Summary

This chapter surveyed how organisms relate to their surroundings and to one another, and the measurable attributes of the populations they form, as laid out in the WBCHSE syllabus for Chapter 11.

The environment of an organism is made up of an abiotic component (physical/chemical factors such as temperature, water, light, and soil) and a biotic component (other living organisms and the relationships among them). A habitat is the physical place an organism lives, while its niche is the functional role it plays there -- its "occupation," including its diet, resource use, and tolerance range; the fundamental niche is the full range a species could theoretically use, and the realised niche is the narrower portion it actually occupies once competitors and other pressures are accounted for.

Populations of different species sharing a habitat interact in characteristic ways. Mutualism (+, +) benefits both partners, as in the lichen (fungus + alga) or fig and fig-wasp. Competition (−, −) harms both species when they depend on the same limited resource; Gause's Competitive Exclusion Principle holds that two species competing for an identical resource cannot coexist indefinitely, though resource partitioning (as in MacArthur's warblers) lets ecologically similar species reduce competition and coexist. Predation (+, −) sees a predator kill and consume prey, shaping both species through an evolutionary arms race of hunting and defensive adaptations, and can preserve overall community diversity through keystone predators. Parasitism (+, −) sees a parasite draw nourishment from a living host without immediately killing it, with ecto- and endoparasites showing distinctive adaptations such as adhesive organs, loss of unnecessary organs, and high reproductive output; brood parasitism, as in the koel and crow, is a specialised form of this interaction. …