Session 9: Coevolution

Transcript of Part 3: Apex predators: Sea Otters and Kelp Forests

00:07.2	My name is Jim Estes,
00:09.2	I am a professor of ecology and evolutionary biology
00:11.1	at the University of California in Santa Cruz,
00:13.1	and I'm very happy to be here today
00:16.0	to be talking to you about the ecological function
00:19.0	of apex predators in nature.
00:20.2	In Part I of my lecture,
00:22.2	I provided a conceptual foundation
00:24.2	for what the questions are
00:26.2	and how we have gone about trying to answer those questions.
00:28.3	In Part II, I'm going to give you
00:30.3	a particular case study.
00:32.1	I'm going to focus on sea otters and kelp forests,
00:34.1	and I'm doing this because
00:37.3	this is work that my lab been engaged in
00:39.2	for the last several decades.
00:43.1	So, this is an outline for Part II of the lecture.
00:46.1	Firstly, I'm going to explain or describe to you
00:50.1	the food webs, the key species.
00:52.1	Second, I'm going to tell you a little bit about the approach
00:55.0	that we have used to understanding the importance
00:58.1	of sea otters in these kelp forests ecosystems.
01:00.2	And then, lastly, I'm going to run through some of the findings
01:02.2	from the work that we have done.
01:05.2	The food web: sea otters are the apex predator
01:09.0	in many coastal kelp forest ecosystems
01:11.1	in the North Pacific.
01:13.3	Sea otters feed on sea urchins;
01:15.1	sea urchins feed on kelp;
01:17.0	kelp is what we call a foundation species,
01:18.3	that is, many other species in the ecosystem depend upon it,
01:22.0	either for habitat or for food or for primary production,
01:25.3	so many other species are linked into the ecosystem
01:28.3	by way of the kelps themselves.
01:32.0	The approach:
01:34.0	how have we gone about trying to understand
01:35.2	the importance of sea otters
01:37.1	in these coastal kelp forest ecosystems?
01:39.2	It's actually very simple.
01:41.1	We have used history as
01:44.2	a large natural experiment.
01:46.1	So, the blue line in this illustration
01:48.3	shows the range over which sea otters
01:51.2	historically have occurred in the North Pacific Ocean.
01:54.1	They were abundant across that range
01:57.1	until about the mid-1700s.
02:00.0	In the mid-1700s, the Pacific Maritime Fur Trade began
02:05.1	with the discovery of the New World by the Russians
02:08.1	and the discovery of vast fur resources,
02:11.1	and in particular sea otter resources, across the North Pacific,
02:13.2	and that began a long period of overexploitation
02:17.3	and extinction of sea otters throughout most of their range.
02:23.1	This illustrates the nature of the Pacific Maritime Fur Trade
02:27.1	and emphasizes that the Pacific Maritime Fur Trade
02:30.2	was the perturbation that we have used
02:33.0	to understand dynamic processes in this system.
02:36.2	The blue line is shown again on this slide,
02:38.3	that shows the historical range of sea otters,
02:41.0	and the little red dots are the locations
02:43.2	of known surviving colonies at the end of the fur trade,
02:48.1	which was over in the early part of the 20th century.
02:51.1	At the beginning of the fur trade,
02:53.0	there were probably at least a million sea otters
02:55.1	across the North Pacific Ocean, possibly many more.
02:58.1	
03:00.2	By the end of the fur trade, there were well under a thousand,
03:03.1	and those few remaining animals occurred
03:06.1	in a couple of isolated colonies.
03:08.0	These colonies were protected by international treaty in 1912, 1913,
03:15.1	right around that time,
03:17.0	and they began to recover.
03:18.3	Subsequently, animals were taken from these recovered colonies
03:21.1	and they were relocated to other parts of the historical range
03:25.0	in an effort to repatriate the species
03:28.1	throughout its natural environment,
03:29.2	and that's shown by the green dots, here.
03:31.2	So, the red dots and the green dots
03:33.2	represent places that otters have been reintroduced,
03:36.1	and all of the places in between
03:38.2	are places where they once occurred but no longer did occur.
03:43.0	All we have done is simply compare the places where they occur
03:46.0	with the places that they don't occur,
03:47.3	and we have watched places that have become recolonized
03:50.2	and how those ecosystems have changed
03:53.0	as otter numbers have built up through time.
03:56.1	These are the locations where the work has been done:
03:58.3	the Aleutian archipelago,
04:00.2	Southeast Alaska and Vancouver Island,
04:03.1	and, as I mentioned before,
04:06.0	simply, what we have done, very simply,
04:08.0	is look at areas with and without sea otters within these large areas
04:11.2	and we have contrasted places through time
04:13.1	as the abundance of otters have waxed and waned.
04:17.2	So, what are the findings?
04:21.2	Very simply, if you look at places where otters are abundant,
04:24.1	what you see are abundant kelps
04:27.1	and relatively few sea urchins,
04:28.2	and if you go to nearby places where otters
04:31.0	once were abundant but are now gone,
04:32.2	you see abundant sea urchins and virtually no kelps,
04:36.1	as shown on the right here.
04:37.3	Amchitka island... these photographs were taken in the early 1970s...
04:41.3	Amchitka island was a place where otter populations
04:44.1	had recovered to what we think was their natural historical abundance.
04:47.1	Shemya island, which is several hundred miles
04:50.2	to the west of Amchitka island...
04:53.0	otters were exterminated on Shemya
04:54.3	and they had not repatriated or recovered on Shemya
04:58.0	at the time that these photographs were taken.
04:59.2	So, you can see visually here, very clearly and simply,
05:02.2	what the differences are between a system
05:05.2	with and without sea otters.
05:07.2	So, what's going on here?
05:08.2	What is going on here is what ecologists call
05:11.0	a trophic cascade.
05:12.1	That is, otters are having a top-down limiting effect on urchins,
05:16.1	urchins are having a top-down limiting effect on kelps.
05:20.2	By adding otters into the system,
05:22.3	it removes the urchins or reduces the urchins,
05:25.0	thus releasing the kelps from control by urchins,
05:29.0	so we have, in a system with otters, abundant kelp,
05:33.0	and we have, in a system without otters, abundant urchins
05:35.2	and relatively few kelps.
05:39.2	So, that's it, but what are some of the broader implications of this?
05:43.1	Well, the broader effects of this,
05:46.0	as I'm going to tell you about today,
05:48.1	largely spin off kelps and what happens
05:51.1	when we have systems with and without the kelp forests,
05:53.1	and I'm going to tell you about
05:55.2	three or four little vignettes of effects
05:58.1	that spin off of this trophic cascade
06:01.0	that are what we call indirect effects of the trophic cascade.
06:03.2	I'm going to tell you about how that is manifested in the abundance of fish.
06:07.0	I'm going to tell you about how it is manifested
06:10.1	in the behavior of various other consumers.
06:14.2	I'm going to tell you about how it influences
06:17.1	what we call the primary production of the coastal system.
06:21.0	And I'm going to tell you how that feeds back,
06:22.3	or has an influence,
06:24.2	on the physical environment through the sequestration of carbon dioxide
06:28.2	through the process of photosynthesis.
06:33.0	These are some data from a study
06:35.1	that my colleagues and I conducted several decades ago
06:38.3	in which we took baby mussels --
06:43.0	mussels are what we call filter feeders,
06:44.3	and these are young mussels
06:47.1	that were grown out at Friday Harbors Labs,
06:49.0	the University of Washington,
06:50.2	translocated to the Aleutian Islands,
06:52.1	and outplanted to islands with and without sea otters,
06:55.2	and thus to islands with and without abundant kelp forests.
06:57.2	And what you can see here is
07:00.1	the difference in growth rate over a one-year period,
07:02.3	the dark bar showing the growth rate
07:06.2	of these filter feeders in places where otters were abundant
07:08.3	and the light grey bar showing
07:11.2	comparable growth rates and comparable habitats
07:13.1	where otters are absent,
07:15.0	and what you can see from this is the growth rates of these filter feeders
07:17.1	are about twice as high where otters are abundant
07:20.0	compared with where they're absent.
07:22.1	Why is that?
07:23.1	It's simply because the primary production,
07:25.0	the abundance of autotrophs, these photosynthesizing kelps,
07:27.3	is much higher in systems with otters
07:30.0	than in systems without otters,
07:31.3	and therefore this consumer, the mussel in this case,
07:34.2	that is eating material
07:37.1	that is being provided by the primary producers,
07:39.2	grows more rapidly where sea otters are abundant compared with where they're absent.
07:43.2	These are data on the abundance of fish,
07:45.2	again, the dark bar showing information
07:48.1	from where sea otters are abundant,
07:50.1	the light bar showing comparable information
07:52.0	from places where they're absent,
07:53.3	and what you see from this is that
07:56.0	the measures that we have of fish abundance
07:58.1	indicate that they're almost an order of magnitude greater
08:01.0	where otters are abundant compared with where they're absent.
08:03.2	So, simply because these animals depend upon kelp
08:07.0	for habitat and food,
08:09.1	kelp is more abundant where otters are more abundant,
08:10.2	and thus fish are more abundant where otters are more abundant.
08:14.0	These are some data on the diet of
08:18.1	another species of consumer in these coastal ecosystems.
08:20.1	In this particular case,
08:21.2	a seagull, Glacous-winged Gull,
08:23.2	and let me just explain a little bit about what the data show.
08:27.0	In this particular panel, what I'm showing you is
08:30.1	the relative proportion of fish versus invertebrates
08:34.0	in the gulls' diet
08:36.0	between places where otters are abundant,
08:37.3	those are the black data, the black bars,
08:41.0	and places where otters are absent,
08:43.2	those are the light bars, the grey bars.
08:45.1	And what you can see is that when otters are abundant
08:47.3	these gulls feed almost entirely on fish;
08:49.2	when otters are lost from the system
08:52.0	they forgo feeding on fish to feed on the now more abundant invertebrates.
08:57.1	These are some comparable data
08:59.2	on the diet of another consumer in the system,
09:01.1	in this particular case, the bald eagle.
09:03.1	And what you can see here again,
09:05.1	by looking at places with and without otters,
09:07.1	is that eagles eat a roughly even mix of
09:11.2	marine mammals, seabirds, and fish
09:14.2	when otters are abundant,
09:15.3	and when otters are lost from the system
09:17.2	the proportion of marine mammals
09:20.1	and the proportion of fish in their diet goes down,
09:21.3	and the abundance of seabirds goes way up.
09:24.0	So, the overall composition of the major things that they feed on
09:28.1	is linked to the effects of sea otters on this coastal ecosystem.
09:32.2	Lastly, I want to tell you a little bit about
09:36.0	some recent work that we've done on the potential sequestration effect
09:39.0	of this trophic cascade,
09:40.2	that is, the enhancement of primary producers by sea otters,
09:43.3	on atmospheric carbon dioxide.
09:46.0	And why would we think that to be an interesting thing to look at?
09:49.1	Because carbon dioxide is the material that fuels photosynthesis.
09:52.3	If you have a plant species in the system that's more abundant,
09:57.1	one might expect that the rate of photosynthesis
09:58.3	is going to be higher,
10:00.2	and therefore the drawdown of carbon dioxide from the environment
10:03.3	and the surrounding oceans is going to be greater.
10:05.2	So, when we look at this, we find that in fact
10:08.1	the sea otter effect is substantial,
10:10.1	that sea otters are responsible...
10:13.2	a system with sea otters will draw down
10:16.0	about 10% of the overlying carbon dioxide in the atmosphere,
10:19.3	compared to a system without sea otters.
10:22.1	Or, if we put this in a slightly different context
10:24.2	and asked the question,
10:26.2	how much of the increase in atmospheric carbon dioxide
10:29.2	that has followed the onset of the industrial revolution
10:33.1	might this accommodate?
10:36.1	It's about half.
10:38.1	We can put this into dollar terms,
10:40.2	because carbon is something that's been valued
10:43.1	on what we carbon exchanges or carbon markets,
10:45.2	and when we do that, and these data are based on
10:48.1	the value of carbon in the European carbon market
10:51.3	as of 2012,
10:53.2	we see that, in the areas that I've been working,
10:56.2	the standing biomass effect of sea otters on kelp
11:00.1	is potentially worth somewhere between 200 and 400 million dollars.
11:04.1	And the potential sequestration effect of that,
11:07.2	that is, the amount of that carbon that's being put into the bank,
11:10.3	and in this case the bank is the deep sea...
11:13.1	carbon is translocated in some cases to the deep sea...
11:16.2	depending upon how much of that carbon that's fixed by the kelp
11:21.1	goes into the deep sea,
11:23.1	that value may range anywhere up to almost a billion dollars.
11:27.2	So, now I want to change gears
11:29.2	and talk about evolutionary consequences of these species interactions,
11:32.3	and why would I do that?
11:34.1	Because evolution is a consequence of natural selection,
11:37.3	and natural selection is going to be affected by
11:42.2	the strength of interspecies interactions,
11:44.1	and we see strong species interactions in this system
11:46.3	that are a consequence of these apex predators,
11:49.1	in this particular case a sea otter.
11:53.1	So, I'm going to focus on one particular part of this trophic cascade
11:56.2	that I've told you about,
11:58.2	and that is the interaction between the herbivores and the plants,
12:01.1	and how is it that sea otters may have influenced
12:04.2	the evolutionary dynamics between these herbivores and plants.
12:09.2	We can see from this illustration,
12:11.2	and from what I've told you before,
12:13.2	that in systems with sea otters
12:16.2	the strength of the interaction between the herbivores,
12:18.2	that is the sea urchins in this case,
12:21.2	and the kelps is going to be very weak,
12:24.2	and when otters are lost from the system
12:27.2	the strength of that interaction is going to increase substantially.
12:30.2	So, how might we approach the question of,
12:33.1	what were the evolutionary consequences of this interaction?
12:36.1	The way my colleagues and I have done this
12:38.2	is simply by looking elsewhere in the world,
12:40.3	where there is a kelp forest that evolved in the absence of sea otters,
12:44.2	so we chose to do this in the
12:48.0	southwestern temperate Pacific
12:51.1	in the area of Australia and New Zealand,
12:53.1	which has a very physically similar system to the North Pacific
12:56.2	in that it's a cold water system that has fleshy macroalgae
12:59.1	that are very similar to kelps,
13:00.3	but it lacks a predator on the sea urchins in that system,
13:04.2	and the other grazers, like sea otters.
13:07.2	So, the first thing that my colleagues and I did
13:11.1	to try to get some sense of the changes in the strength
13:13.3	of plant-herbivore interactions between these various systems
13:17.3	is that we measured the rate of tissue loss
13:21.2	in fleshy macroalgae on the sea floor to herbivory,
13:24.3	and the way that we did this was we simply took a kelp plant,
13:27.1	we put it on the sea floor,
13:29.0	we stuck another kelp plant next to it in a cage
13:31.2	from which the herbivores were excluded,
13:33.1	and we contrasted the rate of tissue loss over 24 hours
13:37.2	between these experimental and control,
13:40.1	caged and uncaged, treatments.
13:42.2	The data that you see here are the relative differences
13:45.3	between grazing intensity in different parts of the world.
13:48.1	So, on the very far right
13:51.0	you'll see data from Shemya Island,
13:53.1	which I told you before is a place where otters are absent in the North Pacific,
13:57.1	and what you can see there is that the rate of grazing,
13:59.2	as indicated by the black bar, is very high.
14:03.1	If you go over to the far left-hand side of this graph,
14:07.1	what you will see are comparable data done from the exact same sort of experimental protocol
14:11.1	at Amchitka Island, where otters are abundant,
14:13.3	and there what you see is that the intensity of grazing
14:16.2	is virtually zero.
14:18.3	And then if you go to Australia or New Zealand,
14:21.0	in this particular case these are data from
14:23.2	four different marine reserves in New Zealand,
14:26.1	what you see is something that was very exciting to us,
14:28.1	and that is that the intensity of grazing
14:32.1	is much less than it is in systems
14:35.0	where sea otters are absent in the North Pacific,
14:36.2	but much higher than it is in systems
14:40.0	where sea otters are present in the North Pacific.
14:42.0	In other words, the intensity of herbivory
14:44.1	in these southern hemisphere kelp forest systems
14:46.2	is clearly higher than it is in natural systems in the North Pacific,
14:50.1	where sea otters are present.
14:52.1	That led us to believe that there would be
14:55.1	some sort of coevolutionary response in the dynamics
14:58.2	between the herbivores and the plants in this system.
15:02.2	What might we expect to see as a consequence of that?
15:05.1	Well, we knew going into this that
15:08.0	the most likely way in which marine plants
15:10.3	are able to defend themselves,
15:12.2	or the most well-known way in which marine plants are known to defend themselves against herbivores,
15:17.3	is through chemical defenses.
15:19.2	And in the case of brown algae,
15:22.0	the kelps and the things that are related to them,
15:24.0	the common group of compounds that does this
15:27.1	are compounds called phlorotannins,
15:29.0	and I've drawn the molecular structure here of one of those phlorotannins.
15:33.2	There are many other molecules and, categorically,
15:36.3	they seem to pretty much act the same way
15:39.2	in terms of the way that they affect herbivores in those systems.
15:43.0	So, this is an illustration that shows
15:45.2	the composition of the various different plant species
15:49.2	in the northern hemisphere,
15:51.0	in the panel on the top,
15:52.3	and the southern hemisphere,
15:54.3	in the panel on the bottom,
15:56.2	and what it shows is the proportion of dry weight of these plants
16:00.3	that is composed of phlorotannins,
16:03.1	and what you see, in contrast in these two different areas of the world,
16:06.1	is that the composition of the plants in terms of the phlorotannin concentrations
16:09.3	is radically different between the North Pacific, where they're very uncommon,
16:14.0	and the South Pacific, where they're very abundant.
16:16.2	So, the overall average percent dry weight
16:20.1	of phlorotannins in southern hemisphere kelp species
16:23.0	is about 10%;
16:25.0	in the northern hemisphere it's less than 1%.
16:28.1	The next question that my colleagues and I asked was,
16:31.1	how are the herbivores in these two different parts of the world
16:35.0	reacting to these phlorotannins?
16:37.2	Now, this was a little bit tricky because
16:40.2	there are lots of other differences between the plants,
16:42.2	and so it wasn't simply a matter of looking at rates of grazing.
16:45.1	What we had to do was isolate the compounds
16:48.0	and then subject the herbivores to
16:52.1	diets that varied only in the composition of these secondary compounds.
16:55.3	So, what we did is we took a green algae called Ulva
17:00.2	that every known herbivore in the marine environment likes to eat,
17:03.1	it's very poorly defended and very nutritious,
17:05.2	we freeze-dried this stuff, we ground it up,
17:08.2	and we put it into little agar discs,
17:10.2	and that's what you see here.
17:12.0	So, these agar discs are then the grazing model
17:15.1	that we exposed to the various herbivores.
17:17.2	We were then able to manipulate the concentration of phlorotannins
17:21.2	s in these discs
17:24.1	and thereby look at how the phlorotannins, in isolation of everything else,
17:27.2	was influencing the grazing rate of the herbivores.
17:31.0	And all we did was simply build these discs
17:33.3	using different phlorotannin concentrations
17:35.2	from different plants
17:38.2	and then look at how herbivores in the northern and southern hemisphere
17:41.1	responded to varying concentrations of phlorotannins.
17:45.1	And here's an illustration that shows the results.
17:48.1	So, what you see here is a whole bunch of data,
17:51.0	it's fairly complicated in terms of there's a lot of material here,
17:53.2	but keep in mind that the big 'NS' values
17:57.1	are indicative of no effect of the phlorotannins,
18:00.0	and the little stars indicate that there was a significant deterrent effect.
18:04.3	So, all of the data on the left side of the vertical line in the middle, here,
18:09.1	are data from herbivores that came from the northern hemisphere,
18:13.0	and all the information you see on the right hand side of the panel
18:16.1	is from herbivores from the southern hemisphere.
18:20.1	And you can immediately see from this that
18:23.2	regardless of whether the phlorotannins came from southern hemisphere algae
18:27.2	or northern hemisphere algae,
18:29.1	they were deterrent to northern hemisphere herbivores
18:32.0	but largely undeterrent or nondeterrent
18:34.2	to southern hemisphere herbivores.
18:36.2	So, I've just encapsulated this in a simplification of that illustration
18:41.3	to show that in the North Pacific
18:44.1	we see a strong deterrence effect of phlorotannins,
18:48.3	regardless of where the phlorotannins are from,
18:52.1	whether they come from the northern hemisphere or the southern hemisphere,
18:54.1	and in Australasia we see either a weak deterrence or no deterrence effect
18:58.2	regardless of where the phlorotannins were from.
19:01.2	So, based on all of what I have told you,
19:03.3	we have come to this view of the coevolutionary dynamic consequences
19:09.0	of sea otters in the North Pacific.
19:11.0	In the southern hemisphere,
19:13.3	we have a two trophic level system,
19:15.3	we don't have an ecological analogue of the sea otter.
19:18.2	As a consequence, the intensity of herbivory on the plants is high.
19:22.2	As a consequence of that,
19:25.0	the plants appear to have evolved defenses in the form of high concentrations of phlorotannins,
19:29.0	and as a consequence of those high concentrations of phlorotannins
19:32.1	the herbivores, in turn, have evolved resistance.
19:35.2	So, we've had a strong coevolution
19:38.2	of defense and resistance
19:40.2	in the south hemisphere kelp forests.
19:42.3	In the northern hemisphere, we've had a third trophic level
19:45.3	in the form of the sea otter.
19:47.2	The sea otter reduces the herbivore,
19:49.3	thus it breaks this coevolutionary arms race,
19:52.1	and as a consequence what we see are
19:55.2	really poorly defended plants
19:58.1	and we see herbivores that have not developed an ability
20:01.0	to resist those defenses because they've never had to.
20:06.2	So, I've made the argument that there has actually been
20:10.0	important coevolutionary going on in this system
20:12.1	that's a consequence of an apex predator.
20:14.2	What are some of the ecological spinoffs
20:16.2	or other effects that this might have had
20:18.3	on other species and patterns in the system?
20:21.1	I'm going to spend a little bit of time telling you about that now.
20:25.1	So, this is an illustration that shows
20:28.1	the co-occurrence of the abundance of plants,
20:31.1	that is, kelps, and herbivores,
20:33.3	that is, urchins,
20:36.0	in both the northern hemisphere and the southern hemisphere.
20:38.1	So, the panel on the top are
20:41.2	data from Southeast Alaska in the northern hemisphere.
20:43.1	The panel on the bottom are data from New Zealand.
20:47.1	In the upper panel, the filled symbols
20:50.1	are data from places where sea otters are abundant;
20:53.0	the open symbols are data from places where sea otters are absent.
20:57.2	And what you can see in the northern hemisphere is that
21:00.1	when sea otters are present
21:02.1	the abundance of sea urchins is very low
21:04.2	and the abundance of kelps is high,
21:06.2	and when sea otters are present...
21:09.0	or, I'm sorry, absent,
21:11.2	the abundance of kelps is very low
21:13.2	and the abundance of sea urchins is very high.
21:15.2	And in this graph, which we call a state-space diagram,
21:19.0	which shows the abundance of two co-occurring species
21:20.2	in relation to one another,
21:23.0	all the data points occur at the perimeter,
21:25.1	that is, they occur in this sort of hyperbolic relationship
21:27.2	along both of the two axes,
21:29.1	but you never see any place in the North Pacific
21:32.2	where both urchins and kelps co-occur in high abundance,
21:37.2	or very few, and in these particular sites where we sampled, none.
21:40.3	If you go to New Zealand and make the same measurements,
21:43.2	you see a radically different pattern in terms of
21:46.2	the way in which herbivores and plants live together,
21:48.2	and you can see this from the symbols in this illustration.
21:51.3	The symbols are not important,
21:54.0	all that's really important for you to note here
21:56.0	is that this state-space diagram is populated extensively
21:59.2	by points where both herbivores and plants are abundant.
22:03.1	So, this, we believe, is one of the ecological spinoffs
22:06.3	of this coevolutionary arms race that I told you about
22:10.2	that is driven by the existence of sea otters in the North Pacific Ocean.
22:15.1	What about other species?
22:17.0	How might they have been affected?
22:19.1	We aren't terribly sure of this,
22:22.0	but one example that is very intriguing has to do with
22:25.0	what we call the Hydrodamaline sirenians.
22:27.0	So, the Hydrodamaline sirenians
22:29.2	are a lineage of manatee-like or dugong-like creatures
22:33.1	that are fairly closely related to tropical dugongs
22:36.3	in the tropical Pacific.
22:38.2	So, dugongs, as you may know,
22:40.3	as sea grass feeders, and they live in the tropics,
22:43.0	but with the cooling of the poles,
22:46.1	what happened was that a lineage of that family of dugong and sirenians
22:51.1	formed what are called the Hydrodamalines,
22:53.2	and the Hydrodamalines radiated into the North Pacific
22:56.1	and they became kelp feeders.
22:58.1	And what's interesting about this particular radiation of mammals
23:02.0	is that the only place in the world that it occurred
23:05.1	is in the North Pacific,
23:06.3	and the only place in the world where we have flora
23:09.0	that seems to be good for herbivores to eat
23:11.1	is also in the North Pacific.
23:13.2	So, we have imagined, and the argument can be made,
23:16.0	that the evolution of the Steller sea cow was, in fact,
23:19.0	a consequence of the fact that sea otters occurred in this system
23:21.2	and created a food resource that made it possible for them to do this.
23:26.2	Another group that's interesting in this context are the abalones.
23:29.0	The abalones are an old group of gastropod mollusks,
23:33.1	and here are pictures of abalones,
23:35.1	and one of the most interesting thing about abalones
23:37.2	is the tremendous variation in maximum body size across species.
23:42.2	So, you see that here in this illustration.
23:44.0	The figure on the left is a tropical abalone
23:46.2	or a warm water abalone,
23:48.2	Haliotis varia,
23:50.3	and the part of the abalone you see on the right
23:53.0	is a cold water abalone from the North Pacific,
23:55.1	this is the red abalone, Haliotis rufescens,
23:58.3	and you see the radical different in body size.
24:00.3	So, the question is,
24:04.1	how might the evolution of the food resource of abalones,
24:07.1	which are kelps,
24:09.1	influence their body size and the evolution of maximum body size?
24:12.1	And you can see that in this illustration.
24:14.1	So, what this illustration shows is
24:19.1	information on the extant (currently living) abalone faunas
24:23.1	from all the different oceans of the world,
24:26.1	and the data on the maximum shell length
24:29.0	or the maximum body size from these different faunas.
24:31.1	And what you can see is that in Australia, New Zealand,
24:35.3	the Indo-Pacific, the Mediterranean,
24:37.3	South Africa, and everywhere,
24:40.1	the maximum body size of abalones is substantially less
24:43.1	than it is in the North Pacific,
24:46.3	and we believe that this has quite a bit to do with the fact that
24:51.1	the food resources that these abalones have evolved
24:54.1	feeding on is a very nutritious food resource
24:57.2	and that, again, is a consequence of the existence of the sea otters in the system.
25:01.2	So, to wrap up, let me recap
25:04.0	what I have told you about sea otters in kelp forests.
25:06.0	I've told you something about the approach that we have used,
25:08.2	that my colleagues and I have used in understanding the dynamics of this system
25:13.0	and in particular how sea otters fit into those dynamic processes.
25:16.1	We have done that by first modularizing the food web,
25:19.1	focusing on species that seemed to matter
25:22.0	so far as the sea otter is concerned,
25:23.3	and then we have used a perturbational analysis
25:26.1	and in this particular case we've used the North Pacific Maritime Fur Trade
25:29.3	to manipulate the abundance of otters in the system
25:32.1	and to observe dynamic consequences
25:35.1	of their ecological interactions by doing that.
25:38.1	The general findings are that
25:41.1	we have discovered what we call a trophic cascade,
25:43.1	that is, an interaction that starts high in the food web
25:47.2	with this apex predator, the sea otter,
25:49.2	and flows downward through sea urchins and kelp,
25:52.1	that this trophic cascade has what I call serpentine influences
25:56.3	on many other ecological processes and species in the system,
26:00.2	I've given you an overview of a few of those,
26:03.2	and that these strong ecological interactions
26:05.2	have led to natural selection and evolution.
26:10.2	I should acknowledge both my collaborators
26:13.3	and some of the support that we have received for this work over the years.
26:17.1	My collaborators are posted on the left
26:19.3	and the major funding sources for the work that we've done
26:23.1	are listed on the right.
26:25.1	Thank you for your attention and I hope you'll come back and join us for Part III,
26:27.2	which will be an exploration of other large species of apex predators
26:31.1	and other ecosystems.