Cover image: all animals move; some move a lot more than others. © Aljabakphoto, 2015
Welcome to the eighth instalment of 100 Key Concepts. Last time, we looked at the strategies organisms use to survive. Now, we’ll explore movement, which can be key to survival.
65 MOBILE AND SESSILE
If an alien were to categorise Earth’s animals, they might start in any number of ways: those which have eyes and those which don’t; those with armour and those with soft bodies; or those which move around and those which stay put. Mobile animals are capable of moving around, though not necessarily of choosing a direction. Sessile animals are rooted in place, and can’t move. The ability to move determines what kind of lifestyle an animal can have. A goose can fly hundreds or even thousands of kilometres in search of fresh grass to eat, but it has to cope with a host of dangers: unfamiliar predators, unrecognisable landscapes, and constantly changing weather patterns. Meanwhile, a sponge, placidly sitting on a coral reef in a shallow tropical sea, usually has to deal with only minor changes to its environment, but needs to find all of its food, and mating opportunities, in one place. Some animals explore both options during their lifetime: you might be used to the idea of a sea jelly floating gently in the current, in the medusa phase of its life cycle, but they spend their childhood as polyps anchored to the sea floor, looking very much like their close cousins, the sea anemones.
If an animal is mobile, it’s almost guaranteed that it will undergo dispersal at some point in its life. Dispersal is the movement undertaken by an animal, specifically for the purposes of breeding. Young animals leave the place they were born through natal dispersal; adults move to their breeding-grounds through breeding dispersal. This movement allows for the flow of genetic material between different parts of a population, providing it with structure. Many animals show sex-biased dispersal, where one sex travels further than the other. In the scrub-forests of Florida, you might catch sight of a flash of blue feathers belonging to the (aptly named) Florida scrub jay. These birds breed cooperatively, with young from previous nesting attempts sticking around their parents to care for their younger siblings. Generally, females travel further from the nest when dispersing, while males stick close by, in hopes of inheriting their father’s territory. In elephants, the reverse is true: most females stay in the herd they were born in, developing intense emotional bonds with their relatives, while males spend most of their time alone, wandering between herds and looking for mating opportunities.
Despite the similar-sounding name, dispersion is very different to dispersal. Dispersion is an adjective: it describes how organisms are distributed within their environment, which determines how individuals interact with each other. There are three general types of dispersion. The most common is clumped dispersal, where organisms stick together in patches, like fish on coral reefs or zebras around watering holes. Then there’s uniform dispersion, where organisms are all the same distance away from each other. Many seabirds, like gannets and penguins, show uniform dispersion in their nesting colonies, building their nests just far apart to avoid being pecked at by neighbours. The rarest form is random dispersion, which can be difficult to tell apart from a very diffuse, clumped dispersion. Trees growing in a forest are often randomly dispersed.
Like many terms, migration has a much more specific definition in ecology than it holds in the dictionary. Migration explicitly refers to directional movement undertaken by animals for a purpose – so wandering around looking for a place to rest wouldn’t count. Migration determines the flow of animals’ life cycles, on a variety of scales. Some organisms migrate in time with the seasons, like the Arctic tern, which travels between the Arctic and Antarctica each year, experiencing summer in both hemispheres and winter in neither, an annual round trip of up to 90,000 km. Other organisms migrate in time with their life cycles: salmon, born in upland rivers, make the journey downstream and out to sea as they grow up. Years later, they famously return to the exact spot they hatched in to breed, and die after doing so.
You might have noticed a bit of a theme here. Aside from looking for food and shelter, organisms move in order to breed, and to increase their chances of doing so successfully. Next time, we’ll be looking into the world of family life: how organisms reproduce, and what they do afterwards.