You know, just once I would love to see some ecological research that found a really simple answer that explained the phenomenon under study. Alas, we live in a complicated world where it is difficult to make robust predictions. I guess it keeps us ecologists in jobs. For example, possums are the biggest pest species in New Zealand wild areas, controlling their numbers must be a good idea. Introduced stoats are a devastating predator in most of our ecosystems, again controlling them must be the best thing to do? Turns out that (surprise, surprise) it's just not that simple. There is the cliche that 'nature abhors a vacuum' and that applies to ecosystems as well. Remove a species from a local area and suddenly a different species has a new opportunity. The remaining species might benefit because you have removed its main predator and so numbers will boom (called mesopredator release) or because you have removed a competitor for resources and so numbers will boom (mesocompetitor release).
Wendy Ruscoe from Landcare Research and a host of authors, including Richard Duncan from Lincoln University have examined the issue of meso-release in New Zealand ecosystems where there is a complex interplay of introduced pest species that degrade the natural ecosystems. There work has just been published in Ecology Letters.
The group worked in forests of the North Island of New Zealand where they looked at interactions between a top predator (the stoat), two mesopredators (rats and mice) and an omnivore (possum). These are all species that are often controlled to reduce population numbers. (Note that in New Zealand the word 'control' generally means 'kill'). The team set up areas of stoat removal, possum removal, possum and rat removal and a control with no removal. Populations in the areas were assessed before and after control operations. The results were convincing. The removal of stoats (a top predator) caused no response in the two mesopredators (mice and rats). The removal of the omnivore (possum) released rats and their populations grew. The removal of rats led to a growth in mice numbers. The net result for the ecosystems was that removing possums and rats (while useful in the short term) probably had little long term benefits as other competitive species filled the void and continued to cause problems for the native biota. The lack of an impact from the removal of the predator was of great interest and suggests that competition, not predation, is the important process that shapes communities. So wildlife managers need to think in terms of multi-species control if they want to effectively manage ecosystems in the long term. Which is a reasonably non-complex answer for a complex process!
12 September 2011
02 September 2011
The Third Combined meeting of the Australian and New Zealand Entomological Societies Conference has just concluded here at Lincoln University. Four days of good talks and plenaries. About 200 participants, of whom about a third were postgraduate students, made for much excellent discussion. I was involved in the opening symposium on New Zealand biogeography and whether we should consider Australia to be the mothership or sistership of our biota. I talked through a rough overview of the geological history of Zealandia and the implications for our biota. I strongly urged that we need to start thinking in terms of a Zealandia biota in addition to a Gondwanan biota. I also reiterated the geological evidence for the Oligocene drowning of Zealandia that saw almost all of New Zealand underwater about 23 million years ago (for previous blogs on these issues see here , here , here , here and here). I'm pleased to report that the concept of Zealandia and of a dramatic drowning seem to be gaining traction as measured by the content of other talks at the conference. There is one aspect, though, that I would like to address: the curious case of the buoyant moa.
Pete Cranston delivered the opening keynote address to the conference just prior to our symposium. He spoke about austral chironomids (freshwater insects) and what they tell us about the ecological and biogeographical history of our part of the globe. All very interesting and delivered in his usual witty manner. At one point Pete thought to make a point about the likelihood of the Oligocene Drowning having occurred. One common response that I get to the idea is "yeah but what about the moa?". Moa were gigantic birds once common in New Zealand until driven to extinction by humans and are related to other large birds around the southern hemisphere. People find it hard to comprehend how big flightless birds could colonise an area across a big water gap. Pete illustrated this problem with the moa buoyancy hypothesis that was put forward by Rich Leschen in 2008 (you can find it here - scroll down to page 5). Although meant in a humorous sense, moa have natural floatation abilities to get across water gaps (and are spacious enough to bring other passengers), it is really saying "what a silly idea the drowning is to even contemplate that moa could colonise New Zealand". Of course I beg to differ.
I could come up with several comebacks. Kiwi, another ratite, clearly arrived well after the Zealandia broke away from Gondwanaland (as determined by molecular dates) and are more closely related to emu than to moa! So they got across the water OK. We now know that South American tinamou (small and flighted birds) are part of the ratite group and that the ancestors of each ratite regional group were able to fly and colonise these regions. Island species also often share a common evolution trajectory which results in gigantism and flightlessness in birds. So there is a method for moa to get across the water OK and one that explains why the are so large. Finally, we should not fall into the trap of assuming that modern traits were always the same. I think I'll call this the 'Fat explorer hypothesis'. New Zealand is fast catching the USA as the most obese country in the world. If I was looking at modern New Zealand and thinking about how humans arrived in our shaky islands, I might think that clearly, based on what we know about early waka and the sailing ships of Cook and the size of present day New Zealanders, that we would have been much too large to have fitted into these vessels and so there is no way we could have got here in that manner. Perhaps our ancestors lived in Gondwanaland and walked (waddled?) into Zealandia before it split apart? Of course this is a silly arguement, present morphology (large and round) is not a full-proof predictor about previous morphology (small and wiry). Clearly, our ancestors were much smaller and quite happily fitted their ships. In recent times, their descendants have have burgeoned until we see the size of our present populations. A good analogy for the moa perhaps. All good fun!