What drives plant diversity?

Plant biodiversity is invaluable because it balances ecosystems, protects watersheds, mitigates erosion, moderates climate, provides shelter for animals, and is a resource for new food crops and medicines.

The first vascular plants evolved about 420 million years ago. Over 350 thousand species of vascular plants are currently known, but there are many more actors on the stage than roles that can be played.

In a new paper, Roderick Hunt of University of Exeter and Ric Colasanti of Bournemouth University attempt to find a minimal set of conditions and processes that can support long-term, high plant biodiversity.

To represent plant life the authors used 19 functional types, each representing a broad grouping of plant species that shares the same role within ecosystem function.  The functional types are represented using the C–S–R classification of plant species: C (competitor), S (stress-tolerator) and R (ruderal). There are many intermediates, but at each of the three extremes Competitive plants have a high probability of vegetative growth under conditions of high resource and low disturbance; stress-tolerant plants have a high probability of surviving low resource and low disturbance; and ruderal plants have a high probability of growth from seed under high resource and high disturbance. There are no plant types that can survive low resource and high disturbance. Each of the 19 functional types occupies its own characteristic part of C-S-R space.

The authors used cellular automata (CA) methods of modelling to find a minimal set of conditions and processes that can support long-term, high biodiversity. The CA characterizes the growth, maintenance, and regeneration of each plant type with respect to its sensitivity to variation in resource availability and physical disturbance.

“We theorized that three drivers would be necessary to support plant diversity” says Hunt. “These were the existence of a range of functional types, heterogeneity in the availability of resource and disturbance, and invasion by propagules from an external source.”

Simulations began with virtual plants representing all 19 functional types in equal proportions. Environmental drivers in the form of resource and disturbance were then randomly applied, alone or in combination, over 1,000 life cycles to determine whether each plant would maintain its presence, grow, or decline. The biodiversity of each outcome was measured.

The CA model confirmed that realistic, long-term patterns of plant biodiversity could be sustained by including a range of functional types, by manipulating resources and external disturbances, and by allowing invading propagules. While each of these drivers was individually able to promote plant biodiversity temporarily, all three were necessary simultaneously for its long-term maintenance.

“Computer modelling allows us to reproduce complex community dynamics over a multiplicity of generations. It is rarely possible to explore the emergence of high-level evolutionary processes like these in the physical world” explains Hunt.