There’s a lovely article on BBC Nature today about how ants sow the seeds of the Cape. Researchers have been investigating the Fynbos habitat of the Cape region of South Africa and the climatically similar south west of Western Australia. These biodiversity hotspots have a large proportion of plants that are myrmecochorous – their seeds are dispersed by ants.
Apparently this is an adaptation to poor soils. Rather than shed its seeds into infertile ground, some plants develop seeds with an elaiosome – a fleshy bit designed to attract ants. The ants collect the seeds and take them back to their nest. They eat the elaiosome and discard the seed. The soil in and around the ant nest is enriched by their poo, and the seeds gets a nice rich soil to grow in.
The photo above is of some seeds that I saw being prepared for storage at the Millennium Seed Bank. If I remember correctly (I would have to dig out my notes to check), those colourful seeds on the left have elaiosomes to attract ants.
Discussing it with Rhizowen this morning, I was told that borage seeds also have elaiosomes. I duly tripped out into the garden to have a look at my borage plants, but they have only recently started flowering, and no seed pods have formed yet. My packet of borage seeds is in Malvern, so I can’t even tip those out and have a look ;)
I imagine it hasn’t been the easiest year for ants, as it has been so wet. Most gardeners would be thrilled not to have to deal with ants nests, and they can be a bit of a pest. I used to feed the ones I found to the chickens. Princess Layer was particularly fond of ant cocoons.
Of course, ants aren’t the only seed dispersal mechanism. Many seeds are encased in fruits designed to attract birds. The bird gets a tasty treat, and the seed passes through the gut and is deposited away from the parent tree, in a handy pile of fertiliser. Chillies developed capsaicin as a defence against being eaten by mammals, whose digestive systems would harm the seeds. Birds are immune (and some humans enjoy the effects!).
Dandelion ‘clocks’ are designed to disperse seeds on the wind. I once read a report of an investigation into the best weather conditions for dandelion seed dispersal. I can’t find it now, but if I remember correctly (my poor memory is being taxed this morning!) a sunny day with a light breeze took them furthest. If it was very windy they tended to go straight up and back down. Or something like that.
If you’ve ever wondered why so many islands have coconut trees, it’s because the seeds float and are dispersed by water. They can travel hundreds of miles across the ocean before being washed up on a suitable shore.
Rodents are also good at dispersing seeds. Although you may think mice have eaten your way through your entire sowing of peas, it’s possible that they’ve taken some off to store for later. Rodents don’t have the best memories (mind you, neither do gardeners, which is why we label plants and rows) and some of their caches are forgotten and left to germinate. Although the plant has to accept some losses, as some seeds are eaten, it benefits from its seeds being moved to new locations – where some of them will thrive and start new populations.
The study of the links between animals, plants and their environment is ecology. Chris Packham recently presented a stunning tv series called Secrets of Our Living Planet. I only managed to catch two of the four episodes, but they are absolutely fascinating. In the first one we saw brazil nut seeds being dispersed by the agouti – a small rodent with the only teeth sharp enough to crack through the brazil nut’s tough outer shell. When it bores its way through, it eats some of the nuts straight away, and scurries off to bury the rest in secret places.
I’m fairly sure that there was another species that relied on elephants to disperse its seeds, its original seed dispersal animal of choice being one of the megafauna that died out. Orangutans do a lot of dispersal work, as well.
Not all seeds that depend on animals for dispersal offer anything in return. Some have hooked or barbed seeds and hitch a ride on fur (or trouser legs). These fare dodgers are fairly benign, but there are hard seeds shaped like caltrops that are designed to embed themselves into animals’ feet. Ouch.
So why did the chicken cross the road? It was playing its role in a diverse ecosystem, and had some seeds to disperse ;)
Autumn is coming and plants all across the country are setting seed, or preparing to. What seed dispersal mechanisms have you spotted in your garden?