Ecology is usually considered from the perspective of the specific geographic environment that is being studied a moment: tropical rain-forest, temperate grassland, arctic tundra, benthic marine, the entire biosphere, and so on. The subject matter of ecology is the entire natural world, including both the living and the non-living parts.
Biogeography focuses on the observed distribution of plants and animals and the reasons behind it. More recently ecology has included increasingly the human-dominated world of agriculture, grazing lands for domestic animals, cities, and even industrial parks.
Industrial ecology is a discipline that has recently been developed, especially in Europe, where the objective is to follow the energy and material use throughout the process of, e.g., making an automobile with the objective of attempting to improve the material and energy efficiency of manufacturing. For any of these levels or approaches, there are some scientists that focus on theoretical ecology, which attempts to derive or apply theoretical or sometimes mathematical reasons and generalities for what is observed in nature, and empirical ecology, which is concerned principally with measurement. Applied ecology takes what is found from one or both of these approaches and uses it to protect or manage nature in some way. Related to this discipline is conservation biology. Plant ecology, animal ecology, and microbial ecology have obvious foci.
Reasons to study ecology
There are usually four basic reasons given to study and as to why we might want to understand ecology:
1. First, since all of us live to some degree in a natural or at least partly natural ecosystem, then considerable pleasure can be derived by studying the environment around us. Just as one might learn to appreciate art better through an art history course so too might one appreciate more nature around us with a better understanding of ecology.
2. Second, human economies are in large part based on the exploitation and management of nature. Applied ecology is used every day in forestry, fisheries, range management, agriculture, and so on to provide us with the food and fiber we need.
3. Third, human societies can often be understood very clearly from ecological perspectives as we study, for example, the population dynamics (demography) of our own species, the food and fossil energy flowing through our society.
4. Fourth, humans appear to be changing aspects of the global environment in many ways.
Ecology can be very useful to help us understand what these changes are, what the implications might be for various ecosystems, and how we might intervene in either human economies or in nature to try to mitigate or otherwise alter these changes. There are many professional ecologists, who believe that these apparent changes from human activities have the potential to generate enormous harm to both natural ecosystems and human economies. Understanding, predicting and adapting to these issues could be the most important of all possible issue for humans to deal with. In this case ecology and environmentalism can be the same.
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Ecosystem
An ecosystem consists of the biological community that occurs in some locale, and the physical and chemical factors that make up its non-living or abiotic environment. There are many examples of ecosystems — a pond, a forest, an estuary, a grassland. The boundaries are not fixed in any objective way, although sometimes they seem obvious, as with the shoreline of a small pond. Usually, the boundaries of an ecosystem are chosen for practical reasons having to do with the goals of the particular study.
Components of an Ecosystem
The parts of an ecosystem can be listed under the headings “abiotic” and “biotic”.
Abiotic components:
Sunlight, Temperature, Precipitation, Water or moisture, Soil or water chemistry (e.g., P, NH4+)
Biotic Components
Primary producers, Herbivores, Carnivores, Omnivores, Detritivores
All of these vary over space/time
By and large, this set of environmental factors is important almost everywhere, in all ecosystems. Usually, biological communities include the “functional groupings”. A functional group is a biological category composed of organisms that perform mostly the same kind of function in the system; for example, all the photosynthetic plants or primary producers form a functional group. Membership in the functional group does not depend very much on who the actual players (species) happen to be; only on what function they perform in the ecosystem.
Processes of Ecosystems
This figure with the plants, zebra, lion, and so forth illustrates the two main ideas about how ecosystems function: ecosystems have energy flows and ecosystems cycle materials. These two processes are linked, but they are not quite the same (see Figure 1).
Figure 1. Energy flows and material cycles.
Energy enters the biological system as light energy, or photons, is transformed into chemical energy in organic molecules by cellular processes including photosynthesis and respiration, and ultimately is converted to heat energy. This energy is dissipated, meaning it is lost to the system as heat; once it is lost it cannot be recycled. Without the continued input of solar energy, biological systems would quickly shut down. Thus, the earth is an open system with respect to energy.
Elements such as carbon, nitrogen, or phosphorus enter living organisms in a variety of ways. Plants obtain elements from the surrounding atmosphere, water, or soils. Animals may also obtain elements directly from the physical environment, but usually, they obtain these mainly as a consequence of consuming other organisms. These materials are transformed biochemically within the bodies of organisms, but sooner or later, due to excretion or decomposition, they are returned to an inorganic state. Often bacteria complete this process, through the process called decomposition or mineralization
During decomposition these materials are not destroyed or lost, so the earth is a closed system with respect to elements (with the exception of a meteorite entering the system now and then). The elements are cycled endlessly between their biotic and abiotic states within ecosystems. Those elements whose supply tends to limit biological activity are called nutrients.
The Transformation of Energy
The transformations of energy in an ecosystem begin first with the input of energy from the sun. The process of photosynthesis captures energy from the sun. Carbon dioxide is combined with hydrogen to produce carbohydrates (CHO). Energy is stored in the high-energy bonds of adenosine triphosphate, or ATP.
The prophet Isaah said “all flesh is grass”, earning him the title of the first ecologist because virtually all energy available to organisms originates in plants. Because it is the first step in the production of energy for living things, it is called primary production. Herbivores obtain their energy by consuming plants or plant products, carnivores eat herbivores, and detritivores consume the droppings and carcasses of us all.
Figure portrays a simple food chain, in which energy from the sun, captured by plant photosynthesis, flows from trophic level to trophic level via the food chain. A trophic level is composed of organisms that make a living in the same way, that is they are all primary producers (plants), primary consumers (herbivores) or secondary consumers (carnivores).
Dead tissue and waste products are produced at all levels. Scavengers, detritivores, and decomposers collectively account for the use of all such “waste” — consumers of carcasses and fallen leaves may be other animals, such as crows and beetles, but ultimately it is the microbes that finish the job of decomposition. Not surprisingly, the amount of primary production varies a great deal from place to place, due to differences in the amount of solar radiation and the availability of nutrients and water.
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Energy transfer through the food chain is inefficient. This means that less energy is available at the herbivore level than at the primary producer level, less yet at the carnivore level, and so on. The result is a pyramid of energy, with important implications for understanding the quantity of life that can be supported.
Food chains with green plants, herbivores, and so on are referred to as grazer food chains because living plants are directly consumed. In many circumstances, the principal energy input is not green plants but dead organic matter. These are called detritus food chains. Examples include the forest floor or a woodland stream in a forested area, a salt marsh, and most obviously, the ocean floor in very deep areas where all sunlight is extinguished 1000’s of meters above. In subsequent lectures, we shall return to these important issues concerning energy flow.
There are many food links and chains in an ecosystem, and all of these linkages can be referred as a food web. Food webs can be very complicated, where it appears that “everything is connected to everything else”, and it is important to understand what are the most important linkages in any particular food web.
Biogeochemistry
The term Biogeochemistry is defined as the study of how living systems influence, and are controlled by, the geology and chemistry of the earth. Thus, biogeochemistry encompasses many aspects of the abiotic and biotic world that we live in.
There are several main principles and tools that are used to study earth systems. Most of the major environmental problems can be analyzed using biogeochemical principles and tools. These problems include global warming, acid rain, we environmental pollution, and increasing greenhouse gasses. The principles and tools can be broken down into 3 major components: element ratios, mass balance, and element cycling.
1. Element ratios
In biological systems, important elements are referred as “conservative”. These elements are often nutrients. By “conservative” it means that an organism can change only slightly the amount of these elements in their tissues if they are to remain in good health. For example, in healthy algae the elements C, N, P, and Fe have the following ratio, called the Redfield ratio after the oceanographer that discovered it:
C : N : P : Fe = 106 : 16 : 1 : 0.01
Once these ratios are known, one can compare them to the ratios that one measure in a sample of algae to determine if the algae are lacking in one of these limiting nutrients.
2. Mass Balance
Another important tool that is used is a simple mass balance equation to describe the state of a system. The system could be a snake, a tree, a lake, or the entire globe. Using a mass balance approach whether the system is changing and how fast it is changing can be determined. The equation is:
NET CHANGE = INPUT + OUTPUT + INTERNAL CHANGE
In this equation, the net change in the system from one time period to another is determined by what the inputs are, what the outputs are, and what the internal change in the system was. The example given in class is of the acidification of a lake, considering the inputs and outputs and internal change of acid in the lake.
3. Element Cycling
Element cycling describes where and how fast elements move in a system. There are two general classes of systems that we can analyze as mentioned above: closed and open systems.
A closed system refers to a system where the inputs and outputs are negligible compared to the internal changes. Examples of such systems would include a bottle or our entire globe. There are two ways the cycling of materials within this closed system can be described, either by looking at the rate of movement or at the pathways of movement.
Rate = number of cycles / time, as rate increases, productivity increases
Pathways-important because of different reactions that may occur
In an open system, there are inputs and outputs as well as the internal cycling. Thus, the rates of movement and the pathways can be described, just as the closed system, but a new concept called the residence time can also be defined. The residence time indicates how long on average an element remains within the system before leaving the system.
Rt = total amount of matter / output rate of matter
Controls on Ecosystem Function
There are two dominant theories of the control of ecosystems.
The first, called bottom-up control, states that it is the nutrient supply to the primary producers that ultimately controls how ecosystems function. If the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will cycle faster).
The second theory, called top-down control, states that predation and grazing by higher trophic levels on lower trophic levels ultimately controls ecosystem function. For example, if there is an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity “cascades” from the top levels of the food chain down to the bottom trophic levels.
There is evidence from many ecosystem studies that both controls are operating to some degree, but that neither control is complete. For example, the “top-down” effect is often very strong at trophic levels near to the top predators, but the control weakens as one move further down the food chain. Similarly, the “bottom-up” effect of adding nutrients usually stimulates primary production, but the stimulation of secondary production further up the food chain is less strong or is absent.
The Geography of Ecosystems
There are many different ecosystems: rain forests and tundra, coral reefs and ponds, grasslands and deserts. Climate differences from place to place largely determine the types of ecosystems we see. Mainly the dominant vegetation influences how terrestrial ecosystems appear to us.
The word “biome” is used to describe a major vegetation type such as tropical rain forest, grassland, tundra, etc., extending over a large geographic area (Figure 3). It is never used for aquatic systems, such as ponds or coral reefs. It always refers to a vegetation category that is dominant over a very large geographic scale, and so is somewhat broader than an ecosystem.
Temperature and rainfall patterns for a region are distinctive. Every place on earth gets the same total number of hours of sunlight each year, but not the same amount of heat. The sun’s rays strike low latitudes directly but high latitudes obliquely. This uneven distribution of heat sets up not just temperature differences, but global wind and ocean currents that in turn have a great deal to do with where rainfall occurs.
A schematic view of the earth shows that complicated though climate may be; many aspects are predictable (Figure 4). High solar energy striking near the equator ensures nearly constant high temperatures and high rates of evaporation and plant transpiration. Warm air rises cools, and sheds its moisture, creating just the conditions for a tropical rain forest. Every location has a rainfall- temperature graph that is typical of a broader region.
Figure 4. Climate patterns affect biome distributions.
Certain plants are distinctive of certain climates, creating the vegetation appearance that is called biomes. High precipitation is not possible at low temperatures — there is not enough solar energy to power the water cycle, and most water is frozen and thus biologically unavailable throughout the year. The high tundra is as much a desert as is the Sahara.
Figure 5. The distribution of biomass related to temperature and precipitation.
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