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.