Ecology Chapter 8
Population Distribution and Abundance
From Kelp Forest to Urchin Barren: A Case Study
• the nearshore waters of some Aleutian islands harbor marine communities called Kelp
forests, which are dense clusters of kelp that rise from their holdfasts on the sea
bottom toward the surface, producing what feels like an underwater forest
• other islands do not have kelp forests but the sea ﬂoor is carpeted with sea urchins
and support few kelp or large algae
• Urchin barrens: ares with large # of urchins and have less species than do kelp forests
it is suggested that urchins feed and can consume vast quantities of algae which
causes the urchin barrens
• this hypothesis has been tested 2 ways:
1. studies showed that kelp forests were not found in regions where there were lots
2. the effects of urchins was tested in an experiment that put kelp and urchins
together. The plots that had urchins remained kelpless while other plots without
urchins thrived with kelp
Population: Group of interacting individuals of the same species living in a particular
• 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.
• Interactions within populations include sexual reproduction and competition.
• in species that reproduce sexually, a population might be deﬁned as the group of
individuals that interact by interbreeding
• however, for species that reproduce asexually, a population must be deﬁned by other
kinds of interactions, such as competition for common sources of food
Distribution: Geographic area where individuals of a species occur.
Abundance: Number of individuals in a given area.
• 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 density = 125/
• 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. Abundance change over time and space
• the number of individuals in a population changes over time and abundances differ
place to place
• populations are dynamic in another sense as well: Individuals move from one
population to another, sometimes traveling great distances
Dispersal Links populations
• Species vary in their ability to disperse.
• In plants, dispersal occurs by seed movement. The distance moved can be very
• Other species, such as whales, can move thousands of kilometers in a year.
• Some populations exist in isolated patches that are linked by dispersal.
• This can result from physical features of the environment, or human activities that
• Example: Heathlands in England have been fragmented by human development. What are Individuals?
• a population may cover a single area whose extent depends on the capacity of the
species for dispersal, or it may cover a series of spatially isolated patches linked by
Clone: identical copies of itself (the trees,coral, sea anemones, frogs, ﬁshes,lizards, and
many insects can clone themself)
• one deﬁnition of individual is the product of a single fertilization event
Genet: single genetic individual. ex, a grove of genetically identical aspen trees is
actually a genet
• Individuals can be deﬁned as products of a single fertilization: The aspen grove
would be a single genetic individual, or genet.
Ramets: independent members of a genet If members of a genet are independent physiologically, each member is called a
Distribution and Abundance
Habitat Suitability Limits Distribution and Abundance
The distributions and abundances of organisms are
limited by habitat suitability, historical factors, and
What factors make habitat suitable?
Abiotic Features of the Environment
• aspects of the abiotic (nonliving) environment, such as moisture, temperature,
sunlight, soil pH, salt concentration, and available nutrients, set limits on whether a
habitat will be suitable for a particular species
Creosote bush is very tolerant of dry conditions and occurs widely in North American deserts.
Saguaro cactus can tolerate dry conditions, but not cold temperatures and has a more limited
Biotic Features of the Environment
• species that depend completely on one or a few other species for their growth,
reproduction, or survival cannot live where the species on which they depend are
absent • organisms can also be excluded from areas by herbivores, predators, competitors,
parasites, or pathogens, any of which can greatly reduce the survival or reproduction
of members of a population
In Australia, an introduced cactus became a pest species, spreading over vast areas.
A moth that feeds on cactus was then released, and distribution and abundance of the
cactus has been greatly reduced.
Interactions between Abiotic and Biotic Features
• in many cases, abiotic and biotic features of the environment act together to determine
the distribution and abundance of a species
The range of the barnacle Semibalanus balanoides is restricted by temperature. But
competition from other species precludes it from some areas with suitable
Disturbance: is an abiotic event that kills or damages some individuals and thereby
creates opportunities for other individuals to grow and reproduce
•ﬁres, droughts, ﬂoods, and windstorms are all examples
ex. Many plant species persist in an area only if there are periodic ﬁres. If humans
prevent those ﬁres other competitors that are not as tolerant of ﬁres but are superior in
absence of ﬁres
History and Dispersal Limit Distribution and Abundance
Evolution and continental Drift
• events in the evolutionary and geological history of Earth have had a profound effect
on where organisms live today
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.
• Continental drift explains the distributions of some species.
• Wallace (1860) observed very different animal species on the Philippines and New
Guinea, even though they are close together. •
Dispersal Limitation: a species’ limited capacity for dispersal can prevent it from
reaching areas suitable habitat
Example: The Hawaiian Islands have only one native mammal, the hoary bat, which
was able to ﬂy there.
Dispersal and Density
• dispersal effects abundances as well as distributions
• when individuals disperse from one population to another, the density of the
population they leave decreases and the density they join increases
• dispersal can also be effected by population
• Many species of aphids produce winged forms (capable of dispersing) in response to
crowding. • Ex. Desert pupﬁsh live in pools that are sometimes connected after heavy rains.
• Dispersal may result in better chances for survival and reproduction than staying in
crowded pools with limited food.
Many organisms alter their rates of dispe