Imagine an episode of CSI-Silurian. The team of detective-scientists are investigating a case of wholescale graffiti in the middle Silurian, and they’re looking for fingerprints. Someone – or something – has taken a knife to the land and carved out a network of rivers and streams.
The usual suspect is quickly identified: the climate. In particular, climate’s main handy-man rainfall. But the detective-scientists suspect something more sinister, with the arrival of vascular plants on the scene. Could meagre plants have contributed to the large-scale river patterns?
This very question has been asked by geomorphologists for a while. Bill Dietrich and Taylor Perron put a spotlight on the question in a 2006 Nature review article. Asking if life leaves a tangible geomorphic signature, their main conclusion was that natural ecosystems probably don’t create any novel landforms – no hillvers or gullakes – just that they change the magnitude and frequency of those landforms that already can exists – hills, rivers, gullies and lakes.
But the question remained: How do the magitudes and frequencies change? And do these changes bear a uniquely biological signature?
In a recent paper of mine, Collins and Bras (2010), I argue that if you look at river systems from arid to humid climates you will see a uniquely biological signature. In particular, as arid climates give way to semi-arid, the number of rivers dissecting a landscape would decrease because the expanding vegetation offsets the erosion power of increasing rainfall. But as semi-arid climates give way to sub-humid and humid, rivers begin to dissect more of the landscape, because the heavier rainfall now overwhelms the resistive plants.
This geomoprhic pattern has been observed many times, but the biophysical chain of events had not been pinned down in as much detail. This detail was possible because of the use of a numerical biophysical model, albeit a simplified one. If there is one single culprit here, it is plant transpiration, which fundamentally alters the landscape’s water balance and thus its erosivity,
Despite their comparatively small stature and life expectancy, then, the evolution of vascular plants in the middle Silurian probably started to alter the trajectory of landform evolution, and fundamentally so. While I did not focus on this point in my paper, it is an easy inference to make.
Dietrich WE, & Perron JT (2006). The search for a topographic signature of life. Nature, 439 (7075), 411-8 PMID: 16437104
Collins, D., & Bras, R. (2010). Climatic and ecological controls of equilibrium drainage density, relief, and channel concavity in dry lands Water Resources Research, 46 (4) DOI: 10.1029/2009WR008615