Part 1 - Island biogeography: landscapes as binary systems
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.
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 the journal 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
Is colonization (immigration) rate more strongly affected by island size or by its distance from the mainland, and why?
Is extinctionrate more strongly affected by island size or by its distance from the mainland, and why?
Should the rate of immigration increase or decrease as species accumulate on an island, and why?
Should the rate of extinction increase or decrease as species accumulate on an island, and why?
-note the intersection points on the curves: each is an equilibrium point, which means:
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.
Would a larger island have a higher or a lower extinction rate relative to a smaller island, and why?
Would a distant island be subject to higher or lower colonization rates than islands near the mainland, and why?
What does IB have to say about community composition?
The species-area relationship is one of the most widespread and well-supported patterns in all of ecology (indeed, Thomas Schoener considers it to be one of the few “laws” in ecology). We do not have time in this course to go into this topic into depth; for more details, take my Community Ecology class (BIOL 4310/5310) in fall of even-numbered years.
Island biogeography theory wastested empirically by Simberloff and Wilson 1969:
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:
SLOSS debate: see Diamond 1975, Terborgh 1976, Simberloff and Abele 1976)
See also Thomas et al. (1990) for northern spotted owls, Newmark (1987, 1995, 1996) for mammals in US and African parks, and many other studies (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. 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.
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. This led to a debate:
Diamond, Terborgh, and others - area per se:
Simberloff and others - conservation of habitat diversity and for “spreading of risk”
Conservation biology, in particular, took to the IB approach. Click here for some IB-based 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.
Part 2 - Island biogeography comes ashore
Traditionally in ecology: patches were viewed as terrestrial analogs of oceanic islands in that both are seen as being surrounded by an inhospitable matrix: this is termed the patch-matrix 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 because:
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) –
modeling study of small mammals - Bender and Fahrig (2005)
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, where 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).
“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)
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