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why is the world green?

When we stop and look around at our natural environment, what do we see: green. Unless you’re living in a concrete jungle, it’s there, whether you notice it or not. So, while climate conditions, such as rainfall allows some parts of land to be green with plant life, and other areas brown and arid, why does land persist on being green? Plants are used by a wide range of animals, so why is so much food left unused?

The answer to this question can be broken down by two explanations: top-down effects and bottom-up effects.

Top-down effects

This is where predators, who are at the top of the food chain, limit herbivores, who are at the bottom. This allows plants to persist, because the number of herbivores are controlled. Essentially, this might explain why herbivore populations might get large enough to destroy all plant life, causing a brown environment.

This hypothesis was first suggested in 1960 by Hairston, Smith & Slobodkin, who suggested that herbivores do not consume all of their food supplies because predators keep their numbers in check [1]. Except in instances where human actions removed predators leading to insect outbreaks, it seems plausible that predators maintain herbivore numbers. However, of course there are arguments to this:

Some herbivore species don’t face many predation threats, and in most cases have evolved to avoid the amount of predation required to maintain population numbers.

One example of this, is the caterpillar of the cinnabar moth: these creatures have developed striking colour displays that serve as warnings of poisonous chemicals to deter predators, such as the common cuckoo, which have allowed them to browse without protection. Because they have little trouble evading predators, these caterpillars can completely exploit their plant hosts by eating all leaves and flowers. However, even with this remarkably successful defence, they cannot change the whole environment from green to brown because they just eat a few closely related plant species.

Two studies have also looked at the larger scale effects of herbivores. In 1982, a study by Hill and others in Wales excluded sheep from patches of vegetation [2]. As a result, the vegetation inside the exclosure (like an enclosure, but with the purpose of keeping things out rather than in) grew taller than the more heavily grazed surrounding vegetation outside of the exclosure. Interestingly, there was also a notable difference in plant species composition in and outside of the exclosure.

In 2006, Terborgh and others carried out a “natural experiment” in Venezuela using islands to control for predator pressures [4]. The islands that were observed had different conditions due to their size: larger islands were able to support viable populations of medium sized mammals, such as primates, which predated significantly on the main herbivore species - leaf cutter ants, whereas some islands were too small for larger herbivores so leaf cutter ants dominated. As a result, those smaller islands with uncontrolled leaf cutter ant populations had lower densities of tree saplings than larger islands with herbivore control via predators.

However, even with the absence of major predators of herbivores we still see a green world, signifying that top-down effects do not fully explain why the world is green. Vegetation is still available in systems without predators, but the physical structure and species composition is altered. Therefore, there must be something else limiting herbivore consumption of plants.

Bottom-up effects

This is where plants themselves limit the effectiveness and abundance of herbivores, through a variety of mechanisms. Because vegetation still occurs in top-down systems, it seems that herbivore population densities are instead limited by their food supply, not predators, so bottom-up effects play a more crucial role. Plant species with low reproductivity and few defences would go extinct, leaving us with plants that can persist with herbivores, even without predator influence. So, how do they do this?

  • Physical deterrents: plants have evolved many forms of physical deterrents over time, from thorns (for example, on roses), spines, stings, and just general height that prevents herbivores from being able to reach. New Zealand has an excellent example of height in the horoeka (lancewood) tree (Pseudopanax crassifolius). The juvenile form has brown leaves with cardboard thickness and very sharp edges that prevent herbivore browsing, predominantly as a defence mechanism from moa. Once it reaches three metres in height (out of a moa’s reach) it transforms into a normal looking tree, with green foliage. This change in juvenile and adult form is known as heteroblasty!

  • Poisonous chemicals: plants often use secondary plant compounds, which are chemicals produced to deter herbivory. They work in two ways: (i) individual herbivores will poison themselves (or cease eating) in the short term if they consume too much plant material thereby leaving the rest of the plant, and (ii) long-term chemicals affect herbivore population size. Therefore, plants may be green, but it does not mean they are readily available for consumption. There are many examples of this type of defence mechanism such as phytoalexins in soybean plants which are used to deter pest species, and alkaloid chemicals which are only produced when the plant is attacked – these are commonly known to humans as caffeine and cocaine, to name a few.

  • Poor nutrient quality: plants try to make poor food for animals, even if they’re non-toxic. For example, an insect may contain ten times more nitrogen in its body chemicals than in the leaf it’s consuming, so in order to obtain enough nitrogen to maintain body levels, it must eat large amounts of plant material. This may not be worth it for the insect, in terms of time and energy in eating that much plant material, and instead will graze on a plant with a higher nutrient concentration.

  • Palatability: plants will try make their foliage less palatable, or less tasty so herbivores avoid them. If a herbivore has to work harder to gain as much energy by taking more time to search for palatable plants, it will leave less offspring. Eventually, the population size of the herbivore with shortage of palatable resources is lower than one with an abundance of palatable material, resulting in less plant herbivory. One example of unpalatable mechanisms are tannins, which are less tasty and may break down into toxic chemicals in the consumer’s gut. Another, is tough leaves produced through structural mechanisms such as lignin and cellulose, which make these plants hard to digest.

  • Spatial distribution: plants spread out over space to evade detection from herbivores. Several studies have expressed the success of this method; for example, in the 1950’s a study on two mite species (one herbivorous and one predatory) found that when food was arranged closely together the predatory mites quickly caused the prey mite population to crash and become extinct [3]. However, then the food was complexly spatially arranged, the predatory mites could not find all the prey mites before they reproduced. In the same way, insectivores may struggle to find all the plant individuals if they are widely scattered.

  • Apparency: plants may hide from herbivores. In a simple example, long-lived plants such as trees may be more easily found by herbivores than small, short-lived plants such as annuals. If a plant is more apparent, it will likely invest in chemicals that reduce digestibility (tannins, lignin) because they are likely to be found by specialist herbivores that have evolved ways of neutralising toxic compounds. In contrast, less apparent trees should invest in toxins to deter and eradicate herbivores if found.

In it’s very essence, this is why the world is green. To some extent, top-down effects from predator to herbivore to plant, controls herbivore populations that prevent a brown world. More likely, the influence of plants themselves affect the ability of herbivores to completely consume all plant material, through mechanisms such as physical and chemical deterrents, plant quality and distribution. But, it is together that these theories explain the presence of greenery in our world, with the answer lying in a combination of mechanisms. So, next time you’re outside or in the middle of a lush forest, take a look around. The last thing we want to do is take plants and our beautiful green world for granted – or, would you prefer a brown world?

[1] Hairston, N., Smith, F. & Slobodkin, L. (1960). Community structure, population control and competition. American Naturalist, 94, 421-425

[2] Hill, M.O. et al. (1992). Long-term effects of excluding sheep from hill pastures in North Wales. Journal of Ecology, 80, 1–13

[3] Huffaker, C.B. (1958). Experimental studies on predation: dispersion factors and predator–prey 36. oscillations. Hilgardia, 27, 343–383

[4] Terborgh, J. et al. (2006). Vegetation dynamics of predator-free land-bridge islands. Journal of Ecology, 94, 253–263

Lena is studying an honours degree at AUT. Her research is comparing behavioural differences of Nemo fish in captivity and the wild. As a part time job, she teaches at the university. One of her favourite pastimes is to banter about current environmental issues. However, this tends to be only with people who agree, so it’s generally one-sided.

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