Infestation of insects and a great many people starved. We cannot alter the food web significantly and expect mankind to thrive. Close proximity means more easily spread disease. Tundra, coniferous forests, deciduous forests, grasslands, deserts, rainforests, chaparral, etc . Example of ecosystems at different altitudes.
Ecosystems are specific environments where there is a well-organized food web.
In between the ecosystems are ecotones , where there is a transitional environment. Species in an ecotone may be completely absent from either of the ecosystems on either side of this habitat. Widely varying conditions in temperature, salinity, nutrient level, degree of oxygenation, etc. Hardy little creatures live there. Estuariane realm, continental shelf, Open Ocean, Coral reefs, Hydrothermal vent biome.