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.

Thursday, 29 March 2012

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?

Saturday, 24 March 2012

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/

Sunday, 4 March 2012

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.