I would like to recommend three books that illustrate the impact of the ever increasing numbers of human introduced species into habitats not their own; they analyze the effects of some of these introductions, describe measures used to control or prevent the consequences of such introductions, and make suggestions about what you can do.
+ Nature Out of Place: Biological Invasions In The Global Age by Jason Van Driesche and Roy Van Driesche, 2004
+ A Plague of Rats and Rubbervines by Yvonne Baskin. 2003
+ Bringing Nature Home: How You Can Sustain Wildlife with Native Plants by Douglas W. Tallamy and Rick Darke. 2007
These books show us that ecosystems are like symphonies. It takes millions of years of co-evolution to develop a fine tuned system, richly complex and delicately balanced. If you took a well crafted symphony and added a snippet of music from another composition, the introduction wouldn’t fit with the rest of the work. The composer could weave this introduction into the symphony by tweaking the alien fragment and the related parts until it all worked well together once again. It would take some time and effort but it can be done. However if a large number of musical fragments are introduced it becomes impossible to repair the piece. Likewise ecosystems have been dealing with introduced species all along; it may take many thousands of years, perhaps millions, but eventually the non-native species become part of the ecosystem. Unfortunately, in recent times, humans have been moving species around, intentionally and unintentionally, at unprecedented rates. The numbers of species invading new environments far exceed the pace of evolution; ecosystems suffer serious disruptions as a consequence.
Tallamy’s book, in particular, had a profound effect on me. I wish I could summarize its main premise in just one sentence; I will try: most land birds (and many other creatures) need insects and, in turn, insects need native plants in the immense majority of cases. In other words, introduced plants provide no food for native insects and as a consequence birds go hungry.
He presents abundant proof of this thesis, for instance, in one study done in Oxford, Pennsylvania “native plants supported 30 times more caterpillar biomass than alien plants supported.” Other work shows that the Asian reed Phragmites australis sustains 170 species of herbivores in its native land while it feeds only 5 species in North America. More than 300 years have passed since its introduction and it is still no good at sustaining native herbivores.
I can assure you that, if you are a gardener and a bird lover, you will never grow a non-native plant again after reading this book.