An ecosystem is the totality of all the plants and animals, including humans, that live in a habitat. The foundation of the habitat is the soil and water, and even the air. Simply put, the world we live in is an ecosystem.
The habitat can be as small as the pond in my backyard or as large as the Mississippi River basin that touches 35 states and includes 41 percent of the surface area of the United States. Indeed, when you consider air as part of the habitat, planet Earth can be considered one ecosystem.
I'll focus on freshwater ecosystems.
Clean water is at the top of the list of services provided by a freshwater ecosystem. We need clean water; fish do too. Nutrients - principally nitrogen and phosphorus - and toxic chemicals are flushed into the water. These chemicals originate from natural sources when rain and snowmelt leach chemicals from soils and rocks. Increasingly, man adds nutrients and toxic chemicals as effluents from waste-water treatment plants and industries and as runoff from heavily fertilized agricultural lands and lawns. In unaltered ecosystems (few of which remain), biological processes detoxify chemicals, remove some nutrients and convert other nutrients to organic matter that cycles through the food web to produce fish.
A comparison of eutrophication, fertilization and pollution may help you better understand ecological services.
Eutrophication - natural accumulation of nutrients
Imagine a lake with no stream flowing in or out and a pristine watershed. Nutrients from the watershed continually enter the lake and enhance the growth of aquatic plants. Some of the organic matter in the plants moves through the food web to produce a variety of animals, including fish. Some of the fish get harvested, but many of the fish, as well as the plants and other animals, die. Bacteria and fungi decompose the dead plants and animals, and the nutrients are cycled back into the lake.
Over time the nutrients accumulate in the lake, and the lake produces more plant life that, in turn, produces more animal life. The process is called eutrophication. It is a completely natural process, and lakes with more nutrients - those that are more eutrophic - produce more fish.
When the soil has insufficient nutrients to grow plants, we add them with fertilizer. Fertilization is considered beneficial because it produces more crops or maybe greener, healthier lawns.
The same principal is often used to increase fish growth and production in ponds managed for recreational fishing. Fertilizer is applied to nutrient-limited water to increase plant production that, in turn, produces more fish. As on land, fertilization, when done properly, is beneficial.
Pollution is an adverse effect of chemicals added to our waters. Some of these chemicals are toxic - they kill aquatic life. These toxins, ranging from simple elements like mercury to complex chemicals like pesticides and chlorinated hydrocarbons, come from a variety of sources including industries, mines and agriculture. Although much remains to be done, the influx of toxic chemicals to our aquatic resources has been greatly reduced by legal actions.
Nutrients can also be pollutants. The "death" of some lakes has been clearly tied to the introduction of nutrients. The sequence typically is that too many nutrients produce too much algae. The algae die, decompose and strip the water of oxygen. The process occurs in lakes small and large. The consequence is that once-productive waters no longer produce fish, and the water may only be used for human consumption after expensive treatment.
Scientists are also learning that some of the algae that thrive under excessive-nutrient conditions produce toxins that kill animals and may make the water unusable for human use.
Eutrophication, fertilization, pollution - what's the difference?
Eutrophication, fertilization and pollution all involve adding nutrients to a water body. Quite simply, the difference is the rate of nutrient additions and the ability of the ecosystem to assimilate and process the nutrients. This is a vital ecological service provided by aquatic ecosystems.
Nutrients added at natural rates or carefully added by humans at desirable rates can be beneficial. Nutrients added in excess of what the aquatic ecosystem can process and assimilate cause ecosystem meltdown and the loss of ecological services, like water to drink and fish to catch.
Altered ecosystems, altered ecological services
Healthy ecosystems provide a full realm of ecological services, but altering ecosystems can change how they function and the services they provide. The alteration may change habitat, such as changing the flow in streams by dams or water withdrawal, changing the timing and duration of water on the floodplain of the Mississippi River, drastic fluctuations in reservoir water levels or draining or isolation of wetlands. Removing or replacing one or more species of plant or animal can disrupt the food web and the flow of energy through the ecosystem. At some point, the ecosystem begins to fail, and ecological services are lost.
Is the loss of a species, such as a snail darter or a pallid sturgeon, only a political weapon or a miner's canary that warns of an ecosystem losing its ability to provide essential ecosystem services?
Is the rapid expansion of Asian carp simply because they produce a lot of progeny or could it be that habitat changes have simultaneously created conditions conducive to these fish proliferating while at the same time suppressing predators that potentially could control their numbers?
The evidence is rapidly mounting that activities on the watershed affect processes in the water. Can eroding soils that lead to turbid water and sedimentation directly or indirectly, via changes in the food web, suppress game fish and favor less desirable fish?
If you think about ecological services, you quickly come to the conclusion of how vital they are, not only to anglers, but to all people whether they venture onto the water or not. What we do know about ecological services is that they are the product of healthy ecosystems, and ecosytems are more easily and economically conserved than restored - if restoration is possible.