FRAGMENTATION

• What is the process of fragmentation?


  Large, continuous area of habitat broken into smaller, isolated patches, result of natural events or anthropogenic activities (e.g., urbanisation, agricultural expansion, deforestation)
• Islands


  Natural isolated patches of habitat within a larger landscape
• Once a population has established on new island, subsequent dynamics depend on the balance between rates of immigration, speciation and extinction
• Trends of insular size evolution

  • No large carnivores
  • Large mammals evolve to become dwarfed/small versions
  • Small mammals evolve to become larger/giant versions
  • Birds evolve to become flightless and larger
• Ecological explanations for size evolution on islands

  • Intensified intraspecific competition = due to limited space/resources
  • Release from predation = islands lack natural predators, allow species to be larger
  • Dwarfism facilitates high population density = allows an increase in population size
• Null hypothesis for size evolution on islands

  Random evolution (chance), due to random genetic drift rather than specific ecological pressures
• Adaptive radiation


  A single ancestral species rapidly diversifies into multiple new species, each adapted to different ecological niches
• Taxon cycle


  Species (of marginal habitats) will colonise islands, adapt into separate subspecies due to individual island conditions, specialised populations decrease in size, small populations are more likely to die out/higher risk of extinction, endemic species of particular islands evolve, creates ecological space for new colonists/new wave of colonisation
• Lerner et al. (2011) - Hawaiian honeycreepers


  • Ancestor was finch-like with thick bill, adapted for generalist diet
  • Geographic isolation on islands
  • Adaptive radiation led to evolution of many various species
  • Filled various ecological niches = insectivores, nectarivores and seed eaters
  • Specialised adaptations = evolution of tubular tongue for nectar feeding
  • Himatione spp = specialised for feeding on nectar from the dominant ohia tree
• Species-area relationship


  Log-log plot = straight line

  Continental samples are higher and have shallower slope than island samples

  Converge at very large areas, large island ≈ continent

  Diverge with decreasing area, small islands are more depauperate for equivalent area of mainland

  
• Explanation for species-area relationship

  • Larger areas have more habitats

  Passive sampling → larger sample include more species, larger area samples a greater proportion of available species

  Dynamic equilibrium theory of island biogeography – MacArthur & Wilson (1967): The number of a species on an island is a balance between the immigration of new individuals and the extinction of older individuals

  
• What is a metapopulation?

  Network of extinction-prone spatially-separated populations of a single species, each unable to sustain itself without immigration from other patches
• Characteristics of metapopulations


  • Habitats are patchy, rarely homogenous

  Populations are rarely continuous and have spatial structure

  Populations die out – population size affects the rate of extinction

  Population growth rate (r) affects population dynamics = r > 0: excess individuals that can migrate to other patches, r < 0: die out in long-term

  Medium dispersal, low heterogeneity and high stochasticity ∴ rare (Fronhofer et al., 2012)

  
• Metapopulation dynamics


  • Source-sink dynamics = in well-connected heterogeneous patches, some areas (source patches) export individuals to other patches (sink patches) through dispersal and colonisation, can rescue patches from decline and recolonise extinct patches

  Mainland-island systems = large stable (mainland) population provides individuals that migrate to small extinction-prone (islands) populations

  Immigration rate > extinction rate = system persists

  As fragmentation ↑, connectedness ↓, immigration ↓, extinction ↑, I < E

  Movement impact between patches: None = set of isolated, genetically and ecologically independent populations, High = single genetically and ecologically uniform population, Intermediate = true metapopulation structure, some genetic exchange between patches but not complete homogeneity

  The probability that an empty patch will be colonised depends on its area/size and distance from the nearest occupied patch

  The degree to which patches are connect affects the ability of the species to disperse and colonise new areas, connection needs to be applicable to specific species
• Habitat loss follows linear progression, but ecological impacts are highly nonlinear

  
• Combination of habitat loss and fragmentation have additive effects, exacerbate risk of species extinction
• Conservation thresholds


  Landscapes have thresholds of conservation value
• Efficient conservation


  • Large habitat area – support more species

  High habitat quality

  Connected spatial arrangement – preserving or creating corridors

  Matrix habitat lethality – the quality of the intermediate habitat between patches (matrix habitat) affects species movement, less hostile matrix habitat can improve dispersal

  Area-dependent dispersal – species' dispersal abilities depends on size and arrangement of patches

  
• Diniz-Filho et al. (2021)- Red Deer on Jersey


  Red Deer on Jersey

  • Looking at quantitative genetics
  • Jersey was isolated by rising waters of Atlantic at end of Ice Age due to melting of the ice caps
  • Pre-isolation fossils = size was normal
  • Post-isolation fossils = rapid evolution of dwarfism, 6-fold weight reduction in 6000 years
• Lomolino et al. (2021) - island rule


  Negative correlation between mainland and island body sizes
• What is the most challenging part of colonisation of an island?

  Establishment (survival, reproduction and maintenance) - moving individuals of a species to an island (via natural migration or human-assisted translocation) is the easy part
• Desmet (2018) - conservation thresholds


  Landscapes have thresholds of conservational value, these thresholds can be applied to quantitative target setting for systematic conservation planning, assessing persistence
• Extinction threshold


  Minimum amount of habitat required for a population to survive, 20% habitat amount, result of Allee effects
• Percolation threshold


  Minimum amount of habitat required for a population to successfully move in a landscape, 59% habitat amount, result of the effect of the spatial arrangement of patches and dispersal
• Didham et al. (2012) - habitat fragmentation paradigm

  Hierarchical model of potential direct and indirect causal paths through which the amount and the spatial arrangement of habitat can affect a measured response variable
• von Seth et al. (2022) - Chatham Island Black Robin (Petrocia traversi)

  Inhabits fragmented environment off coast of New Zealand, faced near-extinction due arrival to islands on multiple occasions not leading to long-term establishment
• Species-area relationship equation


  $log S =$$z. log A+$$log k$
• Powered species-area relationship equation

  $S =$$k.A^z$