Grass: a quiet accomplice

Various species of grass growing together in a grassland area.

Grass has been around for 30 million years. There are 9 700 species of grass of which 967 occur in Southern Africa with 329 endemic species. [1] This makes grass the fourth largest plant family on earth. From a day-to-day perspective the importance of grass to humans is often underestimated. One needs only to think what they ate for breakfast this morning or dinner last night and you will realise that grass is an important part of every meal. Rice, oats, maize, wheat, barley, rye, sugar and more are all grasses and are an important cornerstone of daily diets. Even the animals that we eat are dependent on grass, therefore we are indirectly getting the energy they have converted from the grass. The role of grass in the energy cycle and nutrient cycle is extremely important.

Due to the low growth parts of the plant, grass is able to withstand fire, drought and grazing. Using these destructive forces to gain advantage over other plants growing around it.

Of the grasses the most important in terms of contemporary human culture is the grass we know as wheat. Wheat, a prime agitator of the Neolithic revolution, was first cultivated in the Fertile Crescent at least 10 500 years ago. This ultimately resulted in the success of humans over other species. Wheat enabled humans to spread out and sustainably increase their population densities into Europe and Asia and eventually the entire world. The domestication of grass also led to the domestication of livestock and this further contributed to the success of humans. [2]  

Grassland scene consisting of a few types of grass. There are grasses filling all the niches in the grassland ecosystem. Some are bushy, some grow low to the ground, while some form dense stands.

Today grass is widely cultivated due to human activity. Sugar, maize, rice and wheat are the most commonly cultivated grasses globally. The amount of land that is under the cultivation of these grasses is enormous. The environmental impact and socioeconomic implications of the widespread cultivation of grasses is significant and very interesting to think about. From an ecological perspective it is clear that globally biodiversity is negatively affected because of these crops.

These burnt out trees are testament to a past fire. These trees were over five years old. No longer saplings, yet they succumbed to the flames. 

From the perspective of the grasses however, their success is really astounding. They have, in a sense, managed to use humans to spread globally and colonize land that they would never naturally be able spread to. They managed this trick by offering a high-energy sugar in exchange for worldwide cultivation. The relationships of these grasses to humans can be seen as one of obligate mutualism. Humans would not be able to continue the way they currently live without these grasses and the grasses, such as wheat, rice, maize etc., as they currently exist would not be able to survive without people.

This Aloe will survive the fire damage, its point of growth is well protected by its fleshy leaves and compressed growth form. The Acacia saplings that grow around this aloe have burnt and most will not survive.

Despite getting some help from people, grasses have done very well for themselves. They have been around for about 30 million years. The origin of grass must have been humble: a small plant adapted for dry conditions competing with other desert scrub plants. Over time more species appeared and began to compete with and replace dry woodland areas. About four million years ago grass had begun to produce savanna. [3] Today approximately 40 percent of the earth’s terrestrial surface is covered by grass. The success of grass has to do with the co-evolution of grazing animals, the resistance to frost and the ability of grass to use fire to its advantage as well as the ability of grass to be drought resistant.

Even on rocky hills like this, over time grass will replace the woody plants.

The growing points of grass are low on the plant and these are often below the ground. With other plants, the growth points are above ground and susceptible to fire damage as well as frost damage. Over time fires and frost will eliminate the small trees and shrubs and favour the grasses as dominant on the landscape. Within any grassland system there will be many other plants that are not grasses that are adapted for the hostile conditions of growth created by the grassy cover.

Euphorbia clavarioides is a herbaceous plant adapted to grassland areas. 

Xerophyta retinervis, this is another plant adapted to grassy areas. The stem is fire resistant and can survive sustained drought and spells of frost.

The ability of grass seeds to lay dormant in or on the ground is the way in which grass overcomes the problem of drought. Most trees and shrubs are not drought resistant and over time the grassland will grow and these plants will be displaced. In well established grassland, trees and shrubs often only grow in protected areas and drainage lines.

Some trees are specifically adapted to grassland areas. This Cussonia paniculata has deeply fissured and corky bark. This offers it some resistance to fires and cold. The other trees in the photograph may have survived the fire, but they too will eventually be replaced by grass.

Africa, because of the large areas of grassland and associated savanna is home to 75 different bovids and of these 72 are antelope. This is an incredible diversity. These animals came into existence about 25 million years ago which is well within the era of grass. Even though not all antelope are grazers, they are just an example of the kind of diversity that a rich food source can sustain.   

The Blue Duiker, Cephalophus monticola, one of the smallest antelope. This is a forest species which browses on fallen fruit and leaves. Even though it is not a grazer this species indicates the diversity which has been enhanced by grass.

 

 

The Eland, Tragelaphus oryx, is the largest antelope. This animal which can way up to 940kg is a major consumer of grass. Even though they do browse, grass makes up a substantial portion of their diet.

On a global level, grass plays a very important role. There is a theory that the expansion of grassland globally has played a role in the cooling of the earth over the last 30 million years. The theory is summed up as follows by McCarthy and Rubidge (2005) in the following way:

Grasslands store far less carbon than forests in actual plant material, but grassland soils are usually far richer in carbon than forest soils, so grasses effectively remove carbon from the atmosphere and store it in soils. This may in part be due to frequent fires, which produce copious charcoal that is not easily decomposed by bacteria and becomes incorporated into the soil. […] Grasslands are much lighter in colour than woodland and they reflect a greater proportion of solar radiation into space, contributing to cooling. The air over grasslands is generally much drier than over woodland because trees tap deep-water sources and pump the water into the atmosphere by transpiration. Water vapour is a powerful greenhouse gas, so more grassland means drier, and thus cooler air. (The Story of Earth and Life. Struik: Cape Town. Page 261.)

This grassland scene captured in the Suikerbosrand Nature Reserve clearly shows how much lighter grassland is compared to woodland. Note how shrubs and trees are limited to protected areas.

The rise of grasslands led to a decline in forests and many animals had to adapt to this change. Grazers emerged and colonized the grasslands and carnivores followed. Human evolution was also significantly affected by the domination of grass plants. Paleoanthropology generally accepts that the ancestors of modern humans left the forests to become savanna creatures, and that this move from forests to the grassy plains was what resulted in bipedalism. [4]

Grass has been an important factor for the emergence of the human being. It played a role in the primordial past when hominids were still finding their legs and it played a major role in the development of modern culture through the early beginnings in the Neolithic period. It has also produced partner animals on which we are reliant and which have also helped humans rise to the point where we now find ourselves. On a global scale, grass may be responsible for cooling our planet and keeping the temperatures at an optimal level. Without grass, humans would not have developed, as the forests would not have receded forcing our distant ancestors to colonize the open areas.

Our culture and biology is so closely tied to grass that the global colonization of the human animal would not have been possible without the domestication of specific species of grass. One thing however is for certain, when humans are long gone, grass will continue to quietly grow and be a powerful source of energy driving biological diversity and sustaining a multitude of organisms.

This harvester termite, Hodotermes, is a major grazer of grassland. Insect consumption of grass is greater than that of mammalian  consumption in natural areas. These termites diets consist of 94% of grass and they can consume 1-3 metric tons of forage per hectare per year. 

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[1] van Oudtshoorn, F. 2012.  Guide to Grasses of southern Africa. Briza: Pretoria. Page 10.

[2] For a more detailed and very stimulating discussion of the role of wheat in human culture see Jared Diamond’s book Guns, Germs and Steel.

[3] McCarthy, T and Rubidge, B. 2005. The Story of Earth and Life. Struik: Cape Town. Page 261.

[4] Anyone can try a simple experiment to see that advantages of bipedalism first hand. Next time you are in a grassland environment, when surrounded by long grass, go down on your hands and feet and see how different the environment is from down there. The adaption of standing on ones rear feet provides security as well as a vantage point from which one can see the surrounding environment, thus contributing to the success of the species.