Lecture 13 + 14 Flashcards
Species richness
The total number of species in a community
An island is newly formed by a volcanic eruption. You want to follow what happens with plant communities on the island. What type of succession is this?
Primary succession
Anthropologists have long wondered what happened to the inhabitants of Easter Island. It is clear that part of the island was land used for farming. What likely occurred after the island was depopulated?
Secondary succession
What term describes structured analyses that use large numbers of data sets to discern consistent trends?
A meta-analysis
Any species that has a low abundance but exerts strong effects upon the community through multiple trophic levels would be considered what?
A keystone species
What do you call a series of similar communities that are in different stages of succession all in order?
Chronosequence
If all trophic levels are being controlled by the abundance of resources available to plants it would be called _____________.
Bottom up control
Some marine mussel communities do not change even after strong storms. They might be considered ______________?
Resistant
Two major ways ecologists think about food webs:
- relation to community structure
- energy flow
Trophic level
Position in food web as determined by number of energy transfer steps
Carnivores (2º consumers)
Omnivores
Herbivores (1º consumers)
Primary producers
Food Web Complexity Varies among Communities
In part because of how ecologists study and represent communities
- not all species and not all species interactions are equally important
Strong interactors
Species that play a major role in determining community structure and function
- the loss or introduction of a strong interactor results in significant community changes
Foundation species
A strongly interacting species that comprises a large amount of the biomass in a community
ex. Douglas fir in PNW old growth forests, blue mussels in PNW rocky intertidal zone
Keystone species
A species whose impact on the community is large and disproportionately great relative to its biomass
ex. pisaster starfish in rocky intertidal zone of PNW US coast
Removing the keystone species (starfish)
Results in a loss of biodiversity
ex. following the removal of Pisaster, number of species fell. Pisaster promotes species diversity by preventing competitive exclusion by Mytilus (the blue mussel, a dominant species). Limiting resource is space.
Beavers –another keystone species
Due to habitat modification rather than grazing/exploitation
- few wetlands when beavers were almost extinct
Consider the interaction between phytoplankton and zooplankton. Would you expect:
A) the control by phytoplankton on zooplankton is most important (ie, more phytoplankton production leads to more zooplankton, because zooplankton are food limited).
B) the control by zooplankton on phytoplankton is most important (ie, more zooplankton leads to less phytoplankton, due to grazing).
C) the interaction is weak, with phytoplankton controlled mostly by light and nutrients, and zooplankton controlled by fish predators.
D) the control by zooplankton on phytoplankton is strong only when primary productivity is low.
E) the interaction is strong, but sometimes zooplankton control phytoplankton while other times phytoplankton abundance regulates zooplankton.
The interaction is strong, but sometimes zooplankton control phytoplankton while other times phytoplankton abundance regulates zooplankton
Bottom up effect in food webs
Zooplankton biomass increases with increasing algal biomass (measured as chlorophyll) suggesting bottom-up control
Primary producers -> primary consumers (herbivores) -> secondary consumers
Trophic cascade
Chains of interactions extending among multiple trophic levels through top-down effects
Indirect effects
Effects that are transmitted via the food web to species beyond those with which the exploiter immediately interacts
ex. increase in zooplanktivores, decrease in zooplankton, increase in algae
ex. decrease in zooplanktivores, increase in zooplankton, decrease in algae
Arrows flip
Why were some ponds more green than others? Note that all ponds have the same inputs of nutrients (fertilizer).
Minnows were present in ponds that were more green. Increase in minnows, decrease in zooplankton, increase in algae -> more green
What might happen if we add a fourth trophic level (large predatory fish eating the smaller fish)?
More large fish, less minnows, more zooplankton, less algae
-> arrows flip
-> tertiary consumers, top down effect to secondary consumers, primary consumers, then finally primary producers
Dramatic top-down effects of cod fishing off Nova Scotia
- cod fishing increased leading to a crash in the stock
- cod food increased in abundance
- zooplankton declined in abundance
- phytoplankton increased
Whistling thorn-tree savannah
With native ants, elephants didn’t eat the trees. The invasive ants replaced the native ants and trees were being eaten.
Low visibility, better for lion activity and zebra kill occurrence, worse for zebra density
High visibility scenario, worse for lion, worse for zebra kill occurrence, better for zebra density
Why is the terrestrial world green?
- they have 3 trophic levels
- predators keep herbivores low
Succession
A gradual change in community structure (e.g., species composition) over time, initiated by a disturbance
Disturbance can be
Natural and abiotic (wind storm, fire)
Natural and biotic (insect breakout, disease epidemic)
Anthropogenic (deforestation for agriculture, oil spill)
Disturbances occur on a continuum of perturbation
Frequency and intensity
Disturbance severity and leads to secondary succession
Least and most common to medium severity and rare
- windstorm, insect breakout
- fire
- agricultural clearing (middle ground on chart)
Disturbance severity and leads to primary succession
Rare and medium severity to very rare and high severity
- mining
- glaciers and volcanoes
Primary succession on newly exposed geological substrates
At first, no soil and no existing plants or other organisms (no seeds)
ex. Hawaii: 4 million years of succession and soil development
- not surprisingly, primary succession is slow
Secondary succession following disturbance that does not destroy soil (terrestrial systems)
ex. agriculture
Absent human maintenance, cities and suburbs move through rapid secondary succession: ex. 15 years after Chernobyl nuclear meltdown, Ukraine, 1986
Why is the speed of secondary succession far faster than primary succession?
- existing soil, for supporting rooted plants
- existing soil, for holding onto water
- existing soil, for nutrients (nitrogen, phosphorus)
- seed bank in soils
- often, nearby source of potential organisms (depends on spatial scale of disturbance)
Species that dominate early in succession have different traits than those late in succession
Early in succession with shorter life spans and shorter reproductive periods: r strategists
Later in succession, longer life spans and longer reproductive periods: k strategists
Succession occurs in aquatic communities, not just terrestrial ones
ex. Exxon Valdez, Prince William Sound, Alaska, march 1989
- r strategists came back in early succession
- k strategists still gone
- recovery takes around 150 years
True or false: Dominant species tend to have more of the biomass in a community than keystone species
True. Dominant species are those that are common in the community and therefore make up a great deal of the biomass. Keystone species have a very high impact on the community even though they make up a small portion of the biomass.
The connection between food web complexity and stability is more complex than once thought. Currently we think that populations are actually _________ stable when the web is complex and the aggregate whole community might be _________ stable when the web is complex.
Less, more
Beavers build dams that create flooding that creates abundant aquatic habitat. They might be considered ______________?
Keystone species
Which of the following might be considered primary succession: (You may choose one or more answers)
- Colonization of a newly formed volcanic island.
- Regrowth after timber harvest.
- Colonization after a severe hurricane.
- Colonization of exposed land after glaciation retreat.
Colonization of a newly formed volcanic island
Colonization of exposed land after a glaciation retreat
Which of the following is the correct sequence of dominant vegetation in a community undergoing succession?
Annual weed -> herbaceous perennials -> shrubs -> early successional trees -> late successional trees
Annual weed -> herbaceous perennials -> early successional trees -> shrubs -> late successional trees
Annual weed -> shrubs -> herbaceous perennials -> early successional trees -> late successional trees
Herbaceous perennials -> annual weed shrubs -> early successional trees -> shrubs -> late successional trees
Annual weed -> herbaceous perennials -> shrubs -> early successional trees -> late successional trees
Which of the following is an example of a trophic cascade?
A mountain lion preys upon deer and as a result the abundance of a grass species dramatically increases.
A large population of European rabbits grazes on an annual weed species and dramatically reduces their population.
A marine mammal feeds on baleen and significantly reduces their abundance in the Pacific Ocean.
A plant virus reduces the number of native plants in a community.
A mountain lion preys upon a deer and as a result the abundance of a grass species dramatically increases
According to Hairston et al., why is the world green?
Because of top-down control on herbivores
Who challenged the ideas of Hairston et al. and what did they contend?
Murdoch challenged the ideas of Hairston et al. and argued that the world is green because herbivores are limited by the defences plants have evolved.
If one were to plot species numbers in a community after a major disturbance over a long time period… (select one or more answers)
- Species numbers would be low right after a significant disturbance.
- Species numbers would be highest midway through the time toward climax community.
- Species numbers would decrease some time after the disturbance when strong competitors colonized.
- Species numbers would theoretically be hump shaped with time.
- Species numbers would be low right after a significant disturbance.
- Species numbers would be highest midway through the time toward climax community.
- Species numbers would decrease some time after the disturbance when strong competitors colonized.
- Species numbers would theoretically be hump shaped with time.
True or false: K - selected species tend to be pioneer species (those that show up early in the process of succession) and r -selected species tend to be climax species (those that are common in late succession).
False. The opposite is true. R type species tend to reproduce and colonize areas quickly. K type species exist at the climax when the strong competitors dominate.
It would be most difficult for a species to invade a community where the resource utilization of current resident species ________________.
Overlaps
Experiments performed in the rocky intertidal zone by Robert Paine showed that the experimental removal of a predator had dramatic consequences to the structure of the community. What was the ultimate conclusion of this study?
- That predators can prey so heavily that they eventually reduce species richness to nearly zero. As a result species richness is reduced.
- That prey need spatial and temporal fluctuations to coexist, otherwise the community structure will collapse. When fluctuations do not occur, then species richness is significantly reduced.
- That predators have no impact on the structure of lower trophic levels.
- That predators can mediate coexistence by reducing the number of individuals of a species in the community that tend to dominate and as a result increase species richness more than if they were not present.
That predators can mediate coexistence by reducing the number of individuals of a species in the community that tend to dominate and as a result increase species richness more than if they were not present.
Assume that resources for a community of species are available on a continuum. A common species of bird utilizes a particular portion of this continuum. What term defines the length of the portion of resources that this bird uses?
Niche breadth
What is species richness?
Species richness is the number of species in a community
According to Joseph Connell’s theory regarding disturbance, when would communities be expected to contain the most species?
When the frequency disturbance is at intermediate levels
What term characterizes the equilibrium point for species richness of an island?
S*
Which scientists are credited with the theory of island biogeography?
Edward O. Wilson
Robert H. MacArthur
In theory, the number of plant species _____________ as the size of islands increases.
Increases
Species-area relationships
In 1807, Alexander von Humboldt recognized that larger islands harbour more species than smaller islands
ex. Galapagos islands
‘Islands’ need not be islands of land in a sea of water
- lakes are islands in a “sea” of land
- mountaintops are high altitude islands in a low-altitude “ocean” of land of different climate
- gaps in a forest canopy where a tree has fallen are islands in a “sea” of trees
Equilibrium model of island biogeography
The number of species on an island is an equilibrium between rates of immigration and rates of local extinction
Immigration rate
Rate of arrival of new species to an island
Local extinction rate
Rate at which species are completely lost from an island
Equilibrium model of island biogeography graph
S* (equilibrium number of species) x immigration rate, P (total number of species in source pool) at bottom right axes, extinction rate on the right
Immigration of new species exponential decrease
Species extinction exponential increase
Immigration rates affected by
Distance between the island and the source of new species, and by island size (bigger “target”)
Extinction rates affected mainly by
Island size
Far from source of species, or small island graph
Starts from lower on y-axis and slower exponential decrease
The rate at which species on an island go extinct is
Faster on smaller islands
- on the far right of the immigration rate/extinction rate graph
of species present on island x immigration rate/extinction rate graph
Far/small islands on the bottom left
Near/large top left
Large island bottom right
Small island upper right
Immigration for smaller islands, start lower, decrease exponentially
Extinction for large islands, exponentially increasing, ends lower than small island line
of species present on island x immigration rate/extinction rate graph intersections
Small immigration x small extinction = S* for small, far islands
Large immigration x large island extinction = S* for large, near islands
Evenness
Degree of similarity in relative abundance of different species
Shannon diversity index
H’ = -SUM pi * ln (pi)
i=1
-> -S on top of Sigma i = 1
Incorporates both species richness and evenness
Gives more weight to common species
H’
Value of Shannon Index
s =
Number of species in community
Pi
Proportion of the ith species
ln (Pi)
The natural log of Pi
Calculating the shannon diversity index
- Sum up the total number of species
- Calculate the proportion: number/total
- Ln(Pi) take the natural log of the proportion
- Proportion * Ln(Pi) -> should be negative
- Sum of all of step 4 -> should be positive
At large spatial scale, biodiversity is:
- very high in tropical forests
- somewhat lower in savannah and seasonal dry tropical forests
- lower yet in temperate forests and grasslands
- very low in boreal forests
- moderately low in tundra (but higher than in boreal forests)
Many reasons have been suggested to explain this gradient in diversity (25+ hypotheses)
Most of these do not hold up well to close scrutiny
Productivity hypothesis
Rationale: There is greater productivity in the tropics, and species richness is limited by the partitioning of energy among species
(Evapotranspiration is used as a surrogate measure of photosynthesis)
Seems to fit the pattern for terrestrial biomes (but so do many other possible explanations)
The productivity hypothesis seems to fit the pattern for terrestrial biomes (but so do many other possible explanations). What about ocean biomes?
Productivity hypothesis does not work at all for ocean biomes:
- subtropical gyres: very low photosynthesis and very high biodiversity, compared to high latitude waters or upwelling systems
- deep oceans have high biodiversity, yet no photosynthesis (and very low rate of input of energy through sinking of organic detritus)
Temperature hypothesis
Warmer temperatures more favourable for higher rates of biological activity (plants, microbes, and cold-blooded animals), allowing for more competition and so high biodiversity
- this fits the pattern for terrestrial biomes (tropical rain forests vs. boreal forests, for example)
- also fits for many ocean biomes (subtropical gyres are warmer and more diverse than high latitude waters)
-> but not all: high biodiversity in cold deep oceans (4ºC)
More likely explanation for temperature hypothesis
Terrestrial tropics are richer in species than temperate regions at least in part because the tropics have existed over long and uninterrupted periods of evolutionary time, whereas the temperate regions are still recovering from the Pleistocene glaciations when temperate biotic zones shifted in the direction of the tropics.
This also fits pattern of high biodiversity in deep oceans, which were not affected at all by the geological past over the past millions of years. Very, very constant environment in deep ocean over geological time scales.
Another possible explanation for temperature hypothesis:
More opportunities exist for specialization in a non-seasonal environment than in a seasonal environment. Biodiversity tends to increase as climatic variation decreases.
Both terrestrial and ocean biomes fit this pattern (no seasonality in deep ocean, and less in subtropical gyres than in high latitude waters).
-> these are not mutually exclusive and the controls on large-scale, global patterns of diversity remain debated by ecologists
Disturbance usually
Lowers diversity, and diversity increases during early succession, peaking at mid-succession
Soon after disturbance: low, increases over time, peaks then decreases long after a disturbance
Very frequent disturbances among patches in a landscape mosaic keep most patches in
Early stages of succession (low biodiversity), but if disturbance is very rare, succession will go to climax community across the landscape (with lower biodiversity than at mid-succession)
Intermediate disturbance hypothesis
Communities contain the most species when the frequency of disturbance is neither too high nor too low
What about productivity and nutrient availability? Considered now at the local scale; we already discarded the idea that higher productivity causes greater biodiversity at the global, latitudinal scale.
Reminder: nitrogen additions reduce biodiversity in grasslands
Likely mechanism is that the nitrogen increases photosynthesis, which may increase competitive exclusion (decrease in species richness)
Very long-term fertilization experiment
Species diversity consistently declined over time in fertilized grass, in contrast to the near-constancy in the control plot
In lakes and coastal marine ecosystems, increased nutrient inputs leads to higher primary production
“Eutrophication” is excess production
Leads to low biodiversity, and dominance by r-strategists
Climatic and environmental stability probably allows greater biodiversity (remember deep ocean waters)
What about the converse: does greater biodiversity (and associated complexity) lead to greater ecological stability?
Resilient community
One that returns rapidly to something like its former structure after that structure has been altered
Resistant community
One that undergoes relatively little change in its structure in the face of a disturbance
Fragile stability community
It remains essentially unchanged in the face of a small disturbance but alters utterly when subjected to a larger disturbance
Dynamically robust community
It stays roughly the same in the face of much larger disturbances
Coral reefs are highly diverse. Which ecological stability are they?
How does this inform our understanding of the question “Does greater biodiversity (and associated complexity) lead to greater ecological stability?”
Fragile stability
- in a stable and predictable environment, a community that is dynamically fragile may nevertheless persist
- in a variable and unpredictable environment, only a community that is dynamically robust will be able to persist
Ecological stability, we might expect to see:
- complex, biodiverse, and relatively fragile communities in stable and predictable environments
- simple and robust communities with low biodiversity in variable and unpredictable environments (or those subject to frequent disturbances, or to large nutrient inputs)
Intertidal and shallow marine bottom communities exposed to oil pollution
Biodiversity decreases, and relatively sensitive organisms (often k-strategists) disappear
Remaining community has low biodiversity dominated by more resistant, r-strategist species
Eutrophication from excess nutrient inputs has same effect on the community
Interaction of nitrogen and oil
Eutrophication from nitrogen alters benthic community structure, lowering diversity and selecting for non-sensitive species. How does oil interact with this?
Nutrient-polluted, eutrophic coastal ecosystems are less sensitive to oil pollution
One ramification: offshore oil development does more ecological harm in areas with little nutrient pollution (Alaska, offshore New England) compared to those with more nutrient pollution (Gulf of Mexico)
You are assigned the task of determining species richness in a given area. The lead researcher has told you that the number of samples you have to take is not limited. As you collect data, what pattern should you expect to see?
- The number of species identified will start out slowly but rapidly increase after 2 days of data collection.
- As you collect data, species richness will decrease
- The most common species in this space will likely be identified first, followed by more rare species.
- All the species in this space will be identified by a time period that is related to the product of the square root of number of search and total area.
The most common species in this space will likely be identified first, followed by more rare species.
In which of the two examples of a community is species richness greater: A community with 100 species that can all be found equally within a 25 km radius of the area, or a community spread across the same area that has 100 species, in which 5% of the individuals are rare species.
The species richness is the same
According to the theory of island biogeography, how would the number of species found on a small island far from mainland compare to a larger island or a similar sized island that is closer to mainland?
The number of species on the small far island will be less than on the near larger island.
According to the predictions that make up the theory of island biogeography, how long does it take for species to reach equilibrium?
There is no set time for an island to reach equilibrium.
When looking at a simple model of species richness based on a single resource, species richness may be determined by: (Choose one or more answers)
- The range of available resource.
- The level of specialization of each species on the resource.
- The level of overlap of the usage of the resource by each species.
- The level of interspecific competition.
- The range of available resource.
- The level of specialization of each species on the resource.
- The level of overlap of the usage of the resource by each species.
- The level of interspecific competition.
True or false: According to the intermediate disturbance hypothesis we should expect the lowest levels of diversity at intermediate levels of disturbance and the highest levels of diversity when disturbances are frequent or rare. This is because when disturbances are frequent we get many r type species, and when disturbances are rare we get many K type species.
False. The intermediate disturbance hypothesis predicts the highest levels of diversity at intermediate disturbance rates, because there is a mix of r and K species.
True or false: Biodiversity generally tends to be high in low latitudes, and low in high latitudes.
True
Which of the following are possible reasons why we see the existing trend in species diversity in terrestrial systems?
- The climate is more favourable at higher latitudes.
- Lower latitudes lack seasonality and have greater species specialization
- The tropics have existed for longer uninterrupted periods
- Temperatures are higher at higher latitudes
- Lower latitudes lack seasonality and have greater species specialization
- The tropics have existed for longer uninterrupted periods
True or false: Ponds with high levels of nutrients (such as nitrogen or phosphorous) tend to be more diverse than ponds with moderate levels of nutrients.
False.Think about eutrophication. Very high levels of nutrients tend to result in r-strategists dominating which leads to lower biodiversity.
