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Question

Identify the incorrect statement.
I. During plant succession, some species colonise an area and populations become more numerous.
II. Secondary succession is a slow process when compared to primary succession.
III. In the successive seral stages, there is no change in the species composition of the ecosystem.

A
I and III
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B
I and II
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C
II and III
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D
I only
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Solution

The correct option is C II and III
Ecological succession is a predictable, steady and gradual change in a species composition of a given area with respect to the changing physical environment. It ultimately forms a climax community which is stable and in near equilibrium with the environment.
In an area, the entire sequence of communities that successively undergo change are called sere(s). Each individual community formed during the transitions is called a seral stage or seral community.
In each seral stage, new species invade the area, due to changes in the physical environment made by the preceding species. Hence, statement III is incorrect.

The sequential progression of species during ecological succession is not a random process. As new plant species invade the area, they modify the habitat by altering the amount of sunlight reaching the ground, mineral composition of the soil etc. These changes allow entry of other, more complex species that are better suited to the new or modified habitat succeeding the earlier species. This implies that during ecological succession some species take over the area and increase in number. Therefore, statement I is correct.

Succession where a newly formed area is colonised for the first time by a group of living organisms or species is known as primary succession. For example, newly cooled lava, newly created pond reservoirs, etc. However, if life starts at a place after the area has lost all the life forms existing there, the process is called secondary succession. For example, abandoned farm lands, burned or cut forests, lands that have been flooded, etc.
Primary succession is a rather slow process as life has to start from nothing whereas secondary succession is faster because it starts at a place which had already supported life before and will have at least some soil or sediment already present. Thus statement II is incorrect.

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