Evolution & Ecology Research Centre (University of New South Wales)

Global Scale Ecology

Plants and animals employ very different ecological strategies in different ecosystems around the world. In our lab, we collect data using a combination field work and synthesis of data from the literature. We use these data to quantify ecological patterns at the global scale.

By understanding how ecological traits and biological processes vary along present-day climatic gradients, we hope to improve our ability to predict how human-induced environmental changes (such as global warming) might influence plant and animal communities in the future.

This poster describes three of our recent projects.

GLOBAL PATTERNS IN SEED SIZE

Angela Moles (UNSW), David Ackerly (Berkeley), Cam Webb (Yale) John Dickie (Kew Gardens), John Tweddle (Nat. Hist. Museum, Lond.), Andy Pitman (UNSW), Mark Westoby (Macquarie)

We collected seed mass data for 11,481 species × site combinations from around the world.

We found a 320-fold decline in geometric mean seed mass between the equator and 60o. This decline is not linear. There is a sudden 7-fold drop in mean seed mass at the edge of the tropics.

There are many factors that might contribute to this latitudinal gradient in seed mass, such as a) the predominance of large growth forms in tropical systems, b) the denser shade in tropical vegetation, c) the predominance of biotic seed dispersal syndromes in tropical environments, d) the higher productivity of tropical systems. We quantified the ability of each of these factors to explain the latitudinal gradient in seed mass.

The strongest correlates of seed mass were plant growth form (R² = 0.44) and vegetation type (R² = 0.29), followed by net primary productivity (R² = 0.22) and seed dispersal syndrome (R² = 0.20). This suggests that there may be a general switch in plant strategy (from large, large seeded species to smaller, small-seeded species) at the edge of the tropics.

Our next project on the latitudinal gradient in plant traits will focus on maximum plant height. We already have height data for 21,000 species from ecosystems all around the world.

Further reading:

Moles, A. T.; Ackerly, D. D.; Tweddle, J. C.; Dickie, J. B.; Smith, R.; Leishman, M. L.; Mayfield, M. M.; Pitman, A. J; Wood, J. & Westoby, M. (2007) Global patterns in seed size. Global Ecology and Biogeography. 16:109-116.

THE WORLD HERBIVORY PROJECT

Angela Moles (UNSW), Lesley Hughes (Macquarie), William Foley (ANU), Lissy Coley (U Utah) and 50 others

The idea that interactions between plants and animals are more intense in the tropics underpins many of our ideas about global patterns in plant and animal traits, and about latitudinal gradients in biodiversity. However, this idea has never really been tested.

In the world herbivory project, we established 75 study sites in all sorts of ecosystems (including tropical rainforests, arctic tundras, deserts, boreal forests, heathlands and savannas). These sites spanned a wide range of latitudes, from 75o North (Eastern Greenland) to 55o South (Southern Patagonia).

At each site we recorded:
          ● herbivory on mature and expanding leaves
          ● environmental conditions (soil fertility, rainfall, temperature, canopy closure)
          ● plant physical and chemical defences
          ● invertebrate abundance
          ● pre- and post-dispersal seed predation
          ● plant diversity

We studied the four most dominant plant species at each site. This gave data for a total of 300 species.

We will use these data to:

1) Make the first direct and global test of the idea that there is a latitudinal gradient in the strength of plant-animal interactions

2) Quantify the relative importance of different biotic and abiotic factors in determining the amount of herbivory plants receive.

3) Determine whether the strength of interactions between plants and animals is a better predictor of plant diversity than are abiotic factors.

Results:

We are still processing images and plant samples, but it is clear that plants really do lose a greater proportion of their leaf area to herbivory in the tropics.

GLOBAL PATTERNS IN PLANT TWINING DIRECTION

Will Edwards (JCU), Angela Moles (UNSW) and Peter Franks (JCU)

We had three hypotheses about the factors that might influence plant twining direction:

H1: Twining direction is determined by which side of the vine first contacts a host stem

H2: Twining direction is determined by the apparent movement of sun across the sky

H3: Twining direction is determined by the Coriolis effect

If H1 was correct, 50% of stems would twine anticlockwise, 50% clockwise, regardless of location.

If H2 was correct, stems would twine clockwise in the northern hemisphere, and anticlockwise in the southern hemisphere, with a mix of directions in the tropics.

If H3 was correct, stems would twine clockwise in the northern hemisphere, and anticlockwise in the southern hemisphere, with a rapid transition at the equator.

Results: 92% of stems twined in an anticlockwise direction. Twining direction was not related to latitude or hemisphere. We therefore rejected all 3 hypotheses.

Our new hypothesis is that the global tendency for plants to twine anticlockwise might be a global scale manifestation of the chirality of all biological molecules (amino acid chirality ® chirality in tubulin proteins in cell walls, ® twist in cells, ® twist in stems). This hypothesis still needs to be tested.