Mutualism is an interaction that confers benefits on both the interacting species. Two examples where organisms involved are commercially exploited in agriculture are:
(i) Mycorrhizae close mutual association between fungi and the roots of higher plants.
Fungi help the plant in absorption of nutrients, while the plant provides food for the fungus.
(ii) Lichens represent an intimate mutualistic relationship between a fungus and photo synthesising algae or cyanobacteria.
Here, the fungus helps in the absorption of nutrients and provides protection, while algae prepare the food.

The termite provides shelter and space for the protozoan Trichonympha to live. The protozoan present in gut digests the wood, which termite feeds upon.
In the absence of Trichonympha, the termite is unable to digest wood and hence dies. Thus, the association of two given animal species represents mutualism.

The type of association the lianas have with the trees is commensalism because the plant gets the support of tree without affecting, harming or providing any benefit to the tree.

The ecological principle behind the biological control of pest insects is based on the ability of predator to reduce prey population in that habitat

(a) Hydrophyte. Because it is not able to tolerate deficiency of water. It is partially or completely submerged.
(b) Xerophyte. Because it prefers dry and hot climate with low rainfall. Thus have succulent leaves.
(c) Halophyte. Because it grows in saline habitat.
(d) Mesophyte. Because it prefers the areas with high moisture content

Because light intensities of stronger illuminations are absorbed by the exposed floating plants of the lake/water bodies so only the weaker illuminations reach the submerged, some light is reflected at water surface and a part is absorbed by upper layer of water.

Heliophytes are those plants which grow in strong light so are commonly called Sun plants e.g. sunflower, while sciophytes are those plants which prefer to grow in lower light intensities or shade loving plants e.g. money plant, fir etc. 

Contractile vacuole helps in maintaining salt and water level called osmoregulation. Because of the cellular environment of a freshwater organism such as Amoeba, Paramecium being hypertonic, the water diffuses inside the cell constantly and gets collected in the contractile vacuole, which squeezes the extra water out of the periodically.
Thus, keeping the internal environment constant. While in case of marine protozoans organisms, this does not occur due to high salt concentration. These organism live in isotonic conditions in sea water. Thus there is no need for contractile vacuole.

(i) There must be 'resource partitioning', i.e. by feeding on the same resource at different times.
(ii) They must have changed the pattern of foraging.

Zero population growth rate refers to the situation when the number of deaths and emigration is equal to the number of births and immigration in a give period of time.