Ecology Quiz 8 Flashcards
Biogeography:
Patterns in richness:
Biogeography: study of patterns of species composition and diversity across time and space.
- Patterns in richness: that we see globally are striking - amazon has a lot more tree species than a boreal forest
Biodiversity:
Simplest concept of biodiversity:
Limitations:
Biodiversity: different variety of living organisms present in the biosphere.
Simplest concept of biodiversity: species richness (# of species found in the local community).
Limitations:
Boundaries for measuring species richness are arbitrary (quadrat - plants, transect - forest, etc). This is problematic as richness is correlated to sample size.
Richness is only one aspect of biodiversity: genetic diversity, functional diversity(grasses - distinct roles in ecosystem) and phylogenetic diversity(represents the summed branch lengths of the evolutionary tree connecting species within a set, frequently defined by geographical proximity) (not always related to taxonomic diversity).
Taxonomic(species richness - amazon high), functional, and phylogenetic diversity(five diff oak species and then 3 diff pine so 2 phylogenetic)(more homogenous even)
Large scale patterns of species diversity and composition set constraints for smaller scales:
Global:
Regional:
Landscape/Local Scale:
Global Scale: Rates of speciation, extinction, and migration drive differences in species diversity and composition. Dictated by climate which is determined by perception and temp which effects everything down the line.
Continental Drift: Isolation on different continents or in different oceans by long distances and over long periods (large spatiotemporal scales)
Regional scale:
What is a region: large in spatial scale, extending over several kilometers and containing large # of habitats and communities.
Areas from which species, over time, have a good probability of colonizing a local community.
Landscape - Local scale:
Dispersal limitation, microclimate and species interactions determine local scale species diversity (alpha diversity).
Beta diversity: change in species number and compositions, or species turnover, from one place/time to another.
Historical Biogeography:
What century?
The Father Of Biogeography
Revealed (at least) two important global patterns:
Phylogenetic analyses:
Biogeography was born with scientific exploration in the 19th century.
Alfred Russel Wallace (1823– 1913): “father of biogeography”. Best known, along with Charles Darwin, as co-discoverer of principles of natural selection.
Primary contribution was the study of species distributions across large spatial scales.
Wallace published The Geographical Distribution of Animals in 1876 - Noted that mammals of Philippines were more similar to those in Africa (5,500 km away) than to those in New Guinea (750 km away)
Wallace overlaid species distributions and geographic regions and revealed (at least) two important global patterns:
- Earth’s land mass is divided into six biogeographic regions.
- Notice the gradient of species diversity with latitude.
He deduced 6 biogeographic zones without knowing about the plate tectonics or continental drift, which would come along nearly 100 years later. He did not really understand why these patterns should exist, but he noted them anyway. These biogeographic regions correspond roughly to Earth’s major tectonic plates.
Tectonic plates are sections of Earth’s crust that move or drift (continental drift).
Phylogenetic analyses largely confirm the distinctiveness of regions identified by Wallace - 250 mya all of Earth’s land masses made up one large continent—Pangaea
Positions of the plates and continents have changed dramatically over geologic time.
Pangaea first split into two land masses, Laurasia to the north and Gondwana to the south. - Cretaceous Period 100 mya
Gondwana separated into present-day South America, Africa, India, Antarctica, and Australia, Laurasia later split up into North America, Europe, and Asia. - Early Tertiary Period 60 mya
Vicariance:
Continental drift has resulted in
Vicariance: evolutionary separation of species due to a barrier such as continental drift
The legacy of continental drift can be found in fossil records and in existing taxonomic groups.
e.g., polar bears entirely Arctic, penguins entirely Antarctic e.g., the genus Pinus (pines) are entirely Northern hemisphere, but ancient Araucariaceae are Southern hemisphere.
Continental drift has resulted in unique fauna and flora in some regions: The Neotropical, Ethiopian, and Australian regions have been isolated for a long time and have very distinctive species.
North America and South America were connected about 6 million years ago; there has been some movement of species since then (Great American Interchange).
Latitudinal diversity gradient - biodiversity highest towards BLANK
Why do regions of the Earth vary in the # and types of species they contain? Why are there more species in the tropics?
Latitudinal diversity gradient - biodiversity highest towards tropics
An explanation for why species richness of most taxa increases from the poles to the equator.
Primary Productivity(production of plant biomass)? * Thermal Stability? * Geographic Area? * Evolutionary History? * Evolutionary Rates? * Biotic Interactions? * and many more
Why are there more species in the tropics?
Mittelbach et al. (2007) summarized the hypotheses in three categories:
- Species diversification rate is higher in the tropics. - more species bc faster rate of evolution
- The tropics have the most land area on Earth, and temperatures are very stable
- Thermal stability hypothesis: Terrestrial species diversity is highest in the tropics because this area is also the most thermally stable—temperatures remain uniform year-round.
- Many land masses that currently are home to tropical biotas have been tropical at or near the equator for most of geologic history
- Perhaps the tropics are more diverse because they have more land area than other latitudes? - more land mass
- Do land area and temperature influence species diversity?
Rosenzweig (1992) argued that a larger and more thermally stable area should decrease extinction rates in two ways:
– Increased population sizes decreases the chance of extinction.
– Increased geographic ranges also reduces risk of extinction. - Species diversification time
Diversification rates are similar, but evolutionary time is greater in the tropics. - had more time compared to higher latitude(extratropical) bc cold glacier
- The tropics are thought to have been more climatically stable over time, and species have had more time to evolve.
- Temperate and polar regions have undergone severe climatic changes such as glaciation, disrupting species diversification.
The tropics are a cradle and a museum for speciation
Supported by a study of modern and fossil marine bivalves (Jablonski et al. 2006).
Most extant taxa (A) originated in the tropics and (B) spread toward the poles. - Productivity - Higher productivity results in more abundant resources in the tropics like biomass. - less likelihood of going extinct
- Terrestrial productivity (rate of production of new biomass) is highest in the tropics.
- Higher productivity should promote larger population sizes, which will have lower extinction rates.
- Productivity: at least on land is higher in the tropics
- Higher productivity towards equator despite lack of nutrients: high sunlight, perception/water, and warm temp which plants love leadinging to high productivity
Productivity and species richness - shows that higher productivity is not a perfect hypothesis
- Biotic interactions hypothesis: Many tropical species: extensive and highly co-evolved interactions (Schemske et al 2009)
Hypothesized that abiotic factors primarily limit species in the temperate zone, while biotic interactions primarily limit species in the tropics.
Perhaps tropics are more diverse because taxa there have higher rates of speciation because of more intense biotic interactions and their selective pressures.
Theory of Island Biogeography:
Islands:
Distance affects rate of colonization:
Why do larger areas contain more species?
First Experimental Test:
Island Biogeography Theory: Differences in species richness among islands is a function of distance from mainland and island size
Islands: Characterized by size and by isolation or by distance to nearest similar habitat. Patterns of species diversity on islands. Islands used as a model system to explain distributions on the mainland as well as on islands.
- Ocean islands and habitat islands. Embedded in sea or unlike habitat or a matrix.
Distance affects rate of colonization: Differential dispersal affects likelihood of absence/presence of species:
- Greater distance = lower rate of colonization.
- Remote islands have fewer species than islands close to the mainland.
- Lower distance and lower size less diversity and opposite
Why do larger areas contain more species?
Larger areas can support larger populations which have lower probability of extinction.
First Experimental Test:
- Simberloff and Wilson worked with mangrove islands in Florida, where they manipulated whole islands.
- Islands fumigated with insecticides to remove all insects and spiders.
- After one year, species numbers were similar to numbers found before the experiment. Also, islands closest to a source of colonists had the most species, and the farthest the island had the least.
Community assembly processes
Community Composition: Series of Filters. These factors act as “filters,” which exclude species from (or include species in) particular communities.
Community Composition: Series of Filters - Distribution and abundance of species in communities is dependent on:
- Regional species pools & dispersal
- The regional species pool provides an upper limit on the number and types of species that can be present in a community. Dispersal or immigration may add to the pool - all species found in a given area and the dispersal component, all species able to colonize a habitat or community - Abiotic conditions - filters out any species unable to tolerate the abiotic conditions
- A species may reach a community but be physiologically unable to tolerate the abiotic conditions of the environment. → filter species unable to tolerate the environmental conditions - Species interactions
- The final cut requires coexistence with other species. Species may be excluded from a community by competition, predation, parasitism, disease, or lack of a mutualist (i.e. limiting similarity by McArthur). → requires coexistence
Resource partitioning (niche differentiation)
Resource partitioning: Competing species are more likely to coexist if they use resources in different ways. The less the overlap, the more specialized species have become, and the less strongly they compete.
If species have a high degree of specialization, it can result in less competition and high species richness.
More species can be “packed” into a community if overlap is small.
Or, if the resource spectrum is broad, a diversity of resources would be available for use by a wide variety of species.
In a field study, Robertson et al. (1988) mapped soil moisture and nitrogen concentration and found variation over small spatial scales.
If the two maps are combined, patches corresponding to different proportions of these two resources emerge.
This suggests that resource partitioning could occur in plants
Non-Equilibrium Theory: G. E. Hutchinson
“The Paradox of the Plankton”
Non-Equilibrium Theory: environmental conditions checking the superior competitors. When disturbance, stress, or predation limit dominant competitors below carrying capacity, competitive exclusion can not occur, and coexistence is maintained.
- Equilibrium: competitive exclusion
- Non-equilibrium: no competitive exclusion; coexist
G. E. Hutchinson considered non-equilibrium theory with his paper “The Paradox of the Plankton” (1961)
- Phytoplankton communities in freshwater lakes have very high diversity (30–40 species) despite limited resources and homogeneous environment.
- How?? Phytoplankton compete for the same resources (CO2, P, N, etc.), and these are evenly distributed in the lake. So why didn’t one species force others to extinction?
- Conditions in the lake changed seasonally, which kept any one species from outcompeting the others
- As long as conditions in the lake changed before competitively superior species reached carrying capacity, coexistence would be possible.
Lottery model: Sale
Great Barrier Reef
Lottery model: all about chance and maintaining species diversity by chance
Previous hypotheses assume an underlying competitive hierarchy.
What if species have equivalent interaction strengths?
The lottery model (Sale 1977) emphasizes the role of chance.
Assumes: All species with equal chances of obtaining resources, which allows coexistence. - random dispersal into a community
Example: Great Barrier Reef: extremely high fish diversity. Many species with similar diets, making resource partitioning unlikely. New territories open unexpectedly after deaths of occupants—by predation, etc. - priority effect who ever colonized environment has greater chance of keeping it
Unpredictable as to what species will recruit to any location or at any time (chance).
Resource Ratio Hypothesis (Tilman):
Diatom
Resource Ratio Hypothesis (Tilman): species coexist by using resources in different proportions.
Two diatom (algae with silica shells - silica builds the cells and phosphate important for all life) species were grown in media with different SiO2:PO4 ratios.
Cyclotella dominated only when the ratio was low, Asterionella dominated when the ratio was high.
Intermediate disturbance hypothesis: Joseph Connell
Intermediate disturbance hypothesis: Important factor for maintaining the biodiversity and Highest diversity at intermediate levels of disturbance
WHY? Bc dominant species
- At low levels of disturbance dominant species lead to competitive exclusion
- At high levels of disturbance it is too stressful leading to high rates of extinction in the community
At intermediate levels of disturbance dominant species are ‘held in check’
allowing for less competitive species to exist in the community
This increases the diversity of the community
Wayne Sousa (1979) tested IDH using intertidal boulder communities Disturbance gradient:
Large boulders are disturbed less frequently by tidal action
Small boulders very frequently
Level disturbance: large boulders, intermediate boulders(most amount of species), small boulders
Positive Interactions and Species Diversity: Hacker and Gaines
Hacker and Gaines (1997) incorporated positive interactions into the intermediate disturbance hypothesis (e.g. facilitation) - dune communities EMF
Hacker and Gaines tested this in a New England salt marsh:
Three intertidal zones had different species composition. The Middle zone had the greatest species richness.
All plant species were moved to all three zones.
High intertidal zone: competition with Iva led to exclusion of most transplants.
Low zone: physiological stress (frequent inundation) was the main controlling factor.
