(a) What is ecological succession ? Explain the different types and theories of ecological succession. (20 marks) (b) Explain…

(a) Ecological Succession: Types and Theories

Ecological succession is the directional replacement of one biotic community by another in a habitat until a relatively stable climax is reached. It proceeds through seral stages: pioneer, intermediate, and climax. Typical sequence: bare substrate → pioneers → herbs/shrubs → forest → climax. Primary succession begins on newly exposed, lifeless substrates such as lava, sand dunes, glacial till, or bare rock; secondary succession follows disturbance that destroys the community but leaves soil, such as fire, flood, or fallow land. Succession is also autogenic (organisms modify their own habitat) or allogenic (external factors such as climate or water level); autotrophic (photosynthetic energy input) or heterotrophic (energy from organic matter); and hydrarch (aquatic to terrestrial, pond → marsh → forest) or xerarch (dry substrate to forest, sand → grassland → forest). In a hydrosere the stages are phytoplankton → submerged plants → floating plants → reed swamp → sedge meadow → woodland → forest; in a lithosere they are crustose lichens → foliose lichens → mosses → herbs → shrubs → forest. Clements’ monoclimax hypothesis treated succession as a predictable, organismic sequence ending in one climactic community determined by climate. Gleason’s individualistic/continuum hypothesis stressed independent species responses to microsite conditions, so communities are mosaics rather than discrete units. Tansley’s polyclimax hypothesis accepted multiple climaxes shaped by soil, moisture, and topography, and Whittaker’s climax-pattern hypothesis sees the climax as a continuum of communities varying along environmental gradients. Connell and Slatyer’s (1977) models describe the mechanisms: facilitation (pioneers make conditions easier for later species), tolerance (later species establish under low resources), and inhibition (established species resist newcomers). Egler’s relay floristics holds that each seral stage prepares the site for the next; initial floristics composition holds that the climax is largely determined by the first colonists. As succession proceeds, biomass, species diversity, nutrient retention and stability rise, and the P/R ratio approaches 1 (Odum).

(b) Aggressive and Territorial Behaviour in Animals

Aggressive behaviour is a threat, attack, or defensive response directed against conspecifics or other species. It may be offensive, to gain mates, food, or rank, or defensive, to protect self, offspring, or resources. Much aggression is ritualized: stags lock antlers, Siamese fighting fish flare fins and gill covers, and rhesus macaques use postures and vocalizations before contact, reducing injury while deciding dominance. In the three-spined stickleback, the red belly of a male acts as a sign stimulus releasing attack by territorial males (Tinbergen), and in hens a pecking order (Schjelderup-Ebbe) stabilises dominance and lowers fighting. Territoriality is defence of a bounded area. Ethologists (after Nice) distinguish Type A (mating, nesting and feeding), Type B (mating and nesting only), Type C (small nest-site territory, as in colonies), and Type D (mating only, as in leks). Territoriality is maintained by signals such as song, scent, or display; if an intruder crosses the boundary, the resident escalates until retreat or departure. Songbirds defend breeding territories mainly against conspecifics; damselfish defend algal patches against herbivorous fish; langurs defend home ranges against other groups. Tigers and other big cats mark territories with urine spray and scrapes. Benefits are reliable access to food, mates, nesting sites, and reduced competition; costs include energy, injury, predation risk, and lost foraging time.

(c) Indian Major Carps and Their Polyculture

The Indian major carps are Labeo rohita (rohu), Catla catla (catla), and Cirrhinus mrigala (mrigal); Labeo calbasu (kalbasu) is sometimes added as a fourth, minor species. They are distinct from exotic carps such as Cyprinus carpio (common carp), Ctenopharyngodon idella (grass carp), and Hypophthalmichthys molitrix/nobilis (silver/bighead carps). Polyculture works because these species occupy compatible feeding niches: Catla is mainly a surface zooplankton feeder, Rohu is a mid-water/column feeder on phytoplankton, detritus and small organisms, Mrigal is a bottom feeder on detritus and benthic algae, and common carp is a bottom detritus feeder. Vertical distribution in the pond: surface, Catla; mid-water: Rohu; bottom: Mrigal/common carp. ICAR-CIFA-recommended composite culture uses about 5,000 fingerlings/ha, commonly 3 Catla : 4 Rohu : 3 Mrigal, with common carp added at a lower proportion, e.g. 3:4:3:2, adjusted to pond productivity. In the six-species composite culture developed by ICAR-CIFA, the three Indian carps are combined with silver carp (surface phytoplankton), grass carp (macrophytes) and common carp (bottom), giving about 3–5 t/ha/year under semi-intensive management and considerably more with high inputs. Seed is produced by induced breeding with pituitary extract or synthetic hormones such as Ovaprim. Operationally, ponds are dried, desilted, cleared of predatory and weed fish (for example with mahua oil cake), limed, manured with farmyard manure and, where permitted, inorganic fertilizer, stocked with graded fingerlings, given supplementary feed such as bran, oil cakes, and formulated pellets, and harvested by partial or full harvest at 8–12 months. Yield advantages arise from better use of the water column and reduced interspecific competition. Disease management requires graded stocking, avoidance of overcrowding, water-quality monitoring, quarantine of new fingerlings, and balanced nutrition; polyculture does not automatically reduce disease and can increase parasite exposure if density is high.