Moving to: m/AskMbin!

Plankton isn't an animal or insect though, it's algae and bacteria

My vote goes to worms. Without them huge amounts (like the vast majority) of land will become dead after a few years. Worms are very underrated

You and I thrive in oxygen, because we evolved in its presence, but oxygen is a really potent corrosive chemical that destroys a lot of life. When blue-green algae first showed up and started dumping oxygen everywhere, it in turn was a cataclysmic event for life on Earth.

https://en.wikipedia.org/wiki/Great_Oxidation_Event

The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust,[2] was a time interval during the Early Earth's Paleoproterozoic era when the Earth's atmosphere and the shallow ocean first experienced a rise in the concentration of oxygen.[3] This began approximately 2.460–2.426 Ga (billion years) ago, during the Siderian period, and ended approximately 2.060 Ga, during the Rhyacian.[4]

The sudden injection of highly reactive free oxygen, which is toxic to the then-mostly anaerobic biosphere, may have caused the extinction of many existing organisms on Earth — then mostly archaeal colonies that used retinal to utilize green-spectrum light energy and power a form of anoxygenic photosynthesis (see Purple Earth hypothesis). Although the event is inferred to have constituted a mass extinction,[7] due in part to the great difficulty in surveying microscopic organisms' abundances, and in part to the extreme age of fossil remains from that time, the Great Oxidation Event is typically not counted among conventional lists of "great extinctions", which are implicitly limited to the Phanerozoic eon. In any case, Isotope geochemistry data from sulfate minerals have been interpreted to indicate a decrease in the size of the biosphere of >80% associated with changes in nutrient supplies at the end of the GOE.[8]

Probably be pretty bad for us, but I suppose if you're an obligate anaerobic organism, you'd be having the best situation since a couple of billion years ago.