What species cannot survive without the aid of another species?
The species that cannot survive without the aid of another species are known as obligate symbionts. These organisms have evolved to have a mutually dependent relationship with another species, which means they rely on each other for survival.
To find examples of obligate symbionts, you can start by looking at certain insects. For instance, the yucca moth and the yucca plant have a mutualistic relationship. The moth relies on the yucca plant for a place to lay its eggs and for food, while the plant relies on the moth for pollination. Without each other, they would not be able to reproduce and continue their respective life cycles.
Another example is the cleaner fish and their host fish. Cleaner fish, such as cleaner wrasses, remove parasites and dead skin from the bodies of larger fish while gaining a meal. The host fish benefit from the cleaning service and, in turn, provide the cleaner fish with sustenance. Without the cleaner fish, the host fish may suffer from increased parasite loads, while the cleaner fish would have no food source.
These are just a couple of examples, but there are many other obligate symbiotic relationships found in nature. You can further explore specific ecosystems, such as coral reefs or deep-sea hydrothermal vents, where various species rely on each other for survival.