitat patches
All patches have an equal chance of receiving colonists (hence the spatial arrangement of the patches does not matter)
All patches have an equal chance of extinction
A Metapopulation Can Go Extinct Even When Suitable Habitat Remains
Human actions (such as land development) often convert large tracts of habitat into sets of spatially isolated habitat fragments
Such habitat fragmentation can cause a species to have a metapopulation structure where it did not have one before
If land development continues and the habitat becomes still more fragmented, the metapopulation’s colonization rate (c) may decrease because patches become more isolated and hence harder to reach by dispersal
Extinction and Colonization Rates Often Vary Among Patches
As the impact of Lande’s work on the northern spotted owl suggests, the metapopulation approach has become increasingly important in applied ecology
But metapopulations in the field often violate the assumptions of Levins’s model
Isolation by distance occurs when patches located far from occupied patches are less likely to be colonized than are nearby patches
In H. comma, distance from occupied patches had a strong effect on whether patches vacant in 1982 were colonized by 1991:
Few patches separated by more than 2 km from an occupied patch were colonized during that period
Among patches occupied in 1982, Thomas and Jones found that the chance of extinction was highest in small patches (most likely because small patches tend to have small population sizes) and in patches that were far from another occupied patch
Isolation by distance can affect the chance of extinction because a patch that is near an occupied patch may receive immigrants repeatedly, which may increase the patch population size and make extinction less likely
This tendency for high rates of immigration to protect a population from extinction (by reducing the problems associated with small population size) is known as the rescue effect