Generally, the more flowers, and the more nectar and pollen plants contain, the better they are liked, and many non-native plants score very highly here.
Furthermore, gardens with a succession of flowers selected to bloom throughout the year provide important resources for pollinators when our native species have finished flowering. Surprisingly, the nearby presence of gardens actually improves pollination in some crop species. The one study that added native plants to gardens to test their impact on pollinators found no change, and the pollinators still went for the richly flowering non-natives.
This suggests that pollinator species like bees and some hoverflies can benefit greatly from non-natives, because they can find food for all their life stages in gardens. This may not be true for insects which have herbivorous larval stages like butterflies.
Herbivores and non-native plants
There is rather less evidence for herbivores in gardens, and it is more equivocal. There is no doubt whatsoever, that many non-native plants can support many wildlife species. In Jennifer Owen’s Leicester garden, eleven of the top 15 species eaten by herbivores were non-native. American studies, focussing only on the use of woody trees and shrubs found strong evidence that native plant species were much more valuable than non-natives. It isn’t clear how relevant these studies would be to typical British gardens, but read our leaflet (opposite) to find out the arguments each way.
Other American, and European studies have come to different conclusions, finding many herbivores using or even depending on non-native plants, while some insects seem to be shifting their preference towards common non-native plants. In particular, insects used to eating one sort of native plant, are much more likely to accept a foreign food plant if it is closely related to the one they are accustomed to. As Jennifer Owen observed, if a plant is in the right family, it doesn’t matter a lot whether it’s native or alien. With so many garden species originating from the same ecozone and related to our natives, this means a lot of potential.
However, while non-natives definitely support many generalist or “polyphagous” species, there remain specialist insects, including some moths and most butterflies, that need specific native plants. It is very likely that planting certain native plant species would be of benefit to some target insects, provided that they are living in your area, can find your garden, and find enough of the plants to be useful. This would need careful thought and planning, but could be a valuable project.
There isn’t much evidence that changing gardening practice to concentrate on native species alone would have any great value, and some of the benefits of gardens for pollinators would be lost. Above all, we must remember that typical gardens, with a preponderance of non-natives have tremendous biodiversity, above that of other British semi-natural habitats. But it is also fair to say we need more research, such as the Plants for Bugs project, and that carefully planned native planting might be of value for some species.
Leaflets available as pdf files from this page:
Other pages in this site to look at for this topic:
Plants and Planting: Native or non-native?