World record plant species richness

Plant communities with very high species richness fascinate biologists. For one, the number of species in your plot/site is a bragging right. In papers about herb-rich woodlands in western Victoria, for instance, I've often included the statement that 'these communities are amongst the most species rich in temperate Australia'. Indeed, we've found up to 42 species per square meter and 105 species in 1000 m2 (see here for details).


Herb-rich woodlands in western Victoria are dominated by grasses,
herbs and  geophytes.  (Photo: John Morgan)


High species richness also fascinates biologists because the coexistence of large numbers of species is of theoretical importance. How can so many species occur in same area when no two species can occupy the same niche?

A new paper by Bastow Wilson et al. in the Journal of Vegetation Science examines the world records for species at all the spatial scales at which biologists quantify species richness. They searched the published literature on species richness in quadrats (no small task) and found that plants can really pack it in.

At small scales, the mountain grasslands in central Argentina have been recorded to have up to an astonishing 89 species per m2 . Several meadows in Romania and the Czech Republic were nearly as rich. At larger scales, the world record for the number of species was 942 species in 1-ha in a tropical rainforest from Ecuador. This makes the mind boggle!

What I found most fascinating was thinking about why such high numbers of species can coexist. Wilson et al. provide an interesting mechanistic explanation. While it is clear that grasslands have smaller plants than rainforests and hence, can pack more species into a smaller area ('the species packing effect'), the two communities are intrinsically different in another key way.


A typical grazed mountain grassland from Argentina.
(Photo courtesy of Jodi Price)


The highest richness grasslands across the globe all share a common feature - they are all subject to repeated disturbance (by mowing, grazing or fire). This leads to more symmetric competition between dominant and subordinate species and hence, slower competitive exclusion. In a local sense, we see exactly the same ecological phenomenon in frequently burned C4 grasslands (where richness is very high at small scales) compared to unburned grasslands (where richness can be very low).

Tropical rainforests, by comparison, have a more stable environment, the disturbances being mainly patchy treegap formation. Their richness has been explained by continous speciation in a stable ecosystem that favours subtle differences in life history.

This paper highlights that we still have a long way to go if we want to understand the mechanisms that underpin co-existence. It also highlights that simple biological surveys are paramount if we want to identify biodiversity 'hotspots'. Could we, for instance, readily identify where the global records for bryophyte diversity would be? And at what sampling scale would this occur?




The productivity-richness debate continues.....

A few months ago, Science published a paper by Peter Adler et al. that I helped co-author, examining evidence for a relationship between species richness and productivity. Grime (1973 Nature 242:344) was one of the first to note that a unimodal relationships between local community diversity and local resource productivity existed in grasslands in the UK; such unimodal relationships have attracted attention ever since and, it's probably safe to say, the unimodal diversity-productivity relationship is “widely accepted” in ecology. We challenged this notion by conducting standardised sampling in a replicated, global study. For herbaceous communities (mostly grasslands), in 48 study sites across five continents, we found no clear relationship between productivity and fine-scale richness within sites, within regions, or across the globe, and concluded that "rather than investing continued effort in attempting to identify a general productivity-richness relationship, ecologists should focus on ...investigating the complex, multivariate processes that regulate both productivity and richness".

Now, Science has published two technical comments on the Adler et al. paper, as well as a reply led by Jim Grace that I was a co-author on. These two papers criticise our work, in the context (primarily) of our analyses and assumptions. It's good to see that we've generated some debate about this topic, but I tend to think our study holds up despite these critiques.

Pan et al. claim that, properly analyzed, the Adler et al. data show a strongly linear diversity-productivity relationship, while Fridley et al. claim both that the data are “clearly deficient” as a test for a humped diversity-productivity relationship, and that they show a “clear” humped diversity-productivity relationship.
The links to the papers are: Pan et al.: http://www.sciencemag.org/content/335/6075/1441.1.short  and Fridley, Grime, Huston et al.: http://www.sciencemag.org/content/335/6075/1441.2.short

We reply in the paper by Grace et al. reply: http://www.sciencemag.org/content/335/6075/1441.3.short.

In short, we believe that debate over the productivity-richness relationship has been strongly influenced by the way that scientific motives influence how theories are evaluated. Attachment to particular explanations can result in a tendency to overlook inadequacies and contradictions and lead to a reliance on “theory demonstrations,” which selectively sift through data to find supporting evidence. Both Pan et al. and Fridely et al. seem to have taken this approach in their commentary of our paper. By contrast, “theory investigations” have a different motivation – to evaluate the explanatory adequacy and limitations of theories so as to improve them. This is what our original paper set out to do. Theory investigations are challenged to be either exhaustive in their examination of evidence or to rely on unfiltered samples that represent the variation nature has to offer. Given the scale of our study, and the complete absence of a unimodal relationship, we think we are on fairly solid ground.

I thought I'd leave an "unbiased" assessment of the current debate from one of my favourite ecology bloggers, Jeremy Fox (who writes the Oikos Blog - a really dynamic and informative blog about all things ecological - he's definately worth bookmarking). His commentary is a stark reminder that science should always question dogma or, as he puts it, zombie ideas:    http://oikosjournal.wordpress.com/2012/03/22/trying-to-save-a-zombie-idea/

Recent papers in the Lab - 2011

Our Lab primarily studies aspects of plant conservation ecology. Most of our work occurs in south-eastern Australia where there are many challenges to the conservation of biota: habitat fragmentation, changing climate, invasion by non-native species and altered disturbance regimes. Currently, I have students working on a variety of themes including multi-decadal vegetation dynamics, the role of endogenous and exogenous disturbance on species diversity, theories that underpin species coexistence and community assembly and plant invasions.

We've had a moderately productive 2011, as evidenced by the publications that have come out of the Lab. I thought I'd put links to them here to make them more accessible. Click on the link and that should take you to the journal website where you'll hopefully be able to download the PDF.

1. Adler et al.. 2011. Productivity is a poor predictor of plant species richness. Science 333: 1750-1753.
2. Briggs, A.L. & Morgan, J.W. 2011. Seed characteristics and soil surface patch type interact to affect germination of semi-arid woodland species. Plant Ecology 212: 91-103.
3. Firn et al. 2011. Abundance of introduced species at home predicts abundance away in herbaceous communities. Ecology Letters 14: 274-281.
4. Geddes, L.S., Lunt, I.D., Smallbone, L.T. & Morgan, J.W. 2011. Old-field colonization by trees and shrubs following land-use change: could this be Victoria’s largest example of landscape recovery? Ecological Management and Restoration 12: 31-36.
5. Morgan, J.W., Cutler, S.C. & Wong, N.K. 2011. Life-form species-area curves in temperate eucalypt woodlands. Plant Ecology 212: 1047-1055.
6. Schultz, N.L., Morgan, J.W. & Lunt, I.D. 2011. Effects of grazing exclusion on plant species richness and phytomass accumulation vary across a regional productivity gradient. Journal of Vegetation Science 22: 130-142.
7. Venn, S.E., Green, K., Pickering, C.M. & Morgan, J.W. 2011. Using plant functional traits to explain community composition across a strong environmental filter in Australian alpine snowpatches. Plant Ecology 212: 1491-1499.

Fighting fire with fire

In 2009, on the hottest day on record in Victoria, and at the end of a decade-long drought, the state experienced one of the worst bushfires since european settlement. Known as the Black Saturday fires, over 170 people died - the worst natural disaster in Australia's history. For many biologists familar with the ecology of southern Australia - one of the most fire-prone parts of the world - it was perhaps not unexpected that such a fire erupted on a day when the Fire Danger Index was literally off-the-scale.

Fighting (bush)fire with (prescribed)fire in the Top End
Photo: John Morgan

What could be learnt from such a day?

After the fires, the Black Saturday Royal Commission was established and it was to be the most comprehensive inquiry into bushfire in Australian history. Lots was discussed about fire ecology, house design, emergency warnings, fire preparedness, etc. Amongst the many recommendations, one of the most important was the recommendation pertaining to forest fuel management by prescibed burning.

The Royal Commission recommended a tripling of the annual prescribed burning in Victoria to reduce the risk of another Black Saturday. This equates to 5%, or 390,000 ha, of the state being burnt each year. This is now government policy, but there’s a fierce scientific debate over whether it will work, and the practical and ecological consequences of burning so much more of the state’s public land.

An excellent report on the ABC's Background Briefing went to air a few days ago where prominent fire ecologists - including Mike Clarke, Andrew Bennett, Ian Lunt, Kevin Tolhurst - talk about the ecological implications of setting targets for burning. It seems to me this is an arguement about "quantity versus quality" of prescribed burning and how asset protection needs to be weighed up with ecological impacts. It is well worth listening to. Follow this link  - http://www.abc.net.au/radionational/programs/backgroundbriefing/2012-02-19/3829372 -  to download the audio file. Questions we might ask ourselves include: does fuel reduction burning work to reduce the size, frequency and intensity of bushfires? What evidence is there that fuel reduction burning impacts (positively, negatively, neutrally) on native ecosystems? Which are the most resilient ecosystems to fuel reduction burning and why?

Contingencies ....in time

Everything happens somewhere. And in nature, the somewhere matters.

One of the most obvious things about the discipline I work in is that the results of one experiment or observation are not always replicated elsewhere across the globe, the nation, or even the state that I live in. Ecologists often talk about the fact that their findings are "contingent" on the organisms being studied (for example, short-lived perennial herbs versus long-lived trees will probably have very different demographies), the setting of the study (e.g. alpine versus lowland grasslands probably cycle nutrients in different ways) and the history of the system under investigation (e.g. evolutionary history and recent landuse legacies will affect successional pathways after disturbance).

This can make it all very confusing to come up with simple, widely applicable and generalisable predictions about the way in which natural ecosystems respond to climate perturbations and the impacts of utilisation. I guess that is why there has been a rapid growth in the use of meta-analyses to tease out ecological principles from spatial contingency, as well as the growth in global collaborative networks. I, myself, am loosely affliated with three such networks - NutNet, ITEX and Shrub Hub - whose chief aim is to study questions pertinent to ecologists working in similar systems across the world, using standardised protocols, to see if ecological patterns emerge beyond the local-scale.

One of the things that is also true about nature is that different things happen in different years. As someone who studies seedling regeneration processes, I know this only too well. But I think we tend to under-estimate the importance of temporal contingencies in our ecological thinking.

Just how much does the timing of an experiment or observation cloud our view of key ecological processes? If we study seedling recruitment in a decade of drought, for example, are the outcomes of such a study sufficiently robust to propagate models that forecast extinction risk? The answer is probably no. But collecting data is a hard excercise. Ask any student at the completion of their doctoral thesis about whether they want to do it all again and you'll get a stern look no doubt.

The Button Wrinklewort in remnant native grassland, Victoria
(Photo: Donna Burns)

Returning to seedlings as a case in point. I've been following the seedling recruitment of the endangered grassland daisy Button Wrinklework for almost 20 yrs now. In 1991/2 (crikey, that makes me feel old), I observed a large seedling cohort in the two populations I have followed ever since. I even wrote up a paper on this (see here) and concluded that "....... seedling recruitment is not restricted to climatically favourable years (i.e. is not episodic) but rather, is potentially on-going provided suitable microsites are available for seedling survival."

How wrong was I! My long-term observations of recruitment in the Button Wrinklewort has revealed that there has NEVER been another recruitment year like 1991/2. This probably has a lot to do with the fact that south-eastern Australia experienced a 13-yr drought from 1996-2009 where rainfall was below-average in just about every year. Rather than observe on-going recruitment, seedlings have been entirely episodic. The number of adult plants have declined in some populations and not been replaced by seedlings (whereas I had suggested declines might occur where microsite limitation limits survival; while this is certainly true, you've got to have seedlings before you have this problem). Interestingly, the recruitment event I observed in 1991/2 was very successful. Many of the seedlings that emerged in that year went on to become reproductive. A couple are even still alive today! This is what Peter Chesson calls the "storage effect" - infrequent recruits are stored in the population and these buffer the population in the intervening years / decades when recruitment is far less successful.

This simple example highlights that temporal studies are crucial if we are to adequately understand the dynamic nature of species populations and their interactions with climate and other species. Of course, this is incredibly difficult to do. Permanent plots, data archiving, and good old fashioned persistence are the keys here. These three things are also terribly unfashionable in ecology today. We need answers now to the pressing conservation questions that face our society. But we also need to acknowledge that what data we do have will be imperfect and model outputs are not final nor definitive. And we need scientists (like me) to champion the need to conduct long-term, old fashioned studies. Otherwise we potentially face a future where decisions are made on data that is temporally contingent and this could have serious implications for the future of the species we are interested in preserving. Particularly in a world undergoing a changing climate. Something to ponder!