[A pikachu would've taken nothing on his person, but not me. ]

I think you mean "a wild pikachu" or "an ordinary pikachu" or whatever you're going for specifically, given you say he's still identifying as a pikachu.

[ I had a small green backpack, made out of handcrafted leather]

Also, if you have talking sapient animals, you either don't want to use leather or you need to explain whose skin got handcrafted into the backpack.

[ that was dyed green to match Faraday Island's temperate environment. ]

A temperate environment is one that has a summer/winter cycle, and by extension a lot of leaf litter. There's a reason rabbits and squirrels aren't green. Actual camouflage would be brown or green/brown camo patterns.

[ It had essential survival tools, like firestarters, a flashlight]

Again, this is supposedly a pikachu. Firestarters are things that can produce tiny sparks and material that can catch. A pikachu can produce enough electricity to char regular wood easily. Similarly, he shouldn't have any difficulty being his own flashlight.

If your fic is meant to be about a pokemon as opposed to a human, writing them just like a human is a mistake. For example, a pikachu would probably want emergency batteries – a pikachu out of electricity can't start a fight, generate light, or defend themself, and he could drain the charge out of those in an emergency.

[Whatever vital nutrients there was had to be dug out below the lava flow layer.]

Uh. Volcanic soils are famous for being extremely rich, and volcanic islands are lush as a result. If this is supposed to be a nutrient-poor island, make it another type.

[A trained wilderness expert could tell this trail was reserved for electric mice pokémon by the random burnt marks on the ground.]

Reserved means only electric mice pokemon are allowed to walk there. It sounds like you probably just mean used by, and not actively banned from use by anyone else (how would something like that even be enforced?)

[Forest fires almost never happen here due to the cold and rain, but they do occur. The Faraday Island Fire Department will come in and put out the blaze, even if the forest fire is only reserved to one tree. Single tree fires can occur during thunderstorms. Lightning would ground straight down the spine of a fat spruce tree and the core would smoulder. Sometimes, it would burn holes through the bark, which creates a chimney effect that kindles the burn into a full blown blaze, which can risk spreading to other trees. Sometimes the tree contains the fire and it fizzles out, or the firefighters put them out. The trees were important for the island's integrity, it keeps the soil from washing away into the sea. So forest fires pose a risk for Faraday Island's long-term future. All forest fires are put out without exceptions, single tree or no.]

I really think, if you wanted to write a story that's about hiking around in nature, you should've done more research on hiking and nature. This reads like you got all your information from some old, outdated book.