Dispersing from point A to point B, particularly when point A is hundreds of kilometres from point B, is probably not something one would normally consider plants to be capable of. Often when we think of mobility, we think in terms of the individual–the Canada goose migrating south, the white-tailed deer leaving a wooded area and entering an old field, the bull elephant traversing its great home range in search of a mate.
Granted, you’d be hard pressed to find an individual plant able to pick up its roots and head south for the winter. However, when we consider populations as the unit of interest, the mobility situation for plants is quite different. Plant populations actively leave certain areas and enter others. As environmental conditions change, a given area may no longer be amenable to the growth of certain species. Past climate change events (e.g., ice ages) are good examples of situations in which plants have migrated out of certain areas. Examining the historical pattern of plant distribution in the landscape provides interesting information about past climatic conditions in an area.
Most of us can, of course, grasp the idea of how a given plant species might leave an area: easy, it dies out. But how exactly is the entre performed? When an area that has been burned or logged begins to come back to life, where is this life coming from? Seeds are at the heart of the story. Every plant has some mechanism by which propagation occurs such that future generations are produced and the species is given the chance to live on. And many plants have specific mechanisms designed to allow for dispersal of their seeds to areas quite far from the parent plant. This helps to ensure genetic diversity, and provides the opportunity for seeds to arrive, by chance, in new areas that may be appropriate for colonization, thus expanding the range of the species.
Of course, rooted as they are, plants don’t have the ability to move their seeds around the landscape on their own. They rely on other factors to do this for them: seed-dispersal factors may be biological or non-biological, or both. As we’ll see, animals play a big part in seed dispersal, but so do more basic elements like wind, gravity, and water.
Plants that rely on animal-based dispersal mechanisms display one of two traits: they either offer something to the animal from a nutritional standpoint, or their seed-dispersal mechanisms have to be a little more underhanded, getting the animal to transport the precious parcels unawares.
Acorns fall into the former category. Oak trees produce a tasty and nutritious offering–the acorn–which seed predators like squirrels and blue jays simply can’t resist. Luckily for the oak tree, these animals are long-term planners: they collect and cache (by burying) a great number of acorns for later eating. Often burying the acorns in areas ideal for germination, these animals can forget up to half of the seeds they’ve stored away. Many of these buried acorns are deposited in locations at a good distance from the parent tree, thus aiding in a very real way in the movement of oak populations throughout the landscape.
Of course, not all plants that rely on animal dispersal offer their helpers such sweet rewards; in fact, some plants? animal-dispersal mechanisms can be a downright drag for the furred or feathered individual involved. As many of us have likely experienced, burrs have a way of grabbing on, and holding on. What is it about burrs that gives them such sticking power? A seed’s morphological characteristics (a.k.a its physical traits) often give quite a bit of information about how it might travel from point A to point B. This is certainly the case with burrs, in which those all-too-familiar little ball-shaped seed cases are covered in visible hooked spines–just perfect for adhering to fur, clothing, hair (!), or anything a hook can grab on to. And, given the ability of a burr to become impossibly entangled in fur, feather, or hair, an animal may carry the plant’s seeds quite a distance, aiding significantly in this nuisance plant’s distribution.
Relying on wind dispersal may not seem to necessitate the same level of sophistication as animal-based dispersal requires; but plants have developed very intricate mechanisms for making the most of this dispersal factor nonetheless. Take the common milkweed: once mature, its pods burst open and release hundreds of small, lightweight seeds, each one attached to a number of long, silky hair-like plumes. Even slight breezes are able to set the light, plumed seed in motion; and once It’s on the go, the seed’s plumes act as a parachute, billowing out and allowing the seed to stay windborne for surprisingly long periods of time–and thus surprisingly long distances.
And where would the world be without water? Well, without coconuts, for one. With their hard, protective shell and hollow interior, these tasty treats are able–once they’ve matured and fallen from the tree with a little help from gravity–to float in whatever body of water they may have been lucky enough to land in, and disperse to distant lands by way of ocean or river currents.
The diversity and adaptability of living things never ceases to amaze. Seeds, with their remarkable ways and means of moving throughout the landscape, provide just one more reason to keep on marvelling at the natural world.