Landscape Ecology
Part 1 - Island biogeography: landscapes as binary systems
Terms/people:
Island (insular) biogeography
Robert MacArthur
Dan Simberloff
Nestedness
SLOSS
E.O. Wilson
Jared Diamond
MacArthur & Wilson 1963, 1967
History
Islands have long been objects of mystery, intrigue, and scientific curiosity: they are
isolated, have obvious boundaries, often have exotic and strange biota (including
tendencies towards gigantism, dwarfism, and flightlessness); inspired Darwin, Wallace
islands vary in size and isolation from a source of colonists (e.g. mainland)
biogeography: study of the distribution of organisms
"Insularity is...a universal feature of biogeography. Many of the principles [seen on
islands] apply in lesser or greater degree to all natural habitats." (MacArthur and Wilson
1967)
theory of island (or insular) biogeography: MacArthur and Wilson 1967 (original paper
was in Evolution in 1963 but didn’t cause as much of a stir as the 1967 book)
-one of the most influential ideas in ecology (became a paradigm)
-modeled population persistence and community diversity as a function of area and
isolation
-study the graphs given in this link: note that the immigration curves decrease with
increasing richness whereas the extinction curves increase (know why these are so)
-note the intersection points on the curves: each is an equilibrium point
Island biogeographic theory describes patterns in species richness on islands as a function
of island area and isolation (distance from the mainland). The key tenets of island theory
relate the rates of local extinction and colonization to the number of species S on an
island. Colonization or immigration rate is expected to decrease as a function of
increasing species number S, because as more species occupy an island it becomes more
difficult to add a new species. Extinction rates are expected to increase with S, because
when there are more species present there are more chances that one of the species might
go locally extinct, each added species will be competing over limited resources, etc. The
expected number of species lies at the intersection of the colonization and extinction
curves. All other things being equal, large islands would be expected to have lower
extinction rates than small islands, because large islands would tend to support larger
populations and extinction probability would be lower for larger populations. Likewise,
islands distant from a mainland source of propagules would be subject to lower
colonization rates that islands near a source. Thus, all other things being equal, large
islands support more species than small islands, and near islands support more species
than far ones. These constitute the area and isolation effects of island biogeographic
theory. The area effect is often expressed as a species-area curve, by convention graphing
species richness against island area; that is, S = cAz, with z~0.25 in theory. This general
relationship is termed the species-area relationship: larger areas tend to support more
species.
island biogeography theory tested empirically by Simberloff and Wilson 1969 (also
Wilson and Simberloff 1969 [methods paper on defaunation technique]):
S&W 1969 - arthropods on red mangrove (Rhizopa mangle) islands in Florida
Keys (relatively simple system, "easy" to manipulate [although still an enormous
undertaking!])
Simberloff’s Ph.D. work - surveyed arthropods, then National Exterminators of
Miami constructed tents around islands and fumigated (fogged) them with methyl
bromide to defaunate them, then monitored islands for patterns of arthropod colonization
(pre vs. post treatment comparison along with a control vs. tt comparison); results:
richness usually recovered, but species identity was often different!
See Gotelli (2001) and Schoener’s chapter in Losos and Ricklefs (2010) for additional
examples of empirical tests of IB.
The metaphor of terrestrial "habitat islands" in a "sea" of other land uses was not lost on
MacArthur and Wilson, nor their followers (e.g. MacClintock et al. 1977, Burgess and
Sharpe 1981). Original IB theory pertaining to oceanic islands had terrestrial analogs:
patches (indeed, the first illustration in MacArthur and Wilson 1967 is of deforestation in
Cadez Township, WI!). Importantly, the two main predictions of island theory--the
species/area relationship and isolation effects--are borne out by at least some field
studies. (Click here for an example in the Amazonian rainforest.)
The application of island theory to terrestrial habitats launched a debate that raged over
the issue of whether the goal of maximizing species richness would be served better by a
single large or several small reserves (the SLOSS debate: see Diamond 1975, Terborgh
1976, Simberloff and Abele 1976). Although this issue has never been completely
resolved, it did serve the purpose of pointing out to many ecologists that the prediction of
IB theory--that is, species richness--might not be the most valuable currency for
conservation applications.
Simberloff 1976: tested SLOSS by using chainsaw to manipulate mangrove island size in
FL
found equivocal results: arthropod richness declined as island size decreased
(species/area effect), but SL not always > SS
No consensus:
SS > SL:
Simberloff and Gotelli 1984: found SS > SL for plant species in the US
Quinn and Harrison 1988: found SS > SL for animals in US national parks
SL > SS:
See Thomas et al. 1990 for northern spotted owls; Newmark 1987, 1995, 1996 for
mammals in US and African parks.
And many other studies (e.g. Virolainen et al. 1998, Oertli et al. 2002, Tscharntke et al.
2002, etc.)
Perhaps the lack of consensus comes from different response variables being measured:
richness vs. diversity, abundance, extinction risk.
nestedness
As it turned out, ecologists (and especially conservation biologists) often weren't really
interested in S, the total number of species at equilibrium; they often were interested in
which species were present. A growing concern was with so-called "area-sensitive"
species: species that were found to be symptomatically rare in or absent from small
and/or isolated habitat patches. In studies of forest birds, a subset of area-sensitive
species consistently emerged (Forman et al. 1976, Whitcomb et al. 1981, Lynch and
Whigham 1984). In particular, the area effect emerged only for this select subset of
species. The number of habitat generalists showed no relation to area, and edge species
richness actually decreased with increasing patch area (Whitcomb et al. 1981). IB theory
was mute as to the mechanisms that might explain this.
Exceptions to IB (see Forman 1995) may occur if only edge species are present (speciesarea relationship appears to be valid only for interior spp, not edge spp). (See Quinn &
Robinson 1987.)
Debate:
Diamond, Terborgh, and others argued that we needed to take action quickly
against ever-mounting biodiversity losses (biodiversity crisis), didn’t need pedantic
waffling; argued for conservation of area per se: said area was a useful surrogate for
many other variables and was unsubstitutable in and of itself.
Simberloff and others argued that application of only half-baked ideas can do
more harm than good; said that conservation of habitat diversity and for spreading of risk
would ultimately be more ecologically and economically feasible than conserving area
per se.
Being in a smaller patch isn’t necessarily a bad thing: there may be fewer predators in
smaller patches compared to larger ones (Hovel and Lipcius 2001).
Conservation biology, in particular, took to the IB approach. Click here for some IBbased conservation reserve “design principles” from Jared Diamond.
But terrestrial systems are not perfect analogs of oceanic islands! Therefore:
-patches are not islands: "no park is an island" (Janzen 1983)
-movement of individuals among patches (patch colonization and extinction rates) are not
simple functions of distance and area
And so, we're now saddled with the paradigm of island biogeographic theory, which is
now mainstream in ecology and especially conservation biology, yet the theory itself
offers surprisingly little of actual utility to landscape ecologists. Our task now is to make
the transition from island biogeographic to mosaic theory and the mechanisms of
community response to landscape (habitat) pattern--and then to try to find some insights
that we can apply to landscapes in general.
References:
Bierregaard, R.O., Jr., T.E. Lovejoy, V. Kapos, A.A. dos Santos, and R.W. Hutchings.
1992. The biological dynamics of tropical forest fragments. BioScience 42:859-866.
Brown, J.H. 1971. Mammals on mountaintops: nonequilibrium insular biogeography.
Am. Nat. 105:467-478.
Diamond, J.M. 1975. The island dilemma: lessons of modern biogeographic studies for
the design of nature reserves. Biol. Conserv. 7:129-146.
Forman, R.T.T. 1995. Land Mosaics. Cambridge University Press, Cambridge, UK.
Gilbert, F.S. 1980. The equilibrium theory of island biogeography: fact or fiction? J.
Biogeogr. 7:209-235.
Gotelli, N.J. 2001. A Primer of Ecology, 3rd ed. Sinauer Associates, Inc., Sunderland,
MA.
Hovel, K.A., and R.N. Lipcius. 2001. Habitat fragmentation in a seagrass landscape:
patch size and complexity control blue crab survival. Ecology 82:1814-1829.
Janzen, D.H. 1983. No park is an island: increase in interference from outside as park
size decreases. Oikos 41:402-410.
Laurance, W.F., S.G. Laurance, V.F. Ferreira, J.M. Rankin-de Merona, C. Gaston, and
T.E. Lovejoy. 1997. Biomass collapse in Amazonian forest fragments. Science 278:11171118.
Losos, J.B., and R.E. Ricklefs, eds. 2010. The Theory of Island Biogeography Revisited.
Princeton University Press, Princeton, NJ.
MacArthur, R.H., and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton
University Press, Princeton, NJ.
May, R.M. 1975. Island biogeography and the design of wildlife preserves. Nature
254:177- 178.
Oertli B., D. Auderset Joye, E. Castella, R. Juge, D. Cambin, and J.B. Lachavanne. 2002.
Does size matter? The relation-ship between pond area and biodiversity. Biological
Conservation 104: 59-70.
Quammen, D. 1996. The Song of the Dodo: Island Biogeography in an Age of
Extinctions. Touchstone, New York, NY. [a highly recommended book]
Quinn, J.F., and S.P. Harrison. 1988. Effects of habitat fragmentation on species richness:
evidence from biogeographic patterns. Oecologia 75:132-140.
Simberloff, D.S. 1976. Experimental zoogeography of islands: effects of island size.
Ecology 57:629-648.
Simberloff, D.S. 1988. The contribution of population and community biology to
conservation science. Annu. Rev. Ecol. Syst. 19:473-511.
Simberloff, D.S., and L.G. Abele. 1976. Island biogeography theory and conservation
practice. Science 191:285-286.
Simberloff, D.S., and L.G. Abele. 1982. Refuge design and island biogeographic theory:
effects of fragmentation. Am. Nat. 120:41-50.
Simberloff, D., and N. Gotelli. 1984. Effects of insularization on plant species richness in
the prairie-forest ecotone. Biological Conservation 29:63-80.
Simberloff, D.S., and E.O. Wilson. 1969. Experimental zoogeography of islands: the
colonization of empty islands. Ecology 50:278-296.
Stouffer, P.C., and R.O. Bierregaard, Jr. 1995. Use of Amazonian forest fragments by
understory insectivorous birds. Ecology 76:2429-2445.
Thomas, J.W., E.D. Forsman, J.L. Lint, E.C. Meslow, B.R. Noon, and J. Verner. 1990. A
conservation strategy for the northern spotted owl. Report of the Interagency Scientific
Committee to Address the Conservation of the Northern Spotted Owl. Portland, OR.
Terborgh, J. 1976. Island biogeography and conservation: Strategy and limitations.
Science 193:1029-1030.
Tscharntke, T., I. Steffan-Dewenter, A. Kruess, and C. Thies. 2002. Contribution of
small habitat fragments to conservation of insect communities of grassland–cropland
landscapes. Ecol. Appl. 12:354-363.
Virolainen, K.M., T. Suomi, J. Suhonen, and M. Kuitunen. 1998. Conservation of
vascular plants in single large and several small mires: species richness, rarity and
taxonomic diversity. J. Appl. Ecol. 35:700-707.
Whittaker, R.J., and J.M. Fernández-Palacios. 2007. Island Biogeography: Ecology,
Evolution, and Conservation (2nd ed.). Oxford Univ. Press, Oxford.
Part 2 - Island biogeography comes ashore
Terms/people:
Patch-matrix paradigm
Contrast
Connectivity
mosaic paradigm
amount
placement
context
John Wiens
The patch-matrix approach to landscape structure
Traditionally in ecology: patches viewed as terrestrial analogs of oceanic islands in that
both are seen as being surrounded by an inhospitable matrix: this is termed the patchmatrix approach to dealing with environmental structure. Its roots are in island
biogeography. The matrix is the "background," although not all landscapes have an easily
identifiable matrix. Quote from Richard Forman (1995): "When you’re in the middle of
nowhere, you’re probably in the matrix."
The P-M approach has been adopted in population genetics, conservation (especially with
respect to reserve design), and many other fields. Why?
Main benefits of P-M –
Assumptions of P-M –
But terrestrial patches are not true analogs of oceanic islands! The P-M approach is
unrealistic for several reasons (discussed in class).
Context –
Forys and Humphrey (1999) example –
Margules and colleagues have conducted several studies (e.g. Wog Wog forest
system; Davies et al. 2001) –
Roth et al. 1996 (streams in Michigan) –
Others:
modeling study of small mammals - Bender and Fahrig (2005)
Fahrig (1997)
Habitat loss increases the proportion of a population that must spend time in the matrix
(Fahrig 2002). This is assumed to invoke a cost, usually increased mortality. Therefore,
simply quantifying habitat area (p) doesn’t tell the whole story (see figure here).
Furthermore, there is a widely held notion that fragmentation will affect good dispersers
less than more sedentary species. Several empirical studies contradict this, however (see
Fahrig 2007 for examples), because crossing the matrix invokes a cost.
An alternative to the patch-matrix approach: the landscape continuum model In landscapes where patches are not discrete or obviously defined, patches may
not be easily differentiated from the matrix. The landscape continuum model was
developed by Sue McIntyre and Richard Hobbs (1994, 1999) in response to this issue originally for semi-cleared grazing and agricultural landscapes in Australia that had small
fragments of woodlands and isolated, scattered native trees.
The idea is that patches and corridors are too difficult to define in such settings
because single trees or a set of isolated trees may provide important habitat, so these
small elements should be considered rather than absorbed into the background matrix.
The model contains four broad cover classes:
- Intact cover - containing >90% cover and having low levels of modification
- Varigated cover - containing 60-90% cover and variable levels of modification matrix still consists of suitable habitat
- Fragmented cover - containing 10-60% cover and variable levels of modification
- matrix is now unsuitable or "destroyed" habitat
- Relictual cover - containing < 10% cover and usually high levels of modification
Note that these categories are continuua (percentages); habitat exists along continuua.
The thrust of this model is that landscapes are more complex than described by the
patch-corridor-matrix model, where patches and corridors are only isolated islands of
habitat within a matrix of unsuitable habitat.
Limitations of the landscape continuum model - the landscape continuum (LC) model
is just as subject to issues of scale as the P-M model –> what is variegated to one
organism may not be to another, and what is fragmented at one scale may not be at
another. Likewise, the landscape continuum model also considers only binary measures
of habitat - suitable vs. non-suitable. In some cases, a plurality of approaches may be
necessary (Price et al. 2009).
The P-M model focuses more on the elements of the landscapes--the size and shape of
patches and corridors--whereas the LC model tends to focus more on whole landscapes.
However, both consider the matrix to be the dominant element on the landscape, both are
subject to similar issues of scale, and both suffer from similar limitations. Therefore,
most landscape ecologists subscribe to another approach (“We can now move beyond the
stage of patches-in-an-inhospitable matrix…Why couldn’t the patch-corridor-matrix
model be enriched or even replaced by a functional mosaic model, in which the landscape
is composed of such places portraying movements and flows?” Forman 2002):
An alternative to the patch-matrix approach: the mosaic approach
We should realistically view landscapes as quilt-like mosaics, not patches embedded
within a matrix: the mosaic approach: overall landscape structure has ecological
consequences
• one of the people most associated with the mosaic approach is John Wiens
• the term "patch" still used in mosaic approach
• mosaic approach considers amount, placement, and connectivity of landscape elements
(Dunning et al. 1992, Taylor et al. 1993)
• P-M still currently the dominant paradigm, but the mosaic view is forcing a paradigm
shift/scientific revolution (but more in terms of lip service than true mosaic analyses)
Farina (2006) makes a distinction between the terms landscape and mosaic. Many people
use the terms synonymously, but Farina considers landscape as “a combination of
material and un-material properties” whereas mosaic “represents the material components
alone.”
The mosaic approach inherently recognizes the importance of patch context. The
interpatch matrix becomes a heterogeneous unit (cf. a homogeneous one in P-M). Its role
is more than simple background because dispersal between patches depends on matrix
(mosaic) properties and the contrast between patches and the mosaic: if matrix-patch
contrast is high, the boundaries are discrete and hard, meaning that dispersal becomes
difficult (see e.g. Bolger et al. 1997, Baum et al. 2004).
References:
Baum, K.A., K.J. Haynes, F.P. Dillemuth, and R.T. Cronin. 2004. The matrix enhances
the effectiveness of corridors and stepping stones. Ecology 85:2671-2676.
Bender, D.J., and L. Fahrig. 2005. Matrix heterogeneity can obscure the relationship
between inter-patch movement and patch size and isolation. Ecology 86:1023-1033.
Bolger, D.T., A.C. Alberts, R.M. Sauvajot, P. Potenza, C. McCalvin, D. Tran, S.
Mazzoni, and M.E. Soulé. 1997. Response of rodents to habitat fragmentation in coastal
southern California. Ecol. Appl. 7:552-563.
Davies, K.F., B.A. Melbourne, and C.R. Margules. 2001. Effects of within- and betweenpatch processes on community dynamics in a fragmentation experiment. Ecology
82:1830-1846.
Dunning, J.B, B.J. Danielson, and H.R. Pulliam. 1992. Ecological processes that affect
populations in complex landscapes. Oikos 65:169-175.
Estades, C.F. 2001. The effect of breeding-habitat patch size on bird population density.
Landscape Ecology 16:161-173.
Fahrig, L. 1997. Relative effects of habitat loss and fragmentation on population
extinction. J. Wildl. Manage. 61:603-610.
Fahrig, L. 2002. Effect of habitat fragmentation on the extinction threshold: a synthesis.
Ecol. Appl. 12:346-353.
Fahrig, L. 2007. Landscape heterogeneity and metapopulation dynamics. Pp. 78-91 in:
Key Topics in Landscape Ecology (J. Wu and R.J. Hobbs, eds.). Cambridge University
Press, New York, NY.
Farina, A. 2006. Principles and Methods in Landscape Ecology: Towards a Science of
Landscape. Springer, New York, NY.
Forman, R.T.T. 1995. Land Mosaics. Cambridge University Press, Cambridge, UK.
Forys, E., and S.R. Humphrey. 1999. The importance of patch attributes and context to
the management and recovery of an endangered lagomorph. Landscape Ecol. 14:177-185.
Hansson, L., L. Fahrig, and G. Merriam, eds. 1995. Mosaic Landscapes and Ecological
Processes. Chapman and Hall, London, UK.
McIntyre, S., and R.J. Hobbs. 1999. A framework for conceptualizing human impacts
on landscapes and its relevance to management and research models. Conservation
Biology 13:1282-1292.
Price, B., C.A. McAlpine, A.S. Kutt, S.R. Phinn, D.V. Pullar, and J.A. Ludwig. 2009.
Continuum or discrete patch landscape models for savanna birds: Towards a pluralistic
approach. Ecography 32:745-756.
Prugh, L.R., K.E. Hodges, A.R.E. Sinclair, and J.S. Brashares. 2008. Effect of habitat
area and isolation on fragmented animal populations. PNAS 105:20770-20775.
Roth, N.E., J.D. Allen, and D.L. Erickson. 1996. Landscape influences on stream biotic
integrity assessed at multiple spatial scales. Landscape Ecology 11:141-156.
Taylor, P.D., L. Fahrig, K. Henein, and G. Merriam. 1993. Connectivity is a vital element
of landscape structure. Oikos 68:571-573.
Wiens, J.A. 1995. Landscape mosaics and ecological theory. Pp. 1-26 in: Mosaic
Landscapes and Ecological Processes (L. Hansson, L. Fahrig, and G. Merriam, eds.).
Chapman and Hall, London, UK.