Bird Species richness per 110x110 km grid square (so, strictly speaking, alpha diversity) -most species live there!

Transcription

1 We "know" there are more species in the tropics Why are the Tropics so biodiverse? And the tropics are special: 1. Oldest known ecological pattern (Humboldt, 1807) 2. Well-known by Darwin and Wallace 3. Driven much ecological research Bird Species richness per 110x110 km grid square (so, strictly speaking, alpha diversity) -most species live there! 4

2 Rosenzweig, 1995 G&S, /191 studies of taxa (to the end of 2002) showed a latitudinal gradient (71%) -at all spatial scales -aquatic, terrestria; plants and animals -both alpha diversity (so communities are richer) and gamma diversity (entire landscape is richer) major exceptions: aquatic plants, parasite communities, parasitoid wasps, marine birds and marine mammals The tropics are species-rich and: 1. In the middle (mid-domain affect) 2. Bigger. More area = more species (just the interprovincial Species-Area curve again) 3. Older. More time = more species (older on two scales) 4. More stable. More specialization = more species (environmental variance): Monday's workshop 5. Richer. More energy = more species (environmental mean) a. productivity (so interprovincial SAR again!) b. actual rate of evolution is higher

3 maximum range size General Explanations MDE 1. Null model: Random ranges thrown down on 2-D map give rise to latitudinal patterns, due to hard barriers at the edges: this is known as the mid domain effect (or MDE) (b) uniform range sizes on a 'domain (e.g. continent). x is a latitude, and dotted line crosses the species to give spp richness (c) resulting latitudinal gradient. (b) produces small ranges towards the edges, which is not what we see on a global scale. Also, larger ranges produce stronger latitudinal curves (three curves in c)] Colwell & Lees, TREE 2000 But Mid-domain effect can operate at other scales. richness contour reef size MDE + Area is best model 11 Ecol. Lett

4 The tropics are species-rich and: 1. In the middle (mid-domain affect) Are the "tropics" bigger? 2. Bigger. More area = more species (just the interprovincial Species-Area curve again) 3. Older. More time = more species (older on two scales) 4. More stable. More specialization = more species (environmental variance): Monday's workshop 5. Richer. More energy = more species (environmental mean) a. productivity (so interprovincial SAR again!) b. actual rate of evolution is higher 14 Most of the world is tropical... Another look at the same pattern... gradient we see is due to autocorrelation

5 Log10.spp Log10.spp General Explanations Species richness, Area and Latitude 1. Null model - hard borders at the extremes (not perfect fit, sort of ignored) r 2 = More area: leads to higher net diversification rates (for some reason) Observational evidence from Rosenzweig 1995: 1. Pacific has more species than Atlantic 2. has more savannah species than 3. has more tropical tree species than World s Birds (Gaston & Blackburn, Macroecology,01) Area predicts richness latitude predicts richness too Log10.area 19 20

6 Log10.area Log10.spp take the residuals of richness on latitude Here, area and latitude are not correlated area predict richness after controlling for latitude area predict richness after controlling for latitude?? Log10.area 23 Log10.area 24

7 Log10.spp take the residuals of richness on area Latitude predicts richness after controlling for area?? Log10.area Latitude predicts richness after controlling for area 2. So for birds, both area and latitude affect species numbers independently. So Rosenzweig doesn't have total story...?? 27