LECTURE 8: POPULATION DISTRIBUTION AND ABUNDANCE
Population – group of interacting individuals of the same species living in a particular area
Interactions within populations include sexual reproduction and competition
o Influence each other
Populations are dynamic – distribution and abundance can change over time and space. Understanding
the factors that influence these dynamics help us manage populations for harvest or conservation
Distribution – geographic area where individuals of a species occur
Abundance – number of individuals in a given area
o Abundance can be reported as a population size (# of individuals), or density (# of individuals
per unit area)
o Example: On a 20-hectare island there are 2,500 lizards.
Population density = 125/hectare
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 is not fully known, an area is delimited based on best available knowledge of the species.
Species vary in their ability to disperse
o In plants, dispersal occurs by seed movement. The distance moved can be very small
o 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 subdivide populations
o Example: Heathlands in England have been fragmented by
For some species, it’s hard to determine what an individual is
o We see this a lot in plants, but it can be seen in animals
Individuals can be defined as products of a single fertilization: The
aspen grove would be a single genetic individual, or genet
If members of a genet are independent physiologically, each
member is called a ramet
o A ramet is a “subset” of a genet
Distribution and Abundance
The distributions and abundances of organisms are limited by
habitat suitability, historical factors, and dispersal
o Abiotic features: moisture, temperature, pH, sunlight,
Availability limits the distribution of some species
o Some species can tolerate broad ranges of physical conditions, others have narrow ranges
o Creosote bush is very tolerant of dry conditions and occurs widely in North American deserts.
o Saguaro cactus can tolerate dry conditions, but not cold temperatures and has a more limited
o Biotic features: organisms are affected by herbivores, predators, competitors, parasites, and
o In Australia, an introduced cactus became a pest species, spreading over vast areas.
o A moth that feeds on cactus was then released, and distribution and abundance of the cactus has
been greatly reduced
Abiotic and biotic features can interact to determine distribution and abundance
The range of the barnacle Semibalanus balanoides is restricted by temperature. But competition from
other species precludes it from some areas with suitable temperatures.
Some species distributions depend on disturbance—events that kill or damage some individuals,
creating opportunities for other individuals to grow and reproduce.
o Example: Some species persist only where there are periodic fires Seeds that are designed to respond to fires, fire-dependent in order to avoid competition
with other species
o Evolutionary history and geologic events affect modern distribution of species
o 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.
o Continental drift explains the distributions of some species.
o Wallace (1860) observed very different animal species on the Philippines and New Guinea, even
though they are close together
Were not historically close together, separated by expansive body of water
o Dispersal limitation can prevent species from reaching areas of suitable habitat.
o Example: The Hawaiian Islands have only one native mammal, the hoary bat, which was able to
o Dispersal limitation has also been shown in plant species
o Dispersal can also affect population density, and vice versa
o Many species of aphids produce winged forms (capable of dispersing) in response to crowding
As crowding increases, the number of winged form aphids increases as well
o Desert pupfish live in pools that are sometimes connected after heavy rains
o Dispersal may result in better chances for survival and reproduction than staying in crowded
pools with limited food
Geographic range—the entire geographic region over which a species is found
Many species have a patchy distribution of populations across their geographic range
There is also great variation in species ranges:
o Many tropical plants have small ranges. In 1978, 90 new species were discovered, restricted to a
single mountain ridge in Ecuador.
o Other species, such as they coyote, have very large geographic ranges
o Some species are found on several continents
o Few species are found on all continents except humans, Norway rats, and the bacterium E. coli
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
Not all habitats within a range are suitable,