Mega-organisms are an extremely rare occurance in the modern galaxy and represent one of the greatest potential threats to terraforming efforts on a hospitable world. In basic terms, mega-organisms are colossal creatures, reaching sizes far greater than even the biggest organisms in a typical ecosystem. Such organisms can weigh hundreds of thousands of kilograms and may be well over a hundred meters in height. These organisms are almost solely found on worlds with an extremely populated biospshere, as the sheer amount of biomass required to sustain a mega-organism is far greater than a typical ecosystem could possibly support. While these organisms are not necessarily predators, or even aggressive in anyway, even passive species represent a significant threat just through sheer mass.

The role of these mega-organisms in the local food web can range from extremely obvious to largely unclear. While some mega-organisms are quite obviously apex super predators, feeding on anything large enough to sustain it, others may sustain themselves through more subtle interactions with the local environment. Some of these creatures may even be sessile, reliant on processes like photosysnthesis, symbionts, or local chemical reactions to sustain itself. In the same way, it can be unclear what effect removing the mega-organisms will have on an ecosystem. While many of these creatures number fewer than 100 on the entire planet, their sheer size makes the true effects of their removal hard to ascertain.

Many researchers have also speculated that some mega-organisms may even be intelligent, playing an even more crucial role in their ecosystem than already believed. However, given the inherent danger of mega-organism carrying worlds, as well as the organisms themselves, it is often impossible to actually investigate such hypotheses.