Seed dispersal means the movement or transportation of seeds away from the parent plant. This science article investigates how plant seeds hitch a lift from animals, birds and humans.
There are three principal methods of seed dispersal:
1) ‘Hitchhiker’ seeds attaching themselves to feathers and fur
Seeds hitch a ride by attaching themselves to the feathers of birds such as this ‘Black Noddy’ from Australia….
to the fur of dogs….
to the bodies of cows….
… and onto the bodies of humans!
‘Hitchhiker’ seeds use a variety of methods of attaching themselves so they don’t fall off as they are being transported. Hitchhiker seeds are often contained inside prickly dried fruits called ‘burrs’. ‘Burrs’ attach themselves by different methods which include…
double hooks …
spears with barbs…
‘Spikes’ can be incredibly painful when they stick into the soles of your feet as you walk barefoot!
‘Spears'(without barbs) are another way in which seeds hitch a lift. Each of the ‘spears’ of this ‘rip gut grass’ has microscopic hairs that point backwards. They are difficult to remove after they become embedded in animal fur or human socks!
‘Claws’ are also very effective; ‘devils claws’ are capsules (pods) containing the seeds of Proboscidea plants. The seed capsules are actually dry fruits which lie around on the ground waiting to hitch a lift from larger mammals such as humans!
These dry fruit pods contain the seeds of a double clawed species of Proboscidea called the Proboscidea parviflora.
The Proboscidea parviflora is a low lying plant which produces purple flowers.
This is what the fruit of this plant species looks like as it grows.
As well as being great hitchhikers, Devil’s Claw seed pods make interesting sculptures!
2) Eating fruits containing seeds
This method of seed dispersal involves animals and birds eating fruits…
….including the seeds contained inside the fruits.
Seeds are then transported to different locations inside the bodies of animals and birds. After falling to the ground in excrement they are ready to germinate!
Sometimes smaller seeds are transported in smaller amounts of excrement….
… and sometimes larger seeds are transported in huge amounts of excrement!
In the same way that flowers attract pollinators by offering food rewards of sweet nectar….
….so plants attract seed dispersers by offering fruit as a reward for eating the seeds.
The blackberry is an example of a fruit eaten by many different species of bird.
Over millions of years birds have learnt that the only blackberries worth eating are the sweet and fleshy black ones. It is only the ripe blackberries that contain mature seeds that are ready to germinate. Birds know that the unripened green and red fruits, containing the immature seeds, will taste sour and will not be pleasant to eat.
Blackberry plants have therefore evolved a way of discouraging birds from eating immature seeds that stand no chance of germinating.
The Waxwing is a bird that enjoys a diet of fruit. This Waxwing is eating a species of berry (the rowan berry) that is fully ripened after turning red.
Waxwings vary their diet according to what fruits are in season at any time. In summer and early autumn they eat strawberries, mulberries, raspberries, blackberries and cherries; in late autumn and winter they eat juniper berries, rosehips and mistletoe berries.
It is important to the survival of the rowan tree that its fruits do not evolve to be too big. If the fruits on rowan trees grow too big Blackbirds could not swallow the fruits (or ‘berries’) and the seeds inside the fruits could not be dispersed.
Other organisms which eat fruits include Agouti rodents from South America….
Spiny Tailed Iguanas which eat red cactus fruits…
and Squirrel Monkeys from South America.
Bats are also superb seed dispersers. The excrement of frugivorous bats plays a vital role in the maintenance of species-rich tropical forests. After eating fruit, bats excrete in mid flight as they return to their roosts. Bat excrement falls all over tropical forests under their flight paths; seeds containing many different species of tropical tree fall to the forest floor in bat excrement where they can germinate.
3) Harvesting seeds and transporting them for later consumption
This method involves animals gathering seeds, transporting them and storing them ready for consumption at a later date.
Squirrels gather acorns from oak trees…..
and hide them underground or under fallen leaves.
The only ‘problem’ (for the squirrels, not the trees!) is that they sometimes forget where they have hidden their seeds. Forgotten seeds remain uneaten; uneaten seeds can germinate.
Clark’s Nutcracker is a species of bird that harvests the seeds of pine cones. After harvesting they transport the seeds in pouches underneath their tongues. They store their seeds underground for later consumption.
Clark’s Nutcrackers have evolved long, sharp bills giving them the ability to prise seeds out of pine cones.
Clark’s Nutcrackers store seeds in the ground in secret caches of up to 15 seeds. They have amazing spatial memories and can locate their cache sites even when those sites are buried under deep snow.
Clark’s Nutcrackers often store far more seeds than they need. This means that if some of their caches are stolen by squirrels or other ‘thieves’ they will not go hungry in the middle of winter.
Uneaten seeds hidden underground in caches frequently germinate, helping ensure the survival of different species of pine tree. Whitebark Pine trees depend exclusively on Clark’s Nutcrackers for their survival.
Mammals and birds are not the only organisms that harvest seeds. Ants do as well!
Harvester ants collect large quantities of seeds and transport them back to their nests. As they carry the seeds some get accidentally dropped along the way. Misplaced seeds often germinate.
This is an example of how the storage of seeds by a bird species does not directly assist the parent plant to reproduce.
Acorn Woodpeckers work in groups to harvest the acorns from oak trees.
They place surplus acorns in store ‘granaries’ near the center of the group territory. Granaries are normally dead tree trunks; the birds peck holes in the tree trunks and hammer the acorns into the holes.They tightly wedge the acorns in the holes making it difficult for ‘acorn thieves’ to ‘steal’ their supplies of acorns.
The woodpeckers work as a group to aggressively defend their acorns from rivals.
Oak trees produce copious quantities of acorns; they rely on other seed dispersers, such as squirrels, to ensure that their acorns are dispersed.
This is an example of how a species plant can trick organisms into dispersing seeds which are of no nutritional value.
The Saga tree (Anenanthera pavonina) from India’s tropical rainforests produces bright red berries. When birds swallow Saga berries the birds derive no nutritional benefit. Berries from Saga trees closely resemble more nutritious berries from neighboring species of tree.
Saga trees greatly benefit from this seed dispersal service; they invest little energy in producing their seeds.
An ‘anachronistic’ species of plant produces seeds designed to be dispersed by animals or birds long extinct.
Take the avocado plant. In the wild avocado plants have evolved to produce large fleshy fruits that fall to the ground directly beneath the parent plant.
If the avocado plant’s seeds were to germinate beneath the parent plant they would be in direct competition with the parent for water, light and nutrients. In evolutionary terms it is a massive waste to create large, fleshy fruits which remain uneaten, rot on the forest floor and which have little chance of germinating.
Extinct mammals from the Pleistocene era (2,588,000 million to 11,700 years ago ) which used to live in the Americas. The medium sized giant ground sloth Megalonyx jeffersonii, was one such mammal. It was named Megalonyx jeffersonii to honor the third president of the United States, Thomas Jefferson (1743 –1826).
The Megalonyx jeffersonii was a large, heavily built ground sloth about 3 meters long which readily ate and dispersed the fleshy fruits of the avocado….
with its golf ball sized seeds.
After the extinction of giant ground sloths it was humans who became the main dispersers of avocados; avocados have now been planted all over the world to satisfy humans’ taste for avocado fruits.
Natural populations of wild avocados are now very rare.