How do ecosystems respond to natural disasters such as fires and floods?
This really depends on the ecosystem, as each ecosystem may respond differently.
How an ecosystem responds is going to depend on the ecosystem, as each ecosystem may respond differently, and also on the extent and magnitude of the natural disaster.
While natural disasters may seem like negative forces to humans, there are some ecosystems where a natural disaster is actually crucial to the ecosystem. For example, lodgepole pines require fires to reproduce. Without the hire temperatures found in fires, their cones do not open and release seeds. Read more about how fires help boreal forests here.
Another example is flooding. Floods can and do damage some species, but others rely on periodic floods. For example, red river gums, a type of eucalyptus tree, requires periodic floods. It is well-adapted to periods of low water availability, but these trees need a flood every now and then to saturate the clay they're rooted in. Seeds are dispersed on higher lands during floods where they would not reach without the flood. Read more about them here.
In other instances, a hurricane with winds that are very high may uproot trees and other plants. This is going to impact all of the other organisms that may depend on those trees and plants for survival (whether they depend on them for shelter or food or some other reason).
The 2004 tsunami in Southeast Asia damaged fragile coral reefs. Thankfully, those coral reefs recovered very quickly (see here). A substantial amount of saltwater from the ocean also penetrated terrestrial ecosystems, increasing the salinity of the soils. Read more about the environmental impacts of this natural disaster here.
Environmental damage after 2004 tsunami:
After a natural disaster, the community will often go through ecological succession and return to the state it was pre-diaster. Healthy ecosystems will bounce back, as natural disasters have occurred on earth for a long time.
Thus, natural disasters can severely damage some ecosystems, but other species may benefit and some species and ecosystems rely on these natural disasters. Healthy ecosystems are adapted to withstand natural disasters over the long time.