Epiphytes & Milkweed
Reading:
Epiphytes are plants that grow on other plants. In the rain forest, they sprout from seeds or
spores carried by the wind or animals and take root on the trunks and branches of the canopy
trees. Epiphytes are not parasitic: they gain nothing from the trees except support and access to
sunlight. However, under favorable conditions, they may form dense, water-logged, tangled
mats of roots and stems, weighing up to several thousand kilograms. This weight is often
enough to break the limbs of the more fragile trees. Some tree species have developed
adaptations to protect against the growth of epiphytes. Two known mechanisms are (1) the
frequent shedding of bark layers that periodically removes young epiphytes, and (2) the
presence of certain plant-manufactured chemicals that inhibit the growth of other plants.
In his explorations of the Costa Rican rain forest, Donald Perry developed an apparatus
consisting of a platform base built on one tree and connected to two other trees by means of
ropes. From this base, a system of ropes and pulleys attached to a harness would allow an
investigator to access previously unexplored areas of the canopy and to move relatively
unrestricted with minimal contact or interference with rain forest organisms.
During his studies, Perry noticed that some trees were completely free of epiphytes,
although the species were known not to shed bark or to manufacture plant-inhibiting chemicals.
Other scientists had speculated that the Azteca ants, which live mutualistically with certain
species of rain forest trees, were responsible. These aggressive ants were known to swarm and
attack other insects and mammals that might damage their trees. To test the effect of epiphytes
on these trees, Perry tied some moss and other small epiphytes to the limbs of one such tree.
Perry found that the ants did destroy epiphytes.
1. What is an epiphyte and where are they usually found?
2. Are epiphytes parasites?
3. What are two ways trees prevent the growth of epiphytes?
4. What observation(s) would help support Perry's hypothesis that ants prevent the growth
of epiphytes?
5. Why might ants want to destroy epiphytes?
6. How might Perry's invention help scientists discover new species?
Reading:
Milkweed is a plant commonly found throughout fields and pastures and along roadsides in
eastern and central North America. It gets its name from the milky white sap that oozes when
the plant is broken or cut. Milkweed plants bloom in June and July. When fertilized, the flowers
form large seedpods that open in the fall. The following observations were taken from a
scientist's field study of milkweed plants from spring through fall.
In the summer, the sugary nectar secreted by the milkweed's flowers attracts many bees,
butterflies, moths, and a variety of smaller insects that carry away pollen when they depart.
Milkweed nectar seems to be the major source of nutrition for several species of small moths,
flies, mosquitoes, and ants. Monarch butterflies, which visit in large numbers, lay their eggs on
milkweed plants, and the hatching caterpillars feed on the leaves. As fall approaches, milkweed
bugs begin to attack the developing seeds, and milkweed beetles eat the foliage.
Aphids, which suck milkweed sap, are found throughout the year. Crab spiders do not feed
on the plant itself, but rather on most of the insects that visit the plant. In the two to three weeks
while the milkweed plants are in bloom, successful adult female crab spiders may increase ten
times in mass before laying their eggs on the inner surface of leaves. Some species of flies and
wasps, which feed on crab spider eggs, visit the plants periodically. Harvestmen, also known as
"daddy longlegs," recover the remains left by predators.
7. Based on the scientist's observations, formulate two possible hypotheses about the
effects of crab spiders on the survival of the milkweed plant.
8. How would you describe the symbiotic relationship between the milkweed plant and
aphids?
9. From the scientist's data, infer which abiotic factor affects milkweed. Explain how the
data support your inference.
10. Based on the scientist's observations, what is one food chain that begins with a
milkweed plant? List the organisms and connect them with arrows.
11. How would you characterize the interactions between the milkweed and the organisms
that visit or live on the plant?
12. Did this scientist perform quantitative or qualitative research?