Chapter 6

Communities, Formations, and Biomes

Biogeographic Patterns

• The initial approaches to explaining biogeographic patterns, developed in the 16th and 17th centuries, were based on the Bible.

• All the plants and animals on Earth were represented on Noah’s Ark

• Therefore the New World should have the same plants and animals as the Old World

• Age of Exploration – Captain Cook and Sir Joseph Banks – 1000 new species of plants

• So many new plants and animals discovered in the 18th century, how could they have all fit on the ark?

• Buffon (1756) “The Earth makes the plants; the Earth and the plants make the animals”

• Buffon’s Law - environmentally similar but isolated regions have distinct assemblages of plants and animals – first principle of biogeography– Exception – cosmopolitan species

• Buffon’s Law led to…

• Biomes – subdivisions of the biosphere based on similarities in vegetation structure and climate

• Community – assemblage of all organisms living in a prescribed place or habitat

• Associations – groups of plant species commonly found in similar habitats

• Ecological equivalents – widely separated but physiognomically and structurally similar species and vegetation formations– Von Humbolt

• Life zones – elevational and latitudinal bands of similar climate and vegetation

• Superorganism community concept– Clement– Species in a community have coexisted for long periods of

time and evolved together– Each species depends on the others for survival

• Individualistic community concept– Gleason– Communities are areas of similar habitat where species

coexist because they have somewhat comparable environmental tolerances and resource demands

– Species do not have to occur together– The youthfulness of many present plant associations means

they didn’t evolve together as superorganisms

Six basic vegetation structural types

• Brown and Lomolino (1998)• Forest – trees with continuous canopy• Woodland – widely spaced trees with shrubs,

grasses, or herbs• Shrubland – continuous shrubs• Grassland – grass• Scrubland – widely spaced shrubs• Desert – sparse xerophytic plant cover and bare

ground

Figure 6.6

Figure 6.1

Tropical Rain Forest• Found between 25°N and 25°S latitude• No distinct dry season, Frost free• Equal insolation• Major centers of distribution: South and Central

America (50%), Africa (20%), and SE Asia and eastern Australia(30%)

• Occurs in 70 countries• Highest biodiversity – 50 to 80% of species on 6

to 7% of Earth’s land surface• 300 tree species per hectare, but only 2

individuals per species• Structural complexity• Seed dispersal

Tropical Rainforest

• Epiphytes – rooted in other plants – not parasites, but do compete for light

• Lianas – rooted in soil• Stranglers – born epiphytes, become

lianas – hemiepiphytes – the strangler fig• Poor soil quality – rapid nutrient cycling• Shallow, butressed root systems• Time stability hypothesis• Pleistocene aridity and refugial hypothesis• Podocarpus

Tropical Seasonal Forest

• Between Tropical Rainforest and 30° latitude

• Monsoon rainfall pattern• Little seasonal temperature change• Mainly deciduous canopy• Large, fleshy canopy leaves• Strata – canopy, shrub, sapling• High biodiversity

Tropical Seasonal Forest

Tropical Savanna

• Centered around 20° latitude• Winter dry season• More seasonal temperature variation• Grassland with scattered trees and shrubs• Trees have thick bark to avoid water loss• Seasonal die off of some species• Fewer trees because of water availability,

large grazing animals, and lightning

• Many bird species, large grazers, predators

• Sahel and Serengeti

• Threatened by desertification, over grazing, poaching, changes in fire regimes

Desert• Found in areas dominated by subtropical

high pressure cells, rainshadows, and continental areas

• Tropical deserts receive rainfall from the Intertropical Convergence Zone (ITCZ)

• Cool deserts receive rainfall from winter midlatitude cyclones

• Biodiversity depends on rainfall• Few trees• Most vegetation along waterways and

roadways

Figure 6.13