Collaborators

Jennifer Rudgers - (website)

Judith Bronstein - (website)

Bronstein 1994 TREE

Species interactions

• Species interact in a variety of ways

  

• Outcomes of species interactions range from - to +

  Source: Principles of Animal Behavior

Interaction outcomes are important for:

• Individual growth and reproduction - e.g., herbivory




• Population growth - e.g., fox and hare




• Formation and dynamics of food webs - e.g., interaction strengths important for stability




• Evolution - e.g., interactions alter traits important for interaction

Mean interaction outcomes

What about variation in outcomes?

Species interaction outcomes vary a lot

Within a species interaction, outcomes are not just +, 0, or -

Outcomes within a species interaction can range from - to +

What is meant by variation in interaction outcomes?

• Not error variation in outcome in one context

  

• The variation in outcomes among more than one context

  

Variation in interaction outcomes - example

• Not error variation in outcome in one context

  

• The variation in outcomes among more than one context

  

• And so on for other types of contexts: time, space, etc.

Two points:

Variation in outcomes is common




Variation in outcomes is important

Variation in outcomes is common

Mutualism

Herbivory outcome for Acacia trees varies with ant species identity (Palmer et al. 2008)

Variation in outcomes is common

Competition/facilitation

Outcomes vary from competition at low elevation to facilitation at high elevation (Callaway et al. 2002)

Variation in outcomes is important

for populations

Variation in outcomes is important

for communities

Variation in outcomes is important

for evolution

And people are calling for examination of variation

Questions

How do different species interaction types differ in variation in outcomes?




What are the relative importance of drivers of variation in outcomes?

Does variation in outcome differ by ecosystem type?

Two major distinctions can be made:

Types of species interactions, and

Gradients along which interaction occur

How do different species interaction types differ in variation in outcomes?

• Mean strength

  • Mutualisms weaker than antagonisms (Morris et al. 2007)

  • General sense in literature that mutualisms less important because so variable (Sachs & Simms 2006)

  • Weak interactions the most variable (Berlow et al. 1999)

• Interaction complexity

  • Predation more specialized than mutualism (Gomez et al. 2010)

  • Strength greater with fewer interactions (Edwards et al. 2010)

Gradients that drive variation in interaction outcome

• Same two species

  • Abiotic: Nutrients

  • Space: Across sites

  • Time: Across hours, days, or years

  • Third party presence: Two species w/ or w/o a third species

• Just one species is the same

  • Species identity: Sp. A interacts with either sp. B or sp. C

Meta-analysis

• Web of Science search

• Experimental studies only

• Interaction outcome w/ & w/o competitor, predator, or mutualist

• Error estimates & sample sizes available

• Response variables: abundance, population growth, reproduction, etc.

• Responses measured over >1 year, population, or species, etc.

E.g. Getting data from a single study

E.g. Getting data from a single study..cont.

Data analyses

• Weighted general linear models

• Magnitude (CV)

• CV ~ context type + species interaction type + err

• CV ~ context type + species interaction type + ecosystem + err

• Sign change

• sign change ~ context type + species interaction type + err

• sign change ~ context type + species interaction type + ecosystem + err

Final dataset - 353 papers

How do different species interaction types differ in variation in outcomes?

How do different species interaction types differ in variation in outcomes?

How do different species interaction types differ in variation in outcomes?

Why is mutualism more variable?

• I don't know, but...

• A few possibilities:

  • Conditionality seems to be discussed more in the mutualism literature, but perhaps just what mutualism people are interested in?

  • Possible that species interactions are misidentified? -> Predation is likely predation, whereas not so for mutualism and competition

  • Mutualism is just more variable = but what is it that makes this so? Any ideas?

What are the relative importance of drivers of variation in outcomes?

• Same two species

  • Abiotic: Nutrients

  • Space: Across sites

  • Time: Across hours, days, or years

  • Third party presence: Two species w/ or w/o a third species

• Just one species is the same

  • Species identity: Sp. A interacts with either sp. B or sp. C

What are the relative importance of drivers of variation in outcomes?

** No difference for sign change

What does this mean?

Drivers of variation in outcomes X interaction type? - Sign

Drivers of variation in outcomes X interaction type? - Sign

Mutualism > predation & competition along spatial and 3rd party axes

Makes sense at least in some mutualisms, e.g. ->

Drivers of variation in outcomes X interaction type? - Sign

Predation < competition & mutualism along abiotic and temporal axes

• Predator-prey with largely mobile species, escape abiotic constraints

Mobility

Results qualitatively the same when accounting for mobility

Does variation in outcome differ by ecosystem type?

Reasoning for ecosystem differences

Oceans buffer abiotic variables more so than freshwater and marine

• Which could lead to less variation in interaction outcomes in oceans

Any ideas?

Conclusions

• Types of species interactions differed in outcome variation

  • We can't treat different species interactions as equivalent

  • In interaction webs, it may be most important to understand variation in mutualistic links

• Types of gradients differed in outcome variation

  • Some sources of variation in species interactions should be given priority (i.e., species identity), especially in new study systems

• Types of ecosystems differed in outcome variation

  • Outcome variation: terrestrial >= freshwater > marine

Future work

• Add other species interaction types

  • Herbivory

• Do any variables correlate with variation in species interaction outcomes?

  • Do body size ratios predict variable outcomes?

How does variation in outcome vary with latitude?

• Interaction outcomes are thought to increase in strength nearer the equator (Schemske et al. 2009)

• Stronger biotic interactions nearer equator => Greater biodiversity

• What about variation?

• Stronger mean interactions are thought to be associated with less variation (Berlow 1999)

• Stronger mean nearer equator => less variable nearer equator

How does variation in outcome vary with latitude? - Predation

Latitude

How can we test these findings in the field?

• What are the consequences of variation in outcomes?

  • Probably very difficult to manipulate variation in outcomes

  • Modeling perhaps?

  • Any ideas?





• What are relative importance of mechanisms for variation in outcome?

  • A bit easier

  • Manipulate many gradients discussed here:

  • Abiotic gradients (e.g., nutrients)

  • Third party presence (e.g., presence of predator in plant-herbivore interaction)

Thanks to...

• Data

  Tens of authors who provided data

• Data collection

  Yosuke Akiyama, Neha Deshpande, Rohini Sigireddi, Prudence Sun, Edward Realzola, Rameez Anwar, Matt King, Roy Wu, Than Vu

• Discussion

  The Rudgers-Whitney lab, Jennifer Rudgers, Ken Whitney, Volker Rudolf, Dennis Cox, Steve Hovick, Tom E.X. Miller, Elizabeth Elle, Lisandro Benedetti-Cecchi

Taxonomic bias

Though can't account for it due to lack of replication across levels