Biogeography

What is biogeography?

The study of the spatial distribution of flora and fauna of the biosphere through geological time, past and present.

What does biogeography aim to determine?

What drives the patterns of species distribution and the processes of life.

What disciplines is biogeography influenced by?

Ecology, environmental sciences, evolution, climatology, geology, system dynamics.

What event(s) catalysed biogeographical thinking?

Induced by paradigm shifts, most predominantly creationism → Darwinian evolution (1859) and continental drift theory (Wegner, 1912). The founding of Gondwanaland and Pangea supported evidence for/provided an explanation into historical species dispersal.

What is Buffon’s law (1749)?

Different regions of the world with similar environments are inhabited by different species’ assemblages. This is due to species dispersal, which causes them to become modified by their new environments and adapt to their new living conditions (soil, climate, food availability).

What is species range?

The geographic area where a species naturally occurs. Can be broad (cosmopolitan), narrow (endemic), or divided into numerous separated areas (disjunct).

What is endemism?

Species found only in one location, signalling long-term isolation. Endemic species are highly vulnerable to extinction from habitat loss, climate change and invasive species.

What are ecological niches?

A species’ role and environmental requirements, including climate, food and competition.

What is the Grinnellian Niche - the environment as determinant of distribution? (Grinnell, 1917)

The ecological niche of a species is defined by the set of environmental conditions and habitats that permit a species to persist.

Emphasis on abiotic factors: climate, temperature, moisture, substrates.

What is the Eltonian Niche (1927) - species’ functional role?

The ecological niche of a species is the functional role a species plays in a community. Less based on physical constraints and barriers, and more associated with biotic interactions (predation, competition, parasitism, tropic levels).

What is the Hutchinsonian Niche - the n-Dimensional hypervolume? (Hutchinson, 1957)

The niche can be defined by all environmental conditions that allow a species to maintain a positive population growth in a certain environment.

Introduces fundamental niche (a state of the environment whereby abiotic conditions would allow for indefinite survival in the absence of other species) vs. realised niche (constrained by competition, predation and dispersal barriers).

The realised niche can expand or contract based on changing biotic interactions e.g. non-analogue climates.

Example of non-analogue climates - Veloz et al, 2012

Pollen-based taxon assemblages from 15 - 21ka BP show that pollen abundant in areas with non-analogue late glacial climates have since shifted their realised niches to present. Some pollen species had a greater abundance in areas with greater seasonal variations in temperature and solar insolation, whilst others had stable abundances through time. Recognises that a realised niche at any one point often represents only a subset of the ecological conditions in which a taxon can persist.

Difficulties with interpreting fundamental vs. realised niches

Niche similarities between species sharing the same geographic space may be biased if environmental landscapes are not evenly distributed in an area of geographic space (G-space limitations).

Environmental space may be truncated if species distribution models are based on geographically-restricted data (e.g. studying a species only within one country, rather than its entire range). This incorrectly assumes that the species cannot tolerate conditions outside of the observed range.

E-space does not always show the fundamental niche because the edge of the environment may have a higher risk for taxa to live there. It is difficult to ascertain whether a species is able to survive at the edge of its fundamental or realised niche once they reach the edges of their environmental space, but choose not to occupy any further.

What is a pattern

A non-random spatial or temporal variation in nature.

• Driven by Earth’s interior energy (plate tectonics, orogeny, volcanism); solar energy inputs and changing atmospheric composition (influencing solar insolation and climates); ocean circulation (also influencing global climate over longer timescales, and marine species’ distribution).

What is a process?

A mechanism driving the observed patterns.

• Biotic processes: competition for resources, facilitation, predation, mutualism.

• Historical climatic influence: Mid-Eocene sauna 55Ma, characterised by temperatures 10°C warmer than present, enabled tropical biomes to extend to higher latitudes. Conversely, the onset of glacial conditions can open up land bridges, which provides terrestrial species with access to other landmasses.

  • Russia and Siberia connected to Alaska across the Bering Strait during the Pleistocene Epoch (2.6Ma - 11.7Ka), producing the Beringia subcontinent - an unglaciated steppe-tundra biome, supporting diverse megafaunal wildlife.

  • Greater Sunda Islands (Java, Borneo, Sumatra) connected with the Malay Peninsula during the LGM.

What is scale?

The spatial, temporal or organisational level at which patterns are observed and processes operate.

• Small-scale: competition, dispersal, limitation, microhabitat variation.

• Large-scale: speciation, plate tectonics, climate dynamics.

What is spatial autocorrelation - Tobler’s First Law (1970)?

Environmental similarity increases as distance falls. Climatically-similar reigemes also often support structurally and functionally similar vegetation even in geographically-separated areas (convergent evolution). Environmental conditions are generally similar along major geographical gradients (latitude, elevation, depth, area, isolation)

• Elevation gradients operate similarly to tropical-polar gradients due to changes in temperature, air pressure and precipitation

• Geological and topographic features are influential on a local scale - e.g. the distribution of mesoic forest vs. dry habitat on the windward/leeward side of a mountain. The rain shadow from the Southern Alps on New Zealand’s South Island characterises the arid climatic conditions on the East coast and the wet rainforest on the West coast.

  • 12m of rainfall annually in Fjordland region vs. 1m annually in Canterbury (Sinclair, 1997)

• Wet + hot → increased rates of evapotranspiration, often predicting higher flora abundance and greater biodiversity.

Spatial and temporal correlation - and exceptions

Phenomena which occur on a large spatial scale also take place over a long period of time.

However, natural disasters can unsettle ecosystems instantaneously.

• Chicxulub impactor triggering the K-Pg extinction 66Ma, wiping out 75% of terrestrial life and up to 95% of marine life on Earth due to feedback cycles (Covey, 1994).

• Permian-Triassic extinction 255Ma through Siberian Traps LIP, resulting in rapid climate change, ocean acidification and anoxia.

What is dispersal?

The movement away from an original range. Also known as range extension. Affects biogeographic patterns when it results in immigration.

What is active dispersal?

Dispersal requiring the organism’s own energy. Larger organisms have increased dispersal potential per movement (vagility).

What is passive diserpsal?

Relies on external forces - wind, water, ice/vegetation rafting. Can result in rare colonising events over long distances, such as through plant/animal rafting across water during storm events. Sea ice is especially useful for inter-continental dispersal; supported the dispersal of driftwood and vascular plants to Arctic islands, although several different dispersal agents played a role in this transfer such as migrating geese, foxes, and strong prevailing winds. Reduction of sea ice due to global warming will strongly impact the northward migration of northern boreal species, which may favour competing species, lead to higher genetic differentiation and stochastic extinction. (Alsos et al, 2016)

What is ecological dispersal?

Frequent and short-range dispersal, often with limited impacts on long-term distributions. Does reduce competition and moves species only modest distances within similar habitats.

What is historical dispersal?

Rare and long-range, such as exceptional movements across physical barriers. Associated with endemism, provincialism and disjunction, since species are found in geographically-restricted areas.

• Opening of the Suez canal 60 years ago initiated dispersal between Asia and the Mediterranean (Lessepsian migration between formally-isolated marine ecosystems, threatening vulnerable fisheries in the Eastern Mediterranean due to invasive species. Has been beneficial as an artificial climate corridor for Leesepsian species, where native biodiversity has declined) (Dov Por, 1978)

What is jump dispersal?

A rapid movement over a physical barrier, over a short period of a species’ lifetime. Often triggers diffusion and sometimes adaptive radiation (adaptation to exploit ecological niches in new environments, associated with phenotypic differentiation and rapid speciation).

• Darwinian Finches: once they colonised the Galapagos Islands 1-2Ma, finches diverged into 14 different species with distinct beak types to eat different food sources (and therefore, occupy individual niches).

• Bipolar disjunction in Empetrum shrubs is consistent with mid-Pleistocene bird migrations, resulting in the dispersal of crowberries on a large scale (Popp et al, 2011).

  • Circumpolar boreo-Atlantic distribution, with a disjunct presence in S. America and South Atlantic islands e.g. the Falklands, Tristan da Cunha

Definition for a niche (Chase and Leibold, 2003)

The environmental conditions that allow a species to satisfy its minimum requirements so that the birth rate of the local population is equal to or greater than its death rate, along with the set of per-capita impacts of that species on these environmental conditions.

What is a sink habitat?

A habitat in which a species could not survive unless supported by immigration from a highly productive source habitat, since the death rate exceeds the birth rate.

MacArthur et al (1972) - niche theory

Expanded on Hutchinson’s approach; investigated how similar coexisting species could survive within a given community. Focused on interspecific resource competition. Measured through:

• Niche breadth: the variety of resources or habitats used by given species

• Niche partitioning: the degree of differential resource use by coexisting species

• Niche overlap: the overlap of resources used by different species

• Niche assembly: the colonisation and organisation of species with different resource use in new or abandoned habitats.

Hubbell (2001) - “Unified neutral theory of biogeography”

A null hypothesis to the niche theory; the niche concept is no longer necessary for understanding broad-scale ecological patterns. They are instead driven by stochastic dispersal, immigration and extinction events.

What is Gause’s principle of competitive exclusion? (1934)

Two species occupying the same niche in a homogenous environment cannot coexist. Determined this after his experiments with Paramecium, which demonstrated that even when two species can thrive independently, when put in the same habitat one will thrive and drive the other to extinction.

What is an anthrome?

An anthropogenic biome.

Impacts of human dominance over ecosystems

Humans can structurally alter species ranges, richness and interactions.

Enquitst et al (2019)

Anthropogenic climate change and land use change have had profound consequences on rare plant species. Under RCP 8.5, 156,000 plant species are threatened by climate change.

Ortega, 2014

Areas with a higher abundance of rare species have human footprint values which are 1.6x greater than the global average - these regions have increased damage incurred on them. Range contractions especially prevalent in the Tropics, expected to impact 30% of species.

What is skewed abundance distribution

Common species are often a few orders of magnitude more abundant than rare/endemic species. Rare species are highly vulnerable and more likely to be depleted to extinction. Although common species dominate the structure and energy flows, rare species are keystones in driving trophic change.

What are dispersal corridors?

Broad, continuous habitats that promote unrestricted movement for most taxa. They maintain biotic similarity by enabling frequent two-way dispersal.

What are dispersal filters?

Routes that only permit certain species to pass, generally based on traits such as physiology, size, behaviour, or habitat tolerance. Creates uneven biotic exchange, with some groups moving freely whilst others are excluded.

• e.g. Beringia land bridge, which enabled mammals to move from North America to Asia and vice versa, yet they couldn’t reach as far down as South America to influence biodiversity there because of the difference in environmental conditions.

What are sweepstake routes?

Rare, accidental or hazardous dispersal mechanisms over topographical barriers.

• e.g. storms (bringing extreme precipitation and strong winds), ice rafting, unusual currents, island hopping

• Sweepstake events create stochastic patterns, often enabling unexpected colonisations and isolated biotas with low genetic diversity compared to their original populations.

Passive dispersal through ocean currents

Interannual current variability e.g. ENSO, NAO periodically alters where and when long-distance dispersal is possible.

• NOAA Fisheries, 2022: In the NH, El Nino events result in observations of tropical, warm water species moving north (extending their range); cold water species move north or into deep water, restricting their range. Loss of commercially-important species along the Pacific coast of the USA due to changed marine habitats.

Treml et al, 2008

Population connectivity (the exchange of individuals amongst marine populations) is largely determined by the dispersal of larvae and juveniles between distant patches of suitable ocean habitats.

• The scale of dispersal in the Pacific is usually 50-150km.

• El Nino events increase overall connectivity - enhancing larvae dispersal from Papua New Guinea to eastern Indonesia.

• La Nina events may enable rare dispersal patterns from Papua New Guinea to the Great Barrier Reef.

• Dispersal barriers i.e. ocean current patterns may be responsible for population isolation, explaining genetic differentiation and the existence of endemic species on Pacific islands.

What is migration?

The temporary, often seasonal occupation of a region when conditions are suitable, followed by a mass-movement to a different region when the environmental conditions shift.

• E.g. vertical migration of planktonic species due to the specialised temperature and salinity (abiotic factors) which dictate the conditions that they thrive in.

Behavioural/psychological constraints to dispersal

• Reluctance to cross open areas

• Habitat avoidance

• Strong site fidelity

Ecological barriers

• Lack of suitable habitats

• Intervening obstacles

  • Topographic features e.g. mountain range, desert, large open body of water

• Dissemination barriers

  • Distance and mechanism of spread (wind/water/rafting) may be insufficient to bridge to suitable habitats, decreasing ranges to smaller than the fundamental niche.

Prevents movements across/through inhospitable environments.

Physiological barriers

• Limits of temperature, salinity, oxygen tolerance, moisture

• Prevents survival during dispersal

  • e.g. plankton, whose dispersal is constrained by temperature and salinity in most species.

Adaptation/trait diffusion is the exception, but this is less understood as a process (Ward, 2021).

Great American Biotic Exchange (GABI)

A consequence of a culmination of a >10Ma tectonic uplift process, coupled with decreased sea ice due to N. Hemisphere glaciation, which led to the closure of the Central American Seaway. This formed a land bridge (Panamanian isthmus) between N. and S. America, and consequently barred trans-oceanic dispersal.

Influence of the Darian Gap during GABI

Filter of the Darien Gap prevented the dispersal of small rodents from N. to S. America, but the filter in the opposite direction prevented the dispersal of larger primates, rodents and marsupials. Resulted in an asymmetrical exchange - a large proportion (>50%) of N. American biota in S. America, but not vice versa (10%). Partially attributed to the savanna-adapted species from N. America, which made these species better suited to cross the hot and humid isthmus.

Why were there high extinction rates in S. America during the GABI?

North American mammals had more specialised carnivore teeth and smarter brains than the S. American marsupials, allowing them to predate the S. American species.

What is community saturation?

The dynamic equilibrium of species richness, where no new species can be added without extinctions.

What evidence against community saturation was provided through the GABI by Pinto-Sanchez et al, 2014?

Native species were not displaced or made extinct to accommodate invasive species crossing the Panamanian isthmus. Biotic interchange of hylid frogs dramatically increased local richness, with no significant decreases in the local richness of clades from one region where they co-occur with species from another region. Extinctions of incumbent species may occur due to competition or due to niche occupation (competitive exclusion).

What does the success of dispersal depend on?

• Accurate habitat selection, with dispersers generally using chemoreception (mapping ‘odour landscapes’) and sensory cues to locate suitable microhabitats

• Availability of microsites (soil, substrates, shelter, host species) enabling survival and growth

• Ability to reproduce quickly, self-fertilise and/or persist at low populations.

• Environmental cues (light, moisture, temperature, phenology) that triggers settlement, germination and breeding.

• Low competition or predation threat, allowing small populations to flourish in the short-term.

What do dispersal curves show?

In both cases, the number of species arriving per capita decreases with isolation (distance from source).

Passive dispersal = negative exponential curve

• steep fall-off near the source with very few long-distance arrivals except typically from sweepstake routes.

Active dispersal = sigmoidal curve

• High immigration nearby, sharp mid-distance drop, long tail of far-distance arrivals.

How have humans influenced dispersal?

Created new barriers through habitat fragmentation, resulting in disjunction - roads/cities have altered landscapes.

Promoted long-distance introductions through trade, transport, shipping and deliberate releases.

• Introduced species can increase extinction rates for native taxa through competition, predation and disease

• Human mediated dispersal is predicted to accelerate biotic homogenisation, making distant regions more taxonomically similar.

Positive example of human introductions: wolf ecology in Yellowstone, Wyoming. (Farquhar, 2025)

Northern Rocky Mountain wolves were reintroduced to Yellowstone in 1995, causing a trophic cascade of ecological change.

As the reintroduction of the wolf increased the predatory pressure on elks, they became less likely to remain in one location and intensely browse on young willow, aspen and cottonwood plants during the winter months. In turn, this enabled beaver populations to increase, spread and build new dams and ponds, influencing stream hydrology.

Has also influenced populations of ravens, eagles, coyotes, bears and fish.

What is a species?

A group of population held together through genetic exchange (in sexual species) or ongoing ecological cohesion and shared fundamental niches (in asexual species).

What is microevolution and why is it important?

Arises from small and gradual changes in allele frequencies across generations within a population - produce variations in traits. Occurs due to DNA mutation and recombination.

Important for creating long-term divergence between lineages, and can influence a species’ ability to live in an environment.

What mechanisms drive microevolution?

Mutation: introduces novel genetic variants that can accumulate differently in isolated populations.

• Somatic mutations have no impact on evolution since they occur in non-reproductive cells.

Natural selection: favours different traits in different environments, promoting adaptive divergence.

• House sparrows in Northern USA are larger than those in the South, likely because larger-bodied birds can survive lower temperatures than smaller-bodied birds.

Genetic drift: random fluctuations, especially important in small/founder populations.

• Decreases genetic diversity

Restricted gene flow: allows differences to accumulate rather than homogenise.

• Most useful when there is genetic segregation from the rest of the population of a species i.e. island biogeography.

These evolutionary processes lead to progressive genetic divergence among populations over time.

What does gene flow entail?

High gene flow homogenises populations, preventing differentiation. Barriers (physiological, ecological or psychological) limit gene flow and enable divergence to accumulate. Allopatric speciation can occur when isolation persists across many generations.

What is allopatric speciation?

Physical barriers interrupt gene flow between populations, and instead lead to genetic drift on either side of the large physical barrier. Gene flow isolation may result from vicariance (barrier formation) or colonisation of disjunct regions via long-distance dispersal, both enabling independent evolutionary trajectories.

Speciation may not occur if the barrier doesn’t persist long enough for microevolution, which can take numerous generations.

How do jump dispersal events affect speciation?

Jump dispersal events can isolate small founder populations, creating opportunities for rapid speciation through genetic drift and novel selective pressures (e.g. predation, climate change, urbanisation).

What is sympatric speciation?

Occurs within a single geographic region, requiring strong ecological pressures (e.g. different ecological conditions in soils for plants), behavioural separation, or assortative mating (a non-random mating pattern where individuals choose partners with similar phenotypes).

Example of sympatric speciation - Kentia Palkm and Curly Palm (Savolainen et al, 2006)

Sister species of palm trees split on Lord Howe Island because they occupy different soil types - calcarenite vs. volcanic soils. Since one plant was better adapted to low-lying alkaline soil whereas the other prefers acidic soil at a higher altitude, this meant that the conditions influenced the growth of each plant. They also have different flowering dynamics; the male flower appears approx. 6 weeks before the female flower, which restricts gene flow and therefore isolates the reproduction of the two species.

Ecological selection and phenological shifts can combine to create strong reproductive barriers even without topographical separation.

What is parapatric speciation?

Occurs when populations diverge along a spatial or environmental gradient whilst still maintaining partial contact, albeit in different niches.

Example of parapatric speciation - The greenish warbler (Irwin et al, 2001)

Greenish warblers exist in a ring of populations around the treeless Tibetan Plateau, with adjacent populations not interbreeding. Along the northern meeting point in the Himalayas, the two terminal taxa are highly divergent in song and display behavioural reproductive isolation due to their differences in playback song. Therefore, despite existing in the same geographical area, the two taxa act as different species.

What is hybrid speciation?

Hybridisation between distinct species can generate new lineages with their own evolutionary trajectories, ecological roles and geographic distributions.

What is adaptive radiation?

The rapid diversification of multiple species from a single ancestor when ecological space/resources become newly available. Radiations often follow the colonisation of new/underutilised regions, or the removal of competitors after extinction events.

Rangel et al., 2018 - cradles, museums and graves in South America

Historical patterns of species richness of persistent (‘museum’) species capture the broad features of contemporary species richness for birds, mammals and plants.

Provided evidence for ephemeral circum-Amazonia arcs of dry climates connecting the Andes with the Atlantic forests. These acted as dispersal corridors and refugia from extinction during climate shifts. This gradual warming promoted speciation, whilst rapid/more extensive warming in the same region drove extinctions.

What is extinction and what can trigger it?

Extinction is the termination of a lineage, when its population disappears and its ecological and evolutionary identity ceases to persist. Ecological extinction ca be seen long before the last individual in a lineage is gone, such as when the conditions required for the species to survive are altered.

Geographic range collapse typically precedes extinction, with populations disappearing from peripheral regions and surviving last in small refugia.

What ecological processes drive extinction?

Habitat fragmentation and reduction diminish population sizes, increasing extinction risk through demographic fluctuations and genetic drift.

Ecological interactions (competition, predation, disease) and environmental changes can push species beyond their ecological tolerances.

What processes drove the extinction of the woolly rhinoceros?(Fordham et al, 2024)

Early Holocene habitats became fragmented and degraded, with surviving woolly rhino populations confined to scattered patches of suitable climate. Although the number of habitat patches was large, the distance between them was vast, which weakened source-sink dynamics. Persistent low-level human hunting continued inside these remaining patches, which increased mortality. Population size and range sharply declined until 9.7ka BP, when the woolly rhino finally went extinct.

Extinction in the fossil record (link to Quaternary environmental change) (Stewart, 2009)

The Late Quaternary palaeoecological record represents how organisms and environments responded to climate change. At the LGM, extreme global cooling resulted in population declines of smaller carnivores and other mammals, which drove mammoths from Europe (Stuart et al., 2002). These extinctions generally take place through range contractions into smaller isolated populations. This places them at higher risk due to their inability to buffer environmental or demographic variations.

Causes of a sixth mass extinction

Primarily driven by human activity - the unsustainable use of land, water, energy, and anthropogenically-induced climate change.

• 40% of land has been converted for food production (above the planetary boundary for land system change)

• Agriculture is also responsible for 90% of deforestation and accounts for 70% of the planet’s freshwater use

• Climate change exacerbates the challenges associated with food production which stress species, whilst simultaneously creating conditions that make their habitats inhospitable.

  • increased droughts and floods have made it more difficult to maintain crops and produce sufficient quantities of food in the Sahel belt, increasing food insecurity.

Evidence supporting a sixth mass extinction (Barnosky et al., 2011)

Modern vertebrate extinction rates already exceed those that produced the Big Five mass extinctions, when expressed over equivalent timescales.

• The species extinction rate is estimated between 1,000 and 10,000 times greater than natural extinction rates.

• It will take 2-3 millennia for 75% of all critically endangered species to go extinct at current rates.

• Under natural background rates, the number of vertebrates lost in the past century have declined at a rate 8-10x faster than they should have.

What is historical contingency?

Historical contingency recognises that evolutionary and biogeographic outcomes often correlate with specific sequences and timings of past events. Differences in the order/timing of tectonic shifts, climate changes, or dispersal events can lead to dramatically different present-day patterns.

What is a community? (Mittelbach and Schemske, 2015)

A group of species that co-occur in space and time and that have the potential to interact.

What is ecological succession?

A biological communities’ re-assemblage and change in structure over time, following natural or anthropogenic disturbance.

What is the difference between primary and secondary succession?

Primary succession occurs in the land where there is no initial vegetation, where surface soil cover is absent and pioneer species come from outside of the environment. Secondary succession occurs in a land that has primary vegetation and in places where soil cover is present. It often occurs following environmental disturbances, such as fires or farming, which allows for rapid recolonisation following the onset of succession.

Origins of succession - Cowles, 1899

Studied the chronosequence of vegetation growth along sand dunes, moving from bare sand beach, to grassland, to mature forests.

Superorganism facilitation theory - Clements, 1916

Proposed a climax state for communities, which represents the permanent end-stage of succession. After a disturbance, all ecosystems will eventually return to its pre-disturbance characteristic assemblage of species.

Gleason, 1926 - Individualistic succession model

Species will appear in a given system, increase in abundance, decrease in abundance, and then disappear. Unlike Clements (1916), he argues that successional communities are not super-organisms, whereby all species in the climax community work together to maintain a stable composition. Instead, species respond independently to changing environmental conditions (e.g. climate, soil quality, moisture), dispersal and chance events. Rather than having a fixed climax community, the species which grow in a particular area are merely those who can pass through the filter of local environment conditions and the availability of propagules.

Evidence towards individual success model - Davis, 1983

The rate of movement of invasive white pine and hickory species in the USA in response to climate change since the LGM are not equiformal, ranging from between 100m to 400m per year.

Mechanisms of succession - “Relay” floristics, Clements (1936)

Succession is the predictable replacement of species over time. Early colonisers (arriving via propogules) prepare the sute for the next group. Reaction occurs through modifying the physical conditions of the site by vegetation, enabling the next stage.

Mechanisms of succession - Facilitation, tolerance and inhibition (Connell and Slatyer, 1977)

Facilitation - early species ameliorate conditions for later ones.

• Alter conditions such as soil nutrients, light accessibility, water availability)

Tolerance - later species tolerate low resources, and early species do not hinder them, meaning that early species are not prerequisites for later species to establish.

Inhibition - earlier species prevent later species from establishing until they are killed by disturbance/herbivory, such as through shading, allelopathy or space pre-emption.

• Occupying a habitat limits competition for resources.

How do mechanisms of succession in real landscapes alter successional change?

• Life history traits: arrival time, dispersal, rapid early growth, longevity

• Facilitation: soil improvement, shade availability

• Competition: resource depletion

• Inhibition: litter/shade suppression

• Herbivory: browsing shifts competitive balance

• Type and intensity of disturbance: primary succession disturbance after a magmatic extrusion vs. secondary succession re-colonisation after land use change

• Climate variability

• Presence of invasive species; native and non-native plant diversity may increase during secondary succession, indicating that invasive plants can enter new communities through processes very similar to native species (Sakai et al, 2001). This can alter the normal successional trajectory of a system.

Means that no single successional process or model adequately describes successional change. (Walker and Chapin, 1987)

Why do succession assumptions using space-for-time chronosequences often fail?

Chronosequences use spatial contrasts among sites to assume that the spatial patterns represent how a landscape will change over time. Most studies on succession have used space-for-time substitution and short time periods post-disturbance (up to 50 years)

Sites rarely differ only in time - ignores climatic shifts, multiple disturbance regimes, land-use history, geology etc. These critical factors also play a role in why remote sensing and long-term modelling are required to improve the validity of successional claims made solely through SFT substitution.

What are problems with chronosequence models using time-for-space substitutes?

• A lack of permanent plot records spanning >100yrs

• Paleoecological studies often have a low temporal and spatial resolution

• Dendrochronological reconstructions provide limited information about the early dynamics of non-tree species.

Alternative methods - resource-ratio hypothesis (Tilman, 1985)

An extension of geographical theory of plant competition for resources in spatially heterogeneous habitats and along spatial gradients. This means that succession results from a gradient through time in the relative availability of limiting resources.

• Each plant species is a superior competitor for a proportion/ratio of limiting resources.

• The species available to survive at the lowest equilibrium level of a limiting resource will dominate competition.

• Two species can coexist on two limiting resources if each species is more limited by a different resource - e.g. phytoplankton with phospherous and nitrogen.

• Light vs. soil resources in forests dictate the species composition - whether they require more light or richer soils.

Fastie, 1995 - succession at Glacier Bay, Alaska

Successional chronosequencing at the Glacier Bay fjord in southeastern Alaska, due to glacial till exposure to biotic colonisation. Recently exposed surfaces of unconsolidated glacial deposits were colonised by woody species, whilst older surfaces (35-45yr old) have a 100% cover abundance of shrubs and cottonwood trees - Sitka alder/spruce is the dominant species on surfaces >100yrs old.

The soils revealed during deglaciation are poor in organic matter and therefore have very harsh growing conditions for vegetation. However, the Sitka alder tree can grow in these soils, meaning that they can undergo nitrate fertilisation in their roots to convert atmospheric nitrogen into ammonium compounds to build up proteins. Through nutrient cycling as leaf litter and dead roots decompose over time, the growth of the tree increases the concentration of nitrates in the soil. This eventually fertilises the soils enough for other vegetation, which require more organic matter in the soil, to grow.

Glacier bay timeseries 1916 - 2017 (Buma, 2017)

Expected via a chronosequence-based hypothesis a predictable succession sequence following classic facilitation models: mosses → Alder→ Spruce → Pine.

Reality:

• No support for facilitation model

• Stochastic early assembly dominates, with most species arriving after deglaciation and remaining stable for 50+ years

• Inhibition (inability for new species to colonise), not facilitation, controls trajectories. Spruce and Pine species monopolised light and prevented later establishment. Actually reduced biodiversity in the long-term by blocking out sunlight which prevented further growth.

• No consistent sequence across plots; long-term divergence driven by early events within the first <25 years.

• Ecological stagnation due to very limited seedling establishment of canopy species, which caused communities to become locked in without successional advances.

What is the BAM diagram?

An abstract representation of geographical space. Shows the intersection between biotic, abiotic and mobility (environmental) restrictions which dictate where certain assemblages exist. Represents the ideal living conditions for different species.

• Hutchinsonian n-dimensional hypervolume niche

What factors dictate whether a location is an island?

Oceanic islands - ocean-formed, remote

Continental fragments/land-bridge islands - previously connected

Habitat islands - insular ecosystems (e.g. mountaintops, lakes, caves, springs, forest patches)

MacArthur and Wilson, 1967 - equilibrium theory of island biogeography (ETIB) model

Species richness increases with island area.

• Larger islands support more individuals, resulting in lower extinction rates.

• Larger islands also tend to contain more habitat types, which increases biodiversity through facilitating more ecological niches.

• There is a greater likelihood for spatial heterogeneity in larger islands as well, which enables species to co-exist despite having similar niches.

Species richness also decreases with island isolation

• Isolation affects immigration rates because the probability of dispersal declines with greater barrier distance.

• Aligns with models on passive vs. active dispersal mechanisms; negative exponential curves and sigmoidal curves due to a very low quantity of long-distance arrivals.

Assumes a dynamic equilibrium, where the immigration rate (declining with increasing richness) = extinction rate (increasing with increasing richness).

Empirical evidence supporting the ETIB model - Western Australia (Schrader et al, 2022)

Study of 156 native flowering plant species on 15 islands from 164m² to 19km². Determined that species richness was in equilibrium despite high temporal turnover. The species most susceptible to turnover were on average shorter and had lower seed mass than the persisting species, since these species are more susceptible to extinction during unfavourable conditions. The selectivity of filters remained stable over time, meaning that new colonists had traits very similar to those of extinct species.

Empirical evidence against the ETIB (Morrison, 2010)

Studied plants on islands prone to tropical storms. Determined that there was a non-equilibrium state over a course of 17 years, with extinction rates during hurricane periods much higher than immigration rates. Resulted in indirect effects such as decreased nutrient availability, an increase in temperature and a decline in rainfall - shifting ecological conditions potentially out of the favourable environmental niches which some species inhabit.

Empirical evidence against the ETIB (Brown, 1971)

Insular ecosystem of the Sierra Nevada, Great Basin and Rocky Mountain regions of the USA make them montane islands - arid desert surrounded by rocky mountain terrain.

Recurrent colonisation is not occurring due to limited dispersal/immigration from nearby regions as a result of very different abiotic/environmental factors between the two microbiomes. Post-Pleistocene extinctions as a result of climate change reduced species richness across all three boreal landscapes, but not enough to restore an equilibrium. The environment can be said to be saturated currently because the montane islands were colonised during periods where climatic and habitat barriers (which now prevent dispersal) were temporarily abolished.

Whittaker (2000) - three new hypotheses towards non-equilibrium states (General Dynamic Model; GDM)

1. Dynamic equilibrium - same as Wilson and MacArthur

2. Dynamic non-equilibrium: a constant turnover of species composition as the result of disturbance.

3. Static equilibrium: turnover is lacking or so minimal so as to make the species composition appear static over time. Often found in birds, who have a high dispersal capability and populations are more likely to emigrate rather than be threatened to extinction (rescue effect).

Island progress through stages, which integrate geological ontogeny with biogeographic processes:

1. Emergence and growth

2. Maximum area/elevation → peak habitat diversity (consenus with Wilson and MacArthur, where island size correlates with biodiversity levels)

3. Subsidence, erosion, reduced area → biodiversity decline.

Island biogeography and succession in Hawaii

Primary succession following volcanic events on Kilauea, Hawaii. Colonisers including blue-green algae, mosses and lichens first invaded the newly-formed cinder cone following the eruption in 1959. By 1980, new shrubs began forming, distributed ground-walking Hawaiian geese - a keystone species for vegetation recovery following volcanic devastation.

Succession patterns in Hawaii do not follow Clement’s (1916) superorganism facilitation model - instead, primary succession climaxes, remains stable for several thousands of years, and then falls due to extreme phosphorous limitation and soil leaching. Provides evidence supporting the resource-ratio hypothesis (Tilman, 1985)

What is island syndrome?

Selective immigration filters colonists based on dispersal ability, whilst selective extinction removes vulnerable groups e.g. large-bodied, resource-limited species who arrived via a stochastic migration/dispersal.

High endemicity due to in-situ speciation, which over-represents certain lineages.

Altered interspecific interactions (e.g. simplified food webs, missing predators, limited competition) which further reshapes community cohesion and amplifies disharmony, thereby enabling certain species to flourish.

Island evolutionary anomalies

Isolation produces evolutionary transformations, owing to altered selective regimes, limited predators and simplified interactions.

• Flightless birds

• Ground-foraging bats

• Plants evolving woody forms

• Transformation of body size (dwarfism and gigantism).

Bergon et al., 2014 - unimodal relationship between species richness and productivity

In unproductive ecosystems with low plant biomass, species richness is limited by abiotic stresses such as insufficient water and mineral nutrients because harsh conditions filter out many species, facilitating a small subset with specialised traits to survive. In highly productive conditions that generate high plant biomass, competitive exclusion by a small number of highly competitive species filters out less successful plants.

Fraser et al, 2015 - agree with global unimodal model

Disturbances such as fire, grazing or flooding can amplify additional stress in low productivity environments where conditions are already harsh. In high productive environments, changing abiotic conditions enables specialised species to further suppress others (exert their competitive dominance). This compounds the negative effects of the disturbance where intermediate productivity promotes coexistence, and therefore can reduce the ecosystem’s resilience to disturbances.

Chase and Leibold (2002) - linear relationship between productivity and species richness at larger spatial scales due to the ‘More Individuals Hypothesis’

At local ecological scales, non-linear patterns may dominate because competitive interactions intensify with high resource availability. Conversely, on larger scales, greater species compositional dissimilarity means that the ecosystem is more likely to obtain multiple stable states, which reduces competition. Higher productivity areas also contain more habitat types, leading to species competing with each other less. This means that energy availability is the fundamental constraint to species richness, which allows a linear relationship between productivity and biodiversity to emerge.