Quiz 2: Life on Land

Geology/Geography/Oceanography/Atmospheric Sciences

Daniel Jansen's study of the guanacaste tree led him to the conclusion that the dispersion of the seeds of the tree was carried out by the large herbivorous animals including ground sloths, camels and horses following the end of Pleistocene about 10000 years ago. However, these animals became extinct probably due to over hunting by humans. As a result, for thousands of years, the seeds produced by the guanacaste tree were not dispersed effectively in the absence of the dispersers. However, it continued to produce a large number of fruits. This led to the significant reduction in the number of Guanacaste trees in the region. There would have been numerous other plants which were dependant on these large herbivorous animals for the dispersal of their seeds. After the extinction of these animal species, these trees too would have suffered low dispersal leading to an adverse effect on the plant population size. Some of them may also have got extinct. However, many of these plant species survived, like the guanacaste tree. This fact suggests that some alternative mechanisms of seed dispersal may have existed or alternative dispersers may have done the job of seed dispersal for these trees. Alternative means of the seed dispersal could be through multiple different vectors such as rodents and the other small animals. Also, the plants would have had to depend more on abiotic factors such as wind and rain for seed dispersal. Some trees are known to have evolved special structures on their fruits. Some examples of such structures are long and curved outgrowths which can easily cling on to animal feet and body parts. Hence, they get dispersed with the moving animals.

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A soil profile is the vertical arrangement of different layers of the soil from top soil to bedrock. It is composed of four distinct horizons as shown and described below: img O horizon : It is the topmost layer that is primarily composed of organic matter. The upper layer of O horizon contains fallen plant litter like leaves, flowers, fruits, twigs, and dead organisms. The lower layer of O horizon contains fragmented and partially decomposed organic matter. The decomposition is carried out by soil microbes, nematodes, mites, and other small burrowing animals. A horizon : It is composed of a mixture of the inorganic minerals and organic matter. It is rich in the organic material and biological activity. The burrowing animals like earthworms mix minerals and organic matter from the O horizon into the A horizon. Clay, iron, aluminum, silicates and soluble organic matter are slowly leached from A horizon. B horizon : It is mainly composed of deposits of leachates coming in from A horizon. The deposits may form distinct colors and banding patterns in this horizon. C horizon : It forms the deepest layer of the soil. It is composed of the weathered parent rock material. Sand, silt, and clay are produced by weathering of rocks due to action of water, frost and plant roots. It also contains the rock fragments. Beneath the C horizon is the bedrock.

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