Unformatted text preview: Lecture 21 - Chap 22 Island Biogeography Chapter 22 Introduction - Island Biogeography
• Geographic Ecology Island Area and Species Richness
• MacArthur defined geographic ecology as geographic the search for patterns of plant and animal life that can be PUT ON A MAP Preston found the Preston found fewest bird species living on the smallest • Nilsson et al. found Hierarchically, it’s above the level of landscape ecology.
Vast breadth Chapter 22 only focuses on a few aspects of geographic ecology of
1 islands and the most and species on the largest islands island area was the best single predictor of species richness among
‘ woody plants, carabid beetles, and land snails. 2 Habitat Patches on Continents: Mountain Islands
• As Pleistocene ended and climate warmed, forest and alpine habitats contracted to the tops of high mountains across American Southwest. - Woodlands, grasslands, and desert scrub, invaded lower elevations Montane "islands" surrounded by warmer lower altitude landscape Once continuous forest converted to Once series of island-like fragments associated with mountains: Montane environment Warming climate geographically restricts montane forest montane forest
3 4 1 Lecture 21 - Chap 22 Island Biogeography Mountain “Islands” Mountain Area and Species Richness • In isolated mountain ranges, Lomolino et al. Lomolino et found more species of mammals that were adapted to mountain habitat in the larger areas of montane habitat
5 Lakes as Islands
• Lakes also can be considered as habitat islands. Differ widely by degree of isolation. - Tonn & Magnuson found the number of species increases with the surface area of an insular aquatic environment Insular means - removed from others (like an island)
6 Lakes as Islands in a Sea of Land
Species Richness in Fish
• • • Lakes as Islands
Tonn & Magnuson looked only at lakes in Tonn Magnuson Wisconsin. What about at a larger scale? Barbour and Brown compared 70 lakes Brown throughout the world. Lakes ranged in size from 0.8 to 436,000 km2. There are so many lakes in the area, that Tonn & Magnuson could look at lakes matched by chemistry, depth and sediment type, but that differed by size
7 - They found a positive relationship between lake area and fish species richness - just like in Wisconsin - Relationship applies across large scale, too 8 2 Lecture 21 - Chap 22 Island Biogeography • • • Isolation reduces bird diversity on islands. Williamson compared island area and species Williamson compared richness in Azore Islands & Channel Islands richness Islands Influence of isolation on bird diversity is clear.
This mode of dispersal reduces richness at large distances due to exhaustion of migrant 9 birds Marine Islands In contrast, distance from shore doesn’t seem to reduce fern diversity
10 Marine Islands
Terrestrial birds must fly across water to reach islands
MacArthur & Wilson Wilson • These ferns produce large quantities of small, light spores that are easily dispersed of great distances in the wind Equilibrium Model of Island Biogeography
• MacArthur and Wilson: Built an MacArthur Wilson equilibrium model explaining patterns of species diversity on islands as a result of immigration & extinction. They reasoned that rates of immigration would be highest on new island with no organisms Equilibrium Model of Island Biogeography indicates number of species present determined by immigration versus extinction As species began to accumulate, the rate of immigration (of new species) rate of would decline because fewer of the new arrivals would be new species 11 12 3 Lecture 21 - Chap 22 Island Biogeography Equilibrium Model of Island Biogeography
• Equilibrium Model of Island Biogeography
• M&W predicted that the rate of extinction M&W extinction would rise with increasing number of species on an island for three reasons: Presence of more species creates a larger pool of potential extinctions As number of species increases, population size of each must diminish • Point where two lines cross predicts the number of species that will occur on an island. Predicted rates of extinction on islands in this model determined mainly by island size Large near islands support the highest # Large near Small far islands will support the lowest # Small far Small near and Large far will support an Small near far intermediate number
14 • • • As number of species increases, potential for competitive interactions between species will increase 13 Red= Immigration
Immigration is higher on Blue= Extinction
Extinction is higher on a
• Species Turnover on Islands near islands
Small isolated islands (far) have the fewest species small island Equilibrium model predicts that species composition on islands is fluid / flexible Species composition changes over time This change is called species turnover Large islands that are close to a mainland have the • most species Diamond tested the model for birds on nine California Channel Islands and found: - A stable equilibrium as a result of immigration and extinction
16 15 4 Lecture 21 - Chap 22 Island Biogeography • Changes in populations over 50 years based on censuses by Howell and later Diamond. Howell had limited access to Santa Rosa and San Miguel -. he Experimental Island Biogeography
• Simberloff and Wilson studied insect Simberloff Wilson recolonization of mangrove islands in the Florida Keys. • might not have observed all species present there at the earlier 17 time • Chose 2 stands of mangroves as control islands, and 6 others as experimental islands.
18 - Defaunated islands & followed recolonization Species number returned to previous levels • but species composition changed considerably - Consistent with the equilibrium model of island biogeography
• • Colonization of New Islands by Plants Rydin and Borgegard found variation in Rydin found species richness of plants correlated positively with island area and accounted for 44-85% of variation in species richness among islands. Both small and medium islands continued to accumulate species 19 -Had more species than small islands - Found it difficult to separate effects of area from effects of habitat diversity Large islands attained equilibrium of immigration and extinction. 20 5 Lecture 21 - Chap 22 Island Biogeography Manipulating Island Area
• • Simberloff tested the effect of island area on tested species richness by removing mangroves below high tide level. In all cases where island area was reduced, species richness of arthropods decreased Manipulating Island Area • each reduction in area Richness on control island increased slightly. Islands with reduced area lost species with Showed that area of island has positive influence on species richness - Increased area led to increased richness 21 22 • - replacement by individuals of same species
• Island Biogeography Update Brown and Kodric-Brown found higher Brown Kodric-Brown found immigration rates to near islands can reduce extinction rates. Contrary to the original MacArthur-Wilson model, distance from source CAN alter Also, Lomolino found island area can have a Also, found significant effect on immigration rates. Contrary to the M-A model, large islands can
23 Island Biogeography Update
• • EXTINCTION rates
• Area and isolation (M-W model) are only two of several environmental factors that affect island species richness. Continued testing of an hypothesis often modifies the original hypothesis or lead to a better understanding of occasional exceptions
24 • offer larger target for immigrants 6 Lecture 21 - Chap 22 Island Biogeography Latitudinal Gradients in Species Richness
• • Most groups of organisms are more species-rich in the tropics. Numerous hypotheses trying to explain: Time Since Perturbation More species in the tropics because tropics are older and disturbed less frequently Productivity High productivity contributes to high species richness
25 Latitudinal Gradients in Species Richness Environmental Heterogeneity More heterogeneity in tropics, thus more potential habitat areas and niches Favorableness Tropics have more favorable environments Niche Breadth and Interspecific Interactions Interactions (several contradictory hypotheses that are hard to test) • Brown suggests that the ultimate causes must be physical differences.
26 Latitudinal Gradients in Species Richness
• Continental Area and Species Richness
Rosenzweig found a strong found positive relationship between AREA and SPECIES DIVERSITY.
• Speciation and extinction rates. Tropics richness is greater due to higher rates of speciation and / or lower rates of extinction. There is much more land area in the tropics than in any other ecological zone. 27 - The tropical land area has similar temperatures so that there can be wide dispersal without large change in temperature 28 7 Lecture 21 - Chap 22 Island Biogeography Historical and Regional Influences
• Diversity of Temperate Trees
• area affect Latham and Ricklefs: Reported striking Ricklefs contrast in diversity of temperate zone trees that cannot be explained by the Temperate forest biome in Europe, Eastern Asia, and Eastern North America all have roughly the same area, but support vastly Latham and Ricklefs: Must examine conditions that these regions faced during the last glacial period. During last ice age, temperate trees in Europe had southward retreat largely cutoff by mountains that form east-west different levels of biological diversity - Eastern Asia: 3x NA and 6x Europe oriented barriers - Lower species richness as a consequence of higher extinction rate 29 30 Mountains and Temperate Forests Historical and Regional Influences Appalachian Mountains in N.A. run northsouth, thus temperate trees had an avenue of retreat as temperatures become colder - In Eastern Asia - also no mountain barriers
• 31 - After dispersal lines were cut by glaciation, speciation continued in Asia Concluded from various lines of evidence that most temperate tree taxa originated in Eastern Asia and dispersed to Europe and N.A.
32 8 ...
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