Santa Catalina Island Biogeography
“Biogeography is the study of the facts and patterns of species distribution.
It’s the science concerned with where animals are, where plants are, and
where they are not” (Song of the Dodo, David Quammen).
“Islands have been especially instructive because their limited area and
their inherent isolation combine to make patterns of evolution stand out
starkly. It’s such an important truth I’ll repeat it: Islands give clarity to
evolution. On an island you might find those enormous tortoises, those
flightless birds weighing half a ton, those pygmy chameleons and hippos.
Generally you will also find fewer species, and therefore fewer relationships
among species, as well as more cases of species extinction. All these factors
result in a simplified ecosystem, almost a caricature of nature’s full
complexity. Consequently islands have served as the Dick-and-Jane primers
of evolutionary biology, helping scientists master enough vocabulary and
grammar to begin to comprehend the more complex prose of mainlands”
(Quammen, pg. 19).
Lying roughly 20 miles off the coast of Southern California, Catalina Island
has had many organisms float and fly right to it. Where as other organisms
may have brushed the shores and proceeded to a different destination. How
Catalina acquired its different species of organisms is quite amazing and
dependent on many different processes and chains of events.
First of all the type of island determines how many different species will
inhabit it. Islands can be broken into the following categories:
Continental islands once part of the mainland, varied in their
geology, usually inhabited by land mammals.
* Immigration of species is faster than evolution
Oceanic islands “built” over an oceanic plate, remote and have
never been connected to the mainland, inhabited by few land mammals.
* Evolution is faster than immigration
Catalina is an oceanic island. However, it is relatively close to the mainland
for an oceanic island and is fairly large in size. This has had a profound
influence on the organisms that inhabit it. The closer an island to the
mainland the higher the probability of dispersed organisms to travel the
short distance. The larger the island the chances of organisms landing on
the island increase tremendously. This chance rather than logic theory is
referred to as Sweepstakes Dispersal. There are hundreds of thousands of
organisms dispersed from the mainland each year, and very few actually land
in a hospitable environment. The odds of an organism landing in a hospitable
environment are equivalent to winning the lottery. Millions of people play and
there is only one winner, possibly two. Like dispersal to islands it is chance
not logic in determining the winner.
The three main modes of dispersal of organisms to islands are:
Wind seeds, insects, spiders, birds and bats may disperse this way.
Because Catalina Island is relatively close to the mainland many seeds
and small creatures may have dispersed to the island in this manner.
Wing seeds may get caught in the feathers of a bird or travel to the
island within the bird’s digestive system and be deposited on the
island in a nice fertilized package.
Water larger animals such as squirrels, fox, snakes and frogs may have
rafted over during periods of flooding on the mainland. Creatures may
take refuge on floating logs and other debris, which is then swept out
to sea (also known as rafting). Seeds may also use this means of
dispersal. Bladder pod, a plant found on Catalina probably traveled
this way across the channel, it can float in seawater for up to 4 weeks.
Once organisms colonize an island they begin to adapt to their new
surroundings immediately. There are common adaptations and patterns that
occur on most islands.
 Rarely are large grazing animals native to oceanic islands. Therefore,
plants tend to lose their thorns and other means of protection from
grazing animals.
 Due to the difference in climate, soil, less competition and grazing, some
plants that are bushes on the mainland become treelike on islands
through a process known as arborescence. Examples of arborescence
occurring on Catalina are Toyon and the Island Scrub Oak.
 Niche shifts are also common on islands. These occur because of fewer
numbers of species and absence of competition therefore, creating
empty niches. An animal can then fill a new niche that is different from
the one it occupied on the mainland. Animals may also fill two niches like
the Beechy Ground Squirrel, which occupies both the arboreal and
terrestrial niches on Catalina.
Some animals on islands evolve over time to become smaller or larger
creating either a dwarf or gigantic species  gigantism and dwarfism.
On Catalina we have both dwarf and gigantic species. The Catalina Island
Fox is a dwarf species while the quail and squirrel are gigantic species.
There is no one answer as to why this occurs. One theory on gigantism is
that on islands generally there is reduced predation and competition
within the species that would normally draw on available food and shelter.
This may allow an animal to get much larger than it could on the mainland.
A larger animal may also store water and fat better, survive hungry and
cold times, and finally give birth to larger babies who will also have a
better chance at surviving. Dwarfism may occur because larger animals
with no population control, often the case on islands, result in over
population, leading to malnutrition of the group, which may cause stunting
of growth in individuals, and over time may lead to dwarfism. Smaller
animals may also be more successful on an island that has fewer food
sources. Therefore producing smaller offspring that would have a better
chance of survival.
 Some plant and animal species have evolved and adapted enough to
become a new species or subspecies; these new species are endemic to
Catalina Island. Species can be endemic to islands, regions or continents.
It simply means that species is unique to a specific area. When looking in
a guidebook make sure to look closely whether a plant is endemic to
Catalina or all of the Channel islands.
Catalina Island’s 8 endemic plants include:
Ironwood, St. Catherines Lace, Catalina Island Live Forever, Island
Mahogany, Yerba Santa, Manzanita, Wallace’s Night Shade, Catalina Island
Bed Straw.
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**Remember scientific theories are not wrought in stone, please try to
keep current on changing theories and ideas and teach them to your
students as such.