Ecology Lecture No. 8: Population Distribution & Abundance
Thursday October 4 , 2012
-A population is a group of interacting individuals of the same species living in a particular area.
Interactions within populations include sexual reproduction and competition. Populations are dynamic
with distribution and abundance privy to change over time and space. Understanding the factors that
influence these dynamics helps us manage populations for harvest or conservation.
Distribution & Abundance:
-Distribution is the geographic area where individuals of a species occur. Abundance is the number of
individuals in a given area. Abundance can be reported as population size (the number of individuals), or
density (the number of individuals per unit area). E.g. On a 20-hectare island there are 2,500 lizards with
a population density of 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 isn’t fully known,
an area is enclosed based on the best available knowledge of the species.
Change In Abundance Over Time & Space:
-Species vary in their ability to disperse. In plants, dispersal occurs by seed movement, though the
distance a seed is moved can be very small. 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. For
example the heathlands in England have been fragmented by human development.
-Individuals can be defined as products of a single fertilization. The aspen grove would be a single
genetic individual, or genet due to its tendency to create clones of itself. Clones can form by budding,
apomixis, or horizontal spread. If members of a genet are independent physiologically, each member is
called a ramet.
-Habitat suitability is comprised of abiotic features (moisture, temperature, pH, sunlight, nutrients, etc.)
and biotic features (herbivores, predators, competitors, parasites, and pathogens). Some species can
tolerate broad ranges of physical conditions, others have narrow ranges. The creosote bush is very
tolerant of dry conditions and occurs widely in North American deserts. Saguaro cacti can tolerate dry
conditions, but not cold temperatures and have a more limited distribution.
-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. -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.
Dependence Upon Disturbance:
-Some species distributions depend on disturbance—events that kill or damage some individuals,
creating opportunities for other individuals to grow and reproduce. E.g. Some species persist only where
there are periodic fires.
-Evolutionary history and geologic events affect the modern distribution of species. 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 can also help explain the distributions of some species. Very
different animal species were observed on the Philippines and New Guinea, even though they are
presently close together.
-Dispersal limitation can prevent species from reaching areas of suitable habitat. For instance, the
Hawaiian Islands have only one native mammal, the hoary bat, which was able to fly there. Experiments
suggest that dispersal may result in better chances for survival and reproduction than staying in
crowded pools with limited food.
-Geographic range is defined as 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: Many tropical plants have small ranges. In 1978, 90 new species were
discovered, restricted to a single mountain ridge in Ecuador. Other species, such as the coyote, have
very large geographic ranges.
-Some species are found on several continents. 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