When one plant grows taller than another it can cut out its life support system – the light that powers plant metabolism. Plants compete to capture this crucial resource by growing tall, producing large leaves and having many layers of branches with leaves to capture as much light as possible as it filters through the canopy.
Those species that can make efficient use of other resources like water, and the critical soil nutrients of nitrogen and phosphorous, are at a competitive advantage for light which is needed to grow, survive and reproduce.
There are some downsides to competing for light. Having large leaves can put a plant at risk of losing too much water through the tiny pores on its leaves as it respires. Growing tall can put a plant at risk of being eaten by grazing animals and necessitates investment in strong stems and branches to withstand wind and rain. If soil nutrients and water are abundant, however, growing tall is an excellent strategy for winning the competition for light.
When fertiliser is applied to a grassland it creates exactly the right conditions for plants to race for light. Competitive grasses are the first winners, they grow tall with lots of leaves and exclude slower-growing and shorter plants in the grassland like orchids and sedges.
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Fertilisation can therefore lead to a reduction in plant diversity as the community becomes dominated by the competitive grasses. Recent work by PhD student Caroline McKeon has shown that fertilisation and exclusion of grazers leads to rapid loss of species in an orchid-rich meadow in the Burren.
Grassland diversity
While the competition for light has long been inferred by ecologists as an important cause of the decline of grassland diversity after fertilisation and lack of grazing, a fascinating experiment in Germany has conclusively demonstrated this mechanism of diversity loss. Researchers put LED lights into the understorey of a grassland experiment and showed that plots with the lights did not lose diversity even when fertilised and where sheep were excluded.
Conservationists have long recognised the value of using grazers. Conservation grazing can be done using wild or domestic herbivores and sometimes a succession of different grazers are encouraged. In coastal Fingal, large and relatively unfussy herbivores such as Highland cattle remove large plants and prepare the pasture for the arrival of brent geese in winter, which are choosier and graze on smaller plants. Together the geese and the cattle ensure that the light-bullies do not take over the grassland community.
There can be too much of a good thing, however, and overgrazing is also a threat to diversity. If grazing is too intense then very few species can manage to grow and set seed before getting eaten, and large heavy herbivores can damage the soil through compaction, again reducing the opportunities for species to get the nutrients and water they need.
Intensive livestock farming relies on the application of fertilisers and high stocking density on pastures with one productive grass species. High nature value farmland, at the other end of the spectrum, uses low stocking densities, low or no fertilisation and results in a diverse semi-natural grassland. While both systems are grazed, thereby reducing light competition, the grazing intensity and application of fertilisers determine whether one or many species persist.
Deer vs tree
If there are no grazing animals to remove the grasses then even taller plants have an advantage, and shrubland and woodland can eventually develop if the right seed sources are close by. In natural woodland systems where predators are excluded or have gone extinct, herbivores such as deer can become over-abundant. Woodland or forest regeneration is prevented by deer eating the young native trees.
Species like rhododendron, which are not favoured by the deer, are untouched and can dominate the understorey, eventually displacing the native woodland. If diverse native woodland is the desired ecological state then, in the absence of native predators, fencing or culling are needed to give native tree seedlings a chance to establish.
The interplay of light, grazing, fertilisation and predation-shape agricultural pastures, semi-natural grasslands and woodlands, it is a delicate dance to get the balance right.
Yvonne Buckley is an ecologist, Irish Research Council laureate and professor of zoology at Trinity College Dublin