Explanation:

Ecotone:
An ecotone is a zone of junction or a transition area between two biomes [diverse ecosystems]. It is where two communities meet and integrate. For e.g. the mangrove forests represent an ecotone between marine and terrestrial ecosystem. Other examples are grassland (between forest and desert), estuary (between fresh water and salt water) and river bank or marsh land (between dry and wet).
Ecad:
These are also called epharmones or habitat forms which are environmentally induced variations. They belong to the same genetic stock or species and the variations in their morphology (in shape, size, number and reproductive capacity) are induced by the environmental influences. The variations are not fixed but are temporary, somatic and reversible. If one type of ecad is transplanted into environment of another type of ecad, the differences would disappear.
Ecological Niche:
Ecological Niche refers to the unique functional role and position of a species in its habitat or ecosystem. In nature, many species occupy the same habitat but they perform different functions. A
niche is unique for a species while many species share the habitat. No two species in a habitat can have the same niche. This is because of the competition with one another until one is displaced.