Island Biogeography and Ecological Tolerance: Study Notes

Island Biogeography

• Definition: Study of ecological relationships and community structure on islands or isolated habitats (actual islands, Central Park as an island in a city, oases) and how isolation shapes biodiversity.

• Core rules observed in island biogeography:

  • Larger islands support more total species due to more food, habitat, resources, and therefore more niches; larger populations and lower extinction rates also contribute.

  • Islands closer to the mainland support more species because colonization occurs more readily.

• Consequences of these rules:

  • More colonizing organisms leads to greater genetic diversity and healthier populations.

  • There is an overall positive relationship between island size and species richness; there is an inverse relationship between island distance from the mainland and species richness.

• Richness vs. diversity:

  • Richness = the number of species in an ecosystem.

  • A common illustration shows a mainland, a large island near the mainland, a nearby island, a far island, and a small distant island; the large near island tends to have the most species due to combined effects of size and proximity.

• Conceptual illustration: island area and species richness are positively correlated; larger islands tend to harbor more bird species and other taxa as area increases.

• Example: Galápagos Islands as a classic example of island biogeography, speciation, and adaptive radiation in response to different islands’ ecological conditions.

  • Colonization and subsequent isolation lead to speciation driven by different environments on each island (arid zones, transitional zones, etc.).

  • Adaptive radiation occurs when a single colonizing species radiates into many species adapted to distinct niches (e.g., finches with different beak shapes for different foods).

  • Resource partitioning reduces direct competition among closely related species.

• Concepts connected to island biogeography:

  • Adaptive radiation: a single lineage diversifies into multiple species with distinct beak shapes and feeding strategies to exploit available niches.

  • Resource partitioning: division of resources among coexisting species to minimize competition.

  • Limitations due to island constraints: limited space and resources create unique selective pressures and narrow niches.

• Practical implications and habits of thought:

  • Larger, closer islands maximize species richness and genetic diversity, whereas distant, small islands tend to have fewer species due to limited colonization and smaller available resources.

• Practice FRQ idea (historic prompt reference): describe the process by which island habitats are colonized and how distance from the mainland influences the number of species that colonize.

  • Answer scaffold: Closer islands experience higher immigration rates, leading to more species arriving and establishing populations; farther islands have lower immigration rates, reducing colonization and species richness. Also mention that higher immigration can increase genetic diversity and reduce inbreeding depression.

• Species-area relationship (quantitative note): a common model is S = c A^{z} where

  • S = number of species,

  • A = island area,

  • c and z are constants that depend on the system; this captures the positive relationship between area and richness.

Distance to mainland and colonization

• Closer to mainland equals higher species richness due to continuous or more frequent immigration.

• Proximity enables continual migration of individuals, contributing to a healthier, more diverse, and genetically diverse population reservoir on the island.

• The farther away an island is from the mainland, the fewer species are likely to persist or colonize due to reduced dispersal opportunities.

Ecological tolerance and the abiotic niche

• Definition: The range of environmental conditions (abiotic factors) that an organism can survive, grow, and reproduce.

• Key abiotic factors shaping tolerance ranges: Temperature, Salinity, Nutrients, Flow rate, Sunlight and other environmental variables relevant to an organism.

• Example: Salmon tolerance often described as a temperature range; a typical range is from 6^{\circ}C\le T \le 22^{\circ}C but can vary by population.

• Concept of a tolerance bell curve:

  • Preferred (optimum) niche is at the peak where performance (growth, reproduction) is highest.

  • Outside the optimum, performance declines; mild departures may still be tolerable (physiological stress), but extreme deviations lead to higher morbidity or mortality.

  • Zone of intolerance: environments outside which the organism cannot survive.

• Tolerance curve implications:

  • The broader and more flexible the tolerance, the more resistant a population may be to disturbances like climate change.

  • The narrower the tolerance, the more susceptible the population is to environmental fluctuations.

• Range of tolerance concepts:

  • Fundamental niche: the range of abiotic conditions under which an organism can survive, grow, and reproduce (potential niche without biotic interactions).

  • Realized niche: the actual conditions under which the organism lives, considering biotic interactions like competition, predation, and symbiosis.

• Niche and adaptation interplay:

  • An organism’s niche is defined by its tolerance to environmental factors, and its realized niche is shaped by competition and other ecological interactions.

Indicator species and keystone species

• Indicator species:

  • Species used to monitor ecosystem health because their presence/absence or population trends reflect environmental conditions.

  • Examples:

  • Trout as an indicator in rivers/streams due to preference for cold, well-oxygenated water; declines signal warming water or reduced dissolved oxygen.

  • Birds often reflect habitat loss or pesticide exposure (e.g., DDT effects described in Silent Spring leading to egg fragility and population declines).

  • Butterflies indicate plant community health and pollination dynamics; changes in plant communities impact butterflies.

  • Frogs and other amphibians are sensitive to water quality because they respire through the skin and develop in water; pollution, drought, UV exposure, and parasites can severely impact them.

• Keystone species:

  • Not necessarily abundant but have a disproportionately large impact on community structure and ecosystem function; often ecosystem engineers.

  • Classic examples:

  • Sea otters regulate kelp forest ecosystems by preying on sea urchins, protecting kelp.

  • Beavers create ponds and wetlands by building dams, altering hydrology and habitat structure.

  • Elephants modify habitats through feeding and movement, influencing vegetation structure and nutrient cycling.

• Takeaway: removing or significantly reducing indicator or keystone species can cause cascading effects and degrade ecosystem health.

Generalists vs specialists

• Generalists:

  • Thrive in a wide range of conditions; adaptable to environmental changes; not extremely specialized in any single resource.

  • Tend to be more resilient to rapid change but may not excel at any one function.

• Specialists:

  • Highly adapted to specific resources or conditions; excel at particular niche requirements but are vulnerable if those conditions change.

  • In stable environments, specialists can outcompete generalists for their specialized resources; under rapid or large-scale change, specialists often struggle due to limited flexibility.

• Ecological implication:

  • In fluctuating environments, generalists may persist; in highly stable, resource-limited niches, specialists can dominate.

Human activities, disturbances, and FRQ reasoning tips

• When describing human activities and natural disturbances, connect them to ecological range of tolerance:

  • Climate change shifts temperatures outside the range tolerated by many species, causing declines in populations or range contractions.

  • Ocean warming can reduce dissolved oxygen availability, leading to suffocation risks for aquatic species.

  • Drought or altered rainfall changes soil moisture and plant water availability, affecting population sizes and distribution.

  • Coral bleaching is tied to high temperatures that push corals outside their tolerance ranges.

• FRQ writing tips drawn from this module:

  • Explicitly connect the disturbance to the organism’s range of tolerance and explain how this shifts its fundamental/realized niche.

  • Use concrete examples (e.g., fish with decreased oxygen under warmer water; plants with reduced rainfall pulling back distribution).

  • Be specific about the mechanism (e.g., increased temperature reduces dissolved oxygen, causing suffocation).

  • Include real-world relevance (e.g., how climate change drives population declines, habitat loss affects amphibians).

• Note on ethics and ecosystem management:

  • Removing species (e.g., mosquitoes) without understanding ecosystem roles can have unintended consequences.

  • Habitat loss and pollution can have cascading effects on indicator and keystone species, altering ecosystem function.

• Real-world relevance and caution:

  • Amphibians’ sensitivity to environmental change makes them good early warning indicators for ecosystem health and biodiversity loss.

  • The balance between generalists and specialists can shift with human-induced environmental change, affecting resilience of ecosystems.

Quick refresher terms and definitions

• Island biogeography: study of how island size and isolation affect species richness and ecosystem structure.

• Species richness: the number of different species in a given area.

• Niches: the role and position a species has within its environment, including requirements and interactions.

• Fundamental niche: the full range of abiotic conditions a species could potentially occupy.

• Realized niche: the actual conditions under which the species exists in nature, given biotic interactions.

• Range of tolerance: the spectrum of environmental conditions under which an organism can survive.

• Zone of tolerance: area within the range of tolerance where organisms can survive, with optimal conditions yielding maximum performance.

• Optimum: the most favorable conditions for an organism’s growth and reproduction.

• Physiological stress: stress experienced when conditions move away from the optimum but are not yet lethal.

• Indicator species: species used to assess the health of an environment.

• Keystone species: species with a disproportionately large impact on ecosystem structure relative to their abundance.

• Generalist: species able to thrive in a wide range of conditions.

• Specialist: species highly adapted to a narrow set of conditions.

• Adaptive radiation: rapid diversification of a lineage into a variety of adapted forms.

{S = c A^{z}}

• Species-area relationship as a quantitative summary: larger area (A) tends to support more species (S) with system-specific constants c and z.