Monday, June 2, 2014

Gonna hitch a ride...on a duck

image: Deviant Art
What if you lived on a small island, and you were out of food or just felt like finding some companionship?

And there were other islands nearby, but you couldn't swim?

And let's say there were gigantic sea gulls that flew among the islands regularly, so you could just grab onto them and hitch a ride to the next island?

And maybe a few monkeys or dogs did the same thing? Stay with me here -- this is, in fact, how a number of little creatures get around.



Movin' Out


The food, territories, and potential mates in the areas where animals are born and raised are usually already claimed (by, say, their mom and dad, or their neighbors, or their older siblings), so at least some of the individuals in a given population will have to move around the landscape or seascape if the species is to survive long-term.

It's a tough world out there, and animals (or plants) have figured out many ways to disperse, or move within and between areas with the resources they need to survive. But the road isn't always usually easy.

There may be ecological barriers, such as the wrong habitat, to cross, which requires an animal or plant (hint: seed) to travel either on its own or with the help of some other force, such as wind, water, or other animals.


Hitching for fitness: go with the gull


You can tell Darwin was an observant guy
by those intense eyes
image: Martin Argles/Guardian
You can thank Charles Darwin for inspiring this post. He did more than "just" figure out evolution by natural selection. He tested various natural phenomena, and he may have been the first to propose that tiny critters make their long-distance movements on birds' feet.

Yep, birds are the long-haul trucks for a number of tiny animals that hitchhike their way among wetlands and tidal pools, hungry for snacks and sex.

Darwin figured out this phenomenon out when he suspended a stuffed duck’s foot in an aquarium and observed that the larvae of snails these small invertebrates latched onto it and held on for many hours, even after he removed it from the water.

Why did he do this? He was a curious guy who wanted answers to his many questions about nature!

Shorebirds (e.g. gulls), wading birds (e.g. herons and egrets), and swimming birds (e.g. -- ducks and geese) eat the small invertebrates (those critters with no backbone, like shrimp, snails, and waterbugs) that live on our coasts, tidal pools, estuaries, and wetlands.

Yet some really tiny critters have figured out how to use the birds for transport, hitching rides on the birds' legs, feathers, or even their bills (this has a scientific name, it's called ectozoochory). Some even manage to survive a trip through the birds' intestines and restart their lives where the bird next touches down and poos (it's called endozoochary).

a rotifer: the world's smallest animal
photos: Chloe Givens
a seed shrimp
photo: Utah State University
a water flea, tiny crustacean
photo:Hajime Watanabe

Recently, a research team explored Darwin's hypothesis, studying these little invertebrates (think tiny crustaceans, insects, and other leggy critters < 1mm long, as well as microscopic animals such as rotifers) that live in isolated rock pools attach themselves to the legs of gulls and hang on for dear life when the birds take off.

as tidal pools dry up,
tiny invertebrates must find new ones
photo: NOAA- Nancy Sefton
The little inverts can't cross dry land by themselves. Happily for them, though, the gulls often fly to another rock pool, where the hitchhikers hop off and settle in to see what luck has brought them.

And this mode of transport seems to be common: numerous species of these little invertebrates use bird commuter buses to move among water sources. The study found that the communities of these little creatures in neighboring, but isolated, rock pools in Maine in the U.S. were similar to each other (low beta-diversity for you ecologists). Pools with the most bird movements contained more of the available invertebrate species (the most homogeneous invertebrate communities).

Not to be outdone, snails may move quite long distances (crossing the Central American isthmus from Pacific to Caribbean, or vice versa) either inside or attached to shorebirds. In the same way that land animals arrived at Hawaii, the Galapagos, and other remote volcanic island chains, a few of these little marine animals have crossed the land barrier to make their fortunes on faraway shores.


cool coot feet with scalloped toes
photo: KC Tang

raising a new generation of invertebrate taxis Eurasian coots,
owners of those crazy feet
photo: David Castor, Wikimedia


Plants move too!


And Yes, all this applies to plants as well. Even though plants can't move, their seeds can, and must, through passive dispersal by some other force.

dispersal strategies for different types of (North American) seeds.     image: Encyclopedia Brittanica

Seeds or seedlings often die if they fall right below their mother, so even plants have found ways to get their seeds to land and start growing somewhere else. Small seeds with wings (called samara) might be blown away by the wind; larger seeds are often eaten and later deposited by animals. Either way, the new seedlings won't have to compete with mom for light and nutrients, and any animals that eat or infects that plant species will have to search harder to find the seeds or seedlings if they are spread out across a wide area.

The movements of birds (and mammals) that carry seeds in their guts or stuck to their bodies also influence where the various plant species lucky enough to hitch a ride will germinate. Across the globe, animals are essential vehicles for dispersing seeds of aquatic, savanna, and forest plants, but that's a subject for another post.


Careful: all that observation and world travel
can really take its toll on your skin
photo: Richard Milner Archive

Curious like Darwin?  Have a look--


Here is the original scientific paper describing the latest findings on how the movements of gulls affect whole communities of small invertebrates in rock pools

Or you can try an easier read about the findings in this article in The Economist

Here is a nice review of research on the dispersal of aquatic organisms by waterbirds

Here's a summary of how snails move fast when attached to birds

For all you real invert fans: a video about those perky little rotifers






whistling duck worries: do I have bugs on my feet???
Photo: David Iliff. License: CC-BY-SA 3.0

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