Biodiversity and Ecosystem functioning

Question 1

What is biodiversity?

Definition from the Convention on Biological Diversity (CBD)

Answer 1

Biodiversity is
The variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems

Brings in lots of diff aspects

• Taxonomic diversity – the no. and relative abundance of taxa defined by a hierarchical, evolutionary classification
• Phylogenetic diversity – relationships among taxa based on elapsed time since divergence eg. sum of branch lengths linking species in a phylogeny
• Genetic diversity – nucleotide, allelic, chromosomal, genotypic etc.
• Functional diversity – variation in the degree of expression of multiple functional traits

Question 2

Ecosystem processes/functions

Answer 2

o Ecosystem processes are bio-geo-chemical flows of energy and matter within and between ecosystems.
o Species as the components of the system (parts in the machine)?

Example: Forest primary production

note for services! forests support beneficial ecosystem services by providing timber and regulating climate (Productivity and nutrient cycling)

Question 3

Ecosystem services

Answer 3

o Ecosystem services are the benefits humans derive from ecosystems ie. what does the natural world help us with?
o Ecosystem services are the product of ecosystem functions

Example: Provisioning (timber production), supporting (soil formation), regulating (carbon stores help us prevent greenhouse gases from being released) and cultural services (recreational – ecotourism etc.);

Question 4

Early ideas and concepts on the relationship between biodiversity and ecosystem function

What did darwin have to say

Answer 4

Darwin realised that diverse systems functioned better

Ecologists hadn’t really thought about this question at all
Paul and Anne Ehrlich (1981) made the analogy between species loss and rivets popping in an airplane fuselage.

❖ Redundancy: Are some species functionally redundant? Smoother decline
❖ Idiosyncracy: Any particular series of species loss would show no systematic effect on functioning (no relationship on average)
❖ Debatable as to what graphs of these would look like and some ideas may produce similar relationships because the ideas are so vague and wooly!
❖ What does ecological theory predict?

Question 5

The way to get the super duper most productive communities!!

Two theories

Answer 5

MORE SPECIES! (selection effect theory)

MORE NICHES! (niche effect theory)

Question 6

Why can’t R* theory be used in the long run when studying coexistence?

Answer 6

R* is a theory of competitive exclusion NOT a theory of coexistence that will support biodiversity in the long run.

It only considers the role of the best resource competitor and not the rest of the community
INSTEAD USE NICHE THEORY

Question 7

Selection effect theory and Tilman’s R*

Answer 7

Selection effect theory

• productivity increases with species richness
• Monocultures have the lowest levels of biodiversity
• Due to sampling effect, the more species you put in the community, the greater the chance that you’ll get a species with a really low R* value (best able to use resources!!) in your community, and so the resource will be most effectively used in biodiverse ecosystems!
• using the crude assumption that the more resource you take up, the more biomass you produce, productivity increases with species richness
• graph is top half of a c (tapers off as you add more species)

HOWEVER R* can’t be used for long because it’s a theory of competitive exclusion not coexistence

Question 8

Niche effect theory

Answer 8

• niche theory predicts more diverse community is more productive, but this productivity declines (same as selection theory)
• with niches, different species have different optimum conditions under which they can grow
• you can use all of the resource if diff species specialise to diff bits of it, so more niches from higher biodiversity = more productive communities

eg. temperature and soil - diff species have diff optimums for these. This is the ‘niche’ of the species

Question 9

Humans are causing some species to go extinct (biodiversity is being lost!!)

What will the consequences be for the functioning of ecosystems and the ecological services provided to humanity?

Answer 9

❖ Biodiversity loss reduces the efficiency by which ecological communities capture resources, produce biomass, decompose and recycle nutrients
❖ Diverse communities are more productive because they contain key species that have a large influence on productivity (ie. low R*?), and differences in functional traits among organisms that increase total resource capture (niches!)
❖ Impact of biodiversity is nonlinear and saturating, such that change accelerates as biodiversity loss increases
❖ Loss of diversity across trophic levels has the potential to influence ecosystem functions even more strongly than diversity loss within trophic levels
❖ The impacts of diversity loss on ecological processes is of the same order as many other global drivers of environmental change
❖ As studies look at larger temporal and spatial scales they tend to find stronger evidence for effects of diversity on functioning
❖ Maintaining multiple ecosystem processes requires higher levels of biodiversity than does a single process

Question 10

Describe the 2 empirical tests showing how biodiversity loss can impact ecosystem functioning

What do they show?

Answer 10

Same general trend in both - as biodiversity goes up, productivity goes up in positive but decelerating fashion.

In the lab - Ecotron species knockouts
❖ The Ecotron is a very complicated set of controlled environmental chambers. Set up communities of high, medium and low diversity (diff no.s of species in each)
❖ Initially started similarly, after 40 days productivity diverged – highest diversity communities most productive – supports theoretical idea that losing species harms function of ecosystem (here in productivity!)

In the real world - Experimental grasslands
❖ increased species diversity increased productivity (estimated using biomass)

however, these don’t explain whether dominant (low R*) individuals OR niche complementarity are more responsible for this increase in productivity!

Question 11

How do you determine whether niche complementarity or selection effect is more responsible for the overall ‘biodiversity effect’?

WHICH IS?

Answer 11

❖ Niche theory and sampling theory predict different types of curves (they look p similar tho)
❖ Take the overall biodiversity effect and find out how much is due to niches (complementarity effect) and how much to sampling theory (selection effect)

Both play a role but overall complementarity (niche) effect is twice as strong as selection

Question 12

What happens to ecosystem (single) function as biodiversity increases?

• the scaling problem of biodiversity

Answer 12

Positive but saturating effect of biodiversity - as biodiversity increases, ecosystem function increases
❖ Lots of variation around it
❖ Mixtures are more productive on average than monocultures but sometimes the best monocultures are more productive!!
- ie. on a small scale (short term and simple experiments), it might be that a monoculture is being more productive
- this may make you think that some biodiversity is ‘functionally redundant’
HOWEVER
❖ As studies look at larger temporal and spatial scales they tend to find stronger evidence for effects of diversity on functioning
- on a big scale strong trend for increasing diversity causing increasing ecosystem function
- no species is functionally redundant

Question 13

How do you maintain ecosystem multi-functionality?

Answer 13

❖ Maintaining multiple ecosystem processes requires higher levels of biodiversity than does a single process

Different species are able to…

• support different functions, in different places, at different times
• The broader the scope the larger the fraction of biodiversity that plays a functional role
• positive but saturating effect, but NO SPECIES IS TOTALLY REDUNDANT (all play a role)

Question 14

what’s the diversity-stability debate and why did it arise?

Answer 14

Ideas of how biodiversity may stabilise ecosystem productivity through time.
AND IMPLICITLY
How we expect ecosystems to respond to the loss of biodiversity that we are seeing at such high rates due to human activity!!

❖ Within ecosystems there are communities of species – there are different organisational levels

Why did debate arise?
❖ in part because we can study the stability of different things (organisational levels): fluctuations of individual populations; diversity and composition of whole communities; ecosystem process rates - productivity etc.)
❖ There are different measures of stability: ecological resistance, ecological resilience, temporal stability (both combined!)

Question 15

What is stability?

• what are the different measures of stability

Answer 15

Stability is how well a community is able to respond to external perturbations eg. extreme climate events

Ecological resistance - For a given perturbation, systems that change less are more resistant.
❖ A community that has a sudden decline and recovery to original position is more ‘resistant’ than a community that recovers to same position but declines further

Ecological resilience
For a given perturbation, systems with a faster rate of return to the initial (equilibrium) state are more resilient (also used in other ways*)
❖ Both communities decline by same amount. But more resilient community takes less tome to get back to original point

TOGETHER, resistance and resilience give overall TEMPORAL STABILITY of the ecosystem (not just individual communities)

Question 16

Name 3 ppl that thought that “Diversity begets stability” (conventional wisdom of the 1950s/60s)

What was missing from all of their writing?

Answer 16

❖ Odum’s (1953)
❖ Robert MacArthur (1955)
❖ Charles Elton’s (1958)

o Diverse communities have more routes for energy flow, and more negative feedback loops to control outbreaking populations
o Also more species capable of taking over the ecological roles of others (species redundancy);

Idea that a species that feeds on many prey species is ʻbufferedʼ against chance fluctuations in prey abundance - if a predator loses one prey species, it can switch to another (having many links to diff prey increases stability)
MISSING WAS EVIDENCE!!

Question 17

How and why did conventional wisdom change about the ‘diversity-stability’ debate in the 1970s?

Who was responsible?

Answer 17

Paradigm reversal due to theoretical work

❖ Theoretical work by Robert May (1970) and others suggested that more diverse communities are less stable (completely opposite to conventional wisdom)
❖ In May’s 1973 book he constructed randomly assembled model webs with different no. of species (S), connectance (C) and interaction strengths between predators and prey (β). He disturbed them in different ways (remove species, change abundance), then analysed their stability in response to these perturbations
❖ He showed that individual populations become less stable with:
- More species (S)
- More links between species (C)
- When interactions between species are stronger
❖ Perturbations are likely to propagate further in food webs with more species or with a greater connectivity among species

o Rule of thumb – webs were stable (populations returned to equilibrium after a small disturbance) if you satisfy β(SC)^1/2 <1
• So complexity (more species and/or more connections) appears to decrease diversity

Question 18

What’s the potential resolution to the ‘diversity-stability’ debate ? (how has modern work brought together 50s and 60s and 70s)

Answer 18

The relationship between biodiversity and stability may differ at different organisational levels (populations, communities, ecosystems)

❖ Focus: Temporal stability (work separating resistance and resilience ongoing)
❖ Test: Temporal stability of biomass production of experimental communities of different diversity?
❖ Test: Biomass mean-variance relationship with diversity?
❖ Mechanism: Asynchrony of individual species populations?

Question 19

What are the ‘Insurance & Portfolio’ effects together

Answer 19

❖ Two major ideas – one them proposed by Tilman, other one by Loreau
❖ Related frameworks with some general overlap of key ideas

An ecosystem’s ability to buffer perturbations, loss in species and species invasions is dependent on the redundancy of the species having important
stabilising roles, as well as on the ability of the species in the community to respond differentially to perturbations.

Increasing biodiversity increases the
odds that such species exist in an ecosystem. This idea has been extended to suggest that the greater the variance of species’ responses in a community, the lower the species richness required to buffer an ecosystem.

❖ Usually applied to a single trophic level of producers

Question 20

Tilman’s Portfolio effect

Answer 20

Increasing biodiversity increases the odds that an ecosystem has functional redundancy by containing species that are capable of functionally replacing important species.

Analogy to financial markets (economics) where a diverse portfolio of investments is more stable in the long term due to “statistical averaging”

For long term investments, you should have diversity to gain stability and improve long term outcome.

Question 21

Loreau’s Insurance hypothesis:

Answer 21

Increasing biodiversity increases the odds that at least some species will respond differentially to variable conditions and perturbations.

• Biodiversity provides an ‘insurance’ against perturbations so long as species respond in different ways buffering the ecosystem-level process rates
• A diverse set of traits builds in an insurance effect – bad conditions in one year bad for some, good for others.

Question 22

What’s asynchronous fluctuation and what effect does it have on biomass production over time?

Answer 22

When only 2 species, likely that they’ll have different years that are better or worse for them, so they fluctuate over time.

Asynchronous fluctuation - when you have loads of species, it’s much less likely that they’ll be a year where all the species are having a ‘bad year’, causing a massive drop in total biomass of the ecosystem

Diverse communities have less stable individual species populations, but asynchrony ‘fills in the gaps over time’ stabilising overall ecosystem productivity

therefore
Total biomass production over time is more stable when species populations fluctuate asynchronously (and this in turn results from having lots of species!!)

Question 23

Difference in population and ecosystem stability of depauperate and diverse ecosystems

Answer 23

❖ Depauperate ecosystems (species poor) have more stable individual populations but less stable ecosystem productivity
❖ Diverse ecosystems (species rich) have less stable individual populations but asynchrony ‘fills in the gaps over time’, stabilising ecosystem productivity

This means diverse ecosystems are both more productive and more stable than depauperate versions…

Question 24

How do you test temporal stability?

• what’s the difficulty?
• how to get around it?

Answer 24

Species and communities can differ in how well they resist external perturbations and how resilient they are (ie. in their temporal stability!)

Difficulty in testing it
❖ Even in the absence of external perturbations populations will fluctuate over time
❖ Current research is trying to separate these effects

How to get around it
❖ Easier to compare the temporal stability of more versus less diverse communities
❖ Mostly using experimental grassland ecosystems (previous lecture) - where increased species diversity increased productivity (estimated using biomass)

Question 25

What are the different measures you use to quantify temporal stability?

Answer 25

❖ Correlation: +1 if two species were perfectly synchronized over time, -1 when asynchronous (ie. When one has a good year, the other has a bad year), 0 when independent

Question 26

Finally trying to quantify the diversity-stability problem…. to see who’s right!! (old or new)

What are the different measures you use to quantify temporal stability?

How do you use them to calculate temporal stability of an ecosystem?

and how can you plot this to see its relationship to diversity?

Answer 26

❖ Temporal mean (μ): Average annual biomass production (horizontal line). Generally positively related to diversity! So can be scaled and used as a measure of diversity.

❖ Temporal variance (σ²). This variability of the pop around the mean can be used as a measure of stability.

❖ Temporal standard deviation (σ)

❖ Coefficient of Variation: σ/μ (bigger = less stable). Another measure of variability….

You can inverse it to get a measure of temporal stability!!!
❖ Temporal stability μ/σ (smaller = more stable)

Question 27

Back to the experimental grasslands example

We know that increasing species biodiversity increases productivity (estimated using biomass), but what is the relationship of this mean with the variance (ie. with the overall ecosystem temporal stability?)

Answer 27

WE HAVE ACTUAL EVIDENCE rather than just poncy theory from Odum, MacArthur, and Elton (50s and 60s) and May (1970s)

Grassland evidence that

Diverse ecosystems are more stable…
- Temporal sd (σ) is much lower for mixtures than monocultures – this means that when you have lots of species, ecosystem is much more stable (backs up OLD theory)

…Even though individual populations are not more stable

• looking at species population stability using temporal stability (μ/σ) shows opposite pattern
• individual species populations less stable in diverse ecosystems than in monocultures (this makes sense because we know small populations are less stable!) (backs up NEW theory)

Putting these together allows us to resolve the picture!
❖ Modern experiments support old theory that more diverse = more stable
❖ However, in line with 1970s, at species level more diversity = less stable

Question 28

why are diverse ecosystems are both more productive and more stable than depauperate versions

Answer 28

Because average production increases with diversity more strongly than does the variation in productivity