Why are fungi important decomposers?
they produce many spores.
they can grow in many different environments.
they produce mycelia.
they recycle carbon and inorganic minerals by the process of decomposition.

Fungi are recyclers of organic and inorganic compounds, they can do this by the decomposition process. They are able to do this because they are absorotropic heterotrophs, that is, their way of obtaining food is releasing extracellular enzymes to the environment where they grow, which perform a partial digestion of the substrate, so that the products of this digestion can be absorbed by the fungus to be used in different metabolic processes. This characteristic makes fungi have a fundamental role in the carbon cycle, since part of the products of this digestion remain in the soil and can be reused by plants to obtain nutrients and thus resume the cycle.

it’s lava came from an explosive eruption

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.