Tom Wessels: The Ecology of Coevolved Species

Shortly after World War II, Western scientists came upon the principle of self-organization. So, what self-organization is, it's a process that happens in complex systems while they're developing. And as they're developing, these systems are not just getting bigger, but they're getting more complex. And that complexity is derived from the parts within the system, through time, becoming more and more specialized and more and more tightly integrated together, such that each component or part, like a species in the ecosystem, while doing what it needs to do to support itself, creates conditions that support the whole. And as a result, these self-organizing systems grow increasingly stable, resilient, and energy efficient. We all went through this process as well in our developmental phases. So, we started life as a single microscopic cell. Now, our bodies have something like 254 different cell types. And the specialization goes even further. So, for example, some nerve cells will communicate just with a muscle cell. Other nerve cells will communicate just with a sensory cell. And then there are neurons that connect motor neurons to sensory neurons. So, our cells are really, really specialized. But luckily for us, they're tightly integrated, such that each cell doing what it needs to do for its own purposes creates conditions that sustain the whole. And as a result, we are very stable. Our bodies are very stable. The internal temperature, right at 98.6, blood pH at 6.8, blood sugar levels carefully controlled and very stable. And we're resilient. If we get sick or we get wounded, we heal. So, we're a great example of this process of self-organization. But this film is going to look at this process and how it plays out in ecosystems through evolutionary time via the process of co-evolution, which is a self-organizing process. Now, in nature, the bottom line currency is energy. And it's a finite resource. It's not unlimited. So, natural selection is always selecting for organisms that develop adaptations or strategies or behaviors that make them more energy efficient, because they can support a larger population on the same fixed amount of energy. And natural selection is also forcing out species that are energy inefficient. And this is the primary driver of co-evolution, which, as we'll see, causes ecosystems to become more and more complex, more and more species-rich, and more and more stable and resilient. Now, ecologists have warned against moving organisms from one part of the world to another part of the world, because that generates young ecological interactions where the species have had no time to co-evolve. And those interactions are usually harmful for one or more parties, and they're always very energy inefficient. So, a good example of this would be the accidental introduction of the chestnut blight fungus into the United States in 1904. That fungus came in on Japanese chestnuts that had co-evolved with the fungus, so that it was a very mild, mild form of parasitism. But the American chestnut in this fungus had never interacted. And it's probably one of the most dramatic things that's happened to the temperate deciduous forest biome in eastern North America in probably thousands, maybe millions of years. So, at the time of that introduction, American chestnut was one of the most common trees east of the Mississippi River. In parts of its range in the Appalachians, there were stands that might have as many as one out of every four trees being American chestnut. And these chestnuts grew to immense size. We have pre-blight photographs of chestnuts with trunks up to 14 feet in diameter, which is quite hard to imagine for a broad-leaved tree. It produced the most edible nut of any tree in North America. It was truly the signature tree of the temperate deciduous forest, and it only took about 30 years for that fungus to almost completely eradicate it. Now, that was obviously a real bad outcome for American chestnut, but I'd like you to contemplate the fungus. If you are a parasite, the worst thing you could do is eradicate your host population. Because if you do that, you're doing it in your own population. So, that was a very bad thing for that fungus to do. Probably the worst thing it could do. It would have been much better served. It didn't kill off its host tree. It would have lived longer. It would have reproduced more. There'd be more of a resource for their offspring. And so, it did just about the very worst thing it could do. Now, if two species make it through their initial contact, natural selection is going to kick in and drive co-evolution to force them to adjust their ecologies and become more energy efficient. And this usually results in those relationships growing to become more and more beneficial through time. So, one of my favorite examples of this involves two species that we don't have around here, but occur down in Central America. And that's the Bull's Horn Acacia Tree and its resident Azteca acacia ant. During their co-evolved time together, the acacia tree has developed three morphological features to service its ants. The first is the thorns that used to ward off the mouths of herbivores have grown huge and swollen and pliable. They no longer protect the tree, but the ants will drill into those thorns, hollow them out, and that's where they raise their young. And on every leaf stem is an open sap well or nectary where the ants go and they can drink the sap and get water and the carbohydrate energy resource they need from the sap. And then, around every margin of the leaflets of the acacia, these little globules called Belchian bodies, which the ants harvest for their nutrition, and those grow right back. The ants become so specialized to the acacia that they can only survive on acacia sap and Belchian bodies. They can't eat anything else. But in return, the ants give the acacia the most advanced plant defense system of any tree in the world. They have a really venomous sting that wards off all herbivores. These trees never get munched on at all. But not only that, if vines try to crawl up the trunk of the acacia, the ants will go down, they'll chop those vines down and kill them. If surrounding trees try to encroach in the acacia's space, the ants will go over on those surrounding trees and defoliate them and kill them as well. So these two species are now in a mutualism, which means it's obligatory for survivorship. They cannot make it without interacting with each other. They need each other to survive. And what's dramatic about this example is that from the mandibular structure of these ants, it's known that they're derived from leafcutter ants. So way long ago, when the ancestral acacia and the ancestral ant first came together in a young ecological relationship, it might not have been a very pretty picture. Trees may have been being defoliated, which was energy inefficient. And somehow natural selection kicked into gear and caused these two species to evolve right up to this present-day mutualism. Now another type of interaction that happens in ecosystems is interspecific competition. The competition between individuals of different species. And ecologists look at this relationship as being harmful to both parties because in their competitive struggles, these organisms are losing energy. So if they can adjust their ecologies to not compete, they can get an energy boost. And this happens through what is called niche separation. So a species niche is its ecological role. So we could talk about the foraging niche, how organisms go about getting food. So if two species are competing for food and they can separate their foraging niche, they can do better. There's three ways they can do this. One way is through microhabitat separation. A habitat is the ecosystem organism exists in, but microhabitats are subsets of that habitat. So we can see this in birds like white-breasted nuthatches and black-capped chickadees, both feeding on insects on the same tree at the same time, but not competing because the nuthatch has specialized to forage on the trunks and the chickadee has specialized to forage in the branches. And this way they've separated their niches through specialization. Now another way that species can separate their niches is through temporal separation. When they're active foraging at different times of the day or the season, so this could involve owls at night and hawks during the day. It could involve in our calcium-enriched woodlands, species of vernal wildflowers that are very ephemeral. They grow up and do all their photosynthesis in April and then die back down below ground level and a new set of understory plants comes up after the canopy leaves come on in those forests. And a third way it can be done is through resource partitioning. This can be seen in Darwin's finches down the Galapagos Islands where large beaked finches only forage on large seeds and small beaked finches only forage on small seeds in the same foraging flocks and in this way do not compete. But what you'll see is through this process, they've become more specialized. Now the last example I'll give involves pollination. If we were to go out to a wildflower meadow here in New England and we wanted to chronicle how many different insect pollinators we had all the way from May through till November, I don't know the actual number but I wouldn't doubt, I wouldn't be surprised if there were a thousand different species, all these different species of butterflies, moths, bees, wasps, hornets, beetles, flies, ants, all pollinating either at different times of the day or night or different times of the season or in different ways. And what this builds up is a lot of repetition of function. In all the critical roles in nature, we don't just have a handful of species doing those roles, we have hundreds to thousands of them. So if we're talking about pollination, hundreds of thousands of pollinators. Decomposing organisms, thousands of species of fungi, bacteria, wood boring beetles. We're talking about photosynthetic plants, hundreds and hundreds of them. And this is what builds up the stability and the resiliency in these ecosystems, because if any one species should go extinct, there's so many others doing that critical role, the system is fine. So the way this sort of works is, driven by energy efficiency, co-evolution is forcing species through time to become more specialized, which means their niches are getting smaller, which means ecosystems can support more and more species, which builds up this repetition of function and in turn builds up the resiliency and stability of these ecosystems. So that's what we're going to be looking at with examples in the field of all these examples of species that have worked out more mutually beneficial relationships and as such, build up this incredible amount of biotic diversity where each species doing what it needs to do for its own purposes, creates conditions that support the whole. We're in a hemlock forest here, and also in here we have some scattered clumps of the mountain laurel, which is an ericaceous shrub, a member of the heath family. And with the presence of the hemlock and the mountain laurel, we know there's mycorrhizal fungi in the soil here. So we have four functional groups of fungi. We've got decomposing fungi, really critical for recycling a lot of the nutrients that go back into the soil. Parasitic fungi, which can attack living organisms. Lichenizing fungi. These are fungi that form lichens. And then finally, the fourth group of the mycorrhizal fungi. And mycorrhizae are fungi that interact with the roots of photosynthetic plants. And that's where they get their energy resource from. They actually extract carbohydrate energy from those roots. But they're not parasites. Because of their extensive mycelium that goes through the soil, any plant that interacts with a mycorrhizal fungus can increase nutrient and water uptake up to tenfold. And for certain groups of our plants, they're so co-evolved, they have to interact with mycorrhizae. So all of our regional coniferous trees, like the hemlock here, has to act with mycorrhizal fungi. All the members of the heath family, like the mountain laurel or blueberries or cranberries, have to interact with mycorrhizal fungi. And finally, also all of our orchids have to interact with mycorrhizal fungi. Because these three groups have become so co-evolved, they can't make it on their own. Now, mycorrhizal fungi, we've learned a lot about them in the last 20 years. It was known that they could link up plants and that nutrients could move back and forth through that mycorrhizal network. But over the last 20 years, it's been found that not only nutrients, but energy moves back and forth between plants, even between plants of different species. And not only that, that phytochemicals that can carry information to alert plants about things that are happening far away can go through that mycorrhizal network. In fact, it's now being called the wood wide web because of that transfer of information through that. Mycorrhizae are a huge reason why our forests are resilient as they are. I remember when I was in undergraduate school back in the late 60s, we were taught that trees like these hemlocks solely competed. They were just individuals. They were competing for light in the canopy, which of course is true, but they were also competing for nutrients and energy and things like that and water. And what we know now is that the story is way more complex than that. Yes, they might compete to get in the canopy, but underground, they are all networked up and energy is going back and forth. Around me, there are some younger hemlocks. Studies are showing now that some of these trees wouldn't make it if they weren't linked to bigger trees by the mycorrhizal network. In other words, larger trees, energy is going out through that network to support the regenerating understory trees. So they're a huge part of why our forests function the way they do. Here we have a hemlock stump that is absolutely solid because the stump is still alive, even though it has no ability to photosynthesize. It has no needles. So the question becomes, where is it getting its energy from? Well, generally, mycorrhizal networks are not quite enough to keep a stump like this alive through mycorrhizal connections. However, trees growing within 15 feet of each other generally root graft right on. And those root grafts can happen between species. I've seen hemlocks root grafted onto yellow birch, yellow birch root grafted onto red maple. So this tree was probably root grafted onto this hemlock that's over to my left prior to it snapping off or being cut down. And that root graft has been supplying energy with it to keep it alive, and it's allowed it to basically build all this callus wood in here, trying to heal over this wound. And I've only seen one hemlock stump that is completely healed over in this way, completely covered the wound. And as long as the root graft holds in that stump, it'll probably put on maybe an inch in height a century, but will never, never leaf out. Here we have another root grafted hemlock stump from a tree that snapped off. The remains are on the ground here, and you can see how decayed they are. But this is still absolutely solid. And what I'm looking at up here, this is the callus that was growing over the wound that was caused by the snapping of this tree. And each one of those lines represents one year's worth of growth on this callus wood. And I counted the number of lines in here, and I got 19. So this thing broke off 19 years ago, but because of its neighboring tree here, the root graft is keeping the stump alive. I'm standing next to this hemlock, which has a lot of rows of holes cutting across it, probably one of the most deformed barks on hemlock I've ever seen. And this is from the work of the yellow-bellied sap sucker, a native bird that drills through the bark of trees to drink the sap. And it'll revisit these sap holes frequently, and if they're insects attracted to the sap as well, it will consume them too. The sap sucker has four favorite tree species that it really goes after in this part of New England. That's paper birch, apple, basswood, and hemlock. And it's always puzzled me why hemlock is in one of their top four preferred tree species. I would have thought sugar maple or something like that. But obviously the sap sucker knows something I don't know. There's something attractive about this tree. Also, not only does the sap sucker visit these wells, but 34 other bird species have been seen visiting these wells, as well as many different species of insects and even some mammals like squirrels. And one of the birds that frequents these sap wells consistently is the ruby-throated hummingbird. Now it might look like something like the hummingbird is a parasite of the sap sucker, in that the sap sucker does all this work to drill through the bark, and then the hummingbird eats some or consumes some of the sap. But actually the relationship is more complex than that. The hummingbird is very aggressive about protecting trees like this, if they have a good sap source. And it will drive off a lot of the other consumers, insects and other species of birds and things like that. And studies have shown that actually the hummingbird consumes less sap energy than it saves for the sap sucker by driving off these other species. So this is an example of a mutually beneficial interaction. It's not obligatory, but both species are benefited by this interaction. And the hummingbird will never disrupt a sap sucker. It wants the sap sucker to do its work to open up new sap wells and everything else. So they will never harass a sap sucker, but they will harass most other species of birds. We are in a calcium-enriched, circum-neutral forest with a pH, soil pH around 8. And the trees in the canopy above me are bitternut hickory and white ash and sugar maple. And these are really special spots to come to in the springtime because the understory greens up early in April. So I'd like to talk a little bit about interspecific competition. This is competition between individuals, different species. And if the competitive struggles are strong enough, it really is a very energy inefficient thing for species to do. So natural selection causes species to move away through competition through what's called niche selection. And a species niche is sort of its ecological role. So species that have similar roles start to specialize to change their roles so they no longer compete. So in here, mid-April, we would come in here and this whole slope would be emerald green from the leaves of the wild leek, which I'm holding here, as well as plants like the delicate Dutchman's breeches and the spring beauties, which are in bloom now with five-petaled white flowers where the petals have pink stripes on them, and also trout lily. All of those species are what are called vernal ephemerals. They grow up in early April. They do all their photosynthesis until about early to mid-May. So another week from now, these species will not be visible here. They'll have died back below ground. And then a whole second new group of plants comes up, mostly the blue cohosh, which is this one over here, but also things like maidenhair fern and bainberry and Virginia waterleaf. So you get these two different cohorts of plants. And what they're doing is called temporal separation. They're separating their niches temporally so that they're active at different times of the season. And in this way, these plants do not compete. So vernal ephemerals grow up. They do all their photosynthesis. In about five weeks, die back below ground level, a whole new cohort of plants comes up, does their photosynthesis, and they can coexist without energy losses from competition. And it also makes the understories of these calcium-enriched forests some of our most diverse in terms of herbaceous flowering plants. As I've mentioned, it's not a good idea to be moving organisms from one part of the world to another because we create these young ecological interactions that are usually detrimental for parties involved and very energy inefficient. So this is an example of one of these young ecological interactions. This vine that's growing up in this spiral pattern is Eurasian bittersweet. And it was brought over in this country about 100 years ago. But we created a young ecological interaction that way. This is sort of our northern version of kudzu. This vine will grow right up over the tops of trees. It will completely smother their access to sunlight and kill them. And there's one area I've seen in southern New England where a forest that used to be about 5-10 acres in size has completely been killed off. And all the trees died. Their logs and trunks fell to the ground. And now you have a mat of just bittersweet covering about 5, 6, 7 acres that's about 8 feet high and nothing else. Now, what you'll notice if we go to that site is that it wants to twine up. It's a climbing species. So it starts twining around itself. And what happens is that flowering and reproduction and berry production go way down. This is not a good thing for the bittersweet. It would be much better off if it just stayed on the trunk without killing its host tree. It would live longer. It would reproduce more. And it'd be in much better shape. Eventually, I think it's going to be forced into that. Natural selection is going to push it that way. And it'll become a functional member of our ecosystem. But the problem is co-evolution takes a long time. We're talking about probably thousands of years. Now, something else we can notice in this site is there's all this bittersweet in the ground. So someone came in here at one point and tried to cut these vines, but they didn't herbicide the cut portion of the vine that's going down in the ground because you need to kill off the root system if you're going to get rid of this. That's the only way to do it. Because if you just cut the vines, it clones out by its roots and just explodes. And that's what's happening on this site. So what could have been easily treated early on would have been just to weed the vines that they're just starting to come in. But someone cut them, and now it's a big problem. So the bittersweet can be identified by that spiral growth of the vine. None of our native species do that. Also, the newly emerged leaves tend to have a very pointy tip on the leaf, while the leaves that are older and been around longer tend to be more egg-shaped. And so it's a very clear sort of leaf dimorphism that helps identify this plant. Behind me here, we have a cottonwood tree, and on it is the vine that's growing up its trunk. On the left-hand side is Virginia creeper. That is a native species of vine and has co-evolved with trees to stay out of their canopies. It doesn't mess around with the photosynthetic capacity of the tree, and in that way, stays in the trunk and doesn't harm the tree in any way, shape, or form. And that only makes sense. If you can benefit your host, you're benefiting yourself because it means your host is going to live longer, you're going to live longer, you're going to reproduce more, they'll be more of a resource for your offspring. It only makes sense to do this and not, like the bittersweet, kill off your host, which in the long run is not beneficial for the host tree nor the bittersweet itself. Virginia creeper is pretty easily identified. It is a compound leaf that has five leaflets, sort of like fingers radiating out from the palm of a hand. And then in the fall, it turns a beautiful crimson color, so you can easily idea it growing up the trunks of trees in the fall. But again, a native species and one that does not harm its host tree. Here we have a young quaking aspen, and up at the top, there's some dark masses on the petioles of the leaves. These are aphids. And there's also ants on this plant that are basically tending these aphids. Ants probably have more mutualistic interactions with other organisms than probably any other group of species I can think of. We don't have them around here, but down in the tropics and subtropics, we have leafcutter ants. And those ants harvest those leaves, they chop them up and make compost beds in which they grow mushrooms. And many of these fungi they grow will only grow on compost that has the ant saliva in it. So again, it's a mutualism that's obligatory for both species. But ants aren't the only organism besides humans that garden. They're probably the only organism that herds animals and milks them in those secretions. If let's say this section of the plant doesn't have very good sap in it anymore, the ants will just lift up the aphids and they'll move them to another section. And then they milk them of their honeydew, and that's where the ants get their sustenance. Many of these aphids have so co-evolved with the ants, they can't make it on their own. The ants can even, actually in the fall, take the eggs down underground, overwinter with them underground, then bring them back up on the plants in the springtime. And then there's also ants in mutualism with plants that disperse their seeds. And these are plants that have these ileosomes, which are just a food portion that's sort of glommed onto the seed. The ants collect these seeds, they eat the ileosome, and then they discard the seeds, dispersing them. Right here is a young black cherry, and this is just about 10 feet away from the aspen that had the ants and aphids on it. This also has an interesting relationship with ants. At the top of the leaf petiole, just below the blade of the leaf on black cherry, these little knobs, and they're actually sapwells or nectaries that attract ants. And they're usually activated times when caterpillars would be around defoliating the cherries. So they're not always active. But when caterpillars are active on these black cherries, the black cherries then make those nectaries become functional, attracting ants, which then tend to drive off the caterpillar. So this is, again, a mutually beneficial interaction between ants, once again, and the black cherry. So here we have a red oak and on it growing a fair amount of crustose lichen. Lichen is a mutualistic interaction between species of Ascomycetes fungus and algae, or at least so it was thought until recently. Just very recently, research has shown that actually there's two fungi that form lichens together. So this not only has a species of Ascomycetes fungus, but also a species of Basidiomycetes fungus and a species of algae. The algae can be free living, but when it becomes lichenized like it is here, it can dramatically increase its population and colonize all sorts of substrates it could not colonize on its own. So it's beneficial to the algae. For the two species of fungus that are in this lichen, it's obligatory. They can only survive in relationship with their correct algal partner because that's the only way they get their energy. So this is a mutualism because it's obligatory for the fungi. Now there's also a relationship happening between the lichens and the red oak. It was thought for many years that lichens growing on the bark of a tree like this were just a commensalistic relationship. That's a relationship where the lichens are benefiting, they have the correct substrate, but it wasn't affecting the oak at all because it was just growing on dead bark tissue. But recent studies have shown that actually lichens growing on trees is a benefit to the tree because lichens get a lot of their nutrients out of the air. Some lichens can take in nitrogen gas and fix it in usable nitrogenous compounds, but they can also absorb calcium, potassium, magnesium, phosphorus, both out of the air as dry fall or through water absorbed from rain or fog. And what that means is it builds up sort of a layer of nutrient in the surface of the bark and under heavy rains, when water might sheet down the trunk of a tree like this, some of that nutrient goes in liquid soluble form right down to the roots of the tree. Or if the lichens die and fall off, they fertilize the ground below the tree as well. So many species have evolved barks to attract lichens as they get older. Because usually when trees are young, they have photosynthetic bark. They don't want lichens covering that up. But as they age, they develop barks to attract lichens. And oak is really good at this. Oak is one of our few trees that has a bark on living specimens where you cannot remove it. Any other tree can chip off pieces of bark. You try to do it on the red oak, it's almost impossible, which allows for this dense layer of lichens to grow up. So a limiting factor for lichen is actual light. The only time they're functional is when they're wet and then light becomes a premium. So sometimes on roadside oaks that have a lot of light accessibility, they get this verdant carpets of lichen covering their trunk, just completely covered. And again, that's a benefit to the tree because that increases its nutrient availability for its roots. So on the ground here, we have a lichen, an epiphytic lichen. Epiphytic lichens are lichens that grow on plants, epi meaning on the surface, and phyte meaning plant. So I'd mentioned that they add nutrient to the soil, either by just water running over them and leaching nutrients out that goes down with the rainwater, or when they fall off of a tree and get down to the ground, then they'll decompose and release their nutrients that way. In certain forests, like those in maritime areas that get a lot of fog, something like 40% of all the nutrients entering the soil are coming in through epiphytic lichens. So they're an important part of the health of these forests. And again, if you can benefit your host, like these lichens doing with the trees they're growing on, then you're benefiting yourself as well. Oaks have been around a long time. They're in one of our older family of trees, the Phygaceae. So they've had millions and millions of years of time to co-evolve. And I think studies have shown that there are more co-evolved interactions with insects on oak trees than pretty much any other of our regional species. One such interaction is the oak-apple gall. This is a gall that grows on the midrib of oak leaves on the underside of the leaf. It was always thought that this was a parasitic relationship because the gall grows out of the energy of the leaf, but recent studies are showing that the amount of photosynthesis the gall does when it's growing, because it's green, is equivalent or greater to the amount of energy that's embedded in it. So if in fact it's greater, that means that the oak leaf that's supporting that gall is getting energy boost. It's not an energy drain. And so this again would be an example of something that probably started as a pretty nasty relationship that has co-evolved to being a much more beneficial one. So again, a mutually beneficial interaction. So co-evolution has been working in our regional forest ecosystems for millions and millions and millions of years, and I think we're probably in our infancy of really understanding the complexity of these forest systems and how they work. Basically the best rule of thumb is to mimic nature in whatever we do. For example, in nature as forests develop, trees fall over, logs come down, you build up what's called coarse woody debris. This becomes a really important essential resource for all sorts of decomposers. So we don't want to go out in a forest and pick that stuff and get rid of it, because you simplify the forest ecosystem by doing that. I think we have a lot to learn in terms of maybe our forest operations when we're doing extraction of wood products and stuff, to try to do a better job of really mimicking how ecosystems function. There's a lot of what we're doing nowadays, especially with whole tree harvesting, is we are really moving away from self-organization and resiliency and stability.