Friday, May 20, 2016

Do fire ants benefit native birds?

Invasive species are a serious threat to native ecosystems around the world.  Humans increasingly transport species to places they do not belong, often with disastrous consequences.  Exotic species can kill off, outcompete, or displace native species, and may alter the physical makeup of the environment in ways natives can’t cope with.

Once established, invasive species can be difficult or expensive to eradicate.  Welcome or not, they are usually here for good.  But it is not always bad news.  Sometimes the presence of an invasive can benefit populations of beleaguered natives.

Lake Erie Water Snakes (Nerodia sipedon insularum), for example, once federally listed as threatened, staged a remarkable comeback after they began feeding on invasive Round Gobies (Neogobius melanostomus).  The snake, limited to the Lake Erie Archipelago and Ohio’s Catawba Peninsula, declined in the 20th century due to habitat destruction and human persecution.  Today, snake populations are recovering and they have been delisted, with help from the superabundant invasive fish which now makes up 90% of their diet.

One of the most conspicuous invasives in the Southern US is the Red Imported Fire Ant (Solenopsis invicta).  Mention fire ants to an Oklahoma landowner, and they are likely to wax poetic about the evils of this alien invader.  But most complaints boil down to two things—1) the ants sting, and 2) they make unseemly dirt mounds in an otherwise perfectly manicured lawn.  Stings and difficult lawn maintenance may annoy some humans, but they hardly count as negative impacts on native ecosystems.

This week some coauthors and I published a paper in Biology Letters, where we examine the value of fire ants as food for a popular native bird, the Purple Martin (Progne subis).  Based on work in southern Oklahoma, we found that fire ants are the primary food item for nestling Purple Martins.  Winged fire ant queens and males make up 60% of the prey items fed to young chicks and 30% of total prey biomass, and are captured on 30% of hunting trips.  No other insect comes close to this importance in Purple Martin diets.

We studied the diets of Purple Martins in southern Oklahoma

Purple Martins fed their chicks gobs of invasive fire ants

Fire ant queens are an ideal prey—nearly 90% of their body weight is made of ready-to-eat fats, sugars, proteins, and flight muscle, and they fly through the atmosphere in massive mating swarms.  By hunting fire ants, Purple Martins cut their foraging time in half, thereby saving energy and reducing their exposure to predators.

The swollen abdomens of fire ant queens are packed full of nutrients, and the thorax is filled with meaty flight muscle, making them a nutritious prey for aerial predators.
By capturing fire ant queens, Purple Martins cut their foraging time in half, regardless of how high in the atmosphere they hunted (figure adapted from original article)

This foraging benefit to the birds may translate into more robust populations.  We found that Purple Martins are most abundant in regions of the US where fire ants have invaded.  Over the past 50 years, southeastern Purple Martin populations have been stable or increasing, whereas the birds have declined over most of the rest of their range in the US and Canada.  It is possible that these southern populations are subsidized by the arrival of the invasive, yet nutritious, fire ants.  Several other insect-eating bird species—Barn Swallows (Hirundo rustica) and Northern Rough-winged Swallows (Stelgidopteryx serripennis)—show similar patterns, with healthy populations in the south and declining ones elsewhere.

Red imported fire ants are not native to the US.  They should have been left in South America where they evolved.  But they are also not the evil aliens we make them out to be, steamrolling across our native ecosystems.  They do seem to impact some native ant communities and, yes, they do sting.  But they have been here for 80 years, and have yet to eliminate any native species.  Besides, there is nothing we can do about them anyway.

Since they are here, probably permanently, it is nice to know that at least some of our native species are putting them to good use.

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