GEOG20009 Lecture Notes - Lecture 19: Topographic Isolation, Ecological Niche, Biogeography
15 May 2018
School
Department
Course
Professor
LECTURE 19: ISLAND BIOGEOGRAPHY
ISLAND TYPES
• Islands are a natural laboratory for biogeography
• Islands: any piece of sub-continental land surrounded by water (not size related)
o Grouping of related islands = archipelago
• Types:
o Depositional islands: consist of sediment, quite young, dynamic – barrier & fluvial
islands
o Artificial islands
o Tropical/Reef islands: created by coral growth, then colonised by plants
o Atoll (Volcano and Reef)
o Volcanic islands: lots formed all the time, good for biogeography
o Ecological islands: habitual islands – ecosystems island from surrounding land
▪ Often due to anthropogenic isolation and fragmentation
o Elevation islands: topographic isolation
o Continental v Oceanic islands – whether land is on continental shelf or not
▪ Microcontinental island – continent is rifted
ISLAND BIOGRAPHY AND ECOLOGY
• Basic characteristics of insular biotas (species richness):
o Species-area relationship
o Species-isolation relationship
o Species turnover
• Number of species inhabiting an island represents a dynamic
equilibrium between opposing rates of immigration and extinction
ISLAND BIOGRAPHY AND EVOLUTION
• Immigration and Extinction: selective over long timescales
o Biased subset of mainland species
• Increasing island area: larger capacity, habitat diversity and barriers
• Increasing island distance: lower gene flow (higher isolation from mainland)
• In situ speciation = selective
• Islands are centres of endemism and diversification – yet only account for 3.6% of global land area
Island Biography
• Body size important feature: relates to resource requirements, reproduction,
survival - ecological functions of organisms
o In isolation, body sizing is a very dynamic trait
• Body size of mainland v island populations – depends on pre-existing body
size = island rule
o Small body sizes often leads to gigantism
o Large body sizes lead to dwarfism
• Optimal body size – depends on ecological conditions – to fill environmental
niche vs Island Rule
Dispersal Loss
• Moas in New Zealand: nine species of flightless birds endemic to NZ
o Lack anchoring structure for flight muscles
o Colonised as flying birds, then loss of flight was evolutionary
▪ Character displacement and loss of dispersal capabilities
o Evolution of body size in isolation towards ‘optimal size’
Gigantism
• Vika (species of Giant Rat) – existence of species suspected extinct for over 2 decades
o Solomon Islands: evidence of holes in coconut, then found giant rat living in tree canopy
o Evolved to large body size whilst living in isolation
Topics
• Islands – importance
and types
• Evolutionary island
biogeography
• Evolution on an island
continent
Ecological pressure (predation, niches already filled) vs ecological release (new niches not filled in species-poor
islands) = diversification & adaptive radiation
Document Summary
Islands: any piece of sub-continental land surrounded by water (not size related) Island biography and ecology: basic characteristics of insular biotas (species richness): Species turnover: number of species inhabiting an island represents a dynamic equilibrium between opposing rates of immigration and extinction. Immigration and extinction: selective over long timescales: biased subset of mainland species. Increasing island area: larger capacity, habitat diversity and barriers. Increasing island distance: lower gene flow (higher isolation from mainland) Islands are centres of endemism and diversification yet only account for 3. 6% of global land area. Island biography: body size important feature: relates to resource requirements, reproduction, survival - ecological functions of organisms. In isolation, body sizing is a very dynamic trait: body size of mainland v island populations depends on pre-existing body size = island rule. Large body sizes lead to dwarfism: optimal body size depends on ecological conditions to fill environmental niche vs island rule.
