Note: This is a science outreach piece belonging to a series I wrote for the newsletter of the Fredericton Botanic Garden. I’d be happy to see it modified for use elsewhere and so am posting the text here under a CC BY-NC 4.0 license. If you use it, though, I’d appreciate hearing where and how.
It’s spring, and our Garden is beginning to turn green. That sounds utterly unsurprising; and yet, lurking in that simple observation is one of the deepest mysteries in the science of ecology. Why, exactly, is the world green?
I don’t mean why the colour green. That we more or less understand – green wavelengths of light are reflected, not absorbed, by the chlorophyll plants use to capture light for photosynthesis. What I mean instead is this: given that every plant is fed upon by herbivores (both insect and vertebrate), why do we see such large standing crops of green plant tissue? Why don’t herbivores nibble every garden to the ground? This does happen occasionally, for instance, when invasive lily beetles find your lilies, or when an outbreaking pest like spruce budworm reaches its population peak. But it’s rare enough that the world is, in most places and most of the time, green.
The persistence of plant populations in the face of herbivory has puzzled ecologists for a very long time. We don’t have a simple answer. There are two major possibilities, although neither is a slam dunk:
- It may be that the world is green because herbivores also get eaten. The logic here is that herbivore populations can’t overexploit their host plants because their own populations are kept in check by predators. If this is correct, we can make a simple prediction: when predators are removed, herbivores should increase and plants decline. This certainly happens sometimes. In eastern deciduous forests, for example, humans have suppressed wolves and cougars, and white-tailed deer populations have exploded. The consequence: devastation of plants on the forest floor. But predators can’t be a universal explanation, because we can turn the same logic on cases where predators are attacked by predators of their own. In such a case, top predators should suppress the intermediate predators, leaving herbivores unchecked to overeat their plant hosts. Such four-member food chains are not uncommon, so how do plants at their bases survive?
- Alternatively, it may be that the world is green because plants taste bad. This isn’t always intuitive to us, because we like our fruits and our salads, but most plants are well defended against herbivores. These defences can be physical (like thorns or toughness) or chemical (with a myriad of chemicals distasteful or toxic). So perhaps the green world isn’t the buffet for herbivores that it seems. But (and you knew a “but” was coming): why don’t herbivores simply evolve to circumvent these defences? We know many examples of such evolution; think about monarch butterflies feasting on toxic milkweeds, or giraffes stripping leaves from thorny acacias. The evolutionary process of defence and counter-innovation is an arms race, so we need to explain why plants are winning. Perhaps the nature of chemistry is that it’s cheaper to make a toxic chemical than it is to detoxify it, but it’s not obvious why this should always be true.
Actually, there are two mysteries here. The first is why the world is green. The second is why, given decades of work by a lot of very smart people, we haven’t solved the first one! That second mystery, though, shouldn’t really be unexpected. Complex systems show remarkably complex behaviour, and our natural world is one of the most complex systems we know – with thousands of interacting species in a spatial mosaic living under ever-shifting physical conditions. Among the complexities: some plants remain unexploited because they are difficult for herbivores to find, especially in diverse communities (plants in monocultures tend to be much more vulnerable). Herbivores often need to match the timing of plant development, so plants can grow green before herbivores catch up. And even within species, plants can vary substantially in defence (and herbivores in countermeasures). An apparently simple question like “why is the world green”, it turns out, is really not simple at all. So please enjoy our Garden’s greenness, and marvel at the complicated and fascinating biology that makes it so.
© Stephen Heard (email@example.com) June 8, 2017, but licensed CC BY-NC 4.0. Photo: The Hal Hinds Memorial Garden, courtesy James Whitehead & the Fredericton Botanic Garden.
To read more: a recent scientific paper reviews this question and its possible answers; see http://dx.doi.org/10.1080/17550874.2016.1178353.
Other Botanic Garden newsletter outreach pieces:
- The biology of plant galls
- Nature’s pharmacy? Medicinal plants in the Garden
- Why goldenrods don’t make you sneeze: the biology of pollination
- How plants prepare for winter
- What’s in a (Latin) name?
- Spring, light, and strategy on the forest floor
- The garden of insects
- There’s fungus among us
- Life under the snow
- The botany of henna