Ecology Notes – Oct. 4/12
- 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, which means their distribution and abundance can change over time and space.
Understanding the factors that influence these dynamics helps us manage populations for harvest or
conservation. 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 (number of
individuals) or density (number of individuals per unit area). For example, on a 20-hectare island,
there are 2500 lizards. The population density is 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 delimited based on the best available knowledge of the
species. Beetles at different sites in NY had varied abundances from one site to another and from one
year to the next.
- Species vary in their ability to disperse. In plants, dispersal occurs by seed movement. The distance
moved can be very small. Other species, such as whales, can move thousands of km 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. An example is
heathlands in England that have been fragmented by human development. There were smaller but
more numerous fragments in 1978 compared to earlier.
- For some species, it’s hard to determine what an individual is. Trees can be different genetic
individuals or they could be part of one tree produced asexually from the roots buds of a single
individual, and these are called clones. Examples of asexual reproduction that produce clones are
budding (clonal offspring detaches from parent), apomixis (clonal offspring are produced from
unfertilized eggs), and horizontal spread (clonal offspring are produced as the organism grows).
- 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.
- Distribution and abundance: The dsitributions and abundances of organisms are limited by habitat
suitability, historical factors, and dispersal. Habitat suitability involves abiotic features, such as
moisture, temperature, pH, sunlight, and nutrients. Some species can tolerate broad ranges of
physical conditions, and others have narrow ranges. There is a larger range for creosote bush, due to
cold tolerance. 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 so has a more
limited distribution. Organisms are also affected by biotic features such as herbivores, predators,
competitors, parasites, and pathogens. 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 – can only live in colder temperatures.
But competition from other species precludes it from some areas with suitable temperatures.
- Some species distributions depend on disturbance, which are events that kill or damage some
individuals, creating opportunities for other individuals to grow and reproduce. For example, some
species persist only where there are periodic fires.
- Historical factors: Evolutionary history and geologic events affect modern distribution of species. For
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 observed very different animal
species on the Philippines and New Guinea, even though they are close together. They have only just
recently been close together.
- Dispersal: Dispersal limitation can prevent species from reaching areas of suitable habitat. For
example, the Hawaiian islands have only one native mammal, the hoary bat, which was able to fly
there. Dispersal limitations have also been shown in plant species. Dispersal can affect population
density and vice versa. Many species of aphids produce winged forms (capable of dispersing) in
response to crowding. The proportion of aphid offspring with wings is greater if their mothers were
also reared at high densities.
- Desert pupfish live in pools that are sometimes connected after heavy rains. Experiments suggest
that dispersal may result in better chances for survival and reproduction than staying in crowded
pools with limited food.
- Geographic range is 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, ninety 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 as monarch butterflies, migrate long distances
between summer and winter habitats and it is important that we ensure conditions favourable for
them at both locations. For some species, it is difficult to fi