Page 666 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

630 part IV Soils, ecosystems, and Biomes
In some regions, simply planting multiple crops brings more stability to the ecosystem—this is an important principle of sustainable agriculture.
A modern agricultural ecosystem requires enormous amounts of energy, chemical pesticides and herbicides, artificial fertilizer, and irrigation water. The practice of harvesting and removing biomass from the land inter- rupts the cycling of materials into the soil and depletes soil nutrients over time, a loss that must be artificially replenished.
Biodiversity on the Decline
Human activities have great impact on global biodiver- sity; the present loss of species is irreversible and is ac- celerating. Extinction is final no matter how complex the organism or how long its existence. We are now facing a loss of genetic diversity that may be unparalleled in Earth’s history, even compared with the major extinc- tions that punctuate the geologic record.
Since life arose on the planet, six major extinctions have occurred. The fifth one was 65 million years ago,
TABlE 19.2 Known and Estimated Species on Earth
whereas the sixth is happening over the present decades (see Figure 12.1). Of all these extinction episodes, this is the only one of biotic origin, caused for the most part by human activity.
Presently, about 270000 species of plants are known to exist, with many more species yet to be identified. They represent a great untapped resource base. Only about 20 species of plants provide 90% of food for hu- mans; just three—wheat, maize (corn), and rice—make up half of that supply. Plants are also a major source of new medicines and chemical compounds that benefit humanity.
Table 19.2 summarizes the numbers of known and estimated species on Earth. Scientists have classified only 1.75 million species of plants and animals out of an estimated 13.6 million overall; this figure represents an increase in what scientists once thought to be the diver- sity of life on Earth. The wide range of estimates places the expected species count between a low of 3.6 million and a high of 111.7 million. Estimates of annual species loss range between 1000 and 30000 species, although this range might be conservative. The possibility exists
Categories of living Organisms
Number of Known Species
Estimated Number of Species
Working Estimate (×1000)
Accuracy
High (×1000)
low (×1000)
Viruses 4000
1000 50 400
Very poor Moderate Very poor Poor
Moderate
Moderate
Bacteria
4000
3000
50
1000
Very poor
Fungi algae nematodes
Mollusks Others
72 000 40 000 25 000
70 000 115 000
27 000 1000 1000
200
800
200 1500
150 400
100 400
100 200
200 250
Protozoa
40 000
200
60
200
Very poor
Plants
270 000
500
300
320
good
arthropods:
Crustaceans
40 000
200
75
150
Moderate
arachnids
75 000
1000
300
750
Moderate
insects
950 000
100 000
2000
8000
Moderate
Chordates
45 000
55
50
50
good
Total
1 750 000
111 655
3635
13 620
Very poor
Source: United nations environment Programme, Global Biodiversity Assessment (Cambridge, england: Cambridge University Press, 1995), Table 3.1–3.2, p. 118, used by permission.
Georeport 19.4 Will species adapt to Climate Change?
a 2013 study reveals that in order for vertebrate species to adapt to projected changes in climate by the year 2100, they will need to evolve their niche requirements 10000 times faster than rates in the past. Using genetic data for over
500 species of terrestrial vertebrates, including frogs, snakes, birds, and mammals, spread out over 17 evolutionary trees, the scientists examined how long each species took to shift its climatic niche under past environmental conditions. They found that over about a mil- lion years, species were able to adapt to a temperature difference of 1 C°. These results suggest that adaptation may not be an option for species survival in today’s rapidly warming climate.