Climate and Life on Earth IV: Macroecology Flashcards
Define large scale patterns in diversity
- trends in the distribution of species across the earths surface
- macroecology
Define species-area curves
how the number of species recorded increases as the area studied increases
Describe latitudinal gradients
the trend towards increasing diversity as you move from the poles to the tropics
Describe The Power Function
- as you survey increasing areas, the number of species recorded increases
- increase is not linear
- typically, a 10-fold increase in area leads to a doubling in the number of species
How is the Power Function linearised?
- taking logs
- S = cAz becomes Log S = Log c + z Log A
- z is the slope and log c is the intercept
Species area curves can be seen
- for islands of different areas within an archipelago
- for pieces of single biotas, e.g. states within USA
- for interprovincial areas, e.g. continents
Describe the three main mechanisms used to explain the Power Function
- The habitat diversity hypothesis
- The passive sampling hypothesis
- The equilibrium model of island biogeography
Describe the Habitat Diversity Hypothesis
- if there is a wider range of habitat types, a wider range of species can live in an area
- larger areas have more habitats, and species diversity increases with habitat diversity
Describe testing of the habitat diversity hypothesis
- area/habitat diversity can be separated
- keep habitat constant and vary area within sampling sites
or - keep area constant and vary habitats within sampling sites
Describe the passive sampling hypothesis
- often used as a ‘null model’
- assumes the probability that an individual or a species occurs on an island is proportional to island area
- assumes that islands sample individuals randomly and independently
Describe the failings of the passive sampling hypothesis
- fails to explain the diversity on small islands
- given enough time, all species should be present on small islands– nothing limits diversity
- does not predict turnover of species
Describe the Equilibrium hypothesis
- closer islands will have more species than isolated islands
- large islands will have more species than small islands
When there are more species…
fewer species are potential colonists
When there are more species, competition leads to
increased extinction
Equilibrium no of species depends on
area and isolation
Distant islands are
more difficult to reach, so they have lower immigration rates
Small populations on small islands are
more prone to extinction
More realistic models have
curved immigration and emigration functions
Described curved immigration rates
Strong dispersers colonise easily, then rates of colonisation slow
Describe curved extinction rates
- slow initially (many empty niches)
- increases as island becomes saturated with competing species
Describe the assumptions of the equilibrium hypothesis
- evolution does not influence species richness
- isolated islands are less likely to be colonised
- big islands have big populations
- big populations are less likely to go extinct
- colonisation and extinction is independent of species composition on the island
Describe the predictions of the Equilibrium hypothesis
- substantial turnover in species composition, but species number should be relatively constant through time
- number of species present on an island should decrease with increasing isolation
Give examples of the Equilibrium hypothesis in action
- Birds on the Channel Islands (California)
- mangrove islands
- Bahama Islands Birds
How are species-area relationships used in conservation
- predict how many extinctions will occur following area reduction
- suggest best ways of slowing the loss (reserve design)
Describe the assumptions of conservation wit regards to species-area relationships
- species richness is the primary object of conservation interest
- area is the main cause of variations in species richness
Predicting how many species will go extinct if area is reduced
depends on z
Describe designing nature reserves
- larger areas have more species
- SLOSS controversy
SLOSS controversy
- should you have a Single Large reserve Or Several Small ones of a similar area?
- depends on ‘beta diversity’
beta diversity
the degree to which species overlap between areas
Describe the basics of latitudinal gradients
- species are not spread evenly over the earth’s surface
- habitats with greatest biodiversity on the land and in the sea are both tropical
Describe tropical rainforests
- cover 7% of the earths surface
- account for >50% of the earth’s biodiversity
Describe coral reefs
- cover 1% of the earth’s surface
- account for 25% of all marine fish species
Describe the biotic explanations for latitudinal gradients
- productivity
- competition
- predation
Describe the abiotic explanations for latitudinal gradients
Time & Stability
Describe the Productivity or Species Energy Hypothesis
hottest, most humid places have the greatest net primary productivity
Describe the failing of the Productivity Hypothesis
- tropical seas have low productivity but high richness
- eutrophic lakes have high productivity but low richness
- high productivity does not necessarily lead to high biomass/abundance
Describe competition in latitudinal gradients
- constant climate in tropics
- most species grow toward their carrying capacity
- intense competition
- narrowing of niche breadth
- closer species-packing
- more species can coexist
Describe the failings of competition in latitudinal gradients
comparative analysis of strength of competition and niche breadth difficult
Describe predation, parasitism and herbivory in latitudinal gradients
- predator-mediated coexistence
- more intense mortality from ‘natural enemies’ in the tropics (stable climate)
- population sizes of prey kept low
- reduces competition: allows overlap in resource use and prevents any one species becoming locally dominant
- many species can coexist
Describe time and stability in latitudinal gradients
- communities diversify with time
- temperate regions have younger communities (glaciation)
- tropics relatively unchanged for at least 150 x 10^6 years
- stable in the sense of low seasonality
- empty niches outside tropics
- no ecological or evolutionary time for species to re-invade or evolve to fill them
Describe the failings of time and stability in latitudinal gradients
- many exceptions where stable habitats have low diversity (e.g. tropical mountain tops)
- many tropical environments have seasonally fluctuating environments
Why is explaining the lateral gradient so difficult?
- many of the explanations are circular
- most of the explanations make (similar) qualitative, not quantitative predictions
- data are often messy (many sources)
- sample size is small (n = 1)
- doing experiments is difficult
Tropics may have…
- lower extinction rates
- increased speciation rates
Describe the potential lower extinction rates of the tropics
larger populations and refuges from environmental change
