Island biogeography: lecture topics

Introduction to island communities: Empirical patterns of species diversity on islands

MacArthur & Wilson Dynamic Theory of Island Biogeography Theory Tests Criticisms/problems

Environmental application of island biographical theory: Conservation Biology

community patterns of isolated oceanic islands like Cocos

Depauperate fauna, flora Disharmonious fauna, flora

Defined as non-representative types of organisms, including absence of typical mainland types

Cocos Island, for example, has no native land mammals, no amphibians, no bees, only one butterfly species, two lizards, four resident landbirds; and a disproportionate abundance of melastome plants (>12 species)

Proportionately many endemic species Closest relatives found nearby--Galapagos,

mainland Central & South America

Cocos Island, Costa Rica: Remote (isolated) island in E. Pacific Ocean (6.25º N. Latitude)

Aerial view of Cocos Island (rainforest covered) (Photo T.K. Sherry)

Only four Resident landbird species, of which Cocos Finch shown here (used as example of ecological release = niche expansion in prior lecture) (Photos T.W.

Sherry & T.K. Werner)

Dispersal is important aspect of which species (and how many) occupy oceanic islands

Colonization process, movement to newly created (or scoured) volcanic islands well documented Krakatoa in Java Straight Surtsey near Iceland Mt. Saint Helens colonization well documented

Waif dispersal = arrival by chance, based on wind, ocean currents, flying off course, floating on debris, etc. Thus, the kinds of species on islands are non-random

subsets of mainland pool of potential dispersers (waif dispersers are those species best adapted to disperse, survive during transport--e.g., r-selected!)

Wind, sea-dispersed organisms disproportionately abundant initially on Krakatau Island; all represent waif dispersal

Next pattern: Species-area curve illustrates dependence of species richness on area. Example shown here for herpetofauna (reptiles & amphibians) of Greater Antilles and nearby smaller Caribbean islands

Mathematics of species-area curve Number of species on islands increases with area of

island (same relationship holds for areas of mainlands!) Most prevalent mathematical form of relationship:

S = C*AZ

S = number of species on island, A = island area, C, z are fitted constants

I.e., log(S) = log(C) + Z*log(A) This is equation for a line on logarithmic axes:

log(S) = y axis log(A) = x-axis, log(C) = y-intercept = constant

Z-values = slope, determined empirically: average = 0.3; range: 0.15-0.35

Generalizations about z-vales (slope) of species-area curves

Mainland (“habitat island”) z-values (0.15-0.25) tend to be lower than real (oceanic) islands--why? (mainland areas at all scales tend to have “transient individuals”, because dispersal barriers reduced on mainland “islands”)

Spp. that disperse well tend to have lower z-values E.g., on Nevada mountain tops, birds’ z = 0.165,

mammals’ z = 0.326 Deserts below not such a barrier for birds, which disperse

more easily, show less effects of isolation (i.e., lower z; next slide)

Z-values (slope in species-area curve) tend to be lower in groups that disperse well (such as birds, graph b) than in groups that show more effect of isolation (such as mammals, graph c)

What’s biological basis for species-area curve? Smaller areas tend to have less habitat (e.g., Lesser Antilles

islands tend to be homogeneous dry forest, whereas greater antilles have desert, rainforest, high mts.)

Area effect per se? Some studies have shown statistically that area has an effect I.e., controlling for effect of habitat heterogeneity by considering

at any one time only islands with similar heterogeneity E.g., Barbuda (160 km2) has 20 resident landbird species vs.

Anguilla (90 km2) has 11 spp. (both are 300 m high islands, with similar habitats)

Smaller populations on smaller area tend to go extinct? (Very recent empirical support, but still weakly supported)

Smaller islands less of a target for colonizing species?

Species-area relationship extends to herbivores feeding on host tree species as “islands”

Final pattern: Isolation effect, shown by fewer species on isolated islands, in species-area curve for birds of warm ocean regions: red triangles represent isolated islands (>300 km from next largest land mass) (from Paul Slud)

Cocos Island, Costa Rica

MacArthur & Wilson (1967): Dynamic theory of island biogeography

Attempt to synthesize into a model empirical knowledge from foregoing discussion

Inductive theory that integrates immigration and extinction processes

It’s an equilibrium theory that predicts number of species at point that immigration (gain) rate equals extinction (loss) rate of species on islands

It’s one of most successful paradigms/models in ecology Graphical model: plot of I = immigration rate, E =

extinction rate, both as a function of number of species on island

Graphical model of one variation of MacArthur-Wilson Dynamic Theory of Island Biogeography

Things to note about model?

First, remembering axes is key to understanding model s-hat is equilibrium number of species...is this a dynamic

equilibrium? What is maximum I value, and why? Why does I curve

decrease? Why does E curve increase? Why are curves probably curvilinear, and not linear? What effect in the model of island size, immigration

constant? What effect in the model of isolation (distance effect),

island size controlled?

Effect of island area, distance held constant

Effect of island distance (isolation), area held constant

Both isolation (distance) effects on immigration, and island size (area) effects on extinction, combined into one model--showing different predicted equilibrium species richness values

Dynamic theory of island biogeography makes variety of deductive predictions (some new to this theory)

If decrease area, should decrease number of species If increase distance, should reduce number of spp. Constant no. of species, if island is in fact at equilibrium Existence of species turnover (new ones arrive, others go

extinct) Turnover rate should be greater in near, small islands than

far, large ones

No. Species on Island

Rate (immigration, extinction, turnover = T)

I

ET

Dan Simberloff tested MacArthur-Wilson predictions (turnover, area effect, equilibrium) with defaunated mangrove islets, Bay of Florida

Simberloff’s results, from defaunated mangrove islets

Simberloff’s results...

There exists an equilibrium species diversity on the islands (return, post-defaunation, to pre-treatment species diversity, based on island area)

Experimental reduction of mangrove areas caused decrease in species (OVERHEAD; Fig. 19.12, Stiling text)

Turnover of species composition, due to continual immigration and extinction on each island (data not shown)

Experimentally reduced areas for rainforest birds, Manaus, Brazil (Tom Lovejoy’s Minimum Critical Size of Ecosystems study)

Result? Many animal species disappeared from forest fragments, especially rainforest interior species; big edge effects on trees & other species

Criticisms of MacArthur-Wilson theory Few tests conducted to date...and they don’t usually identify

underlying processes (immigration, extinction) Underemphasizes evolution on islands (Mark Williamson:

continental ids. dominated by ecological processes, oceanic ids. by evolution) E.g., Cocos Id., Costa Rica, is distant (>300 mi. from

mainland in Pacific Ocean), dominated by endemism & adaptation--->low turnover rates of landbirds

Speciation important in archipelagos such as Gapalagos, Caribbean Ids.--e.g., birds, reptiles, amphibians, plants

Ecological barriers to colonization (David Lack’s idea): Disharmoniousness makes environment hostile to

colonization by some organisms Thus colonization is more than just dispersal!

Cocos Island: Vagrant, starving great blue heron (due to disharmonious-ness of fauna, harsh climate) would be unlikely to colonize successfully. Thus successful dispersal does not guarantee successful colonization.(Photo T.W. Sherry & T.K. Werner)

Additional criticisms/additions to MacArthur-Wilson theory

Rescue effect (Brown & Kodric-Brown): distance affects not just immigration, but also extinction rates...near islands have immigration that reduces chance of extinction

Target effect (Whitehead and Jones): area affects not just extinction, but also colonization...larger islands provide larger targets to colonists

Importance of history E.g., landbridge islands such as Trinidad have not yet relaxed

(extinctions still ongoing) to expected = lower equilibrium species richness of island its size

Rickleffs work on molecular phylogenies: History more impact on Caribbean bird distributions than equilibrium

Relaxation effect: Number of species

on continental islands decreases

back towards MacArthur-Wilson

equilibrium over time, from super-

saturated condition when island

formed by post-Pleistocene sea-

level rise

Applications of island biogeography theory? Design of parks & natural preserves so as to maintain

native diversity Parks = “islands” in sea of inhospitable (e.g.,

agricultural) habitats Best designs to maximize area and minimize dispersal

distances between units of park: Next slide Related design feature: Parks with top predators better

than those without (b’cause top predator helps maintain diversity of organisms at lower trophic levels)

Important implications for field of Conservation Biology!

Application of island biogeography ideas for design of nature preserves

Better design Worse design

a.

e.

b.

d.

c.

Corridor

Proximity

Proximity

Less subdivision

Area

f. Edge effect (perimeter)

Conclusions: Many patterns of species richness on islands well

known for a long time--e.g., spp-area relationship MacArthur & Wilson synthesized these ideas into a

predictive, mechanistic model, “dynamic theory of island biogeography”

Many deductive predictions made from this model, and many of these are supported by both observational and experimental tests

Some problems with model are well documented, and these add to our understanding of when model is applicable, rather than invalidating model

This is well supported equilibrium model, attesting to balance of extinction with origination processes influencing community structure

Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.