Measuring complexity in soil ecosystems

Monika GorzelakNovember 24th 2014

Objectives

• How can I ask a CAS question in my research?• Soil is complex• Should behave as a complex adaptive system• Explore whether that is measurable in my

experiments/research

Outline

• Revisit “response diversity”• Historical context Tilman Cedar Creek• Current theory• Introduction to mycorrhizal networks• My experiment(s) + CAS

Response diversity

Definition• Diversity of responses to environmental

change among species that contribute to the same ecosystem function

• Diversity within functional groups is important to the adaptive capacity of ecosystems; not just species richness

Concepts of diversity in plant ecosystems

• Tilman 2001• Hector 1999• Large scale field experiments that

manipulated diversity and measured fitness outcomes

• Debate

Cedar Creek

• Long term experiment• Temperate grasslands, in Minnesota• 16 species, chosen at random into plots in

increasing species richness• Measure productivity (biomass)• Increased diversity = increased productivity

BIODEPTH

• Europe experiment• Same design, 32 plants• 8 sites across Europe• Hector et al. (2002)• Same results: increased diversity results in

increased productivity (biomass)

(Loreau, 2010)

Mechanisms

• Complementarity effect: functional niche complementarity, permanent association of species leads to better collective performance

• Selection effect: trait variation as initial effect upon which selective process promotes dominance of a particular species

• Both at work (Loreau et al. 2001); complementarity more likely (Cardinal et al. 2007)

What functions?

• Root depth and architecture (Dimitrakopoulos and Schmid 2004)

• Nutrient use efficiency (van Ruijen and Berendse 2005)

• Increase input and retention of nitrogen (Fargione et al. 2007)

• ***grasslands only!

Bonfante and Anca 2009

More functions, more interactions

Mutualisms

• Nitrogen-fixing bacteria (classic example affecting plant diversity, fix nitrogen, facilitate co-existence of other species by generating a nutrient source)

• Mycorrhizas. Symbiosis between soil fungi and plant. Plant provides fixed carbon, soil fungus provides mineral nutrient uptake

Microscopic structure

Colonized root tips

• Potential for different types of connections because different species

Scales of complexity in soil

• Spatial– Roots create heterogeneous structure*– Networks

• Temporal– Within a trophic level– Between trophic levels (different time scales

apply)

Root architecture

Networks

(Beiler et al. 2010)

Scale-free network

(Beiler et al. 2010)

Temporal (within trophic level)

• Behaviour* (within life span, quick changes)• Adaptation (positive/negative feedbacks)– Expression of genes to exploit resources– Improve fitness– Adaptive niche construction (Callahan 2014), vs.

homogeneous medium environmental modification

• Evolution (“permanent” changes to genome)– Longer term

Growth curve example

Medium term (mal) adaptation

Adaptive niche construction

• “Opposite” of a growth curve experiment• More likely in a heterogeneous environment• Modification of a local environment to confer

fitness advantage, passing the local improved environment onto offspring

• Recently demonstrated in a lab setting using bacteria (easier to do with bacteria because they have short generation times)

Soil trophic levels

• 1st level: primary producers (plants)• 2nd level: decomposers, mutualists, pathogens• 3rd level: shredders, predators, grazers• 4th level: higher level predators (carni)• 5th level: even higher level predators (?)• Also: food webs, organisms can play at

different levels (nematodes eat fungi and are eaten by other fungi...next slide)

Fungi eat nematodes!

© George L. Barron

Temporal scales across trophic levels

• Plants interact with mycorrhizas (2 trophic levels)

• Bacteria interact with both• Reaction times differ between these 3

components (scale: years, months, days)• (Food webs: there are other players, I’m

ignoring them)

Seasonal changes in fungi

(Pickles et al. 2010)

My project: what we know

• Douglas-fir connect with mycorrhizas which form networks

• Douglas-fir can transfer resources and signals through its mycorhizal network

• Douglas-fir preferentially transfers carbon to kin

• Douglas-fir transfers defense signals via the mycorrhizal network in response to herbivory

My project: what we want to know

• Do Douglas-fir transfer defense signals preferentially to kin in response to herbivory?

• Need to look at 3 factors:– Mycorrhizal network (yes and no)– Kin vs stranger– Herbivory (insect, manual defoliation, no)

Treatments

MN=mycorrhizal network, using mesh bags that allow fungi to pass, but not roots

Kin vs strangerdefoliation

insect

control

MN no-MN no mesh

Kin vs stranger

Kin vs stranger Kin vs stranger

Kin vs stranger Kin vs stranger Kin vs stranger

Kin vs strangerKin vs stranger

No-MN=mesh bags exclude roots and fungi, eliminating the mycorrhizal network, whileallowing the passage of water and bulk soil flow including nutrients

No mesh=control for unknown impacts of including the mesh**changes to spatialheterogeneity I hope to be able to detect by considering complexity of the soilin the design and trying to quantify it under these conditions

Design: Two recipient

D = donor

Rs = recipient, stranger

Rs = recipient, kin

Design: Tripartite

D = donor

Rs = recipient, stranger

Rs = recipient, kin

Thoughts on design

• Tripartite is more complex• Should I expect different results? Its essentially

the same experiment...• Dilution of label added to Donor• Change to the spatial heterogeneity of the soil

structure (can I see this change by monitoring the bacteria...if they change, will they impact the system and possibly alter responses to treatments?)

Complexity in soil: Relevant concepts

• Spatial heterogeneity causes adaptive radiation in bacteria (increase diversification)

• Priority effects (first bacteria arriving takes priority, impacts habitat in the future)

• Time-scale differential between plants, fungi, and bacteria

Apply treatments:Herbivory, mycorrhizal networks

Bacteria

• Productivity (measure biomass)• Diversity • species diversity• Functional diversity

• Time-scale comparisons• Bacteria at start• Bacteria at finish

Diversity and Productivity

• Aboveground plant diversity unchanged• Soil spatial structure is altered, allowing for adaptive

radiation• Parallel to Tilman: will productivity increase with

increased diversity (in bacteria rather than plants?)• Plant stress response have different effects on

bacterial community systems (is high diversity system more resilient?)

• Will kin vs stranger change the diversity/productivity of bacteria in the system?

Questions

• Does the bacterial community change in response to – the presence/absence of a mycorrhizal network?– herbivory (with and without a mycorrhizal network)?– Density of seedlings (and therefore changes to root

architecture)?– Can these changes be seen temporally over the

duration of the experiment?– If layered on top of the original experiment, can these

measures inform signal transfer through a MN?

Thoughts on measuring complexity?

• What should I add?• Is there a better measure of soil complexity

(bacteria will be most responsive)?• Change the experiment to include explicit

measures of complexity• Do a separate experiment to tease out the

factors– Consider no plant pot, single plant pots etc

Functional diversity in bacteria

• Too much going on in the design? Simplify with a lab experiment?

• mimic Tilman experiment, but upside down. Keep above-ground constant, inoculate the soil with known species, use fake roots, fake soil (simplify the system), look for productivity increases and changes in diversity

The end.