Your question appears to ask about invasive species more than biodiversity on the whole.

On the former, an invasive species can completely destroy local populations of indigenous species and become established. Sometimes just partially, sometimes completely.

The endgame of biodiversity? Evolution has no endgame, it just is. A constant adaptation where some species die out and new speciation fills any spaces created by their absence. Nature abhors a vaccum.

Will global biodiversity decline significantly to a few "core" species that are flexible in multiple climate zones or environment?

xkcd.com/1338/

It already has.

There's no design, so there's no intent. We can predict what might happen, which ecological areas will collapse, which species will thrive, and as humans we can intervene to try to prevent disasters we se coming. The "endgame" is the heat death of the universe, at which point all life will be long dead.

The end game is to become perfectly adapted to their environment, at which point changes may slow way down, like with sharks or cockroaches. If new niches open up (like a local species going extinct), existing species will fill the gap and adapt to it, becoming new species over time, eventually competing with everyone else like usual.

I guess the ultimate ultimate goal is to expand across the galaxy and eventually escape the death of the universe, but in the short term, its being well adapted to the local environment.

• The rate at which pathogens, predators, etc. evolve to exploit a species is dependent on the size of the species’s population—and once a pathogen anywhere in the world evolves to spread through the species, it can take out the whole population at once.

• The more different species fill the same niche around the world, the more likely it is that some of those species will successfully adapt to changing conditions. The local ecosystems with species that don’t adapt might collapse temporarily, but the successful alternatives can replace them and re-diversify.

So reduced biodiverity makes the remaining species and ecosystems more fragile—and species like humans that are dependent on all the remaining globally-distributed monospecies are as vulnerable as all of them combined.

Yeah, there's no endgame. We and all the species who live today are the product of yesterday's climate. And tomorrow's species will be the product of today's climate. Rinse and repeat. The endgame of life will be some large celestial body crashing into earth. Or our sun swallowing the inner planets.

The emerald ash borer is a great example of the damage invasive species can cause.

There's no conscious goal to biodiversity. But over time, life will grow and reproduce to harvest as much of the universe's energy as possible and make as many copies of itself as possible.

Think bacteria on a petri dish, humans on the earth, and really, all living material in the universe.

First, you want local organisms to support the local ecosystem. Various plants and animals depend on others. For example, if a bird eats one fish but not another, and the bad fish takes over, the birds die out.

The problem with a monoculture is that if there is any disease that affects them, it's very easy for it to spread and collapse the whole population, leaving it completely lifeless.