The term Ecology, and it's identification as a unique field of study began over a century ago as the study of individual species and their relationship with their environment and each other. While the field of study began as a study of natural systems, it has expanded to be come a foundational platform for other applied fields of study such as climatic change, environmental toxicology and sustainable agriculture. For the sake of this introduction, I would like to discuss the principles of ecology as they relate to these three applied fields.
The first principles of ecology relate to an organisms nutrition, or acquisition of nutrients as we might say with plants. Most ecology texts start off with a description of nutrient cycles, and their relationships to different organisms. As a note, we ( humans ) as animals, do not get our primary nutrients, carbon and nitrogen, directly from the atmosphere, but must acquire them indirectly by consumption of other nutrient sources. More specifically, plants, through a process known as photosynthesis (photo - light, synthesis - assembly ) use the energy of the sun to capture atmospheric carbon dioxide and convert it to carbohydrate( CHO ). Plants then, are referred to in ecology literature as "autotrophs", ( auto - self, troph - feeding ) and become the base of our conceptual food chain, or pyramid as it is very often represented in ecology texts. Other organisms, animals usually, are represented at higher levels in the pyramid as they are dependent upon autotrophs for nutrition and are referred to in the terms of ecology as heterotrophs ( hetero - other ). So then, we are heterotrophs, and reverse the process of photosynthesis.
We mentioned that nutrients flow through an ecosystem, or group of organisms that make up a particular habitat in cyclical fashion. Decomposers function on a process called respiration which is the reverse of photosynthesis. In respiration, Oxygen from the atmosphere is combined with carbohydrate from consumption to produce carbon dioxide and water and return them to the environment to complete the cycle.
Nitrogen, like carbon cycles through an ecosystem from the atmosphere, but there is one difference of note that I would like to make, which is that plants do not have the ability to fix nitrogen from the atmosphere into a form that can be used for the construction of amino acids ( amino is a chemical group containing a nitrogen atom and hydrogen ) to be used in construction proteins. Nitrogen is fixed by bacteria, very often in the roots of plants, that work in cooperation with plants to produce nutrients. When two organisms work in cooperation for mutual benefit, it is referred to in ecology terms as symbiosis ( sym - together ) . Examples of symbiosis are not only all around us, they are inside us. There are bacteria in our intestines that perform beneficial functions for digestion, and that is why the taking of antibiotics can be disruptive to our digestive system.
Water, while not usually considered a nutrient, moves in a cycle like nutrients, and with the advent of climatic change impacting our ecosystems, it is important to consider the water cycle an important foundation of ecology. Water is not only the house that many animals live in, it provides a cooling system that moderates the temperature in many ecosystems, and changes in the water cycle can impact or destroy ecosystems.
Evaporation and currents are not the only important transportation mechanisms in the water cycle, trees themselves are often overlooked. Trees provide the important function of reaching into the water table and transporting water to the atmosphere in a process known as transpiration. There are two great examples, one is island rainforests of the equatorial region. One can look at a weather satellite image and easily confirm that the air above forests contains much more clouds than air above the ocean. The other prime example is the Smokey mountains which are named for their transpiration capacity. In the case of rain forests and smokey mountains, the trees provide not only an ecosystems supply of carbon and nitrogen nutrients, but water as well. When ocean temperatures have increased to the point where sea water is moving a large amount of water into the atmosphere by way of evaporation, the result is very often a hurricane.
In more recent years, interest has moved from cycling of not only nutrients through the biosphere but the movement of anthropogenic, or man made chemicals through the biosphere, or community of living things. The field that would become known as environmental toxicology was launched by Rachel Carson's publication of Silent Spring  in 1962. The anthropogenic compound that the book focused on was DDT, a commonly used pesticide, and it's impact on bird populations. Carson, an ecologist and author introduced the world to the profession and study of ecology and launched not only a new profession, but a global awareness of the environment that would become one of the hallmarks of the 1960's.
In the interest of brevity, I can only mention one more example related to environmental toxicology, and that is the case of the Beluga whale of the St. Lawrence and Pierre Beland . During Dr. Beland's work as a population ecologist working with Beluga whales, Beland had the painful experience of being transformed from an ecologist to an environmental toxicoligist. The experience is documented in his powerful book, Beluga, a farwell to whales. To give away the findings of Belands inquiry into the unusual mortality rates in the whale populations, it seems that many of the toxic compounds found in the St. Lawrence river are fat soluble, and as such accumulate in the milk of mammals. The concept of bioaccumulation, or accumulation of high concentrations of toxins in the top level of the food chain had already been documented. For example, birds of prey are at the greatest danger as documented by Rachel Carson and her studies related to DDT. Beland's shocker was that mammals are not of less susceptibility but of greater susceptability because toxins accumulate in females milk and are passed in high concentration to offspring.
As the Beluga whale is a leading aquarium attraction, for example at the Shedd aquarium, and highly intelligent, the story is particularly painful, not only for the whales, but for all mammals, many of which are suffering epidemics that parallel the Beluga's experience.
In the same way that Beluga's whales were impacted by release of toxic compounds that could enter the food chain, other ecologists are seeing their ecosystem of interest impacted by agricultural practices, both modern and traditional. Sustainable agriculture involves integration of ecological principles into agricultural practice, for the minimization of impact on the natural environment. The word sustainable implies that when knowledge of the environment and local ecology are not considered, the agricultural effort itself tends to collapse. Sustainable agriculture involves better management of water resources, better management of nutrients ( fertilizer ) and awareness that ground water quality is jeopardized by many traditional agricultural practices.
In addition agriculture over large areas has a tendency to change the environment, or contribute to climatic change. I prefer not to use the term "global warming", because for many people, that seems to provide an incentive if they would like a bit more moderate winters. In particular, water vapor is a factor in moderating temperatures, and warm areas tend to be cooler with more water vapor and conversely, temperate areas may be more moderate with additional water vapor. Condensation of water to snow or rain has the affect of preventing rapid temperature drop as a principle of thermodynamics. In other words, climatic change is likely to make winters in Cleveland colder dear, not warmer.
I hope this served as a practical introduction to ecology principles, with an eye toward how people with an interest in ecology might pursue a career in the field if the so chose to enter. I consider it very likely that ecologists will be the engineers of the future.
 Carson, Rachel, Silent Spring (1962 ) Houghton Mifflin here
 Beland, Pierre A Farewell to Whales,(1996) Lyons Press here