Ecosystems are just that- systems. Like the inner workings of an antique clock, every piece plays its part in keeping things moving and functioning. No single species or even a handful of species can make an ecosystem "tick." It's the interactions between organisms, each filling its own niche and performing many different ecosystem services, that determine the health of the ecosystem.
How Do Native Plants Fit In?
The complexity of these interactions developed over millions of years of coevolution between species. Sure, an introduced plant may still serve some ecosystem functions, such as providing pollen and nectar to generalist pollinators, or fixing nitrogen in the soil to make it available for other plants to use. However, that plant is unlikely to also provide food to herbivores like caterpillars, who typically can only feed on one or a handful of plant species. Even many bees are specialists, relying on a narrow range of plants for the pollen they need to rear their young. If these host plants aren't present, not only do the insects suffer, but insectivores like birds are also limited.
The complexity of these interactions developed over millions of years of coevolution between species. Sure, an introduced plant may still serve some ecosystem functions, such as providing pollen and nectar to generalist pollinators, or fixing nitrogen in the soil to make it available for other plants to use. However, that plant is unlikely to also provide food to herbivores like caterpillars, who typically can only feed on one or a handful of plant species. Even many bees are specialists, relying on a narrow range of plants for the pollen they need to rear their young. If these host plants aren't present, not only do the insects suffer, but insectivores like birds are also limited.