This version corrects the key grammatical problems: The majority of local marine mammals are predators, which stand high in the food chain, resulting in the ecosystem losing its balance. Bear in mind: "Mammals" should be plural in this context, but you could start the sentence differently, like "The main local marine mammal..." to make it singular, but main does not mean majority so I have not gone for this version. If going plural, change "is" to "are". "Above the food chain" implies they are not a part of the food chain. More accurate is to say they are at the top, or near the top, or high up, within the food chain. "Stand high in the food chain" is what I went for, for simplicity and to keep changes minimal, but you could equally say it a bit differently, for example "stand high up in the food chain" or "rank highly in the food chain" etc. "Resulted" is for talking about the past, but so far this sentence seems to be talking about the present moment (now) because it uses words like "is" or "are". If you change "are predators" to "have been predators" then you can say "which resulted in", but if we are using present tense, with "are predators", then "resulting in" matches that tense, for good grammar. Now I've explained every change I've made, and offered some similar alternatives, so you should be fine. But there's always more that can be done, to polish the wording, for efficiency & finesse, by native speakers' standards. For example: "which stand high" can become "positioned high". And "resulting in the ecosystem losing its balance" can become "causing the ecosystem to lose its balance." And you might want to change "predators" to "apex predators" if you want to refer specifically to those at the very top of the food chain, who have no predators above them; but obviously don't do that if it's not correct information - I'm only offering the option as a way of demonstrating how to use richer wording.

The majority .... are.