Biodiversity in a Global Context
Robin Allaby
Outline
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Determinants of the world’s major biomes
Species diversity across the biomes
The latitudunal gradient of diversity
Global threats to diversity
Cook’s 2nd voyage 1772-1775
Johann Forster Observations Made During A Voyage Around the World (1778)
Holdridge Life Zone Classification
Holdridge Life Zone Classification
Tundra
• short growing season (50 days)
• permafrost
• low precipitation (< 40cm/year)
• low productivity (1g/m2/day)
• seasonal difference in temperature
Boreal
• 2 month summer
• precipitation (25-100cm/year)
• warmer summer (up to 23˚C, frost
free)
Cool temperate
• 4-6 month growing season
• 15-25˚C July isotherm, -10 - +15˚C
January isotherm
• still dominant winter/summer
(temperature driven) cycle
• range of precipitation levels
Cool temperate
Temperate rainforest
Cool temperate
Grasslands, steppe
Warm temperate
dry forest
wet forest
• transition to both temperature and
precipitation driving season
• 6-12 month growing season,
almost no frost
Warm temperate
sclerophyllous community
• drought tolerant
• fire adapted
Fynbos, chaperal,
mediterranean, heath
Hot desert
• precipitation < 25 cm/year
• high insolation
• high diurnal variation
• very high temperatures (up to
50˚C)
Savannas
• season driven by precipitation
• short rainy season 3-4 months
• long dry season 8-9 months
• precipitation 50-200cm/year
• warm all year round
Tropical seasonal forest
• rainy season 6 months +
• dry season 6 months –
• precipitation 150-200cm/year
Tropical rainforest
• precipitation > 200cm/year
• precipitation > 10cm/month
• temperature 21-30˚C, diurnal
variation wider than seasonal
• contants high humidity
Montane environment
The effect of altitude:
Tundra-like within rainforest
region:
• temperature range wide
• high UV
• low oxygen levels
Aquatic systems
• oxygen
• nutrient
River systems
• linear dependency
• rate of flow
oligotrophic
eutrophic
Aquatic systems
• seasonal stratification
Lake systems
Aquatic systems
• deep
low nutrient
low productivity
many species
• shallow
high nutrient
supplied by rivers
high productivity
Ocean systems
Biodiversity across biomes
Scales of diversity
• α diversity – small scale, homogenous area
• β diversity – rate of change in species
composition across habitats
• γ diversity – rate of change in species
composition across landscape gradients
Endemism in different taxa
Plate tectonics
Papua New Guinea
Eurasian e.g.
Parsonsia
Australoid e.g.
Nothofagus
Latitudinal gradient
Must be a difference in one or more of:
• speciation rate
• extinction rate
• immigration
Gradient causes 1: Area
Gradient causes 2: energy
More energy /cm2 = more biomass
Gradient causes 3: disturbance
Greater distal
disturbance
Global threats to biodiversity
Global threats to biodiversity
A worsening situation
Millenium Ecosystem Assessment