Top Down Effects Are Elusive in Saprotrophic Food Chains

Recent studies have delved deeply into the long-known large differences between aquatic and terrestrial food webs with meta-analyses that combine hundreds of individual studies. Aquatic food webs have much higher rates of herbivory and produce less detritus than terrestrial food webs. This key difference has been known since the 1960s, but its implications are immense, varied, and still a subject of much ecological research. The major producers in aquatic systems are phytoplankton, which are much more nutritious to herbivores than higher plants, the major terrestrial producers. Phytoplanktons are much smaller than their consumers and have high population growth rate; the opposite is true in terrestrial ecosystems - higher plants are mostly larger and have lower population growth rates than most terrestrial herbivores. More nutritious and more rapidly growing populations of primary producers lead to much greater transfer of primary productivity to herbivores in aquatic systems than in terrestrial systems.

Phytoseiulus macropilis (predatory mite)

The ultimate basis of the nutritional difference is that the medium of phytoplankton, water, is roughly of the same density as their cells. Buoyancy carries them to the surface where they compete for light. Land plants are not buoyant in air, and need rigidity in order to maintain upright stature in the competition for light. Cellulose and lignin, which comprise a large fraction of the biomass of higher plants, give rigidity and comprise a large fraction of the biomass of land plants. Cellulose and lignin have a surfeit of carbon to nitrogen. They have a high C/N ratio, low nutritive value to animals, and are avoided or largely undigested by many herbivores. This shunts primary productivity to detritus, consumed mostly by bacteria and fungi in the saprotrophic channels of terrestrial food webs (Figure 4).

The difference in body composition of primary producers between aquatic and terrestrial systems means that detritus is more nutritious to saprotrophs in the sea and lakes than those on land. Saprotrophic animals and microbes have a body composition of nutrients that is much closer to phytoplankton than to the high carbon to nitrogen ratios in wood or straw (which comprises the stems of grasses). In stoichiometric terms given by R. W. Sterner and J. L. Elser in the important book, Stoichiometry, the average ratio of carbon to nitrogen (C:N) ranges from 7.7 to 10.2 in detritus in oceans and lakes, respectively, while on land C:N ratios of detritus are an order of magnitude higher. Sterner and Elser suggest that the reported C:N ratios in much of the terrestrial data are probably underestimates; terrestrial saprotrophs get even more carbon and less N than the data show!

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