• ​IF an ecosystem suffers frequent disturbances, many of the ecological relationships are disrupted and organisms may find it difficult to survive & reproduce. Some species die out.

• IF an ecosystem is rarely disturbed, then it is stable and the number of species remains relatively unchanged.

• Intermediate levels of disturbance may actually stimulate microevolution and speciation!

​How can IDH stimulate Microevolution?

​The IDH proposes that species diversity is generally maximized if disturbance is neither too rare nor too frequent because species that thrive at both early and late successional stages can coexist.

​On a larger scale, disturbances of intermediate frequency and/or intensity may generate diversity. For example, a fire of intermediate or mixed severity could increase diversity by generating spatial heterogeneity within a landscape.

This is because a mixed severity fire will result in a complex of patches in a landscape. Patches differ in severity of burn and include:
- unburned patches
- low severity burn patches
- moderate severity burn patches where perhaps one-third to two-thirds of the vegetation is killed
- high severity burn patches where almost all the vegetation is killed .

In this case, the IDH may not hold on a local scale (within patches) because species richness within a small area may decline (for example, in severely burnt areas). However, on a larger landscape scale a fire of intermediate severity may increase species diversity. This is because areas or patches that differ in burn severity, and that are at different stages of post-fire change, would sustain more species with different disturbance sensitivities.

​spatial heterogeneity - the uneven distribution of characteristics or features across a landscape or population.