March 2009

 

PLANT - ANIMAL MUTUALISMS:

Seed Dispersal

 

I like to ask students what are the advantages and disadvantages for a young plant that is underneath its parent and what are the advantages and disadvantages for a young plant that is away from its parent.  What do you think? Do the young have more disadvantages or more advantages if they grow under a parent? What about young plants that grow away from the parent? 

 

The main advantage for the young to grow under a parent is that the parent did well enough to reproduce so that the habitat must be good.  The disadvantages are more numerous and all relate to problems of a high density of seedlings and immatures under or next to a parent plant.  High density means high competition for water and nutrients with each other and with the much larger parent.  High density can also lead to disease transmission and make it easy for enemies like herbivores to find.  In addition the parent may cast too much shade for survival or optimum growth of seedlings.  Finally a caterpillar that eats a few leaves of the parent tree causes little harm but if it should fall on a small seedling below the parent, the caterpillar could eat it all.

 

For all of these reasons most plants have evolved mechanisms to disperse their seeds. Usually each seed has some accessory structures to help dispersal by biological (biotic) or non-biological (abiotic) agents.  The combination of seed and dispersal enhancing parts is called a propagule

 

DISPERSAL BY WIND AND WATER 

dandelion seed head:
wind-dispersed propagules




Many readers know about the propagules of dandelion, which are dispersed by wind, an abiotic agent.  We have a few dandelions in south Florida but on the Refuge the main wind-dispersed species are salt bush in the aster family, milkweeds, willows, and some grasses.  Like dandelion their propagules have feathery plumes that are easily carried by the wind.

The most important plant at the Refuge that is dispersed by water, another abiotic agent, is the cypress tree.  Its propagules drop into the water during the wet season, float, and are stranded at the water's edge as the dry season progresses.  With sufficient light they germinate but they require several years of drought to grow tall enough so that their leaves will be above water during the next wet season; only then will they survive.

  

DISPERSAL BY HITCH-HIKING 

Hitchhiking propagules have hooks or spines that stick to the fur of passing mammals.  Hitchhiking dispersal is a commensalism since the plant is helped but the mammal gets no benefit.  The most common example at the Refuge is Spanish needles but the most famous is the cocklebur.  It was the cocklebur that gave someone the idea for Velcro, a million dollar invention.  Here is a limerick I wrote about the function of spines and hooks on the cocklebur propagule.

cocklebur propagule adapted for hitch-hiking               

We observe the fruit of the cocklebur

And infer what the tiny hooks are fur

     Whether defense of some sort

     Or animal transport

In truth, cockles really stick to fur

 

 

A test of the hypothesis that cockleburs
are adapted for hitchhiking dispersal




Actually the defense hypothesis has some truth since it really hurts if we try to bite on the fruit to get at the seed, which is edible.

 

 

 

 

DIRECTED DISPERSAL BY MAMMALS OR BIRDS 

Just as biotic pollination is more reliable than abiotic pollination so too is biotic dispersal more reliable than abiotic dispersal.  For example, to be carried any distance by wind the seed on the propagule has to be small and a small seed has a low chance of germinating and surviving even if it reaches a favorable spot. 

The most efficient means of dispersal is when an animal eats a fruit propagule and either spits out or defecates the indigestible seeds at another location.  In some cases the germination of the seed is enhanced from its passage through the digestive tract. In addition the early growth rate of the seedling may be increased from the nutrients in the feces. 

Reliability of dispersal is further increased when mammals or birds co-evolve with plants to disperse their seeds.  Do you remember why we bird-watch but not mammal watch (January column)?  If so, can you hypothesize what kinds of fruit propagules mammals would favor and thus disperse and what kinds birds would favor and thus disperse?  In particular, what would the color, smell, taste, and size of fruit and seeds be to attract birds?  And what kinds of fruit propagules would mammals seek and disperse?  And how should the plant display its fruit propagules to favor birds or mammals?   

Brazilian pepper propagules, hypothesized to be bird-dispersed



For fruits that co-evolved with birds we hypothesize clusters of many small fruit at the end of thin pendulous branches.  We might even expect thorns on the branches to discourage small mammals like squirrels.  When ripe, the fruit should be held on the plant rather than drop to the ground. This will make it difficult for mammals to get to the fruit.  In addition the fruit might be bright colored and will certainly have no smell.  When eaten, the flesh of the fruit might have a bad taste.  This will not affect birds but could discourage mammals. And the seeds should be many and small so the bird can easily swallow them and hard enough that they are not ground up and killed in the gizzard.

test of hypothesis that small red propagules
on tips of branches are bird-dispersed      

You can test these hypotheses if you watch mockingbirds or robins or cedar waxwings.  They really feast on elderberry, dahoon holly, and Brazilian pepper here in Florida and hawthorn and small crabapples up north.  Their tiny seeds are passed intact and end up in bird scat which is recognized, you may remember, by the white uric acid. None of these fruits is palatable to us and we do not see mammals eat them or find their seeds in mammal scat.

coco plums on a bush


 

We hypothesize that fruit propagules that co-evolved with mammals will be single, medium to large size, and on large branches that medium-sized mammals can climb.  The fruit might drop to the ground when ripe which makes it accessible to even larger mammals.  The fruit might be dark and dull colored and have an obvious smell which is pleasant even to humans who have a poor sense of smell.  When eaten the flesh might have a sweet or sweet and sour taste; even if the taste is not pleasant taste to humans, we do not expect it to be awful.  We expect the seed within the fruit to be single, large, and hard and possibly taste bitter to discourage the mammals from trying to chew them.

The pleasant-smelling cherry propagule has        
evolved to encourage mammals to eat the fruit     

You can test these hypotheses if you examine mammal scat for seeds.  Since mammals are hard to watch at night when they feed, you might also look for fallen fruit and scat under plants.  At the Refuge at the right season we find lots of seeds of cocoa plum or pond apple and occasionally grape in raccoon scat.  Near the Florida shore sea grape is a classic mammal dispersed fruit.  Up north mammal dispersed fruits include ginkgo and paw paw (a relative of pond apple).

We can use ourselves to test whether a fruit is mammal-dispersed. How does the fruit flesh smell and taste?  And what is the hardness, size, and taste of the seeds?  And if you are adventuresome, you can test whether the seed survives passage through your intestine.  Alternatively, feed the fruit to your dog!

 

CHILE PEPPERS:  THE ULTIMATE BIRD-DISPERSED FRUIT 

Chile pepper propagules and birds are a specialized dispersal mutualism.   The bright red fruits are small with no smell and many small seeds.  To discourage mammals they are displayed at the end of thin branches, never drop to the ground, and are so hot to the taste that, even if starving, mammals reject them. Apparently, the wild pepper has evolved the hot taste to stop mammals from eating the fruit and for attracting birds. The plant gets an advantage of attracting birds since the chilies pass through its intestine not only unharmed but also with germination and growth of seedlings enhanced.  

In closing I offer my limerick that describes how I teach students about co-evolved fruit propagule dispersal.

        

  A fun-loving teacher, TP,

  Exhorts his students not just to see.

        Tests of touch, taste, and smell

        Range sublime to pure hell;

  Plums, berries, and chilies, tee hee

 

 

 

REVIEW QUESTIONS      

a. Chile peppers are co-evolved to be dispersed only by mammals. Birds dislike their taste and will not eat them.

b. Plants have evolved propagules to disperse their seeds because there are many more disadvantages than advantages for small plants to grow under their parents.

c. Dispersal of plant propagules by animals (biotic agents) is much more efficient than dispersal by wind or water (abiotic agents).

d. Hitch-hiking propagules, like cocklebur or Spanish needles, are examples of a co-evolved mutualism.