Lecture 37: Biomes, especially tropical rain forests

READING: 1028-1043, 1135-43, 1135

1. What is a biome, and why should we care?

a. a biome is a large climatic region with a characteristic set of plants and animals, i.e., containing a recognizable community

b. biomes are determined by interaction of climatic factors, especially temperature and rainfall (Fig. 50.3, and on handout) (learn from reading how seasons and rainfall patterns are generated)

c. biomes may be aquatic (Fig. 50.9, 50.11) or terrestrial (50.15, 50.16)

d. "Productivity" = net annual g/m2 of new plant material produced

e. productivity varies dramatically among biomes

f. the more productive biomes usually have more species, play pivotal roles in conservation efforts

g. tropical rain forest is the most productive biome on earth.

2. Tropical rain forest

a. This biome has combination of high average precipitation and temperature

b. rainfall typically 200-300 days/year (even up to 365!)

c. 4% of land surface area (and shrinking!), but 20% of earth's net carbon fixation!

d. 65% of world's 250,000 flowering plant species (and many yet to be discovered!

e. many rare and specialized species

f. a 13km2 rain-forest preserve in Costa Rica has been shown to have 450 tree species, 1000 other plant species, 400 bird species, 58 bat species, 130 amphibian and reptile species (compare to 2-10 tree species in temperate forest)

g. fogging one tree (Dr. Terry Erwin, Smithsonian), led to discovery of hundreds of arthropod species. Erwin estimated 163 beetle species/ species of canopy tree

3. Soils and nutrients in rainforest

a. soils of tropical rain forests contain few nutrients. Most nutrients in the system locked up in lush vegetation (contrast to temperate areas with deep, rich soil, rather poor "standing" plant crop.

b. topsoil layer very shallow, most roots in top layer (contrast with deep roots of temperate trees)

c. large trees less securely anchored by roots -- need other structures to anchor \ them-- either have very broad roots, or buttresses

d. recycling of nutrients by decomposers a key part of ecosystem in tropics

e. if forests are cut down, remaining soil is very poor, only supports agriculture for a few years! Rainforest may not be able to grow back because soil is so poor, becomes easily compacted

f. clearing the rainforest destroys valuable resources for extremely short-term gain. Nearly impossible to go back to rainforest after clearing.

4. Plants

a. extremely high species diversity

b. multi-tiered canopy structure (very tall canopy trees, mid level trees and understory trees

c. light is at a premium! There are special adaptations to this

i. leaf arrangement to maximize exposure to light

ii. vines use trees for support-- can grow to canopy!

iii. epiphytes ("on top of- plants"). Vascular plants that grow on another plant. Entirely adapted for arboreal life

d. tropical plants a rich source of medicine because they produce many bioactive molecules (probably evolved as defenses against insects and pathogens)

5. Animals

a. 75% of animal biomass is insects-- mostly ants and termites!

b. however, some very unusual birds and mammals also

6. Rainforest conservation must be a top priority in the 21st century! (more later)