247
environment, however, largely owing to the reasons these species were domesticated
in the first place: relatively large seed or fruit size, and high productivity. Annual
cereals like wheat produce large amounts of big nutrient-rich seeds because the
plants that sprout from the seed must survive wherever the seed falls and complete
the entire reproductive cycle in a single growing season—making these seed
characteristics competitively advantageous for the plant.
Before wheat was domesticated, the plant also benefited from having much of its
seed drop quickly upon ripening, hiding it from depredation by animals, and preventing it from getting too dry for germination by protecting it in the soil. During
the domestication of wheat, therefore, humans selected for individual plants that
held more seed on the plant, because that seed was more easily harvested. This
selection process resulted in those and other related genetic characteristics being
carried forward to the next generations of domestic wheat. The suite of characteristics that have been bred into domestic plants has thereby made the plants more
desirable to humans in a variety of ways, but not always in ways that conserve water
and energy resources during their growth, harvest and processing. As a result of this
dynamic, many domesticated annual plants would not thrive without human cultivation because the characteristics that have been bred into them are not adaptive for
survival of the plant in the absence of human intervention.
In contrast to the dominance of annual plant species in conventional agriculture,
natural ecosystems are more characteristically dominated by perennials including
trees, shrubs, vines, perennial flowering herbaceous plants and perennial grasses.
While annual plants sometimes play a significant role in natural ecosystems—particularly in climates that provide a competitive advantage to plants that can go dormant in seed form during dry summers—annuals in nature more typically rely on
regular disturbance to be able to compete with their relatively well-rooted, longlived perennial neighbors.
Fruit- and nut-producing trees, shrubs and vines are the primary counter- example
to annual plants in agriculture. The longer lives of these plants are inherently coupled with a longer establishment phase before they can yield food. This tends to
average the effects of climate, natural moisture regime, soil and other factors that
influence vegetation growth. While these types of vegetation are sometimes grown
through intensive human intervention that sustains them in locations where they
otherwise would not grow (e.g., citrus trees in areas where frost occurs), generally
these plants grow more sustainably and with less human intervention than annual
crops. A variety of agroforestry practices have been developed that conserve use of
energy and water and couple the production of tree crops with annual or perennial
plants or with livestock grazing. These practices can reduce the amount of nitrogen
fertilizer that is lost from the soil—and that potentially impacts human groundwater
wells and health—because the deeper and more extensive root systems of trees can
uptake excess nitrogen that annual plants are unable to use before it runs off.
The development of new perennial food crops is a growing area of research as
discussed in Chap. 21, because of their potential for conserving energy, water, and
natural ecosystem functions. Conventional perennial plants used for food or livestock forage, such as potatoes and alfalfa, are replanted either every year or every
9 Ecosystems and Ecosystem Services
environment, however, largely owing to the reasons these species were domesticated
in the first place: relatively large seed or fruit size, and high productivity. Annual
cereals like wheat produce large amounts of big nutrient-rich seeds because the
plants that sprout from the seed must survive wherever the seed falls and complete
the entire reproductive cycle in a single growing season—making these seed
characteristics competitively advantageous for the plant.
Before wheat was domesticated, the plant also benefited from having much of its
seed drop quickly upon ripening, hiding it from depredation by animals, and preventing it from getting too dry for germination by protecting it in the soil. During
the domestication of wheat, therefore, humans selected for individual plants that
held more seed on the plant, because that seed was more easily harvested. This
selection process resulted in those and other related genetic characteristics being
carried forward to the next generations of domestic wheat. The suite of characteristics that have been bred into domestic plants has thereby made the plants more
desirable to humans in a variety of ways, but not always in ways that conserve water
and energy resources during their growth, harvest and processing. As a result of this
dynamic, many domesticated annual plants would not thrive without human cultivation because the characteristics that have been bred into them are not adaptive for
survival of the plant in the absence of human intervention.
In contrast to the dominance of annual plant species in conventional agriculture,
natural ecosystems are more characteristically dominated by perennials including
trees, shrubs, vines, perennial flowering herbaceous plants and perennial grasses.
While annual plants sometimes play a significant role in natural ecosystems—particularly in climates that provide a competitive advantage to plants that can go dormant in seed form during dry summers—annuals in nature more typically rely on
regular disturbance to be able to compete with their relatively well-rooted, longlived perennial neighbors.
Fruit- and nut-producing trees, shrubs and vines are the primary counter- example
to annual plants in agriculture. The longer lives of these plants are inherently coupled with a longer establishment phase before they can yield food. This tends to
average the effects of climate, natural moisture regime, soil and other factors that
influence vegetation growth. While these types of vegetation are sometimes grown
through intensive human intervention that sustains them in locations where they
otherwise would not grow (e.g., citrus trees in areas where frost occurs), generally
these plants grow more sustainably and with less human intervention than annual
crops. A variety of agroforestry practices have been developed that conserve use of
energy and water and couple the production of tree crops with annual or perennial
plants or with livestock grazing. These practices can reduce the amount of nitrogen
fertilizer that is lost from the soil—and that potentially impacts human groundwater
wells and health—because the deeper and more extensive root systems of trees can
uptake excess nitrogen that annual plants are unable to use before it runs off.
The development of new perennial food crops is a growing area of research as
discussed in Chap. 21, because of their potential for conserving energy, water, and
natural ecosystem functions. Conventional perennial plants used for food or livestock forage, such as potatoes and alfalfa, are replanted either every year or every
9 Ecosystems and Ecosystem Services