Spring 2014 Community Ecology
Symposium
Sharon P. Lawler
UC Davis Department
of Entomology and
Nematology
Professor of Aquatic
Entomology and
Community Ecology
Sarah Bolinger
Community Ecology
April 2014
Education
B.A., Lehigh University
M.S., Rutgers University
Ph.D., Rutgers University

Worked with well-known community ecologist
Peter J. Morin on competition in aquatic
systems between insects and vertebrates

Continued to work with him on studies of
population dynamics in laboratory protist
microcosms
Research Interests

Insect-vertebrate competition in aquatic
systems

Food web architecture and population dynamics

Diversity effects on ecosystem function


Ecotoxicology – effects of toxics on stream
ecology
Mosquito control
MODEL SYSTEMS

Laboratory protist
microcosms to
research population
and metapopulation
dynamics; food chain
architecture

Ecotron research

“Ecology in a Bottle”
Protist Microcosm Studies



Microcosms are small, bounded habitats
containing the desired number of organisms
Used to study ecological interactions on a scale
that is highly replicable and easily controlled
Some population dynamics scale well; others
don't
Protist Microcosm Studies

KINGDOM
PROTISTA

(may actually be
8 separate
kingdoms)

Eukaryotes that
don't fit into
other kingdoms
Euglena
Paramecium
Publications
1993 Lawler, Sharon P,. Morin, Peter J. Food web architecture and population dynamics in
laboratory microcosms of protists. The American Naturalist, 141(5): 675-686.
1993 Lawler, Sharon P. Species richness, species composition and population dynamics of protists
in experimental microcosms. Journal of Animal Ecology, 62: 711-719.
1993 Lawler, Sharon P. Direct and indirect effects in microcosm communities of protists. Oecologia,
93: 184-190.
1995 Balciunas, Dalius and Sharon P. Lawler. Effects of Basal resources, predation, and alternative
prey in microcosm food chains. Ecology, 76(4): 1327-1336.
1995 Morin, Peter J. and Sharon P. Lawler. Food web architecture and population dynamics: Theory
and empirical evidence. Annual Review of Ecology and Systematics, 26: 505-529.
1996 Morin, Peter J. and Sharon P. Lawler. Effects of food chain length and omnivory on population
dynamics in experimental food webs. Food Webs - Integration of Patterns & Dynamics, 218-230.
1996 Holyoak, Marcel and Sharon P. Lawler. The role of dispersal in predator- prey metapopulation
dynamics. Journal of Animal Ecology, 65: 640-652.
2000 Holyoak, M., S.P. Lawler and P.H. Crowley. Predicting extinction: Progress with an individualbased model of protozoan predators and prey. Ecology, 81(12): 3312-3329.
2004 Orland, M.C. and S.P. Lawler. Resonance inflates carrying capacity in protist populations with
periodic resource pulses. Ecology, 85(1): 150-157.
2005 Holyoak, M. H. and S. P. Lawler. The contribution of laboratory experiments on protists to
understanding population and metapopulation dynamics. Advances in Ecological Research, 37: 245271.
Food Web Architecture and
Population Dynamics


In theory, food chain length and presence of
omnivory are important to population dynamics
Food web theory was controversial because
experimental evidence of the effects of food
chain length and omnivory (and other food web
characteristics) on population dynamics were
relatively few at the time
Lawler and Morin 1993
Food web architecture and population
dynamics in laboratory microcosms of
protists.


Look at population dynamics in protist
microcosms

Do protist communities in longer food chains
experience more instability?

Does the presence of omnivory by top predators
destabilize population dynamics?
Complications: stability as evaluated in model
systems is harder to measure empirically
Lawler and Morin 1993


Look for parallels between model behavior and
measurable dynamics in experimental
populations (used as proxy for stability)
Dynamics used:

persistence time

temporal variability of population size

return time
Experimental Setup

Detritus-based food webs

Bacterivorous ciliates


Tetrahymena pyriformis

Colpidium striatum
Facultatively omnivorous ciliate

Blepharisma americanum
Experimental Setup, cont.
Testing effect of position in food web on stability of
population of a species


Compare mean abundance and temporal variation in
abundance in populations of bacterivores (T. pyriformis and C.
striatum) when each is top predator (short food chain length) or
penultimate predator (long food chain length)
Long food chains also differ in whether top predator is omnivore
or nonomnivore
Experimental Setup, cont.
Testing effects of omnivory

Two conditions for facultative omnivore B.
americanum: 1. feeds only as bacterivore 2. feeds as
omnivore

Compare population dynamics between the two

Also compare to population dynamics of a
nonomnivore top predator

Lastly, compare effects on prey population stability by
looking at population dynamics in bacterivores preyed
on by omnivores vs. nonomnivores

Do longer food chains and food chains
containing omnivory show signs of unstable
population dynamics?
Results and Conclusions
Results and Conclusions





Addition of top predator reduces abundance of bacterivores
Blepharisma increases more rapidly and has higher mean
abundance when feeding as omnivore; max population was the
same
Population dynamics of bacterivores vary more in longer food
chains except in one case
Omnivore abundance varies less than that of nonomnivores at
third trophic level
Blepharisma shows greater variation when restricted to
bacteria, because of slower growth of these populations
Results and Conclusions



Tentatively support population fluctuation and
extinction increase with increased chain length
Omnivore study seems to indicate that species
feeding at multiple trophic levels better endure
fluctuations in prey abundance
Generalization requires more research, but it's
important that real communities of organisms
display theoretical food web phenomena
Lawler and Morin 1995

How to find experimental evidence of issues
within food web theory:

Factors limiting food chain length

How length and complexity affect trophic
cascades

How length and complexity affect population
dynamics
Experimental Evidence of Food Web
Theory


Lots of theoretical work existed, but much less
experimental work – why?

Hard to get from long-lived organisms in
natural systems

Skepticism exists over whether food web
models are accurate and even applicable to
natural systems at all
Lawler and Morin: Theories are testable,
especially in artificially constructed
environments (ie microcosms)
Food Web Theory: Background

Elton: Food chains are short


Hypotheses: energetic transfer efficiency;
dynamic instability
Omnivory

Lotka-Volterra models destabilize with
omnivory

Thought to be uncommon until recently
Food Web Complexity and
Population Dynamics



Relationship between complexity and stability –
positive or negative?
Empirical studies hard to interpret
No studies vary connectance while holding
species richness constant
Testing Food Web Theory – Future
Work

Show quantitatively that dynamics are similar
between model and real system

Study food chains longer than 2 or 3 levels

Increase species richness in studies

More work needed in studies of:

Omnivory effect on population dynamics

Effects of nutrient enrichment

Effects of more complex food chains on
trophic cascades
Contributions of Laboratory
Microcosm Studies
Holyoak, M., and S. P. Lawler 2005. The contribution of laboratory experiments on protists to
understanding population and metapopulation dynamics. Advances in Ecological Research,
Vol. 37: Population Dynamics and Laboratory Ecology 37:245-271.




Huge variety of protists useful in constructing communities
Many protists make good analogs of larger species with similar
ecological strategies
Studying protists is convenient – short generation time, high
replicability
Protist study has been historically important in verifying models, like
Gause's studies of logistic growth and competitive exclusion
How useful are natural microcosms for study?
Srivastava, D.S, Kolasa, J., Bengtsson, A. Gonzalez, Lawler, S.P., Miller, T.E.,
Munguia, T, Romanuk, Schneider, D.C., Trzcinski, M.K. 2004. Are natural
microcosms useful model systems for ecology? Trends in Ecology & Evolution,
19(7): 379-384.


Whole ecosystem vs. laboratory microcosm studies:
tractability vs realism in ecological study
Can natural microcosms help circumvent the conflict?



Potential for replication; natural boundaries; small
size; short generation time of most organisms
Some questions well-suited to these systems:

How does diversity affect ecosystem function?

How does the metacommunity affect species
richness?
So why use natural? How good is the external validity,
actually?
Ecotron
Climate-controlled
facilities for
ecological
experiments
16 chambers
Uses of Ecotron
Create simplified communities to study in a lab
Real-life simplified model of nature
Because of climate control, experiments can be
replicated across the chambers and statistical analysis
is more robust
Species Diversity and Ecosystem
Performance


Naeem, S. et al. 1995. Empirical evidence that declining
species diversity may alter the performance of
ecosystems. Philosophical Transactions of the Royal
Society of London Series B, 347: 249-262.
Hector, A., J. Joshi, S.P. Lawler, E.M. Spehn, and A. Wilby.
2001. Conservation implications of the link between
biodiversity and ecosystem functioning. Oecologia, 129:
624-628.
Ecotron Experiment





Direct manipulation of diversity
Replication: 14 chambers; all
conditions held constant except
diversity – high, med, low
Four trophic levels
Keep at least one member of each
trophic group and functional group
Look at effects on identified
ecosystem processes

Community respiration, productivity,
decomposition, nutrient retention,
water retention
Using Mesocosms to Study Effects
of Diversity


Loss of a whole trophic level
or functional group has clear
impact, but what about part?
As biodiversity declines, will
ecosystem function change?
Know about causes of
diversity, and about
biogeochemical cycles and
energy in ecosystems, but
what about how diversity
affects cycling and energy
flow?
Four hypotheses at the time
Results and Conclusions
“Higher diversity systems had more dense, more complex
canopies, higher numbers of earthworms and insect
herbivores, greater rates of CO2 flux, greater productivity
and greater accumulation of phosphorus and potassium.”



Doesn't appear to be an artifact of particular plant community
used
Limitations
Some caveats when attempting to extrapolate results to natural
systems, but it appears that affecting diversity can cause
ecosystem function to change even if trophic structure is
unmanipulated, but changes vary across functions. Also it
appears that if loss of diversity affects canopy structure, CO2
and productivity are affected.
Other Research


Cascade frogs

What are indirect effects of introduced trout on
Rana cascadae?

Competition for prey appears to be limiting
populations of R. cascadae
Mosquito control
Literature Cited
Lawler, Sharon P,. Morin, Peter J. 1993. Food web architecture and population dynamics in
laboratory microcosms of protists. The American Naturalist, 141(5): 675-686.
Naeem, S. et al. 1995. Empirical evidence that declining species diversity may alter the
performance of ecosystems. Philosophical Transactions of the Royal Society of London
Series B, 347: 249-262.
Morin, Peter J. and Sharon P. Lawler. 1995. Food web architecture and population dynamics:
Theory and empirical evidence. Annual Review of Ecology and Systematics, 26: 505-529.
Srivastava, D.S, Kolasa, J., Bengtsson, A. Gonzalez, Lawler, S.P., Miller, T.E., Munguia, T,
Romanuk, Schneider, D.C., Trzcinski, M.K. 2004. Are natural microcosms useful model
systems for ecology? Trends in Ecology & Evolution, 19(7): 379-384.
Holyoak, M., and S. P. Lawler 2005. The contribution of laboratory experiments on protists to
understanding population and metapopulation dynamics. Advances in Ecological Research, Vol. 37:
Population Dynamics and Laboratory Ecology 37:245-271.
Joseph, M., J. Piovia-Scott, S. Lawler and K. Pope. 2011. Indirect effects of introduced trout on
Cascades frogs (Rana cascadae) via shared aquatic prey. Freshwater Biology, 56: 828-838.