If genetically modified organisms are able to survive and reproduce in the environment, some of them will succeed in spreading like 'aliens' throughout natural populations. This process might not be immediately obvious or externally apparent. However, by the time the problem is evident, it may be too late. Genetic engineering can thus endanger the conservation of natural species.
An example: a gene in fruit flies (Drosophila melanogaster) was manipulated using the gene scissors CRISPR/Cas to make it similar to one found in monarch butterflies (Danausplexippus). By changing only four base pairs in total, the fruit flies became resistant to toxins produced by certain plants. As a result, the flies can absorb the toxins and become poisonous to predators. Mass release of such flies could have serious consequences for the food web and ecosystems.
This example shows that minor changes to a single gene can have significant impacts on nature, even if no additional genes are inserted into the genome.
If such organisms are not strictly regulated in genetic engineering law, they can escape unnoticed into the environment; this is how genetic engineering becomes a threat to species protection.