On an evolutionary scale, the equilibrium number of species results from the balance between rates of extinction and speciation

Transcription

1 Island Biogeography On an evolutionary scale, the equilibrium number of species results from the balance between rates of extinction and speciation Island biogeography: the study of the abundance and diversity of species on islands, or island like habitats (mountain tops, ponds, forest patches ) The theory of island biogeography: On an ecological scale, the equilibrium number of species results from the balance between rates of colonization and local extinction (depends on the # of species present) Evidence for species equilibrium 1. Survey of arthropods on small mangrove islands at different distances off Florida (determined species diversity) 2. Fumigated islands to kill all species 3. Re-censused islands for 2 years to look at colonization 1

2 Evidence for species equilibrium 1. The number of species accumulates quickly and then tapers off 2. The equilibrium number of species is similar to the original values for each island 3. The species are turning over (changing) although the number of species stays the same Number of species present Predefaunation surveys 4. Why? The equilibrium theory of island biogeography The species equilibrium will be reached when rates of immigration match extinction - species composition may change, but the number will stay the same Rates of immigration by new species will decline as the number of species already there increases (e.g. it is increasingly unlikely that a new colonist represents a new species for the island) Rates of extinction increase as the number of species on the island increases a) more species to go extinct b) competition and predation are likely to increase with increasing diversity 2

3 The equilibrium theory of island biogeography All mangrove islands were small, but the ones closest to the mainland had the highest equilibrium number of species Why would distance change the equilibrium number of species? The equilibrium theory of island biogeography Immigration and extinction curves vary as a function of island size and distance Far islands will have slower colonization rates Small islands will have higher extinction rates 3

4 The equilibrium theory of island biogeography Immigration and extinction curves vary as a function of island size and distance small large S S S S Tests of the equilibrium theory of island biogeography Islands closer to Australia (the mainland source) have more species 4

5 Tests of the equilibrium theory of island biogeography Islands in the East Indies: bigger islands have more species Wooded habitats: larger fragments have more species Application of island biogeography theory to conservation Habitats are becoming more and more fragmented (like islands) Can we manage so as to maximize diversity within these fragments? Park design assumptions: a) immigration will be minimal since unprotected areas surrounding parks will be destroyed minimal immigration b) Species equilibrium numbers will be driven by extinction conclusions: a) small parks will have higher extinction rates 5

6 Application of island biogeography theory to conservation Extinction rates in US National parks Park/Park Assemblage % loss of mammals Area (km 2 ) Age (years) Bryce Canyon Lassen Volcanic Crater Lake Manning Provincial Mount Rainier Rocky Mountain 31 1, Yosemite 25 2, Sequoia-Kings Canyon 23 3, Olympic 6 3, Small parks have : a) higher rates of extinction b) species with low populations are most vulnerable (predators, jaguars, wolves ) c) Extinction of predators can alter diversity of prey Glacier- Waterton Lakes Grand Canyon Grand Teton- Yellowstone Kootenay-Banff- Jasper-Yoho ,627 4,931 10,328 20, Habitat fragmentation Roads bring colonists who clear the land (Rondonia, Amazonian Brazil) Roads reduce biodiversity by: a) making barriers to dispersal (many organisms wont cross a road or even go near the edge of their habitat b) increasing colonization by weedy species and invasives 6

7 Park design : how can we minimize loss? better worse why? 1. Size extinction less 2. Number Immigration more 3. Distance Immigration more 4. Connectivity Immigration more (corridors) Effects of connectivity (corridors) Plant species diversity for patches of long leaf pine habitat that were unconnected or connected to other patches via corridors 7

8 Effects of connectivity (corridors) unconnected Connected patches had higher diversity of native species (green bars) But no difference for exotic species (yellow bars) connected At the beginning of the experiment, plants were removed and recolonized over time Park design : how can we minimize loss? better worse why? 1. Size extinction less 2. Number Immigration more 3. Distance Immigration more 4. Connectivity Immigration more (corridors) 4. Shape Extinction less with less edge effects 8

9 Edge effects: size and shape Effects of shape and amount of edge habitat Plant species diversity for patches that had lots of edges (winged) had lower species diversity than those with less edge effects (rectangular) 9

10 Minimum critical size project, Amazon, Brazil (size and edge effects) In partnership with cattle ranchers and loggers, created habitat islands of different sizes (1-100ha) and monitored changes in diversity Smaller fragments lost diversity faster than bigger ones, but diversity in all fragment sizes declined Edge effects were enormous (changes in the abiotic environment near edges, such as increased light, drought stress, wind ) caused higher mortality and species loss Characteristics of species that make them vulnerable to extinction rarity dispersal ability diet specialization 10

11 Characteristics of species that make them vulnerable to extinction trophic status life span r (reproductive ability) Conservation biology Understanding how to manage ecosystems to maximize preservation of biodiversity 11