Tuesday, April 3, 2018

What do we know about flying ants?

When I began my research career I had the snobbish idea that I would not study a particular organism, site, or system.  I would instead come up with a grand question, and study any aspect of the world I needed to in pursuit of the answer.  This, I thought, was the way real scientists work.

Nearly a decade in, I have mostly tossed that archaic idea.  Rather than imposing ideas upon the world, I am more productive when I let nature guide me.  The best way for me to discover is to pick one thing in the world around me and try to figure out how it works.  Each answer reveals more questions, setting off a chain reaction of discovery.  An added consequence of this inspiration from the world is love for it.

I have come to realize, for example (despite my earlier snobbish pretensions), that I love ants.  Queens in particular—the mothers, explorers, and true individuals of ant societies.  I don’t just mean that I find them interesting or practical to study (although they are endlessly fascinating and useful).  When I see them in the wild, read about them, think about them, I get pangs of excitement and sympathy.  I occasionally dream about them.  On optimistic days, I even dare to think I understand them.

In most ant species, young queens have wings and fly to find mates or disperse to new nest sites (Aphaenogaster flemingi queen, photographer April Nobile, from AntWeb)

About a year ago, letting my mind wander during a particularly boring meeting, I realized I had devoted a good chunk of my life to understanding what it means to be an ant queen.  I spent years trying to wrap my mind around that dangerous part of their life when they leave their birth nests to fly through the atmosphere to find mates and new homes.

“The swarms of some ant species are among the more dramatic spectacles of the insect world.”
Bert H̦lldobler & E.O. Wilson РThe Ants

One of the only other people to devote their research so thoroughly to ant flight was the 20th century ecologist Mary Talbot.  And our knowledge of ants has advanced a lot in the decades since Talbot’s pioneering work.

“…the brief interval between leaving the home nest and settling into a newly constructed nest is a…dangerous odyssey that must be precisely timed and executed to succeed.” – The Ants

My idle musings led eventually to a review paper, published recently in Myrmecological News.  In the paper, I use new conceptual developments to weave together all that we know about what flying ants do in the air.

“No encompassing theory exists to explain the extreme variation in the patterns of mating behavior so far observed.” – The Ants

Several discoveries stand out.

Queens face hard choices.  They can, for example, choose to store lots of fat and protein in their bodies before they leave their nest to fly.  Doing so makes it easier to survive alone while they are trying to rear their first offspring.  But it may also make them less able to evade predators, find mates, or discover a suitable home.

Queens carry heavy burdens.  The pressure to load up on nutrients and still be able to fly has led to the evolution of extreme weight-carrying ability.  Some ant queens can carry ~1.5 times as much weight as other flying insects.

Ants can fly longer than we thought.  Some queens can fly for over an hour straight and some males may fly repeatedly over several days or weeks.  Queens of some species occasionally fly 20 miles in one go.

Ants are part of the atmosphere.  Queens and males enter the atmosphere in their millions, flying high to find mates, ride on air currents, and search for new homes.  There they form a major food source for birds, dragonflies, bats, and other predators.  They also transport energy and materials, including toxins like mercury, long distances across landscapes or among food webs.

I have spent much of my career trying to understand what ants do in the air (Dorylus driver ant male with author, photographer Alex Wild)

There is still much more that we don’t know about ant flight, especially when you consider that there are probably over 20,000 ant species in the world.

“…the reproductive behavior of ants is still a poorly explored domain with rich possibilities for general evolutionary biology.” – The Ants

In other words, there’s plenty of room for me and everyone else to continue exploring (and loving) ant queens.

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.