Mosquitoes on the move: part two
Mosquitoes on the move: part two
What does climate change mean for one of our deadliest foes? (8 minute read)
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One of the things I enjoy most about my summer holidays in Auckland is the weather. As much as I love living in Wellington, it never gets very warm and I seldom go anywhere without a jacket. Auckland is different. Anything that I pack with long sleeves invariably stays in the suitcase. I can feel the heat of an Auckland summer warming me right through, from my skin to my bones. I love it.
But not everyone does. I know people that loathe summer and find Auckland’s heat stifling. They would far rather spend their summers further south, and they look forward to the cooler days of autumn and winter. I can’t imagine that myself, but temperature is an individual thing – we have different likes and dislikes.
The same can be said of different species of animals and plants. Some species do well in the steaming heat of the tropics, for example the coconut palm and the stunning blue morpho butterfly. Others, like the Adele penguin, inhabit places so cold that humans only survive there by wearing layer upon layer of clothing. Some species, like humans and rats, are supremely tolerant and can be found almost everywhere. Others have very specific requirements, something you’ll know if you’ve ever tried to grow a particularly finicky house plant.
Humans have been observing the distributions of animals and plants, and connecting the distributions with environmental conditions, such as temperature, for much longer than recorded history. However, western science first began to do this in the late 19th and early 20th century. In more recent years, the techniques have become increasingly sophisticated. Now, scientists use complex computer models to understand the distribution of species and to make predictions on where they are likely to survive.
Such models are useful, because where a species actually occurs is not necessarily the only place it could live. Radiata pines are native to a small area of coastal California and a couple of islands off the coast of Mexico, and yet they are now grown in temperate areas around the world. Chickens, now kept worldwide, are native to the jungles of south-east Asia. New Zealand’s beloved pōhutukawa tree has become a problem in the Cape region of South Africa.
I was fascinated when I heard pōhutukawa had become invasive in South Africa. I had long loved plants from the area – my favourite pot plant is a South African blood lily which I’ve kept for 30 years. When I worked on invasive weeds in New Zealand, there were a number of species from South Africa too, from Agapanthus with its spectacular blue or white flower heads, to the weedy relative of asparagus which smothers the undergrowth of small patches of forest, to the viciously-spined boxthorn which was the bane of my life when I worked on reserves in coastal Marlborough. Was there something about South African plants which made them grow well in New Zealand, and vice versa?
The answer, it turned out, was both yes and no. There were certainly plenty of South African species which were invasive in New Zealand, but there were species from other places too – Australia, Europe, Asia and the Americas. There was nothing particularly special about South Africa, apart from my own interest in the country’s flora. But there was a pattern, something that became apparent only when I looked more closely at exactly where most of New Zealand’s invasive weeds came from.
As a whole, South Africa isn’t much like New Zealand, but there are areas that have a similar climate to New Zealand, and the species which are a problem here come from those areas. The pattern is even clearer if you look at Australia. It ranges from tropical rainforest in the north, to parched desert in the centre and even the occasional snow-clad mountain in the south. But almost all the Australian plants which are a problem here come from south-eastern New South Wales, Victoria and Tasmania – the areas with a climate most like New Zealand.
You can see the pattern on the interactive map I have linked to here. This map is something I used constantly when I worked at Biosecurity New Zealand. At a glance, this map allowed me to see how similar other parts of the world were to New Zealand, in terms of climate. My biosecurity colleagues and I used this map, along with other information, to make predictions about how likely it was that a particular pest or disease would establish in New Zealand, if it arrived here.
In my biosecurity work, I never worked on mosquitoes although I had colleagues who did. However, the principles are the same. Looking at where the troublesome species are now helps us understand where they might become a problem in the future.
The obvious point is one that I made two weeks ago – that the most dangerous mosquitoes tend to be tropical species. But it’s a bit more complicated than that. It’s true that some of the deadliest mosquitoes, like the African malaria mosquito, a type of Anopheles, are largely tropical. But the picture becomes more complex for the two most important Aedes mosquitoes, the yellow fever mosquito and the Asian tiger mosquito. While they are both problems in tropical areas, they are also capable of surviving in cooler areas. For example, in South America the yellow fever mosquito is found in southern Brazil, Uruguay and the north of Argentina. And the map that I mentioned earlier, the one showing the similarity of New Zealand to other parts of the world, indicates that those areas have a very similar climate to parts of New Zealand.
But looking at current climate comparisons only gives half of the story. Our climate is changing, and as a result pests are changing their distributions too. What does climate change mean for mosquitoes?
The first point that it’s useful to understand is that mosquitoes, and indeed all insects, gradually increase their rate of metabolism as temperatures increase, right up to the ideal temperature for that mosquito species. This is the temperature at which the mosquito can fly the fastest, complete its life cycle most rapidly and bite the most people. But if temperatures move above the ideal, the mosquito is in serious trouble. Often, temperatures which can kill it are not far above the ideal temperature.
This is quite different from the situation with humans and other mammals – we don’t speed up just because someone turned up the heat. Humans are endotherms, commonly termed warm-blooded, and so our body temperature and therefore metabolism is not dependent on the temperature of our environment. Insects such as mosquitoes are ectotherms, or cold-blooded, meaning their body temperature is largely dependent on their environment (although insects are not completely incapable of regulating their body temperature – for example a mosquito is capable of cooling itself off so that it doesn’t overheat as it feeds on our warm blood).
So, as temperatures increase under climate change, mosquitoes are likely to become more of a problem – up to a point. Eventually, though, temperatures get too warm. Mosquitoes will seek shade and cooler areas if it gets too hot, but their survival will also be affected.
However, climate change won’t just make mosquitoes develop faster and bite more often. More important is the impact it will have on mosquito populations over a larger scale, by changing which parts of the world are suitable for mosquito survival, and which are not. In fact, this is happening already.
One change we are already seeing is in the highlands of East Africa and northern South America. Once, these high-altitude areas were largely free from malaria, because they were too cool for the Anopheles vectors. However, with increasing temperatures, the mosquitoes, and therefore malaria, are moving into higher altitudes. Researchers have calculated millions more people, in countries like Ethiopia will be infected with malaria as temperatures rise. More areas of Europe will also become suitable for malaria-carrying mosquitoes.
Climate change isn’t all in the mosquito’s favour. Some parts of South-East Asia may become too hot for the most important of the malaria-carrying mosquitoes there. However, other areas of the region which are currently less suitable will become more suitable. There also remains the possibility than another species will move in and become a problem – with 70 different species able to transmit malaria, it is hard to predict exactly what will happen.
What about the Aedes mosquitoes, which transmit dengue and many other diseases? Here, again, the picture is complex. The Asian tiger mosquito is more sensitive to heat than the yellow fever mosquito. With climate change, therefore, it is likely to move into cooler areas, but disappear from the hottest parts of its current range. As a result, there won’t be more people exposed to this mosquito in future – instead, some people will get a reprieve from this mosquito while it becomes a new threat to others.
The yellow fever mosquito, on the other hand, is more heat tolerant. Under climate change, it will spread to cooler areas, but will still persist in hotter areas. Overall, it’s likely to benefit from climate change. It will expand more than it retreats.
New Zealand is currently on the cool side for both the Asian tiger mosquito and the yellow fever mosquito. As a result, climate change is only likely to make the country more suitable for both species, and the same applies to the Anopheles mosquitoes which carry malaria.
What can we do, then, in the face of an increasing threat from disease-carrying mosquitoes? We have the trapping programme I wrote about two weeks ago, but is there anything else? In fact, humans have a number of weapons against mosquitoes and the diseases they carry. But just as the threat is expanding and we need them the most, these weapons are starting to lose their power. Our deadliest foe is fighting back. It’s part of a bigger issue with the use, or overuse, of pesticides and some medicines — an issue has been on my mind for some time. And so it will be the subject of my article in two weeks.
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Stunned to hear that Pohutakawa are now invasive in South Africa!
I just read that the Asian tiger species was found in southern Portugal carrying the Zika virus. Case in point!