Dynamic Equilibrium in Organisms, Populations, and Ecosystems
I. Dynamic equilibrium in multicellular organisms:
Organisms live in a world of changing conditions. But, to remain alive, every organism
needs to keep the conditions inside of itself fairly constant. An organism must have ways
to keep its internal conditions from changing as its external environment changes. This
ability of all living things to detect deviations and to maintain a constant internal environment
is known as homeostasis. An obvious change that has occurred in thecourse of evolution is the development of larger multicellular organisms from microscopic, single-celled ones. Is there an advantage to being multicellular? Being microscopic and single-celled makes it difficult for an organism to maintain homeostasis. Having a multicellular body makes possible many types
of protection against changes in the environment. In other words, an organism with many cells is able to have structures and systems that protect its individual cells from external changes, thus helping it to stay alive. To maintain homeostasis, organisms actually must make constant changes. That is why homeostasis is often referred to as maintaining a dynamic equilibrium. Dynamic means “active,” and equilibrium means “balanced.”
One of the most fascinating facts about our bodies is that each of our many, many cells is surrounded by liquid. The smallest blood vessels in our bodies, the capillaries, are close to every cell. There is a small amount of space between the capillaries and the body cells. This space is filled with fluid. The fluid thatsurrounds cells is made up mostly of water, with many substances dissolved in it. This intercellular fluid is important in helping to maintain stable conditions inside each of our cells. Many materials are exchanged between the cells and the fluid. In turn, materials may be exchanged between the fluid and the blood in the capillaries. All of this is done to make sure that each and every body cell is able tomaintain homeostasis and remain healthy.
II. Dynamic equilibrium in populations (species within the same geographical area):
All populations are dynamic- they change in density and dispersion over time. To understand these changes scientists must know more than populationdensity and dispersion. Three other important measures include natality, mortality, and life expectancy. All populations undergo equilibrium shifts. Some fluctuations are clearly linked to environmental changes such as pollution of their environment and climate change.
Population density also undergoes equilibrium shifting due to cycles within their environment, such as the abundance of producers during a given year. As an oceanic example: a producer such as algae/limu is in great abundance due to a boost in potassium and phosphorus, two important biogeochemicals. Then primary consumers, sea urchins, feed on the abundant algae. The sea urchins are able to have more offspring. Next, the secondary consumers, sea stars, are eating more of the sea urchins. Now there is more “food” on the primary trophic level for the secondary trophic level. Soon the carnivorous sea stars can produce more progeny because they are acquiring more nourishment. But over time because of the carnivores’ rapidly increasing reproduction the once abundant herbivores are now mostlyeaten and the sea stars begin starving. The once abundant limu, sea urchin, and sea star food chain now show a decrease in density.
III. Dynamic equilibrium in ecosystems:
Ecosystems can be small or large. Our entire planet is covered with a variety of different, sometimes overlapping, and often interdependent ecosystems. Major global ecosystems are referred to as biomes. A tropical forest is a mid-sized ecosystem, which itself contains a diverse array of smaller ecosystems, and which in turn connects with global ecosystems. These layers of ecosystems are in dynamic interactions with each other, and influence which equilibrium state each maintain).
Ecosystems are said to be “self-regulating” or “self organizing” in that each contains feedback mechanisms which function to maintain the components of the system in one or other of its equilibrium states. An equilibrium state demonstrates the stability of ecosystems. Even in these stable states the components of ecosystems are in dynamic exchanges, and these exchanges involve the predictable build up of energy or materials which cycle the ecosystem either within a single equilibrium or between various equilibrium states. Ecosystems tend to cycle between these states of change and stability. Ecosystems of different sizes are interconnected and affect each other. As ecosystems at one level ebb and flow between different stable states, they each create fast and slow cycles relative to their neighbors. Smaller ecosystems are generally characterized by faster cycles of change and stability, and larger ecosystems by slower cycles, with timeframes as long as a millennium or more.
Multiple stable states characterize most ecosystems. If disturbances or perturbations occur from either internal or external sources which tend to drive an ecosystem away from its current equilibrium state, then the ecosystem’s regulatory feedback mechanisms work to maintain the current state, or to bring the ecosystem to one of its other typical equilibrium states. Which state is prevalent at any particular time has an impact on related ecosystems. Depending on which equilibrium state is prevalent, there will be more or fewer plants or animals in that ecosystem (or more of one type and less of another), more or less food available, more or less waste absorption, more or less nutrient cycling, or more or less energy.
In addition to the relatively predictable ebbs and flows of ecosystem cycles, less frequent and predictable external disturbances also occur (lightning induced fires sweep through a forest or grassland; a volcanic eruption spews tons of material into the atmosphere; a desert riverbed is flooded). These disturbances stimulate ecosystems to change within their existing equilibrium state, or if the disturbances are great enough the system may move to one of the other typical equilibrium states. When disturbances occur with regularity (although their timing and extent may be unpredictable, such as with fires) they are incorporated into the ecosystem’s self regulating mechanisms. These adaptive mechanisms may either provide protection against the disturbance (e.g. fire resistant bark) or rely on the disturbance to maintain itself (e.g. fire induced bursting of pods to release seeds).
The geographic ranges of most plant and animal species are limited by climatic factors, including temperature, precipitation, soil moisture, humidity, and wind. Any shift in the magnitude or variability of these factors in a given location will impact the organisms living there. Species sensitive to temperature may respond to a warmer climate by moving to cooler locations at higher latitudes or elevations. Although the response to warming is generally understood, it is difficult to predict how concurrent changes in other climatic factors also affect species distributions. Despite the uncertainties, ecological models predict that the distribution of world biomes will shift as a result of the climate changes associated with increased greenhouse gases (IPCC, 1998). The distribution and size of the populations of plants and animals within those biomes will also change, with potential consequences for the functioning of ecosystems and for humans who are dependent on many ecosystem goods and services.