BiodiversityTropical-Temperate Gradient

Dr. Mark A. McGinleyFulbright Visiting Scholar

University of Malaya2010

Biodiversity Challenge

• Who has more biodiversity, Texas or Malaysia?

Biodiversity Challenge!

• Area – Texas - 696,241 km2

– Malaysia 328,600 km2

• Number of Ecoregions– Texas- 13– Malaysia- 12

• Therefore, Texas is larger and has more habitat diversity than Malaysia.

Lessons From Biogeography

• Species-area curves show that larger areas often contain more species

• Species richness is correlated with habitat diversity

• Thus, predict that there will be more species in Texas than in Malaysia

• Not surprising- Everything is Bigger in Texas!!!!

Biodiversity Challenge

Because of its large size and habitat diversity, Texas is one of the most diverse states in the USA.

#1 in number of bird species (477)#1 in the number of reptiles species (149) # 2 in the number of mammals species (159)# 2 in the number of vascular plant (4,509).

How About Malaysia?

• Mammals = 313 species• Birds = 746 species• Vascular plants = 15,500 species

Terrestrial BiodiversityLocation # terrestrial

mammals# birds # reptiles # vascular

plantsTexas 159 477 149 4509

Malaysia 313 746 ? 15,500

Texas data from The Nature Conservancy of TexasMalaysia data from Wikipedia and www.mongabay.com

Much Greater Terrestrial Species Richness in Malaysia Than in Texas!

• Texas is an unusually species rich region in the US so it is not a problem with Texas.

• What causes there to be so many more species in Malaysia than in Texas?

Temperate-Tropical Gradient

• It has been well known for over 200 years than there tend to be more species of organisms living in tropical regions than in temperate regions– Temperate-tropical gradient in species richness• One of the most studied questions in biology

– But still don’t understand it very well– Over 30 different hypotheses have been proposed

Temperate-Tropical Gradient in Species Richness

• In many groups of organisms there are more species found in tropics than there are in the temperate regions.

• Why?– Suggests that there is some sort of physical,

biological, or geographical factors going on in the tropics that cause there to be more species

Tropical-Temperate GradientNull Effect

• It is possible that the pattern might have nothing to do with biology– Instead the pattern might arise because the

tropics lie in the center of the globe

– Mid domain effect– Colwell, Willig

Null Models in Ecology

• It is possible that many of the patterns that ecologists have used to suggest that ecological interactions such as competition are structuring ecological communities are merely the result of random chance.

• Null Models are used to investigate this possibility.

Mid-Domain Effect

• If species vary in their latitudinal ranges (i.e., some species are found across a large range of latitudes and others are found across a very small range) then if the location of species on the globe is random then there will still be more species in the tropics.

Mid-Domain Effect

• The mid-domain effect probably doesn’t explain all of the pattern, but we need to examine whether tropics are more diverse than expected by random chance.

• Other factors can help to explain the “extra diversity”

How Should We Try To Study What Factors Cause Greater Species Richness in the Tropics?

Go back to the library analogy for biodiversity

How we should attempt to understand patterns of diversity depends on whether ecological communities are full or empty.

Tropical-Temperate Gradient

• Full– Number species = number of niches• Why more niches in tropics?

• Not Full– Number of species = number of species added –

number of species lost• Need to focus on biological processes that increase the

number of species (speciation and immigration) and those that decrease the number of species (extinction)

Temperate-Tropical Gradient

– To understand why there are so many more species in the tropics than in the temperate regions we need to think about• Genetics• Evolution• Ecology• Biogeography

Who Has More Money?Me or You?

Money Example

• What determines the amount of money that someone has?

• How much money they can store (silly, but possible)

• How much money they make (ringgit per year)• How much money they spend• How long they have been making and

spending that amount of money

Biodiversity

• Making money = adding new species– Speciation– Immigration

• Spending money = losing new species– Extinction– (not emigration – because rarely do all members

of a species leave a region)

Making New Species“Speciation”

• Allopatric speciation– Requires isolation by “geographic barrier”– Requires long periods of time– Thought to be the most common mechanism in most groups of

organisms• Sympatric speciation

– Does not require isolation– Can be “instantaneous”

• Hybridization• Host shifts

• For more info on speciation see http://www.slideshare.net/secret/3eX72KxuoWZn6R

What Determines Rates of Speciation

• Probability of forming geographic isolating barriers– And maintaining them long enough for speciation

• Rate of selection– Rate of mutation– Generation time

More Time For Speciation in the Tropics

• Some people have proposed that the reason that there are more species in the tropics is that the temperate zones have been colonized more recently so there has been less time for the evolution of new species

Higher Rates of Speciation in the Tropics

• Data from plants show that plants in lower elevations have higher mutation rates than plants from higher elevations– Suggest higher metabolic rates in warm

temperatures causes higher rates of mutation

Lower Rates of Extinction in the Tropics

• Tropics are the largest biome in the world– Large area allows for large population sizes– Large population sizes correlate with lower rates

of extinction– If a species does go locally extinct, then it has the

possibility of being “rescued” by immigration from another location in the tropics.

Higher Rates of Speciation in the Tropics

• Because tropical plants live where it is warmer, they might have higher rates of mutation than plants in temperate regions• Higher mutation rates might allow for faster natural

selection• Would allow for more allopatric speciation

Higher Rates of Speciation in the Tropics

• Plants are ectothermic whereas mammals maintain their bodies at a constant level no matter where they live.

• Thus, mammals may be less affected by this than plants.

• Compare mammal species from tropics to temperate zone– Found faster evolution

• Cool climates caused less mutation• Red Queen Hypothesis

Higher Rates of Speciation in the Tropics

• Some researchers have found no relationship between rates of mutations and latitude in birds

• Suggest that the differences in rates of evolution and speciation might be due to differences in generation times.– Tropical species breed more often than temperate

species so they should evolve faster.