Fragmentation & Island Biogeography
Definitions & Core Concepts
Island (Ecological Definition) - Any area of suitable habitat that is isolated from similar habitat by a barrier that effectively restricts organism dispersal and movement. These isolated patches can function as ecological islands for specific species or groups of species.
The nature of the barrier is highly variable and can include natural formations like large bodies of water, mountain ranges, or expanses of unsuitable land cover (e.g., desert surrounding an oasis). Human-made barriers such as urban developments, highways, or clear-cut agricultural fields also serve this function. Even subtle differences in height, like gutters on a roof creating small 'islands' for moss or insects, can be relevant at certain scales.
Island-hood is inherently species-specific: a barrier for one species (e.g., a river for a flightless insect) may not be a barrier at all for another (e.g., a bird or a strong-swimming fish). This emphasizes that the definition depends on the dispersal capabilities and habitat requirements of the organism in question.
Fragmentation - The process, primarily driven by human actions (e.g., clear-cutting forests for timber, conversion of land for housing developments, road construction), that breaks large, continuous tracts of natural habitat into smaller, disconnected, and isolated patches. This essentially creates "terrestrial islands" within a modified landscape matrix.
Consequences are profound and include:
Altered movement patterns: Species may be unable to move between patches, leading to genetic isolation and reduced gene flow.
Altered predator–prey dynamics: Predators may have increased access to prey in smaller patches, or prey may be trapped. Edge effects can also alter these dynamics.
Changes in disease patterns: For example, increased Lyme disease risk in fragmented eastern deciduous forests is linked to changes in host populations (white-footed mice, deer) and reduced predator efficacy.
Reduction in patch size and increase in edge habitat, leading to loss of interior habitat specialists.
Forest Fragments as Islands - A specific example of fragmentation. A clear-cut area within a larger forest matrix acts as an open-habitat “island” for species that prefer or require sun-loving conditions (e.g., certain herbaceous plants, butterflies, early successional birds). The surrounding intact forest then serves as the ecological barrier for these open-habitat species.
Conversely, for forest-dwelling species, the clear-cut acts as an inhospitable matrix, isolating the remaining forest patches. Similarly, a pond inside a clear-cut creates an “aquatic island” for unique assemblages of fish, amphibians, and aquatic invertebrates, with the surrounding terrestrial clear-cut as its barrier.
Corridor - A linear or narrow strip of similar habitat that connects two or more larger patches of habitat. Corridors are crucial for facilitating the movement of individuals between isolated patches, thereby promoting gene flow, allowing species to track shifting resources, and potentially aiding in recolonization after local extinctions.
Species Richness vs. Diversity - These terms are often misused. Richness refers specifically to the number of different species present in a given area or community. Diversity is a broader concept that includes richness plus evenness (the relative abundances or proportions of each species). A community with many species but dominated by one or two is less diverse than a community with the same number of species where their abundances are more evenly distributed. Many lecture slides erroneously used “diversity” when they meant “richness” when discussing island biogeography principles.
Empirical Patterns
Species–Area Relationship - One of the most fundamental patterns in ecology, stating that larger areas tend to contain more species than smaller areas. This relationship is typically expressed as a power law: , where is the number of species, is the area, and and are constants. The value (slope on a log-log plot) usually ranges from to . This pattern has been extensively verified across numerous taxa and ecosystems:
Plants on the Galápagos Islands: Larger islands consistently host more plant species.
Flowering plants in England: Larger land areas or habitat patches support greater floristic richness.
Birds in North America: Explains why larger national parks or nature reserves typically have higher bird species counts.
This relationship holds true not only for true islands (land masses surrounded by water) but also for arbitrarily defined plots within contiguous landmasses, though the slope (z-value) may differ.
Oceanic vs. Terrestrial Patches - When comparing ecological islands, true oceanic islands (landmasses genuinely isolated by an inhospitable oceanic matrix) accumulate species at a faster rate (exhibit a steeper slope in the species-area relationship) than terrestrial habitat patches of equal area that are merely surrounded by a different, but often less inhospitable, habitat matrix (e.g., a forest island surrounded by farmland).
Geologic Origin Matters - The geological history of an island significantly influences its initial species richness and subsequent colonization dynamics.
Volcanic islands: These islands typically emerge from the ocean as sterile landmasses, devoid of life. Richness therefore grows slowly via primary succession, as pioneer species gradually colonize and modify the barren substrate, making it suitable for subsequent arrivals.
Sea-level rise islands (e.g., continental shelf islands like the Aleutians): These islands are formed when rising sea levels inundate previously contiguous land, isolating existing high-elevation areas. They begin with a pre-existing mainland biota, meaning they inherit a much higher initial richness and a more mature ecological community compared to volcanic islands of similar age and size.
MacArthur–Wilson Equilibrium Model
This seminal model of island biogeography proposes that the number of species on an island represents a dynamic equilibrium between the opposing forces of immigration of new species and the extinction of existing species.
Immigration Curve - This curve represents the rate at which new species successfully colonize the island over time. It is plotted with the rate of immigration (new species per unit time, ) on the y-axis and the current number of species on the island on the x-axis.
The rate starts high when the island is initially empty or has very few species, because almost any arriving individual represents a new species. As the number of species on the island increases, the pool of potential new colonists from the source diminishes, and arriving individuals are more likely to already be represented on the island, causing the immigration rate to decline. It eventually reaches when the island contains all species from the available regional source pool (all species capable of reaching and surviving on the island).
Extinction Curve - This curve represents the rate at which species go extinct on the island. It is plotted with the rate of extinction (species extinctions per unit time, ) on the y-axis against the number of species on the x-axis.
The extinction rate increases with the number of species on the island. This is primarily due to increased interspecific competition for limited resources (space, food, light), which intensifies as more species are crammed onto a finite island. Additionally, a larger number of species means a larger pool of species susceptible to random demographic fluctuations, environmental stochasticity, or disease, further contributing to higher extinction rates.
Equilibrium ()** - This is the point where the immigration curve and the extinction curve intersect. This intersection predicts the long-term, stable number of species that an island will maintain. This equilibrium is dynamic, meaning that while the number of species remains relatively constant, immigration and extinction events are continuously occurring; new species are arriving, and existing species are going extinct, but these two processes balance each other out over time.
Distance & Size Effects (Graph Interpretations)
The MacArthur–Wilson model elegantly incorporates the effects of island distance from the mainland source pool and island size on the equilibrium number of species, .
Near vs. Far Islands (holding size constant):
Near islands: Are closer to the mainland source pool, leading to a higher rate of immigration. This results in the immigration curve being shifted upwards. Consequently, the equilibrium point (where the higher immigration curve intersects the extinction curve) is at a higher . Thus, near islands are predicted to have a higher species richness.
Far islands: Are more distant from the source, reducing the rate of immigration. This shifts the immigration curve downwards. The intersection with the extinction curve occurs at a lower . Therefore, far islands are predicted to have lower species richness.
Small vs. Large Islands (holding distance constant):
Small islands: Possess fewer resources and less diverse habitats, leading to more intense competition and higher population vulnerability. This results in a higher rate of extinction, shifting the extinction curve upwards. The equilibrium point (where this higher extinction curve intersects the immigration curve) is at a lower . Thus, small islands are predicted to have lower species richness.
Large islands: Offer more diverse habitats, greater resource availability, and larger population sizes, which buffers species against stochastic events and competition. This leads to a lower rate of extinction, shifting the extinction curve downwards. The intersection with the immigration curve occurs at a higher . Therefore, large islands are predicted to have higher species richness.
Student Exercises Suggested - To solidify understanding, students should practice drawing:
Extinction curves for near/far islands (these curves typically don't change with distance, only with size and inherent island characteristics).
Immigration curves for large/small islands (these curves typically don't change with size, only with distance).
It's critical to understand which curve (immigration or extinction) is affected by distance and which by size to correctly predict changes in .
Colonization Phases
Islands do not simply reach equilibrium instantaneously; rather, they undergo a series of ecological phases during colonization and community development, which can overlap:
Non-interactive - This is the initial phase following colonization. Early arriving species face minimal biotic interactions (e.g., competition, predation) because populations are small and resources are abundant relative to the number of individuals. Changes in species richness during this phase are primarily driven by the balance between immigration of new species and any intrinsic extinctions (e.g., due to inability to establish).
Interactive - As more species establish and populations grow, interspecific interactions become increasingly important. Competition for resources intensifies, predation dynamics develop, and mutualistic relationships may form. These interactions begin to significantly influence species composition, relative abundances, and the overall diversity profile of the island.
Assortative (Successional) - Community structure continues to change through ecological succession processes. This involves a directional change in species composition over time, driven by mechanisms such as:
Facilitation: Early colonizers modify the environment, making it more suitable for later successional species.
Tolerance: Later species are able to tolerate the conditions created by earlier species, neither being hindered nor helped.
Inhibition: Early species inhibit the establishment of later species, potentially through competition or allelopathy.
Species are sorted based on their ability to persist through these changing conditions.
Evolutionary - On sufficiently old and isolated islands, in situ speciation (the evolution of new species from existing island populations) and adaptation (local evolutionary adjustments to island conditions) become significant forces. This can lead to the development of endemic species not found anywhere else and further modifies the island's overall species composition and richness beyond what can be explained by immigration from the mainland.
All these phases operate simultaneously, but their relative dominance shifts over an island's ecological age. Non-interactive processes might dominate early on, while evolutionary processes require much longer timescales to become apparent.
Experimental Evidence
Simberloff & Wilson (1960s) – Florida Keys Mangrove Islands - This classic experiment provided crucial empirical support for the MacArthur–Wilson equilibrium model.
The researchers meticulously cataloged all arthropod species on several small, isolated mangrove islands in the Florida Keys, ranging in size from approximately to feet across.
They then fumigated entire islands by enclosing them in large circus-tent-like structures and applying insecticide, effectively "resetting" or defaunating the islands of their arthropod inhabitants.
Following defaunation, they observed and meticulously monitored the recolonization process:
Near islands (closer to the mainland source of arthropods) recolonized much more quickly and typically regained approximately their pre-defaunation species richness (which was in the range of species). Interestingly, some near islands initially "overshot" their original richness before stabilizing, possibly due to a temporary influx of species that couldn't persist long-term.
Far islands (more distant from the source) recolonized more slowly and consistently settled at a species richness level below their original pre-defaunation richness. This directly supported the model's prediction that distance impacts the equilibrium number of species.
The regional source pool for arthropods in the Florida Keys was estimated to be around species, yet individual islands typically only hosted a small fraction, illustrating the island effect on richness.
This experiment demonstrated the dynamic nature of island equilibrium, showing that while species composition might change through turn-over (new arrivals replacing local extinctions), the total number of species could remain relatively stable.
Case Study – Mt. Rainier National Park (WA)
This case study illustrates the principles of island biogeography in a terrestrial, human-modified landscape.
Mt. Rainier National Park was established around , serving as a protected area for its unique biodiversity, but its boundary also implicitly defined an ecological island.
The known western Washington mammal pool (regarded as ext{100%} of potential species for the region) comprises species.
The maximum expected mammal species for the park, given its habitat variety and size (habitat limit), was estimated to be around species (approximately ext{79%} of the regional pool).
Observed Species Richness Trends:
1920s: The park supported roughly mammal species (about ext{58%} of the regional pool).
1970s: The number declined to species.
1980s: A precipitous drop to only mammal species occurred. This dramatic loss was primarily attributed to surrounding land sales and development outside the park boundaries. The conversion of adjacent natural habitats into farmlands, timber clear-cuts, and residential areas created an increasingly hostile and impermeable anthropogenic barrier around the park, effectively leading to its “islandization” and severing connectivity with regional mammal populations.
Restoration (1990s): Recognizing the severe biodiversity loss, conservation efforts led to land re-purchases around the park, and critical habitat corridors were re-established to connect the park with larger land blocks and other protected areas. This strategic intervention allowed for increased dispersal and gene flow.
Current Status: As a result of these restoration efforts, Mt. Rainier National Park now supports approximately mammal species, very close to its historically estimated maximum, demonstrating the effectiveness of applied island biogeography principles in conservation.
Designing Nature Preserves (Thought Experiment Results)
Applying principles from island biogeography, ecologists have proposed optimal designs for nature preserves to maximize species richness and minimize extinction rates within protected areas:
Optimal Traits for Maximizing Species Richness (SLOSS Debate: Single Large or Several Small):
Large area: All else being equal, a single large preserve will support more species, have larger population sizes (less prone to extinction), and encompass more diverse habitats than several small preserves totaling the same area. This directly aligns with the species-area relationship.
Single, contiguous block: A preserve shaped as one large, unbroken unit is preferable to multiple disjunct fragments. This minimizes internal “edge effects” (the altered environmental conditions at the boundaries between two habitat types), which can negatively impact interior-dwelling species.
If multiple patches are necessary: If preserves must be multiple patches due to land availability, their spatial arrangement matters:
Close spacing: Patches should be as close to each other as possible to increase inter-patch dispersal and reduce isolation.
Triangular clustering better than linear: A clustered arrangement, triangularly or circularly, supports better connectivity and gene flow among patches compared to a linear chain of patches, which can be vulnerable if a single link is broken.
Corridors linking patches superior to isolation: The presence of narrow habitat corridors significantly enhances the functionality of a network of preserves. Corridors facilitate movement, gene flow, and recolonization, effectively increasing the