Quiz 20: Succession and Stability

Geology/Geography/Oceanography/Atmospheric Sciences

Ecologists have observed an increase in species diversity and changes in species composition during succession. Johnston and Odum studied the changes in species richness of birds with changes in plant populations during succession. Their study showed the following changes in plant population during succession in a forest ecosystem: img As, the composition of the plant species changed; there was an increase in plant diversity. There were similar changes in the population of birds. The composition of bird species changed with time, and there was an increase in the bird diversity. In addition, MacArthur correlated the complexity of the environment with the species diversity. His observations indicate an increase in diversity of birds with the increase in foliage height diversity. Moreover, succession in the forest ecosystem studied by Johnston and Odum began with plants of short stature such as grasses and weeds. They were followed by shrubs and small trees of intermediate stature and finally by tall trees. Thus, the foliage height diversity went on increasing with succession. This increase in foliage height diversity in turn led to an increase in the diversity of bird species.

A variation in the normally distributed observations can be estimated by using standard deviation or standard error. These estimates are not useful for analyzing distributions that are non-normal. The interquartile range can be used to indicate the variation in such samples. The interquartile range can be asymmetrical around a median. Though 50% of the observations fall in the interquartile range along with the median, the observations included are on the basis of the rank and not on the magnitude of the observations. The ranks whose values are included in the interquartile range are equally distributed around the median. However, the observations show non-normal distribution. Hence, the interquartile range may include values asymmetrically distributed on either side of the median.

The studies on the succession in the rocky intertidal zone, observed an increase in the number of species with time. The studies on the small boulders indicate that the community became steady within 1 to 2 years and showed about 5 species. This community will not remain steady indefinitely. Moreover, it may undergo fluctuations due to the disturbances, intraspecific, and interspecific interactions. A disturbance may lead to destruction of the climax community and mark the beginning of succession. In addition, the studies have shown the intertidal boulders were first colonized by the green alga Ulva and species of barnacles. They were followed by the red algae of species Gelidium and species of Gigartina. The observation of succession on small boulders indicated about 5 species in the climax community after 1.5 years of succession. When Sousa continued his observations of boulders for a period of 3 years, he found Gigartina canaliculata to be the predominant species. The species was found to over grow the other species. Furthermore, large boulders in Sousa's study site are stated to be dominated by the red alga Gigartina canaliculata. The boulders supported an average of 2.3 to 3.5 species instead of 5 species observed on the small boulders. The number and types of species on the boulders was found by Sousa to be dependent on many factors, such as the boulder size, the frequency of disturbance, and the stage of succession. The difference in number of species on the two boulders is due to following reasons: • The size of the two boulders is different. • Difference in size of boulders leads to differences in disturbances experienced by the boulders from the waves during the storms. Small boulders experienced maximum disturbance, and large boulders experienced minimum disturbance. Ecologists have shown that intermediate level of disturbance favours maximum species richness. Intermediate level of disturbance will be experienced by boulders of intermediate size. • The observation of boulders after 2 years indicated about 5 species. Observations made later showed predominance of Gigartina canaliculata and its overgrowth on other species. Thus all these factors have a role in controlling the number of species on the boulders.

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