


S1: there's just one handout today on the lecture, and the exam's coming up, Wednesday, uh do you have any questions, coming up, on the exam? the review will be tonight, uh bring questions to the review cuz you'll really generate, uh, you know what information is given out, so, do that, uh go over your study questions, that you got there's there's example exams, on the web or on the web page for two eighty-one, and uh, i looked over the the first exam and this, the population stuff is on there, but there's a foraging problem and obviously, we haven't talked about foraging so, don't worry about things that we haven't discussed at all. so, any questions (coming up?) everyone's
S2: is the exam is in this building?
S1: exam will be here in this room at this time, and uh you'll be handed out the exam that you write on so you don't need to bring uh books or anything no calculators. be no need for calculators you can just show all your work and uh, so no calculators yeah?
S3: is today's lecture on the exam?
<SNEEZING SU> 
S1: i'm sorry what?
S3: is today's lecture on the exam?
S1: uh oh. good question i'm sorry it is not on the exam. okay? it will include up to last Wednesday's lecture. good questions. <P :04> but this will be on the next exam.
S4: do you have any of the handouts that are, outside your office, (xx)
S1: oh uh, i don't. if there we're missing some handouts that people need i guess the best thing is to tell me or email me and i could try to xerox some more but, i don't. um they're all there outside the door. basically. but if we're missing something i could try to replace them. or you could get them you know maybe from somebody (xx) okay so everyone's all set? for Wednesday? okay well today, i'm going to talk about population regulation. we've been talking about populations, demography, life histories, and uh, when I talk about population growth i showed you, uh this figure, where if we look at the patterns of population growth for different species, uh we find in some cases we do get a nice logistic pattern, where we have the model that fit very nice where the species grows to a carrying capacity, and then settles down around this equilibrium, but more often what we see are, populations that fluctuate, but around some, sorta K value or at least some, uh asymptote here. and the populations fluctuate within bounds. and this is what we mean by regulation. that is there's some factor that's causing the population, to stay within bounds rather than, completely crashing and going extinct, or going into exponential growth, uh indefinitely. and, uh we saw this then in the logistic equation, our population increases, up to this K, and stays there. and if the population, uh for some reason goes above K, then our little density-dependent term, will become negative and it'll start going down if it's below K, this'll be positive and growth will pu- put it back up to K. so we have an equilibrium, then. <P :06> and this equilibrium, in the density uh, th- or the density dependence in our logistic equation, is due, to... this density dependence, is due to what? or whatta we assume it's due to? it's in our models we just kind of assume i said the models should uh <NOISE DISTURBANCE> supply a logically reasonable parameters, well they assume it's due to intraspecific competition. for resources. that is what slows the growth. and keeps it around K and it becomes, relatively more intense as you get, near K. so in this one we have what we call K selection too. but uh, today what we're going to talk about, is that other factors can also act in a density-dependent manner, and this is when we start looking at species interactions. for example you could also have like predation, or predators, acting in a density-dependent manner. that is as the population reaches_ is_ gets more and more abundant, it may be easier for predators to find the prey, they might eat more of them, and so it gets proportionately greater, uh as a density-dependent factor. we could also have interspecific competition, that is between two species that becomes more intense as they reach their carrying capacity, or you could have things like disease, or parasites, that could all act as density-dependent factors. now they don't necessarily act as density-dependent factors but these are biotic interactions that could, show this density dependence. and then they could set the population, at some other K i think i gave you a little K, a saturation K, level... so. and uh to remind you, this density dependence occurs because either our birth rate, or our death rate is going to change, as the density or the numbers, change. and so for a population to slow down as it grows, larger what has to happen to birth rate? it has to go down right? so we have the birth rate declining, and and or, the death rate may increase, as the population goes up and that's what will cause the population to, uh stabilize, and what is this point right here? <P :06> [S5: zero growth ] birth equals death it'll be K. at zero population growth. that's when the population stabilizes right? it's right here. [S6: (xx) ] <P :05>so this is what we call a density-dependent relationship of birth rate or death rate or both. now this is in contrast to what is called density-independent... relationship and this would be where our rate, either birth or death shows no relationship to density. kay? it's neither negative or positive and it may just be kind of variable around this line. okay? so that's density independent. and uh when we have density-independent factors which i mentioned in the uh, uh lecture on life histories, what we see is a population, so if it's density-independent factors, the population N over time, you'll go_ the population may go into exponential growth, and then it might get hit by some catastrophe like a flood or blizzard or whatever then it might decrease and then it'll start going into exponential growth again, it may get knocked down by a bad, summer and then it may go into exponential growth and so, you end up with a pattern like this, rather than what we see with the logistic, where it stabilizes around some, carrying capacity. so this is where we have R selection right? and, the important thing t- here to realize is this density-independent factor, will determine the population, so we have determination, of population, but there's no regulation. <P :04> so regulation is something that keeps the population within some bounds. <P :07> now there's another interesting uh, density dependence that we can find, and that has been found, and this is where we have our birth or death rate, and let's just look at birth rate, and what happens is we have inverse, density dependence... and by this we mean that as the population is small gets smaller, the birth rate actually declines. so (this is) the birth rate. and then at some critical threshold point <P :06> the regular density dependence will kick in and it will decline as the population gets larger. so this is our regular, uh density, dependence that we think about in uh stabilizing population growth, but, you can have this kind of inverse density dependence, and this is called, the Allee effect. named after Allee who first described this. so what's the problem with this? well if the population gets stro- as it gets smaller as the birth rate starts declining what's gonna happen to the population? <P :04> it's likely to go extinct, right? so this is a real concern this could also be death rate too where death rate increases, as the population gets smaller. so, there's a problem_ pop- uh, possibility of extinction local extinction of this population. and so this is of great concern to conservation biologists. if you find this, effect and how could this happen? well it could be as the population gets smaller, it's harder to find mates. and as a result reproduction declines. or you could have species that are group hunters like lions, where they hunt together and they're more successful if they hunt together. if there are too few, they may be unsuccessful in hunting, and death rate could increase, birth rate could decline as this population gets smaller. and also uh some organisms really depend on large groups for some kind of social behavior. like the passenger pigeons. when they reach a certain size smaller s- size of population, they're just uh they didn't uh mate, and uh reproduce. so their social behavior, was disrupted. and so, this, effect is, of great concern and it has been found in fact, uh in certain species. so what we're very interested in here then is regulation, that will stabilize, the population. in contrast to this Allee effect, where this destabilizes. at low population. or density independence where we don't have this regulation we just have determination. <P :06> now in thinking about regulation we have some stabl- stability, but it can fluctuate within bounds, what is very fascinating to population biologists, is that you can have this fluctuations(sic) or, uh within bounds but they form cycles, which are regular, oscillations or fluctuations, over time, and uh, this has been found, this pattern of cycles has been found in many species, uh this is on your handout, and here's an- a, a caterpillar a pine looper, uh and you can see we have these fluctuations but it turns out they are, in a a pattern, uh where we have peaks, what about every ten years. okay? and lows, about every ten years. here are voles, and these are small like mouse-like, um, mammals, and uh here they have, uh, cycles where the peak, and the trough is, is like every three to four years you have a peak, population. and then uh also red grouse here's an a bird, and here we have peaks that occur like every four to eight years, and finally the classic snowshoe hare, which you've probably all been introduced to, where the population of snowshoe hares peaks about every, uh eight well, nine to ten years. so these patterns then as i told you in one of the first lectures is that, really we hafta look for patterns and when we see these patterns, they demand explanation so how can we explain, these population cycles? and uh, these species have acq- garnered a lot of attention, because people really want to understand why do we get cycles? why don't we get the stability at just one, um, uh value. and it is interesting that most of the cycles that we see, in species do occur in very simple environments, particularly in the boreal forest, or in tundra. in northern or more northern, areas. now voles do cycle here for example in Michigan. so but they seem to be in relatively simple, systems at least that's what we think so far. so we have these cycles, let's go back to thinking about our model can we take that simple logistic model? i told you it's a really simple model, then could we generate cycles with that? well it turns out, we can and how does that work? well let's take the logistic, and we have this here, change in population, over time equals, do you all have this memorized now? little R times N, okay times our density-dependent factor, mkay? and remember i said one of the assumptions of the logistic was i'll get instantaneous, density effects. so the population now, if we add one more individual to that, that density effect is gonna occur right now. as soon as we add an individual, it's gonna kick in and start changing, this uh, population growth. kay that was that was instantaneous. sorry i spelled that wrong. and uh, in fact, we may not always have this instantaneous effect instead there may be a time lag, or delay. so that instead what we find, is we have our population <P :06> where, we have the N at some previous time... affects the population at time now. okay? so, let's, uh, let's look at how this would, well first of all let's ex- let's give you a biological example. uh one example is just within a species and we can look at daphnia, and these are little water fleas they're very common in lakes very important fish food, and in daphnia, the adults of daphnia these little creatures, actually store food, as fat globules in their bodies. okay? and as a result when we have the population growth we have our logistic growth here, the uh it should be slowing down here, as they imme- start coming up to the resources K, but in fact these daphnia, have stored all of this food, so the population, can actually increase, keep on increasing above what it should be set by by resources. kay? okay and then it gets too far above and then, it starts kicking in we start getting negative uh, growth, but again, the population now starts to decline, and it's based on the previous population. the density effect is based on the previous population so it's gonna decline below, the carrying capacity. okay? so as a result, we can see that we can start getting cycles because of this delay, or this time lag, in the logistic. now another way we could get a time la- we could call this delayed density dependence then. <P :06> and this is usually put in the equations where we just use uh, a term tau <P :08> okay. but that's, it's just the gi- you need to know the idea of this whole thing, you're not gonna actually, uh, do this. and uh, so we have this delayed density dependence, and we could also get it, through another kind of a a a predator something like this. okay? or we could have a time lag, because say predators increase, but then they increase, and they stay at a high population, and uh, start forcing the population, then down below the saturation K. and so we get cycles and i'll, i'll go over that, again in a minute. so we can see then how a time lag, can start to give you cycles. and i have on your handout, where people have done this with our simple logistic, equation, where they put in a one-generation time lag, and here's the population growth with no time lag, and here's the population growth with one time lag. and we get these nice, cycles. okay? and it turns out that as we increase, the, uh time lag, okay? and if we have high R, and we increase time lags, these cycles get bigger and bigger, the oscillations get greater. okay? so here's an example of that, and, what finally happens, which is really fascinating, is that instead of these nice regular cycles which are called stable-limit cycles... where we have these cycles going on, and on over a long period of time, we start getting really violent fluctuations, and, we can end up with population growth, that might look like something like this. <P :06> and this is called, chaos. <P :06> so this very simple equation the logistic, remember it's a very simple equation, just by adding time lags and having a high little R, we can generate this really complex population behavior. it not even regular cycles anymore it's just violent oscillations. and yet it is generated by a very simple deterministic, equation. so here is N over time. so we have this simple deterministic equation, <P :05> we add a time lag, and we can get this chaos, or chaotic behavior. <P :06> and uh, it looks totally unpredictable. but is it's deterministic. and this was a really exciting and somewhat disturbing finding, because we're very interested in how often does density dependence population growth occur how important is density dependence, and here we have this model, and it doesn't give us just a sweep, logistic, it gives us this totally unpredictable-looking pattern. and uh it's uh this whole idea of chaos has come into many different fields now it's in physics it's in chemistry, uh it's in many different areas and it's really like a revolution, in trying to think about how the world, works and this actually came out of, Robert May who was a physicist who became an ecologist, and he explored these models, and he's the one who really started this by thinking about the simple population models, he came up with this, aspect of chaos. which is uh, very exciting and has implications in all these different fields. so, it is interesting then that we can take our simple little model, and we can generate very interesting, behavior. okay. in fact if we take a a real population cycle, this is with little lemmings, and they're like voles, uh and they have these cycles that where they have a peak about every three or four years, they have taken the original data, and this is on your handout, and uh fit, one of these logistic equations with a time lag, and they're able to predict or generate i should say, the population cycle, pretty well, that you see, in nature. and so just with this tau, or this value T, uh, we can fit, the natural population cycles. so, that's all really well and good, what can we say from that? does that mean we've explained it? well no we we could be right for the wrong reason right? maybe these cycles are due to something totally different. or maybe there is some density dependence in there. and maybe it's just within this population, or maybe it's created by density-dependent factors outside this population like predators. or food. so this has become the big question then, is what factors actually cause these population cycles? is there a delayed density dependence and is that wha- is that really what's causing the cycles and what are the factors, uh involved, with that? so uh, what i want to do then, is, give you some approaches that have been used, in trying to explain, these population cycles, in different species. and <P :06> oh first i'm gonna show you this, this is in contrast now to what has happened in some cases, uh this is a reindeer population, this is just to remind you of the difference(sic) things that could happen, these are reindeer that were introduced to the Pribilof Islands off Alaska, and here we see on one island the population went into exponential growth, and then it crashed. and uh, went extinct. okay? here's another island, the population grew s- kinda slowly and then just kinda stable- stabilized at a very low, low level. so w- what determines these different population behaviors? what factors, were going on, and then uh, so we can ask, about what factors cause these cycles, but really the way we do it can be applied to any kinda population. not just cycling populations. but these kinds of populations, as well. if i had the, first slide, <P :07> uh okay. can we push that over? that's great. (alright) cuz i had to show you my reindeer slide this is_ i was in Sweden this summer in Northern Sweden, and uh here are the reindeer the, uh Laplanders or Saami actually uh, herd these and so forth but uh they're_ during the summer they wander all around and, uh, browse, and, they're all over. it was wonderful. very exciting to see these, running around they're small, they look like, baby deer. it was really cute. and so that's, an example of reindeer. and uh, here is the area in Northern Sweden where this, where i saw these reindeer, and this is just to remind you that these cycles often occur in very simple habitats and this is more of a boreal habitat, where we have, mostly just uh spruce, and a few birch, and uh and some areas just hav- tend to have just grasslands, and many of these populuns(sic) are also in tundra, where it's mostly just, sedges and, uh, a few shrubs so these simple environments seem to, somehow foster, the, cycle, cycles... so first i'm gonna talk, a little bit about what are called microtine cycles, and microtine is uh, uh these are microtine rodents, and they include the lemmings, that we (were talking) about and voles, uh lemmings are more northern voles occur here if you go out into any field you'll f- see voles, and they do show cycles, right here in Michigan as i mentioned. and uh, and here is a lemming, aren't they cute? i think they're, adorable, but uh anyhow, these lemmings are very very interesting, and they occur in northern environments all over through uh, Canada and Scandinavia, uh, and, what is amazing is they have these dramatic population cycles. and the peaks tend to occur about every three years, and the population increase is just phenomenal. and uh, this has been um, known, historically for a very long time, and in the uh nineteen, twenties, Charles Elton, who was a very well known, ecologist in Britain at Oxford, got interested in these population cycles of lemmings in Scandinavia. and he was going to Norway, and uh, here they were well known for what are called their marches to the sea. and what you would see during these peaks, there'd be so many lemmings the ground would just be s- covered seething with lemmings, and they would reach the edges of the fjords, in Norway, at the edge of the ocean, and they would jump in, like uh, Gary Larson so nicely, shows here in this little cartoon, and so these were called the marches to the sea and like what what's going on with this? uh and they're fascinated Elton, and i'm gonna read a little bit from his, paper on this, so he said <READING> every few years these, lemmings migrate down from the lowland in immense numbers to the edge of the ocean, and the lemmings march chiefly at night, and may traverse more than a hundred miles of country before reaching the sea, into which they plunge, unhesitatingly, and continue to swim on until they die. even then they float so that their dead bodies form drifts, on the seashore. </READING> so this is the sort of pattern that grabs your attention. right? and we really want to try to explain, what is going on. and, first of all these marches to the sea are very dramatic, but in fact it's just a matter of dispersal. the population is getting really high, and so animals are trying to leave and find better areas, with food. and so there's a natural dispersal, and it turns out some populations, happen to disperse towards the en- edges of these cliffs (and) the ocean, and they basically get pushed over, or happen to fall over whatever they're obviously not committing suicide as some people have called these, suicidal marches and they aren't marching to the sea, they're just trying to disperse, and find better habitats. so these uh, uh drownding in the sea does not, explain the cycles that's not what causes, these cycles, uh these peak numbers, to decrease. it's not that many, individuals that are involved. and also these decreases, are equally dramatic. when they're in a low, you can hardly find a a a lemming. you can go out and search, and, you may not see a single one. so very dramatic, population cycles. and, people have tried to explain this for a long time, and i like this explanation, that i wanted to just, tell you about this was in fifteen thirty-two, and there was actually an explanation written about this, and the explanation is shown in this wood cut, and their explanation was that the lemmings fell from the sky, and were eaten then by ermine ermines which are these uh, weasels that turn white, in the summer, i mean in the winter sorry, they're, they're pretty, pretty too, but uh and here's the picture of these weasels eating, these little lemmings. okay. and the interesting thing this is a little idea about how science the the time difference that we have, because, in uh this explanation was given in fifteen thirty-two, and, it was confirmed, in fifteen ninety-nine, by, eyewitnesses, who were reliable men of great veracity or truth. so that was the state of science in the fifteen hundreds and see how long it took, for them, to even test this, uh, hypothesis. and of course Elton didn't, really buy this explanation as i don't think anyone here is going to do. and so he wanted to look for, what factors are actually determining, these population cycles, that are just so, so dramatic, and, one of the first factors, that uh, he thought about and others, was uh, could it be weather? that is somehow, there's a cycle in the weather and these are the good years and those are the bad years. mkay? and people looked at this for a long time, but there just aren't any weather patterns that are that regular for that long. so it just has never held up. and we just cannot explain, these cycles this may determine some of the population numbers at times, but it does not explain this sorta regulation, uh with cycles. so, people have dismissed that. now it is very interesting, that if you go back and look at the, hare-lynx cycle, this hare cycle, which is, uh nine to ten years, nine to eleven years i guess and then it just varies, uh, people did think there is something, that varies on that kind of cycle do people know what that is? [SS: <UNINTELLIGIBLE ANSWERS> ] i think i hear it sunspots. so people have actually looked to see, if for the snowshoe hare, this is related to sunspots. because they tend to flare up about ele- every eleven years. okay? so these other cycles, they wouldn't explain the microtine rodents but maybe it would explain the snowshoe hare. and it turns out that, this did fit for quite a while but then things got out of synchrony. and, it didn't work. so, the evidence is not looking good for that... kay... so people started looking at other factors particularly biotic factors, and, one of 'em was the thought of well what about predators or predation, uh, such as these, little weasels these ermines on the microtine rodents? because that could cause, uh cycles if you look at uh how predation might change, as the population grows, we might see uh, cycles occurring, and, let's just look at, just with you think about it makes sense, we have a high prey population, and, the predators then eat a lot and they're doing really well so they, reach a very high level, but then, as they eat up the prey the prey start declining, and then the predators can't get as much food, and they start declining even through lower birth rates or higher death rates, and, and then when the predators get rare, the prey can start increasing again. so you can see where we might have, the predation, actually, set up, cycles, in their prey. and people have done uh predator-prey models, and uh Dr Warner's gonna be talking about this aspect, he'll be starting your lecture next week and you'll talking about all these interactions, uh and including predator-prey, models so, he'll talk about that a little more but i think intuitively you can see, how predation could cause a cycle. and so what you can get, with uh predator-prey interactions, are what are called coupled, oscillations... you may not get them but you can, get these, in models and they've actually done laboratory experiments where they've been able to find, uh these coupled oscillations in uh, insect and as a parasitory. so and this is really due to a time delay. kay? or a time lag. because a predator's lagging, behind the prey. okay? so we're back to our sorta idea of how, we could generate cycles in our models. so we can see through models then, that predators could cause cycles, but is that what's causing the cycles we actually see, in nature? and, we tend to focus on, predators because they're so cool, i guess, but let's also think there's another sort of predator-prey, uh, uh interaction going on here, and that is that these animals, eat plants, the lemmings eat sedges, the hares browse on twigs, and so we could also imagine this, same oscillation going on, with the food, or plants, and food quantity, and quality, that is how nutritious the food is, and the herbivores. so we basically have a predator-prey, uh cycle. that could be, through effects on the food. the food (that) the, lemmings eat out the food, the food de- uh, decreases the lemmings decrease, and then the plants can recover. so you have again this lag time then between the food, and the herbivores. so, we really have two big biotic factors, that could cause these cycles, and that is, predator-prey, or food, interactions. so people um, have set out to study that, and, for some reason i can't get my handouts (xx) the, study i'm gonna talk about next, is the snowshoe hare study, and its main predator here which are lynx <P :06> and this uh, let's see (and) the next slide (xx) <P :04> here's a snowshoe hare, this is in the summer, in the winter it turns white, so it, is camouflaged against the snow, and uh, and they show these, uh nine to eleven-year cycles and here's, a lynx. which is one of the main predators not the only predator. but uh, one of the major predators. and, we have these records, of the hare and lynx cycles, and if we look at this we see, exactly what i told you, for the hare, as it increases, the lynx increase, but with a lag time, when the hares decline the lynx decline but with a lag time. and so we have the lynx, tracking the hares. so they sorta track, the hare cycles. but are they the cause of the cycles? or are they just following along with these hare cycles that are caused for some other reason? <P :04> kay now it fits very nicely but maybe that's not, the actual the lynx don't really cause these these cycles so that's our question. so, um, there's been a lot of research, done on this oh i know what i wanted to mention. and they have these incredible records that go back to eighteen forty-five. and the reason we have this long, time span, is because of the Hudson Bay Company and fur-trapping records. kay? because that's a problem with a lot of our ecological studies that we don't have long-term data and when we're looking at population cycles we need long-term, kinds of studies. and uh the Canadian government actually has uh, they have, convinced, the Canadian government that this is really an interesting problem, and the government has put in millions of dollars into a huge, uh uh, huge research projects trying to understand what caused, these cycles, in um Canada. and there's a_ one major person is a guy named Charles Krebs who's done a lot of work on both microtine rodents and, on the snowshoe hare, uh, cycles. and, uh he wanted to look then at the effect of predation, and food, which for the snowshoe hare, are twigs. they tend to browse on twigs of um, different shrubs and, low trees and so forth. and now um, actually i want to do a little aside here, if you think about predation, you would think that with the predation it's obvious why_ of course it must cause these cycles i mean when predators kill the prey, the prey have to decrease right? so it would seem almost like it's an obviosity(sic). but that's not necessarily true because you could have predation, where the lynx are killing all these hares, but it's possible they're just killing hares that would have died anyhow. and that the cycle's really caused say by food, and the predators are just causing what we call secondary mortality. <P :06> and this just means that the predators kill the prey, that are doomed to die anyhow. for other reasons... mkay. so it may be we see these nice coupled oscillations but the predators don't have anything to do with them. kay? now the predator cycles are just following the prey cycles. so this is uh, something we need to find out and the way, to test whether it's predation or food is to do field experiments. and this is what Krebs and actually many m- a whole group of people have been working on this. and, we're also dealing with a system, uh i think we can just actually, stop the slides, yeah thanks (we can get) a little more light here. um, and the scale of this is a real problem because when you think about lynx, i mean those, animals can (go) over large areas snowshoe rabbits can go over large areas, and uh, the experiments that i'm gonna talk about today, they had plots that were a kilometer squared. kay? so we're talking like big-scale, experiments. and, they had, two plots, a kilometer squared each, where they added food, for the hares, and they did this by adding commercial rabbit chow, it's known to be good, for bunnies, and so they increased both the quantity and the quality... kay. and then they had only one plot, where they did a predator exclusion, and here's where th- i mean there weren't any replicates on this but you can imagine how difficult it is to exclude all the predators in a kilometer square, of area, and uh they went and they live-trapped out these predators, uh, there were also, coyotes, and lynx and uh, well two of the major predators and they put_ did this, they live-trapped 'em and then they had an electric fence, to keep them out, kay? and then they had um, one plot, where they had both, food, and, predator exclusion... they also had, two plots, where they added fertilizer. and this was to increase the plant quality. in case plant quality, and quantity was driving these cycles so this was actually through the plants, the natural food that the snowshoe hares are eating and then they had three controls. okay so, not many replicates but i said it's uh hard in field ecology to get, the kind of replicates you can do in lab just because these experiments are so, huge. and what they did is they_ and then the time scale, they followed these, hares' population patterns for eight years, and they followed it then, through uh the start of a cycle through a peak, and through, to another low. mkay? so this was the time-frame of the experiment. so um if you think about any lab people, um, i don't know how many of you start out intending to do an eight-year, project where you're gonna get results in like eight years. okay so, what'd they find out? okay well i have this on your handout this little figure, and here we're just going to look at, what was the effect of these different treatments on the survival rate? this is our L-X. cuz that's gonna tell something about whether the population's gonna, increase or decrease. and if it's in the decline phase, if L-X increases it means the decline will be less, or it may mean be that it'd even stop that decline. okay? so what did they find out? well first of all, fertilizer had no effect it was not significant... so it seemed like even though the twigs sort of disappeared and there is some change in the plants they couldn't seem to, change, the plant effect. and doesn't mean it doesn't happen but somehow this fertilizer did not work. okay? and then they looked at food the rabbit chow and they found out that in fact survival rate increased, over the controls and that was significant. so what this means, is that this decline, then was less if you added food. okay? <P :06> and so this would then affect that population cycle. but the cycle, is still going on. okay well then they looked at this, predator exclosure and here, they found out that the survival, rate was even greater. okay? so again, we have this decline is less <P :04> but the cycle is still there. the pop- the hares is still going to, go through this decline. so yes, but, uh, it doesn't stop the cycling. and then they did looked at the experiment where they combined both of those, and this was the most significant. and the interesting thing here is if you can just look at this, if we take the food effect, and add that to the predator effect, it would be about here. okay? so when you do both of these, you don't just get an additive effect, that is that we add the food effect to the predator effect, but we get an even greater effect than you would predict by this additive, effects. by adding the two effects so this is called an interaction. so we seem, to be that the predator, and the food, interacted and so we have a significant statistical interaction. that is we're getting a bigger effect than we would predict by either one, uh food or predator, removal alone, and here they found, that the decline, virtually stopped. not completely, but pretty much, and it pretty much stopped the cycles. <P :04> mkay. so the conclusion then from this incredible set of experiments, is that it may, be not just is it predation or is it food, but rather it's an interaction between the two of them. and, why might there be, what is the biological reason for interaction? <P :04> well, they don't know, but some of the hypotheses are, that when you have uh the predators are abundant... it may be that the hares, perceive this risk, of predation, and they may forage less, they may eat less, and so their survivorship goes down, and uh Dr Werner's gonna be talking about this, because to avoid the predation they don't go out and eat as much, or it could be that as food quality goes down... or, as food quality goes down they're more susceptible to predation... that is they can't escape, as easily. they're not in such good condition. or it could be, that instead they're searching more for good food, so they're more exposed to predation. so all of these factors then, could, uh cause this interaction, effect that they believe they saw. now we only have the one, one replicate but, um these are all hypotheses, as to why they saw that effect. and, so this is one of our most complete studies, uh let me just mention that the, for the, microtine rodents, they have very similar results if they remove all the predators, they can pretty much stop the cycles, but there also appears to be a food effect, so this may be general, uh that both interact, but, the end of the story still, is not in. people are still studying this, and uh, well i guess i'll end there it's very fascinating and we're still trying to figure out what's going on. so...
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