(a) Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem Processes are named as ecosystem services. For examples, healthy forest ecosystem purifies air and water, minimize droughts and floods, cycle nutrients, generates fertile soils, provides wildlife habitat, maintains biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values. Though value of such services of biodiversity is difficult to determine, it seems reasonable to think that biodiversity should carry a healthy price tag. 

(b) Based on the nature of the habitat - whether it is water (or very wet areas) or it is on very dry areas - succession of plants is called hydrarch or xerarch, respectively.

Differences : Hydrarch succession takes place in wetter areas and the successional series progress from hydric to mesic conditions. As against this, xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions. 

Similarity : Both hydrarch and xerarch successions lead to medium water conditions (mesic), neither too dry (xeric) nor too wet (hydric).