Wild Side: Answers about plants

Wild Side: Answers about plants

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How do plants grow in inhospitable places?

A plant pushing its way up through sand. — Photo by Danielle Zerbonne

The great outdoors can produce baffling mysteries. MV Times Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to onisland@mvtimes.com.

I don’t get this: plants that grow in seemingly inhospitable places… saltwater, sand, sidewalk cracks…

How does this happen? How do they survive?

The whole answer:

Plants have evolved a huge arsenal of methods to allow to them to inhabit inhospitable spots. But the first question is, why do they want to live in inhospitable spots in the first place? The answer to that one is, simply put, to avoid competition. If you can grow under conditions that would kill most other plants, then you don’t have to worry so much about being shaded out by a taller plant, or having one with a bigger root system suck up all the water and nutrients.

I could go on for weeks about the various approaches plants take to make adversity less adverse. Here are just a few examples to illustrate how creative evolution has been.

Taproots. A long, thick central root plunging deep into the ground takes a lot of resources to grow. But once you’ve got one, you can reach moisture in the soil that is too deep for other plants to reach. You’re resistant to being pulled out by an animal that wants to eat you. And if a grazing animal does bite off your leaves, you have enough energy stored in your tap root to begin resprouting instantly. The ubiquitous dandelion is a great example, so successful largely because it produces a hefty tap root.

Thick skin. Many plants that grow in harsh environments have developed a thick skin, called a cuticle, on their leaves and stems. This tissue, usually waxy and impermeable, helps prevent unnecessary water loss that could kill the plant. In addition to keeping water in, a thick cuticle can help keep harmful substances out. Many plants adapted for life by the sea, for example, are resistant to the toxic effects of salt spray by virtue of a thick skin. And a thick cuticle can also help protect more delicate internal cells from damage from relentless sunlight.

Seasonality. In most places, conditions vary widely during the course of the year. And it’s not just the physical surroundings — the weather, amount of wind, or day length — that change: the amount of vegetation varies, too. So many plants have simply evolved a life cycle centered early or late in the season, when temperatures are moderate and when few other plants are active competitors. A great example would be the wood anemone, a little white wildflower of our woodlands that puts up leaves and blooms early in spring, before the trees have leafed out. In this way, the anemone gets the energy-intensive part of its life cycle over with before trees start hogging all the incoming sunlight.

You get the point: in the natural world, adversity represents opportunity for organisms that can find a way deal with the situation better than most competitors do. It isn’t that the well-adapted plants don’t suffer in inhospitable settings. It’s just that they suffer less than their competitors do, and that works out to be an advantage.

Seven-spotted Ladybug on beach plum.

These colorful but homely creatures are the larvae, or immature stage, of a ladybug. As most gardeners are aware, ladybugs are highly beneficial insects (at least from the human perspective) because both adults and larvae eat a wide range of insects that are harmful to plants. They specialize in preying on soft-bodied insects such as aphids, scale insects, and mites, which suck vital juices from plants and can also carry disease. Most ladybugs pass through four distinct larval stages as they develop, before morphing into an adult, and the energy source that powers all that growing and molting is the caloric content of a ladybug’s prey.

There are somewhere around 500 species of ladybugs in North America and, to make a rough guess, perhaps 20 species on Martha’s Vineyard. These particular larvae appear to be those of the seven-spotted ladybug, which is a Eurasian species that has been introduced widely in the U.S. to help control agricultural pests. It is now widely established — so widely, in fact, that it may be out-competing many of our native ladybugs, such as the two-spotted ladybug that is the official state insect of Massachusetts. (I bet you didn’t even know we had a state insect!) Many, perhaps most ladybug species are mildly toxic, producing chemicals that give them a foul smell and taste. The bold, distinctive patterns these beetles show — black spots on a red or orange background, or in some cases black spots on red — serve to warn away would-be predators that have had first-hand experience trying to eat a ladybug.

The plant the larvae are on happens to be a beach plum, but this is not just coincidence. Beach plum, like its close relatives the shadbushes and wild cherries, is notorious for attracting sucking and leaf-eating insects of a wide variety. The plants of this family, in other words, furnish a prey-rich environment for ladybugs, and as a result are good places to look for these most excellent beetles.

Luna moth attracted by light.

You think a porch light is attractive to moths? Try putting out an ultraviolet “black light” sometime! Many kinds of moths just can’t resist black light; in fact, biologists studying moths routinely use ultraviolet light sources to attract moths for collection or observation. But while the tendency of moths to fly toward light, and toward ultraviolet light in particular, is well known, nobody has come up with a fully satisfactory explanation. Most likely several factors play a role, and it’s also probably that different moth species respond to light for different reasons. And it’s worth keeping in mind that, for most of the evolutionary history of moths, all or most of the light at night came from the moon, stars, or the sky glow before sunrise and after sunset. So today’s situation, with artificial light sources popping up everywhere in moth habitat, is not what moths evolved to experience.

Moths are simple animals, and their needs in life boil to the usual basics. They need food, they need a mate, and it’s helpful to have some way of finding their way around. Explanations have been offered that relate to all three of these needs. For example, many adult moths feed on nectar from flowers. And many flowers glow under the ultraviolet let that makes up part of incoming sunlight (it’s the flower’s way of advertising for pollinators to come visit). So there may be some primitive tendency in moths to fly toward ultraviolet light in expectation of a meal. This tendency, then, would bring moths in to human light sources, at least ones that emit part of their energy in the ultraviolet range.

Many kinds of moths may also patrol large areas to find mates, and in order to be efficient about it, they would want to avoid going over the same area repeatedly. One way to do this is simply to fly in a straight line, and some moths may use the moon or bright starts to help this. If you head towards the moon, or keep it on, say, your starboard beam, you’ll keep moving in a roughly straight direction. But if you try this with a light source that is much closer than the moon, you either arrive there and then don’t know what to do next, or you end up circling the light in an effort to keep it at the same angle. Both mechanisms could bring moths to light.

Finally — and I get in over my head on this one — some biologists noted a similarity between the frequency of ultraviolet light and the resonant frequency of some of the chemicals, called pheromones, that moths produce to lure in potential mates. In other words, ultraviolet light somehow reminds moths of the irresistible smell of a mate, even though one stimulus is a visual one and the other is a scent.

So take your pick: any, or all, or none of the above. Many kinds of moths are attracted to light, and that’s really all anybody knows for sure. But there is also agreement that in today’s environment, moths are paying a price for their fondness for light. The moths around your porch light are essentially wasting their time, hanging around in a sterile habitat that is unsuitable for them to live or reproduce in. Disruption of moth life cycles by artificial lighting is a real problem, and it’s one you can help with by turning outside lights off unless you are actively using them.