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
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
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?
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.
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.
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.
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
