l20-Metapopulation Dynamics and Island Biogeography

Introduction and Ecological Observations

  • Class Observations: Raccoons and Seed Dispersal

    • The lecture begins with observations from an animal sanctuary webcam featuring raccoons, colloquially known as "trash pandas."

    • Raccoons are observed eating shelled walnuts. In a natural setting, if a raccoon smashes a hard-shelled nut—causing seeds to shoot off into the forest—this represents a form of seed dispersal known as dyszoochory.

    • Other animals identified in the footage include an opossum, a tree squirrel, and an armadillo from a rural property.

  • Metapopulations Context

    • The lecture covers metapopulations, drawing from textbook Chapters 8 and 19.

    • Ecology is characterized as a non-linear topic involving feedback loops between population dynamics and life history traits.

Island Biogeography

  • Definition of Biogeography

    • Biogeography is the study of relationships between geographic area size, the degree of isolation of that area, and the species diversity (richness) found within it.

  • Definition of an "Island" in Ecological Terms

    • An island is any portion of isolated habitat surrounded by a different, potentially hostile environment.

    • Examples:

      • Land Islands: Landmasses surrounded by water (e.g., the islands of New Zealand).

      • Mountaintops: Species evolved for the alpine zone (cool temperatures, thin air) cannot survive at lower elevations. The valleys between peaks act as hostile barriers.

      • Lakes: Aquatic habitats surrounded by terrestrial environments. For aquatic species, the land is a hostile barrier.

  • The Species-Area Relationship

    • Ecologists observe that species richness increases with larger geographic areas.

    • Continents like Australia exhibit higher species diversity than smaller island systems like New Zealand.

    • Mechanisms for High Diversity in Large Areas:

      • Resource Base: Larger areas provide more resources.

      • Population Stability: They support larger populations, making them less vulnerable to extinction by sheer numbers.

      • Niche Diversity: There is a greater diversity of ecological niches.

      • Resilience to Stochastic Events: Environmental perturbations (storms, earthquakes) are less likely to affect the entire landmass. A storm hitting the Chatham Islands will affect a much higher proportion of the land than the same storm hitting Australia.

  • Trophic Rank and Area

    • The relationship between richness and area increases with higher trophic rank.

    • The slope of impact is steeper for predators compared to herbivores, and steeper for herbivores compared to plants.

    • As area decreases, predators are lost faster than lower trophic levels due to bottom-up food chain control (large islands have more biomass to support multiple levels).

The Simberloff and Wilson Experiment (1969)

  • Experimental Design

    • Conducted by E.O. Wilson and Daniel Simberloff in the Florida Keys (Florida Bay).

    • The study site consisted of mangrove islets (tiny islands) in shallow tidal areas.

    • Methodology:

      • The researchers selected islands of varying sizes and distances from the Florida mainland.

      • They used scaffolding and plastic sheets to enclose entire islets (like fumigating a house).

      • They sprayed pesticides to kill all arthropod fauna (spiders and other invertebrates).

  • Results and Conclusions

    • Species counts increased until they reached an asymptote roughly equal to the original species count prior to fumigation.

    • While the number of species remained consistent, the species composition (the specific types of creatures) changed.

    • The experiment proved that species richness is a result of island size and isolation, not simply the age of the island (time allowed for accumulation).

Equilibrium Theory of Island Biogeography

  • Dynamic Balance

    • Species richness is a balance between immigration/colonization (arrival of new species) and extinction.

  • Rate Factors

    • Immigration Rate: Higher for islands close to the mainland.

    • Extinction Rate: Higher for small islands due to fewer resources and higher vulnerability to shocks.

  • The Four Equilibrium States

    • Small/Far Islands: Lowest species richness; slow colonization, high extinction.

    • Large/Near Islands: Highest species richness; fast colonization, low extinction.

    • Small/Near vs. Large/Far: Intermediate richness levels.

  • The "Target Effect" (Class Experiment)

    • Small islands are small targets, making them physically harder for wind-dispersed seeds or animals to hit.

    • Increased distance reduces the likelihood of dispersers surviving the journey across the matrix.

Metapopulation Theory and Landscape Dynamics

  • Definition of Metapopulation

    • A subdivided population consisting of a series of discrete subpopulations behaving independently but connected by occasional dispersal (immigration and emigration).

  • Scale of Dynamics

    • Patch Scale (Local): Governed by birth (BB) and death (DD) rates. Individuals interact routinely within the patch.

    • Landscape Scale: Governed by dispersal and colonization. The survival of the metapopulation depends on the recolonization of patches where local populations have gone extinct.

  • Patch Characteristics

    • The landscape is viewed as a matrix (non-habitat) containing potentially suitable habitat patches.

    • Patches vary in size, quality, and isolation.

  • Necessary Conditions for a Metapopulation

    • Suitable habitat must be patchy.

    • All patches must be at risk of extinction.

    • Recolonization must be possible.

    • Patch dynamics must be asynchronous (they do not all respond to environmental cues at the same time).

Connectivity and Dispersal

  • Movement Types

    • Dispersal can be unidirectional (e.g., wind-blown from Australia to NZ) or bidirectional within a local metapopulation.

    • Successful colonization requires the ability to move, the tendency to disperse, and survival across the hostile matrix (e.g., snails crossing mowed fields with bird predators).

  • Types of Connectivity

    • Structural Connectivity: Based on the physical arrangement of patches (e.g., a riparian corridor or vegetation along a river).

    • Functional Connectivity: Based on how well the landscape facilitates movement, accounting for the organism's behavioral response to physical structures.

    • Note: Structural connectivity does not always equal functional connectivity. Human-engineered wildlife overpasses must be placed with biological knowledge to be effective.

Mathematical Framework for Metapopulations

  • Occupancy Rate

    • To understand the fraction of patches occupied at any given time (pp), we use:

    • p=1−emp = 1 - \frac{e}{m}

    • Where ee is the probability of extinction and mm (or cc) is the probability of colonization.

  • Persistence Requirements

    • For a metapopulation to persist, the fraction of occupied patches (pp) must be greater than zero (p > 0).

    • This requires that the probability of extinction (ee) be less than the probability of colonization (mm).

Case Studies in Metapopulation Dynamics

  • Sweden Cricket Population

    • Study of crickets in various-sized patches ranging up to 6464 hectares.

    • Findings: Extinction was common in patches below 11\text{ hectare}. Larger patches (22 to 6464 hectares) had only extant (present) populations.

    • The maximum recorded distance for a cricket to successfully recolonize a patch was 280 m280\,m.

  • New York City Camera Traps

    • A network of camera traps from Downtown Brooklyn to Nassau County studied coyotes and deer.

    • Found that patch occupancy was related to patch size and the hostility of the urban matrix (concrete vs. suburban greenery).

  • Source-Sink Dynamics

    • Source: A patch where population growth is positive; individuals emigrate to other patches.

    • Sink: A patch with negative population growth. The population only persists due to constant immigration from source patches.

  • Cat Predation in Dunedin (Otepotee)

    • House cats were monitored via GPS collars.

    • For species like fantails (PıˉwakawakaPīwakawaka) and bellbirds (KorimakoKorimako) in urban environments, the catch rate by cats was higher than the estimated total population.

    • These urban areas are sinks; the birds only remain present because of immigration from nearby native forest sources.

  • UC Campus Nest Success

    • In 20202020, native species (fantails and silver eyes) had lower nest success on campus compared to literature standards.

    • Increased predator trapping by the EnviroSoc group has since led to a rise in native bird nest success.

  • Freshwater Systems: Galaxids and Trout

    • Introduced trout act as a sink for juvenile galaxids by preying on them.

    • Galaxid populations are more diverse and include smaller individuals when trout are absent.

    • Barriers like waterfalls can protect galaxid populations from trout, but this effectively creates habitat fragmentation, preventing the galaxids from moving and functioning as a metapopulation.

Conservation and Reintroduction

  • Tui on Banks Peninsula

    • Tui were lost from Banks Peninsula around 19901990.

    • Because Tui are reluctant to fly across large areas of open land (low functional connectivity), human intervention was required.

    • They were reintroduced in 20092009 and 20102010 and are now thriving in areas like Akaroa.

    • Current research focuses on restoring linear features (native hedgerows) to reconnect the Southern Alps to Banks Peninsula.