Quiz 12 :
The species of plants differ with respect to the seed size and the number of seeds. The variation in seed size was found at all geographical locations. The advantages and disadvantages of seed size were responsible for maintenance of variation. Small seeds have been found to have following advantages: • A plant producing small seeds can invest in large number of seeds. This increases the chances of survival of the plant. Small seeds have been found to be vulnerable to stress. Hence, the large number ensures at least few survivors in the disturbed environments. • Under favorable conditions small seeds can establish themselves large numbers in new areas. • Small seeds can easily disperse without any assistance or through wind. This helps in dispersal of species over long distances and increases the geographic range.
The sample variance/sample mean ratio for a species is used to determine its pattern of distribution. The sample variance/sample mean ratio is equal to 1 in a randomly distributed population. The ratio is less than 1, in case of regularly distributed population and greater than 1, in case of clumped distribution. The pattern of distribution of a species can be statiscally confirmed by the chi-square test. Chi-square test can be used to find out whether the sample variance/sample mean ratio for each species significantly differs from 1. The values of chi-square can be determined for each species included in the study from the sample variance, sample mean and the degrees of freedom. To determine whether the sample variance/sample mean ratio is significantly less or significantly greater than 1, the calculated chi square values are compared to two critical values namely upper critical value and lower critical value. Only one critical value has been defined for species B. Though the calculated value of chi-square for species B is below the upper critical value, the lower critical value is not defined. Hence, the results of chi-square test for species B are doubtful to confirm the regular distribution of the species.
A limited range of resources and energy are available for the living organisms. With the limited resources, the organisms have to grow, sustain and reproduce. The allocation of energy for reproduction or the reproductive effort is related to many factors such as the mortality rate, the age of reproductive maturity, the clutch size and the size of the offspring. The ecologists have made an observation that a species that produces smaller offspring produces them in large numbers and a species that produces larger offspring produces in smaller numbers. Both the cases have advantages and disadvantages depending on the circumstances. The smaller offspring have been observed to be vulnerable to environmental stress and show reduced fitness. Hence, production of such offspring in large numbers increases the chances of survival. Natural selection will favor the small and numerous offspring under following conditions: • Establishment in new areas with favorable conditions • Dispersal through wind in case of plants • High mortality rate of the species • Low parental care leading to high mortality of juveniles • High predation of the species • Short life span of the species