Biodiversity in the Anthropocene

• Extinctions tend to take place much more on islands than on continents
• Reptiles are more at risk to extinctions on islands
• Islands are central to our understanding of continental biogeographical patterns

Island types:

• Continental islands
  • Emergent fragments of continental shelves, separated from continents by shallow waters e.g. British Isles, following post-glacial rise in sea level. Classic land-bridge islands may have a full complement of species when they first become islands, but then extinction may follow if the island is too small to support all
• Continental fragments
  • Once part of continents but now separated by tectonic drift from continents by deep water; long isolated e.g. Sicily, Crete, Cyprus, Cuba, Jamaica, New Zealand
• Oceanic islands
  • Origins in submarine volcanic activity, never connected to mainland e.g. Iceland, Hawaiian archipelago. Classic oceanic islands start off 'empty' and fill with animals/ plants which disperse to them - founder effect

• Typically, a species colonising an island will establish itself through a small founding population
• Founders have only a subset of the genetic variation of the entire species with no new introduction of genetic variation
• Genetic bottleneck - narrowing of gene pool
• Leads to bias in the island population on which evolutionary processes then operate
• Genetic variability then increases after estalishment through mutation and resorting of genes
• Founder effects: e.g. ecological release, loss of dispersal powers, change in size (dwarfism/gigantism)
  • Change in size e.g. giant tortoises of Galapagos and Indian Ocean islands
  • But size variation is not solely restricted to islands, dangerous to infer a general rule from a few examples e.g. giant tortoises also existed on African mainland but are now extinct
  • Did large sizes evolve after the founders reached the islands or was larger size an advantage of dispersing to the islands?
  • 'Island rule' - small animals get larger and large animals get smaller, holds for mammals, birds, snakes and turtles
  • Suggestion that size is the most suited for the species to exploit the resources on that island

• Occurs when a newly colonising species encounters a new environment in which competitors or other interacting organisms are absent e.g. predators
• Response is a loss of 'unnecesary features' e.g. defensive traits. Also an increase in variation in features such as beak morphology
• e.g. fruit bats on Fiji are more diurnal than those on less isolated islands as there are no predatory eagles
• Loss of such traits is sometimes associated with lack of fear as shown by the extinction of the dodo and other species of flightless birds
• Ecological release also occurs in ecosystems today e.g. loss of top predator may change behaviour patterns e.g. Wolves in Yellowstone National Park

• Founding populations had to disperse considerable distances to reach some island e.g. Hawaiian islands nearest continent is N. America which is around 4000km away
• However, once isolated some species of birds and insects have developed flightless forms e.g. 18/20 endemic species of beetles on Tristan da Cunha in S Atlantic have reduced wings
• However, caution is needed as flightless species are also found on continents (e.g. ostrich)
• Some island plants also have reduced dispersal abilities by comparison with their closest relatives

• Radiation in terms of spreading out rather than in the physical sense
• 'The outcome of speciation and adaptation in the context of ecological opportunity' and 'evolutionary diversification during adaptive radiation involves two processes: proliferation of species from an initial ancestor to many descendants; and the adaptation of species to use different parts of the environment' (Losos and Rickleffs, 2009)
• Evolutionary development of distinct species (or varieties) occurs from a single ancestral form where radiation occurs due to the availability of empty niches (niches in the sense that there aren't other competing species which require those same resources)
• Availability of these niches allows a form of ecological release, allowing for diversification which can sometimes lead to speciation within a lineage (new species) or variety (polymorphism) with a single species
• Likely to occur on remote islands lying close to the periphery of a group's dispersal range - which limits the likelihood of another individual of the same species arriving at the island
• E.g. Hawaiian honeycreepers - estimated that around 25 original colonists resulted in 135 native species of which 81% are endemic (Losos & Rickleffs, 2009)

• Endemism occurs a lot on islands
• Data not known for all categories of animals, nor all regions/islands of the world
• But data is reasonbaly well knwon for land snails, lizards, birds and mammals
• Why do islands have such high levels of endemism?
  • Not just down to adaptive radiation, but also allopatric speciation
  • Allopatric speciation is in some sense adaptive radiation sped up
  • Sympatry: two or more species overlap in their distribution
  • Allopatry: two species with geographically separate distributions
  • Allopatric speciation: a new species arises in a separate geographical area. Strongly associated with islands given the isolation from mainland, but within an archipelago there may also be allopatry at the inter-island scale
  • Parapatric speciation: a new species arises in a zone of contact (hybrid zone) between two species
  • In reality, the distinction between the three may be blurred

• Species dispersal
• Spread of taxa by people as they have crossed the world
• Variously known as exotics, aliens, non-natives
• Sometimes deliberately introduced to new locations: domesticated animals and cultivated plants
• Sometimes accidentally introduced
• Some introduced species become establish or 'naturalised', some become invasive or pests
• Islands may be particularly threatened by introduced species
• Agriculture also spreads as people spread
  • Neolithic transition (wheat, barley, pigs, sheep, cattle)
  • Trade and exchange (maize, potatoes, tomatoes)
  • Impacts landscapes on various timescales
  • Impacts indigenous species
  • Impacts culture

• Biological invasions are one of the top 5 divers of extinction together with habitat destruction, over exploitation, climate change and pollution
• Also linked to declining status of species threatened with extinction
• Degrade ecosystem functioning: altering trophic interactions, nutrient cycling, habitat structures
• In terms of economic damage, estimated to cost Britain £1.7 billion a year (ceh.ac.uk, 2020)
• Impact on human health e.g. in 1991, more than 10,000 died of cholera in Peru when ballast water containing Vibrio chlorae microbe was accidentally discharged into drinking water
• Predation example: 'The lighthouse keeper's cat effect - rats, mice, mongoosesm mustelids
  • Predation may lead to community level changes via alterations to trophic cascade or may eliminate a few species
  • Islands may be at risk as their native species lacked predators prior to introductions
  • E.g. Gough Island, one of the most important seabird nesting sites in the world - invasive mice have grown 50% larger than usual as they have no natural predators, predate eggs and chicks of birds including critically endangered Tristan albatross
  • E.g. snakes on island of Guam, possibly stow away via military cargo in 1940/50s. By 1968 had spread across islands with 12-15,000 snakes per square mile reported in 2001. Vertebrate fauna of Guam had evolved in absence of native predators so lacked defence mechanisms. Snakes consumed bats and birds so there was loss of pollinators and seed dispersers. Considered responsible for extirpation (local extinction) of all breeding populations of seabirds, 10/13 native forest bird species, 5/10 lizards). Insect outbreaks due to loss of insectivore birds. (Whittaker & Fernandez-Palacia, 2007)