In lectures and on our arboretum field trip we have discussed and observed several levels in ecology. I would like to first review those levels.
Biome. This is a region of the planet's surface that is occupied by a particular collection of ecosystems. The biome is marked by an assemblage of organisms that is dominated by particular kinds of vegetation. Most of the biomes are named after this vegetation. On our field excursion, we visited the Eastern Deciduous Forest biome.
Ecosystem. Our Eastern Deciduous Forest biome, is made up of a collection of communities interacting with the environment. In other words, the biome is comprised of a collection of ecosystems. Each ecosystem demonstrates a particular community and its attendant environmental features and interactions. Our local biome includes upland forest, lowland forest, creek bed, pond, and meadow ecosystems. Each of these ecosystems has its own community of organisms that interact with each other and the environment.
Community. Each community, such as the upland forest community, holds a group of populations of various species that interact with each other. The upland forest community is dominated by canopy trees, understory shrubs, and forest-floor herbs. It is at the community level that trophic pyramids and food webs, represent the interactions. The community is dominated in numbers, biomass, and energy by the producer level of the trophic pyramid and food web.
Population. Each population in a community is composed of a group of individuals of a particular species that interact and that usually interbreed. In our upland forest community, there are populations of maple trees and beech trees, for example. The oak trees represent a problematic set of "species" in that they interbreed to form a single hybrid swarm population. The concept of species is often difficult to apply in the case of plants.
So far we have focused our discussion on abiotic factors (CO2, H2O, light, minerals, climate) at each of these levels in ecology. Today we will turn our attention to the biotic factors in those levels.
We have observed organisms in communities interact in a competitive way. The competition is generally observed for acquiring some limiting abiotic factor in the environment. So plants compete for water, light, minerals, and carbon-dioxide for example. Some plants are better-able to compete than others in a given portion of an ecosystem. These species exclude their competitors from that part of the ecosystem; this is called competitive exclusion.
One example of competitive exclusion is observed between two species of duckweed (Spirodela and Lemna which are aquatic aroids). Each species grows nicely in a chosen medium in a particular environment by itself. However when both are put into competition in that very same medium and environment, one species dominates to the detriment of the other. In this case Lemna gibba excludes Spirodela polyrhiza.
Did you observe competitive exclusion in any case in the arboretum? Yes, certainly grasses were excluded in the dense shade under the canopy trees. This was obvious as we moved from the exposed grassy meadow into the upland forest. Further evidence that light, rather than other factors, was the basis for grass being excluded was observed as grass grew nicely beneath gaps in the canopy caused by tree-fall, trails, or creek bed. A similar relationship was observed in the distribution of multiflora rose (Rosa multiflora). Arguably this might be more due to the position of seed dispersers in the environment rather than competitive exclusion. Conifers were observed struggling to survive in the shady upland forest, but these were depauperate and incapable of reproduction even after several years of growth.
Yet, a very inhospitable place for an oak seedling is under the canopy of conifers! In the taiga biome, where pines dominate, the shade is dense, and breakdown of the crop of needles that is shed each year releases acids into the soil. These acids probably inhibit grasses and other plants from growing in a pine forest...even in most gaps! This chemical inhibition of one species by another is called allelopathy.
Perhaps the most commonly known case of allelopathy is the production of growth inhibiting juglone by walnut (Juglans regia) trees. As walnut leaves and fruits break down in the soil beneath the trees, juglone is leached out and accumulates in the soil. This chemical inhibits growth of grasses and vegetables. If your tomato plants will not produce fruits and seeds, you might check to see whether a nearby tree is a walnut...it could be a biotic factor in your garden! You will notice that it is not a competitive exclusion, rather it is allelopathic exclusion.
An allelopathic relationship was discovered by an ECSU student a few years ago in my lab. Steve Lamoureaux worked for two semesters and one good summer with me to demonstrate which of the essential oil components in fennel seeds inhibit root hair growth, root elongation, hypocotyl elongation, and seed germination in lettuce. Fenchone was the active allelopathic agent though it is only the second-most abundant component of the oil. Anethole was inactive even though it is the most abundant component of fennel seed oil. Part of the project was done with fennel populations naturalized in Bermuda. Steve won a Johnson award for his βββ presentation and the plaque is displayed in the Goddard hallway. I wish I could say we published that work...it certainly was worthy of it...but Steve moved on before we could take the project to that natural conclusion. However you can read more at this location.
Plants are not always "fighting" each other in competitive or allelopathic ways...
Species always live together in communities, but some species interact in a much more intimate way. We call these more-intimate interactions a symbiosis (sym-= together, -bio- = life, -sis = process). There are at least four different types of direct interaction that I will lift up today.
Mutualism exists when two species interact to the benefit of both species. Perhaps the most obvious mutualism was observed yesterday in the arboretum as we discussed lichens. Here the fungal partner (haplobiont) receives carbohydrate and more from the algal partner. The algal partner benefits from the fungal partner providing water for a terrestrial habitat for the alga. The fungal partner also decomposes the rock or other substrate and supplies released minerals for the algal partner.
Other mutualisms exist at the root-soil interface for higher plants. Legumes, such as the clover in your lawn, are in a symbiosis with Rhizobium a bacterium that can fix atmospheric nitrogen into a form that the clover can utilize. The legume supplies the carbohydrates and so on from photosynthesis to the bacterium for respiration, etc. The legume root nodule carries out repspiration at high rates and has heme-iron to mop up any extra oxygen gas, providing an anaerobic microhabitat for the Rhizobium.
Other plant species live in association with fungi. The fungi live either on the surface (ectomycorrhizae) or on the inside (endomycorrhizae) of the plant roots. The plant provides photosynthate (carbohydrate, amino acids, etc.) to the fungi below ground, and the fungi decompose soil particles and detritus to provide the plant with essential minerals.
For those of you who visited Belize, you may have observed the mutualism between Pseudomyrmex ants and Acacia trees. The trees provide the ants with beltian bodies for protein and carbohydrate. The ants provide aggressive defense against herbivores and against shading by adjacent plants. They literally keep the plant "in the sun," thus assuring their supply of rich beltian bodies.
In the arboretum we observed hymenopteran insects visiting jewelweed flowers. The flowers provide nectar and pollen as food for the insect. The insect carries pollen from one flower to the next to achieve pollination. The pollination examples are very interesting mutualisms. We will learn about some bizarre ones later in the semester.
Commensalism is a symbiosis in which one partner benefits but the other gains nothing (nor loses anything) from the association. The grape vine (Vitis labrusca) that you observed climbing a partner tree is mostly a commensalism. The grape vine benefits from the tree's support to grow into the canopy from the forest floor. The tree gets virtually nothing out of this, nor do grape vines cause much harm to the trees.
Other vines, such as kudzu (in the southern US) and poison ivy (in the northern US) are probably NOT partners in a commensalism. These aggressive vines can completely envelope the tree canopy...overgrowing and shading it. These examples are closer to:
In this symbiosis, one partner benefits (parasite) at the expense of the other partner (host). In the arboretum we observed a leaf miner that spends the larval phase of its life between the two epidermal layers of a leaf, feeding upon the mesophyll cells. This feeding of the parasite reduces the productivity of the leaf in a permanent way...it is detrimental to the host.
Another example of parasitism that involves grape vines is quite famous. Grape vines can be a host for the parasite aphid (Phylloxera infestans). This was unknown to early American explorers. These people arrived in North America to lots of very-robust grape vines. They named one part of the eastern coast Vinland in honor of the vines. These plants were quite productive compared to the European grapes that the explorers knew about. However, the wine made from the American grapes had an odd unpleasant taste. Today we would call it the "Welches" flavor...or a grape-y flavor. In the wine the flavor is called foxy flavor after the fox grapes that produce it (Vitis labrusca). French explorers were quite excited by the high yield and robust growth. They figured the off-flavor would be eliminated by moving the vines to the "superior" soil in France's vineyards. They potted some up and shipped them back to France. Indeed the grapes kept their high yield and robust growth...but alas, the flavor was still foxy. What was not clear at first was that the potted grapes from North America also brought the root aphid (Phylloxera infestans) to France. The American grapes had evolved over the millennia in contact with this parasite and so had evolved an effective defense. Unfortunately for the French, their wine grapes (Vitis vinifera) had not evolved in contact with Phylloxera. The French grape hosts had no defense against the exotic alien aphid and the vines were dying rapidly. Germany and Italy closed their borders and trade with France in a vain attempt to keep out the pest. Thousands of acres of vineyards were lost. The solution was found in grafting...joining the aphid-resistant rootstock of the American grape to the better-wine making French grape scion. So don't let some Frenchman get too snobbish about how good French wine is and how poor US wine is; remind him that his French grapes are really Franco-American grapes!
Other common parasite pests of plants include rusts, smuts, and scab fungal diseases, various wilts and rots by fungi and bacteria. Some of the pests have specific host/parasite relations with particular plant species. Some have been studied extensively, most poorly. The garden pea, Pisum sativum can be a host to a particular wilt known as Fusarium. A fusarium spore lands with its chitin cell wall against the wall of a pea leaf cell. The epidermal cell of the host pea plant can detect the carbohydrates in the chitin. These carbohydrates are called elicitors. When these elicitors are detected, the pea plant responds in turn by making a phytoalexin. The Pisum phytoalexin is called pisatin. For less-than-virile fusarium species, this pisatin prevents the growth and establishment of the fungus on the pea plant. Virile strains of Fusarium have evolved a gene producing a monooxygenase. This enzyme is produced and secreted by the Fusarium spore and it degrades the pisatin produced by the pea. This virile fusarium then continues its attack and the pea wilts and dies.
Herbivory is often treated separately from symbioses, but I prefer to think of it as just an extreme form of parasitism. Some herbivores eat just one kind of plant, so the relationship is just as specific as in the case of parasites. The difference is in what the parasite does. The herbivore literally eats the plant tissue...some parasites do and some don't do that. An example of a symbiotic herbivory would include the relationships between pandas and bamboo and koalas and eucalyptus trees. It would be harder to defend as a symbiotic relationship how generalist herbivores feed in a community. Poor examples would include woodchucks, rabbits, and deer in a vegetable garden.
Plants can defend themselves mechanically from some herbivores. Species with spines or thorns or very tough bark are good examples. Cacti possess both spines and thorns. One good example is the Opuntia cactus. Its spines are long, sharp, and hard...capable of piercing through a sneaker and into your foot. Its thorns are tiny glochidia with backwards-pointing barbs; they create a wound that festers for weeks as it is almost impossible to remove them from the skin without breaking them off. This well-defended plant was accidently released by humans onto Australia. It had no herbivores on the island continent to control it, so the cactus rapidly crowded out native species. Finally the government imported an insect, Cactoblastis cactorum, from South America where Opuntia is native. The moth quickly decimated the population and the cactus is clearly under control now. ECSU students learned an important lesson however when we visited San Salvador Island this past June. Opuntia is a native resident of San Salvador and the plant has an obligate herbivory relationship with a native iguanid lizard, Cyclura rileyi. The iguana can avoid or tolerate the spines and thorns and feeds almost exclusively on the fruits of this cactus. Somehow the South American Cactoblastus moth arrived a few years ago on San Salvador. It crashed the population of iguanas on the cays around San Salvador. Humans, ferile goats, rats, dogs, and cats have virtually eliminated the iguanas on San Salvador's main island. The iguana and the cactus are now endangered and threatened, respectively. On one cay, the iguana population is still flourishing however, thanks to visitors from San Salvador's Club Med. They have always brought lettuce, apples, and so on to the cay and "feed the animals." This illegal act has probably saved that population from extinction. On that key the iguanas are virtually "tame" and walked right up to us begging for food. On a nearby cay, they were very wary and we could hear them crashing through the shrubs upon our approach but we never could see them there.
Many species of arthropod larvae besides Cactoblastis feed upon plants. Our farm stores are filled with a diversity of pesticides that we can use to control various arthropod herbivores. Plants have evolved their own batteries of pesticides. They hold these internally until an herbivore takes a bite of plant tissue...the herbivore then gets his "dose" of biochemical toxin. Chrysanthemum coccineum is known to produce the toxin pyrethrum. We now extract that poison from plants and sell it as a "safe and natural pesticide" (from a certain point of view this is an oxymoron).
Other examples of plants producing pesticides include tobacco plants (think Enfield, CT) producing nicotine (a potent neurotoxin). Consider opium poppies that make morphine and codeine, marijuana that makes delta-9-THC, coca plants producing cocaine, peyote cactus producing mescaline, and oaks producing tannins. All of these examples include toxins that are lethal to small herbivores, such as insect larvae. For a big animal such as a human, small doses of these drugs are more "fun" than they are lethal.
Do plants ever defend themselves by "biting back?" Well, not in the strictest sense. However there are a range of plants that are competitive only in the acidic swamps of the world. The acidic water in the swamps makes soil minerals unavailable to plants. The good competitors in this environment have evolved carninvory as their competitive edge. These plants attract, capture, and digest small animals. The digestion is not for carbon supply or energy supply, rather it is to release and capture minerals such as calcium, iron, and magnesium for the plant. Because the plants digest an animal for their minerals, these plants are sometimes called carnivorous plants. If you consider the energy costs involved in digesting an animal and reabsorbing minerals from it, you realize that carnivorous plants are typically very small...shorter than a half-meter. The movie Little Shop of Horrors was complete fiction from a botanical perspective. So you should not fear Venus' fly trap, sundew, pitcher plant, butterwort, or bladderwort.
Finally, we might wonder if plants ever alter the abiotic factors in an environment resulting in a change in the composition of the community. Rather than just be "affected" by the environment, plants do alter certain abiotic factors in some stunning ways. Of course, as a plant changes an abiotic factor that once made it competitive in a particular environment, its own change makess the species less-competitive relatively speaking. The change increases the chance that another species will replace it by improved competition.
Initially the earth's land surfaces were uncolonized...all life was in the oceans. As organisms invaded the terrestrial environment, much of that surface was rock or gravel or sand. The first organisms to arrive in that environment are called pioneer species. These species have characteristics that permit them to survive in the virgin inhospitable surfaces. However, many pioneer species have evolved ways to improve their growth and increase their reproductive capacity by releasing acids from their roots to degrade rocks, release minerals, reduce gravel to sand, and sand to silt, and silt to clay. In other words, the crevices where these pioneer species colonize become filled with fine mineral particles. Detritus from the leaves of the pioneers mixes in with these mineral particles to add humus particles and to create soil. These pioneer species that colonize and establish soil are the first step in primary succession.
The presence of soil created by the action of the pioneer plants, improves the chances that the pioneer species will now have to compete with other species from other areas of the environment. Areas once occupied by pioneers are quickly populated by early settler species. As these settlers make further improvements in soil (perhaps by neutralizing exceedlingly acidic soil pH) and water availability, yet other species will dominate the landscape. Each wave of replacements in succession is called a successional sere. Because this progression of seres started with bare rock or bare sand, this kind of succession is primary succession.
When an area previously populated is denuded by fire or human activity, the re-establishment of new communities in that area is called secondary succession. Again, each sere alters the abiotic conditions in this area by its own growth and reproduction, making it likely that it will face further competition and be replaced by the next sere. Connecticut was once clear-cut of trees and was farmed. With the opening of the midwest at the frontier, Connecticut farmers could no longer compete. The old fields of the farms were left to "go back to nature." What has appeared was a succession of species in secondary seres...back to Eastern Deciduous Forest biome...but now with stone walls marking the field borders...a testament to the past.
In BOTH primary and secondary succession, the seres are replaced until a particular sere seems to be stable and to stop changing the abiotic factors. The community present in an area at the end of the succession of seres is called the climax community. This community, because of the now much-more-complex habitat and multiple seres of colonization, establishment and displacement cycles, is very diverse. Climax communities in each biome possess large arrays of species, demonstrate complicated food webs, and multidimensional trophic pyramids. Whether a succession series has reach the climax community or not is often controversial. Sometimes penultimate seres can be very long in duration and our perspective (limited to the present and prohibited from the future) can lead us to wrong conclusions.
Furthermore, what may have appeared as a climax community can be disrupted significantly by perturbation by humans. Scientists of yesteryear would have described the "climax" forest of Connecticut as Oak and Chestnut. The accidental release of chestnut blight, and further environmental alteration gives us instead, an oak/hickory forest. Just 10 years ago we might have counted dogwood as a very diagnostic feature of the understory in our forests of New England. Dogwood anthracnose is wiping out that population, perhaps viburnum or sassafras will now take over. Our "climax" forests are thus changing before our eyes...an oxymoron.