Species diversity

Question 1

Why are so many species in the tropics?

Answer 1

Two hypotheses
1. Higher productivity in the tropics
2. Tropical environments older and more stable (more time accumulate to species)
-> these are global-scale processes

Question 2

Global tropical biodiversity is not evenly distributed

Answer 2

See page 10 - 12
Southeast Asia highest plant density but lowest area
over represented in species diversity in relation to rest

Latin America many plant species but lower density

Question 3

Why is Southeast Asia so overrepresented regarding species diversity?

Answer 3

P. 13

It remains under-collected. New species are discovered every year

Question 4

Why is there such a high species diversity in Xishuangbanna in temperate southern China?

Answer 4

Despite a 6 month dry season this region shows great diversity (124 spp in 1 hectare)

In the dry season, fog forms after sunset and fills the valleys. ‘Fog drip’ contributes more than a third of dry-season rainfall.

Question 5

Why so many species in the tropics? Summary

Answer 5

The origin of species diversity in the tropics may best be explained by the age and area of hot, wet environments, which has allowed a huge pool of rainforest tree species to evolve, but then became ‘restricted’ to the tropics after the Eocene

Question 6

Processes / mechanisms promoting the coexistence of species

Answer 6

1. Niche differentiation
2. Growth-survival trade-offs
3. Negative density-dependence
4. Intermediate disturbance
5. Neutral Theory

Good evidence for 1 - 4, evidence for 5 is mixed but it could help explain the coexistence of many very similar species in each layer of the forest

Question 7

Niche differentiation

Answer 7

All species exhibit optimal dimensions of resources, and this is exhibited in differences in where and when the species can be found. Consider the complex structure of tropical forests and the specialists that live in certain spaces of the forest

Partitioning of resources is a fundamental basis for the niche concept

p. 22 - 25

Question 8

Growth-survival trade-offs

Answer 8

The ability to grow fast and have a long lifespan (defend itself) is a zero-sum phenomenon. A plant cannot do both (limited resources)
Coexistence is promoted by spatiotemporal partitioning across species with different growth rates and lifespans

pages 27 - 29

Question 9

Negative density-dependence

Answer 9

The higher the density is, the higher the mortality is

the mechanism will only work if the natural enemies are species-specific. if generalist enemies are more important then it makes no difference which species you grow next to

pages 31 + 32

Question 10

Intermediate disturbance

Answer 10

Pages 34 + 35

Tropical forests and coral reefs are in a constant state of non-equilibrium.
Maximum diversity will be attained when disturbance is sufficient to increase habitat heterogeneity and opportunities for disturbance-specialists, but not so much that they dominate.
Thus, “intermediate disturbance”

Question 11

Neutral Theory

Answer 11

pages 37 + 38

Neutral theory is based on the premise that all species are ecological equivalent, i.e. have the same per capita birth and death rates, and the same chance of immigration from the regional species pool

Even random variation in birth, death and immigration would eventually drive all but one species to extiction, but this process would be extremely slow and could be balanced by a very low rate of speciation

This theory essentially abandons ecology and has been met with controversy and criticism