Ecology

Ecosystems An ecosystem is the result of all the relationships among the living and non-living parts of a particular environment. An ecosystem with a large biodiversity will contain many types of organisms. In a healthy ecosystem, each organism fills a niche, i.e., each has a certain place and role in the ecosystem. For example, owls live in wooded areas and eat rodents, worms live in the soil and digest its nutrients while keeping it loose for the growth of plants, and trees provide cover for all of these.

In a healthy ecosystem, resources remain properly distributed and competition among the organisms is at a level that does not decrease too many of their resources too quickly. But, competition for a niche may increase if a new organism enters an ecosystem and encroaches on the space and food already used by an existing species. In that case, there may be a loser as one of the competitors experiences a population decrease or perhaps even a local extinction. Human activity often allow species to come in contact that would otherwise hardly impact one another. Think of the rabbits introduced to Australia. With no natural predators, their numbers exploded. Native grass eaters suffered as the rabbits ate large percentages of their food plants. Another example is the development and accidental release of experimental Africanized bees in Brazil in the 1950's. As they spread northward, they pushed out or took over colonies of native bees. On a personal level, just about any time we enter some new area, we may be unknowingly importing some organism with us that remains there when we return home. What is the effect of this? Sometimes, nothing obvious but we must be aware of our potential to upset ecosystems.

Non-living things like sunlight, temperature, and wind levels are abiotic elements of the ecosystem. The biotic elements of the ecosystem are all its living things.

One indicator of the overall health of an ecosystem is the health of its indicator species. An indicator species is an organism whose health reflects the state of balance among the other components in that environment. Some researchers use a common amphibian, the frog, as an indicator species. Frogs and other amphibians can be found in a wide variety of environments from warm swamps to city parks to mountainous terrain. Their body structure allows them to live in the ecotone (zone) where terrestrial and aquatic environments meet. As tadpoles they are part of an aquatic food chain; as adults they fit into a terrestrial food chain. Because they are part of two worlds and part of two food chains, frogs are especially useful as living measuring sticks of the health of their environments.

Food Chains A food chain or food web is a diagram that shows what eats what. It has a number of trophic levels. Plants form the foundation of the food chain. They are in the 1^st trophic level and are called producers. Plants we often forget about are the microscopic ones living in fresh or salt water. The bases of some food chains include detritus, e.g., the sludgy matter at the bottom of ponds. Detritus contains wastes (from living animals and plants) and the remains of dead plants and animals. Food chains containing detritus also contain decomposers that feed on it. In doing so, they create nutrients that feed plants and algae. Moving up through the chain or web we encounter the 2^nd trophic level containing the primary consumers that feed on the plants. Above them are secondary consumers, in the 3^rd trophic level. The 4^th trophic level contains the tertiary consumers and finally, the 5^th trophic level contains a single top predator.

Animals that feed on plants are herbivores, those that feed on other animals are carnivores and animals in the chain that can feed on both plants and animals and are called omnivores. A general term for an animal is heterotroph because it can not make its own food as can a plant which is an autotroph.

A well developed food web looks like a pyramid - the biomass decreases as we move up the web. It takes thousands of plants to support a few dozen primary consumers which can support just a few secondary consumers which are food for even fewer tertiary consumers, etc. This is because transferring energy up through the food web is an inefficient process. Think of how much of a plant's energy is needed by a rabbit to chew, digest, produce wastes, create the body heat it needs to live, race away to safety, or to breed. If the rabbit just ate slowly and frequently, created minimal wastes, slept often, and moved about slowly, it would be much fatter when it was eaten, perhaps by a fox. But, since the rabbit has a more active life than this, much of the energy that entered the rabbit as plant tissue is unavailable to the fox. Similarly, when an eagle catches a fish, much of the energy the fish absorbed in its food will have been used up during its normal life processes and is not available to the eagle. Cattle farmers do not want their cows running around because too much of the energy the cows absorb from eating hay would be lost as it changed into heat energy and muscle motion and so the cows would become too lean. In the bygone days of the huge cattle drives across the Great Plains of the United States, a major concern of the cattle owners was how thin the cattle would be when they reached the markets in distant cities. A person eating beef from one of those cows would be getting hardly any of the energy the cow got from its food. The cattle owners hoped there would be time to let the cows stand around and eat for a few weeks to fatten them up. Beef from those cows would contain much more of the energy the cows took in when they ate.

Eating mostly plants may not be for all of us but is certainly an efficient route to obtaining energy for our life processes! Eating shark meat is one of the most useless ways of getting energy. Now of course a shark could provide many meals but, if we ate the fish lower in the food chain that were needed to support the shark, we would have food for weeks. Think of farming and cows again: all the care, cost and time to raise a few cattle compared to the much lower cost of just the few weeks to grow the grain and corn that fed them. The cattle food would support more of us for a much longer period than the beef from the cows themselves.

Decreasing Populations When the population of an indicator species diminishes, the simplest reason is that its habitat is being negatively affected. But, what is responsible? Often it is a complex accumulation of effects related to human development. We drain swamps and clear woodlands. We build roads that crisscross the countryside. We develop industries that release a huge range of chemicals. Some liquids are responsible for water pollution. Certain gases cause air pollution. Still other gases are linked to climate changes and endangering the protective layer of ozone encircling the Earth.

Q. Amphibians have delicate, heat sensitive, moist skin through which gases can easily pass. How would the effects of human activity listed above affect a frog's health?

Of course, the continued loss of population leads to extinction! Extinctions are both natural and manmade. Natural extinctions in the past have been dramatic, sometimes with about an 80% wipe out of species. A common hypothesis about the disappearance of the dinosaurs is that an asteroid crashed into Earth, causing huge tidal waves, earthquakes and fires. Dust from the impact and from fires would blot out the sun for years - temperatures would plummet and plants would die off. Soon, so would most animals in the food chains. Today, humans and their activities are the major cause of extinctions. Some predictions suggest that each half hour, another species becomes extinct. A major irony in this whole thing is the loss of species we have not even discovered, especially in sections of rain forest being destroyed for land development. And who knows what new medicinal cures may be lost because the plant whose bark or flower or root contained a special chemical has been cut down and burned.

Q. Summarize the extinction of any two organisms. There are thousands from which to choose!!

Attempts to counterbalance population losses often center on breeding programs. In some situations, all members of a small, extremely endangered population are captured and held in protective custody (captive breeding) until careful management can help their numbers to increase to the point where of them can be released back into the wild.

Food chains are complex things and difficult to monitor. A certain organism may been seen as just a pest because its place in a food chain is not recognized or appreciated. There were, and still are, loud, nasty debates over the reintroduction of wolves into areas through which they historically had roamed for thousands of years, e.g., Yellowstone National Park in Montana. After the wolves had initially been driven and hunted out some years ago, the food chains in those areas had become very imbalanced as the populations of some organisms swelled and others declined. Cows and sheep, introduced species, were important to the farmers in those areas but caused further distortions in the food chains. Elk and coyote populations rose. The coyotes ate too many smaller predators lower in the food chain and the elk ate hillsides bare. In a similar situation elsewhere in the US Midwest, the populations of large mammal predators had been reduced, perhaps to protect cows, and so the populations of deer and elk again became oversized. In this case, the deer and elk began competing with the cows by muscling in and eating the hay put out for the cows. In some west coast urban settings, distorted ecosystems have allowed increases in the coyote population. As their regular food sources began to dwindle from overeating, they took a liking to dogs and cats. One thing is clear - the results of just a single change to a food chain may have negative effects that multiply.

Cycles Nature has been recycling forever, and very effectively.

The Carbon Cycle

The burning of fossil fuels in homes, cars and industries releases CO₂ into the atmosphere. Plants absorb the CO₂; photosynthesis allows them to use sunlight to break apart the CO₂; the O₂ is released into the air while the C collects in the plant's tissues as it grows. (The presence of C in plants is why they burn.) When animals eat plants, the C goes into them. Next, respiration occurring inside the animal combines the C it obtained from the plant tissues with O₂ it inhales from the atmosphere. When the animal exhales, it releases CO₂ back into the atmosphere. When an animal dies, it's C goes into the soil where decomposers turn it into CO₂.

The Nitrogen Cycle

Nitrogen gas is the major component of our atmosphere, making up about 78% of it. When certain soil bacteria absorb the N₂, they "fix" it, i.e., change it into NH₄⁺ (ammonium ion). Lightning flashes also fix N₂ into NH₄⁺. As the NH₄⁺ builds up in the soil, other bacteria called nitrifying bacteria change the NH₄⁺ into the NO₂^- (nitrite) ion. A second group of bacteria then change the NO₂^- into the NO₃^- (nitrate) ion. With the creation of the NO₃^- ion, the plants have nitrogen in a form they can use. When grazing animals eat the plants, they absorb the NO₃^- ions. When they die, bacteria use the process of ammonification to change the N compounds in their tissues back into the NH₄⁺ ions that gather in the soil. These NH₄⁺ ions will be recycled into the NO₃^- ions again. (And happens to these NO₃^- ions again?) The cycle is completed when denitrifying bacteria change unused NO₃^- ions back into N₂ gas that goes up into the atmosphere.

The Oxygen Cycle

This cycle is simple to state: plants absorb CO₂ from the air and, through photosynthesis, release O₂ back into it; animals absorb the O₂ put into the air by the plants and release CO₂ into the air. What is waste from one organism is a necessity for the other.

Pesticides A pesticide is a chemical that kills organisms some consider "pests". Weeds, insects, fungi and rodents are common examples of pests. Someone labels an organism a pest when it interferes with their work or quality of life to an "unacceptable" degree. Sometimes this label may seem arbitrary, e.g., certain weeds are beautiful plants except in a lawn, but there are real dangers posed by some pests. Diseases and food shortages are the results of the activities of some insects and rodents. First generation pesticides were a few chemicals that occurred in soils, rocks or plants. Second generation pesticides are made by the thousands in laboratories. An improvement is their faster rate of decomposition but a downside is that some, but not all, target specific types of pests. Have you ever accidently sprayed bug repellent on your lips? Remember the burning and numbness? Should it have affected you or just the mosquitoes? And sometimes, pesticides get into areas where they are not wanted. What about spraying pesticides on weeds along the edge of the garden but wind drift carries some onto a bed of expensive roses and they all die?

Q. What organisms are killed by: a) insecticides b) herbicides c) fungicides d) bactericides?

A major problem with pesticides is bioamplification. This occurs when pesticide molecules are passed up the food chain as things get eaten. At each new trophic level, the consumers there concentrate more and more of the pesticides in their tissues because they eat so many prey from the lower level. As the concentration level increases, at some point the consumer will begin to suffer - perhaps reduced fertility, cancers, early death, metabolism changes, deformations. Will this issue become important to us only when we eat something with a large amount of accumulated pesticide? Also, what about the few organisms that survive the application of pesticide? Will they become resistant super-pests and multiply?

Q. Imagine you are sprayed with just a bit of pesticide and now have a "pesticide card". You get in your lab group. A predator comes along and eats all of you, collecting your cards. How much pesticide is in that person now? A predator from a higher trophic level eats all those people and collects all their cards. How much harmful chemical is in that organism now?

Soil Structure Plants are the lungs of the Earth but, in spite of their enormous importance to life on this planet, most of the soils that support them are only about two meters thick and many are much thinner. Soil is a sequence of rather well defined layers. The upper layer is litter made of partially decomposed grassy material. Next is topsoil with its humus made of decaying plant and animal matter. The humus gives topsoil its dark color, acidic properties and is a rich supply of nutrients and minerals. Rocks in this layer are small and scattered. Water and air are important components of this layer. The subsoil has more and larger rocks with less humus, so it is lighter in color. Minerals are common in subsoil. Below this layer is bedrock. Where bedrock is exposed, the combined action of physical weathering (wind, water), chemical weathering (water, chemicals), plant root action, and the activities of animals and microorganisms gradually turn it into soil. Because these actions and agents are less effective as we look deeper into the ground, it takes thousands of years to create a soil depth that can support flourishing plant communities.

Water on the surface is called surface water; when it percolates down it becomes ground water. As the water descends, it carries down nutrients and minerals - this leeching becomes a problem in some cases. Plant root systems try to correct the problem by reaching far down and bringing the needed chemicals back toward the upper soil levels. The depth where the earth becomes saturated with water is the top of the water table.

Biomes A biome is a grouping of ecosystems that contain similar plant life. Around the world there are six major biomes and Canada is large enough and diverse enough to contain four of these: tundra, boreal or coniferous forest, temperate forest and grassland.

Tundra: very cold, 8 to 10 week growing season, low precipitation, thin soil, permafrost low growing sedges, dwarf shrubs, lichens, mosses, grasses lemmings, hares, foxes, caribou, musk ox, migrant birds north of the tree line

Coniferous Forests: seasonal, long cold winters, precipitation heavier in summer, acidic soils coniferous trees, a few deciduous trees, thin undergrowth moose, elk, deer, mice, bears, foxes, martens Northern North America and Europe

Temperate Forests: seasonal, warm summers, medium precipitation, well developed soils broad-leaved deciduous, some coniferous, moderate undergrowth deer, foxes, black bear, skunks, snakes, amphibians, soil organisms Western and central Europe, eastern Asia and eastern North America

Tropical Rainforest: always warm, heavy rains, thin soils many broad-leafed evergreens, dense canopy, little undergrowth huge colorful variety of life, monkeys, small to large cats, small mammals zone around the equator

Grasslands: seasonal, light to moderate rainfall, deep soils, frequent fires variety of grasses, some woods bison, lions, rhinos, kangaroos, goats, horses, jackals, termites subequatorial belt in Africa, India, North and South America

Deserts: very dry, hot days, cold nights, thin porous soils widely spaced thorny bushes, cacti, rodents, lizards, insects, owls, small birds North and SW Africa, Middle East, SW United States, Northern Mexico