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

Ecological succession 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. Diagram: 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). 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 climax pattern hypothesis accepted multiple climaxes shaped by soil, moisture, and topography. Mechanisms include facilitation (pioneers make conditions easier for later species), tolerance (later species establish under low resources), and inhibition (established species resist newcomers). 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.

Aggressive and territorial behaviour 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. Territoriality is defence of a bounded area. Ethologists distinguish Type A (conspecific defence), Type B (heterospecific defence), Type C (defence against both), and Type D (both, with seasonal or priority variation). 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. Benefits are reliable access to food, mates, nesting sites, and reduced competition; costs include energy, injury, predation risk, and lost foraging time.

Indian major carps and polyculture The Indian major carps are Labeo rohita (rohu), Catla catla (catla), and Cirrhinus mrigala (mrigal). 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. Pond diagram: 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. Operationally, ponds are dried, desilted, 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.

Thus, succession explains how communities reorganize through predictable mechanisms, while aggressive and territorial behaviour shows how individuals secure resources; in aquaculture, the same principle of niche partitioning is applied to raise carp yields sustainably.