let's talk about some of the factors
that contribute to the uneven
distributions of life on earth
why do different organisms have
different ranges and different
distributions first let's imagine the
planet without humans what factors would
govern where you find different kinds of
life on Earth
without our influence in such a world
life would still exhibit distinct
distribution patterns distinct ranges
and different overlapping ranges the
ranges of some species are large some
are small but the key here is that the
distributions would in fact clump there
are some places on earth where the
distributions overlap in larger numbers
than in other places and this is because
every species has its own preferred
place to live and sometimes the
preferences overlap more in some places
than in others now it doesn't mean the
species are thinking about it and saying
Hawaii's nice this time of year but it
does mean that every species tends to
have a characteristic range there's a
set of conditions in that range that
allows the organism to get its food to
reproduce and do all the things that
members of a given species have to do to
survive in their habitat preferences are
determined by three main factors
physical biotic and evolutionary let's
have a look at each of these physical or
environmental factors are things like
the geography of the place is it
freshwater or saltwater does the
organism prefer living up in the air or
underwater does it prefer living in
mountains in deserts flat lands or in
caves physical factors also include
things like climate temperature and
rainfall are two of the things that
drive the distributions of organisms
they can govern where certain life forms
can live if we look more closely at
temperature it's not only whether it's
hot or cold all the time like a certain
distance from the poles or near the
equator but what the range of the
temperatures might be it can be very
cold now
and very warm during the day or vary by
season temperature can change with
elevation it gets colder as you go up
mountains organisms that prefer those
different temperature regimes as we call
them have specific adaptations for that
regime another thing about these
physical factors is that they can be
actual barriers to the movement of
organisms they can physically restrict
organisms to a particular place mountain
ranges even rivers can be barriers in a
terrestrial environment some organisms
can't cross them there are even barriers
in the ocean fresh water outflow say
from the Mississippi River makes a
barrier different ocean species that
like saltwater are separated on the east
and west sides of the dangerously fresh
outflow at least dangerous to these
marine animals anyway or islands they're
isolated by the water around them a
tortoise living in the Galapagos for
example wouldn't be able to get back to
South America or some other landmass
without crossing the ocean and tortoises
are pretty good at floating in the ocean
but no one would argue that it's their
preferred way of life so the ocean is a
barrier to movements of many terrestrial
organisms we're actually talking here
about something called Island
biogeography which a lot of scientists
love to look at because it provides a
laboratory in which to study evolution
we can study how the barriers change the
properties and ranges and distributions
of different organisms that are
restricted to living on the islands
because the ocean and even large lakes
are such good barriers to dispersal
let's talk now very briefly about how
biotic factors living factors influence
the distribution of other life here's an
example there are places in the ocean
that are essentially deserts where
there's low productivity low biomass low
biodiversity without high species
richness or large populations of
plankton things like whales are going to
have a hard
likewise on land no grass no antelopes
those are very simple examples of how
one type of life can dictate where other
kinds of life might have their preferred
habitats another great example going
back to the whales this is what I love
is people think of whales as these sleek
things that are going through the water
at high speeds but when you look closely
at the front end of a humpback whale
there's an entire little ecosystem
living there
everything from barnacles to these well
you can imagine whales are big so
they're lice are big too so they have
this type of crustacean these these lice
that live in between the barnacles and
all over the whale if you had lice that
big you'd probably notice but the whales
just seem to deal with it basically the
host whale is the preferred habitat for
these parasites the range of the lice is
going to depend on the range of the host
the final factor in the uneven
distribution of life is evolution of
course evolution is not a ladder it's
not a line of change it's actually a
bush as life evolves the pathways of
change make a sequence of branches and
twigs that tell us that life is
constantly diversifying so you can see
how this would very rapidly become a
very complicated overlay on top of the
fact that life depends on other life to
live and the fact that physical factors
can govern we're living things live
chance is another element of change that
feeds into the complicated branching
sequences that lead to the diversity and
location of life on Earth if we go back
to the Galapagos again our tortoises
floated to the Galapagos and were
presented with a habitat that was a
little bit different from the one that
they enjoyed on the South American
mainland like the Rolling Stone said you
can't always get what you want but the
tortoises discovered that it was enough
the tortoises were no longer in their
preferred habitat but some adapted to
the new circumstances they survived
and reproduced that is the population
evolved they arrived at this new habitat
by chance and then adapted to a new home
that eventually became their preferred
habitat okay so we've looked at some
natural factors that contribute to the
patterns of life on the planet now let's
put humans back into the equation we
can't ignore the subject because we
humans are clearly an integral part of
our environment whether we want to be or
not the activities of humans can
restrict ranges of naturally occurring
species and in other cases can actually
increase the sizes of ranges we exploit
organisms we use them for our own
purposes
way back when agriculture was invented
for example we chose plants that we
wanted to grow in order to enhance our
ability to get food we started raising
animals for our own use as well and as
humans move from place to place we
purposefully brought these organisms
with us and those organisms had their
ranges extended estuaries are often
brought up as examples of invaded
environments San Francisco Bay is one of
the most invaded estuaries on the planet
in terms of introduced new species
you're seeing types of organisms that
were introduced by human activity by
accident primarily through the movement
of ships that carried those organisms to
docks and into our harbors now those
organisms find themselves in a place
where they can live and they proliferate
throughout the estuary sometimes a
single species can totally dominate the
habitat which brings us to the question
of huge biomass but low species richness
we love to grow crops but what we like
to do with our crops is to make sure
it's only that one species that grows in
those many thousands and thousands of
acres or hectares to be provocative I
often say that the single greatest
environmental change to be perpetrated
on the North American continent
is corn because corn is grown in a
monoculture nothing else is allowed to
grow with the corn nothing that eats or
competes with the corn is allowed to be
there so the species richness of the
millions of acres of corn field across
the u.s. is extremely low about as low
as you can get there's high biomass sure
but what was there before we planted all
that corn much higher species richness
those prairies were very bio diverse
that richness is preserved in some small
pockets here and there but you can
obviously see that the biomass of those
wild populations is much lower and their
ranges much smaller humans can do other
things in terms of decreasing the sizes
of organisms ranges simply by increasing
their own range humans can change and
shrink the ranges of other organisms we
build roads and big highways that can
form barriers to natural movements of
organisms we're also changing ranges by
changing the watercourses of certain
rivers we can change entire habitats by
making a lake where a river once existed
decreasing the number of species and the
sizes of the ranges of the organisms
that used to live along those river
channels human activity changes where
life on Earth finds itself and where it
can go sometimes more than natural
factors or evolutionary chance - we do
it by accident we do it on purpose but
there's no denying our enormous impact
on how biodiversity is distributed on
earth