Fires can result from both human disturbances and natural causes. an immediate effect that wide-spread fires have on any ecosystem is

Fires disrupt the equilibrium of an ecosystem. The results of a fire include a resetting of plant succession. Many ecosystems have adapted to a certain frequency and intensity of wildfires, and many plant species actually require fires as part of their life cycles. Fires can open up forests and grasslands that would be too congested and shaded to allow young grasses or trees to take root. Fires in fact act as a stimulus for some seeds to germinate. These would typically be pioneer plants that are able to grow in fairly harsh and varied conditions. Fires also remove accumulated dead tree matter that can accumulate in a forest, and if wildfires are too infrequent, fires and be very intense when they do happen as they are fed by the accumulated material. 

Hope the answer is b