Dragon fruit. (Photo by R. Butler)
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SEEDS and FRUITS
Many of the seeds and fruits produced by canopy trees fall to the ground where they provide food for seed gatherers (rodents, birds, fish, etc) and create a natural seed bank in the leaf litter. There are two growth strategies once seeds reach the ground. One strategy is to produce large seeds with food reserves enabling the seedling to survive in the low-light conditions of the understory. The second method characteristic of many pioneer species is to produce huge numbers of small seeds which only germinate under certain conditions (usually light gaps). When a light gap opens, these seeds sprout and the seedlings rapidly grow to once again plug the hole in the canopy.
Since seeds are the agents for future growth, many have defenses against predators to ensure some will be left untouched to germinate. Plants reduce herbivory and seed eating with structural defenses like thorns, stinging hairs, or spines and the production of toxic compounds which interfere with digestion or are unpalatable. Often the seed is surrounded by an edible fleshy material to make it palatable to birds and mammals who can disperse it, but the seed is toxic or indigestible so it will be passed out or discarded. For example, the cashew has a sweet and juicy stem but a toxic seed, so monkeys eat the fleshy stem and discard the seed.
Many rainforest trees, especially those of the forest interior, produce large cauliflorous fruits that grow directly out of the surface of the tree. This mechanism enables the tree to grow large, conspicuous fruits that attract larger animals. One of the largest cauliflorous fruits is the jackfruit of Southeast Asia which may be three feet long (1 m) and 110 pounds (50 kg). The jackfruit produces a strong odor that attracts nocturnal mammals like the flying fox. A famous cauliflorous fruit of Southeast Asia is the durian, a thorn covered fruit that is said to "taste like heaven but smell like hell." The odor of the durian is so offensive that its presence has been banned in some public places. Nevertheless, the durian is an important export for Malaysia, Indonesia, and Thailand, which exported nearly 1 million metric tons in 1992. Both cocoa and coffee beans grow as cauliflorous fruit.
Seeds and fruits provide an important food resource for forest-floor animals. To survive, a seed must elude these predators and many do so by being dispersed by specialized birds, mammals, and fish. These dispersers eat the fruit flesh without destroying the seed. Other seeds escape by being poisonous. Some of these toxins are useful to insects, which sequester them for their own purposes, and to humans. For example, many seed toxins have been chemically isolated and used to formulate drugs for humans, while umpteen insects naturally incorporate these compounds to render themselves toxic or use these toxins to produce scents to attract mates. For example, the male butterflies of the Danaiidae family convert alkaloids in the crotalaria plant into a scent to attract females.
Many large seeds have structured defenses against seed predators. In some cases seeds designed for gut passage by large mammals will have a sturdy seed coat, while bird-dispersed large-seeded fruit will often have seeds that are chemically defended. For example, the fruit of the nutmeg splits open to reveal a large seed covered with an orange-colored aril. The seed is highly aromatic, and the odor acts as a warning and toxic deterrent.
Review questions:
- How do animals help the reproductive cycle of plants?
- Why are many seeds poisonous?
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Continued: Light Gaps, Seedlings, Shrubs
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
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WEEKLY NEWSLETTER
INTERACT
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Recent news
Some toilet paper production destroys Indonesian rainforests, endangering tigers and elephants
(02/09/2012) American consumers are unwittingly contributing to the destruction of endangered rainforests in Sumatra by purchasing certain brands of toilet paper, asserts a new report published by the environmental group WWF. The report, Don't Flush Tiger Forests: Toilet Paper, U.S. Supermarkets, and the Destruction of Indonesia's Last Tiger Habitats, takes aim at two tissue brands that source fiber from Asia Pulp & Paper (APP), a paper products giant long criticized by environmentalists and scientists for its forestry practices on the Indonesian island of Sumatra. The brands — Paseo and Livi — are among the fastest growing, in terms of sales, in the United States.
Tropical ecologist: Australia must follow U.S. and EU in banning illegally logged wood
(02/09/2012) Australia should join the widening effort to stamp out illegal logging, according to testimony given this week by tropical ecologist William Laurance with James Cook University. Presenting before the Australian Senate's rural affairs committee, Laurance argued that the massive environmental and economic costs of illegal logging worldwide should press Australia to tighten regulations against importing illegally logged timber at home.
Humans drove rainforest into savannah in ancient Africa
(02/09/2012) Three thousand years ago (around 1000 BCE) several large sections of the Congo rainforest in central Africa suddenly vanished and became savannah. Scientists have long believed the loss of the forest was due to changes in the climate, however a new study in Science implicates an additional culprit: humans. The study argues that a migration of farmers into the region led to rapid land-use changes from agriculture and iron smelting, eventually causing the collapse of rainforest in places and a rise of grasslands. The study has implications for today as scientists warn that the potent combination of deforestation and climate change could flip parts of the Amazon rainforest as well into savannah.
Green groups: government moving too slowly on protecting Canada's Great Bear rainforest
(02/08/2012) Three environmental groups have submitted a letter to British Columbia Premier, Christy Clark, to ask the government to speed up the process of implementing the Great Bear Rainforest Agreement, which is meant to ensure 70 percent of old-growth forest is maintained.
Majority of protected tropical forests "empty" due to hunting
(02/08/2012) Protected areas in the world's tropical rainforests are absolutely essential, but one cannot simply set up a new refuge and believe the work is done, according to a new paper in Bioscience. Unsustainable hunting and poaching is decimating tropical forest species in the Amazon, the Congo, Southeast Asia, and Oceana, leaving behind "empty forests," places largely devoid of any mammal, bird, or reptile over a few pounds. The loss of such species impacts the whole ecosystems, as plants lose seed dispersers and the food chain is unraveled.
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