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Introduction Plants, being stationary, require a mobile mode for
seed dispersal. Modes for seed dispersal include self-projectile mechanisms, wind,
water, and animals. Dispersal by animals falls into the category of plant-animal
interactions, a subject of interest because of the reciprocal adaptations that can be
observed. These reciprocal adaptations lead to coevolution, or change in organisms
as a result of their interactions with each other.
Animals as Dispersal Agents
Plants that depend on animals for
dispersal have seeds that are adapted to traveling on the outside or the inside of the
animal. Seeds with burrs or hooks can attach to an animal’s fur. For
seeds that are transported internally, plants provide an attractive fruit pulp reward in
return for the ride.
The process of internal seed dispersal
begins with the plant producing fruits. Next, an animal is attracted to these fruits
based on their characteristics (such as odor), and begins to feed. After these
fruit-eating animals (also known as frugivores) ingest fruit, they process them in their
digestive tracts, and then regurgitate seeds or deposit seeds in their feces.
Behavior, both innate and learned, is
involved in every step of the dispersal process. First, the stimulus of being hungry
makes the animal begin searching for food. While in this state of searching, the
animal is receptive to stimuli (e.g., color, odor) that may signal the presence of food
items. If the fruit encountered has an acceptable taste, the animal will probably
choose to eat it. Where seeds are deposited depends on how long the animal stays at
the plant feeding, and where the animal goes afterwards.
Not all animals that eat
fruit are appropriate dispersers. The effectiveness of an animal as a seed dispersal
agent depends on how many seeds it disperses, and how it treats these seeds. Animals
that visit a plant for fruit more frequently may be more reliable than those animals that
eat fruit more rarely. Some fruit-eating animals are considered seed
"predators" if they kill seeds by digesting the seed along with the fruit
pulp. On the other hand, seeds can sprout better after passing through the digestive
tract of certain frugivores. In addition to enhancing sprouting, an effective
dispersal agent deposits seeds in appropriate habitats for their survival to reproductive
adulthood. The survival of a seed greatly depends on where it lands. Seeds
that move farther away from other seeds have greater success because they can better
escape resource competition, interbreeding with parents, and post-dispersal mortality.
Because not all animals that eat fruit are equally good at dispersing seeds, plants should
change fruit characteristics to discourage frugivores that are seed predators, and
encourage frugivores that are effective dispersers.
Fruit Characteristics and Forager Choice
Fruit characteristics (or fruit traits)
that foragers use to choose fruit as food items include toxin content, fruit appearance,
and nutrient content. Together, the presentation of these traits is called the
fruiting display. Foragers learn to recognize these traits as cues to trigger
selection or avoidance of certain fruits as food items. The reliability of the
behavioral response depends on the consequences of eating the fruit selected.
Animals will prefer fruits that fulfill nutritional requirements, but avoid those that are
toxic. Reciprocal change (coevolution) between plants and frugivores can occur when
behaviors of foragers select fruit traits that in turn select for foraging behavior.
Toxins. Some fruits contain toxins to
deter seed pathogens or predators. Animals can learn which fruits are toxic, and
thus learn to avoid them. Toxins can also limit the amount of time a more
toxin-tolerant frugivore spends feeding at the plant. Because animals are limited to
the amount of toxins they can handle over a period of time, a forager must stop eating a
food item when maximum toxin load is reached. The foraging animal may then leave to
find an alternative source of food or to seek antidotes. By forcing an animal to
leave early, the plant may be able to ensure that its seeds will be deposited far enough
away to avoid inbreeding, competition, and pathogens as mentioned above.
Fruit appearance. Although toxin
content limits ingestion of fruit, appearance can both limit and encourage it. Fruit
appearance provides salient cues for recognition of fruit as a food source. Pigments
such as carotenoids, flavonoids, and betalains give fruits their colors, and volatile
compounds provide odors. In general, bird-dispersed fruit are red or black, and
mammal-dispersed fruit have distinctive odors. The morphology (size and shape) of a
fruit may also influence forager choice. Animals are limited to food items small
enough to fit in their mouths and throats, especially birds and reptiles that tend to
swallow fruits whole. In addition, birds may also consider the bulkiness of seeds
when choosing fruit. Not only does seed bulk take up space in the digestive tract,
it also may need to be jettisoned before flight.
Nutrient content. Finally, fruit
nutrients may be important in influencing forager choice. For most temperate fruits,
energy in the form of sugars is the most abundant nutrient. Other vitamins and
minerals may also be influential. For example, rose hips are known to be high in
vitamin C. Nutrients may be more indirect in effect than appearance. Fruit
appearance provides the cue for fruit choice, but nutrients provide the positive feedback
that maintains an animal’s preference for the fruit.
Conclusions
By understanding what influences an
animal’s decision to choose fruit, we can attempt to predict whether or not a
plant’s fruiting display will lead to successful dispersal. The fruiting
display must balance maximizing gains from attracting effective dispersers while
minimizing losses from also attracting ineffective dispersers. Thus, we can
determine the effectiveness of a fruiting display by the number of seeds successfully
dispersed per effort for the entire fruit crop. Effort can be determined by
identifying the factor that influences an animal’s decision to choose the
fruit. For example, if the animal selects fruit on the basis of energy content, then
effort is the amount of kilojoules of energy of the entire fruit crop. Effectiveness
is then measured in units of numbers of seeds dispersed successfully relative to the
amount of energy of the fruit crop.
In conclusion, foraging behavior
is an important component of the plant-seed disperser interaction. Foraging behavior
determines how fruits are chosen, and where the seeds are deposited. Toxin content,
fruit appearance, and nutrient content may influence how fruits are selected and where
seeds are deposited. By examining the behavioral components of the seed dispersal
process, we can better understand how coevolution between plants and their dispersers may
take place.
Additional Reading:
Herrera, C.M. 2002. Seed dispersal by
vertebrates. Pages 185-208 in C.M. Herrera and O. Pellmyr, editors.
Plant-animal interactions: an evolutionary approach. Blackwell Science
Ltd. Malden, Massachusetts, USA.
Schupp, E.W. 1993. Quantity, quality, and the
effectiveness of seed dispersal by animals. Pages 15-29 in T.H. Fleming and A.
Estrada, editors. Frugivory and seed dispersal: ecological and evolutionary
aspects. Kluwer Academic Publishers, New York, USA.
Stiles, E.W. 1989. Fruits, seeds, and dispersal agents. Pages 87-122 in W.G.
Abrahamson, editor. Plant-animal interactions. McGraw Hill, New York, New
York, USA.
http://www.cas.vanderbilt.edu/bioimages/pages/fruit-seed-dispersal.htm
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