"This" modifier

Question

Biologists have found that all tadpoles of that species begin life as omnivores, feeding mainly on organic debris in their soon-to-be-dry pool in the desert, but that occasionally one tadpole eats another or eats a freshwater shrimp. This event can trigger changes in the tadpole's physiology and dietary preference, causing the tadpole to become larger and exclusively carnivorous, feasting on other animals including members of its own species.

I uploaded a question asking the meaning of but that and got an answer that it means except. So I assume that the first sentence means this: [Biologists have found that tadpoles begin life as omnivores and eats mainly organic debris, except when they eat shrimp or one another.]

Now what makes me confusing is this event. What does this event means in this paragraph?

1. eating organic debris
2. eating one another or eats a freshwater shrimp

For me it sounds like the first one because eating one another or freshwater shrimp is followed after but that in the first sentence which makes eating organic debris sounds like the main idea of the first sentence. Can someone help?

Answer 1

Biologists have found that ..., but that...

They have found two things of a contrasting nature.

"This event..." is a good example of careless writing in which the exact event being referred to by "this" is uncertain. Whenever one is tempted to begin a new sentence with "This..." it is always a good idea to read back a few sentences and ask oneself if it is completely clear what the "This" refers to.

Answer 2

It's referring to the eating of one another or the fresh water shirmp. Judging the the rest of the context, the sentence was intended for "the consumption of one another or fresh water shrimp" to be the main idea. Without context, it might be a little harder to tell.