Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Friday, 10 November 2017

Guest Post: Coming out of the shadows – do ecologists need to think more about light?

In the first of a series of Guest Posts, my PhD student Sue Bryceson (S.Bryceson@latrobe.edu.au) tackles the question: what do we really know about light? And how might it shape the understorey of eucalypt forests?

Every seed catalogue and every nursery pot label indicates the optimum sun exposure conditions for the plants they sell: ‘sun’, ‘shade’ or ‘partial shade’ (which usually means ‘full sun for one part of the day’). Australian plant growers have inherited this standard practice from Northern Hemisphere thinking, and it permeates horticulture, botany and by default, ecology. I’m starting to think that this needs to be refined.

As many a tourist to Australia will bemoan, it can be hard to find shade from sun or shelter from rain under eucalypts compared to the protection offered by a plane tree, elm, or oak. A well-known characteristic of our iconic tree is that the leaves angle downwards, a trait that enables them to avoid the intensity of the midday sun. This characteristic also means that when the sun is high in the sky, the leaves cast dappled shade.

I started trying to characterise eucalypt shade compared to the shade of exotic species. What is the shade like, and how does it affect what can grow under it? Does this affect the dynamics of eucalypt-dominated ecosystems?  So I photographed a 1 x 1 m white quadrat under eucalypt woodland shade (top image), then posterised the images (centre) to quantify the shade levels. I did the same thing under exotic canopies (bottom).



I wanted to know more, so I took a photo every 15 minutes for 4 hours under a dozen different woodland canopies in Melbourne, native and exotic. The images below show one of this series.

The shade patches shift randomly and rapidly. There’s no temporal pattern, just blasts of light amid splotches of shade. Anything growing under this dappled shade would need to be able to cope with full sun conditions ‑ although only for minutes at a time ‑ rather than the muted light of steady shade. Any understorey shrub or groundcover plant that can take advantage of these minutes of strong light could have a competitive edge.

An agricultural study in the tropics found that full sun plants like melon and capsicum can grow equally well under the dappled shade of a passionfruit (but not deep shade). Studies of light flecks in tropical rainforests typically find that plants can fire up their photosynthesis mechanisms as soon as the sun-specks hit them. It’s not a long stretch to think that this phenomenon is probably happening right through the dappled shade of Australia’s eucalypt woodlands.

How many of Australia’s common eucalypt-woodland plants could grow under the shade of a North American elm or savanna oak? I suspect that the canopy light conditions mean there are clear halos under northern hemisphere trees where the plant life is clearly different to the surroundings, whereas in Australia any such vegetation shift is less apparent.

So as well as looking at soil patterns to explain species distribution in woodlands, perhaps we should also be looking upwards to see what’s going on with the light?

Sunday, 1 October 2017

Linear Native Grasslands and Fire

This short video - Linear Reserves Project - details how conservation of threatened native grasslands on linear reserves such as roadsides is advantaged by burning for asset protection.

The best examples of native grasslands on the Victorian Volcanic Plains are those that have had a long history of frequent fire and little grazing. This has happened almost by accident. The Country Fire Authority (CFA) burns roadsides in summer to mitigate the risk of fire sweeping across that landscape. As it happens, such a regime promotes light sensitive forbs and grasses, including a multitude of geophytes, because it reduces the probability of competitive exclusion by tall C4 grasses. Few exotic grasses move into intact systems under this regime too. It's interesting to see the CFA now promotes roadside vegetation conservation, seeing a mutual benefit of maintaining these endangered ecosystems (because of the reduced fire risk from low biomass grasslands relative to higher biomass exotics).

There's some excellent shots of fire taken by drones, lots on lovely photos of grasslands and their biodiversity, and a cautionary tale about the need to recognise endangered vegetation.

Monday, 4 September 2017

The "wow" moments in ecology

Every now and then you read a paper or, better still, make an observation that makes you step back and go "wow! Isn't that awesome?"

I had one of those moments recently. It involved the a) reading of a paper that b) made me recall an observation I made a few years ago which intrigued me at the time. I'd filed away the observation hoping to re-visit it one day. Maybe that day has arrived! Such moments reminds you of why you do what you do (in my case, studying the dynamics of ecosystems) and the excitement that can be generated by new discovery.

All good questions in ecology stem from observations of nature that generate testable hypotheses. For me, natural history informs what I do on so many levels. Observing the form of plants invariably gets you thinking about the function of plants. Looking at a stand of trees gets you thinking about why there are no seedlings. Compiling a species list for a 1m2 quadrat and getting to 42 plant species invariably gets you to thinking about the mechanisms that allow so many species to coexist in such a small area. It's how careers are built - curiosity demands explanation.

Recently I stumbled across a paper in Ecology - in a new (and welcome) section of the journal titled The Scientific Naturalist - that excited me no-end. A bunch of ecologists (Dell et al. 2017) had been burning pine savanna in the USA when they stumbled across one answer to a question that has rarely been asked: where do invertebrates hide to survive fire? Typical answers include: they shelter under rocks, down burrows, under bark. But, it seems that survival might actually include climbing / walking / jumping up tree trunks to escape the direct flame zone.

Sticky tape acts as an effective trap for grasshoppers and other invertebrates moving up trunks to escape fire

Using sticky tape sourced from the local hardware store, wrapped around tree trunks to trap insects as they move up trees (you can figure out direction of travel, abundance and species composition using this simple method), the researchers describe a hitherto poorly documented phenomenon. Insects moving up trees makes sense if fire is frequent and flames are low. But it can't be a chance thing; something must trigger this response. Wow! How cool is this??

What's really great is Dell et al's explanation for the behaviour they observed. I'd have imagined that insects would be responding to chemical cues from smoke. Given there are over 60 different compounds in smoke, it makes sense - to me - that one of these chemicals might trigger such a uniform and rapid response. Smoke drifts ahead of the flame zone and therefore provides some time for insects to respond.

But no, the authors suggest that insects respond to the sounds of an approaching fire. We have a testable hypothesis right there! Apparently the frequency ranges associated with fire lie well within the arthropod hearing range. Who would have guessed!

So how does this relate to my own observations? Well, I read the paper and realised that I too had seen this behaviour. But I'd seen so much more!!! When burning tropical savanna at the Territory Wildlife Park in Darwin a few years ago, I was stunned when we saw ants, beetles, FROGS and GECKOS all using the trunk of a smooth-barked eucalypt tree as a veritable highway as a fire moved closer. They were all going one way - up the trunk. Not far, but definitely up. In these fires, being 2-3 m up a tree will be usually outside the flame zone and, as long as they can withstand the convective heat for a short period, then their chance of survival seemed pretty good.

The curious observation of animal movements up trees in the face of oncoming fire needs to be better documented. How common is it? What species typically respond in this way? Do they survive fire? How does the invasion of Gamba Grass into tropical savanna affect this survival strategy? What's the cue or cues that act as triggers for movement? If frogs are responding to the noise of a fire, then this suggests their hearing is more than just about finding a partner (i.e. sexual selection), it's about survival in a fire-prone landscape. Now that's cool! And probably rarely, if ever, been thought possible. I guess that was a bit of a "wow" moment for me.

Dell et al. (2017) An arthropod survival strategy in a frequently burned forest. Ecology doi:10.1002/ecy.1939

Monday, 27 February 2017

It's Official! Kangaroo Grass is Australia's most widespread plant species

Large-scale investment in biodiversity informatics since the 1990s has revolutionized the study of biogeography. For instance, Australia's Virtual Herbarium (AVH; plants) and the Atlas of Living Australia (all taxa) provide immediate access to occurrence records of Australian species at the touch of a keyboard. As a consequence, the unprecedented availability of data in online natural history collections means it is now possible to test previously intractable questions at continental and global scales.

Rachel Gallagher, Macquarie University
One great example is provided in a  recent paper by Rachel Gallagher on "Correlates of range size variation in the Australian seed-plant flora" published in the Journal of Biogeography (http://onlinelibrary.wiley.com/doi/10.1111/jbi.12711/full).  This study provided a taxonomically and spatially comprehensive understanding of how range size varies across a large proportion of the Australian flora. A dataset of 3,061,143 occurrence records (representing 19,277 native species from 1931 genera and 198 families) was accessed from the AVH and used to map and analyse species distributions.

From this, some really interesting findings about the Australian flora could be deduced.
  • The perennial tussock grass Kangaroo Grass (Themeda triandra) had the largest range estimate of any species in Australia, occurring across a whopping 7,114,754 km2 of the continent!! I knew that Kangaroo Grass was extensive, but had never realised it is the most widespread species of all. Rachel speculates on why this may be: "C4 photosynthesis confers major plant productivity benefits in arid and hot environments, where photorespiration is typically high. Water-loss via stomatal opening is reduced as a result of C02 concentrating mechanisms and C4 species can remain photosynthetically active at high temperatures. The development of C4 photosynthesis in grasses has been implicated in their dominance in arid and savanna biomes throughout the globe and may help maintain large range sizes across the widespread Australian arid zone."
Kangaroo Grass - officially Australia's most widespread species!

  •  68% of Australian seed-plant species have ranges which cover < 1% of the continent (<77,000 km2); this is a rather astonishing number of species confined to small ranges and speaks to the concept of  local endemism being very important in Australia.  The smallest ranges were found in Mediterranean ecosystems in the SW corner of the continent. Additionally, a number of species had only one unique occurrence record and therefore shared the smallest range size estimate of 100 km2. While these small-ranged species were drawn from 63 plant families, 27% were orchids.

  • On average, species that had large ranges were characteristically non-woody and were broadly associated with the arid and grassland biomes which dominate at mid-latitudes. Notably, Australia's arid biome supports a greater richness of endemic species than its tropical rain forests, implying a high degree of specialization in the arid-adapted flora.

The study demonstrated clearly the importance of arid conditions in selecting for large range size. Climatic instability has been shown to select for large range size by increasing extinctions of species with small climate niches and poor dispersal capacity. The retention of widespread taxa in response to climate oscillations has left the signature of large range size in current-day assemblages across the globe and may contribute to large range size in the Australian arid zone flora. That is, climatic fluctuations during the evolution of the Australian arid biome may have provided similarly appropriate settings for the retention of large-ranged species.

The study shows what a good dataset, a sharp mind and some nifty computing power can achieve. And it has real-world implications. Given the finding that 68% of Australian plant species have ranges which cover < 1% of the continent, a wider portion of the Australian flora may be more vulnerable to human-threats, such as habitat fragmentation and climate change, than previously acknowledged. Given that range size is inversely proportional to extinction risk, the data that underpin this study can identify which taxa and landscape locations have the smallest range sizes and this should be instrumental in identifying conservation priorities.

It also highlights to me that we still have a poor understanding of why recent 'invaders' to Australia, such as C4 grasses like Kangaroo Grass, have become so widespread in Australian ecosystems. Is it dispersal? Is it plasticity? Is it because we lack ungulates? Is it because of a grass-fire feedback? Is it because eucalypts create dappled shade rather than deep shade?

It's well worth a read!