Reactions in plants and animals, starting in the very low level plankton and bacteria begin to affect wildlife and domesticated animals, and creep up through the soil and waterways, disrupting things so badly that nature cannot flurish. Where man maintains fields and herds and flocks to sustain human society, this is at its base nature, and where this gets disrupted all is affected. Enzymes that are necessary in order to get crops to come to fruit or to bear, and where these get disrupted, mankind would die rapidly. Fully a quarter of all 3rd Density worlds require cleanup during their transformation, having polluted their worlds in this way or having disrupted their ecosystem to the point of devastation, so that by the time of their transformation they are a mess. The ones that do not impact their ecosystems are ones where the intelligent species on them cannot control their ecosystem.
Most worlds are water worlds.
Creatures that swim and paddle about can't really affect their ecosystems. They are at the mercy of their ecosystems. When 3rd Density worlds are populated by hominoids or land based creatures or for whatever reason they are able to manipulate their environment, they ruin it. This is a factor of intelligence. It's the child in the nuclear armaments.