(a) What is "population" according to you as a biology student ?
(b) "The size of a population for any species is not a static parameter." Justify the statement with specific reference to fluctuations in the population density of a region in a given periods of time.
OR
(a ) What is hydrarch succession ?
(b) Compare the pioneer species and climax communities of hydrarch and xerarch succession respectively.
(c) List the factors upon which the type of invading pioneer species depend in secondary hydrarch succession. Why is the rate of this succession faster than that of primary succession ?

Explanation

Single individuals of any species living in a groups in a well defind geographical area share or compete for similar resource, inter-breed and thus, constitute a population.
(b) The size of the population tells us a lot about its status in the habitat. Whatever ecological processes are to be investigated in a population, be the outcome of competition with other species the impact of a predator, it is evaluated in terms of change in the population size.
Population size is no more technically called population density. Population size of a species is the number of individuals of a species per unit volume
Population density = `("Number of individuals ina a region (N)")/("Unit area in a region (S)")`
`implies P Delta = (N)/(S)`
A population at any given time is composed of individuals of different ages. When the age distribution (percent individuals of given age or age group) is plotted for the population, the resulting structure is called age pyramid.
OR
(a) The gradual and fairly predicatable change in the species composition of a given area is called ecological succession. Hydrarch susccession takes place in wetter areas and the successful series progresses from hydric to the mesic conditions.
(b) The species that invade a base area are called pioneer species. The changes which finally lead to a community That is in near equilibrium with the enviroment is called climx community.
In primary Hydrarch succession, the pioneer are the small phytoplanktons, they are replaced with time by free floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees. The climax would be a forest and later water body is converted into land.
Xerarch succession takes place in dry areas and series progresses from xeric to mesic conditions. Primary succession on rock initiated by pioneer lichens which secrete acids to dissolve rock, helps in weathering and soil formation. Later, bryophytes grows which are able to hold in small amount of soil. Later bigger plants takes over ultimately a stable climax forest community is formed.
(c) In secondary Hydrarch succession, the species that invade depend upon the condition of soil availability of water. since the soil is already there, the rate of succession is much faster and hence, climax is reached more quickly.