Chapter 18- Biogeography

Biogeography

the study of patterns of species composition and diversity across geographic locations.

Patterns of species diversity and distribution vary at…

global, regional, landscape, and local spatial scales

Spatial scales are interconnected in a …

hierarchical way

The patterns of species diversity and composition at one spatial scale set…

the conditions for patterns at smaller spatial scales.

What 3 factors influence global patterns of species diversity

1. Geographic area and isolation

2. evolutionary history

3. global climate.

Figure 18.5- Interconnected Spatial Scales of Species Diversity

Rank Spatial Scales from highest to lowest

1. Global Scale

2. Regional Scale

3. Landscape Scale

4. Local Scale

Global Scale includes

the entire world

Components of Global Scale

• Species have been isolated from one another, on different continents or in different oceans, by long distances and over long periods.

• Rates of speciation, extinction, and dispersal help determine differences in species diversity and
  composition

Regional scale

• areas with uniform climate

• the species are tied to that region by dispersal limitations.

Regional species pool

all the species contained within a region

Landscape Scale

• topographic and environmental features of a region.

Components of Landscape Scale

• Species composition and diversity vary within a region depending on how the landscape shapes rates of migration and extinction.

Local scale

equivalent to a community.

Components of Local scale

• Species physiology and interactions with other
  species are important factors in the resulting
  species diversity.

Actual area of different spatial scales depends on…

the species and communities of interest

Example of how different spatial scales depends on the species and communities of interest

Terrestrial plants might have a local scale of 102–104 m2, but for bacteria, the local scale might be more like 102 cm2.

What is the species pool?

The set of all species contained within a region that can potentially colonize local sites.

3 types of species diversity

1. Gamma

2. Alpha

3. Beta

Gamma Species Diversity

Species diversity within a region

Alpha Species diversity

local species diversity of a community

Beta diversity

• Change in species number and composition, or turnover of species, from one community type to another (diversity btwn communities)

• connects local and regional scales.

The regional species pool provides

raw material for local assemblages and sets the theoretical upper limit on species diversity for communities.

Figure 18.6- What determines local species richness

What does slope= 1 represent?

• Red

• All species in a region found in all communities

Figure 18.6- What determines local species richness

What does slope <1 represent?

• Blue

• Community richness is mostly determined by the regional species pool

Figure 18.6- What determines local species richness

What does non-linear slope represent?

• Green

• Local processes can be assumed to limit local species diversity

Figure 18.7 (A) - Marine Invertebrate Communities May be Limited by Regional Processes Components

• Witman and colleagues (2004) considered this relationship for marine invertebrate communities living on subtidal rock walls at a variety of locations throughout the world

• At 49 local sites in 12 regions, they surveyed species richness of marine invertebrates on subtidal rock walls

FIGURE 18.7B: Local vs Regional Species Richness Graph Components

• X-axis = Regional species richness (how many species could possibly live there based on broader lists).

• Y-axis = Local species richness (how many species they actually found in the tiny plots).

• Each dot = one site (out of 49).

• Trend: As regional richness increases, local richness increases too — but local richness is always lower.

• Important result:

  • Local richness never leveled off even at very high regional richness.

  • There was a strong positive relationship (regional richness explained about 75% of the variation in local richness).

What did Witman et al. (2004) discover about local and regional species richness in marine invertebrate communities?

• Local species richness was strongly influenced by regional species richness (~75% variation explained), without saturation at high richness levels.

• The results of this study

  suggest that regional species pools largely determine the number of species present in these marine

  invertebrate communities.

Global patterns of species diversity and
composition are influenced by

geographic area and isolation, evolutionary history, and global climate

Figure 18.8 Alfred Russel Wallace, the Father of Biogoegraphy and His Collections

What did Wallace Find?

1. Wallace’s Line:

   • Wallace noticed a sharp boundary between the faunas of Asia and Australia in the Malay Archipelago (near Indonesia).

   • Mammals (and other species) on either side of the line are very different because they evolved on separate continents.

   • These continents only came close together about 15 million years ago, allowing their species to stay distinct.

2. Wallace’s Two Major Global Patterns:

   • Pattern 1: Six Biogeographic Regions
     → Earth’s land masses can be divided into six major regions, each with distinct groups of species (biotas).
     → These regions differ markedly in both species diversity and species composition. → (Examples: Nearctic, Neotropical, Ethiopian, Palearctic, Oriental, Australasian)

   • Pattern 2: Latitudinal Diversity Gradient
     → Species diversity is highest at the tropics and decreases toward the poles. → (This is one of the most famous patterns in ecology!)

Biographic Regions

Regions that contain distinct biotas that differ markedly in species diversity and composition

What does this image depict?

• Wallace’s Line (black line):

  • Drawn by Alfred Russel Wallace in the 1800s.

  • It marks a sharp boundary between the fauna of Asia (left/west) and the fauna of Australia (right/east).

  • Example:

    • To the west (like Borneo, Sumatra, and Java): mammals like tigers, elephants, and monkeys (Asian fauna).

    • To the east (like New Guinea and Australia): marsupials like kangaroos and tree kangaroos (Australian fauna).

• Weber’s Line (red line):

  • Drawn later by Max Weber.

  • It is a more refined, gradual boundary that runs farther east.

  • It shows that species mixing is more complex, not as sharp as Wallace originally thought, with some overlap (species transition zone).

• Geographic Area Covered:

  • Southeast Asia and Oceania, including islands like Borneo, Sulawesi, New Guinea, and the Philippines.

Why This is Important:

• Historical reason: The sharp difference in animal species occurs because of deep water channels that never dried up, even during ice ages when sea levels were lower.

• Evolutionary reason: Species evolved separately for tens of millions of years on the Asian and Australian continental plates.

• Biogeographic reason: It proves that physical barriers like oceans can create huge biodiversity differences even across short distances.

What did Wallace’s Book: The Geographical Distribution of Animals (1876) identify using the distributions of terrestrial animals

• The land masses can be divided into 6 biogeographic regions.

• There is a gradient of species diversity with latitude

Continental drift

• The process that drive the movement of tectonic plates

• Hypothesized that the continents drifted over Earth’s

Three major types of boundaries between tectonic plates.

1. Mid-ocean ridges

2. Subduction Zones

3. Fault

Mid-Ocean Ridges

• molten rock flows out of the seams between plates and cools, creating new crust and forcing the plates apart

Subduction Zone

• Area where 2 plates meet

• One plate is forced downward under another plate.

• These areas are associated with strong earthquakes, volcanic activity, and mountain range formation

Fault

• Areas where two plates meet, the plates slide sideways past each other

Figure 18.10- Mechanisms of Continental Drift

• Plates:

  • Earth's outer crust is divided into large sections called tectonic plates.

  • These plates "float" on the semi-liquid layer underneath called the mantle.

• Mid-Ocean Ridge (Middle Area):

  • Hot magma rises from the Earth's mantle at mid-ocean ridges.

  • As the magma cools, it creates new oceanic crust, which pushes plates apart (this is called seafloor spreading).

  • The yellow arrows in the mantle show the flow of heat and material driving this motion.

• Subduction Zone (Right Side):

  • Where two plates meet, sometimes one plate sinks under the other — this is a subduction zone.

  • Here, one plate gets forced downward into the mantle and eventually melts.

  • Subduction causes the formation of mountains and volcanoes (shown erupting on land).

• Mantle Convection:

  • Movement of heat (via convection currents) in the mantle drives the movement of the plates.

  • The arrows in the magma layer show circular convection currents: hot material rises, cools, then sinks back down, driving the cycle.

How does continental drift work, and why is it important in biogeography?

Convection currents in Earth's mantle move tectonic plates, forming mid-ocean ridges and subduction zones. Continental drift separated species over millions of years, driving evolutionary divergence and creating distinct biogeographic regions.

Explain this image

• Biogeographic regions are defined by distinct species groups — species within a region are more similar to each other than to those in other regions.

• These regions formed largely due to continental drift and long-term geographic isolation.

• The plate boundaries help explain why continents — and their species — moved and evolved separately.

• Notice how tectonic plate boundaries don't always match biogeographic boundaries exactly — because historical land connections and separation patterns also matter.

Explain Figure 18.11- The Positions of Continents and Oceans Have Changed over Geologic Time

Permian Period (251 million years ago)

• Supercontinent Pangaea existed.

• All the continents were fused together into one giant landmass.

• Species could move freely across connected land areas.

• Very little geographic isolation at this point.

Explain Figure 18.11- The Positions of Continents and Oceans Have Changed over Geologic Time

Cretaceous Period (100 million years ago)

• Pangaea split into two massive continents:

  • Laurasia in the north (North America, Europe, Asia)

  • Gondwana in the south (South America, Africa, Antarctica, Australia, India)

• Significant isolation began between northern and southern species.

• Ocean basins started widening.

• Divergence of species accelerated because of geographic separation.

Explain Figure 18.11- The Positions of Continents and Oceans Have Changed over Geologic Time

Paleogene (60 million years ago)

• Continents moved closer to their modern positions.

• South America, Africa, and Australia were increasingly separated by oceans.

• Distinct faunas developed on isolated continents (example: marsupials in Australia, unique mammals in South America).

• Long-term geographic isolation promoted high levels of endemism (species found nowhere else)

What does this image show?

• Pangea supercontinent surrounded by a global ocean, widespread deserts and mountain ranges, with species freely mixing before continental drift caused isolation and speciation.

• Because all land was connected, species had wide distributions and fewer barriers to movement.

• As Pangaea later broke apart, populations became geographically isolated → led to speciation (formation of new species) and eventually today's distinct continents and biotas.

• Climate patterns were extreme:

  • Coastal areas were wetter.

  • Inland areas were dry deserts due to the huge size of the landmass (continental interiors are far from oceans).

Explain Figure 18.11 (B) The Positions of Continents and Oceans Have Changed over Geologic Time

• A summary of the movements that led to the configuration of the continents we know today. Red arrows are labeled with the time (in millions of years) since land masses joined; black arrows are labeled with the time since land masses separated.

• Key Takeaways:

  • Continental drift continues today — it wasn't just ancient history!

  • The positions of continents have changed dramatically even in the last 100 million years.

  • These movements:

    • Created new barriers (oceans).

    • Allowed species mixing in some cases (like India colliding into Asia).

    • Drove evolutionary changes by separating or joining different biotas.

The Neotropical, Ethiopian, and Australian regions…

have been isolated for a long time and have very distinctive forms of life

The Nearctic region differs substantially from the Neotropical despite…

modern- day proximity.

North America was part of…

Laurasia

South America was part of…

Gondwana

North America and South America…

had no contact until about 6 million years ago; there has been some movement of species since then.

Vicariance

evolutionary separation of species by barriers such as those formed by continental drift

Explain this image Figure 18.12

It shows the evolutionary relationships and geographic distribution of a group of large, flightless birds known as ratites

What is the difference between the Holt et al. (2013) biogeographic regions and the classic six-region model?

Holt's assessment incorporates modern extinction and threat data, showing that current biodiversity crises do not match historical biogeographic patterns.

Identification of marine regions has been hindered by which 2 things?

1. the complicating factor of water depth

2. basic lack of knowledge of the deep oceans.

Biodiversity Hotspots

• Regions with exceptionally high biodiversity that are under threat.

• They are characterized by a large number of endemic specie

Willig and Colleagues (2003) experiment setup

• Tallied the results of 162 studies on a variety of taxonomic groups extending over broad spatial scales (20° latitude or more) that considered whether diversity and latitude showed a

  • negative relationship (with diversity decreasing toward the poles)

  • positive relationship (increasing toward the poles)

  • unimodal relationship (increasing toward mid-latitudes and then declining toward the poles)

  • no relationship.

Results of Willig and Collegues Experiment

• Negative relationships were by far the most common.

• Strong Negative Relationship:

  • For almost every group, the majority of studies (blue bars) showed a negative relationship between latitude and species richness.

  • This confirms the classic latitudinal diversity gradient: more species near the equator, fewer toward the poles.

• Few Positive Relationships:

  • Very few studies (orange bars) found higher diversity at higher latitudes.

• Some Unimodal Patterns:

  • Some groups (especially fishes and invertebrates) showed a unimodal pattern — diversity was highest at mid-latitudes rather than exactly at the equator.

• Very Few “No Relationship” Results:

  • Very few studies found no trend (purple bars are small).

Explain Figure 18.13- Seabirds Defy Conventional Wisdom Global:

Guide to Seabirds of the World. Penguin Random House: London

Key Point	Importance

Seabirds are an exception to the normal latitudinal diversity pattern

Shows that productivity, not just temperature, influences species diversity

Higher diversity in cold, productive waters

Arctic and Antarctic oceans

Lower diversity in warm, less productive tropical waters

• seabird species richness shows a latitudinal pattern opposite to that of most faunas.

  • (A) Species richness among seabirds is high in temperate and polar regions and much lower in the tropics.

  • (B) Species composition also shows strong latitudinal differences.

Why do seabirds show highest species richness at temperate and polar latitudes, contrary to most faunas?

• Because marine productivity is highest at temperate and polar regions, providing more food for seabirds.

• Most life = richest near the equator
  🐧 Seabirds = richest near the poles because that’s where the fish are!

Change in species richness Formula

∆S = D - E

Components of Species Richness Formula

• ∆S is the change in species richness

• D is the diversification rate (number of new species produced)

• E is extinction rate (loss of existing species)

Graph A: Diversification Rate Hypothesis

• The tropics have a higher diversification rate — species are forming (speciating) faster than in temperate areas.

  • So, the difference in slope (how fast the line rises) shows that tropical regions accumulate species at a faster pace.

✅ Key idea: Higher diversification rate → faster rise in species richness.

Graph B: Diversification Time Hypothesis

• Both the tropics and temperate areas diversify at about the same speed, but tropics started diversifying earlier (because they were less disturbed historically).

  • Thus, the tropics have more species now simply because they've had more time to diversify.

✅ Key idea: Earlier start time → more species, even if the rate is similar.

Graph C: Productivity or Carrying Capacity Hypothesis

• Species richness levels off (reaches a plateau) because communities eventually reach their carrying capacity.

  • The tropics can hold more species at equilibrium because they have higher productivity (more energy and resources to support species).

✅ Key idea: Higher productivity → higher carrying capacity → higher species richness ceiling.

Global patterns of species richness are controlled by rates of which 3 things?

1. speciation

2. extinction

3. dispersal

Does species diversification / extinction vary with geographic location? If so, why?

Yes; tropical regions have higher diversification and lower extinction due to climate stability, large area, high productivity, and fewer historical disturbances.

What are the three broad categories of hypotheses proposed by Mittelbach et al. (2007) to explain the latitudinal gradient in species richness?

1. Diversification Rate Hypothesis

2. Diversification Time Hypothesis

3. Productivity or Carrying Capacity Hypothesis

What does the Diversification Rate Hypothesis propose?

Species in the tropics have a higher rate of diversification (speciation minus extinction) than in temperate zones, leading to faster species accumulation over time.

What pattern would the Diversification Rate Hypothesis show on a species richness vs. time graph?

A steeper slope for the tropics compared to the temperate zone.

What does the Diversification Time Hypothesis propose?

Tropics and temperate zones have similar diversification rates, but the tropics have had more time to diversify because they were less affected by disturbances like ice ages.

What pattern would the Diversification Time Hypothesis show on a species richness vs. time graph?

The tropical curve would start earlier but have a similar slope compared to temperate zones.

What does the Productivity or Carrying Capacity Hypothesis propose?

The tropics have higher productivity (more resources) which allows for a higher carrying capacity — meaning more species can coexist.

What pattern would the Productivity or Carrying Capacity Hypothesis show on a species richness vs. time graph?

Both regions’ species richness would level off, but the tropics reach a higher maximum.

How do geographic area and temperature stability contribute to tropical species richness?

The tropics have the largest land area and most stable temperatures, promoting lower extinction rates and higher opportunities for speciation.

1. Species diversification rate

• The tropics have the most land area on Earth
  and temperatures are very stable.

• Large, thermally stable areas should decrease
  extinction rates—population sizes and
  geographic ranges would be larger.

• Speciation by geographic isolation would be
  more likely.

Explain Figure 18.16 — Do Land Area and Temperature Influence Species Diversity?

• The tropics not only have the most land but also the most thermally stable environment.

• These two factors — large area and stable, warm temperatures — promote high species diversity.

• This supports the Diversification Rate Hypothesis and Productivity/Carrying Capacity Hypothesis by explaining why the tropics could have more species.

2. Species diversification time

• The tropics are thought to have been more
  climatically stable over time, and species have
  had more time to evolve.

Figure 18.6- The Tropics Are a Cradle and a Museum for Speciation Extant and fossil marine bivalve taxa were examined to evaluate the hypothesis that longer evolutionary histories in the tropics contribute to the latitudinal gradient in species diversity. (A) Climate zones of first occurrence of marine bivalve taxa (based on families of fossils). (B) Range limits of modern marine bivalve taxa with tropical origins

Jablonski et al. (2006)

• The tropics may serve as a cradle - species originate in there and to other regions

• Jablonski et al. (2006) found evidence of this in marine bivalves.

• Loss of biodiversity in the tropics could cut off the supply of new species to higher latitudes in the future

• Another idea is that most species originate in the tropics and then move to other regions during warm climatic periods.

• Most extant taxa originated in the tropics and spread towards the poles.

• Findings: Most species originated in tropics and spread poleward, while remaining in tropics too

3. Productivity or Carrying Capacity

• Terrestrial productivity is highest in the tropics.

• High productivity promotes large population sizes because carrying capacity is larger.

• Higher productivity leads to lower extinction rates, greater coexistence, and overall higher species richness.

• Productivity could also explain the reverse pattern seen in sea birds.

Mannion and colleagues (2014) looked at…

the fossil record to characterize species diversity by latitude across geological time.

What does the image Mannion deduce?

1. Species richness has generally been highest near the equator (0° latitude).

• Most black circles (low-latitude diversity) dominate over time.

• Indicates equatorial regions consistently had higher species richness throughout Earth’s history.

2. Major mass extinctions reduce global diversity sharply.

• Every time you see a blue shaded region (mass extinction), the red line (species diversity) drops steeply.

• Especially visible around:

  • End-Ordovician (444 Ma)

  • End-Permian (252 Ma, the biggest drop)

  • End-Cretaceous (66 Ma)

3. After extinctions, diversity gradually recovers — but equatorial regions tend to bounce back faster.

• The black circles (equatorial diversity) appear earlier after mass extinctions compared to red ones (high latitude).

• Suggests tropics act as a refuge and cradle for rebuilding diversity.

4. Over long timescales, higher latitudes (30°–60°) sometimes show some bursts of diversity, but they are less consistent.

• Red open circles (high latitudes) are present but less frequent and often later than equatorial points.

Regional differences in species diversity are influenced by area and distance, which determine…

the balance between immigration and extinction rates.

An important concept in biogeography is…

the species– area relationship

Species– area relationship

species richness increases with area sampled.

H.C. Watson plotted first species-area relationship, using plants in the Great Britain...in 1859

• Study: Plotted species richness of plants vs. area size in Great Britain.

• Findings:

  • Species richness increased with area sampled, showing a species–area relationship.

• Importance:

  • Laid the foundation for understanding how area affects biodiversity.

  • Proved that bigger areas support more species

Species–area curves plots…

species richness (S) of a sample against area (A).

The relationship between S and A is estimated by linear regression formula:

S= zA + c

Components of Species–area curves plot formula

S= Species richness
A= Area

z= slope

c= y-intercept

Ecological Toolkit 18.1, Figure A Species–Area Relationships of Island versus Mainland Areas (Part 2)

• The figure shows species–area curves for plants on the Channel Islands (off the coast of France) and on the French mainland (Williams 1964).

• An important characteristic of species–area curves is evident in this figure: the steeper the slope of the line (i.e., the greater the z value), the greater the difference in species richness among the sampling areas.

• The Channel Islands have a much steeper slope than the French mainland areas

• Species–area curves for plant species on the Channel Islands and in mainland France show that the slope of a linear regression equation (z) is greater for the islands than for the mainland areas.

Islands” are really useful for studying…

species-area relationships

An island can be a…

any kind of isolated area surrounded by dissimilar habitat (matrix habitat).

Habitat fragments, such as in the Amazon forest, can be considered as…

islands

All display the same basic pattern

large islands have more species than small islands.

What does Figure 18.19- Species–Area Curves for Islands and Island-Like Habitats show?

• Species–area curves plotted for (A) reptiles on Caribbean islands, (B) mammals on mountaintops in the American Southwest, and (C) fishes living in desert springs in Australia all show a positive relationship between area and species richness.

• Species richness increases with area across many types of isolated habitats — not just true islands.

• Larger areas provide more habitat types, more resources, and lower extinction risks, allowing more species to persist.

• Isolation doesn't stop the area effect — mountains and springs (not just islands) follow the same rule.

MacArthur and Wilson developed a
theoretical model…

The equilibrium theory of island biogeography

The equilibrium theory of island biogeography

The number of species on an island depends on a balance between immigration or dispersal rates and extinction rates.

The equilibrium theory of island biogeography effects on species richness

• Larger islands have more species because they have lower extinction rates (more habitat, larger populations).

• Islands closer to the mainland have more species because they have higher immigration rates (easier to reach).

Explain this figure 18.21- The equilibrium Theory of Island Biogeography:

Small island ; Far Island Species Richness

Fewest species

Explain this figure 18.21- The equilibrium Theory of Island Biogeography:

Large, far island

More Species than small-far, but still limited by distance

Explain this figure 18.21- The equilibrium Theory of Island Biogeography:

Small, near island

More species than distant islands, but limited by small size

Explain this figure 18.21- The equilibrium Theory of Island Biogeography:

Large, near island

Most species overall