Four natural processes whose condition can be used as indicators to evaluate the current state of the ecosystem with regard to our goals as managers are:
- Water cycle
- Nutrient cycle
- Energy flow
- Successional level
The trend in condition of these indicators over time allows us to determine whether the condition of the plant community is improving, deteriorating, or remaining static as a result of management.
Water and Nutrient cycles
Water and nutrients are raw materials allocated to different users in both ecological and economic systems. Just as cash flow is an indicator of the rate of business activity, the rate at which these materials become available and are used indicate the level of biological activity that is occurring in the respective system. If a relatively large proportion of these raw materials are transformed into complex organisms, the system is operating efficiently, and if we are adding more value than any additional costs we incur, profitably. If a high proportion of these materials are lost from the biological system, it will likely be both unsustainable and unprofitable in the long-term. Just as we refer in economic terms to an increase in net worth as an improvement in our economic condition (profit), so we can usually refer to a long term increase in the natural capital of a biological system as improving its ecological condition (e.g. soil moisture holding capacity, organic matter, proportion and productivity of palatable plants, etc.).
Water that falls as precipitation can run off and be channeled through watercourses to the ocean, intercepted by plant material and evaporated, evaporated from bodies of water, or absorbed into the soil where it flows beneath the surface or is absorbed by plants and transpired. Therefore, ANYTHING that softens the impact of precipitation on the soil surface, slows down the rate of overland flow, and allows more infiltration will increase cycling of water through the biological system, if plants are available to use this moisture.
Nutrients are cycled through the biological system by being absorbed and manufactured into plant tissues, followed by consumption, digestion, assimilation, and decomposition by animals and microbes. If nutrients are tied up in several generations of dead plant material, or are transported off the site as a result of accelerated erosion, the nutrient cycle is functioning ineffectively. Anything that encourages uptake of plant nutrients as they become available in the soil will increase the amount available to grazing animals. If that plant material is kept highly digestible and the plants maintained in a vigorous condition, more nutrients can be returned to the soil more rapidly, further enhancing the nutrient cycle. When this rapid turnover, consumption and decomposition is interrupted at any point, the mineral cycle becomes less effective, and “leakage of nutrients from the system becomes a problem.
Indicators of good water and nutrient cycles, include abundant green plant material and a variety of species that promote infiltration into the soil and allow water and nutrients to be absorbed and manufactured into plant tissues under a wide range of climatic conditions. Dead material rapidly comes in contact with the soil so that it can protect the soil surface from the impact of falling raindrops and create a better habitat for beneficial soil microbes. Since microbes and enzymes in the digestive tracts of animals also break down plant material, a high proportion of young vegetation in the plant community that is highly digestible also promotes rapid cycling of nutrients through the biological system. However, soils should not be allowed to compact and the surface structure to be destroyed through the continuous disturbance associated with long graze periods without adequate grazing deferment to allow plants to regrow a full complement of leaves and enough root material to adequately absorb nutrients and moisture from the soil to maintain their vigor and competitive capability with its neighbors.
Energy only flows through the biological system (one-way) and is not recycled. It is captured first in green plants, the primary producers in the system, since they are the only organisms that capture energy from the sun and convert it directly into a useful form through photosynthesis. Consumers and decomposers use the stored energy from the plants either directly or indirectly. These include animals, bacteria, and fungi. Each consumer or decomposer in the food web uses the products produced by one group and converts some of that energy to other useful forms, with some loss as heat. They can, therefore, be referred to as secondary producers. For example, herbivores are plant consumers, but convert some of the plant energy into products useful to humans and other carnivores or omnivores. In that way energy flow through a biological system can be likened to money in an economic system. It is not used up, but only changes hands (system-wide) and it only comes in or goes out (individual business or organism).
Like money, energy can also be stored. However, bear in mind that both money and energy are only useful when used. The only reason for storing energy or money is to use it at a different time or place. Also, like money in an inflationary period, each time energy is stored, it usually loses value. Generally, the faster and steadier that energy flows through the biological system (cash flows through a business), the higher the rate of biological (economic) activity.
High rates of energy or cash flow may or may not also indicate a high degree of system efficiency. A business can have a rapid cash flow without being profitable if it does not add enough value to the raw materials it uses to cover production costs, or if the cash inflows are a result of liquidation of capital assets. Capital assets in a business can be defined as those things such as machinery, real estate, etc. that are used to produce wealth but are not themselves consumed as a normal business activity. The soil organic matter and associated decomposers, seed bank, perennial plant crowns, stems and roots, and animal seedstock can be thought of as the capital assets of the biological system. Just as we lose the potential to produce wealth from the business when we maintain cash flow at the expense of capital assets, we also lose the long-term potential to produce wealth from the ecological system when we allow biological capital assets to deteriorate.
The biological system may also have a high rate of energy flow, but with much of the energy lost for use by humans and other higher organisms as respiration (e.g. not converted to one form to another through growing organisms, but simply produced and then respired through organisms that have ceased growing or through decomposition, disease organisms, etc.) or that are transported off site (e.g. plant material that is washed or blown away). This may happen when “artificial” or stored sources of energy are used to enhance the production of certain parts of the biological community. These “artificial” or stored sources of energy may take the form of fossil fuels, fertilizer, pesticides, etc. and often harm water and nutrient cycles also.
A relatively continuous cover of vigorous plants that can capture and store sunlight energy during a large portion of the year, without the need for high supplemental energy inputs in the form of fossil fuels or fertilizer is one of the best indicators of efficient energy flow. Significant deterioration of capital assets like soil organic matter, topsoil, reproductively active parts of vegetation, etc. would indicate poor energy flows.
Just as there are economic indexes to measure complexity and diversity of economic systems, successional level is a measurement of the complexity and diversity of ecological systems. In a highly developed economic system a more diverse group of producers can provide a greater amount of goods and services to a more diverse group of consumers with a given amount of raw materials and capital assets than a similar sized group of producers in a less developed economic system. Because of this increased diversity and competition, “One man’s trash is another man’s treasure.” So, less wealth leaves the system, and often accumulates.
Likewise, in a highly diverse and developed biological system, more of the raw materials are converted to biologically useful forms because products produced by one group of organisms become the raw materials that other organisms use to live. This increased diversity enhances the cycling of nutrients in the system. Increased productivity of the plant community increases the amount of vegetative cover on the soil. This vegetative cover protects the soil from erosion, increases infiltration, and therefore, enhances water cycles. In the same way, the increased diversity of the vegetation would increase the likelihood that some plants would be available at all times to respond to moisture and temperatures that favor growth and thereby increase energy capture and flow through the system.
The need for diversity is, however scale dependent. That is, species diversity may be required on a landscape scale, but may or may not be required, or even desired, on a given ecological site, much as a variety of businesses are good for the economic stability of a region, but zoning confines certain types of businesses to certain areas where conditions for that type of enterprise are most desirable. When measuring successional level at any scale, species composition of the community is compared to a potential desired community. Those producing relatively close to their potential are referred to as high seral communities, those producing at a lower level are referred to as mid- or low seral communities. The types of plants that dominate a plant community are determined not only by the potential of the ecological site, but also by the frequency and severity of disturbance, the annual precipitation and the depth to which that precipitation penetrates in the soil. The depth of penetration may be affected by the depth of soil, the effectiveness of the water cycle, the seasonality and size of rainfall events, or other factors. Table 1 shows the relationship among these factors and the dominant type of plants that can be expected in many cases in North America.
Table 1: Relationship of Different Environmental Factors to Dominant Plant Type in High Seral Communities.
Frequency, severity of disturbance
- Low to Medium
- Moderate to High
Moisture infiltration (depth)
- Shallow to Moderate
- Moderate to Deep
Dominant plant type
- Perennial short to mid-grasses
- Perennial mid-to tall-grasses with some woody plants
- Tall Grasses and Shrubs Adapted to the Disturbance
These are disturbances to which the grasses and shrubs are adapted, like grazing or fire.
Formerly, we thought of succession as a linear process in which plant communities changed over time with species replacing each other until a climax community – the most productive in the given environment – was achieved. This community was thought to be relatively stable in the absence of disturbance. Disturbance as a result of excessive grazing pressure, drought, or other factors would cause the community to revert to a lower successional level. With removal of the disturbance, the community would then progress through the same stages back to the stable climax community. This view of successional processes, however, has been unsuccessful in explaining plant community changes in some environments, particularly in those where “naturalized” exotic species have become an important part of the plant community, on areas where extreme degradation of the soil has occurred, or where other environmental influences like pollution or species extinction have changed the productive potential of the site.
The concept of linear succession has now been modified, by stating that a site has one to several possible alternate stable states. Each of these stable states can regress from its potential with the ability to recover back to its original productivity and species composition as long as this regression stays within a natural range of variability. However, when this ecological threshold is crossed, the feedbacks that govern relationships among plants change. The community becomes dominated by different species, and the former stable state can no longer be attained in management level time frames without significant alterations to the system. These ecological thresholds usually are associated with soil degradation, the introduction or extinction of a species or a change in the reproductive success of one species compared to others. Therefore, our management actions should first strive to preserve the productive potential of a site.
Seral status tells us something about the species composition of a community in relation to our management goals and the productive capacity of the site. The trend in seral status tells us whether or not we are progressing toward our management goals. Therefore, everything is relative, that is, no particular plant community is good or bad except in the context of what we want, what we have, and what a site is capable of producing. A low seral community may be desirable because of an improving trend, or because it provides a desired condition or product like habitat for a wildlife species, while a high seral site may be undesirable because of a declining trend. Furthermore, if the site has crossed an ecological threshold to an alternate stable state, no matter how much we would like and strive for a condition that used to be true (or that we thought used to be true) our efforts may be unsuccessful, at least in the short term. So, we should evaluate the vegetation of an area in terms of what we need from it (protection of productive capability, forage or habitat for animals using the site, etc.) and what we can realistically expect from the area, given the constraints of environmental conditions and the state of the plant community. Some indicators that a community may have crossed an ecological threshold into an alternate stable state could be a change in basic soil function, a loss of reproductive capacity for some species in the community, introduction of a foreign species that is capable of competing, a change in the basic age or spatial distribution of species in the community, etc. Table 2 summarizes and compares the characteristics of highly developed economies or high seral plant communities with those of developing economies or low seral plant communities.
Table 2. A comparison of developing economies/low seral plant communities with highly developed economies/high seral plant communities.
- Developing/low Seral
- Highly developed/high seral
- Relatively high year-to-year variation in production
- Relatively low year-to-year variation in production
- Average life of organism relatively short
- Average life of organism relatively long
- Small, volatile, short-term pools of resources
- Larger, stable, longer-term resource pools
- Wealth or raw materials often “leak” from the system
- Most wealth or raw materials stay in system
- Large scale fluxes in the “capital assets” of the system
- “Capital assets” remain fairly stable
- Wide environmental variability
- Environmental conditions relatively stable