As humans or Progenitors expand and build colonies on Planet, they inevitably begin to cause ecological damage. Ecological damage can provoke the native life forms and cause other unexpected effects.
The red ecological damage number represents a percentage chance, per turn, of an ecological accident happening in the base's territory.
The more mines, solar collectors, roads, and farms (but not forests, which actually help the ecology) you build, the more the potential damage. Boreholes, Mirrors, and Condensers cause even greater damage. You can reduce or eliminate this type of damage by building Tree Farm and Hybrid Forest facilities.
As your industrial output (minerals) increases, ecological damage will also take place. This effect can be reduced by building the Centauri Preserve, Temple of Planet, and Nanoreplicator or a Tree Farm/Hybrid Forest facilities.
Ecological damage reducing facilities only reduce ecological damage if built [after] your first eco-damage report.
See also Ecology (Advanced).