The Basis of Diversity
Plants and Pests: the Basis of Diversity
By Fran Zak
Pests have played an important role in the history of forests. Tropical forests have a greater diversity of plants and animals than temperate forests (Leigh, 2002). This is due in part to the lack of cold winters and the warm, wet climate that allows for continuous production of vegetable matter all year round. Pests keep plant species in check. Because there are so many pests, it is important for rainforest plants to have their seeds dispersed (carried far away) from the parent plant and also far away from the pests that attack that species (Leigh, 2002).
Tree seeds are dispersed by many species of mammals such as fruit bats that occur in tropical forests worldwide (I saw these fruit bats in Australia -I circled the bats in red to help you see them more easily), Possums and pademelons are dispersers in Australia, while monkeys and agoutis spread the seeds in Panama. Reptiles and birds such as toucans in Panama and parrots, lorikeets, and crimson rosellas in Australia, also disperse seeds.
Plants are not defenseless against pests. The following are some methods plants use to decrease pest damage:
- Just being tough to chew is the best defense of a plant against pests or predators. What we call "fiber" in plants is tough cellulose that is indigestible to many animals.
- Growing spines along the stem and/or on the leaves can be a valuable defense. I have seen the spines on the trunks of trees in both Panama and Australian tropical forests. In Australia, I even saw spines on the leaves of plants. If you were an insect, how easy do you think it would be to climb up trunks that look like this to get to the leaves you want to eat?
- Accumulating toxins in leaves is a good strategy. Biologist Dr. Lissy Coley found that many young leaves in the tropical rainforest of Panama have more toxin than their mature counterparts (Leigh, 2002). There are always trade-offs: making these toxins is a major drain on the plant's resources. Sometimes the insects that eat these poisonous leaves accumulate the toxin in their own bodies, making them toxic to their own predators. This is a photo of the poisonous stinging plant in Australia. You can see all the holes in the leaves where insect pests have eaten these poisonous leaves.
- Making nectar that attracts ants helps plants like Croton and Inga . The ants live in the hollow plant stems and help defend the plant's young, tender leaves against herbivore predators (Leigh, 2002). This is a mutualistic symbiotic relationship, where both the plant and the ant benefit by living together.
- Synchronous sprouting of the leaves of so many plants of the same species that their pests can't possibly devour all the leaves helps guarantee that some will remain. Maturing leaves then develop tough tissues that are difficult to chew, thus deterring many pests. If the plants can also have their young leaves mature very quickly, then the herbivore pests will not have enough opportunity to defoliate them (Leigh, 2002).
- Growing in the sun will decrease herbivore damage since small pest insects desiccate more quickly in the sun. The herbivore pests will die from drying out more often in dry season (Leigh, 2002).
- Growing young leaves that are not green. They lack the photosynthetic pigment called chlorophyll. I saw similarities in these young, reddish colored leaves in tropical forests in both Panama and Australia. Some plants do not provide new leaves with chlorophyll until the leaves have reached full size and have become tough enough to deter herbivore pests. This saves energy in synthesizing chlorophyll when these young leaves may only get devoured by pests. Biologist Dr. Lissy Coley found that young leaves on shade-tolerant plants are eaten twenty times more rapidly than full-grown leaves (Leigh, 2002).
I saw similar types and amounts of pest damage to leaves in tropical forest in both Panama and in a similar ecosystem in northern Australia. I saw interesting pest damage on tree trunks in Australia. Inland from Tangalooma Lodge (in Moreton Bay), on our way to sand tobogganing, we saw Scribbly Gum Trees. These trees are aptly named because their trunks look like someone scribbled graffiti on them. Click here to see photos of damage done by these moths (commonly called Scribbly Moths).
In the tropical rainforest when trees can produce leaves year-around, the pests keep the plants under control. On the other hand, the number of leaves a plant can produce keeps the species-specific herbivore pests under control by limiting their food supply. The tropical rainforest is a balanced ecosystem with great diversity where pest species keep host species in check and vice versa.
Leigh, Egbert. A Magic Web New York: Oxford University Press, 2002.
Pascack Valley HS
Jackie Willis, Ph.D.
Montclair State University
Upper Montclair, NJ
Special thanks to Professional Resources in Science and Mathematics (PRISM) at Montclair State University and Dr. Jackie Willis for making these ecology trips possible and for sharing her wealth of knowledge, her expertise, and her photographs with us.