Lecture 5 - Latitudinal Biodiversity Gradient

exactly 3 things determine how many species are present in an area:

(1) # of species that originated there

(2) # of species that have gone extinct there*

(3) # of species that dispersed into/out of there

*could be bc the species goes extinct globally or became extirpate din area of interest - the sorts of extinctino regimes we think about for this theory are background extinctions, NOT mass extinctions

analogous to 3 controls on population size - birth rate, death rate, moves in and out

also the same as how island biogeography theory uses only immigration and extinction rates to predict species diversity - but with the addition of speciation

equation for species richness

species that originate in area + species that disperse into area - species thta go extinct/are extirpated/leave an area = species richness

equator and diversity

taxonomic and func diversity often higher closer to the equator - why?

latitudinal biodiversity gradient (LBG or LDG)

name for the pattern of more speices in equatorial regions

there are as many as 1-2 dozen explanations including:
(1) geographical area

(2) energy availability

evolutionary speed

tropics are a cradel

tropics are a museum

out of the tropics

climate stability

(1) geographical area hypothesis

observation: there’s more land area within a latitudinal band near the equatori.e. 0-10deg lat than nr the poles i.e. 50-60deg lat

hypothesis says: 

-tropics are largest biome so species in this biome can have large geographic ranges

-specie w larger ranges have more chances at allopatric speciation happening with one of their populations

-speciation rates are higher near equator

one of earliest explanations for LBG (terbogh 1973)

criticisms of geographic area hypothesis

-tropics aren’t considered a single biome by anyone else - ex: geographical areas of the tropics include the biomes of tropial rainforest and tropical deciduous forest

-it’s unclear whether larger ranges are associated w more speciation - some argue that small ranges cause more speciation

(2) energy availability

obersvation: there’s more direct solar energy supplied to tropics throughout the yr

hypothesis says:

-more available energy means more primary productivity

-more primmary productivity means mroe individuals can survive

-extinction rates are lower near the equator

criticisms of energy availability hypothesis

-increase in # of individuals w greater primary proudctivity isn’t enogh to explain how many more species are found near the equator

-there isn’t a clear cut relationship etween primary productivity and taxonomic diversity across lots of studies

(3) evolutionary speed hypothesis

obersvations: nat selection depends on mutations (source of genetic diversity) and diff outocomes measured from one gen to the next - mutation rates correlate w diversification rates for many taxa

hypothesis says:

-higher avg temp means faster growht and shorter generation times

-mutation rate might also be higher near equaotr (Rohde 1992)
- speciation rates are higher near the equator

criticisms of evolutionary speed hypothesis

-careful study of many species found barely any diff in molecular evolution rate between species near equator and far away

(4) tropics are a cradle

observation: climate (especially temp) is relatively stable clsoer to equator

hypothesis says: 

-specialist species can survive better in stable climates - places w high seasonality means specie have to be more genralists in habitat and resource use

-more species can coexist bc of nich separation to minimize competition

- speciation rates higher near equato

criticisms of tropics are a cradle

-there are habitats where environment is stable but diversity is low i.e. tropical mountaintops

-there are habitats where environment is variable but diversity is high (i.e. many tropical places are highly seasonal in precipitation even tho temp is stable)

(5) tropics are a musem

observatoins: climate has more extreme fluctuations towards the ples and past climatechanges made the poles even harsher

hypothesis says: 

-fluctuating and harsh conditions can present kill mechanisms more often away from the equator

-extinction rates are lower near equator

(6) out of the tropics

observations:

-studies on fossil ibvalves found that clades originated in tropics

-species w narrow temp niches can be widespread near equator bc large geog areas similarly warm

hypothesis says:

-tropics are a cradle AND a museum

-speciation rates are higher near equator

-species’ geog ranges expand to include equaotor and eareas next t i (bridge species)

criticisms for out of torpics

-hypothesis was based on marine invertebrate species, which have best fossil record of speciation events and range size changes

-the OOT hypothesis might not explain terrestrial species as well and it will be harder to test if that’s the case

(7) climate variability/stablility (“mountains are ‘higher’ in the tropics”)

winters get colder farther from the equator

• many species at mid and high lats are adapted to survive winter temps (temp niche is wider)

• species near equator don’t have any natural selection to make them adapted to cold temps)

it gets colder as you go up mountains

• species that have adaptations for cold can pass over mountains easier

• species can also wait until summer to pass over mountains when warm

speciation rates higher near equator

mid-latitude mountain

• both bottom and top of mountain get colder in winter and warmer in summer and top of mountain is colder than bottom on any given ay but top of themountain in summer isn’t colder than the bottom ofthe mountain in winter

• species that live in lowlands can cross over mountain in summer w/out passing into an environment ouside their niche limits

equatorial mountain

• top of the montain is colder than the bottom, but temp is constant in each place all year

• species that live in lowlands can never cross over the moutnain without passing into an environment colder than their usual habitat

allopatric speciation is more likely

speciation rates higher near equator

geog area

evolutionary speed

tropics are a cradle

out of the tropics

climate variability

extinction rates lower near equator

energy availability

tropics are a musem

out of the tropics

which is the right answer

probably several of them, answers might depend on what clade you study

out of the tropics and climate variability more likely

OOT includes tropics are a cradle and tropics are a museum