The following note was written for a landscape manager in our neighborhood. A well-intentioned and intelligent person, he had introduced “Mysore raspberry” on a near-by property because he grows them on his own place and enjoys eating them. He has since removed them from the place under his care.
The geological history of the Hawaiian Islands goes back some 75 million years; the botanical history is less lengthy because about 40 million years ago a 10-million year hiatus in the process of hot-spot island production apparently resulted in the drowning of all terrestrial life that had populated the extant islands up to that point as the lands subsided into the Pacific.
Still, 40 million years is a long time. The remoteness of Hawai`i’s location in the middle of the north Pacific is so extreme that the thousands of miles of open ocean in every direction has acted as a filter to make arrival of organisms – before the intervention of humans – a very rare event. Thus, only two mammals that can be considered terrestrial populated pre-human Hawai`i: a bat and a seal. There were no amphibians nor reptiles before people brought them. Only nine or ten forest birds had successfully arrived here, and only 250 plant species. One interpreter of the island’s natural history has compared the successful arrival of a new plant species to the likelihood of a person standing on the edge of an Olympic pool being able to hit with poppy seeds a postage stamp floating on its water. Others have calculated that a new plant species got here on the statistical average of one every 100,000 years.
In short, whether their seeds were light enough to be airborne for thousands of miles, durable enough to survive the freezing temperatures of high altitudes, and fortunate enough to land on ground suitable for their germination and propagation; impervious enough to withstand months of immersion in saltwater and being caught in currents that carried them to parts of the islands that were conducive to their survival and reproduction; sticky enough to adhere to bird feathers or muddy legs, or in fruits or berries tasty enough to be ingested and carried here in avian digestive tracts, pre-human colonization of Hawai`i by new plants just didn’t happen very often.
The islands, though are very diverse in their range of habitats and ecological niches. Once a plant species did establish itself here and began to disperse, it typically adapted to the various circumstances to which it spread, changing its characteristics to do so to the extent that it gave rise to a number of new species. Thus, the 250 ancestral arrivals gave rise to some 2,600 native species – accounting in large measure for the fact that over 90% of Hawai`i’s native plants are found nowhere else in the world. In the absence of grazing animals, the need for protective characteristics (e.g., thorns, toxins, strong tastes, noxious odors) was absent and these tended to diminish or disappear.
Another testimony to the remoteness of Hawai`i is the fact than it was one of the very last places suitable for human habitation that was actually discovered and populated by our kind. That only happened about 1,500 years ago, but the event was marked by significant changes. Pigs and dogs were introduced deliberately, the Polynesian rat and a species of gecko apparently accidentally, and 30 species of plants culturally significant to the settlers all had their several and cumulative impacts. With the arrival of the British, the ships of James Cook’s third Pacific expedition in 1779 – on his way to search for a passage around the upper reaches of North America – the impacts of introduced species accelerated hugely (including micro-organisms infecting naturally immunized human carriers of diseases that began a massive die-off of the native human population).
Observers in the early stages of “post-contact” times commented that scarcely a ship called at the Islands without new botanical specimens to set out. That tendency has not remitted with time. Thus, presently, perhaps 10,000 species of plants are found in Hawai`i that were not here at the time of Cook’s arrival. All of these are likely to have evolved in environments that were much more challenging and competitive than those of Hawai`i. As a consequence, these historic-era introductions strongly tend to have patterns of growth and dispersal that are highly aggressive relative to the native species (comparisons with human culture also spring to mind) and a strong likelihood of displacing them. The Casuarina trees, often in Hawai`i called “ironwood,” lay down a thick mat of its needle-like branchlets that keeps other plant species from thriving; the many species of ornamental ginger spring vigorously from corms inches thick and inclined to mass into impenetrable thickets where nothing else can grow; Eucalyptus trees continuously drop leaves, twigs and branches that change the chemistry of the soil to inhibit the successful propagation of potential competitors; and the list goes on.
Consider as a contrary example the native raspberry, `Akala (Rubus hawaiensis). It was probably one of those species brought to the Islands in the digestive tract of birds. In the course of time, the thorns so typical of the genus have, in this species, become modified so that they are absent or relatively small and soft to the touch. According to the University of Hawai`i, “There are also several introduced species (of Rubus), many highly aggressive invasives, in the Hawaiian Islands: Prickly Florida blackberry (R. argutus), Himalayan blackberry (R. discolor), Yellow Himalayan raspberry (R. ellipticus var.obcordatus), Andean raspberry (R. glaucus), Mysore or Hill raspberry (R. niveus), Mauritius raspberry or thimbleberry (R. rosifolius), and Molucca raspberry (R. sieboldii).” The Mysore is one of the members of the genus Rubus that thrive in tropical climates (most preferring colder winters) and bears cautious monitoring because of its thorniness and vigorous spread (see http://tropicalfruitforum.com/index.php?topic=45.0).
Needless to say, all these are subject to dispersal by birds and pigs feeding on the tempting berries – and their thorny stems and characteristic formation of dense patches makes their presence problematic for those who would protect spaces for native plants, or those who simply find their forest wanderings blocked by them and their formidable defenses.
Nearly everyone who lives in Hawai`i and cares for plants enough to propagate and nurture them has for a variety of reasons regrets about some of his or her botanical introductions. Our ordinarily mild tropical climate shifts the struggle typical in more extreme zones to promote plant growth, to on-going efforts here to control their growth. Without such efforts, yards and fields in Hawai`i become jungles and/or weed patches with startling rapidity.
Many people have a fatalistic attitude toward the growing dominance of historically-introduced specimens, thinking that the tipping point has long been passed; that the shrinking share of habitat for native species will inevitably continue to diminish. That may indeed be the case, but we should not “go gentle” into that frame of mind, but be inclined to err if need be on the side of the native species.