(light mystical music)
- [Instructor] For this video,
I want to focus on the idea of dispersal.
How the ranges of species can change,
and how this can affect biodiversity over time.
As we've seen elsewhere,
the classic model called the vicariance model
proposes that a mother species can be split
into daughter species
when barriers arise from changes in geology,
or climate, or habitat.
Sometimes all three at the same time.
These barriers can lead to divergences resulting
in speciation through the restriction of gene flow.
However, what we'd like to focus on here
is that there's another way to really accentuate
the effects of restricted gene flow,
and that's something called dispersal.
This is basically the way that species of plants,
animals, and other organisms expand their ranges,
their distributions on Earth,
through movements of individuals that increase
the sizes of the ranges of populations,
and, therefore, the ranges of the species themselves.
Dispersal can also lead to speciation.
Species like plants that have a rooted to the ground
or sedentary habit,
like I sometimes have when there's a hockey game on.
Even plants have dispersal stages in the form of seeds
that can be distributed in air, or in water,
or even in and on other organisms.
Bird migration is an obvious dispersal mechanism.
Bird movements can easily result in the establishment
of new populations of a species
where they didn't exist before.
But did you know that spiders can also disperse
through something called ballooning?
Young spiders especially can release fine, silk threads
that are caught by the wind,
carrying the spider aloft and to new territories.
There are many other examples of dispersal.
The spores of fungi that blow in the wind,
and get up my nose,
and give me allergies,
or a sneeze full of bacteria or viruses.
Even that's a dispersal technique
that has evolved among microbes
that increases their ranges during cold and flu season.
Things like corals, sea urchins, and snails also disperse.
Mostly they do this during the earliest stages
of their lives drifting through the water as little larvae.
These larvae can be carried significant distances
and when they eventually settle down on the substrate,
and metamorphose, or change,
from the juvenile dispersing stage
into a small version of the more sedentary adult,
they establish new populations
and expand the species' ranges.
Logs, or pieces, and mats of vegetation
that wash into the ocean from land
can harbor terrestrial organisms that go along
for the ride and are taken out to sea.
The may drift to new places to live,
and if they land and are successful,
this can result in the establishment of a new population.
All these events in nature make it worth asking,
"What happens at the end of these olympian journeys,
"these organismal odysseys?"
If the conditions are right
and the organisms can continue to survive,
a new population can be established in a new place.
The ability to survive and reproduce
in this new place is key.
In the case of sexual species,
individuals need to be carrying viable young
when they arrive,
or find a member of the opposite sex
with which to produce new generations.
If you think of a coconut,
which can travel hundreds of miles floating in the ocean,
washing up on a bare lava shore is gonna have
a much harder time of it than if it washes up
on a sandy beach.
In other words,
the conditions have to be right for the establishment
of a new population.
In general, the greater the distance between
a new population and the original population,
the more likely the gene flow will be restricted
between the two populations,
and the more likely the two populations
will diverge from each other.
And this takes us to the idea of isolation.
Isolation can be very obvious on islands,
but it's interesting to remember not all islands have
to be in the middle of a body of water.
You can have isolation among oases in a desert, for example.
You can have isolation on the tops of mountains
or in the valleys between the mountains,
Or you can have isolation in a fragment of rainforest
that's surrounded by extensively clear cut land.
And these habitat islands can exhibit
the same principles of isolation and restriction
of gene flow that influence speciation.
Amazing patterns of speciation can emerge
in all of these systems,
because there are some pretty basic rules
that emerge logically from thinking about islands,
and dispersal, and isolation
leading to an entire field
of study known as island biogeography.
One of these rules is that islands can be harder
or easier to get to depending on how far away the are.
This is known as the distance factor.
Second, the longer the island has been in existence,
the more likely it is that organisms
have already arrived there,
and the longer they have had to diverge
from their parent population.
That's the time factor.
A third concept is that the smaller the island,
the less likely it is for a species to get there
in the first place.
That would be called the area factor.
Fourthly, diverse environmental conditions
on an island can enhance the island's biodiversity
because there's a greater chance that the right climate,
the right ecological resources will be present.
And we can refer to this as the habitat factor.
A fifth logical rule concerns the location of the island
with respect to things like currents and winds
that allow new energy, in the form of nutrients,
to flow into the system,
supporting ecosystem functions,
or increasing or decreasing the likelihood
of new colonizers arriving,
and I would call that the flow factor.
A sixth factor is just chance and random events
that also play a big role.
I'm not sure there's a name for that one,
so I'm calling it the serendipity factor.
An example of that might be a freak storm
that carries organisms with it.
You might think of other factors or tweaks
to these basic rules.
For example, if you throw in the fact
that organisms differ greatly in their ability to disperse,
you have a rich and complicated overlay
of things influencing the biodiversity
on any particular island.
And why that biodiversity can be so different
from one island to the next.
Here's a simplified graph that illustrates
a couple of these factors.
You've got basically two sets of curves.
One set refers to how close or far
an island might be to the mainland,
and how that affects the rate of colonization.
And the other set refers to whether the islands
are large or small,
and how that's related to the rate,
or probability, of extinction.
The horizontal axis represents increasing species richness.
So you have a couple of interesting
and important intersection points
that mark the lower richness of a small distant island,
compared to that of a large nearby island.
This graph incorporates a couple of other things.
One is the balancing of colonization and extinction.
The more crowded the island becomes,
the more likely it is that extinctions will happen.
It also summarizes the idea that large islands
close to the mainland source of new populations,
places like, say, Madagascar,
will have lots and lots of species.
But remember that on a big island like Madagascar
we have the habitat factor, too.
Populations can disperse on the island itself,
find new habitats,
encounter new barriers,
and all kinds of new species can arise.
And if you look up what's going on in Madagascar,
that's definitely true.
Madagascar has lots of endemics.
Species that arose there and nowhere else.
Also, the geologic evidence is strong
that Madagascar broke free from Africa in the past,
carrying with it subsets of species
that existed on Africa,
and then continued to diverge and evolve on Madagascar.
In contrast, consider the Galapagos.
This archipelago of islands is relatively far
from any mainland,
and sprung up there through volcanism.
These remote islands had almost no life on them
when they first appeared.
There were fewer species arriving there,
but because of the multitude of islands
within the archipelago,
there are subsequent speciation of populations
that make it to one island,
and then island hop from there.
Lots of island groups illustrate these ideas.
Hawaii and the Philippines, for example.
In any of those island groupings you can see
all of these overlapping factors at work.
It's a grand and beautiful view,
I think, of how islands can foster
the formation of new species.
But the graph can also tell ya something about
why so many island species are in such trouble.
The island biogeography curve summarize that as well,
because we've got this word in there, extinction.
Many island populations are vulnerable to extinction.
I've been talking about all of this
with no humans involved,
but if you put humans in the equation,
drop them into that island ecosystem,
who's to say what the dimensions of the effects
will be down the road?
How do the curves get changed by human activity?
What happens to the extinction curves?
How steep will they be?
How big are the effects that humans have
on these systems of endemic species?
Scientists who try to gather data
to make these curves more precisely understood,
also know that these human effects can be big,
especially on islands.
Speciation, that incredible generator
of Earth's biodiversity,
is not really keeping up.