Relationships between primary and secondary productivity

Since secondary productivity depends on primary productivity, we should expect a positive relationship between the two variables in communities. Turning again to the stream study described in Section 17.4.1, recall that primary productivity declined dramatically during the summer when a canopy of tree leaves above the stream shaded out most of the incident radiation. A principal grazer of the algal biomass is the snail Elimia clavaeformis. Figure 17.20a shows how the growth rate of individual snails in the stream was lowest in the summer; there was a statistically significant positive relationship between snail growth and monthly stream bed PAR (Hill et al., 2001). Figure 17.20b-d illustrates the general relationship between primary and secondary productivity in aquatic and terrestrial examples. Secondary productivity by zooplankton, which principally consume phytoplankton cells, is positively related to phytoplankton productivity in a range of lakes in different parts of the world (Figure 17.20b). The productivity of heterotrophic bacteria in lakes and oceans also parallels that of phytoplankton (Figure 17.20c); they metabolize dissolved organic matter released from intact phytoplankton cells or produced as a result of 'messy feeding' by grazing animals. Figure 17.20d shows how the productivity of Geospiza fortis (one of Darwin's finches), measured in terms of average brood size on an island in the Galápagos archipelago, is related to annual rainfall, itself an index of primary productivity.

there is a general positive relationship between primary and secondary productivity

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Figure 17.20 (a) Seasonal pattern of snail growth (mean increase in weight of individually marked snails during a month on the stream bed ± SE). The open circle represents growth at a nearby unshaded stream site in June. (After Hill et al., 2001.)

(b) Relationship between primary and secondary productivity for zooplankton in lakes. (After Brylinsky & Mann, 1973.)

(c) Relationship between bacterial and phytoplankton productivity in fresh water and seawater. (After Cole et al. 1988.)

(d) Mean clutch size of Geospiza fortis in relation to annual rainfall (positively related to primary productivity); the open circles are for particularly wet years when El Niño weather events occurred. (After Grant et al., 2000.).

A general rule in both aquatic and terrestrial ecosystems is that secondary productivity by herbivores is approximately an order of magnitude less than the primary productivity upon which it is based. This is a consistent feature of all grazer systems: that part of the trophic structure of a community that depends, at its base, on the consumption of living plant biomass (in the ecosystem context we use 'grazer' in a different sense to its definition in Chapter 9). It results in a pyramidal structure in which the productivity of plants provides a broad base upon which a smaller productivity of primary consumers depends, with a still smaller productivity of secondary consumers above that. Trophic levels may also have a pyramidal structure when expressed in terms of density or biomass. (Elton (1927) was the first to recognize this fundamental feature of community architecture and his ideas were later elaborated by Lindemann (1942).) But there are many exceptions. Food chains based on trees will certainly have larger numbers (but not biomass) of herbivores per unit area than of plants, while chains dependent on phytoplankton production may give inverted pyramids of biomass, with a highly productive but small biomass of short-lived algal cells maintaining a larger biomass of longer lived zooplankton.

The productivity of herbivores is invariably less than that of the plants on which they feed. Where has the missing energy gone? First, not all of the plant biomass produced is consumed alive by herbivores. Much dies without being grazed and supports the decomposer community (bacteria, fungi and detritivorous animals). Second, not all plant biomass eaten by herbivores (nor herbivore biomass eaten by carnivores) is assimilated and available for incorporation into consumer biomass. Some is lost in feces, and this also passes to the decomposers. Third, not all energy that has been assimilated is actually converted to biomass. A proportion is lost as respiratory heat. This occurs both because no energy conversion process is ever 100% efficient (some is lost as unusable random heat, consistent with the second law of thermodynamics) and also because animals do work that requires energy, again released as heat. These three energy pathways occur at all trophic levels and are illustrated in Figure 17.21.

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