This approach is called selective removal . For example, one can remove selected grass species from a grassland community and observe the reaction by the other constituent grasses to the open spaces. Researchers found that the removal of one grass species, Muhlenbergia capillaris , resulted in the expansion by five other plants, suggesting that this grass species is in direct competition with five other plants and prevents their expansion. Contrast this with the removal of the sedge Fibristylis spadiceae , whose removal led to the expansion of only one other plant, the grass Spartina patens . Interestingly, the removal of Spartina patens led to an expansion of Fibristylis spadiceae , such that only two species appeared to be competing for the same ecological niche.
Many examples exist of a non-native species completely displacing other native species.
For example, the European starling Sturnus vulgaris was introduced into new York City in 1891, and is now found in all 48 states and throughout North America. It expanded at the expense of the eastern bluebird, the yellow-shafted flicker, and the meadow lark. Starlings are simply more efficient at holding and defending nest cavities traditionally used by these other bird species. The American gray squirrel Sciurus carolinensis was introduced into the British Isles early in this century, and has replaced the native red squirrel Sciurus vulgaris , primarily because of its superior adaptability to the niche of herbivore at the canopy level in deciduous woodlands. In reality, however, in natural situations, species that compete for food or space have adapted by reducing the pressures of competition and of dividing up the resources.