To understand food and food systems today, it is necessary to comprehend the basic natural processes that are central to agriculture. If you have already studied the Healing Earth chapters on Biodiversity, Natural Resources, Energy, and Water you have learned information that is fundamental for understanding food and agriculture. In the following discussion we build on this knowledge with more details on food from an environmental science.
The Structure of Food Webs
It is important to recognize that food production is inextricably connected to ecological processes that have evolved in the natural world for millions of years. In any ecosystem, caloric energy (energy gained from food and used for cellular metabolism, or work) is transferred from plants to herbivores and then to carnivores and omnivores through the many feeding interrelationships of food webs.
Terrestrial Food Webs
Review more detailed information about the trophic levels in the Energy Chapter.
Organisms in the natural world are classified into trophic levels. The first trophic level consists of primary producers. These are organisms such as plants that can produce their own food energy through photosynthesis and that serve as food to other organisms. Primary producers are the base of most food webs. Primary consumers (herbivores), are organisms whose main diet consists of primary producers. Secondary consumers and tertiary consumers are organisms that eat the animals who feed on plants (carnivores), or eat both plants and animals (omnivores). Figure 2 below demonstrates the complex interrelationships among trophic levels in a terrestrial African Savanna biome.
Since primary producers are ultimately responsible for fueling all the other organisms in the food web, the amount of biomass produced by plants determines the number of higher trophic-level organisms that can exist in an ecosystem. The most productive terrestrial ecosystems with the greatest overall carrying capacities have rich, fertile soil, abundant rainfall and moderately warm temperatures year-round. These food webs can support five trophic levels and typically occur at or near the equator.
Humans are tertiary consumers, and like all tertiary consumers, human beings ultimately depend upon plants for sustenance and survival. We rely on the minerals that are sequestered in the soil as well as the carbohydrates, fats and proteins produced by plants and transferred through the trophic levels. As plant oils, carbohydrates, and proteins are consumed by animals, they are metabolically synthesized into animal fats and proteins, all of which are necessary for a complete human diet.
Marine Food Webs
Food webs in the ocean share many similarities with terrestrial food webs, but in water the primary producers are not large plants but very small (typically single-celled) floating algae and cyanobacteria. Together these are known as phytoplankton. Because the primary producers in marine ecosystems are so small, they are usually eaten by similarly-sized floating animals called zooplankton. These zooplankton are then eaten by larval fish or larger zooplankton, which are in turn consumed by small fish. Small fish are eaten by bigger fish, seals or seabirds. By the time the energy originally captured by phytoplankton has reached large animals, it has traveled through many tropic levels (Figure 3). Interesting exceptions to this are baleen whales and whale sharks. Despite being some of the largest animals on earth, these marine animals feed almost entirely on microscopic phytoplankton, zooplankton, krill, and small fish.
Corals also contain algae and form the basis of coral reef ecosystems (Figure 4). Coral reefs, which were discussed in the Biodiversity Chapter, occur in shallow, relatively warm water, and support some of Earth's most diverse communities of fish and marine animals. The reefs also act as nurseries for some fish species that spend their adult lives in the open ocean. Though most fish available for human consumption are open-ocean species caught by large-scale fishing operations, near-shore coral-reef species accessible to local fisherman are vital for the survival of many coastal communities throughout the world.
Photosynthesis and Biogeochemical Cycles
Plants and phytoplankton require solar energy from the sun, which was discussed in detail in the Energy Chapter. Through the process of photosynthesis, solar energy is captured and converted into chemical energy in the form of glucose, which is used by plants, algae and cyanobacteria to synthesize more complex organic molecules (Figure 5). Through this process, energy from sunlight is converted into caloric energy, which is then passed onto organisms higher in terrestrial and marine food webs.
Even though individual algae and other marine primary producers are typically single-celled and microscopic, there are so many of them capturing the sun’s energy that they can support a food web containing whales, sharks, fish, and countless other animals that live in the Earth’s oceans.
The transfer of energy up the food web is unidirectional, dissipating as it is transferred, with the sun serving as the energy source. When primary producers receive this energy, they store it in the bonds between atoms. When these bonds are broken apart during digestion or respiration, the energy holding the atoms of the molecule together is released. Most of a primary producer’s stored energy is lost as metabolic heat when it passes from one trophic level to the next, such as when a consumer eats a plant as food.
Most of the energy transferred to living things through food is used for maintaining the basic cell functioning that powers movement. On average, only 10% of the food’s energy is used to build new tissue for growth and reproduction while 90% of the energy is lost as metabolic heat through each successive trophic level. Because of the unidirectional flow of energy, most trophic pyramids cannot sustain greater than 4-5 levels. The top consumers in any given food web are few in number and comprise the lowest biomass (total amount of mass of all organisms) of any trophic level (Figures 2 and 6).
Unlike energy, chemical elements and nutrients do not dissipate and are not unidirectional in their passage through food webs. Instead they cycle throughout food webs. Nutrients are elements like nitrogen, phosphorus, and potassium that are essential to life because they make up the building blocks of organic molecules, which form the basic components of organisms. Nutrients are needed by both plants and animals. They usually enter terrestrial food webs when plants absorb them from the soil and atmosphere, and in marine and aquatic food webs when algae absorb them from water.
All nutrients cycle through the different levels in a food web and are converted into different chemical forms, but they are never completely lost or dissipated. Figure 6 depicts a typical terrestrial nutrient cycle. While many elements, such as phosphorus and sodium, only cycle locally (meaning they remain within the trophic levels of one ecosystem for lengthy periods of time), other elements that are easily converted to gaseous forms, such as carbon and nitrogen, are more broadly cycled on a global scale.
Read more about the biogeochemical cycles of phosphorus, carbon, and nitrogen in the Natural Resources Chapter.
Nutritional elements that are needed in high concentrations for plant and animal growth are called macronutrients. Elements that are needed in low concentrations are called micronutrients. The list of macronutrients is nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen(O), and hydrogen (H). The list of micronutrients (or trace minerals) is iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni).
Plants derive most nutrients from minerals present in the soil. Over a long geologic timescale, minerals are eroded from rocks due to wind and water exposure and leach into soils where they are taken up by plants or carried down rivers (as run off) to the oceans, where they will feed algae (Figure 7).
Soil is the foundational substrate that supports all terrestrial food webs and is defined as the interactive mixture of minerals, organic matter, water, gases and the living organisms that comprise the pedosphere, the outer most layer of earth. Besides creating a physical substrate to support rooting plants and a habitat for soil organisms, soil also functions as a medium for water storage and a recycling system for nutrients and organic wastes.
Read here to learn more about the organisms that live in soil, including bacteria, fungi, arthropods (like spiders and mites), earthworms, and vertebrates (like groundhogs and salamanders).
Soil Food Webs and Organisms
When many people think of soil they often equate it with dirt—tiny pieces of weathered rocks and minerals. Dirt particles are a vital component of soil, but there is much more to soil than dirt. Soil is a living substrate that shares many similarities with above-ground ecosystems. Just as there are trees, shrubs, plants, mammals, birds, arthropods, and microorganisms that form living communities above ground, there are countless micro and macro living organisms that live among the particles of crushed rock and contribute to the difference between dirt and soil.
Soil organisms range from tiny bacteria and wide-reaching mycorrhizal fungi to insects, worms, salamanders and even very small mammals. Together they form a complex ecosystem that is critical to maintaining soil health and to cycling the nutrients and energy that support terrestrial ecosystems. This below ground soil community directly influences plants that provide the base of above ground food webs.
Check out this video about mycorrhizal fungus that teams up with plant roots to increase their surface absorbing area.
When plants and animals die, their decomposition depends on soil-dwelling organisms. Through the decomposition process, nutrients that are bound in organic molecules in the form of plant, animal, fungal or bacterial tissue are mineralized into inorganic forms that can be dissolved in water and taken up by plants. In this way the decomposed plants and animals re-enter the food web in a perfect system of natural recycling.
Soils are classified by soil profiles, particle properties, soil types and soil formations. Each of these are described below.
A soil profile is a vertical section of soil measured from the ground surface to the underlying parent bedrock. Soil profiles contain horizons, which are characteristic horizontal bands, or layers, visible when soil is viewed in cross-section. (see Figures 8 and 9).
Different soils have different profiles depending on how, when and where they were formed. The top two layers of soil, known as the O horizon and the A horizon are the most important for plant growth because they are accessible to the widest range of plant roots and they contain the most nutrients. Together, these layers are known as top soil. A deep top soil can take tens of thousands of years to develop because many of its nutrients come from decomposing plants and animals.
Soils from temperate regions are different from those in the tropics (Figure 9). Many tropical soils have been highly weathered over time and have been exposed to large amounts of rainfall and high rates of decay from constant warm and moist conditions. These soils lose much of their capacity to chemically bind with nutrients. As a result, nutrients are quickly leached out and taken up by the massive plant biomass that these soils support. Tropical soils typically have little accumulated top-soil. In contrast, grasslands have a deep top-soil due to the long roots of their grasses and their high turnover rates.
Despite the comparative shallowness of their tropical soil, tropical ecosystems (especially tropical rainforests) are among the most productive terrestrial ecosystems on Earth. Tropical trees and smaller plants maintain productivity by quickly absorbing the nutrients released from decomposing plants and animals. In this way they holding most of the system’s nutrients in their above ground biomass.
Read this article for a description of soil properties.
The physical and chemical components of soil have a major influence on soil properties, determining the amount and kinds of crops farmers will grow. These properties include soil texture, soil structure, chemical composition, the soil’s pH, salinity, moisture holding capacity and its living organisms. The following paragraphs describe the composition, formation, and fertility of soils in general terms. For a more detailed discussion of soil properties, consult the Closer Look noted here.
Top soil plays a large part in determining an agricultural site’s productivity. To achieve optimum growth a plant typically requires top soil that consists of 45% minerals, 5% organic matter (or humus), 25% water, and 25% air (Figure 10). The organic matter portion can be further subdivided into 80% humus, 10% roots and 10% living organisms. This distribution provides the optimal combination of nutrients, drainage and aeration for plant growth.
Compacted soils are highly condensed, meaning there are few interstitial spaces for air and water and the humus is tightly compressed. Soils get compacted when heavy machinery is driven over agricultural soils or when compacting machines are used in building road ways or building foundations. As a result of such compaction, soil composition is modified and is less conducive to plant growth (Figure 11).
Soil Formation and Soil Types
Soil formation processes determine soil types and their profiles. These processes vary from place to place and depend on a region’s geologic history, climate and patterns of human land use. Factors contributing to soil formation include the original parent material (the bedrock from which the soil has originated), the terrain of the region, the amount of rainfall, the climate, the type of microorganisms present, the amount and type of vegetation present, time, and human influences. These processes act in concert with chemical, biological and physical factors to impact local soil formation. Under warm and humid conditions, soils can form rapidly. Under cold or dry conditions, soils may take hundreds of thousands of years to form. Soil scientists have classified over a thousand different soil types, grouped into one of 12 basic soil types, which are outlined in this chart
Read about how farmers change the pH of their soils to enhance plant growth.
Soil fertility refers to the chemical and physical properties of soil that confer its ability to promote sustained plant growth. A broad range of fertility is found in soils around the world from infertile desert soils to the richest tropical grasslands and river floodplains. Soils are more or less fertile depending on their macronutrient and micronutrient content, their cation exchange capacity, their pH, their capacity to absorb and drain water, and their organic matter content.
Fertile soils tend to contain an abundance of phosphorus, potassium, sulfur, calcium, magnesium, and iron. These soils support a greater variety of plant species than infertile soils. Because plants form the basis of terrestrial food webs and since different insects, mammals, and birds use different plant species for food, variances in soil fertility can impact the entire natural community of a region.
Questions to Consider
- We might think that calling a food nutritious means it tastes good. Why is this incorrect? What makes a food nutritious?
- How would it be more efficient in terms of energy and nutrient transfer in the food web if humans were herbivores rather than omnivores?
- What soil type do you have in your region and is it fertile for growing food?