Rights: University of Waikato. All Rights Reserved. Published 6 June 2012 Download

Many New Zealand birds have declined in numbers, so Dave Kelly and Jenny Ladley of University of Canterbury study the effect this has on native bird-pollinated plants. Dave explains the need to use experiments to answer questions and outlines why we need to look after interactions between plants and birds, not just the individual species.


Famously, New Zealand has had big declines in birds. There are a lot of species that are extinct, and a lot of the other species that are still around are lower in density or they’re only found on offshore islands and bird reserves. And so there’s been speculation all over the world really that if bird numbers went down, that might start to cause problems for plants that rely on the birds.

But the impetus really for this work came from when Jenny Ladley was doing her master’s research, and she was looking at the Peraxilla mistletoes, and she discovered that they have this specialised flower that doesn’t open unless a bird comes by, and Jenny discovered that a lot of the flowers were not getting opened. So because the flower opens when a bird visits it, you can actually tell looking at a plant if birds have been coming through or not – it’s got this visual indicator of how much pollination is going on. And Jenny discovered that those species weren’t getting enough visits at a couple of different sites.

And once we realised the mistletoes were having trouble, we then started widening out saying, let’s look at other native plants that also rely on birds. And so we’ve spent 10, 15 years really expanding the project out and doing a wider survey to see if this is a widespread problem or just a bit of an oddity for one or two species. And the answer is it is a widespread problem – most of the bird-pollinated plants we’ve looked at aren’t getting adequate pollination at least in some of the places they’re growing.

The key thing is, if you want to find out what’s causing, say, the decline of a plant species or a failure of seed production or a shortage of seedlings, you’ve basically got to do some experiments.

If you think pollination might be lacking, then you go in there, you stick some pollen on with a paintbrush, you keep birds off and see if it makes a difference. If you think it might be a lack of seed production, you can put some more seed in and see what happens. If you think that slugs or possums are eating the seedlings, you put a cage on some. And so you go out there – it’s basically the scientific method – you think of all the things that could be going on and you go in there and you tweak them around.

So if you do some hand pollination treatments and then you compare that to what the birds are doing, we can see how well is bird pollination working compared to what we could do. And if you’ve got a big gap, then the birds aren’t doing as good a job.
We need to understand what’s working and what isn’t, so that when we started the work, we didn’t know whether bird pollination was a problem or not. That’s where a really useful way of looking at that is if you’ve got a reserve or somewhere that’s kind of working the way New Zealand used to work.

Now we know it’s a problem, and so what that means is, if you want to look after some of these species, we need to maintain the interactions as well as the individual species. So if you want to look after some of these plants like Peraxilla or Rhabdothamnus, you need to worry about the birds as well as just making sure that the plant is not eaten by a herbivore or something. And if you can get onto these problems before they become critical, they’re much easier and cheaper to look after.