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.

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!