 Okay, welcome back everybody for the last lecture of today's session. It's a pleasure to have here once more, Justin Nico, who will tell us more about Palio V College. Please, Justin. Great. Thank you for having me. Italy looks a lot like my garage, so it's a pleasure to be here though. And let me just share screen. I'm not sure I can hear you. Okay, so can you hear me now? I can hear you well. Maybe it's on Antonio's side. Okay. Okay, so I assume that others can hear me. Okay, Antonio, you can hear me. Okay. And let me share my screen and, okay, so you can see my slides. Okay. Again, thank you for having me. Today I'm going to talk again about a bit of an overview of theoretical paleo paleo ecology. And I say it's a biased overview of theoretical paleo ecology because I'm going to be speaking about things that I know something about, which I guess is a good place to start and that is with respect to reconstructing interactions in the past, and using reconstructed to say something about modern systems. So very briefly I'm an assistant professor at the University of California Merced, and started there in 2016, overlapped with Jacopo at Santa Fe Institute very briefly. And you can see my Twitter handle and website are there in the bottom right. And so I just wanted to give a little bit of a layout for where I was going over the next couple of days for the series of lectures that I'll be giving. Today I want to talk more about understanding extinct ecosystems and why understanding extinct ecosystems is important, as well as how we go about reconstructing past communities with tools from ecological theory. So my goal today is really a broad overview I'm going to be discussing work that a lot of work that other people have done some work that I've been doing, and hopefully convince you that that examining these extinct systems is important and relevant. Tomorrow I'm going to really focus on a particular case study of ancient Egypt over the last 10,000 years or so to understand how the unraveling of that mammalian community over 10,000 years since the end of the Pleistocene can tell us something about how mammalian ecological systems work generally. So part of that is also discussing some mathematical techniques, generalized modeling that can be used to assess the dynamics of nonlinear systems when a lot of the system is unknown. A lot of the particulars of how organisms might be interacting with each other is unknown. So today I'm changing gears quite a bit, focusing on energetic constraints at a much smaller scale and principles of ecological interactions at the scale of physiology really to see if we can say something about very very large scales. In particular macro evolutionary processes such as the evolution of large body size. Most of this work is focusing on mammalian systems except a lot of what I'm going to be talking about today. Okay, so what am I going to be talking about today. Why is understanding extinct ecosystems important. How do we go about reconstructing past communities with with modern tools. I'm covering a lot of ground. First I'm going to focus on just reconstructing ancient communities and I'm going to try to follow somewhat time and some order of time from the, from the earliest life to life as influenced by the arrival of humans on the landscape. So part of this again is just focusing on on reconstructing the structure of interactions. But also one of the one of the advantages of looking into the past that looking into the past can give us is being able to see how communities were structured and organized before and after large mass extinctions. One of the big open questions in our world today is how will communities respond to climate change to anthropogenic disturbances, and we can gain a lot of clues by looking into the past by seeing how communities responded to large disturbances that are on record. Climate change gears a little bit partway through the talk and think about how organisms themselves have structured the biosphere, and what the role of these ancient ecosystems in, sorry, with the role of these ancient ecosystem engineers might have been in structuring communities, and then finally focusing on food webs and the Anthropocene so how have humans more recently influenced the structure of interactions. I'm open to questions being thrown out during the talk I don't know what the. I think you've been waiting till the end of talks and that's fine too, but I can't see my chat windows that's the only. That's the only thing. Okay, I'm not going to belabor the point because I think you've been talking about these concepts really for the last few weeks but of course species interactions reconstructing or understanding species interactions and paleo food webs presents some unique challenges relative to understanding contemporary systems. Now, of course, when we're thinking about consumer resource relationships where we're thinking specifically about the flow of biomass right from one species to another. And there's many different ways that we can measure this in in the past, of course observation is not currently available to us until the invention of some kind of time machine. We can observe in different ways in some systems one of the systems I'll be discussing today that are very well preserved sometimes we can find gut contents we can actually use gut contents to reconstruct who is eating whom. The ratios of stable isotopes is another way that we can look into the past and reconstruct how the flow of biomass and this is really, this was my entrance into science I was, I worked in a stable isotope lab for large part of my PhD. And in those types of situations we can use the chemical signature of bone and tissue, the ratios of different stable isotopes to track biomass flow, because you are what you eat except what you excrete, and that's kind of the rule of stable isotope ecology and, and it allows us because many state ratios of stable isotopes are preserved for long periods of time in bone sometimes fossils. You can go into the past and reconstruct just as you would for contemporary systems. And another big part of this is a lometry understanding how body size dictates who can eat whom in a system, or who can interact with whom in various ways. A lot of this is constrained by a lometry so we can use allometric principles derived from modern systems to essentially constrain how we understand how species may have interacted in the past. And of course, once we have these networks of interactions we can assess how structure has changed over long periods of time or not. How that structure might impact or affect the resistance or resilience of a system to disturbance and other aspects of dynamics that we might be able to infer. And, you know, one of the benefits here is that the story has already been told, right so we can go into contemporary systems and assess measures of dynamics and try to postulate what that means for how that system changes in the future. And then into the past the experiment has been run. So if we say something about dynamics so for example we might go into reconstructive food web and try to say something about the susceptibility of different species to extinction. Well, then we can see if that susceptibility actually results in extinction by looking forward in time so so we have this time this temporal flexibility that paleontology gives us and and looking over long periods of time, and some unique challenges as well. Okay, I think I said, I'm not going to belabor the point but then I belabor the point. Okay, let's orient ourselves let's root ourselves in the history of life. Okay, so earth form 4.6 billion years ago, which is at the bottom of the spiral which looks like it came from a biology textbook because it did. We started the bottom of the spiral and we start moving up forward in time. We have the earliest cells at about 3 billion years but for our interest we're really going to start in the, in the last full turn of the spiral so we're going to start at the Cambrian explosion, and that was about for 500 billion or sorry, 500 million years ago half a billion years ago that we can see I think I can use my pointer I think you can see my pointer. So this is the Cambrian explosion we start here, and then this last full turn of the spiral is really the evolution of complex ecosystems. That's that's the record that we have. And so we're going to start in the Cambrian explosion we're going to spend some time in the Permian towards the end of the Cretaceous which is the end of the rain of the non avian dinosaurs. And after that we're going to zip back to the Devonian the expansion of plants on on the terrestrial landscape, which is a really interesting time that I've been thinking a little bit more about. And then, and then we're going to, you know yo yo back up towards the more recent and think about how the, how humans have have impacted systems and the more recent past. Okay. So the Cambrian explosion. So this is work really beautiful work, I think, done by Jennifer done at the Santa Fe Institute this is a plus biology paper published in 2008, where she and a team of paleontologist essentially reconstructed interactions of species in these beautifully preserved shale fauna. So the Burgess shale is in Canada and that's what's pictured at the top. The other shale is the sheng sheng fauna, I hope I'm probably not pronouncing it right in China dating to around the same period of time. So they were looking at two of these beautifully preserved shale faunas to reconstruct the interactions because they're so well preserved there's a lot of information about who was eating whom in the system. And on that they could reconstruct the interactions and then see if food web organization is in any way comparable to contemporary systems to get a sense of the scale of this question we really have to understand how alien, these systems were. Okay, so I want to spend a few minutes just just looking at the Burgess shale. So these are, first of all, half a billion years old. And I want you to notice how very well preserved they are. But then let's look at some of these species. Micro Mitra looks kind of like a today's, you know, urgent. Eugenia is somewhat urgent like hallucinogenia just the names of some of these species illustrates how strange they are. We don't even know. Well I guess more recent when they found it they didn't know which was up or down I think they have a better sense of that now I still don't. And, Lee and Colea. So, so these are very strange and very different looking species than what we have in shallow intertidal systems today. The British shale was a shallow marine system. Oops. These, the bottom colored images are the more recent understandings of these two different species. Adontogryphus and Nectocaris. The above illustrations are how they were originally conceived. Some were better than others. It's a lot of work trying to reconstruct what was what, even in very well preserved shale fauna. Now, one of the original ideas that is very well described by Stephen Jay Gould in his book beautiful life or sorry wonderful life that I showed on a previous slide. One of his original ideas, and it was a popular idea for a while is that the Cambrian explosion was really this period of massive experimentation, where these different life forms had a lot more morphological disparity compared to similar systems today. So you can see this middle image here is this Gould 1988 1989 interpretation, where the x axis is documenting the morphological disparity of the system, and time is moving from the bottom to the top. And so his idea was that, you know, these systems were were experimenting with very different shapes and very different modes of life. And the way of which were not successful, and those that were successful, of course, gave way to, to life as we know it today. The more recent interpretation of this, however, is that there's about as much morphological disparity in the Cambrian explosion, as there is today, maybe even a little less. And even these very strange species that don't look like they have any modern relative are. Oops. Oh, I think I skipped a slide. I was probably going to pop up later by accident. These, these two very strange looking species are actually early mollusks. And so so even these kind of alien looking organisms that were in the British shale, or in the shale faunas are related to modern relatives, and, and perhaps the systems aren't quite as alien as we thought. So, taking advantage of this preservation allows paleontologists to go back and reconstruct who is eating whom. So this is a very simple illustration of the British shale food web. We can reconstruct, of course, you know the algae species and then those organisms that were specializing on grazing and filter feeding illustrated in the in the brighter green color. The organisms that were scavenging those organisms that were active predators, and we can get a sense of the actual species interactions by observing stomach contents which I'm showing in the lower left here so this is the stomach contents of one of the organisms in the British shale fauna we can actually see inside the soft tissues were not preserved but left impressions in the shale feces allow. To reconstruct interactions the bite marks can be matched to the mouth parts of predators. And of course body size determines to large extent, who is capable of eating whom, in terms of active predation. So if we accumulate all of these different lines of evidence we can, and even wait the different lines of evidence with with our ability to say something about the interaction, you know more certain interactions or less certain interactions where we can reconstruct the food web, and this is what Jennifer done and her team did. So now that they're able to reconstruct the food webs of the shale faunas. What's one way that we can go about comparing whether you know the structure of these systems was different or similar to those today. The way that they went about this is looking at the cumulative link distributions. So here I'm showing link distributions for, let's see 12345 modern food webs, you know, all of the, all of the ones that you've probably seen over and over a little rock lake, you've been at estuary so would park. And one of the, and then the link distributions is illustrated below it. One of the things that you can immediately see of course is that these link distributions are long tailed. They have most species, tending to be specialists in other words they have fewer trophic links to other species, and relatively fewer species being generalists where they're linked to many many other species in the systems in the system with trophic links. So that way that we can more easily compare these systems because they all differ in size and they all differ in link density is to divide to scale the cumulative cumulative link distributions by the average number of links per species. And this is what I'm showing on this slide so now we still have the cumulative distribution on the y axis but on the x axis we have the number of trophic links that that's scaled to the average number of links per species. And so we're moving the distribution to the density of the network. And what we find is that all of the distributions fall on top of one another. So, all of these different systems do seem to be constrained by, or do do do appear to be to share similar constraints in terms of the distribution of specialists and generalists trophic specialists and trophic generalists in the system. So just one thing to note that the distribution tails fall off much more quickly than you'd predict from skill free networks so I've just, you know put on top of this a power law relationship, where, and we see these ecological systems are fall off very quickly. So, so generalism is not following a scale free power law relationship here. Okay, so, so what how does this relate to the Cambrian system. Well we can take these cumulative degree distributions and of modern systems and compare them directly to the Cambrian food webs because this is we have the same type of information. What I'm showing here again is the same x axis and same y axis as before the normalized number of trophic links on the x axis and the cumulative probability distribution on the y. Other squares are modern food webs from eight different sites, and the black and the gray circles are to Cambrian food webs, one from 505 million years ago and one from 520 million years ago. And the message is pretty clear here. They all fall on top of each other. Really kind of blew my mind, because again these are ecosystems at the very beginning of multi cellular complex life. These are some of the first ecosystems that we have record of assembling. And to think that they share such strong structural similarity to modern systems to me is very very striking. But it suggests that there is a fixed a fixedness to food webs that there are similar processes constraining interactions. And that these processes that constrain interactions are truly independent of taxa truly independent of location. I mean this these this is a shallow marine system on the shores of Pangaea. So of course very different than than anything today. Over time this is half a billion years ago, and independent of environment. So, you know these energetic constraints these interaction constraints are apparently they would appear to be very fixed over time and space. I guess I'll let the animation play out here we have a trilobite which bit the dust, bit the dust in the Permian, which we'll get to in a minute here. I guess I'll call it like Harris one of the top predators in the British shale. I guess well, while this is playing I can see if I can see the chat window. Okay, you can see the pointer. Excellent. Okay, so because now yeah. So, so every nature now it's going to replace every nature video has to have a predation event that's like a law of the universe. Now let's let's move forward then and think about communities before and after mass extinction so we've looked back at the Cambrian so I've I've point. I'm showing that here so the Cambrian is about 500 million years ago that's where we've been move forward for you know quite a while. We don't pop up into the very end over here but let's move forward. A couple hundred million years. And what I'm showing on this graph is the extinction intensity so so earth has been marked earth's life on earth has been marked by five large extinction events. It's very likely we are creating the sixth most the sixth mass extinction. But one of the largest the largest mass extinction event was at the end of the Permian. It's called the Permian promo triassic mass extinction. This mass extinction, how are communities structured before the mass extinction, how are they structured after the mass extinction. Can structure tells anything about the dynamics or the robustness of the community. And then we're also, we're going, we're not going to the Cretaceous mass extinction, the asteroid impact unfortunately. But we're going to go to this event that occurred right before the asteroid impact. It's called the in Cretaceous restructuring period. And this is when all the big famous dinosaurs were walking around the landscape and what we see towards the end of the Cretaceous is a very large decline in diversity. There's even a period where sauropods just disappear. They reappear later so they didn't go extinct at that point. It's called the sauropod gap. So strange things were happening at the end of the Cretaceous in terms of restructuring dinosaur diversity, and this was happening right before the asteroid impact sealed their fate. So we're going to visit these two big events in the history of life and try to understand whether you know the changes in the structure of the food web can give us any insight into the robustness of the system before or after. Okay, so again, two big events, the Permian extinction and the in Cretaceous restructuring. Let's look at this in a little more detail. So the Permian extinction. This is also called the Great Dying. This was 251 million years ago. It's, you know, the causes of this are somewhat contentious. I argue that there were asteroid impacts. It's certain that there was massive volcanism, resulting in these, you know, one of the events resulted in the Siberian traps. And it's thought that this massive volcanism occurring over hundreds of millions of years actually triggered global climate change that just altered the landscape destroyed primary productivity and resulted in this. You know, cascade of extinctions from primary productivity to primary consumers to to to those consumers that are eating the primary consumers. Ultimately 70% of terrestrial vertebrates when extinct at this mass extinction event, 96% of marine species when extinct so this is, this is major major towards the end of the Cretaceous we have the end Cretaceous restructuring this was about 72 million years a little more subtle. There was a decrease in dinosaur richness. There's fewer endemic taxa. And one of the big questions has always been were in Cretaceous systems less robust due to this restructuring event. Did this event set the stage for the KT extinction would dinosaurs have gone extinct or non avian dinosaurs because of course birds are dinosaurs would non avian dinosaurs have gone extinct. I don't know if this restructuring didn't happen to this restructuring really erode the robustness of the system, so that the asteroid impact had a larger effect than it would have otherwise. Now for both of these reconstructions, I should mention by the way this is work done by root Narayan and Mitchell. They used a technique called guild level reconstructions where we really can't say to the species level, who was eating whom. Instead, what they went what they did was reconstruct guild level reconstructions of who is eating whom. So the guilds here are, you know, coated blue, and then the species are coated green on the inside so we might not be able to establish species to species interactions, but we can play with confidence based on what we know about modern systems, which guilds were interacting with each other and once we have these guild level reconstructions of the system, then we can randomize species interactions within the guilds and build a, you know, a set of food webs that we can analyze that represent likely, you know, interactions between species as a function of their of their guild interactions. And the question that they went about addressing once they were able once once they reconstructed these these guild level structures from which they could simulate many many different potential food webs of species interactions. They focused on this question of have large perturbations impacted food web structure or function. And they assess this by looking at the effects of primary extinction on the structure of the system. So for example if they go in to their simulated food webs and initiate a primary extinction. Would that result in a series of secondary extinctions. If you remove the only resource of a consumer of course that consumer would be a secondary extinction that that is the result of the primary extinction applied to the resource. And then you can set different cut off levels for how sensitive you think that primary extinction, or how sensitive secondary extinction should be to primary extinctions. And so they initiated a number of primary extinctions as given by this perturbation magnitude so as they're increasing the perturbation magnitude, they're increasing the number of primary extinction extinctions that are imposed on the system. And of course you would always expect that the number of secondary extinctions would increase with perturbation magnitude as you remove more species from the system. More species will secondarily go extinct as well. And the interpretation is that systems with a higher proportion of secondary extinctions are more fragile in other words they're less robust. And so this is the assumptions kind of going into this experiment, taking, taking food webs reconstructed before and after these large disturbances and assessing the effect of perturbations on secondary extinctions. So what do they find. Okay, so I'm showing two sets of results here one for the Permian extinction and the bottom for the incretaceous restructuring. What I'm showing up top is. In this panel labeled f again we have perturbation magnitude on the x axis and the number of primary secondary extinctions on the y. In the Permian before the mass extinction. We have this relatively tight sigmoidal relationship between the perturbation magnitude and the magnitude of secondary extinctions. So of course as you increase the perturbation perturbation magnitude you have an increase in the number of secondary extinctions. This is relatively, you know robust to the number of perturbations at the beginning. We don't have the sigmoidal increase this, this, this really sharp increase in secondary extinctions, until the perturbation magnitude is quite high. In comparison, if we look at Triassic systems again these are systems that were reconstructed following the promo Triassic extinction. So you, you know these are recovering systems. We find something very different we find that well root nirine at all found that, regardless of the perturbation magnitude there's many more secondary extinctions that that you'd expect by removing species. We all have the sigmoidal relationship, but the spread at lower when the perturbation magnitude is relatively lower is much greater in terms of the number of secondary extinctions. And this would suggest that Triassic systems are less robust are more fragile, following the promo Triassic extinction event. There's a similar message from the incretaceous restructuring, although they're plotted on top of each other here so now blue is before the incretaceous restructuring and red is following the incretaceous restructuring. And this would. And so what we see here is, you know, still a sigmoidal relationship again this event is not nearly as dramatic as the promo Triassic extinction event, but we have these elevated secondary extinctions for any given transmission magnitude, relative to how how robust the system was before the incretaceous restructuring. So the message, the messages here then are that large perturbations appear to have left less robust communities. And that declines in robustness may exaggerate extinction events so it's very true, or it appears to be the case, I shouldn't say it's very true but it appears to be the case that it took some time to recover in that the Triassic system immediately following the promo Triassic was less robust. And it's also very possible that the the incretaceous restructuring really set the stage for the effects of the asteroid. When it hit the planet 66 million years ago, signaling the end of non avian dinosaurs. And this work again by Jennifer done focused on fall okay so so 66 million years ago the asteroid hits the planet. Non avian dinosaurs are wiped out and this really opens up niche space for mammals. And this is to a large extent why we're here. And one of the best faunas for these early mammalian communities is 18 million years after the asteroid impact in Germany the missile, the missile fauna. What we're done in colleagues reconstructed these incredibly highly resolved food webs one for a lake community and one for a near shore community, a forest community that was nearby this lake. Nearly 700 species are in these networks which challenges a lot of modern, modern food webs contemporary food webs. And using again a cascade of different lines of evidence from functional morphology got contents damage patterns body size copper lights etc. In these really highly resolved systems, and they show that 18 million years after the asteroid impact that the structure of these systems is indistinguishable from contemporary food webs. It's not a very similar message as the Burgess shale, but this suggests that, even if systems are less robust following mass extinction events as root nirine pointed out following the promo Trias or sorry, yeah, promo Triassic mass extinction event. After the asteroid impact, given 18 million years the system had apparently reassembled to a structure that's no different from contemporary systems. Okay. One of the, I think one of the most interesting things when you look back into the paleo record and reconstruction of life on earth is the role that organisms played in essentially establishing the biosphere. In other words, the environment not only had a large impact on evolving communities evolving communities had a large impact on the environment. One of the most dramatic examples of this and this is this is via engineering these are changes levied upon the environment by evolving species changes to the atmosphere changes to the bedrock changes to rivers. The more detail we uncover from the paleo record the more dramatic. We see species having an impact on a biotic on the biotic system. One of the most dramatic examples of this is the evolution of multicellular cyanobacteria and the oxygen crisis. Okay, so if we look back this is atmospheric oxygen that I'm showing on this graph, before 2.4 billion years ago there was very little oxygen in the atmosphere. Okay, 3.2 billion years ago we have ox oxygenic photosynthesis beginning at around in the most recent estimates, put this almost squarely around 2.4 billion years ago is the evolution of the first multicellular cyanobacteria. Immediately following the evolution of multicellular cyanobacteria and diversification of multicellular cyanobacteria, we have an explosion of oxygen into the atmosphere. And of course this is an inference but the timing suggests that the oxygenation of the atmosphere is due to the evolution of multicellular cyanobacteria. So these are the first global engineers, at least that we have record of that are operating on a on a grand scale, pumping oxygen into the atmosphere. So, understanding these these feedbacks between the biotic and a biotic environment is really vital for understanding the evolution of early life on our planet. And of course understanding the future of our planet. We are engineering on a similarly grand scale we are changing the climate in a much shorter period of time by the way. And we don't really know what the effects of this is going to be. So, understanding the roles of these global scale ecosystem engineers is important for understanding our role, it's also important for understanding the history of life. One of the recent projects that I've been involved with is trying to understand the role of engineers within complex ecological communities. So on the right, I'm showing the cyanobacteria and forest ecosystems on the top, which which I'll get to the evolution of forests in a moment. In the middle here, this is a rock boring shipworm that was recently discovered so it actually digests rock, but it's an ecosystem. It's also an ecosystem engineer on a much smaller scale and paying, paying tribute here to the rock eater from the never ending story and the subset image for those of you who have seen the never ending story. I'm updating myself I get a little bit, but the rock boring shipworm bores through rock and in your streams and it actually creates micro habitat for invertebrates that live within these rocks so it's creating habitat for other species. It's also an ecosystem engineer on a smaller scale. And of course elephants are common examples of ecosystem engineers as they move about changing the landscape and opening habitat for smaller grazing organisms. But what is the role of engineers within complex ecological communities. There's been a lot of theory developed to examine the role of ecosystem engineers and systems, but most of it is within a smaller scale so understanding, maybe one or two species how they might be impacted by by engineers. We wanted to understand how engineers might impact a community of species. And that really required us to think about how to integrate these abiotic interactions into the biotic interaction interactions that we characterize with food webs. And we wanted to really zoom away from just thinking about trophic interactions we wanted to think about interactions more generally so taking into account trophic interactions as well as mutualistic interactions. And so this is a little schematic that details our process of integrating these different aspects into into a species network, where we have three different types of interactions we have eat interactions need interactions and make interactions. And we have two different types of nodes in the network we have species, which are the colored circles, and we have modifiers which are the black, which are the black nodes. The modifiers represent the abiotic changes that are introduced by species. For example, the rock boring shipworm picture to the right creates opening a porous opening in the rock so the porous opening in the rock is the abiotic condition is the modification that species makes to the environment that other species might rely upon. So, species can eat other species species can need other species and from these interactions they create trophic and mutualistic interactions species make modifiers and in that case they are ecosystem engineers, and then other species can eat or need those modifiers. And again the modifiers are somewhat abstract so they're not specific they're not specifically detailing any type of modification but a general modification that species make to the system. With these series of dependencies that species now have between each other and with the abiotic modifiers, we can then establish a an assembly process and a set of dynamics a very simple set of dynamics that dictate how systems are put together and change over time. So on the panel D here I'm just illustrating a very simple food web where we have let's let's just kind of walk through this here. So these are consumers that both are eating this resource. This consumer is an ecosystem engineer and it makes this modifier this modifier this modification to the landscape is being consumed by this species. These two consumers are competing for this resource. Another species that my pointer is on is engaged in a mutualistic interaction with this lower trophic level species whereas the other consumer is just engaged in a trophic interaction there's no service dependency. And now we imagine that another species is colonizing so we have the colonization of this yellow species with kind of this black ring around it to indicate that it's just colonized into the system. Where this consumer is eating it and the modifier is needed by that species. The one of the key ingredients to this assembly framework that I'm understandably describing very quickly is that we allow species to have varying numbers of trophic interactions, but they need what they need. If they lose anything that they need any of the service interactions that they need. Then they go extinct. However, they won't go extinct if they have at least one thing that they eat. So in other words, you have to eat at least one thing to stay in the system but you have to have all of the species or modifiers that you are engaged in service interactions with to stay in the system. So the assembly process then allows this colonizer to come in. And extinction works through primary extinctions work through competitive exclusion. Consider the fitness gains of species that are engaged in service interactions so species engaged in service interactions gain a fitness benefit. Whereas species that have multiple predators lose fitness because they're spending more of their time avoiding predators than they are trying to fulfill their own functions of life. And species that are generalists have a fitness disadvantage relative to species that are specialists. So those rules determine which species in the system are subject for primary extinction. And so if this consumer is goes extinct. Then we see a cascade of secondary extinctions. And this consumer that just colonized into the system is going to go extinct because it's losing the one thing that it eats. And this consumer is going to be subject for secondary constriction as well because it's live, it's losing the species with which it depends on with a service requirement so it has a need interaction with the species. And if this species disappears, it loses that need interaction. And this is also subject for extinction. So this this is my very quick introduction to this model. It sounds like there's a ton of ingredients but it's a relatively simple set of relationships. Okay, so what are some of the things that we found, we found. So as we increase the number of engineers in the system. There's a nonlinear effect on primary extinction and secondary extinction frequencies within the system. And again this is a measure of our, this is one measure of robustness for these communities, and I'll come back in a second and relate it to, you know the fossil record what does this have to do with fossil record. When we increase the number of engineers we're also increasing the number of engineering dependencies, how many species depend on those engineers. And we find that when extinction when engineers are relatively rare. So this is this this one here, this is when engineers are rare. We find that there are higher rates of primary extinction coupled with lower rates of secondary extinction. And that means that extinctions are more common but there have limited magnitudes such that disturbances are relatively compartmentalized. The reason for this is that there's stabilization of consumers in the system because there's redundant resources that that eventually increases the vulnerability of prey to predators, and that's increasing the primary extinction frequency in this case. However, as we increase the number of engineers and with that the number of engineering dependencies, we find that both primary extinction and secondary extinction rates decline. And this core, this corresponds to increase persistence of species in the system. And this has to do with the expanding niche space that ecosystem engineers supply to the system. And when there's many more engineers in our systems there's also more engineering redundancies in the system. So you have multiple species that are engineering the same modifiers in other words they're changing the a biotic environment in the same way. It's similar to having multiple species of tree, which are pumping oxygen into the atmosphere if we lose one species of tree we're not losing oxygen in the atmosphere because so many other autotrophs are doing that. Okay, so, so there does seem to be a very important role. This is again a first pass but there does seem to be an important role of engineers in communities. How can we levy levy that to explore paleo systems. And I think I'm going to end on this note I had I think I have till 1015 am I going up to 1015 or guess you'll stop me at some point. Okay. So how could we levy this ecosystem engineering community model to say something about the past to try to understand past systems. One of the big questions that I would like to really explore is is the is the Devonian. Devonian, we have the evolution of early land plants at the beginning of the Devonian and at the end of the Devonian 60 million years ago, we have forests. Okay, so this is called the Devonian plant explosion. And obviously a really important period in Earth's history. We also have another oxygen bump associated with the evolution of forests. Early tree species these early plants that were living on the land were ecosystem engineers on massive scales. They created the soil. Okay, so we have soil generation, petagenesis. We began the process of weathering the soil. And this served to accumulate carbon into these silicate weathering products. And so it's sucking carbon dioxide out of the atmosphere, placing it into these silicate weathering products. These weathering products were being swept into the ocean and buried in marine sediments. And it's thought that this series of events led to massive climate cooling and glaciation. And there's a large extinction at the end of the Devonian. And it's been theorized that it's tied to this plant explosion. And to this initiation of these terrestrial ecosystem engineers, changing both the soil, the atmosphere, and as a consequence, the marine system. These kinds of extinctions were then then occurred at the end of the Devonian during this 60 million year interlude. And so how would we love you a model like the one that I described, I'll be very quickly to explore this problem. So we can imagine, and I'm illustrating this on the on the bottom left here. So we have, we have a species or a set of species that are that are creating a modification to the environment. These might represent Cuxonia the early, the earliest terrestrial plant that we have on record, pictured in the circle down here at the early Devonian Over evolutionary time, these plants diversify into a large clade of terrestrial land plants, eventually forming these early lipidedendron forests that we have record of these these giant fern like forests towards the towards the late Devonian. Now all of these forests are also contributed contributing modifications to the environment. And I'm drawing a link here between, and I'm just picturing a single modifier here but we could imagine that might be a set of related modifiers. And now these modifications are are impacting other species other species now that are colonizing these forests are depending upon the modifications that these ecosystem engineers are making. And there's also direct exclusion of other species. So, whereas some species may be depending on the modifications that ecosystem engineers are making other species are being excluded from those environments and exclusion is one thing that we haven't really investigated with this initial model of ecosystem engineering within a community context, but I think it's going to be a very important piece of the puzzle. Can somebody tell me how much time that I have, I think, I think I should probably just stop here. The next kind of the next place I'm going and this is going to feed well into what I'm talking about tomorrow is the effects of humans on ecosystems, and I can just begin tomorrow here. That's great. Thank you. Thank you very much. I think we have several questions in the chat. You can read them out or you can invite directly the persons who ask the questions to speak for themselves. Okay, so I'll go back. So, here's an earlier question. What's the reason or the theoretical argument behind this universal law of trophic links. Is it dependent. Is it independent of average biomass per individuals because it's so here's my answer. Because it seems to be independent of system, it seems to not be system specific it's a general relationship that spans space and communities and types of communities and types of systems. It would seem to be relatively universal, or at least it hints at some universality. And then I would say it would likely be independent of average biomass per individuals because I would, unless that also might be relatively consistent. So, you know, there's strong alametric relationships between the amount of biomass per individual in a system that also changes quite a bit from one system to another mammalian system. In mammalian systems, we have damas law in ectothermic systems. Well, damas law corresponds to ectothermic systems as well. And this might be one of the things that could be structuring some of those larger scale interaction patterns that we see. I don't think it's full I think this is I think this is an open question. I'm not sure if I have a good answer for why trophic links would be structured in that way. One of the things that we've thought about exploring is, and we kind of get to this in our in our ecosystem engineering model is that, you know, specialists tend to have short term fitness benefits. They tend to be better at capturing their prey because they have adaptations to capture specific prey. However, over long periods of time, over large perturbations of events, generalists tend to have an advantage because they can adapt when things go bad. And so this this ratcheting between generalism and specialism may very well lead to the types of patterns that we see in the link interaction distributions. So if we have the survival of generalists after a mass extinction, like we see at the KT, when the asteroid impact hits, you know, when the asteroid hits the earth, the organisms that small that survive our small generalist mammals and small generalist organisms, regardless of whether they're mammals or not. So if it's if you have survival of these generalists after these mass catastrophes, but then selection towards specialization over shorter periods of time, that very well might might explain some of that. Okay, what are guild level reconstructions or specifically what are guilds so guilds are organisms that share similar foods. So you might put pollen eating bats pollen eating birds into pollen eating insects into into the same guild. So they may be not closely related to each other phylogenetically but they share similar resources. And so the idea of reconstructing guilds is just finding organisms that that that share the same types of foods. And that's all inferred by the way that's just inferred from paleo reconstruction body size constraint. You know, do you have sharp teeth are you a carnivore, or are you obviously an herbivore, because of the shape of your teeth. So there's a lot of, you know, there's a lot of not guessing but I would say, there's a lot of different lines of evidence that go into that some of some of which might be good evidence and some of it might be somewhat shaky. So we have to be really careful about, you know, what we're assigning what we're assigning and how certain we are of it. So I'm just kind of going down the list, it would be great if you can share some references on any math, any mathematical models about this topic. I assume you mean with respect to reconstructing food webs. You know, I haven't talked yet about dynamics in terms of, of building ODEs I'm going to talk about that a lot more tomorrow. So far, this true. So far, I've only really been talking about structural dynamics. So not imposing change change in biomass over time or change in populations over time but I will tomorrow for sure. So do trophic. Yeah, do trophic network summary features vary before and after large disturbances. There are some structural differences in the permeate promo triassic food webs that I was discussing so before the, the large extinction event at the promo triassic boundary. I don't, I don't have those in my brain at the moment, but they're in the root neuron paper. They're coming off when I run out of time is cell network theory which is introduced. You know, I don't know I'm not familiar with that theory. So I would, I would be interested in. Is that something that was discussed during during this class. Well that would be interesting I'd be interested in learning more about it. Great. Yeah, so so if there's time for any other questions I'm happy to do my best to answer them. Yeah, let's see if there's anyone wants to ask a question they can raise their hands also and speak on the mic. See any of those. Okay, so if that's the case. Thank you very much Justin for this introductory lecture and we look forward to hear more about this in the following days. Sounds wonderful thanks for having me and I'll I'll see you tomorrow. Thank you. See you tomorrow. And why everybody. Bye.