in this module we've been focusing on communities and ecosystems and most recently we were focusing on the types of biotic interactions that happen in biological communities and the influence that those interactions have both on population sizes of different species but also on ecosystem level effect so how does how do species interactions how does the population status of one species affect many other species in the ecosystem as well as the ecosystem services that can be provided by that ecosystem and so what I want to do now is share with you a case study and this is a classic system a classic case study in ecology and I want to share this this case study with you so that we can both review some of the concepts in kind of real world context but also learn a few new key terms for studying candies and ecosystems so the system that I want to tell you about is a coastal marine ecosystem and and it's like I said a classic system in ecology that's used to describe a couple of types of species and some important ecosystem level phenomena that that occur in many systems so I want to use the simplified coastal marine community that includes only three main species to kind of simplify what we're talking about even though there's a lot more complexity in the system these three key players in this simplified system sea otters sea urchins and giant kelp can allow us to to get a sense for some of the changes ecosystem model changes that occur when one or two species are impacted and then we'll end by talking a little bit about the complexity of the system beyond these species alright so I want to begin by introducing you to our three key players so the first species that I want to tell you about is the primary the major primary producer in the system this is giant kelp kelp is a type of algae it's a large brown algae type of seaweed and it can grow really really large so it can grow up to 200 feet from the sea floor up through the water column and it creates littoral forests of kelp and actually this this ecosystem is generally called the kelp forest ecosystem and the importance of the kelp while there there are multiple values of the kelp forest ecosystem but one is that this the complexity of the kelp itself of the forests created by the kelp provides shelter and food substrate for algae other algae to grow on it and refuge for many many species invertebrates fishes marine mammals and also kelp is really important for humans in several ways but the first is that we consume kelp both by eating it but also by using it for for many many different products and I've listed a couple here there are lots of cosmetics that use seaweed there are paint use of seaweed if you or anyone you know drinks milk alternatives so almond milk coconut milk things like that they tend to have seaweed in them carrageenan in order to to give the the nut milk the consistency of realizing that we're consuming seaweed but seaweeds also important for for humans and for the ecosystem in general because like forests on land kelp forests also sequester carbon right so they take up carbon through photosynthesis and store carbon that would otherwise be released into the atmosphere so - really huge ecosystem service that we get from kelp and another is that again these are coastal areas so the physical presence the physical structure of these kelp forests can attenuate the wave energy that's that's coming in towards the coastline which can prevent coastal erosion and protect shoreline areas and so there are a lot of ecological values economic values and and other types of values from some would say aesthetic values that that we can assign to kelp forests alright so moving up the food chain we have sea urchins I'm going to start by saying something really nice about sea urchins because I'm about to disparage them a bunch they're gorgeous right look at them they're absolutely beautiful and we'll get to the fact that they do have some threats that are facing sea urchins as well in certain parts of the world oops sorry but first I'll tell you so sea urchins are omnivores they eat which means that they many different things they eat at different trophic levels they finish the barnacles they eat the detritus so dead decaying matter they eat a lot of algae including giant kelp which is where I start to disparage them there are kind of germs they're pretty cool in terms of their anatomical structure they the habitat that they they tend to occupy is be the rocky ocean bottom and you will also find them in coral reef areas which of course are also coastal marine communities the threats I mentioned is there are some threats to urchins pollution can be can be a threat to to urchins ocean acidification other types of pollution it certain fisheries practices like dredging where essentially a big massive rake is pulled along the ocean floor can physically disrupt and remove actions from their environment and one that I didn't mention here is that in some parts of the world urgent are really valuable food items and so also over harvesting by humans can be a big threat so in some areas urgence are declining in some areas or cheon's are doing fairly well and the problem with urgent doing really well is that when they are unregulated right so when the sea urchin population is not being regulated population growth is not being suppressed somewhat then oceans become over abundant and they will over graze giant kelp seriously over graves giant kelp alright so we have giant kelp sea urchins and now here we are with our top predator in this food chain so this is a sea otter sea otters are in the weasel family in the Mustelidae family they're the largest member of this family they grow up to 5 feet long and up to 70 pounds which is essentially the size of like a small German Shepherd they are said to have the finest fur of any mammal and what we mean by the finest fur is that there it's very valuable for it because these animals live in cold cold web environments they need fine fur to protect them from the elements and so they have between 850,000 and 1 million hairs per square inch of their body which is pretty phenomenal if you think about it they play on many things but primarily one of their preferred pronouns is benthic invertebrates so organ invertebrates that occupy the lower kind of ocean floor areas and specifically one of their preferred food items seems to be sea urchins and you can see this otter right up here eating a sea urchin the Predators include sharks Eagles killer whales and orcas and I put humans here we're not really their predators so much as where they're over hunters but there's a long history of overhunting of sea otters by humans and we'll get to that in a little bit sea otters are a key species in coastal marine ecosystems and specifically actually they are considered keystone species we'll talk about that in a moment actually let's talk about that right now keystone species is a term that I foreshadowed about in the last module or so and I believe it was one of the terms that I asked you to look up for this this module a keystone species as you probably know now is one that has a significant impact in an ecosystem so that's part of it it plays a key role in the ecosystem a very important role in the ecosystem and that impact that it have is has is disproportionate to its abundance in the ecosystem so those are the two important pieces of the definition of a keystone species so first of all it plays a key role it's very important it has a significant impact on the ecosystem is one thing and the second thing is that that impact that it has is disproportionate to its abundance in the system and so often we will have keystone species at higher trophic levels because if you remember our trophic pyramid it's those species at higher trophic levels that tend to be present in lower abundance relative to all of the trophic levels below them right ok so there are in this simple food chain that we're talking about what we're talking about kelp virgins and otters we can talk first about the direct effects that occur from the trophic interactions that these species have with one another so the direct effects are that otters eat urchins right so otters regulate urgent populations and urgent eat kelp and keep in mind that the arrows here the blue arrows right right so that's why the ED the arrow is going from urgent to auteurs even though the otters are eating the urchins it's because the energy is flowing from the urchins and otters when the otters consume them and similarly from the kelp to the urchins so these are the direct effect otters are not having a direct effect on kelp right otters are having a direct effect on oceans oceans are having a direct effect on kelp but because they're regulating the urchin population right so because otters are regulating the urchin population keeping it low keeping them in relatively low abundance the urchins are not able to over graze the kelp and so the Otters are indirectly benefiting the kelp okay so direct and indirect effect is what I want you to get the difference between here all right so now we want to talk about a phenomenon in in biotic communities that happens when we have the population of a species higher up in the food chain change in some way in a dramatic way so in this case we're having the population of otters we're imagining that the population of otters is declining dramatically right we have a big drop in the population size of otters and then what happens is we have a big increase in the population of urchins so when there are fewer otters around the urchin population increases because they're released from regulation right they're released from that predation pressure from the Otters and then of course the result of that is that the kelp abundance of kelp the kelp forest reduces it goes down right the population of kelp decreases because they're all of a sudden now being over grazed by the urchins right so we see this what we call reciprocal population effects we have a decrease up here with the Otters leading to an increase with the urchins which then leads to a decrease with the kelp and this reciprocal the series of reciprocal population effect decrease increase decrease is what we call a trophic cascade okay now we can also have a terrific cascade work the other way and when I say the other way what I mean is we could have the otter population increase so let's say the otter population got really low and then for some reason it's able to rebound again so we have the otter population being revived so it increases dramatically and then subsequently the urchins are being regulated again so the urchin population declines and then as a result of that we have the kelp population regrow okay so trophic cascades can begin with a dramatic decrease or a dramatic increase in the population size of one of these upper predators but what we what you're really an important piece that you're seeing with the trophic cascade is this series of reciprocal population effects so I think of it as like an ABA pattern right a decrease increase decrease or an increase decrease increase okay and so if this happens right if we like what's the big deal right so if this happens and we have a serious decline in the otter population and then we have an increase in the urgent population and then a big decline in the kelp population one of the impacts in this particular ecosystem that can happen is that it can cause kelp forests so here's a picture of a kelp forest it can cause these kelp forests which are really diverse systems that support you know sharks and lots of fishes and otters and seals internals and all sorts of things it can cause these kelp forests to turn into birchen Barrens so it really actually causes an ecosystem transformation from a very violet averse kelp forest to a low biodiversity system that we refer to as urgent Barrens they are barren of most things except for actions okay so that was all theoretical right so far that was theoretical and so now I want to talk about but does this actually happen have we actually seen this trophic cascade in particular with in the sea otter system in the kelp forest ecosystem so first oh definitely about the sea otter populations and how they're doing and how they've been doing in the past so California sea otters and the northern sea otter they're different subspecies of sea otters are listed as threatened in the United States and then globally the sea otter is listed as endangered by the International Union on the conservation of nature so this is the the red list that you've probably heard about so they are to this day listed as threatened in the United States and endangered globally and and what I want to tell you a little bit of the history and I have to look at my numbers so I get get this right for you okay so historically sea otters numbered between several hundred thousand to more than 1 million sea otters that was their global population but due to the fur trade remember I told you that sea otters have this precious valuable fur due to the fur trade the worldwide population plummeted to a total of only 1,000 to 2,000 by the early 1900's so they went from a historical population of you know up to or more than 1 million otters down to one or two thousand otters by the early 1900's and so in 1911 there was an international treaty that put a moratorium on hunting of otters that you know stopped the pressure from the fur trade and so subsequently after this moratorium on hunting we did see the otter populations start to increase over time so what are some of the threats to otters other than human ponting pressure so there's human hunting pressure and you know these this species is found in coastal areas so there exposed to all of the pollution that occurs in coastal areas which unfortunately is a lot so all of the agricultural effluent and industrial effluent everything that's flowing into coastal systems from the adjacent land and there are disease problems especially in high density areas I feel like there's one other that I'm not thinking of but but those are some of the main issues so disease pollution and coastal areas and of course historically hunting from humans all right so now I want to show you some actual data and let's take a look at these good dates so on the x-axis here and we have time and we're starting here at 1910 and going all the way up - not that long ago about 2010 - a hundred years of data looking at otter populations virgin populations and kelp populations which are three key players what I love about this figure is that it's showing this is right around the time it begins right around the time of the moratorium on hunting right so I just told you in 1911 we have this international treaty that stops hunting of otters and this is right around that time so when when the international treaty was agreed on otters were rare they were nearly extinct - so you see that appearance it says right here what's going on with otters on the top the two trend lines that you're looking at here are for in bright green you're looking at virgin numbers and in this kind of like pea green you're looking at kelp numbers right so otters aren't represented by a line and this figure otters are only represented by the words at the top that's important to understand okay so otters are rare they're nearly extinct in 1910 then we have a moratorium on hunting and so it takes a little while but eventually in the 1960s 1970s 1980s otters are really abundant again so after several decades four or five decades without hunting the population is rebounding which is great but let's look at this trophic cascade look for evidence of a trophic cast so when otters were were rare we have really high populations of urgence right so right here really high populations of urchins and really low kelp so that's what we would expect right so we have no otters or low otters higher cheon's low kelp right that's the trophic cascade pattern that we would expect now here as otters become more abundant we have really low numbers of urchins right they're being regulated by those otters and the kelp population is rebounding so it seems like here we have an urchin baron and now the ecosystem is swinging back to another state to the kelp forest state right now related direct your attention to down here to more recently the last couple of decades last few decades all the Simmons's otters are rare again and again we see that pattern that we would expect from the trophic cascade so when otters are rare merchants are abundant and kelp is rare but right now what I want to direct your attention to is why are otters rare in the late-1990s 2000-2010 why otters rare so the first thing you might be thinking is we lifted the moratorium on hunting right and so now we have the fur trade up and up and alive again that's not what happened so you did not lift the moratorium on hunting the international treaty still applies and yes there was some disease yes there was pollution from coastal areas yes there were some oil spills right these are some of the issues that otters were dealing with but it turns out that none of those things could explain why the otters had become so rare again and so this right here is where I want to tell you a little bit more about the the biological community and the ecosystem that we're looking at here so I had said earlier that you know we're gonna look at the simple system we're going to look at otters and oceans and kelp to understand the TripIt cascade but to really understand why otters became a rare again and why we saw the reverse of the trophic cascade once again we have to think about other species in the biological community because it's really not as simple as as I've been explaining it so far let's keep going backwards sorry guys so it's really not so simple we have to think about some of the even larger predators the the quaternary consumers at the very top of the food chain so I had mentioned earlier that orcas killer whales were one of the predators of sea otters they are but historically orcas used to eat the the great well so they would hunt down right whales and some of the other what we call great whales this class of whales that are some of the largest whale I've told you about orcas before net orcas hunt in pods right to take down these prey that are much larger than themselves so I'm not oh and by the way okay so sources used to eat whales and then what happens though in the the mid 1900s - whales it was about 1950 or so the middle of the 1900s when industrial whaling ships from Japan from Russia started to really an United States I started to really decimate the the populations of the large whales of the great whales so the populations were plummeting and so it turns out that the orcas that that were losing their main prey species right these large whales you see their main prey species the orcas lose their main prey species and so they have to shift their their food source to something else and they shifted it to sea lions they shifted it to seals but we were also hunting down the sea lions and the seals and so after some time there aren't even that many seals left for the orcas to eat and so it turns out that right around the 80s and 90s the orchid two sea otters so imagine a species orcas that used to take down huge whales right for food and all of a sudden they're taking down sea otters for food they're going from whales that are much bigger to them to sea otters which are five feet long maximum so you can imagine first of all they're they're putting pressure on sea otters that never used to be there and they have to eat a lot more sea otters to get the same food amount of food that they got from a whale right and so it turns out that this shift in diet by the orcas with one of the major pressures that caused otters to become less abundant in the late 90s these populations are rebounding a little bit but like I said they're still they're still listed as threatened and endangered by the IUCN so they're rebounding a little bit as of course we also have restrictions on seal and sea lion hunting and those numbers are going up as well but it's going to take a number of decades for this system to write itself all right so why then is our the Eagles on this slide right because that seems a little out of place a little weird well this is just a part of the story that I think is really interesting and so the biologists who study bald eagles which incidentally used to also be endangered we're really concerned when the sea otter population started to plummet again in the 1990s because sea otter pups are one of the main food sources that bald eagles use to feed their chicks during the breeding season and so they were concerned that the bald eagle population was going to be negatively affected by the by the lack of food items for the bald eagle chicks it turns out that the bald eagles actually shifted the prey items that they were started collecting for their chicks from sea otter pups to sea birds to other seabirds and specifically ptarmigans were one of the birds that they shifted to and when they did that they actually had an increase in fecundity so this might sound really familiar this is kind of like the Lake Erie water snake right shifting its diet to this invasive species and all of a sudden they have an increase in Sakana t-this is a really interesting thing that happens that some species that are forced to shift their food items to something else sometimes it actually works really well so it turns out that the bald eagles weren't negatively affected anyway this is all just to say that yes trophic cascades have been observed in this system in nature and that the system is a lot more complex and you have to look at so many different factors to really understand the impacts of one species either increasing or decreasing