ESA 2014 Conference talk: Dungeons & Dragons!

Today I gave a talk at the Ecological Society of Australia 2014 Annual Conference about some of my PhD work. About 500 ecologists have descended on Alice Springs, the desert town smack-bang in the middle of Australia, to exchange their most recent work, ideas, revelations and frustrations. The amount and variation of work that's being done here is staggering - I'm lucky to be able to contribute to such a vibrant community!

Below I give you the outline of my talk, just to give you an idea of what I've been up to.

Predation has been associated with changes in behaviour and habitat choice in a varied range of species.

Moose shift their diet dramatically to avoid wolf predation. Birds alter their time budgets and flock size in the presence of a predator. Here in Australia, velvet geckos will forego hiding in crevasses previously occupied by snakes.

Mitigating predation risks represents a trade-off for foraging and reproduction. Different species come up with different strategies and budgets for survival. And so predation becomes one of the drivers that helps facilitate coexistence and high local diversity.

The system is influenced by abiotic factors like rain and fire. However, the effects of these large-scale events are not easily predicted.

In the system we work in, deserts, rainfall and fire are major drivers of the ecosystem. But whether they have an impact on predation and if so, how, is not known. These are questions we’re trying to address and with the Australian desert agamids we find an unusually good opportunity to do so.

The deserts of central Australia contain richer communities of lizards than any other arid regions in the world, with the highest diversity occurring in sand dune habitats dominated by hummock forming spinifex grasses.

We are here (Alice Springs). This red blob on the map is the Simpson Desert, we’re on the western border of it. And the field sites of our work are situated on the eastern end, right on the border of QLD and NT.

This is an aerial view of our camp site and surrounds. You can see the road through the middle which has acted as a fire break. To the left, around our camp there is old, unburned spinifex habitat. To the right, the fire burned most of the spinifex, leaving a sparsely vegetated, sandy habitat. Over time, fires create a mosaic of burned and unburned patches at various stages of regeneration.

Within this setting we find two species of agamid lizards, the military dragon and the central netted dragon. They both occur abundantly throughout the Simpson Desert but in contrasting habitats. The military dragon is dominant in areas where spinifex ground cover exceeds 30%, whereas in areas where spinifex cover is less than 10%, the central netted dragon is dominant.

In addition, each lizard species selects different habitat components (micro habitats). The military dragon selects areas within 30cm of spinifex hummocks, whereas the central netted dragon selects dead wood within sparsely vegetated areas.

The difference in habitat use and behaviour between these species is distinct. Ben Daly has asked the question ‘why?’ in his 2008 Ecology paper. He identified predation as a possible driver for this divergence.

If predation is indeed a driver, the first question that arises is whether predation pressure is different in burned and unburned habitats. The second question is whether the use of different micro-habitats is associated with different predation rates.

These are a selection of predator the dragons encounter: raptors, including the occasional grey falcon if we're lucky, reptiles and mammals.

Just to give you an example of what predation looks like in the field, here’s a central netted dragon we encountered only a couple of weeks ago. This one was scooped up by a raptor in a burned area, thoroughly decapitated and left in a tree .

Instead of offering up live lizards to the same fate, we used models made of plasticine clay to test the differences in predation pressures and predator assemblages.

We used simplified models of both lizard species.

Using models also has the benefit that we can standardize size, sex, appearance, and behaviour which makes the method optimal for comparing between habitats.

300 models, 150 of each species, were set up at 60 sites. Half these sites were burned, the other half were unburned. Sites were a minimum of 500m apart to make sure attacks between them were truly independent.

Within a site, 5 models of the same species were set up within 10x10m. Each of these were set in one of the 5 main micro-habitats the dragons use. One out in the open, one head-first under spinifex, one parallel and close to spinifex, one set on wood and the last on leaf litter of a shrub. 

After 4 trips between November 2013 and last week, a total 1200 models and 14,500 checks we put all the results together.

I have to do a little disclaimer here. This is still hot of the press, the last data was only collected last week and I’m still working through the full analyses – these are the preliminary results of a Generalized Linear model (GLM) I ran to analyze the impact of the variables we were interested in.

For clarity I decided to forego the R graphs and output and instead go back to basics and use very simple bar graphs to explain the results.

When we analysed all data together, this is what we saw:

Burned vs unburned -> has a significant impact on attack rate as expected. However, the impact is the complete opposite of what we expected to see, with unburned, dense habitat incurring a much greater risk of predation.

Unexpectedly, microhabitat selection did not influence attack rates

Military dragons and central netted dragons were not attacked at different rates

Birds and reptiles were by far the most common predators and the only ones significantly impacting the chance of attack. However, analyses could not identify which one was the more important predator.

In summary, only macrohabitat and two sorts of predator seemed to explain a significant amount of variance. And of those, the macrohabitat effect was completely opposite to what Daly found a few years ago in his pilot study; and on top of that we cannot distinguish which of the two predators is more important.

All in all, it felt like we were missing something.

The light bulb moment came when we decided to include rainfall into the model. If almost everything else in the desert is linked to water, maybe it could explain some of the variance in our predation model too.

And it did. We split the data in two, dry times where there had been no substantial rainfall in the previous 6 months and wet times, where there was either rainfall on the trips itself or in the two months before.

One of the most marked changes occurred in attack rates between macro habitats. In the dry times attacks in open habitats surged, matching the results Daly got in his pilots (incidentally done at very dry times).

In wet conditions, the rates completely change. Most attacks occur in denser spinifex grasslands.

Attack rates on micro habitats were also greatly affected by rainfall. In dry times, attack rates in different microhabitats are significantly different, with being out on wood as the most dangerous spot to be and hiding under spinifex as the safest.

Rainfall evens out the odds it seems, with no differences found between the micro habitats.

Dragon species remained un-affected by the split between dry and wet times, not playing a role in explaining predation events. For the purposes of this experiment it’s a good thing. It rules out general size and skin pattern as a cause for predation. If both species inherently incur the same predation risk, we can assume that their contrasting behaviour and habitat use are the coping strategies.

Last but not least, rainfall also explained why we didn’t find a predation difference between birds and reptiles. One turns out to be the dominant predator in dry times, the reptiles, and the other in wet times, birds.

It’s also interesting to look at the difference in raw number of attacks here. Avian attacks are double the attacks by reptiles in dry times. It seems that wet periods are much more dangerous than dry times are.

Birds arrive with the rain and represent a distinct, highly mobile predator group. They seem to attack several models within a site when they find it – and so they even out the attack rates within all micro habitats. They also look like a likely culprit behind the high attack rates in unburned habitat in wet conditions. 

In conclusion, desert agamids have to respond to a changing assemblage of predators.  To build a model explaining predation events on both dragon species we have to include rainfall as a factor, together with macro habitat, micro habitat and predator species.

The next question is how these factors influence the behaviour and habitat use of the dragons and if both species respond differently to different predators. Who knows what behaviour they have come up with!

A Rainy Desert Trip

I write this a day after coming home to Sydney from the Simpson Desert, the place I go to work on the border of Queensland and the Northern Territory. I’m a biologist, a desert ecologist to be exact. 

Yesterday I stepped out of our field vehicle, a very off-road worthy thing, after an unusually long drive back. Something happened you wouldn’t expect in the Simpson. It rained.

It started raining while we were in the desert. Rain is a little magical out there. Dusty, yellow clumps of prickly spinifex grass change colour instantly and become light green. The dust settles and the sand turns a deeper shade of red. Burrowing frogs dig their way out to the surface. Birds fly in for both the water and those frogs. The whole place comes alive.

Burrowing frogs emerging from the sand - Notaden nichollsi

And then it rained more. We started hearing about road closures on the radio, more every day. Rivers that have been dry beds for years start flowing, usually straight over the roads we needed to take to get out. We laughed about it at first and carried on with our work, enjoying the conditions. We found ourselves stuck, flooded in, and it felt like a great adventure!   

When the day we need to leave drew closer we got a little more serious. We all needed to be places, meet people, teach classes, start the first day of a new job, catch that plane home. Our real lives invaded the adventure of being stuck. Eventually we drove out, not really knowing what we were getting ourselves into.

We got out of the desert to the first town, the part we thought would be most tricky. We thought we done it. But from there we were pushed further and further north due to road closures. Keep in mind we were trying to go south, back to Sydney. In far West Queensland towns are easily 200-300km apart. These detours were costing us many hours.

After another very wet night camping beside a flooded road full of frogs we conceded defeat – quite grumpily.

We stopped at the information centre of the nearest town we could reach for the latest updates on the roads. A middle-aged lady beamed at us when we walked in. ‘How GRRREAT is this rain?!’ I admit I raised an eyebrow and produced a frown at that.

It turned out she and her husband own a cattle property. They had been waiting for this rain for 3 years, struggling through that time to keep their cattle alive. She told the sad stories of farmers who couldn’t manage the financial burden and resorted to killing their livestock. While telling us her story the rain got even worse, it was pouring down. She got so excited she started bouncing up and down, doing a little rain dance behind the counter, hopping around in circles.

It lifted my spirits and provided some welcome perspective. The rain was inconvenient for our trip, our jobs and our appointments but it brought life and hope to others. We continued our journey further north, racing the floods, eventually making it to Sydney. 3000km, a day late and one teaching session missed. I think my students will forgive me.

Pictures by Frank Bird

Summer of the Singing Cidada

As I write and draw this I’m sitting on my balcony in Sydney. It’s a warm summer evening, the sun just went down, friends are having a chat in the garden below and all seems peaceful. A cicada starts singing – it’s a pleasant sound, reminding me of warm holidays spent in France. The bug must have been a few gardens away. A closer one starts answering. And before I know it, the tree next to my ear explodes in a shrill cacophony of noise. Gone are the bird calls, gone are the voices from below, it is literally deafening. The summer of the cicada has come to my back garden.

A bit of artistic freedom: this one does not sing in my garden but in South America.
It's called the Giant Cicada (it's huge!) and is considered the loudest insect in the
Western hemisphere. 

It is a bumper year for cicadas, both in the USA and here in Australia. They haven’t emerged in large numbers like this for ten years (in Australia, seventeen years in the US). Species with names like shady underworld figures – masked devil, razor grinder, black prince – are prowling through the suburbs of Sydney and the Blue Mountains in swarms.

The typical lifecycle of a cicada

Why so many at the same time, you ask? Some of this can be explained by the life cycle of these noisy bugs. Eggs are laid in slits in the bark of trees. When they hatch a few weeks later the miniature cicada (called nymph) falls to the ground and buries itself up to 2m deep. It is here that they will spend most of their lifetime, feeding on the sap flowing through plant roots. They grow, occasionally shedding their skin, until they reach maturity. This underground stage can last any time from 9 months to 17 years(!), but most species will take 6 to 7 years. When they emerge, they shed their skin one last time and enter adulthood. This stage is quite short compared to the nymph stage, only a few days to a few months, typically 2 to 4 weeks.

It is not exactly clear why all cicadas decide to emerge at the exact same time – but they do. It most likely has something to do with environmental signs like increased sap flow in the tree roots indicating warmer weather and more rain. This would explain why the cicadas were particularly early this summer: Australia has had the mildest winter on record.

Studies of the only cicada I ever photographed -
a Singaporian fellow from the Pomponia genus

Emerging en masse does have its perks. The sole purpose of the adult cicada is finding a mate and produce eggs. Finding a mate becomes easier if there are millions of you buzzing around the same hectare. Cicadas also use an interesting survival technique called predator satiation. The idea is to overwhelm predators with such an overload they literally cannot eat them all. Mind you, the birds will do their best. This also has its advantages; it has been shown that in years like this birds produce stronger, healthier broods.

The perpetrator in my garden:
Cyclochila australasiae aka Green Grocer
(you can see why). It is one of the
only species adapted to living in urbanised
areas and incredibly loud.

Cicada swarms do very little damage. They are so single-mindedly focused on reproducing that they do not eat. They are not poisonous, they do not sting or bite. They don’t fly into your house and crawl under your bed. The only real damage they can do is to your ears. It’s only the male cicadas that sing, and when they do they can reach over 100 decibels. It’s like a high-pitched airplane coming over. While they have the peculiar ability to turn off their own hearing while singing, I do not. I think it’s time to get some headphones.

Made a new friend!

Me and my friend in the Simpson Desert
Pic by Shaun Doyle

I’d like to introduce you to one of my new friends here in Australia! He is a bit of a loner and somewhat prickly at the best of times, but bear with me and you’ll come to like him too!

His name? Thorny devil. Yes, really. In Latin it gets even worse, Moloch horridus. Moloch refers to an ancient god, revered around 2000BC by a people living on the Southern and Eastern Mediterranean shores (modern day Lebanon, Israel, Palestinian territories, Jordan and Syria). This god was associated with a particular kind of child sacrifice through fire. Not a great start. The last part ‘horridus’ means bristly or secondarily dreadful.

This reptile might look fearsome but it is not as dreadful as its name suggests. In a land filled with deadly dangerous animals, this one uses clever disguise and innovation to survive.

Poisonous? No

Fast? No

Ferocious? Eh.. no. When picked up it might looked slightly annoyed and then just close its eyes.

So how exactly does it survive?

Thorns. Lots of them. Not many animals enjoy swallowing something that looks worse than a cactus.

Holding at your own risk! Pic by Enyi Guo

Camouflage. You don’t see a thorny devil until your foot hovers right over one and you were happening to look where you were walking. Even when you know what you are looking for they are exceedingly tricky to find. Their body colouration is patchy, partly the red of the desert sand, partly the yellow/green (it varies per individual) of the vegetation. When the animal is undisturbed the colours are quite dull. However, when moving or picked up they brighten up considerably as a warning sign to potential predators: do not think about eating me, it’s going to be unpleasant!

Body colouration can rapidly change. Here, the colours are dark and distinct.
Pic by Shaun Doyle

Deception. On its neck, it grows a bump with thorns about the same size as its head. When threatened the thorny devil tends to tuck its real head between its front legs which manoeuvres this bump where the head usually is. Many predators will aim for the head first when attacking. When they get in contact with the spines some might decide they like their stomach without holes and dismiss the attack. The moloch survives without damage to its real head.

Notice the 'extra' head on top of its neck. Pic by Shaun Doyle

Innovation. The thorny devil is endemic to Australia, meaning it is only found there. It thrives in the dry, sandy interior of the continent. One thing is very hard to come by in this place: water. Luckily, the thorny devil has got a rather unique solution to this. In its skin it has a system of small grooves, all leading toward the corners of its mouth. These grooves collect and move water from the entire surface of its body towards its mouth by means of capillary action. The only thing he has to do is swallowing. If you put a moloch with its feet in a dish of water, within a matter of seconds its body will be glistening with moisture and without lowering its head it will start to swallow the water brought to its mouth from its feet. All tested by yours truly of course.

A thorny devil can drink via its feet

You won’t find these animals in captivity often; they are notoriously hard to keep. Outside Australia, only a zoo can get a permit to keep them as they are a protected species. And few zoos actually do keep them. The problem lies in the feeding behaviour of these animals. They are very picky eaters, only feeding on 2-3 species of tiny black ant. And even then, they will only eat those when they are moving in a line, picking them off one by one, consuming around 2000 every day. So keeping one in captivity requires keeping a few large ant colonies of a particular ant species as well, and training them to walk in a line through the enclosure of the thorny devil where most of them will be eaten. It is a daunting task.

In short, I am a lucky person to have such an elusive new friend!