Lecture 3 –
Islands as Biodiversity Hot-Spots

Islands: Ecology, Evolution, & Conservation

Dr. James A. Danoff-Burg

Department of Ecology, Evolution, & Environmental Biology

Columbia University

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

Island Biodiversity

• Characteristic patterns

– Low in richness

• Small number of species present

– Extremely high in endemicity

• Disproportionately rich in species found only there

• Many islands = Hot-Spots

Biodiversity Hot-Spots

• Definition

– Confined to a specific area è a unique biota

– Threatened by human activity

• Typically <30% vegetation intact

– Diversity of endemic species

• Origin of idea

– Conservation International

– Goal: help to provide a conservation ranking

• Will help to prioritize conservation efforts

• Given a limited amount of resources

Biodiversity Hot-Spots

• 1º continental

– 15 of 25

• 10 Islands disproportionately important

• 4 of 5 hottest are islands

– Madagascar, Philippines, Sundaland, Caribbean

Hot-Spots and Historical Biogeography

• How to explain current island biodiversity patterns?

– Historical biogeography

• Using the past distribution and origination patterns to explain current abundance patterns

• Largely = explaining colonization methods

– Dispersals

– Vicariant events

– Historical extinction patterns

• Many have already happened

• Distort historical biogeography conclusions

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

Oceanic vs. Continental Shelf Island Biodiversity

• Split between ocean & continental shelf islands is imperfect

– Really only extremes of a continuum

– Nonetheless, extremes are useful for elucidating process

Oceanic vs. Continental Shelf Islands

• Oceanic Characteristics

– Within a plate – volcanic origin

– Remote

– Isolated from mainland

• Great distance

• Deep seas

– Never been connected to mainland

– Lack indigenous mammals & amphibians

• Continental Shelf Chars.

– Geologically varied

• Sedimentation origin

• Volcanic origin

– Not remote from mainland

– Easily reached by many species

– Probably connected to mainland at low ocean levels

• Land-bridge islands

– Continuous land

– Stepping-stone islands

– Have indigenous mammals & amphibians

Oceanic vs. Continental Shelf: Implications

• Oceanic Chars.

– Fast moving evolution

– More isolated populations

– Evolution > immigration speed

• Continental Shelf Chars.

– Quicker moving evolution than continents

– Not as isolated as oceanic

– Immigration > evolution speed

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

Species Poverty

• Islands are depauperate relative to mainlands

– Fewer species / area

– More marked difference, the smaller the island area

• More depauperate / area with a smaller area

Species Poverty

• Nonetheless, highly important for conservation

– High endemicity

• St. Helena: 59 plant spp., 40 endemic (68%)

• Rapa: 105 land snail spp, 105 endemic (100%)

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

Disharmonic Diversity Patterns

• Definition

– Skewed balance of taxa relative to the mainland

– Superabundance of some higher taxa

– Absence of other higher taxa

Island Disharmony

• Two main features

– Polar climate and biota

– Dispersive subset of source pool

• Disharmony is on a regional scale

– Have some normally constituted ecosystems

– Island as a whole is disharmonic

Disharmonic Nature of Islands

• For both climate and biota of islands: More polar

– Not just climate

• as previously discussed

– Ex: Canary Islands

• Mediterranean flora

• Comparable coastal Morocco

– African tropical forest

Disharmonic Island Biodiversity

• Biota of islands: Dispersive portion

– Of mainland pools

– Only those that disperse well

• Using natural dispersal mechs

– Non-random sampling

• Not simply impoverishment

– Quasi-equal depression of all taxa

Disharmony Through Dispersal

• Characteristic distances for different vertebrate taxa

– Freshwater fishes = 5 km

– Large mammals = 50 km

– Lizards & rodents = 1100 km

– Land birds = 3600 km

• Isolated islands tend to have vertebrate fauna dominated by birds

Disharmony Ramifies

• Disharmony è More Disharmony

– Loss / absence of one taxon è loss or absence of others

– Through broken food web linkages, interspecific interactions, etc.

• Sweepstakes dispersal

– Definition: great fortune to those lineages able to overcome dispersal barriers

• Rapid & prolific speciation with successful colonization

Diversity Filters

• Definition

– No clear line, only a progressive reduction in diversity

• Examples

– Polar trending of islands

– Terrestrial habitat diversity present

• Islands without topography

– Only those that are adapted to the rhythms of the sea can exist here

– Usually marine stranded spp dominate

• Islands with topography have more habitats è greater endemicity

– Dispersal disharmony

– Sweepstakes dispersal

Biogeographic Provinces: Land Masses

• Often put into biotic regions

– Oriental

– Australian

– African

– Palearctic

– African

– Nearctic

– Neotropical

• From Wallace 1876

Biogeographic Provinces: Islands

• Difficult to assign with certainty

– due to differences between taxa

• Consequence of diversity filters

• Example – Wallace’s line

– Based on distribution of mammals

– Oriental vs. Australian between Bali & Lombok

– Heavily debated when using other fauna / flora

Biogeographic Provinces: Islands

• Difficult to assign with certainty due to diversity of source locations

– Example – Hawaiian flowering plants

• 40% Indo-Pacific origin

• 16% Australian origin

• 18% American origin

• 3% Palearctic origin

• 12% Cosmopolitan origin

• 11% Unknown origin

• è Which province does this belong?

Influences on Dispersal

• Geological history of the area

– Separated continental fragment islands

• Have low vagility endemics (frogs)

– Sea-level history

• Distance to source populations

• Ocean or wind currents

– Migration routes

• Habitat diversity of the island

• Biotic dispersal abilities

– Olympic ocean-going elephants

– Birds & butterflies – dispersal aces

Explaining Disjunct Distributions

• School of Historical Biogeography

• Two main explanations

– Dispersalist

• Descendent forms are product of chance long-distance dispersal

• Across a pre-existing barrier

– Vicariant

• Descent forms are a product of chance splitting / isolation

• Due to formation of new barrier

• Both operate

– Need to determine where & how often

• Not separate camps

– Example: Greater Antilles & Jamaica

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

Endemicity

• How things come to be endemic

– Palaeo-endemicity

• Relicts or strandings

• Dispersal / vicariant, then continental extinctions

• Of special conservation significance?

– Neo-endemicity

• Evolution in situ

• Dispersal / vicariant; then speciations

• Often these two explanations are in competition

Trends in Endemicity

• About 1/3 of endangered global floral diversity are island endemics

• Highest endemicity proportions are on:

– Continental island fragments

• Madagascar & New Zealand

– Have less vagile taxa as well as dispersalists

– Larger, higher islands

– Tropical & warm-temperate islands

• Regional island endemics

– Those that are endemic to entire island chains

– Larger chains have more endemics

Taxonomic Trends in Animal Endemicity

• Mammals – uncommon on islands

– Exception: bats & marine mammals

• Birds – excellent dispersalists

– High endemicity

• Lizards – common on islands

– High endemicity

• Butterflies – common on many islands

– Excellent dispersalists, low endemicity

• Other Insects – disharmonic diversity

– High endemicity in general

Today

• Biodiversity on islands

• Continents and oceanic islands

• Species poverty

• Disharmony and biodiversity filters

• Endemicity

• Extinction on islands

The Island Death Toll

• Many extinctions have already occurred

– Problem for historical biogeographers

• Losses due to:

– Climatic changes

– Sea-level changes

– Colonizations & Invasions

– Humans (twice)

Underestimating the Death Toll

• What we think we know now is probably inaccurate or incomplete

– Incomplete fossilization

– Incomplete paleozoology

– Overestimating extinct endemics from fossils

• Impacts on biogeographical explanations

• The picture is likely bleaker than we hope…

– Is the future of conservation too?

Assignments for Next Week

• Next week: Speciation on islands

• Read

– Chapter 4 in Island Biogeography, Robert Whittaker

• Write

– 1 page single spaced essay on Jamaican geological and climatic history and the relative importance of each on determining local biodiversity

– Take into consideration the methods of island formation

– Research island ecological & geological history

– Choose a single taxon (ants, beetles, lizards, birds, mammals)

– Due next week before start of class via email