Managing sites for biodiversity based on plot-level data

A simple, but powerful realisation hit me the other day while I was away on a field course with my 3rd Year Botany students. We went to Cape Conran in the east of Victoria to study fire and diversity relationships in sandplain heathlands. We set the students some questions for which they had to design a research project. They did a great job, so much so that it made me think about how we use scientific data to inform conservation management actions.

One of the questions we asked was: what is the relationship between species number and time-since-last-fire? This is a very 'old' question in ecology, with lots of evidence that species number initially increases after fire, then plateaus off, before declining with increasing time. As such, it is recommended that to maintain diversity in heathlands, they need to be burnt at frequent intervals. Reading the literature, it is evident that fire return intervals of 10-25 years are recommended.



Botanists often quantify the number of species in quadrats of fixed size.
This is SPECIES DENSITY. Often only one or two quadrats per 'site' are sampled. An individual site here is
a heathland with a known date of  Year-Since-Last-Fire

 

Land managers take advice about the ways in which to manage biodiversity from scientists, so it is important that the advice we give is based on solid data from well-executed studies.

At Cape Conran, we asked whether species number declines with long fire-free intervals to test the well-established understanding of heathland ecology. But we decided to take a different approach to that which is usually employed.

In many of the 'fire in heathland' papers I read, I see that botanists compare the number of species in a quadrat (this is known as Species Density) across sites with known fire histories. Importantly, in these studies it is usual that the same sized quadrat is employed across sites to generate Species Density estimates. But this might be a flawed approach. Imagine you set up a 4 x 4 m quadrat in recently-burnt heathland. The plants are very small and many of them can 'pack' into the quadrat. As plants age, they become bigger and hence, it is likely that fewer of them will fit into your 4 x 4 m quadrat. In long-unburnt heathland, heath species are as large as they can grow and even fewer will fit into your (what now seems small) quadrat.

So, while Species Density might inevitably decline with time-since-fire in your small quadrat, this is not the same as saying Species Richness declines across the site. Species Richness is the number of species you find in a defined site - in our case, a heathland - and it is this scale that a manager manages. Hence, is there a mismatch between the scale of evidence that botanists have used to assess fire impacts (the plot level) and what managers actually need to know - how do I manage heathland to maintain their diversity?

To answer this question, we set out to sample Species Richness across sites that contrasted in time-since-fire. We used lots of small 1 m2 quadrats (n = 25) at each of three sites (that differed in time-since-fire: recent to long-unburnt) rather than relying on one or two big quadrats as is usually done. These were spread out across each heathland and all species were recorded. The results were VERY surprising!




Here, students are also quantifying species density
in smaller quadrats, but doing lots
of them across a site

As expected, Species Density was highest in the most recently-burned areas compared to long-unburnt areas (15 vs. 8 species per m2). But then it got interesting. The total number of species observed in each site across our 25 quadrats was the same - 39 (all of them native). So, despite there being many more species in the small quadrats in the recently burned areas, unburnt areas supported the same number of species across the site. Yes, they were 'rarer' and some were different species, but if the aim was to maintain species richness at the Site Level, the level at which managers would be most interested, then the need for fire was not so clear.

And, when using some fancy statistical techniques to assess how much more biodiversity would be present if we had sampled more quadrats (the procedure is called 'rarefaction'), we actually expected more species in the long-unburnt heathland than the recently-burned heathland (60 vs. 41).

It became apparent that frequent fire might actually hold-up succession of sandplain heathland to more complex, diverse systems rather than maintain it. Frequent fire would likely knock out species that have long primary juvenile periods or that take time to colonise after fire. Far from allowing such species to be members of the community, frequent fire might reduce spatial heterogeneity and hence, reduce habitat complexity. Such thinking hasn't been applied to heathland ecology before in south-east Australia and while it is too early to suggest we change our current practices (of frequent fire), this student research hints that maybe a rethink is in order.

And, as I said at the start, it highlights that the relationship between Plot-level data and Site-level responses are not always as clear-cut as might be imagined. Stay tuned........

Thanks to Luke O'Loughlin, David Cameron and Botany students Holly Fiske, Nicole Baboucek, Tony Hampton, Jasmine Thum, Darragh O'Sullivan and Kate McWhinney for collecting and interpreting this interesting dataset from the annual Botany 3 undergraduate trip to Cape Conran.

New study points to the global significance of the Plains-wanderer

Can you spot Australia's most unique bird?
Terrick Terrick National Park
Photo: John Morgan

The Plains-wanderer has for some time been known to be a member of Australia’s ancient avifauna and its nearest, albeit distant, relatives are seedsnipe from South America.  It is the sole member of a Family of birds called the Pedionomidae. It's a species typically confined to native grassland habitats in eastern Australia and, unfortunately, one of the most endangered species of those grasslands. It should be a flagship for conservation and new research tells us why!

Recently, Jetz et al. (2014) published a major review of the world’s 9,993 recognised bird species to determine which species we can least afford to lose in the current extinction crisis if maximum global phylogenetic diversity is to be maintained.  Phylogenetic diversity is a measure of biodiversity which incorporates phylogenetic difference between species and phylogenetic analyses have become essential to research on the evolutionary tree of life. The concept of phylogenetic diversity has been rapidly adopted in conservation planning.

Jetz et al. (2014) developed a hierarchy based on how isolated a species is on the phylogenetic tree which they termed ‘evolutionary distinctness’.  They also included global geographic range, and global endangerment in their metrics.  The summary metric that Jetz et al. (2014) used to rank the world’s birds combines evolutionary distinctness and extinction risk. 

By their calculation, the Plains-wanderer is ranked:
 #1 among Australian birds and #4 in the world!!

As such, these analyses highlight we can ill-afford to lose the species, yet current data suggest that significant declines are being observed, and it's not entirely clear why.

The two strongholds of the Plains-wanderer are the semi-arid (or xeric) native grasslands of the Riverina region of NSW and Victoria’s Northern Plains.  Monitoring in NSW during 2001-2012 has found that the population size has declined by 75% during droughts, then recovered slightly during benign years, and was then recorded at record low levels during the very wet years of 2011-12.  The population has remained at very low levels for over a decade, and this is cause for considerable concern. In Victoria there has been monitoring on Terrick Terrick NP and nearby private land over five years (2010-14).  Numbers declined by >90% during 2011-12 in the wet years (perhaps because breeding was negatively affected, while thickening of grasslands has reduced occupyable habitat) and the numbers have remained at historically low levels.

If ever there was a need to monitor the dynamics of a species of conservation concern, whilst also monitoring its habitat suitability and key determinants of mortality risk (e.g. predation),  then the Plains-wanderer would seem an essential candidate species. Good, basic scientific research is needed to answer simple questions: how long do birds live; are population dynamics cyclic; can suitable habitat be successfully created from scatch? In some respects, a metric of the success of grassland conservation and management will be that species like the Plains-wanderers can be maintained in their habitat, and that their numbers grow rather than decline.

Thanks to David Baker-Gabb for alerting me to the evolutionary distinctiveness of the Plains Wanderer, and for providing information on the population trends of this species.

Reference

Jetz, W, Thomas, G H, Joy, J B, Redding, D W, Haartmann, K and Mooers, A.  2014.  Global distribution and conservation of evolutionary distinctness in birds.  Current Biology (2014), http://dx.doi.org/10.1016/j.cub.2014.03.011.

A new population curve for prehistoric Australia

The arrival of Aboriginal people in Australia marked an important change in the continent's ecology. The loss of the mega-fauna and a rise in the use of fire are common themes that resonate when discussing the role of Aboriginal people in shaping the nature of Australia.

Up until now, most of the discussion has been about when aboriginal people arrived. The numbers fluctuate between 40,000 and 100,000 yrs ago, with a convergence of opinion in recent years (because of better carbon dating techniques) that it was probably about 52,000 yrs BP.

An interesting paper by Williams take this insight one step further.

Using radiocarbon dating techniques on >1700 sites from all over Australia, Williams has tried to reconstruct/assess population growth rates in Australia up until the time of European contact.

Using some modelling assumptions about the initial founder effect (that is, how many people first colonised), he interprets three key things:

1) Australia was settled by thousands, not just a handful, of humans, suggesting deliberate rather than accidental colonisation of the continent. The research suggests that it probably would have taken 1000 to 3000 people to reach the numbers of Aboriginal people observed at time of European contact.
2) the population size grew very slowly - constrained by glacial periods. It wasn't until the Holocene (from about 10,000 yrs BP) that the population grew substantially. The study shows during the glacial maximum 18,000 to 21,000 years ago, the population fell dramatically. The data suggests 60% of the population was lost during this time, a period of extreme dry and cold. It took 9000 years for the population to recover to the same levels.
3) the maximum population size before European settlement was approx. 1.2 M people (assuming a founder population of 1000-2000). This occurred only 500 years ago.



Distribution of archaeological sites contributing radiocarbon data to the study by Williams.
You can see there is a pretty good coverage across Australia.



You can see this population reconstruction clearly here in this figure from the paper. The effects of 'founder size' (how many people might have first colonised Australia) has a big effect on population size at the time of European contact, as does climate. The two graphs show the outcomes based on different model assumptions - they give pretty much the same results. They suggest that population growth really took off at about 12,000 yrs BP; this corresponds with increasing climate stability

This paper (along with others - see here) hints that the big changes in vegetation and mega-fauna extinctions observed in the last 50,000 yrs were probably driven by climate. Landscape burning by Aboriginal people has been linked to significant changes in the geographical range and demographic structure of many vegetation types but there is now an emerging lack of congruence between human activity and fire records during the period 20-40 kya. During a period of consistently low human populations, as posited by Williams, it is difficult to reconcile how Aboriginal people could have had the profound impacts that have been speculated by many.

I can't vouch for the veracity of the work reported by Williams, but it provides interesting food for thought when interpreting the recent history of Australia.


References
Williams (2013) A new population curve for prehistoric Australia. Proceedings of the Royal Society B 280, 20130486.

Sakaguchi et al. (2013) Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia. Proceedings of the Royal Society B 280: 20132182

Ecological Rants

I just discovered Charley Krebs Blog page called "Ecological Rants". It's worth a look.

Krebs is an excellent ecologist who has written one of the best textbooks on ecology (Ecology: The Experimental Analysis of Distribution and Abundance) and, because of his age, has been around long-enough to have seen the field both stagnate, get bogged down and ultimately, make some massive gains about the understanding of the natural world. I've always thought his insights were interesting, even if I don't always agree with him.

There's excellent 'rants' about:
- answering unanswerable questions
- when academics should retire
- conservative politics and science
- biodiversity research
- science and money
- what bureaucrats need to do to let scientists get on with their job

I really like Charley's approach (elder statesman type of thing) and it's great to see (once again) how Blogs can get scientists to reach out to other scientists (and the public) beyond their academic publications.

Shrubs in woodlands - directional or cyclic?

One of the things that ecologists have noticed in (some) grassy woodlands in southern Australia over the last two decades is that woody cover has been on the rise. This can take the form of increases in shrubs (such as Leptospermum spp., Acacia spp.), or dense regeneration of trees.

My own work (and that of my students and other scientists) show this increase might be due to changes in management (such as the removal of stock grazing from long-grazed forests upon reservation for nature conservation), altered fire regimes (usually a decline or exclusion of fire), overgrazing by native and exotic herbivores (that reduce competition from the ground-layer vegetation, as well as encouraging unpalatable species at the expense of palatable ones), and potentially even changes in atmospheric CO2 levels that favour woody plants over grasses.

Increases in woody plants have been recorded at several locations in southern Australia. This increase has generally been thought to be a uni-directional change that requires management intervention to alter this trajectory. So, at Wilsons Promontory National Park for instance, dense thickets of native Tea Tree that have established in swale grasslands and woodlands over the last 40 years are now being actively managed by carefully timed fire to open up the closed shrublands to benefit biodiversity and amenity.

Parks Victoria staff undertaking a burn to manage Coast Tea Tree (Leptospermum laevigatum) at Wilson's Promontory NP
(Photo: Greg McCarthy)

Just the other day, however, I saw evidence that woody plant encroachment isn't always uni-directional.

Herb-rich woodlands at Langi Ghiran
in mid-winter, before flowering.

At Langi Ghiran, there are fabulous herb-rich woodlands. Spring is a sight to behold - indeed, some of the highest species richness levels in Australia have been recorded there. Shrubs appear to be localised and generally sparse.

Hedge Wattle (Acacia paradoxa), a shrub native to the system, began spreading about a decade ago at this site and forming thickets. I'm not sure why it did this - was it climatically stimulated, was there a change in grazing pressure form native herbivores, was this a response to rising CO2?

It established prolifically in the inter-tree gaps, which are common in this woodland (and support much of the plant diversity), and establishes in the absence of fire. It also seems pretty unpalatable to the resident kangaroos and wallabies. I began to think that the end game here would be dense stands of wattle, a decline in diversity, and a landscape transformed visually.

However, when I last visited, the scene had changed a little bit.

Hedge Wattle - the dead looking stuff in the centre of the image - at
Langi Ghiran, 22nd June 2014.

Across 10's of hectares, Hedge Wattle had died, or was senescing. I'm pretty sure this was not because of old age - most shrubs are (and this is just a guess) less than 15 yrs old. We know from work by my Honours student Julia Franco that Hedge Wattles can live for five decades.

Where plants were not quite dead, the phyllodes were brown and desiccated and clearly the individuals looked like they were on the way out.

I first noticed this browning off in summer after a hot and dry spell. Now, in mid-winter, there seems no signs of recovery. This hints (to me) that drought has probably played a key role in the population dynamics of Hedge Wattle at this site. It hints that the population might be cyclical - assuming that the rise and fall of Hedge Wattle has occurred before, but just gone un-noticed.

If this is the case, it suggests a few important lessons:
- long-term observations can help decipher directional versus cyclical patterns in nature.
- if drought is at play here, was this drought 'extreme' and hence, might we expect its frequency (and potential effects) to increase over time?
- drought plays a key role in the dynamics of many woody plant populations but its effects are sometimes under-appreciated.

What else might be going on?? It's probably important to survey Hedge Wattle at Langi Ghiran to see if mortality is linked to particular aspects, slopes or soil types. Perhaps it is exacerbated in close proximity to century old trees. Might smaller plants be more susceptible than older ones (because they have less extensive root systems)? What about density? Are dense populations more or less susceptible to desiccation as individuals compete strongly for water?

Whatever the cause, this site illustrates that woody plant encroachment doesn't always end in permanent site occupation. Trying to figure out where it does would improve our understanding of both the process, and the management response.