My name is Jim Estes.
I am a professor of ecology and evolutionary biology
at the University of California in Santa Cruz,
and I'm delighted to be here today to be able to
tell you about some of the things that my colleagues and I
have learned over the last few decades
about the ecological function of apex predators in nature.
If you look around you in the world,
wherever you're living,
you're going to see a species that we would call an apex predator,
and an apex predator is nothing but a large animal
that feeds on other species
that exists at what we call the top of the food web,
that is, there are no other species above it that feed on it.
These are some pictures of apex predators
that are part of our recent world, but are now extinct.
Some of them you'll recognize.
In the upper-left-hand corner is a picture of a species
that went extinct about 10000 years ago,
the dire wolf.
In the middle you'll see a big bear.
That's a short-faced bear.
That also went extinct not too long ago.
And toward the bottom of the illustration
are some species of things that we would call apex predators
that were around many, many hundreds of millions of years ago.
So, let's take a look at the history of the Earth.
Let's go back four and a half billion years
to the beginning of the Earth
and look at the history of life and when these creatures
that we call apex predators showed up.
Well, life started not long after the Earth was created,
maybe a billion years or so,
things that we refer to or we recognize as life appeared.
But it wasn't until about half a billion years ago that
a major event happened in the evolution of life
that was very important and that was called
the Cambrian Explosion.
The Cambrian Explosion was a very important time
in the history of the evolution of life
because all of the major life forms,
as we currently know them,
either from existing species or from the fossil record,
appeared right at that time,
including apex predators.
So, apex predators have been part of nature
for the last half a billion years.
They've been all around us.
So, this leads to two questions.
One is the question of why,
and that is a question for evolutionary biology,
why being,
why is it that all of a sudden, half a billion years ago,
all of these large apex predators
appeared on the Earth.
The other question is the question of,
do they really matter?
Would the Earth be a different place
had these animals not appeared in our fauna?
And that's a question for ecology
and that's going to be the focus of my lecture today.
So, I'm going to divide this lecture into three parts.
The first part, which I'm in the middle of,
or at the beginning of right now,
is going to be a focus on approach.
How do we answer these questions?
Part II is going to... the second module...
Part II is going to focus on one particular species and ecosystem,
and that is sea otters and kelp forests,
and the reason that I'm doing this is because
that's what my laboratory has been focused on
for the last half-century.
And then in Part III I'm going to look beyond sea otters and kelp forests
to other species and ecosystems
and ask the question:
to what degree are the things that we have learned
about sea otters and their ecosystems
applicable to other species and other systems around the world?
So, for Part I of the lecture,
there are really two major issues that I'd like to discuss.
One is that I need to provide you with
a conceptual template of what it is
that we're looking at.
We need some notion of both the
structure and function of nature
before we can ask questions about how nature works.
And the second is,
how do we actually go about testing hypotheses
that pertain to these purported functional relationships
that involve large predatory animals
or large apex predators?
So, first, the conceptual template,
and the conceptual template has two pieces.
One is what we call structure --
how is nature put together? --
and the second is what we call process --
and that is, how do these pieces that form nature
work when they're all together?
Alright?
So this is a problem.
Uhhh... structure...
we use what we call a food web
to conceptualize the structure of nature,
and on the left of this slide you'll see
one of the world's simplest food webs.
This is a food web that was put together
for the cod system in the northwestern Atlantic,
and what you'll see in this particular illustration,
if you kind of squint your eyes and look at it,
because there's a lot of material there,
are what we call nodes and then linkages between those nodes.
The nodes are species
and the linkages are the interactions between species,
and in this particular case
what we call predator-prey or consumer-prey interactions.
Species feed on other species
and this is the way nature is put together.
And a simple look, or a quick look,
at this simple food web
will immediately emphasize the problem
of understanding these systems
and that is that they are monstrously complicated.
So these are difficult systems to work in
simply because of their complexity.
So, as ecologists, one of the things that we do
is we try to simplify this complexity.
We need to, what I call, modularize the system
or, that is, pick out the species and linkages
that really matter to study.
You can't study them all, there's just too many,
so modularization is one of the tricks
to understanding these systems.
The next point that I want to make
that falls into this conceptual template
is the nature of the interactions
between consumers and prey.
The interactions between consumers and prey
can be either what we call direct,
in which case there are no intervening species
-- a prey is eaten by a consumer --
or indirect,
in which case there are one or more interceding species.
So, for example,
a wolf might eat an elk
and an elk might eat plants,
and therefore the wolf might be having effects on those plants,
indirectly, by way of elk.
And you can easily imagine that
there are lots of potentially very complex
indirect interactions in nature.
So keep that in mind as we go forward.
As we think about these interactions,
the next point that is important to conceptualize
is that the strength of those interactions
might vary tremendously,
and I've tried to illustrate that in this illustration
between two directly interacting species, A and B
-- A might be the predator, B might be the prey...
A might be wolves, B might be elk,
A might be sea otters, B might be sea urchins...
and that interaction can either be what we call
a strong interaction or a weak interaction
-- and I've tried to indicate that by simply
the variation in the weights of the lines
between these two particular cartoon illustrations
of the linkages.
A strong interaction is one where
the interaction has a big effect --
A might have a very strong effect on B
or B might have a very strong effect on A.
A weak interaction would be structurally the same
but the interaction consequences would be lesser.
Focusing now on process,
how do we understand the consequences
and the strengths of these linkages
between two interacting species, a predator and a prey?
The way that we do that is by perturbing the system.
It's the only way that we can really see into this
complex milieu of interactions among species.
So we would either add a species
or we would take a species away,
and we would look at how the consumer-prey interactions changed
and how the food web in general responded
to that change in direct interactions
through the various indirect interactions that might follow.
I'd like to raise two further details
that are important to our thinking
about the nature of food web dynamics.
One is what we call the directionality of forcing
and the other is what I would call
functional relationships.
So, firstly, directionality of forcing.
If we imagine a consumer and a prey,
and that consumer might be an elk and the prey might be a plant,
the consumer might be a wolf and the prey might be an elk,
the consumer might be a sea otter and the prey might be a sea urchin,
or the consumer might be a sea urchin and the prey might be a plant...
alright?...
these two species can interact with each other
in two fundamentally different ways
in terms of the consequence of those interactions.
One is what we call bottom-up forcing.
In the case of bottom-up forcing,
it is the prey that really is
controlling the abundance and behavior of the consumer,
so changes in prey availability and prey abundance
are going to impact the consumer in a similar way.
The other is what we call top-down forcing.
In top-down forcing,
the consumer is having an impact on its prey
by the very fact that it eats it, alright?
Now, in nature, the interactions between consumers and prey
involve both of these forcing processes --
bottom-up and top-down.
When we look at nature
with the question in mind of the importance of apex predators,
we're really interested in top-down forcing,
and so much of what I'm going to be telling you about subsequently
is going to focus on this dimension to consumer-prey interactions.
The last piece of this is
what I would call functional relationships,
and that is, how is it that
a species that is affected by one other species,
let's say species A and B again, here,
let's say we're talking about elk and wolves,
how does the abundance and the functional importance of elk
in a system change
as the abundance of its predator
goes from being absent to very high?
So, that might be a linear change,
so there would be a gradual change in the abundance of elk
as the abundance of wolves changed.
It might be a nonlinear change,
so for example in the bottom-left panel here,
we can see a situation where
there's really a changing abundance of species B,
which in this case we're going to call the wolf,
that there would be really relatively little effect on the food web
until we reach some particular critical point,
and then we would have what we call a phase shift,
a very strong nonlinear transition.
The third dimension to this is
the process that we call hysteresis,
and hysteresis occurs when these functional relationships
that are strongly nonlinear
track along different lines
depending upon whether directionality of forcing,
that is, rather species B is becoming more abundant or less abundant,
is on the increase or on the wane.
So, when we have these functional relationships
that track along different functional relationships
depending upon the directionality of change,
we have a system that is characterized by hysteresis.
Obviously, nonlinear relationships and hysteresis
make our task of understanding species interactions,
and thus the importance of apex predators in nature,
more challenging.
So, in Part I of the talk,
I've gone through really the conceptual foundations
of what it is that I want to get across to you today.
We've talked about large-bodied predators
and that they have been part of our living world
for the last half a billion years.
I've talked about the concept of a food web,
a food web being the way in which species
interact with one another
through consumer-prey interactions,
and the food web being really
nothing more than a roadmap
of who is eaten by whom in nature.
I've told you that to understand and study these complex food webs,
it's often necessary to modularize or simplify them,
that is, to pick a series of species or processes
that we can manage and deal with,
because of the tremendous complexity
of virtually all food webs in nature.
I've told you a little bit about the nature of species interactions
and I've dichotomized
the way in which these processes work
between what I call bottom-up forcing and top-down forcing.
I've told you about two different ways
of thinking about species interactions,
that is, direct interactions
in which there are no interceding species
and indirect interactions
where a species might have an effect on some other species
by way of one or more intervening species.
I have argued that the way in which we understand this dynamic process,
and the only way that we can really see this in nature,
is by perturbing species.
By perturbing species,
we see the consequences of species interactions
that are simply not possible to see otherwise.
And at the end I told you
a little bit about some of the complexities of understanding
the functional relationships between
the abundance of a species and its effect on nature,
and we have discussed situations
where that might vary linearly,
we talked about the possibility that these things vary nonlinearly,
where we have what we call very abrupt phase shifts,
where we see virtually no effect over varying ranges
of particular species abundance
and then a very strong effect,
and we've talked about hysteresis,
which is the notion that the directionality or form...
or the form of these functional relationships
varies with directionality.
So, that gives you a conceptual foundation
for where I'm going to be going for the rest of this talk.
In Part II, I'll be talking about
the work that my lab has done on sea otters and kelp forests
over the last several decades,
and in Part III I'll be expanding the perspective
to other species and systems in nature.
Thanks very much for your attention
and I look forward to rejoining you for Part II of the talk.
Thank you.