Production: Background Information

Note: hot linked bold faced terms are in the Glossary

Understanding Production and its Limiting Factors

In the first unit, we began to familiarize ourselves with some principles of biogeography and how biogeography and the human dimensions of global change are related. One of the questions raised there was how production (in agriculture, forestry, and so forth) may change as the climate and other environmental factors change. In this second unit, we look at this question in detail.

Let's begin with the basic question, what is production? We commonly associate production with yield. The units can be volume or mass. Increases in mass may be defined as the increase in gross biomass (for example, the growth of trees), as the increase in the number of individuals (as in the case of population increase), or as the increase in a subset of mass production (for example, in seed, fruit, or forage yield, a subset of total production).

Organism types, species, and individuals differ in their productivity and in the efficiency with which they convert resources into biomass. Efficiency varies throughout the life cycle of the organism and depends on the environmental setting. The organismic and the environmental factors that influence this efficiency and productivity are called limiting factors, because in combination, they determine (limit) efficiency and productivity. For example, cool-season plants optimize the conversion of atmospheric carbon dioxide to plant carbon at about 20o C. Cool-season plants, like wheat, rice, and soybeans, use the C3 photosynthetic process, in which the first product in the sequence of biochemical reactions involved in photosynthesis has three carbon atoms. C3-plants use some of the solar energy they absorb in a process known as photorespiration in which CO2 fixed by plants gets reoxidized and released again as CO2. Because photorespiration is repressed under conditions of increased CO2, the photosynthesis of C3-plants under conditions of increased CO2 will lead to the production of more biomass (i.e., increased productivity). Warm-season plants on the other hand, like corn, sorghum, millet, and sugarcane, optimize the conversion of atmospheric carbon dioxide to plant carbon at about 35o C. They use the C4 photosynthetic process in which the first product in the sequence of biochemical reactions involved in photosynthesis has four carbon atoms, a more efficient process than the C3 process. C4-plants are optimal photosynthesizers under current CO2 conditions, and thus are likely to be less efficient photosynthesizers in a carbon dioxide enriched world (cf. Rosenzweig and Hillel 1993: 209).

In commercial agriculture, economic yield is usually more important than biomass yield. For example, farmers may choose to plant a low-yield, but high-price crop instead of a high-yield, low-price crop. In this case, efficiency is defined as the return on cash investment rather than biomass production return on energy and carbon input. Much of what drives the intensification of agriculture through technological innovation, resource inputs, and structural changes is the search for financial efficiency, or in other words, profit. For example, government crop subsidy programs are a major driving force that farmers respond to by selecting which crops to grow. This aspect of global biomass production is not predicted by the production potential of the environment and for that reason is not considered any further here. Instead, let's direct our attention toward the non-agricultural biogeographical systems.

What factors govern how much biomass is produced? We discuss six that apply to all biogeographical systems and two additional ones that apply in the case of managed ecosystems and/or agricultural systems.


Genetic factors

Geographic location and site factors Places in the landscape (hills, valleys, and uplands) have local variations in solar radiation, water, and soil resources, and are subject to different types and frequencies of disturbances (processes like fire, wind fall, erosion, landslides, etc. that eliminate or decrease the short- to long-term viability of an organism). By "breaking up" the soil or the interactive web of organisms, disturbances make colonization by new organisms possible. For a summary of recent research on disturbance, invasion, and habitat change see Lodge (1993). On the other hand, already established organisms may or may not offer the kinds of ecosystemic niches, resources, and potential for interactions necessary for an arriving organism to find its new habitat there. The appropriate habitat for plants and animals simply may or may not be available or accessible. Trophic level and biotic interactions The diversity of organisms and their distribution among the trophic levels (position of an organism in the food web) are among the limits to long-term production. The degree of mutual benefit derived from sharing resources is important to long-term productivity. When passenger pigeons and squirrels sustain themselves by consuming acorns, they also provide for the oak's dispersal by not consuming all the acorns they carry away. Patterns of buffalo grass suggest that the bison destroyed plants by wallowing, but this also created opportunities for spreading the grass when the seed burrs carried in the bison's hide until rubbed off in the wallowing process.

Not all sharing relationships are harmonious, nor do all mutually beneficial relationships maximize production. Such a relationship may have benefited buffalo grass populations, but it probably replaced more productive plants. Some plants have a positive growth response to being grazed on, but the grazers' presence is not required. Winter wheat is sown in the fall and is sometimes grazed to stimulate tillering of the newly emerged seedlings before winter dormancy sets in. Tillering, the increase in the number of emergent reproductive and vegetal shoots, creates more robust plants that produce a higher seed yield. The truest form of mutual benefit/dependence is in the symbiotic relationship between two organisms, a relationship in which both organisms mutually require the presence of the other. A good example is the symbiosis between fungi and algae that form lichens. Though these species evolved as independent species, they are now so mutually evolved that neither can survive alone. Similar relations exist between animals and digestive bacteria.

Pests, predators, disease, and other disturbances This is really a special case of the third factor, biotic interactions. Pests, predators, and diseases are aspects of a changing, interactive biotic system, aspects that capture or divert the resources needed for production of a given organism. They may even eliminate some organisms from the landscape. Time Production varies over time as a result of the temporal variability in all of the above factors. A clear expression of this factor is the seasonal change of productivity: temperature, light, and water change in most biomes over the course of the year. Consequently, productivity increases and decreases over the course of the year. Both too little of a resource or too much of any will reduce production. Management The term management implies control of factors of production. Given the complexity of any biotic system, management strategies usually aim to affect only a few of the limiting factors of production. Price While not going into any detail about the market forces that affect agricultural production, it is important to mention that price fluctuations affect which products are grown. Commonly, production is therefore biased toward crops that will yield the most net cash return per unit of land rather than the most biomass. Occasionally, the result is the same, but usually it is not.

The Food Web

Places in which biotic interactions take place -- habitats for short -- can be viewed as systems in order to understand the process of production. The concept of the food web is one example of place as a system. In the food web, each organism and energy reservoir interacts with its surroundings by transfers of mass and energy (Cunningham 1994). The flows of mass and energy can be expressed in transfer budgets. They are altered by direct human intrusion into the process (e.g., harvesting, fertilizing, irrigating) and by indirect human alteration (habitat reduction, forage removal affecting grazers and carnivore production, deliberate or accidental introduction of a new organism, inadvertent changes in atmospheric temperature or chemistry affecting plant life, wildlife habitat, and crops). The alteration of these flows sometimes involves the redirection of resources to other uses (e.g., water capture for agricultural crop irrigation or car washing). Humans also invade the ecosystem energy storage compartments for particular purposes. For example, plowing soil increases the oxidation of soil carbon (organic matter), and forest cutting removes carbon and nutrients stored in the standing biomass.

Modeling Material and Energy Flows Through the Food Web

Modeling is one way to enhance our understanding of nutrient and energy flows. It is important to remember that models are simplifications of reality; they are never as complete and dynamic as the real system we try to understand. Precisely because in building a model we select what we think are the most important elements and processes of a system, models are a helpful means to understand some of the interactions taking place and the relative importance of the elements in a system. The food web shown in the food web slide show is no exception (see FOODWEB.FLC).

Models like the food web can be operationalized as simulation models. We use simulations as tools to understand the interdependence among parts of a system (e.g., between nutrients and production) and to see how a system behaves if one of the elements is changed, or in other words, how sensitive the overall functioning of a system is to a change in one of its components. For instance, we can use such models to explore how production might change if we alter the availability of water or nutrients from the atmosphere. A test of this kind is called sensitivity analysis.

With regard to global change studies, such analyses are extremely useful and are also frequently used in the common case where we do not know what impacts, say, a temperature increase of 2 ºC would have on the productivity of wheat. The questions are: how sensitive is wheat to such a change in temperature, and in which direction will its productivity change; toward higher or lower productivity?

The simple three-compartment simulation model of the nutrient cycle (Gersmehl 1976) we will use in the activities operates under the laws of conservation of mass and energy (which says that mass and energy may change the form in which they appear, but they can't really be lost). This simulation model is formulated as an equilibrium or steady state model in which, after a certain number of iterations, the relative allocation of nutrients among compartments stabilizes. In the real world, forces that control the fluxes among compartments are not constant, not synchronous (they do not occur at the same time), and are seldom predictable. Thus, we cannot use the model to predict the future. We can, however, use it to illustrate how changes in one part of the system cascade through the system to produce different futures, providing all other things are held constant. This, of course, can be done only in the abstract world of models.

With a model such as the nutrient cycle simulation model we can see why plant biomass, soil, and litter conditions differ dramatically over large geographic areas or at different points in time. The quality of the modeling outcomes depends on how adequately we know the relevant attributes of the range of places we try to model.


Human Interactions with the Food Web

Human life depends on the food web. As omnivores, humans depend on intrusion into more than one trophic level. We are special omnivores because we manipulate primary and herbivore productivity to meet specialized wants (agriculture, natural resource management and use). Via mechanized agriculture we try to direct all production toward satisfying our own special wants. The result is substantially reduced diversity, which, in turn, denies resources to other organisms. Monocultures, the most extreme example of such specialized production systems lacking in diversity, are of concern because they affect a wide array of environmental processes and organisms. Anything we do to change the suite of plants and animals or their abundance alters the nutrient and energy fluxes (uptake, respiration, and fallout) of affected spaces. These altered flux rates cascade through the system, affecting other trophic levels.

All human activities that affect land cover change mass and energy balances. Loss of vegetation accelerates erosion and deposition elsewhere. Release of greenhouse gasses alters the loss of long-wave-length terrestrial radiation and is thought to result in climate warming. Many other feedbacks in the global system, their nature and magnitude, remain unknown. Specifically we do not know how much of the greenhouse effect will be mitigated by increases in humidity and cloud cover that reduce incoming solar radiation.

We also do not understand how simple policies aimed at mitigating problems in part of the system stimulate radically different responses from place to place. Some of these mitigation strategies stimulate responses that are the opposite of what the strategies were meant to do. Just as business managers try to minimize tax impacts (by maximizing benefits from the tax code), farmers respond to land conservation reserve programs by conserving some land and at the same time plowing other land to maintain the same base. Alternatively, they may quite intentionally fail to use effective soil conservation measures in order to qualify for another government program aimed at paying farmers to retire vulnerable lands. A third example of unintended effects of mitigation strategies is the case where the government pays farmers not to plant so as to keep marginal pieces of land out of production while maintaining farmers' income. In some dramatic cases, this led to a perceived scarcity of land and consequently caused the land values of both the cultivated and the preserved land to go up. Finally, this resulted in the ironic situation of farmers plowing under the (marginal) "virgin" rangeland to meet the market demands for cultivated land and to boost their incomes. Thus the attempt to conserve land ultimately lead to greater use (and in some cases to degradation) of the land.