why study islands

7/23/2019 Why Study Islands

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ISLAND BIOGEOGRAPHY

Why study Islands?

First biologists and geographers studied them like Wallace (East Indies), Darwin

(Galapagos Islands) and Hooker (outhern !cean)"

#atural e$perimental plots which o%%er di%%erences in si&es, number o% species, isolation,

number o% predators"

Interaction much less comple$ than in mainland habitats"

Due to their isolation e'olutionar processes work at di%%erent rates

ittle or no gene %low to dilute the e%%ect o% selection and mutation causing a 'er high

le'el o% endemism

Depending on scale and dispersal abilit man habitats can be *Islands+ (lakes,

mountaintops, etc")

Islands can ser'e as natural %ield laboratories to stud the relationship between area and

species di'ersit

art o% unintentional e$periments are habitat loss and introductions o% in'asi'e species b

humans, o%ten detrimental conse-uences

!nl with a better understanding o% species.area relationships can we design optimum

conser'ation areas

What types of islands ae thee?

O!eani! islands" which are located o'er oceanic plates and ha'e ne'er been connected to

the continental shel%

#ontinental shelf islands$ which are part o% the continental shel% and can be connected

to the mainland during periods o% lower sea le'el

% Ha&itat islands/ distinct patches o% terrestrial habitat surrounded b 'er di%%erent

habitats but not water

% Non'(aine islands$ which are somewhere between habitat and continental shel% islands

in their le'el o% isolation

Natual distu&an!es of islands

0n relati'e discrete e'ent in time that remo'es organisms and opens up space which can

be coloni&ed b indi'iduals o% the same or a di%%erent species


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Disturbances can be short term and %re-uentl reoccurring like high winds or high rain%all

ome disturbances like E#! e'ents and hurricanes occurring e'er decade or more

with larger impacts on islands

!ther e'ents occur onl between 122 .1222 ears %or e$ample 'olcanic eruptions,tsunamis or earth-uakes

I(pli!ations of s(all foundin) populations

3picall the number o% organisms arri'ing b a chance e'ent on a remote island is small

mall %ounding populations containing onl a subset o% the source population+s

biodi'ersit can cause a genetic bottleneck

tudies on Hawaiian %ruit %lies suggest that %ollowing the arri'al o% a single %emale with

eggs on one o% the islands, strong selection %or %emales with less strict mate selection

genes were more success%ul

eading to a signi%icant shi%t in gene %re-uencies allowing better adaptation to the new

en'ironment (4arson 5225)

3he reduced genetic di'ersit in the %ounder population can also gi'e rise to random

genetic dri%t

Genetic dri%t b can lead to signi%icant changes in a species genetic makeup e'en without

%urther adaptation "

Giants and d*afs

3he Galapagos and Indian !cean tortoises were long regarded as tpical island giants,

but there ha'e been large mainland species, onl man are e$tinct due to humans

6ut a stud on insular species o% mammals %ound that 789 o% island rodents are larger,

possibl due to the absence o% predators (Foster 1:;


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+hee hypothesis fo )i)antis( of island spe!ies ,S!h*ane - Sae ./001

1" redation hpothesis/ either there is selecti'e release i% no predation occurs or there isselecti'e ad'antage to escape a window o% 'ulnerabilit

5" ocial.se$ual hpothesis/ due to high densities that occur among island populations,

intraspeci%ic competition among males and %emales selects %or larger bod si&e

@" Food a'ailabilit hpothesis/ increase in the mean and 'ariance in %ood supplAdemand ratio

selects %or giants


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Loss of dispesea&ility

0n interesting aspect o% man species which dispersed to islands is, that in man cases

the lost their dispersal abilit a%terwards

>an birds became %lightless, e"g" 0ldabran rails, Dodo+s, Bakapo

lants lost their abilit o% wind dispersal on near shore islands in 64 (4od and !'erton

1::;) and elsewhere

Flies lost their wings on 3ristan da 4unha and Gough islandsC elsewhere wing si&es are

reduced

!riginal theor was this occurred due to pre'enting wind loss particular in insects, but

?o%% (1::2,1::


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#esting sites o% se'eral bird species on the !rkne Islands shi%ted %rom cli%%s and trees to

shrubs and %lat ground

Adapti2e adiation

>ost well known e$amples are the Galapagos %inches and the Hawaiian hone.creepers

3he a'ailabilit o% empt niches is 'er important to adapti'e radiation, allowing the

di'ersi%ication which sometimes leads to new species

3here are also cases o% non.adapti'e radiation like the land snail genusAlbinariaon the

Island o% 4rete, which di'ersi%ied without occuping di%%erent niches (Gittenberger 1::1)

Island ende(i!s

>an endemics to islands used to ha'e a much wider distribution, but were replaced in

other habitats, hence not all endemics ha'e e'ol'ed in situ (palaeo.endemics)

!ne e$ample is the t Helena EbonC originates %rom a more widespread species :

million ears ago" ince then the %amil on the mainland has de'eloped awa %rom this

species (4ronk 1:7=)

Whereas species e'ol'ed on islands are called neo.endemics

3he issue/ whether palaeo.endemics are more important %or conser'ation due to a highercontribution to global biodi'ersit

3he number o% plant species endemic to the islands below (@;,822) contribute 1@"79 o%

the worlds higher plant species

0bout =,222 o% these are onl %ound within a single island or island archipelago


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3he percentage o% endemics are the highest %or ancient continental islands like

>adagascar and #ew ealand

Islands contribute disproportionate amount %or their land area to global

plant biodi'ersit

and snails/ onl 7 archipelagos account between ="=.:"29 o% the world land snail

species" In particular larger islands with higher ele'ation harbour man species

(Groombridge 1::5)

Insects/ in Hawaii+ are alone about 1222 species o% %ruit %lies (Wagner Funk, 1::8)"

i&ards/ 4aribbean anoles are small arboreal insecti'ores and one o% the larger and better

studied 'ertebrate ta$a" !ut o% @22 known 0nolis species hal% occur on 4aribbean islands

(osos 1::


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Williams (1:71) %ound a decrease in the number o% mainland bird species with

increased distance %rom the mainland


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?easons %or decline o% species di'ersit with distance

Dependant on dispersal pathwa, terrestrial mammals e$cept bats can onl disperse 'er

limited distances (omolino, 1:75)

6ird species can disperse o'er larger distances, as seen in the e$ample o% resident land

birds (Diamond 1:=5)

Dispersal abilities are also dependant on the tpe o% reproduction a organism uses

Di%%erent estimates %or ocean dispersal without human assistance is/ %reshwater %ish 8km,

elephants and other large mammals 82km, tortoises, snakes and rodents reached the

Galapagos 1122km, bats and land birds reached Hawaii+ @;22km (>enard 1:7;)

3here%ore the %urther an island is %rom the mainland the less species can disperse to it

Isolation %rom the mainland can also be changing o'er time

E$ample o% li&ard species on Islands in the Gul% o% 4ali%ornia (Wilco$ 1:=7)


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!ne o% the most ob'ious traits o% Islands are a limited number o% species, more countable

than on the mainland

3he area a'ailable %or species is also easier de%ined than on continents

Darlington (1:8=) %ound an empirical relationship between Island area and number o%reptile and amphibian species in the West Indies

Darlington (1:8=) %ound an empirical relationship between Island area and number o%

reptile and amphibian species in the West Indies

0s a log.log plot, it is not a cur'e but a straight line

0s a rule o% thumb with e'er 12 %old increase in si&e double the number o% species are

present"

S 3 # A4

Sis number o% pecies

#is a constant which 'aries with the ta$onomic group under stud (ta$a which consist o%

good dispersers (these species also tpicall ha'e rapid population growth) will logicall

accumulate more species on an isolated island, all else being e-ual)"

Ais the area o% the island, and the e$ponent & has been shown to be %airl constant %or

most island situations

represents a parameter %or the slope o% and 0 on a log scale

Geographic 'ariation in #has been obser'ed and loosel re%lects the isolation o% island

groups tpicall studied

3he presence or absence o% maor air or water circulation pathwas nearb increases #

3here are also e%%ects o% gross climatic di%%erence, #is higher in the tropics than %or

islands at high arctic latitudes

4 is also regarded as the the scaling %actor

5in an all out treatment, is related to the distribution o% abundances o% species

3here%ore the number o% species e$pected i% the total number o% indi'iduals increases, as

it would on a larger island, and those species %ollow a reston log.normal distribution o%

abundance (see >a 1:=8)

Interpretation o% these constant can be misleading (omolino 1:7:)


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5in an all out treatment, is related to the distribution o% abundances o% species

3here%ore the number o% species e$pected i% the total number o% indi'iduals increases, as

it would on a larger island, and those species %ollow a reston log.normal distribution o%

abundance (see >a 1:=8)

Interpretation o% these constant can be misleading (omolino 1:7:)

>an studies ha'e looked at and compared &.'alues %or di%%erent habitats

0n earl comparison (>ac0rthur and Wilson, 1:;=) %ound Islands to ha'e & between

2"52.2"@8 whereas non.isolated samples on continents or within large islands had a & o%

2"15.2"1=

3his suggests that an reduction in island area lowers the di'ersit more than a similar

reduction o% sample area in a contiguous mainland habitat

!ther studies (Williamson 1:77) ha'e %ound a less clearl marked di%%erence in &

between mainland habitats and islands

Wh might there be a di%%erence in the species.area relationship between islands and

isolated habitat areas on larger islands or continents

3he inclusion o% transients in species counts %rom small islands* on continents

pecies with large home ranges %or e$ample wol% with


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0 good record o% recolonisation, particularl b bird species %or the Brakatau Islands

a%ter the big 'olcanic eruption in 177@

0 rapid increase in bird species until 1:52, a%ter that number o% species remained

constant, but newcomers replaced alread present species

E6uili&iu( theoy of island &io)eo)aphy

Its based on the combination o% species.area relationship, species.isolation relationship

and species turno'er (>ac0rthur and Wilson 1:;=)"

It proposes that the number o% species inhabiting an an island is based on the dnamic

e-uilibrium between immigration and e$tinction"

3he model is one o% a dnamic e-uilibrium between immigration o% new species onto

islands and the e$tinction o% species pre'iousl established"

3he %ormula is/ St7. 3 St7I78'E

tis number o% species at time t

I is the Immigration rate

J is additions through e'olution

E is losses b e$tinction


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3he immigration rate is decreasing as there are %ewer and %ewer potential immigrant

species remaining in the species pool " 3his decrease is non.linear as the rate at which

di%%erent species can disperse is di%%erent (e"g" tortoise 's bat)

3he e$tinction rate increases non.linearl as %actors like competition, predation, and

parasitism become more important at higher species densities"


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+ests of the e6uili&iu( theoy

In an e$periment imberlo%% (1:=;) censused terrestrial insect species on mangro'e

islands, and then cut the islands into smaller ones b creating 1m di'ides" 3his was

su%%icient to re-uire ump dispersal %rom man insects

3he smaller islands maintained a lower species number according with the e-uilibrium

theor

3here%ore in this stud area as the onl 'ariable was a ke determinant o% number o%

species"

Is the *old that si(ple?

Here are man criticisms o% the E3I6

3he theor ignores autoecolog.but species are not e$changeable units (0rmstrong 1:75,

auer 1:;:)

Data is rarel ade-uate %or testing turno'er (nch and Kohnson 1:=ost turno'er in'ol'es transients (imberlo%% 1:=;)

3urno'er e-uilibrium has not been demonstrated (Gilbert 1:72)

Immigration, e$tinction, and species pool are poorl de%ined (Williamson 1:71,1:7:)

Ignores successional e%%ects and pace, and the hierarchical links between ta$a (6ush and

Whitacker 1::1)


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Su((ay

Islands pro'ide interesting stud areas %or the speciation, dispersal, coloni&ation,

e'olution, radiation etc"

3he simpli%ied island world allows easier hpothesis testing than more connectedcontinental habitats

Islands harbour a disproportional part o% biodi'ersit


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Refeen!es

Whitaker ?K 1::7" Island 6iogeograph, Ecolog, E'olution, and 4onser'ation, !$%ord

ni'ersit ress" 6!!B

Jitousek >, oope , 0dsersen H (eds) 1::8" Islands, biological di'ersit andecosstem %unction" pringer" 6!!B

6rown KH, omolino >J 1::7 6iohepgraph, second edition 6!!B

nch KD Kohnson #J 1:=< 3urno'er and e-uilibria in insular a'i%aunas, with special

re%erence to the 4ali%ronian 4hannel Islands" 3he 4ondor, =;, @=@.@7=

Gilbert F 1:72 3he e-uilibrium theor o% island biogeograph, %act or %iction Kournal

o% biogeograph, =, 52:.5@8

6ush >6 Whittaker ?K 1::1 Brakatau/ coloni&ation patterns and hierarchies, Kournal o%

biogeograph, 17 @

why study islands

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