Autotrophic production

All biological activity within ecosystems (Fig. 2) is supported by the production of organic matter by autotrophs (organisms that can produce organic molecules from simple inorganic substances, such as carbon dioxide and inorganic nitrogen). More than 99% of the autotrophic production on Earth is provided through photosynthesis by plants, algae, and certain types of bacteria. Collectively, these organisms are termed photoautotrophs (autotrophs that use energy from light to produce organic molecules). In addition to photosynthesis, some production is conducted by chemoautotrophic bacteria (autotrophs that use energy stored in the chemical bonds of inorganic molecules, such as hydrogen sulfide, to produce organic molecules). The organic molecules produced by autotrophs are used to support the organism's metabolism and reproduction, and to build new tissue. This new tissue is consumed by herbivores or detritivores, which in turn are ultimately consumed by predators or other detritivores.

Fig. 2 General model of energy flow through ecosystems. (Copyright © McGraw-Hill Education)

Terrestrial ecosystems, which cover 30% of the Earth's surface, contribute a little over one-half of the total global photosynthetic production of organic matter—approximately 60 × 10¹⁵ grams of carbon per year. Oceans, which cover 70% of the Earth's surface, produce approximately 51 × 10¹⁵ grams of carbon per year. See also: Terrestrial ecosystem

Whereas most ecosystems receive sunlight to support photosynthesis, photosynthetic production does occur in the deep ocean and in caves. In a few unique ecosystems, such as deep-sea thermal vents and caves, the production of organic matter is carried out by chemoautotrophic bacteria. These ecosystems harbor surprisingly diverse communities that function in the complete absence of sunlight. See also: Bacteria

Heterotrophic production

The organic matter produced by autotrophic organisms is ultimately consumed by heterotrophs (organisms that obtain energy and nutrients by consuming other organisms). Globally, most organic matter produced by plants and algae is consumed by detritivores. Herbivores consume only about 5% of photosynthetic production, fire destroys another 5%, and a small fraction of autotrophic production is lost through burial within the Earth's crust (to potentially become fossil fuels). The proportion of photosynthetic production consumed by herbivores can vary considerably among ecosystems. In terrestrial ecosystems, much plant tissue is structural molecules, such as cellulose and lignin, that are difficult or impossible for most organisms to digest. In aquatic ecosystems, in which algae dominate, a much greater proportion of photosynthetic production is consumed by herbivores. Algae have little in the way of structural molecules and contain more molecules that can be consumed by grazing organisms. See also: Biomass; Ecological succession

Food webs

Organisms are often classified according to the number of energy transfers through a food web or food chain (Fig. 3). Photoautotrophic production of organic matter represents the first energy transfer in ecosystems and is classified as primary production. Consumption of a plant by a herbivore is the second energy transfer; thus, herbivores occupy the second trophic level, also known as secondary production. Consumer organisms that are one, two, or three transfers from photoautotrophs are classified as primary, secondary, and tertiary consumers. Moving through a food web, energy is lost during each transfer as heat, as described by the second law of thermodynamics. Consequently, the total number of energy transfers rarely exceeds four or five; with energy loss during each transfer, little energy is available to support organisms at the highest levels of a food web. See also: Ecological energetics; Food web; Heat; Trophic ecology

Fig. 3 Food chains are limited by energy transfer. In this illustration, energy is transferred from the Sun to a plant (producer) to a primary consumer and to a secondary consumer. Energy is used by the producer and consumer organisms and is lost to the environment as heat. (Copyright © McGraw-Hill Education)

Biogeochemical cycles

In contrast to energy, which is lost from ecosystems as heat, chemical elements (or nutrients) that compose molecules within organisms are not altered and may repeatedly cycle between organisms and their environment. Approximately 40 elements compose the bodies of organisms, with carbon, oxygen, hydrogen, nitrogen, and phosphorus being the most abundant. If one of these elements is in short supply in the environment, the growth of organisms can be limited, even if sufficient energy is available. In particular, nitrogen and phosphorus are the elements most commonly limiting organism growth. This limitation is illustrated by the widespread use of fertilizers, which are applied to agricultural fields to alleviate nutrient limitation. See also: Biogeochemistry; Fertilizer

Nitrogen cycle

Nitrogen commonly limits the rate of primary production in terrestrial, freshwater, estuarine, and oceanic ecosystems (Fig. 4). In one turn of the biogeochemical cycle of nitrogen, (1) cyanobacteria and certain types of eubacteria transform atmospheric dinitrogen (N₂) into organic molecules, such as protein (nitrogen fixation); (2) organisms are consumed by other organisms, and organic nitrogen is converted into ammonium as a waste product (decomposition); (3) ammonium either is assimilated by plants, algae, and bacteria into organic forms or is converted to nitrate by chemoautotrophic bacteria (nitrification); and (4) nitrate is converted back to dinitrogen by anaerobic bacteria (denitrification). Whereas nitrogen is very abundant in the atmosphere as dinitrogen, only a select group of organisms can transform dinitrogen into organic forms (amino acids), and the transformation requires a lot of metabolic energy. Humans have greatly impacted the global nitrogen cycle by doubling the amount of ammonium, nitrate, and organic nitrogen in the environment through the widespread use of fertilizers and combustion of fossil fuels. This massive change in nitrogen forms has led to problems with acid precipitation, alterations of global patterns of primary production rates, increased production of nitrous oxide (a greenhouse gas), and pollution of aquatic ecosystems. See also: Human ecology; Nitrogen; Nitrogen cycle; Nitrogen fixation; Nitrogen oxides; Water pollution

Fig. 4 General model of the nitrogen cycle in ecosystems. (Copyright © McGraw-Hill Education)

Carbon cycle

Carbon cycles between the atmosphere and terrestrial and oceanic ecosystems (Fig. 5). This cycling results, in part, from primary production and decomposition of organic matter. In turn, rates of primary production and decomposition are regulated by the supply of nitrogen, phosphorus, and iron. The combustion of fossil fuels is a modern-day change in the global cycle that releases carbon that has long been buried within the Earth's crust to the atmosphere. Carbon dioxide in the atmosphere traps heat on the Earth's surface and is a major factor regulating the climate. This alteration of the global carbon cycle along with the resulting impact on the climate, that is, global climate change, is a major issue under investigation by ecosystem ecologists. See also: Air pollution; Applied ecology; Global climate change

Fig. 5 General model of the carbon cycle in ecosystems: (A) carbon dioxide (CO₂) from the air; (B) carbon from decomposition; (C) carbon from fossil fuels; (D) natural events (for example, forest fires and volcanoes) release CO₂ into the air. (Copyright © McGraw-Hill Education)

Jeremy B. Jones, Jr.