Interspecific Competition

Lecture: The Logic of Science

Prof. Stephen C. Stearns

OPEN YALE COURSE EEB 122: Principles of Evolution, Ecology and Behavior

Yale Univeristy

Overview

Competition among species, or interspecific competition, can have an even greater effect on selection than competition within species (intraspecific competition). This is often the case in lower density populations. Different species can have positive, neutral, or negative effects on each other’s fitness, and the effect species 1 has on species 2 is not necessarily the same that 2 has on 1. The effects that cohabiting species have on each other shapes evolution the same way that selective pressures from within a species or the physical environment shapes it.

Part II:

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Interspecific competition

Prof. Stephen C. Stearns

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（Chinese and English subtitles：杨薏琳）

Today we’re going to talk about inter-specific competition. And you will remember that last time I was talking about intra-specific competition, and we were looking a bit at the impact of density on population growth. And I want you to remember that if an individual is encountering increased population density, during its life as it grows from a zygote up through an adult and reproduces, the effect of increasing density will be to decrease its growth rate. It will be smaller when it matures. It will have fewer babies. Often the babies will be of lower quality, and more mortality rates in the population will be higher. So that’s the general impact of intra-specific competition. However animals don’t live in a world and plants don’t live in a world where they only encounter other members of their own species. They‘re often living in a complex ecological community. And so what we want to do is to understand what happens to them in a complex community. And I’m going to do this by showing you some classical descriptive patterns; then move through to some experiments to demonstrate competition; give you an abstract conceptual framework in which to think about this; and return to the complex reality of competition at the end. Okay? So a reminder that it’s often useful, when you thinking about interactions between species, to just use simple logic. You’ll remember in an earlier lecture I said, “Don’t be afraid of using simple logic in constructing you papers.” This is the kind of thing that could lay down a nice simple introductory logical framework for something that you’re dealing with. This particular one is about biological interactions in communities. So if specie’ 1 effect on species 2 is in this column, and species’ 2 effect on species 1 is in this column, and specie 1 have a negative effect on species 2, and specie 2 have a negative effect on species 1, then we’ve got a competition. If specie 1 have a positive effect on species 2, and specie 2 have a negative effect on species 1, then specie 1 is food, and specie 2 is eating it and you’ve got predation,parasitism or grazing, If specie 1 have a negative effect on species 2, and specie 2 have no effect on species 1, we called that amensalism, and you can contrast that with commensalism where the relationship is +0. So here, for example, specie 1might be benefiting:its effect on specie 2 is positive. So specie 2 is benefiting from its presence, but there’s no effect the other way around. So basically what’s going on here is 

specie 2 might be living in specie 1 but not impacting it otherwise. It might be small or it might not...it might just fit in, in a very comfortable way. So this kind of plus/minus framework is a useful overall way to see what we’re concentrating on today,which is competition. And here are some of the natural history patterns that suggested to ecologist that inter-specific competition is important in Nature. This is one of Jared Diamond’s observations, from New Guinea. And this is a thrush. There are a couple of species of thrush there. There are two species in the genus. Usually when two species are in the same genus, they’re ecologically pretty similar, they tend to eat pretty similar things. And the interesting thing about this pattern is that you are going a mountainside, and this species is getting more and more and more abundant, and suddenly it disappears. And if you’re coming down from the top of the mountain,this species is getting more and more abundant, and then suddenly it disappears, right here about 5500 feet elevation; just about one mile high. So it looks like the two species are butting into each other and they’re excluding each other right at the point where they are each doing great. So that’s the suggestive pattern. It doesn’t demonstrate that the reason for the border here is inter-specific competition. Anybody have an idea what else might do that? They each carry a disease that knocks out the other one. Okay? That will do it. It could be that this one actually can resist a predator that eats its eggs, or something like that, and this one can’t resist that predator, and vice-versa. So it could be a apparent competition. Okay? Without this experiment you don’t actually know what is making that sharp line. But it’s suggestive, it looks like it could be competition. Then Robert MacArthur did a lot of work on warbles for his Ph.D.thesis. And the thing that you want to concentrate on in this picture is the different parts of the spruce tree that the different warbles use. So the Cape May...this is a diagram of half a spruce tree, cut in half using the middle and lower inner part. The Blackburnian warblers is using the outer middle third about. The Bay-breasted warble is using about the upper third. The black-throated green warbler is using some about the inner part here,and the Myrtle warbler is down inside the tree, about here, and then down close to the ground. So it looks like the warblers of North America ave taken the spruce tree and they’ve divided it up so that they don’t run into each other too much, and they’re each using a different part. That’s an interesting observation, because if you think about it, what it means is that they are guaranteeing that they’re going to run into more intra-specific competition.They’re avoiding inter, but they’re running into more intra-specific competition. So that if you say, “Hey, this has been caused by competition,” you’re making an implicit assumption about how strong those relationship are. This one is saying inter-specific has probably been more important than intra-specific competition. Okay? So again a very suggestive pattern, but no confirmed by experiment. Now, those sorts patterns led Evelyn Hutchinson to suggest this kind of view of why we find so many species on the planet. He says basically over a long period of time evolution will fill the world up with species, and they will get better and better at exploiting the resources they encounter and the competition will limit the number of species you can pack on the planet. At about the time that Hutchinson was thinking about this, various people, including Garrett Hardin, were enunciating what’s called the competitive exclusive principle; that two species with very similar resources needs in physiology can’t co-exist in the same place, at equilibrium. So the assumptions behind that are that we’re looking at an equilibrium that’s established after a long time; that the competitive principle applies. That means two species cannot occupy exactly the same niche...which is what it looks like is going on with the warblers and the thrushes in New Guinea...and that diversity is determined primarily by competition rather than by predator or by disease or something like that. So let’s see to what extend this kind of thinking has occasionally been confirmed in Nature. And one of the first field experiment, showing that competition was really quiet important, between two species, was done by Joe Connell. Joe Connell is much revered Emeritus Professor at the University of California at Santa Barbara now. He did his Ph.D.thesis at the University of Edinburgh in Scotland, and he worked on barnacles that were living in the rocky intertidal in Scotland. And in particular he contrasted Balanus balanoides and Chthamalus stellatus; and balanus and is big and Chthamalus is small. And the patterns that he found...and he confirmed this by doing manipulation experiment in the field...is that the larvae of Balanus, which is big, settle out over a big tidal range. This is mean higher water; this is mean lower spring water; this is mean lower normal tide range. Okay? So once a month the tide gets this high, and at that point the larvae of Balanus can settle out over the whole tidal range. However, Balanus is sensitive to drying out, and so as these larvae grow up, many of them die because they’re getting desiccated, and so the upper range of Balanus drops. However, it dose just fine over through whole tidal range below that. And down here, the problem that Balanus is encountering is that primarily there are other Balanus around crowding them out. And, by the way, when two barnacles grow right up next to each other, one of them can actually grow under the other and pry it off; so it’ll fall off. So you might think that  barnacles look like extremely boring, slow moving rocks, but in fact they have a little bit of direct competitive activity for space. Chthamalus is a little guy, and what Chthamalus dose basically is it gets a refuge up in the dry part of the upper intertidal. Its larvae can actually settle pretty high up, and it can survive up here, when Balanus cannot. So its problem is primarily getting scrunched by Balanusat lower tidal ranges. And by doing cage experiments and removal, where he actually took one type or the other off the rock, and came back...and he actually mapped them out, so he followed each individual...Connell could show that this was going on. Another famous set of early competition experiments were done by Gause. Gause was a Russian ecologist, who became a Russian epidemiologist, and he did some early work, back in the 1920s, using Paramecia. And he used three species: Aurelia, Caudatum and Brusaria. And what he showed basically is that if you grow them alone, this is their density, that you can measure, per milliliter; and if you grow them together, that Aurelia will exclude Caudatum completely; and if you compete Caudatum and Brusaria together, they can actually coexist. So from these early experiments it did look...by the way, in this circumstance they are both persisting at much lower population densities than they live alone. You can see that from the y-axis. This goes up to 75 than goes up to 200. So they are depressing each other’s densities. You can tell that they’re competing because of the observation. But they manage to coexist, they don’t go extinct. So it looked here like there were two possible options. One wins and the other goes extinct, or they coexist. Now over the last fifty or sixty years, there have been a lot of competition experiments done, and after Coonell, and then later Bob Payne, who is working more on predators, did these removal and caging experiments, people had a frenzy of experimentation out in Nature, where they would remove one species and then see what happened to another. Nelson Hairston did it with salamanders down in North Carolina, on a huge scale in the Appalachian Mountains, and managed to demonstrate that if you pull one salamander out, that the density of the other one increase, and they grow faster and they have more babies. So you can by moving one, you can demonstrate that there’s competition going on. In all those many experiments, one of the most important take-home point is that the results are usually highly asymmetric. That means that the removal of one of the competing species has a much bigger impact on the other; species 1 having a big impact on species 2,and removing species 2 often doesn’t have such a big impact on species 1. So asymmetric competition appears to be fairly common. And it can get so asymmetric that it may be amensalism rather than competition. So in some cases, you get removing one has no impact on the other, whereas you do it the other way around and there’s a big impact, Okay? So there’s a continuum of these kinds of cases. And often what’s going on is that competition for one resources is reducing the ability of a species to compete for another. So, for example, if plants are competing by shading each other out,,then the plant that’s getting shaded is having a harder time building roots, which is giving it difficulty getting the water. So you can see there can be a cascade of effects that inter-specific competition might trigger. And just as the side note, plants have developed an easy warning system to indicate whether they’re coming into competition.they can’t tell them whether they’re dealing with another species or not, but if they’re starting to get shaded by another species, or by another plants,then the ratio of near-red to far- red light, that’s coming into their chloroplasts, is getting shifted by being shaded, and they actually produce a hormone,that gives the signal to the plant, oh, I’m getting shaded in that direction,and they will grow a branch out in the direction,so they’ll grow away from shade and toward light, and they actually have an early warning system that’s detecting competitions. Annie Schmitt, up at Brown University,has done interesting work on that. Now how to conceptualize all of this? Well if we take those observations we had, from the last lecture on density dependence, this guy Verhulst, who was a Belgian demographer, developed a simple modification of the exponential equation that we were looking at. And the key idea there is that asa density goes up, the per capita rate of increase is going to decline. So it’s going to decline linearly until it reaches 0 at K. And if you look at this, right here...so if density goes up, and when N=K, you have 1 minus 1, which is 0, and that means that the rate of change of population per unit time equals something time 0. okay? So it levels out; and that’s what’s going on right here. This is increasing, but the rate at which it’s increasing is being affected by the density. So density is on this axis, K is right here. And as you get that N closer and closer to K, this part of it’s getting closer and closer to 0. So the multiplication rate is dropping and it smooths right out. In fact, in this simple model, the maximum rate of increase is here,when N is equal to K/2. There are the two man that extended that into multi-species competition: Alfred Lotka and Vito Volterra. And Lotka was a demographer at Johns Hopkins, and worked between about 1915 and about 1935 mostly. And Vito Volterra was a really eminent Italian mathematician, who had a son-in-law who has engaged in fisheries management in the Mediterranean, the son-in-law would occasionally have dinner with the father-in-law and would bring to the father-in-law certain conceptual problems dealing with the fisheries, like, “Well let’s suppose that we have two fish species that are competing with each other, but we can conceive of the fishing fleet being a predator acting on them; how should we predict the dynamics?” And Volterra, who really was a pretty profound mathematician, found these problems amusing, and he tend to write down the answers on napkins at dinner, and hand them to his son. So they were more or less throwaway lines for him. Now the way that they...these two guys who came up with the same way of conceptualizing these problems...the way they conceptualized it was this. This is the essence of it right here. They’ve both decided that what they would try to do is basically use the single species framework and just convert the density of the other species into an equivalent number of this species. Okay? And that’s what these alpha competition coefficient are going to do. And so they wrote down differential equations that have a term up here, that includes the inter-specific effects. And if you look at the logistic equation,which had that nice smooth approach to a saturation point, you’ll notice that there is a chunk of these equations that looks just like the chunk of the logistic equation, except they’ve stuck in this little term here. So what they’ve basically saying here is this, I’ll read it out in English. “The rate of change of species 1 is equal to the intrinsic rate of increase of species 1, times the number of species1, which are present, times a factor that you’re using to account for density.” And the way you account for densities take the caring capacity of species 1, and you ask,”How faraway from that carrying capacity are we?” Well we have to subtract the number of species 1 which are there. Okay? So we might be a long away from carrying capacity, because we have some of species 1; and we convert specie 2 into a number that’s equivalent to a number of species 1. And we do the same thing over here. So this is very similar to K-N/K. And you’ll find that basically when this number here, N1+alpha 12N2=K1, we have K1-K1, which is 0/K1. So as this gets larger and larger, the rate of increase is going to smoothly go to 0. So they did posit what is probably the simplest way to make that one species situation into a two species situation, and they did it by assuming that you could just convert one species into the other, in terms of its impact on inter-specific competition.