EEB 122: Principles of Evolution, Ecology and Behavior

Lecture 28. Ecological Communities

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-28

Today we're going to start talking about ecological communities; and this builds on our prior discussion of competition, predation and disease.?

I'm first going to give you the classic view; by classic, I mean the view of community ecology from about forty years ago. Then I'm going to talk about something that has become a very dominant and influential point of view more recently, which is top-down control and trophic cascades. Then I'm going to emphasize the importance of history. So communities are not some kind of isolated microcosm, they develop in the real world and they have a history, and that's important. And they also exist in space.?

So next time, actually, we're going to talk a bit about geography, island biogeography and metapopulations. This is making kind of a jump ahead into that. Dispersal really does make a big difference to the structure of communities; and I'll mention that briefly in this lecture.?

And the take-home message really is that communities, which consist of basically all of the species that you find in a local habitat, and how they interact with each other, have been shown to be shaped by all of these things: competition, predation, parasites and pathogens; so in a real system they all interact. And it is one of the aims of community ecologists, out of this complexity to pull a few take-home messages that they can apply broadly.?

The classical view on competition-driven species packing, on the planet, was developed by Evelyn Hutchinson.?Why are there so many different kinds of plants and animals? And his answer is because evolution has packed them tightly into the available niches.?

• If it has done so, then there should be a limit to the similarity of competing species.?He'd seen these two species living in this pool, and they're not the same size, they're different. And he's wondering what is the evolutionary process which has caused them to have different sizes? And he thinks it's competition.

• And so he measured them, and then he went back and did a literature review and came up with the claim that the body size of the smaller one seems to stabilize at about 75%?as a length measurement of the body size of the larger one.

• And from that he gets a measurement when allopatric and a measurement when sympatric; and basically the summary of all of this is that species differ more when they're living together than when they're living apart. It's as though when they are living together, they're being pushed by something to be different. And that was taken as an estimate of limiting similarity under competition. So that's a classical view.?The world is the way it is because it's shaped by competition.?

The second idea is this idea of top-down control. By top-down, basically the top is the top of the food chain, and down is the bottom of the food chain. So this is about predators eating things in a food chain. And in particular it can interact with competition.?So one of the early ideas was that if a top predator prefers to eat prey that are competitively superior, then predation enables the competitively inferior species to survive.?

• What would happen is if you pulled out the predator, so there was no top level in the food chain, and you just had say the herbivores competing with each other--the things that would be eating algae or plants competing with each other--then you would find that the dominant competitor would exclude the inferior competitor, which would go extinct, and you would have a simpler community.

• So in this sense predation is maintaining biodiversity in the community, and it's making possible for the inferior competitors to do well because they're better at escaping the predator.?

Now energy is flowing up through trophic levels in the food chain.?The primary carnivores are eating the herbivores; the herbivores are eating the plants that are trapping solar energy. And when we talk about top-down control, we're basically talking about either the secondary or the primary carnivores, usually controlling what's going on below them in the food chain, going up this way, and the omnivores and the detritivores are usually off in another loop.?

So the food chain idea, that you go up through a simple linear chain, is actually quite a simplification. If you look at a food web, then things get a little bit more complicated. There are potential flows of matter and energy in quite a different number of different directions in this food web.?So when we simplify that picture and say there's top-down control, I just wanted to have that in the background.?

The?ideas really come from Brooks and Dodson's observations on a natural experiment with alewives, and Payne's manipulation experiments with starfish.?And the effects of predation are seen, under this hypothesis, as propagating all the way through food chains or food webs.?

• You have an alewife in the lake, and you have an array of zooplankton?of different size.?This is the size distribution of things in the lake, when there are no alewives.?You put an alewife into the lake and you shift the size distribution down to the smaller size categories. So the idea behind this is that when there are no alewives, the bigger guys are out-competing the little guys. You put the alewife in to eat the big guys, and it gives the little guys a chance.

• And basically the ones that are taken out are Daphnia and this Mesocyclops; which is a copepod. And the ones then which are allowed to increase in frequency are little things like Ceriodaphnia; that's plankton, which is a rotifer; Bosmina, which is another cladoceran, a relative of Daphnia; and Tropocyclops, which is a small copepod. So simply by putting this fish into, or taking it out of a lake, you completely restructure the community.

• Big things take longer to mature, and small things mature more rapidly. So what's going on here is not just a shift in the size distribution of the species that live in the lake, it's a reorganization of the whole population dynamics and rate of energy transfer of things in the lake as well.

• The?presence of the fish in the lake, causes an induced response in the life history of the zooplankton, and the induced response, which is basically that they shift a lot more energy into reproduction--not of growth, because they're going to get hammered by the predator--that causes them to reallocate their fat reserves in such a way that the ratio of elements in their bodies changes.?

So these kinds of changes have very deep-reaching consequences that cascade through the entire community and affect not only energy flow, but the ratios of elements like carbon and phosphorus and nitrogen, that you find in different parts of the food chain.?

What's going on in the rocky intertidal is that there's a starfish which is eating snails,?limpets,?mussels and?barnacles.And if you take out the starfish, which prefers to eat mussels, then the mussels exclude everybody else; you just get a forest of mussels that covers the intertidal. And you put the starfish back in, and it will eat up the mussels and clear space that allows the others to exist. So this gives you the general starting idea of a trophic cascade.?

Pace, Kohl, Carpenter and Kitchell?define a trophic cascade as an interaction that yields an inverse pattern in abundance or biomass across more than one link in a food web.?So, as an example, if you have a simple three-member food web, if you have abundant top predators, they are reducing the numbers of the middle level consumers, and they're increasing the biomass of the basal producers. So?the enemy of my enemy is my friend, is the idea behind a trophic cascade.?

The?field manipulation experiments are conducted in such a way that?top predator was a spider, and the herbivore was a grasshopper, and the plant was a grass or a forb. And there is another case that the killer whale switched onto sea otters. The sea otters declined. Because the sea otters declined, the sea urchins went up, and because the sea urchins increased, the kelp went down. So this is from that paper by Carpenter and Kitchell and others.?

• Here is a summary diagram of what one means by trophic cascade. So you have a carnivore or a herbivore and a plant.?And this summary diagram actually introduces a new element. Besides just changing the numbers of the population, at the intermediate level,?the?presence?of a?predator--the carnivore here--will?cause a herbivore either to reduce its activity or to shift into a different habitat.

• If you have a lot of spiders, they will cause the grasshoppers to reduce their activity, and that will give an indirect positive effect on the growth rate of the plants. And if you have some grasshoppers that prefer to eat one kind of grass, but if the spider is particularly good at foraging in that kind of habitat, and you introduce a spider, then it will cause the grasshoppers to switch onto the less preferred plant, and this actually causes the community to reorganize.

• You shouldn't think that the effect of a trophic cascade is just on the numbers of the herbivores, or of whatever is at the intermediate level. It changes their behavior, their?habitat preference,?their life histories, their?ecological stoichiometry, and even?the ratios of elements in the freshwater system.

It shows?you that trophic cascades have been now identified in marine, freshwater and terrestrial systems. For example, mantids affect insects which affect plants; lizards affect grasshoppers which affect plants; wolves affect moose which affect balsam fir; mosquitoes, mosquito larvae eat protozoa, which eat bacteria; etcetera.?

Why predators might prefer the competitively superior prey??There's more of them, and so over evolutionary time they may very well have been shaped by evolution to be really good at eating the things that were most common. And so if you are an economically clever predator, you might take the biggest packet of energy available.?

So during the last Ice Age, patterns were created in North America that have allowed people to reconstruct what's going on, and Margaret Davis did excellent work reconstructing the history of the reassembly of the forests of North America, after the ice went off. And Rosie Gillespie is currently doing a similar kind of reconstruction of the assembly of communities in Hawaii. And in both cases there are probes into time. In the case of the deciduous forests, the probes are essentially pollen.

• At the time of the full glacial extent of the Wisconsin Ice Sheet, you'll notice that it probed down well into southern Indiana here, and actually it came--right under here it is out over Long Island. The shoreline had dropped, as water was withdrawn from the oceans into the icecap. We had tundra conditions in southern New York and in Pennsylvania, and you had a spruce pine forest that went all the way down into southern Georgia and north Texas, and then you had a kind of a refuge down here for deciduous trees.

• If you wash the sediments out, pollen has the great advantage--if you're a palynologist--of preserving its shape very well. Pollen is really tough stuff. The plants have evolved a lot of ways of preserving their precious DNA, and that serves us well.?You're not looking at DNA here, you're looking at the various kinds of structures that pollen gets packaged in. The important thing is they're really well preserved in pollen, and you can tell the difference.

• When Margaret reconstructed these things, she discovered that the first things to come back north, after the ice melted off, were spruce and larch. And so this is basically how long ago it was that spruce got this far north: 14,000 years ago, 12,000 years ago; 10,000 years ago it was pushing up into Canada. Larch moving up at a fairly similar rate; up here getting a bit north of Quebec City, about 8000 years ago. So you can reconstruct that pattern for these conifers, and then you can also reconstruct it for hickory and chestnut. So hickory gets to Maine about 4000 years ago, and chestnut only gets there 1500 years ago.

• And, by the way, just as a note on the importance of disease in structuring ecosystems, at one point there was evidently an outbreak of a forest disease that pretty much cleared out an area that ran from Massachusetts to Minnesota. That happened about 10 or 12,000 years ago. So the forests just basically all died, and then they were re-established, and it probably was a disease or a forest insect outbreak that did it. That's also in the record.?

So they come, in at any particular point, at different times. Now that's an important comment on the idea of ecological succession, because a modern community, if you were just to look at it in time, and you were to forget history, you could take communities from across a broad range of North America and they would all have very similar species compositions; they would all have these different hardwood and conifer species in them.?

• It might be that you would postulate?that these species had to be there in a particular mix in order to co-exist.?The historical record indicates that's not true at all. They had quite different sequences of assembly. So it wasn't as though one species prepared the way for the next. One happened along in one place, and another happened along in another place, and they all ended up making communities of a fairly similar nature.?

• In particular, the hardwoods came in, in a lot of different sequences. Some are going fast, some are going slow. So it looks like the tree species you see in an eastern hardwood forest can be assembled in an almost random order.?

• The rates of their dispersal are determined a lot by what kind of seeds they had. Something like a hickory or an oak is going to move north about at the rate at which squirrels bury seeds. So it's going to be about 50 or 100 meters per generation, max. Something like a maple, that has a nice little helicopter seed, can go north on the winds and can move hundreds of miles at a clip.?

So the take-home message from Margaret Davis's analysis of the eastern hardwood forest is species assemblages come together in a potentially fairly random sequence. It isn't as though one thing prepares the way for the next.?

Now let's take a look at what's going on with spiders that are ecomorphs in Hawaii. There is?something going on with these spiders that looks something like the lizards in the Caribbean, the anole lizards that have different ecomorphs in the same community, and that that pattern repeats itself on different islands.?And you should think of the spiders as having started from Kauai and then moved down the chain. And there are two things that can go on. You can either have speciation or you can have assembly.?

• The difference in those words means the following: If you have assembly, it means that a bunch of evolution went on and generated a bunch of species, which we have over here in a pool, and then they migrated; and then communities got put together, with pre-existing species.

• If you have speciation going on, it meant that an ancestor came in and that it speciated locally, on whatever island, and that a community got assembled out of the clade that developed, with the different species in it.

• The diversity of the communities, which is indicated when you have four of them present, appears to be highest--you only, you get that on Maui and you get it here, on Oahu--it appears to be highest in communities of intermediate age. On the Island of Hawaii, which is young, and on the Island of Kauai, which is older, the communities appear to be simpler.

• One of the important things that sort of confirms Hutchinson's view of species packing is that Rosie has never found two species that share the same ecomorph in the same place; so that locally these things sort out.?And the maximum number of spiders you're going to get locally is four. And they're going to be in these different ecomorphs. They're going to be using different parts of the habitat; they're going to be behaving differently; and they're going to be colored differently.?

So the phylogeny indicates that the ecomorphs are arrived at convergently, and the phylogeography indicates that both speciation and assemblage have put them together into communities; and the local pattern indicates that you're never going to get more than four.?

• The historical and the spatial perspective put together suggest that there's no such thing as a local community. Every community on the planet has been influenced by large-scale processes in space and time.

• The regional and the historical forces that are acting on communities are just as important as predation and competition; maybe even more so. So if you want to understand those processes, you have to analyze them at a scale that's big enough to see them. And therefore there is now a discipline which is called macroecology, that tries to study these things at the scale of the planet, if possible.

• At that scale you can see that dispersal is integrating things across regions, and therefore it is causing the process of assembly to take place; dispersal is bringing the species in from other places. And we can trace that process, as you've seen with Rosie's phylogenetic tree, using molecular phylogenetics.?

If you create a landscape in a computer, and simulate this process, there's an interesting pattern that pops up. This is from some of Michel Loreau's work. On the Y-axis we have species richness, and over here we have how much dispersal is going on; how much are things moving around the landscape? And we have a couple of different definitions. We have local diversity and regional diversity.?

• Local diversity is going to be how many species you can count in a local patch, and regional diversity would be how much can you count in a state? Local diversity say for a lake in Connecticut would be how many species are in Lindsley Pond? Regional diversity would be how many fish do we find in the State of Connecticut?

• So the local diversity first increases and then decreases, as dispersal increases. So that means that the uniqueness of each species assembly is going to start going up, as you increase dispersal from 0. And then as you start dispersing right across the whole landscape, you homogenize it. So the uniqueness of a local community goes down.

• The regional diversity stays stable until local diversity hits a peak, and then the regional diversity starts to decline, along with the local diversity. So as you just increase dispersal more and more and more and more, you decrease the opportunities for unique species to exist in places where they're favored by isolation, and you get maximum biodiversity at an intermediate level of dispersal.

Then what we see is that a tropical forest really isn't completely assembled by random dispersal, from some kind of regional pool of species. Some species are much more widespread than expected.?

• If you look across distance, and look at the fraction of the species that are shared between any two points--in Panama, which is a fairly small country, on this scale, you can see that as you go out, the fraction of species that are shared, say if you took it two points that are fifty or sixty kilometers apart, they're only sharing about fifteen percent of their species.

• But if you go into Ecuador and Peru, and you look at the fraction that are shared, across the huge expanse of trees that are available in the Amazon Basin, you find that there are some trees which are shared at great distances. They've spread across the forest and they are right across much of the continent.?

Okay, today I've tried to give you a bit of the intellectual history of community ecology, starting with an idea of communities in some sort of stable equilibrium, determined by competition. Then the idea of top-down control, through predation and dispersal; predation giving top-down control, and then dispersal and assembly in a regional framework. And then the perspective of time, that communities are assembled through time, and that this occurs at a geological timescale, with glaciers coming and going, and at a longer time scale with continents drifting. I think that really the communities are driven mostly by the glacial cycle, rather than by the continental drift cycle.?