The species that invade a bare area are called pioneer species. In a xeric succession, the pioneer species are usually lichens then bryophytes which are succeeded by ferns and some other bigger plants.

Lichen produces lichen acid and carbonic acid which corrodes rock surface and release minerals required for growth. The corroded rock accumulate soil particle by wind and provides substrate for Bryophytes and ferms.

Solar radiation is the ultimate source of energy for the ecosystem except deep hydrothermal ecosystem.

Common edible mushroom (Agaricus) is achlorophyllous (does not possess chlorophyll) and is a heterotroph.

Oceans are least productive because

(i) There is insufficient radiation as sunlight decreases with the increasing depth of the ocean.

(ii) Oceans are nitrogen deficient which is an important nutrient for plants.

(iii) Conditions of high salinity which is not favourable for all plants.

(iv) there is no substratum to support plants.

The rate of assimilation of energy at herbivore level is called secondary productivity because the biomass available to the organisms of next trophic level (consumer) for further consumption is a resultant of the primary productivity which is formed by autotrophs (plants).