The Distribution of Primary Production in Different Ecosystems

The Earth's annual NPP on land is estimated at 59% and the remaining 41% occurs in oceans and other aquatic systems. While estuaries, swamps and marshes, and tropical rainforests are highly productive, open ocean, tundra (arctic and alpine grasslands), and deserts are the least productive. Consequently we could conclude that to feed our hungry millions we should harvest plants in estuaries, swamps and marshes, or clear tropical forests and plant crops. However the grasses in estuaries, swamps, and marshes cannot be eaten by people and they are vital food sources (and spawning areas) for fish, shrimp, and other aquatic life that provide us and other consumers with protein. Therefore we should protect, not harvest or destroy, these plants and their ecosystems.

In tropical forests, most nutrients are stored in vegetation rather than in the soil. When the trees are removed, the nutrient-poor soils are rapidly depleted of their nutrients by frequent rains and by growing crops. Thus food crops can be grown only for a short time without massive, expensive applications of commercial fertilizers.

The rate of organic production, or primary productivity, of the world's solar-powered natural systems has been attempted to be estimated many times throughout the history. The pioneer agricultural chemist and plant nutritionist Baron Justus von Liebig, in 1862, based an estimate of the dry matter production of the global land area on a single sample of a green meadow. Interesting enough, Liebig's estimate of approximately 1011 metric tons yr-1 is very close to Lieth and Whittaker's estimate of 118 x 109 tyr-1 for continental areas.

In 1944 Gordon Riley overestimated ocean productivity, by basing his estimate on measurements in fertile inshore waters. Not until the 1960s, after the introduction of the carbon-14 measurement technique, was the low productivity of most of the open ocean recognized. Because the oceans cover about 2.5 times the area of the land on Earth, it was natural to assume, as Riley did, that marine ecosystems fixed more total solar energy than did terrestrial systems.

Figure 3 shows a comparison of the latitudinal distribution of land and ocean production. For the estimated mean values for large areas, productivity varies by about 2 orders of magnitude (100-fold), from 200 to

Coral World Distribution

Some estuaries, springs, coral reefs Terrestrial communities on alluvial plains Energy-subsidized agriculture

Figure 3 World distribution of primary production in terms of annual gross production (103 kcal m 2yr 1) of major ecosystem types.

Some estuaries, springs, coral reefs Terrestrial communities on alluvial plains Energy-subsidized agriculture

Figure 3 World distribution of primary production in terms of annual gross production (103 kcal m 2yr 1) of major ecosystem types.

20 000 kcal yr~\ and the total gross production of the world is on the order of 1018 kcalyr-1.

A very large part of the Earth is in the low-production category because either water (in deserts and grasslands) or nutrients (in the open ocean) are strongly limiting. Naturally fertile areas (i.e., areas that receive natural energy subsidies) are found chiefly in river deltas, estuaries, coastal upwelling areas, areas of rich glacial till, and wind-transported or volcanic soils in regions of adequate rainfall.

For all practical purposes, a level of 50 000 kcal m~ yr~ can be considered the upper limit for gross photosynthesis. Most agriculture shows low annual productivity, because annual crops are productive for less than half the year. Double cropping (i.e., raising crops that produce throughout the year) can approach the gross productivity of the best of natural communities. Remember, however, that net primary production will average about 60% of gross productivity and that the 'yield to humankind' of crops will be one-third or less of the gross productivity.

Was this article helpful?

0 0
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment