Dwight A. Brown
Department of GeographyUniversity of Minnesota
Minneapolis, MN 55455
Biological resources of substantial geographic concern fall into two categories, natural biological systems and managed biological systems. These two subsets of biological resources will be examined in order. Then, the principles will be applied by analyzing the issues of deforestation and desertification, and crop genetic diversity. The managed biological systems are of most direct concern in support of human life, and as such are the focus of immediate concern when we consider climate change and particularly the potential for human induced global warming.
Before we examine the various issues that surround
biological resources we need to set a common vocabulary and an understanding
of basic processes and fundamental principles.
Figure 3 shows the patterns of populations that reflect this process. Other patterns are disintegrating and reflect the disturbance patterns and the greater success of other species. Most plant populations that have been studied have not established themselves over their full potential climatic range. This is because dispersal processes are driven by the conditional probabilities of:
The place of the most genetic diversity of a genera is assumed to be the sources from which dispersal of a species originated. From the maps of species numbers in Figure 5, we can infer western, southwestern, and southeastern source areas for those grasses. The relative dominance patterns and morphologies of nine major grass species are shown in Figures 6 to 9:
Grasses are not the only plants to display dispersal processes in their abundance patterns. Margaret B. Davis (1983) studied the history of tree movements since the last continental glacier melted. Two distinctly different migration patterns are detected among eastern trees. Pines and hemlock moved toward the northwest from a Mid-Atlantic coast source, and the others moved toward the northeast from Tennessee, where they persisted during the last glacial maximum.
|Eastern tree types and dispersal patterns since the last glacial maximum inferred from pollen analysis of sediments. Isolines represent first arrival times in 1,000 years before present. Shaded areas on maps show modern range of tree types. Post glacial dispersal records based on pollen type analysis are after Davis (1983).|
The climatic and human contexts of these tree migrations is not known, but the differences among these distributional adjustments is increasingly viewed by paleobotanists as evidence for rejecting the deterministic paradigm that held vegetation patterns as directly determined by climatic. They are also used as evidence for natural dispersal rates for trees. These rates of migration indicate that most trees would not be able to keep in step with theorized climate changes over the next 50 years. Even in the absence of major human barriers to migration, the rate of adjustment of trees patterns would lag substantially behind the human-induced climate change. The response of other plant patterns to climate change is also of concern. The water use efficiencies and temperature responses of different plant types cause us to expect different responses. The root of some of the major response differences lies in the photosynthetic process.
Most plants have one of two photosynthetic systems in their leaf structure for fixing atmospheric carbon dioxide into plant carbon. These two leaf anatomies, known as C3 (cool season plants) and C4 (warm season plants) have optimum production temperatures. The optimum temperature for production by C3 plants is about 20°C and 35°C for C4 plants. These differences in the response of productive to temperature were observed in agricultural crops long before the leaf anatomy and biochemical processes were understood. As the production response curves in Figure 5 indicate, C3 plants have the potential to be most affected by global warming. This includes most woody plants and some grasses. Among the agricultural crops most vulnerable to heat stress are wheat, oats, barley, and rye. Beans and other legumes like alfalfa and clover are also C3 plants.
Farmers combat this photosynthetic limitation of small grain crops by planting the annual crop in the fall, allow it to go dormant over winter and complete growth before the temperatures get too hot. Wheat (winter wheat) is successfully planted in this manner, but not all grains survive winter well enough to use this forced adjustment of the plants phenology. As more winter hardy varieties of wheat have been developed, the region of winter wheat production in the United States has slowly drifted northward. Figure 11 shows the current distribution of winter wheat and spring wheat production in the United States. Global warming might necessitate further shift from spring wheat to winter wheat. The losers would be those crops, like soybeans and some small grains, that cannot survive winters.
One factor that adds uncertainty to estimating the
response of C3 crops to global warming induced by greenhouse gasses is
the fact that this photosynthetic system is presumed to have evolved at
a time when the global atmosphere had 10 times the CO2 content
than the modern atmosphere. Experiments show that there is a positive
production response of these plants to elevated CO2 levels.
C4 plants don't share this production response.
Other domestic grass crops are C4 plants (corn and various sorghums). These plants evolved in the tropics and benefit from high summer temperatures. These plants are also more efficient users of water, but global warming may in some areas result in more negative water balances and reduce yields. A strategy for combating climate change is for the area of production to shift with the climate, but there is a rub. The spaces of expected favorable climate will not correspond to the optimum soils, topography, or market location (Figure 11).
MANAGING BIOGEOGRAPHICAL SYSTEMS
Humans interact with all the processes that create plant and animal population patterns. By modifying habitats and selecting desirable agricultural crops, humans affect processes of evolution, adaptation and extinction. We affect the patterns of genetic resources by creating disturbances that allow invasions, and by assisting and blocking dispersal. These actions facilitate both the removal and addition of species to affected areas. We change the environment of species by introducing grazing, by irrigating, flooding, and draining. We add fertilizer or energy to biological systems which changes the nutrient and energy cycles and, in turn, the productivity of a place.
Changing what genetic resources are available
For plants to evolve, there must be a wide variety of genes available within an interbreeding population. Then natural selection acts to favor some gene combinations over others, depending on the particular environment in which individual plants attempt to grow. Selection forces include climate, pests, competitors, and people. For thousands of years, farmers have been selecting and favoring plants that carry traits desirable for food production. Some of these traits include:
Changing patterns of genetic resources
All species are at one time invaders. However, the invasions of some exotic species have been aided by humans, and these have been the subject of much concern. The Russian thistle or tumbleweed was an early, inadvertent introduction into the Plains. The sea lamprey into the Great Lakes, Eurasian milfoil, purple loosestrife, and the zebra mussel are some of the most publicized recent invaders into Minnesota. Fire ants are a significant pest in the Gulf Coast states. Despite all these notorious nuisances, no exotic species is so publicized as the African honey bee, first introduced into Brazil in 1957. Population growth and dispersal allowed it to gain a strong foothold north of the Rio Grande in 1993. Other organism invasions (besides our own species) of historic significance include:
Changing productivity of a place
Although we think about the questions of genetic resources, geographical pattern, and productivity as we consider the agricultural productivity of a place, attributes of place and economics are often govern the selection of what crops are grown where. Hence, not all crops are grown in their optimal location. For example, in Washington County, Texas, grain sorghum is the optimal production crop on the Brazoria soil series. On the Norwood soil its production is just as high but the optimum crop here is corn. Neither are native plant populations distributed plants in a way that conforms with the places that best meet their needs for maximum production. Table 5 illustrates the effect of our emphasis on economic criteria for determining plant location. The table considers 3 plant types and 3 different places. Each plant occurs in each site. The economic value (see column headings with dollar sign) of its natural pattern is not the same as the optimal pattern for economic return. The price of plant 3 is such that it is the favored crop in all sites, even though it is not the optimum producer in any. The optimum producer in Site C is Plant 1. Their optima are coincident. The optimum plant for Site B is also Plant 1, but the optimum plant at Site 3 is Plant 2. In the Real Production block, some plants are not distributed to produce the maximum. The production of the sites is genetically or population limited. That means that the right plants are either not there or there are not enough of them.
Table 5. Optimum production and economic return for plant resources
in places with different site characteristics.
The potential economic yield shows how land management would strive to move genetic material to optimize dollar return. If managers were concerned with maximum productivity, they would use plants indicated in the Genetic Potential Block.
These are monocultures. Greater long-term production
is usually sustained at a higher level and at a lower energy cost when
multiple plants grow together. Two factors are involved here, no
energy is expended to keep the system simple (low weeding expense) and
the different plants are not always in competition for the same resources
at the same time. That means if resources, such as rain or solar
radiation are available at different times and plants have different phenological
calendars (their growth stages have different calendars), some plant is
prepared to take advantage of the resource when others are not. The
result is a more complete use of energy inputs and a greater total biomass
Types of Human Interactions with biological systems
The last category of destructive human interactions
highlights the point that not all biological systems are viewed as a resource.
In some cases the term management means eradicating the organism.
Have you developed the fear of shatter cane or velvet leaf that herbicide
commercials attempt to instill in you? The herbicide brand name Eradicane
obviously focuses on the idea of total elimination. Small pox or
the polio virus are two examples of intentional eradication. A natural
systems view would be that these organisms play a role in creating opportunities
for other populations or organisms to use resources. From micro-organisms
to humans to whales, death of the old creates opportunities for young of
the same species.
Use of natural biological resources range from total exploitation, leading to extinction of species, to management efforts that attempt to maintain the productivity and yield of the system. These efforts usually focus on use of forest products or animal production by grazing range lands. In some cases the effort is directed toward restoration or rehabilitation of biological resources. This is a goal that cannot be accomplished because the boundary conditions that convey new genetic materials into the site are not restored. Wilderness areas are also managed to preserve wildlife habitat, plant resources, or plant patterns.
Agriculture is a direct and intentional attempt to control the dynamics of the biological system. The emphasis is on nurturing selected species. Yet, even here we control only a small part of the factors of production. For those factors we do not control--such as weather and genetic resources--we use game strategies, labor, and technology to mitigate them. In agriculture we may manage one or more trophic levels. The simplest example is cash crop farming which only attempts to control primary productivity. Other farmers feed their crops to livestock. Still others produce feed for dairy cows, sell cream or cheese, leaving skim milk or whey as byproducts to be fed to cattle, hogs, or chickens--the third trophic level. Maximum productivity of each level is controlled by the tax leveled by the necessary respiration and system maintenance of each level. If the population at one trophic level explodes in number, they must necessarily consume their resource base. That resource base depletion results in a crash of the consuming population (an example of the chaos theory model outcome). Biomass growth at each stage depends on the supply of nutrients beyond those required to maintain the system.
We use a number of strategies to mitigate threats
to our goals of controlling biological systems and maximizing their production.
At one extreme of the agricultural management spectrum is high technology
farming, and the other end is nonmechanized farming that tries to mimic
the optimum productive behavior of natural ecosystems. The former
substitutes machines, plant genetics, and chemicals for labor. The
latter, often practiced in the wet tropics, depends on high labor and substitutes
land and time for fertilizer inputs by long periods of fallow that allow
the natural system to restore nutrient levels and reduce pest populations.
Sustainable agriculture as practiced in the Midwest falls between these
DEFORESTATION AND DESERTIFICATION
Deforestation and desertification are two issues that are often linked, but are not synonymous. These environmental changes become issues because of concerns about their long-term consequences.
The primary deforestation concern in the media today is focused on the wet tropics. The purpose for most of the deforestation in the wet tropics is land clearance for small swidden agricultural plots and large farming operations (49%), commercial logging (26%), fuel wood (14%), and grazing (11%). The balance of these causes differs around the globe. Swidden agriculture (slash and burn) is responsible for 70% of closed forest clearing in tropical Africa, 50% in tropical Asia, and 35% in tropical Latin America. Expansion of cattle grazing is responsible for 30% of the Amazon Basin deforestation (World Resources 1994).
The issue of deforestation in the wet tropics has come into wide public view because of its link to the global warming issue (the release of carbon stored in plants into the atmosphere as CO2), and its link to the biodiversity issue. In addition, deforestation leaves soils highly erodible soils exposed to the atmosphere in a climate of frequent and intense rainfall.
In some areas, particularly less developed areas, cutting rates far exceed planting rates. Who is responsible? Often the major economic powers provide the market for the timber and livestock grown on deforested land. While the stimuli to cut forests vary, the following social/institutional reasons have been advanced:
The role forests play in the carbon cycle and its relationship to the issue of global warming induced by greenhouse gas increases is a major issue. The carbon cycle is not independent of nutrient cycles or the hydrologic cycle. Solar energy drives the photosynthesis process of the carbon cycle that converts atmospheric CO2 to plant carbon and the nutrient uptake is conveyed by the hydrologic cycle. Both of these processes are limited by temperature extremes. All three of these cycles are shown in a simplified form in Figure 16. The ability of the soil to supply nutrients to the biomass is one of the controls on rates of forest regrowth. Nutrient uptake by plants is also limited by precipitation, growth stage, and plant characteristics.
Two environmental issues that focus on burning of
tropical rainforests (selva) are the low ability of the soil to support
rapid regeneration and the conversion of huge carbon stores the in standing
biomass to atmospheric CO2. Figure
16 shows general models of the carbon and nutrient cycles. The
general nutrient budgets for several major ecosystems presented by Gersmehl
(1976) are shown in Figure
17. The fluxes are quite large for the selva, except for the
fallout. Burning creates a one-shot influx of nutrients to the litter
(the ash is nutrient rich), but the efficiency of return and holding of
these nutrients in the soil for prolonged uptake by regenerating plants
is very low because of the high throughflow of water and the high iron
and aluminum in the soil overly flocculate materials (nutrients are repelled
and not bonded to soil particles, which allows them to be easily flushed).
The soil nutrient reserves quickly succumb to relentless leaching.
As we move toward the drier desert environment, the rate of nutrient uptake is reduced by the lack of water to carry the nutrients to the plants. As the trees and other vegetation are removed, the amount of biomass converted to CO2 is not large, but the protection of soil and understory plants is lost. Without planting and nurturing new trees, the fuel wood supply is lost, but not all impacts are local.
Christopherson (p. 643) cites deforestation, overgrazing, erosion, improper soil water management, salinization of soils, and ongoing climate change as causes for desertification. He also shows the global extent of the degree of hazard of desertification. What is missing is the impact that desertification in these vulnerable areas has on the global climate, and on local populations. Some of these areas now support high populations, while others are sparsely populated. Deforestation taking place in the areas subject to desertification is primarily for cooking fuel.