A new study of Hawaii's rainforests finds that more trees aren't always better. According to the study, published today in the Proceedings of the National Academy of Sciences, certain species of trees are actually destructive to the biodiversity that is the hallmark of tropical rainforests. Not only do non-native trees crowd out other trees, but they alter the ecosystem as a whole, changing soil fertility, available sunlight, and hospitability to other plants and animals.
The team of researchers, led by Gregory Asner at the Carnegie Institution, used remote sensing technology on aircraft to survey more than 850 square miles of rainforest. They produced three-dimensional maps of native and non-native species, allowing them to analyze the different structure of rainforest created by each.
3-D Imaging of Hawaiian Lowland Rainforest
Invasive tree species are reds/pinks and native trees are green
Source: Gregory Asner, 2008
The encroachment of alien species upon Hawaii's native rainforest has been well documented. In fact, Hawaii is a powerful case study of invasive species, as roughly half of organisms there are non-native, and approximately 120 plant species are designated highly invasive. However, it's been difficult to visualize on a broad scale how the invasive plants are changing ecosystems. The images produced by the recent study reveal that in areas where the native ohia tree has given way to the tropical ash and the Canary Island fire tree, the canopy has become denser, suppressing native understory plants.
A similar problem is caused by the opportunistic Strawberry Guava tree, another Hawaiian invasive plant. The tree is dense and short, creating an intermediate level of thick brush that occludes light to the native plants on the forest floor.
A Promising Technology
Hard numbers of invasive species in countries around the world are difficult to come by since the definition of "invasive" varies from place to place. In addition, the available data on invasive species represent the number of reported alien species, not necessarily the total number. For this reason, it's difficult to determine where the problem of invasive species is most urgent.
The visualization technologies used in this most recent study may be a means of providing the kind of data to understand invasive plant species on a broader scale, including in forests that are remote and largely unmapped. In addition, the airborne sensing can detect saplings and young trees, making it possible to identify a species' migration at an early stage.
Tracking the location of invasive trees and their effects on ecosystems will likely become increasingly relevant as climate change shifts species dispersal. Species--both plant and animal--migrating from one region to another become, in effect, invasive species. Technologies to understand how displaced species alter the native environment may help conservation biologists strike the delicate balance between preserving alien species and preserving native ecosystems.
Top photo by jfredericksen via Flickr
RELATED LINKS:
"Invasive plants transform the three-dimensional structure of rain forests" (PNAS, 3 March 2008)
FAO Database of Invasive Tree Species by Country
"A Radical Step to Preserve a Species: Assisted Migration" (New York Times, 23 January 2007)
EarthTrends
Bioinvasions: Stemming the Tide of Exotic Species
