Monday, March 5, 2018

Invasive acacias can benefit Bornean ants

A primary goal of conservation is to protect intact lands and waters.  High quality habitats like old-growth forests, unplowed grasslands, and unfished coral reefs are essential for the future of the world’s biodiversity.

Less pristine areas, however, also play a role in conservation.  Working landscapes like ranches, sustainably logged forests, easements along highways and power lines, and environmentally friendly farms, can be valuable tools for protecting our natural heritage.  Even lands that have been heavily impacted by farming or development can be restored to some semblance of natural vegetation.

Compared to areas that have never been damaged, however, working landscapes and restored areas often harbor fewer native species or function less effectively.   They are also more likely to contain invasives—organisms that have been transplanted by humans from other regions and that alter or harm native ecosystems.  Invasive species are a leading cause of extinctions, and transporting living things outside their native range is generally not a good idea.  Once established in a new area, many non-native species are difficult or impossible to eradicate.  But when doing conservation on highly degraded or working landscapes—habitats that are not, and may never be, pristine—exotic species can sometimes help deliver conservation benefits that would otherwise go missing.

The degraded landscapes around Gunung Palung National Park and elsewhere in Borneo illustrate this idea.  Over the past few decades rampant development has reduced most of Borneo’s original forest cover to sterile agricultural landscapes covered by oil palm plantations, rice paddies, or slash-and-burn farms.  These open disturbed areas are prone to erupting into wildfires which further eat away at the island’s remnant forests.  Most of the island’s native species, adapted to living in dark wet forests, cannot tolerate these conditions.  The result is that megadiverse rainforests are slowly replaced by barren areas dominated by a handful of plants that thrive in direct sunlight and can survive fire—mostly native alang-alang grass (Imperata cylindrica) and bracken ferns (Pteridium aquilinum).

Human-caused wildfires kill rainforest trees, but favor alang-alang grass and bracken ferns which need direct sunlight and resprout quickly in burned landscapes

After years of logging and burning, Bornean rainforests transition into man-made grasslands.  The condition is hard to reverse since few trees can survive the direct sunlight, frequent fires, poor soil and intense competition from grasses and ferns.

Restoring these areas is difficult, as most rainforest trees need dark, moist environments to grow.  Soils in the man-made grasslands are often impoverished after years of burning and erosion from heavy rains.  And the densely packed grasses and ferns crowd out any tree seedlings that could otherwise survive the harsh climate.

But one non-native tree thrives in these conditions.  Acacia mangium is native to eastern Indonesia, New Guinea, and northern Australia, but is now one of the most common non-native trees in Borneo.  It grows rapidly in open sunny areas (3 to 4 meters per year!), and quickly colonizes areas that have been burned or disturbed by humans.  It is also a legume, able to acquire nitrogen straight from the atmosphere with help from symbiotic bacteria, so it does fine in poor exposed soils.  Best of all, since it needs open sunlight to grow, it does not invade native rainforests, and its seedlings cannot survive under its own canopy.  Once a stand of Acacias grows tall enough to shade out the direct sunlight, the understory can be colonized by native rainforest species without competition from young acacias.  For all these reasons, people often plant Acacia mangium to help restore mines, logged areas, and other degraded landscapes.

Acacia mangium forests have become one of the most common habitat types in disturbed lands around Gunung Palung National Park, and the trees accelerate the transition from grassland to rainforest.  They restore soil that has been burned off or washed away, provide shady areas for native rainforest seedlings to germinate, and by blocking direct sunlight they kill off the alang-alang grass and bracken ferns that crowd out tree seedlings.  Given time, acacia forests revert to native secondary forest as rainforest trees replace acacias in the canopy.

Acacia trees thrive in sunny barren environments and form canopies that shade out and kill alang-alang and bracken ferns, creating the cool dark conditions necessary for many rainforest seedlings.  This Acacia forest began growing only 11 years ago after a wildfire, and is now ~20 meters tall.

Many of Borneo’s native animals can also survive in semi-natural acacia groves.  Last year, we set out to see what ant species called this new habitat type home (find our paper here).  We studied a young acacia forest that had sprung up after a wildfire 11 years before, and was starting to transition to native secondary forest.  Compared to native rainforest, the acacia forest harbored relatively few species.  But almost all of them were native, and a few invasive species were absent that were common just a few hundred feet away in farmlands and towns.  The major exception was the yellow crazy ant (Anoplolepis gracilipes), a notorious invader that has colonized many Pacific and Indian Ocean islands, and which was the most common ant in the acacia forest.  Nonetheless, the ant community here was more intact than nearby open areas.

Semi-natural Acacia mangium forests in Borneo are species poor compared to native rainforest, but still harbor more forest species and fewer invasives than nearby disturbed areas (poster by Sara Helms, photos from AntWeb or taken by author)

Our results suggest that these non-native trees are a useful conservation tool in disturbed landscapes in Borneo.  In highly degraded areas, sometimes a non-native is better than nothing at all.