The birds and the bees… and the trees?

By Rebecca McLeod 25/05/2010

I have perfected the art of ‘purposeful stops’ when I’m out running. It’s nothing that countless runners before me haven’t mastered — the sudden need to tighten a shoelace or stretch that hamstring… Unfortunately the laces on my new shoes are annoyingly well behaved, and so I have been conjuring up other excuses to take a break, like staring intently at some small plant on the side of the track, trying very hard to look like an interested botanist should someone pass by. The other day, on a track near Dunedin I stopped and gazed purposefully upwards to the forest canopy. And what I saw surprised me — most of the leaves were full of holes and were all raggedy around the edges. It seemed that a lot of bugs had been hard at work.

It reminded me of a study that was published in the journal Ecology a few months ago where the question was asked Do trees grow faster in the presence of birds? Many birds obviously feed on insects and these scientists wanted to know whether this predation had a measurable impact on the number of insects and the growth rate of the trees. The reasoning being that leaf loss due to insect munching causes lower rates of photosynthesis, and sap-sucking insects incur a further energetic cost. Sounds like a simple enough question, until you consider the logistics involved with excluding birds from 7 meter high spruce trees!

The ecologists measured and compared tree growth rates between trees to which birds had full access, and those covered by bird-netting (effectively bird-exclusion zones). After a year it was found that more often than not, bird-excluded trees had slower growth rates and around 60% more insect herbivores on them.

New Zealand has suffered a dramatic and well documented decline of native birds. I began to wonder whether ecologists here have ever tried to quantify the potential impact of this decline on the health of our native forests. Here we are all worrying about possums! Could out-of-control insects be doing more damage?

Dave Kelly, an Associate Professor in the School of Biological Sciences at Canterbury University has spent many years researching the ecology of New Zealand forests. He emphasized that studies around the world looking at relationships between bird abundance and tree health do not have very consistent findings, and was quick to point out the complexities of this issue here in New Zealand. True, many of our forest dwelling native birds have declined, but in many cases their ecological role as insect predators may have been taken over by birds such as chaffinches and blackbirds that were introduced by European settlers. ’Some of these exotic insect-eating birds get deep into native forest, so I suspect the insectivore guild of birds is still well represented in most parts of New Zealand despite the big changes in avifauna.’

Dr Kelly isn’t aware of any similar bird-exclusion studies being carried out here and perhaps this isn’t so surprising – it would be near impossible to carry out a well replicated bird exclusion experiment on our forest giants. But there is evidence that insect herbivory is impacting our native plants. For example, the threatened native mistletoe — thought to be in decline due to pressure from possums — actually loses more leaf area to insects than possums on an annual basis. Now, this isn’t to say that possums don’t love tucking into mistletoe. But, be it possums or insects that are to blame for our stressed-out forests, it could be argued that the solution is essentially the same. Insect-eating birds such as kaka, tomtit, fantails and robins have declined in response to predation by introduced mammals, including possums that feed upon eggs. So, controlling possum numbers should lead not only to less forest being consumed by them, but also to an increase in bird numbers and perhaps more natural control on those ravenous insects.

I thought I’d leave the last word to an author of a recent review on bird-tree relationships published in the Annals of the New York Academy of Sciences:

“In sum, many investigations in terrestrial and aquatic habitats, both natural and human-dominated, find that bird predation decreases invertebrate prey populations. Most studies that examined cascading effects of birds on plants found them, in some cases, with positive economic consequences for humans. In stark contrast, bird persecution may have devastating consequences. Although information is only anecdotal, apparently the ’war against the sparrows,’ part of a pest-control campaign launched in China during Mao Zedong’s Great Leap Forward, led to massive increases in pest insects and, thus, crop damage, ultimately contributing to a catastrophic famine from 1958—1962 in which 30 million Chinese died from starvation.” [C. J. Whelan, et. al., Ecosystem Services Provided by Birds, Annals of the New York Academy of Sciences 1134: 25—60 (2008)]