Final answer:
The term 'keystone effect' refers to the significant impact on an ecosystem caused by the removal of a keystone species, which are crucial for maintaining ecological balance and biodiversity.
Step-by-step explanation:
The term that describes the large effects on an ecosystem at all trophic levels due to the removal of a keystone species is known as 'keystone effect'. A keystone species is an organism that has a disproportionate impact on its environment relative to its abundance. These species play a critical role in maintaining the structure of ecological communities, affecting many other organisms in the ecosystem and helping to sustain ecological balance. For example, the intertidal sea star, Pisaster ochraceus, is a keystone species in marine ecosystems. Its removal leads to an increase in mussel populations, which can drastically alter the ecosystem composition and reduce biodiversity.