Ecosystems can be thought of as energy transformers and nutrient processors composed of organisms within a food web that require continual input of energy to balance that lost during metabolism, growth, and reproduction. These organisms are either 'primary producers' (auto-trophs), which derive their energy by using sunlight to convert inorganic carbon into organic carbon, or 'secondary producers' (heterotophs), which use organic carbon as their energy source. Organisms that perform similar types of ecosystem functions can be broadly categorized by their 'functional group'. For example, 'herbivores' are heterotophs that eat autotrophs, 'carnivores' are hetero-trophs that eat other heterotrophs, while 'detritivores' are heterotrophs that eat nonliving organic material (detritus) derived from either autotrophs or heterotrophs (Figure 3). Herbivores, carnivores, and dertitivores are collectively known as 'consumers'.
Classifying organisms according to their feeding relationships is the basis of defining an organism's 'trophic level'; the first trophic level includes autotrophs; the second trophic level includes herbivores and so on. Ecosystem components that make up a trophic level are quantified in terms of biomass (the weight or standing crop of organisms), while ecosystem dynamics, the flow of energy and materials among system components, are quantified in terms of rates.
Typically, ecologists quantifying ecosystem dynamics use carbon as their currency to describe material flow and energy to quantify energy flux. Material flow and energy flow differ in one important property, namely their ability to be recycled. Chemical materials within an ecosystem are recycled through an ecosystem's component. In contrast, energy moves through an ecosystem only once and is not recycled (Figure 3). Most energy is transformed to heat and ultimately lost from the system. Consequently, the continual input of new solar energy is what keeps an ecosystem operational.
Solar energy is transformed into chemical energy by primary producers via photosynthesis, the process of converting inorganic carbon (CO2) from the air into organic carbon (C6H12O2) in the form of carbohydrates. Gross primary production is the energy or carbon fixed via photosynthesis over a specific period of time, while net primary production is the energy or carbon fixed in
crop biomass (high turnover)
Standing crop biomass (low turnover)
Figure 4 Standing crop biomass is not always correlated to production rates. Here, two hypothetical species with populations at equilibrium, where input equals output, have an equivalent standing crop biomass but differ in their turnover rates. Population (a) has high input, high production, and high turnover rates, whereas population (b) has low input, low production, and low turnover rates. In reality, populations are rarely at equilibrium so standing crop biomass fluctuates depending on input rates and the amount of production consumed by higher trophic levels. Adapted from KrebsC (2001) Ecology: The Experimental Analysis of Distribution and Abundance, 5th edn. San Francisco: Addison-Wesley Educational Publishers, Inc.
photosynthesis, minus energy or carbon which is lost via respiration, per unit time. Production by secondary producers is simply the amount of energy or material formed per unit term.
A careful distinction needs to be made between production rates and static estimates of standing crop biomass, particularly because the two need not be related. For example, two populations at equilibrium, in which input equals output, might have the same standing stock biomass but drastically different production rates because turnover rates can vary (Figure 4). For example, on surf swept shores from Alaska to California, two species of macroalgal primary producers grow in the low rocky intertidal zone of temperate coastal ecosystems (Figure 5). The ribbon kelp, Alaria marginata, is an annual alga with high growth rates, whereas sea cabbage, Hedophyllum sessile, is a perennial alga with comparatively lower growth rates. Although they differ greatly in their production rates, in mid-July, during the peak of the growing season, these two species can have almost equivalent stand crop biomasses.
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