Some Basic Ecological Concepts
The term habitat refers to the kind of place where an organism normally lives. It includes the arrangement of food, water, shelter and space that is suitable to meet an organism's needs. You can think of this as the "address" where an organism lives. In contrast, a niche is the "occupation" of an organism. It defines the role of an organism in an ecosystem, such as a "fish-eating wader" for a heron, or a "plant-juice-sipping summer buzzer" for a cicada. An organism's niche may change during different life stages. For example, a tadpole typically lives in the water and eats plant material, while the adult frog may catch insects from the shore.
The source of energy for all life on Earth is the sun. Green plants (and some bacteria) are the only organisms that can directly capture the sun's energy and change it into a form that other organisms can use. Through the process of photosynthesis, plants use sunlight to change carbon dioxide and water into sugar and oxygen. The oxygen is given off into the air, where it is available to other organisms including humans. Simple sugar molecules make energy available to plants and, by forming the basic units of complex carbohydrates, contribute to plant structure. Other organisms then eat the plants, or eat organisms that eat plants, and in doing so indirectly gain the benefit of the sun's energy to run their bodies. The flow of sunlight energy is therefore passed from producers (green plants) to primary consumers (animals that eat plants, such as leafhoppers) to secondary consumers (animals that eat other animals, such as birds); this sequence is known as a food chain. As energy is passed along the food chain, much is used up at each level as it works to run each organism. This energy is given off as heat and results in less energy being available at each stage along the food chain. It takes a lot of grass to support one rabbit, and many rabbits to support one hawk. As a consequence, there are many, many green plants on the Earth, fewer animals that eat plants, and even fewer animals that eat animals; this is known as the energy pyramid. In the bosque, the cottonwoods and other plants trap the sunlight energy and provide it in a form usable by the entire collection of other organisms found there. They provide the foundation for life along the river.
Although sunlight energy is used up as it is passed along the food chain, fortunately there is an abundant supply of this energy. In contrast, the materials from which all living things are made are limited in supply and must be used over and over. The primary building blocks of all living things include only six materials: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. When an organism dies and decomposes, these materials are returned to the system and are used again. The carbon that was once part of a dinosaur's tail may now be in the tomato that you eat for dinner! If these compounds are removed from the cycle in some way, they may become limited in supply. For example, if a tree dies but the wood does not readily decompose, carbon and the nutrients are trapped in the wood and are not available to other organisms. This appears to be happening in the bosque. Without the annual flooding that once inundated the forest, wetting the fallen wood and increasing the rate of decomposition, undecomposed wood is now building up and trapping nutrients. This affects the health of the entire ecosystem.
One very important cycle is the water cycle. Rain that falls on a hillside percolates down into the ground water, or may flow aboveground into a lake or the ocean. Water in the lake or ocean then evaporates, and drops join together into clouds, to eventually fall again as rain. Our use of water greatly affects the water cycle. In New Mexico we remove water from the underground aquifer (water present in the bedrock below ground) much faster than it is replenished. Much of this water evaporates directly into the atmosphere while we use it, and may then fall again somewhere else on the planet, thus reducing the amount of water available locally.
We also impact the cycling of materials by introducing poisons. As materials are cycled over and over, toxins build up. Concentrations of toxins increase along food chains, since a predator eats many prey with the toxin, a process known as biomagnification. These increasing concentrations of toxins often have devastating effects. Some well-known examples include top-predator species such as bald eagles and peregrine falcons that nearly became extinct due to the effects of DDT or other chemicals. Awareness of these problems may go a long way towards helping to keep our cycles clean.
Through the flow of energy and the cycling of materials, all living things are interrelated. A mouse not only gets energy from the seed that it eats, but also gets materials that will help to build more mouse tissue. The mouse breathes out carbon dioxide which is taken in by plants, which in turn give off oxygen used by the mouse. The mouse also depends on plants for finding shelter, and it provides food for a snake or owl. The components of the bosque are interrelated with connections extending to the surrounding uplands as well. Some connections are obvious, such as birds that fly between the bosque and uplands at different times of day or during different seasons, moving materials from one place to another. Others are more subtle, such as water flowing underground. But these connections make our actions even more important. Pesticides applied to our fields may add toxic materials to the river, affecting not only the water itself but also all the organisms that depend on the water.
Change is an integral part of the natural world. Changes may occur over geologic time, such as the transition of the Rio Grande from a series of lakes to the river that we know today, or they may occur over much shorter time periods, such as the transition of a seed to a tree and finally to a fallen log. Change was once an integral part of the natural Rio Grande riparian ecosystem, as the river wandered across the floodplain leaving behind its ever-changing mosaic of vegetation. However, human-induced changes have much different effects on the ecosystem. The rate at which we are causing changes on Earth is much greater than has been known previously, and we do not yet know the ecological consequences of most of our actions. By understanding the ecological systems in which we live, and how we interact with them, we can begin to lessen our impact on Earth.