APES 2.3-2.4: Island Biogeography and Ecological Tolerance

Island biogeography

The study of how isolation, area, immigration, extinction, and environmental conditions influence species richness and diversification in isolated habitats. The term applies to true islands and to isolated habitat patches such as mountain peaks, lakes, fragmented forests, and protected areas.

What qualifies as an “island” in island biogeography?

An island is any habitat suitable for a particular organism or community that is surrounded by unsuitable habitat. Whether an area functions as an island depends on the organism being considered.

Who developed the modern theory of island biogeography?

Robert MacArthur and E. O. Wilson developed the theory in the 1960s to explain and predict species richness on oceanic islands.

What two opposing processes primarily determine the number of species on an isolated island?

Immigration adds species to the island, while extinction removes species. Their opposing rates can produce a relatively stable equilibrium species richness.

Equilibrium species richness

The approximate number of species maintained when immigration and extinction rates balance over time. Species identities may change even when the total richness remains near the same level.

Does equilibrium species richness mean that island communities are unchanging?

No. Equilibrium refers to a relatively stable number of species, not necessarily a fixed set of species. Immigration and extinction may continue while richness remains near the equilibrium value.

Distance effect

The influence of an island’s isolation on immigration. Islands farther from a mainland or other source of colonists generally receive fewer immigrants than nearby islands.

How does distance from a mainland or source population affect species richness?

Greater distance usually lowers immigration because fewer organisms successfully disperse to the island. If island size is otherwise similar, a more isolated island is therefore expected to have fewer species.

How does island area affect extinction probability?

Larger islands generally have lower extinction rates because they support larger populations, more habitat, and more resources. Larger populations are less vulnerable to extinction from chance demographic or environmental events.

Species-area effect

The pattern in which larger areas usually contain more species. Greater area can provide more habitat types, resources, and space for viable populations.

Target effect

The tendency for larger islands to receive more immigrants because they present a larger target during dispersal or offer more visible and suitable habitat. This effect can increase immigration in addition to reducing extinction.

Rescue effect

The reduction in local extinction risk caused by immigration from a mainland or neighboring population. Incoming individuals can replenish a declining population or restore genetic variation.

How does isolation influence extinction through the rescue effect?

Less-isolated islands are more likely to receive immigrants that prevent or reverse local extinction. Highly isolated islands lack this demographic support and therefore have greater extinction risk.

What island is predicted to have the greatest species richness under the basic equilibrium model?

A large island located near the mainland or another source of colonists is predicted to have the greatest richness because it receives many immigrants and has relatively low extinction rates.

What island is predicted to have the lowest species richness under the basic equilibrium model?

A small, highly isolated island is predicted to have the lowest richness because immigration is limited and extinction rates are relatively high.

Why does habitat heterogeneity increase island species richness?

An island containing diverse habitats provides more ecological niches and supports species with different requirements. Thus, two islands of equal area may differ in richness if their habitat diversity differs.

What factors besides area and isolation can influence species richness on an island?

Important factors include the length of time the island has been isolated, climate, habitat suitability and diversity, existing species composition, historical connections to a mainland, ocean currents, nutrient and dust inputs, chance colonization events, and human activity.

How can the duration of isolation affect an island’s present-day species composition?

Longer isolation allows more time for colonization, extinction, adaptation, and speciation. It also permits historical events and chance arrivals to leave a stronger imprint on the community.

Why do initial species composition and historical land connections matter on an island?

Species that colonized when an island was connected to a mainland may leave a historical legacy after isolation. The starting community influences later competition, predation, available niches, and evolutionary trajectories.

How can ocean currents and atmospheric dust affect island communities?

Currents can transport nutrients, seeds, fish, and birds, while airborne dust can supply nutrients. These inputs alter productivity and the probability that organisms successfully arrive and establish.

Why are chance events important in island biogeography?

Dispersal to isolated habitats is often stochastic: an organism may arrive by wind, water, or transport simply by chance. Random colonization can make species composition differ substantially among otherwise similar islands.

Species-area relationship

The quantitative relationship between the area of a habitat and its species richness. It is commonly modeled by $S=cA^z$, where richness increases as area increases.

In the species-area equation $S=cA^z$, what do $S$, $c$, $A$, and $z$ represent?

$S$ is species richness or another measure of diversity, $A$ is habitat area, $c$ is a constant related to the intercept, and $z$ is the exponent that determines how strongly richness changes with area.

How is the species-area relationship linearized for graphing?

Taking logarithms gives $\log(S)=\log(c)+z\log(A)$. A plot of $\log(S)$ versus $\log(A)$ has slope $z$ and intercept $\log(c)$.

What does a larger $z$ value indicate in a species-area relationship?

A larger $z$ indicates that species richness changes more strongly with area. It corresponds to a steeper line on a log-log plot of richness versus area.

Why must species-area relationships be interpreted cautiously when comparing islands with mainland habitats?

The same general pattern may occur in both settings, but the parameters—especially $z$ and $c$—can differ. Isolation, habitat structure, species traits, and sampling scale also influence the relationship.

How was the equilibrium theory of island biogeography experimentally tested in the Florida Keys?

Researchers surveyed arthropods on mangrove islands, fumigated the islands to remove their arthropod communities, and then monitored recolonization. The islands returned to approximately their previous richness, although species composition could fluctuate.

What result from the Florida Keys experiment supported the distance effect?

Mangrove islands closer to the mainland were recolonized more rapidly than more distant islands. This supported the prediction that immigration decreases with isolation.

Island-like systems (ILS)

Isolated habitat systems that are not literal islands but have island-like dynamics, such as ponds surrounded by land, forest fragments, mountain “sky islands,” and habitat patches in developed landscapes.

In an island-like system, what do “mainland” and “matrix” represent?

The mainland is the nearest or most important source of immigrating organisms. The matrix is the surrounding environment through which organisms must disperse, and it may be much more varied than the ocean surrounding a true island.

Why may the area and isolation of an island-like system change over time?

Seasonal changes, water levels, habitat alteration, and landscape development can expand or contract the habitat and change its connectivity. These changes modify both resource availability and immigration opportunities.

How do species-area relationships in island-like systems commonly differ from those on true islands?

Island-like systems often have lower $z$ values than true islands, although richness still commonly increases with area. The constant $c$ can vary among different types of systems and between island-like systems and true islands.

Why did island biogeography become important to conservation biology?

Protected areas surrounded by human-altered landscapes can function as habitat islands. Their isolation and limited area may cause local populations to decline or disappear as communities move toward a new, lower equilibrium richness.

Habitat fragmentation

The division of formerly continuous habitat into smaller, isolated patches. Fragmentation can reduce habitat area, increase isolation, restrict dispersal, and raise local extinction risk.

What is ecosystem decay in the context of isolated reserves?

Ecosystem decay is the gradual loss of species from an isolated reserve as its community approaches a new equilibrium after fragmentation. Larger-ranging species are often especially vulnerable because they require more area.

Single large or several small (SLOSS) debate

A conservation-planning debate over whether one large reserve or several smaller reserves preserve more biodiversity. A single large reserve may support more species because of its area, but several smaller reserves may capture more distinct habitats or populations.

Why is reserve area not the only factor determining conservation value?

Habitat diversity, connectivity, species-specific requirements, population sizes, and the quality of the surrounding matrix can be as important as total area. A smaller reserve can be valuable if it protects unique habitats or improves representation.

Wildlife corridor

A strip or network of suitable habitat connecting otherwise isolated habitat patches. Corridors can promote dispersal, gene flow, recolonization, and the rescue effect.

What is a potential disadvantage of wildlife corridors?

Corridors can facilitate the spread of diseases, pathogens, invasive species, or predators between populations. Therefore, connectivity can have both beneficial and harmful consequences.

How does human activity alter island-biogeography dynamics?

Development can destroy or fragment habitat, increase isolation, introduce nonnative species, change disturbance regimes, and create corridors or barriers. These changes modify immigration, extinction, and equilibrium richness.

Ecological tolerance

The range of abiotic environmental conditions, such as temperature, pH, salinity, or moisture, under which an organism can survive and reproduce.

Range of tolerance

The full span between the minimum and maximum values of an environmental factor that an organism can withstand. Conditions outside this range cause death or prevent survival.

What are the zones within an organism’s range of tolerance?

The optimal range supports the best survival and reproduction. Zones of physiological stress permit survival but reduce performance or reproduction. Zones of intolerance are outside the organism’s viable limits.

Law of tolerance

A species’ abundance and distribution are controlled by the environmental conditions it can tolerate. Both excessively low and excessively high levels of a factor can limit survival.

How does an organism’s optimal range differ from its zones of physiological stress?

In the optimal range, growth, survival, and reproduction are highest. In zones of physiological stress, the organism may survive, but growth, health, or reproductive success is reduced.

Limiting factor

An environmental factor that restricts the growth, abundance, or distribution of an organism or population. A factor can be limiting when it is below the minimum required level or above the maximum tolerated level.

Generalist species

A species with a broad range of tolerance and the ability to use many resources or habitats. Generalists are often more adaptable to environmental change than specialists.

Specialist species

A species with a narrow range of tolerance or highly specific resource and habitat requirements. Specialists are more vulnerable when environmental conditions change or their habitat is disturbed.