R. H. MacArthur E. O. Wilson 1963, 1967 Species richness on islands: Tended toward equilibrium number, Ŝ Resulted from balance between rates of immigration and extinction

Extinction vs. colonization Immigration rate Extinction rate Number of species P Ŝ Size of species pool

More realistically: Inflections due to interactions among species Extinction rate Immigration rate Immigration rates differ among species Competition among species increases as more species arrive P Number of species

Still more realistically: Extinction and immigration probabilities related to island area and distance from source Immigration to near islands more likely than to far vs. Extinction on small islands more likely than on large vs. Archipelago Mainland

Area and distance combined: near small Extinction rate Immigration rate Ŝ Species richness for a far, small island Ŝ Species richness for a near, large island large far P Number of species

Conceptually powerful tool: Can be used to make predictions across a broad array of circumstances

Near vs. far archipelagos: B A Near A B C C B A Far

Distance vs. area: C B A Large A B C Small C B A

Stepping stones: C B A Without A B C C B A With

Effects of clumping: C B A Scattered C B A C B A Clumped

Extinction Immigration Modifications: Area Distance Extinction Immigration Target effect MacArthur- Wilson Rescue effect

Predictions Number of species on an island constant, but turnover is considerable Relationship between island isolation and immigration Relationship between island size and richness Species area curve

Species-area curve S = cAZ Where: S = richness c = slope of curve A = area, and z is really hard to explain (usually 0.2 to 0.4) Number of species Island area

S = cAZ  Qualitative comparisons Archipelago B Predictions of IB hold for both Archipelagos S greater overall for Archipelago B Number of species Archipelago A Island area

log (S=cAZ)  log S = log c + z log A Why? Plot of logS vs. logA is linear z has meaning! Slope of the logS/logA line The rate of increase of species per unit area (or is it?) c is intercept of the logS/logA line Not of a lot of analytical interest (generally) Allows quantitative analysis: Island biogeography’s predictions Comparisons of biotas

c differs, z is the same Predictions of IB hold for both S greater overall for Archipelago A Log species Archipelago A Archipelago B Log area

Archipelago A Log species Archipelago B c and z differ Predictions of IB hold for Archipelago A only Log area

Species-area curves Probably broadest application of IB theory Used to explore relationships between S and: Space Time Body size Habitat diversity Sampling effort Speciation Extinction Coevolution [Rosenzweig, M. L. 1995. Species diversity in space and time. Cambridge University Press]

Caveat Species-area relationships do not validate island biogeography, per se Other reasons S could increase with area: On larger islands, the more you look, the more species you find (sampling issue) Speciation more likely on larger islands Large islands influenced proportionally less by disturbances Fewer catastrophes, more successional diversity Large islands contain more habitats

Pros of IB Elegant Rigorous Currency is tractable Simple A few causes explain multiple effects Rigorous Yields testable, numerical predictions Facilitates quantitative analysis Falsifiable Currency is tractable Species richness Important to research and conservation

Cons of IB Biological meaning? Non-mechanistic No biology involved beyond immigration and extinction rates Very few estimates of extinction and immigration rates, or species turnover exist How well does simple species richness portray community structure and stability? Application to landscape ecology & conservation? Are patches on a landscape analogous to oceanic islands?

What is biodiversity? Number of species? Relative abundance of species? Trophic interactions? Energy/nutrient flows?

How do we measure it? Count things: Estimate interactions: Vertebrates only? Invertebrates, too? Estimate interactions: Food webs? Rates of energy transfer? At what scale? richness area/time

I dunno… No 1 universal approach… Match the approach to the scale of observation Temporal, spatial, ecological Sounds like…. … hierarchy theory?

On landscape scales, maybe…? Competition Predation/herbivory Disturbance Coevolution Succession Parasitism Commensalism Mutualism Complex interactions (indirect effects) Transfer/flow of energy and nutrients Island Biogeography L - n L? L + n

Discussion What is biodiversity, really? How do we understand and measure it on large scales When is island biogeography appropriate to landscape ecology, when not? How much of island biogeography is implicit in our understanding of landscape ecology?