13 October 2011

Lantana: a fuel's paradise?


Given the sheer number of invasive species in New Zealand and the difficulties that we have with them it often feels like things couldn't get much worse for our native ecosystems. Of course things can always get worse and there are some prominent invasive pest species in other parts of the world that we don't have, thankfully, here in New Zealand. One of these is the weed shrub lantana (Lantana camara). Although this South and Central American species is a major problem in various parts of the world, all I knew about it was that it was good at invading bush edges and regularly disturbed environments, hard to get rid of and was the title of a very good movie featuring Geoffrey Rush. Thanks to our new plant ecology lecturer, Tim Curran, I now know a great deal more.

Tim recently moved to our Department of Ecology from a field station in northern Queensland where he was part of the School for Field Studies at the Centre for Rainforest Studies. Lantana is a problem species in this part of Australia and Tim and colleagues have just published a paper in Weed Research on a potentially insidious aspect of this species. Lantana often invades dry rainforest (yes a strange combination but an important habitat nevertheless!) where it dominates the understory. One of the suggested problems that this invasion causes is in changing the fire regime, such that these habitats become more prone to fire. This change could be the result of chemicals in lantana that make it more ignitable (and therefore fires are more likely to start) or, by changing the structure of the understory, more fuel may accumulate that would aid in the spread and duration of fires (making them more damaging).

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Tim and his undergraduate students went to field sites in Forty Mile Scrub National Park and measured fuel bed depths, leaf litter depths, % cover of fuels in areas with and without lantana. They also took lantana specimens back into the lab and measured how much dry matter the leaves and twigs had as well as burn durations.


The results were reasonably clear-cut. Lantana was not found to be particularly ignitable. In fact, lantana burnt faster than many native plants, which would usually decrease the damage from a fire moving through a forest. However, measurements of fuel loads in dry rainforest found that fuel cover was significantly greater in areas with lantana and fuel bed depth changed from 9 cm without lantana to 66 cm with lantana. So, lantana added significantly more fuel to burn in the habitat (so fires would burn longer and more intensely in an area) and also allowed greater access to the canopy layer of the forest (so that parts of the habitat not normally at risk were affected).

This research helps to show why lantana is such a successful weed species. It is a species that is very good at colonising disturbed habitats and this study shows that lantana can cause further disruption of the habitat, allowing itself to become even more competitive. This study suggests that removing lantana from forest edges near frequently burned savannas should be a high conservation process. All in all, this is certainly one species we can do without here in New Zealand!

03 October 2011

The web stays in the pitcher


Some of my earliest biology memories are as a 9 year old at Balclutha Primary School working through food web diagrams. The idea of the interconnectedness and interdependence of life was an extremely powerful idea and I recall the first afternoon we worked on food webs more vividly than most in my primary years (although the day Rolf Harris came, sang and painted a picture was pretty cool; and then there was the great pea-shooter battle one afternoon – but I digress).
Of course at the time it was comprehending that the big things eat the little things who eat the planty things that really sank in. Suddenly the natural world around me started to make sense in a cool and interesting way. From what I see from my own sons’ passages through primary school, the whole food web activity still seems to exist, hopefully influencing future biologists. Which is great, as there is still much to be done on understanding how food webs work. Hannah Buckley (Lincoln University) and colleagues from around the USA have worked on a very simple food web that can be found all around North America, the fluid filled pitcher plant, Sarracenia purpurea. The long-lived (> 50 years) carnivorous plant grows in bogs, sand plains and pine savannahs across much of North America Within this carnivorous plant lives a small community that typically contains 6 arthropod species, 9 protozoans and 17 bacterial species. The beauty of the pitcher system is that the food web is about as simple as you can find, the links between the different species is reasonably well understood and, more importantly, the system is replicated in thousands of sites at a continental scale. That makes for a great natural experiment which they investigated earlier by looking at the variation between populations. In the present study they wanted to see what effect temperature, rainfall, atmospheric nitrogen availability as well as the abundance of the keystone species (the pitcher plant mosquito, Wyeomyia smithii) had on food webs.

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The team collected data from all over North America. At the level of individual pitcher plants, food web complexity was proportional to the volume of liquid in the plant – the more liquid, the more complexity (meaning the pitchers act just like little islands). At the scale of sites over North America there was a less clear cut picture of the pitchers. The further north the site was (moving to higher latitudes) the less variation there was is the various traits of the pitcher food web but, overall, food web structure was not driven very much by climate variables like rainfall and temperature and it seemed that food webs at sites were often built simply by which species historically happen to have arrived in the site in which particular order. This finding is a bit unexpected as it suggests that random processes play an important role at the larger scales. So there’s plenty more work to do on this great little model system. There is something quite satisfying in finding that food webs are still as cool and interesting as that day long ago in South Otago.