BIOL 4120

Principles of Ecology

Phil Ganter

320 Harned Hall

963-5782

Sedimentary Rock in Zion National Park

Lecture 19 Landscape Ecology

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Overview - Link to Course Objectives

Landscapes

  • A Landscape is mosaic of environmental Patches considered together
    • Each patch is embedded in a matrix of surrounding patches that differ in some significant way from the patch under consideration
    • Patches are (relatively) homogeneous within and differ from thier neighbors
  • Patch edges can be
    • Inherent Edges - those set by natural geographic features
      • Inherent edges are more or less permanent, as least as long as the geographic feature is permanent
    • Induced Edges - those that have been created by disturbance (fire, flood, etc.)
      • Induced edges are usually temporary unless the underlying geography has been changed by the disturbance
    • Patches for plants are defined by physical characteristics such as soil type, moisture, etc.
  • Where patches meet, Borders are formed
    • Borders can be
      • Narrow or Wide
        • Wide borders become habitats of their own and are referred to as Ecotones, transition zones between adjacent patches
        • Ecotones are places where hybrids between species characteristic of the adjacent patches are found
        • Classic example comes from wild iris species - one species found in wet soils next to streams and another found in dry soils in the forest with the hybrid found in a zone between forest and stream with soils intermittantly dry and wet
      • Straight or Convoluted
      • Sealed or Perforated
        • Sealed and perforated refer to the ease with which organisms move from patch to patch and so this quality will depend not only on the physical characteristics of the patch but the characteristics of the species being considered.  A sealed border for some species may be perforated for others.
  • Edges have different conditions than do the Interior of the patch
    • Some species may be adapted to edge conditions, others found only in patch interiors
      • Edge Effect - edges often have increased species richness when compared to the interior of the patches because, in addition to the species that live in the patch without regard to interior-edge differences:
        • the edge may have species from the adjacent patch foraging there
        • the edge may have species adapted to the edge conditions
      • Forest edges and interiors are examples of the effect of edges
        • Forest edges have more light than the interior and are drier than the interior
  • Since edges affect the habitat, the shape of the patch becomes important as it will determine the ratio of edge to interior area
    • There is relatively more edge when:
      • the patches are smaller
      • the patches are long and thin rather than circular or square

Island Biogeography

  • Theory that island community compositions are due to a balance of colonization and extinction
    • Much early work done on bare islands either new (volcanic activity) or where a catastrophe has removed most or all of the flora and fauna
    • Many saw succession in the plants that were able to colonize these areas and so islands were viewed as good examples of succession
  • MacAuthur and Wilson viewed islands as dynamic communities, where the species present were the product of constant invasion and local extinction
    • This dynamic process of extinction and colonization leads to species turnover, where it is difficult to predict which species will persist and which will go extinct when comparing different islands, although the total number of species may remain steady
  • They developed some rules for predicting the number of species on an island (either total or some subset &emdash say the bird community on an island)

    MacArthur & Wilson's rules

    • Species number should increase through time, but rate of increase will slow until it becomes zero and the number of species no longer increases (some sort of curve with an asymptote)
      • Number of species at asymptote is an equilibrium between rates of Colonization and Extinction
      • Notice that speciation is not considered - not because it doesn't happen but because the time scale is too short for speciation to have much of an effect
        • Extinction rates depend on island size
          • Smaller islands have smaller population sizes, fewer refugia, and, therefore, a higher rate of extinction
        • Colonization rates depend on the island's distance from the continental source
          • More remote islands have fewer colonists arrive than closer islands
        • When there are islands between the source (the continent) and the island, these may act as "stepping stones" and may increase the rate of colonization for remote islands
    • Other rules have been added, as refinements of the theory due to new ideas and new data
      • When two islands are the same distance from a source the larger island will have a higher colonization rate (the interception arc is larger)
        • this is the Target Effect (bigger targets are easier to hit)
      • When an island is close to its source of species, both the rate of colonization and the rate of extinction are affected
        • extinction rate appears to be lower (or is lower) because a species can be replenished or re-colonized from the source
          • This effect is called the Rescue effect
    • What happens is that new members of a species already on the island arrive, and are counted as natives (one can't tell which organisms are migrants and which are not)
      • The new arrivals increase population size and make extinction less likely
    • When a community's landscape is patchy and each patch is surrounded by unsuitable habitat, then the rules of island biogeography may apply to other situation
      • mountain tops in a mountain chain, small ponds, etc.
      • Habitat islands can be large geographic objects (mountain tops, lakes) or organisms (individual trees)

    Species area relationship

    • Not expected to be linear, but a power function
    • S = number of species, A = area of the island

    • z and c are both constants needed to fit area data (in m2, km2, etc.) to number of species
    • This can be linearized by taking the log of both sides:

    • In this form, you can estimate c and z from a plot of S versus A
      • If the theory of island biogeography is true, this relationship must be true
        • This relationship has been shown to be true for many island communities, but not all
        • Examples: reptiles on Caribbean islands, insects on small mangrove islets
      • However, one expects to find more species with a greater sample size without there being and island effect
      • We expect the relationship to be stronger (larger positive slope) for continental communities that are confined to habitat patches with poor dispersal between patches
        • birds and mammals on mountain tops are in habitat patches
          • birds disperse long distances
          • animals are confined to patch
          • animals should have stronger relationship (and they do)
    • Rates of colonization and extinction lead to a constant number of species when they are equal
        • In the graph below, the extinction rate increases as the number of species present increases - more species means that more will go extinct simply through chance
        • The rate of colonization decreases as the species number increases because the pool of species not yet on the island decreases as the number already there increases

      • species richness remains constant when the colonization rate equals the extinction rate

        • The constant number of species does not mean that the same species are present but that there is Species Turnover so that the loss of species is balanced by the arrival of new species
      • As the distance between islands increases, we expect the colonization rate to decrease (near vs. far islands below)
      • As the size of islands decreases, we expect the extinction rate to increase (large vs small islands below)
        • As colonization and extinction rates change, we get new equilibria in species richness

  • Simberloff and Wilson
    • Fumigated mangrove islets just off of the Florida coast
      • Observed the re-colonization of the islet's animals (mostly insects but molluscs and vertebrates also present)
    • Saw rapid recovery to equilibrium number of species
      • There was an unexpectedly low rate of species turnover, although the rate was not 0
    • First interpreted this as confirming island biogeography
    • Later revised conclusion to say that the turnover rate was too low
        • Felt that biotic interactions (competition and predation, mostly) were more important than the colonization/distance and extinction/area relationships
  • Impact of island biogeography
    • important way of thinking for conservation biologists
      • decisions about how big to make wildlife preserves
      • decisions abut how to arrange smaller conservation areas around larger areas  (steppingstone preserves)
    • problem is that this approach is too simplistic
      • all islands are same type and quality habitat
        • clearly not true
      • dispersal is constant over time and among different members of the community
        • know that dispersal tends to be episodic, often tied to catastrophic events
        • know that species differ in dispersal abilities

Landscape Ecology, Island Biogeography, and Metapopulation Models

  • The application of island biogeography models to situations other than islands is very similar to metapopulation models
    • Similarities
      • both see the environment as fragmented into patches
      • both balance colonization and extinction rates
    • Differences
      • Metapopulation models deal with intraspecific population dynamics
      • Island biogeography models focus on community-level measures, mostly species richness
      • Island biogeography models have patch size and interpatch distance as part of the model, not all metapopulation models incorporate these factors
  • Landscape models are more realistic than either of the above approaches
    • Patch quality, shape, and edge/border effects are incorporated into these models

Disturbance and Landscapes

  • Disturbances are short-term events that disrupt community-level processes and may even alter patch composition of landscapes
    • Examples: fire, drought, windstorms and tornados, cold spells, floods, epidemics (happens in both animals and plants), wars, volcanic activity, rock and mud slides, avalanches, ice storms
    • Individual events have two landscape properties:
        • Intensity of the event - measured in terms of the loss of individuals (biomass) or habitat
        • Scale of the event - the area affected by the disturbance relative to the size of the landscape under consideration
  • Disturbance Regime - the recurring pattern of a particular type of disturbance (i. e. fire regime, hurricane regime, etc.)
    • Regimes have both intensity and scale but add the dimension of frequency - how often disturbances occur
  • Fire
    • Fire frequency is negatively correlated with rainfall
    • Fires are initiated by natural events, usually lightning strikes, or by human activity, sometimes deliberately
      • Some landscapes have fires so-frequently and of such scale and intensity that the species that live there have adapted to a fire regime that prevents other species from invading the community - the fire-adapted community is sometimes called a fire-climax community
    • Underburns are fires of low intensity and small scale burn off the litter and singe the lower trunks of trees but do not burn the foliage of mature trees
      • Greater intensity can damage thek soil's O layer
    • Brush fires are intense fires in shrublands
    • Crown Fires reach the tops of trees and spread from tree crown to tree crown
      • These fires spread faster as the wind speeds are greater above the canopy
      • Greater heat is released and these fires kill trees
    • Firestorms are the most intense fires that produce gale-force winds along the surface as air rushes in to replace the air rising above the fire zone
      • Only occur where a large amount of fuel has accumulated
    • Intense crown fires, brush fires and firestorms can permanently alter the landscape as they can burn off the humic content of the soil which can make the soil unable to support new tree growth
    • Humans influence fire regimes
      • Some cultures have historically initiated fires to maintain grasslands (Northern Australia, Western USA)
      • In the USA, we have historically suppressed fires
        • This can lead to fewer fires, but can increase the intensity when they do occur if fuels accumulate
          • In our western forests, the climate is so dry that decomposition does not, on average, consume the leaf and branch litter that falls each year, which leads to fuel accumulation over the years
        • Fire management now includes periodic, low intensity burns to reduce fuel accumulation
          • Questions remain about what to do about natural fires
            • Suppressing them can lead to worse fires in the future but, when the fires are frequent (in dry or drought years) or threaten human habitation or activity (smoke can make it hard to work outside and can harm those with respiratory problems), suppression may be the proper course
            • Controversy also surrounds the practice of removing dead tree trunks after fire has killed them.  These can be valuable in the short term to logging companies but may delay the recovery of the forest as they may promote new tree growth as they decay
  • Human disturbance is often long-lasting as we clear forest for fields and cover fields with tarmac (roads and parking lots) or buildings

Island Biogeography, dynamic communities, species turnover, Extinction rate, Colonization rate, stepping stones, rescue effect, patches, Habitat islands, Testable hypotheses, falsification, Species area relationship, power function, colonization/distance and extinction/area relationships

Terms

Patch, Matrix, Inherent Edge, Induced Edge, Border, Narrow Border, Wide Border, Ecotone, Straight Border, Convoluted Border, Sealed Border , Perforated Border, Interior, Edge Effect, Island Biogeography, Colonization, Extinction, Extinction Rate, Colonization Rate, Stepping Stones, Target Effect, Rescue Effect, Habitat Islands, Power Function, Species Turnover, Disturbances, Intensity, Scale, Disturbance Regime, Underburns, Brush fires, Crown Fires, Firestorm

Last updated March 21, 2007