Transcript Nelson2Spr2013x

Evaluation of Conceptual Synthesis Model through Trait Beta Diversity
Gregory T.
*
Nelson ,
Brandon Urhammer, Nate Servey, Katie Klundt, Molly Dieterich
Faculty mentors: Evan Weiher, David Lonzarich
Biology Department, University of Wisconsin – Eau Claire
*[email protected]
Introduction and Objective
Results
• Assembly processes are a phenomenon observed throughout taxonomic groups and are capable
of exerting evolutionary forces. 1,7,9,14,16,20,21,23 Due to the ubiquity and diverse biological
implications of community assembly theory, the underpinnings of these processes have been, and
are, a fundamental question of ecology.4,5,6,8,15
• Community structure has been found to affect productivity, implicating assembly to agriculture
through food yields and to climate change through net carbon sequestration.11,23
• There is a great deal of evidence supporting neutral assembly theory which suggests that
communities are shaped stochastically, or randomly.2,18,19 In contrast, niche-based assembly theory
suggests that abiotic and biotic stressors deterministically shape community structure.3,10,11,13
Historically these two ideologies have been contentious and largely mutually exclusive.
• Fresh theorizing posits that both processes are likely occurring to varying degrees under different
circumstances.28 When both neutral and niche-based assembly processes are considered
predictive capabilities for community type and structure are improved.23
• Communities can be characterized simply by species composition. Communities can also be
viewed through a trait-based lens, or be defined in terms of the specific functional traits of the
species they contain (e.g. height). This allows for insights into evolutionary history and ecosystem
function.24
• Beta (β) diversity is defined as the rate of turnover through space, either by species composition
or by trait values.3 This metric is used to quantify community heterogeneity and can be used to
probe neutral (high rates of turnover) and niche (low rates of turnover) assembly (Figure 1).
• Here we utilize the conceptual synthesis model (see below) combined with quantification of
communities through species and traits to illuminate the conditions that promote neutral
and/or niche-based assembly processes. Ultimately, we seek to understand the underlying
mechanisms that dictate where, when, and why species occur.
Neutral theory
Niche-based theory
o Stochastic, random
o Deterministic, predictable
o Birth rate, death rate, immigration rate, dispersal,
and disturbance events are driving forces
o Climactic patterns, environmental gradients, resource
availability, competition, facilitation are driving forces
o Relatively high β diversity
o Relatively low β diversity
Figure 2. Summary diagram of a structural equation model describing the relative influence of environmental
factors on β diversity by species and by SLA. Numbers represent the standardized path coefficient per
parameter. R squared values represent the amount of variation captured within the model. Overall, the model fit
was proficient with and there is little evidence to suggest that the data differs from the model.
Figure 3. Theoretical rationalization of the patterns observed within structural equation model (Figure 2). Each
path that was found to be a good predictor of either βspecies or βSLA has a text box describing how increased values
of each environmental variable will impact β diversities as well as a likely mechanism of influence over β diversity.
Results:
• As the filtering effects of drought stress decrease, more species as well as a greater range of SLA
trait values are able to enter the community, resulting in more turnover for each.
• βspecies increased most strongly with light transmittance and to a lesser degree with soil
moisture.
• The effects of light transmittance were more complex. For βspecies more transmittance likely reduces
the competition stresses in terms of light resources which enables more species to enter the
community, increasing turnover rates. Conversely, as transmittance increases the light levels
become more even between the canopy and the ground which exerts pressures for high light SLA
values, decreasing turnover rates for SLA.
• βSLA increased most strongly with soil moisture and βspecies, and to lesser degree with variability
in light transmittance; conversely it decreased with light transmittance.
Results:
• Because different species often have non-equivalent SLA values, higher turnover in species
composition also resulted in greater turnover in trait values.
Conceptual Synthesis Model
β species
relatively low
turnover
=
deterministic
assembly
β SLA
Discussion
Recognizing that both neutral and niche-based processes occur to varying degrees allows
for deeper insights into the vague and gentle constraints that work to shape communities.
• β diversity was dependent on environmental factors. Under conditions of high stress,
assembly was relatively deterministic whereas under conditions of low stress assembly was
more stochastic. Environmental conditions play an important role in assembly processes.
Methods
• Environmental homogeneity results in deterministic assembly. This is likely due to less
available niches which produces a homogeneous community. Variability in the environment
seems to play a role in assembly processes, but to a lesser degree than the actually
availability of resources.
• The study was conducted along the lower Chippewa River basin in Wisconsin. Community types ranged from dry
upland prairies to low wetland swales. Representative communities were chosen as study sites based on visual
identification of established and dominant plant community types.
• Community sites were sampled using a releve technique. Each releve consisted of four 1 m2 subplots, arranged
linearly 3 m apart.
• Environmental conditions were characterized for each subplot in terms of soil moisture, light transmittance to the
𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛
ground, the coefficient of variance (
) for light transmittance, and the coefficient of variance for
𝑚𝑒𝑎𝑛 𝑣𝑎𝑙𝑢𝑒
soil moisture and subsequently averaged per releve. A total of 21 releves were sampled; mean light
transmittance ranged from 0.8% - 83.0% and soil moisture ranged from 2.8% - 60.0%.
• Plant communities within each subplot were surveyed by identifying all species present as well as by measuring
specific leaf area (SLA) for each species. SLA is a common leaf economic characteristic that is useful in
assessing leaf form and function. For each plant, the upper most fully expanded leaf was chosen for SLA
𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑎𝑓 𝑎𝑟𝑒𝑎 (𝑐𝑚2)
analysis. SLA was calculated as
.
relatively high
turnover
=
• Characterizing community structure through trait values provides interesting insights into the
underpinnings that shape communities, but species characterization is still informative and is a
contributing factor of trait composition.
stochastic
assembly
Figure 1. Visual conceptualization of βspecies and βSLA between hypothetical subplots. Under low rates of
species turnover the species composition of a community is relatively constant. With high rates of
species turnover the species composition of the community drastically changes. Under low rates of SLA
turnover the leaf characteristics remain relatively constant throughout the releve. With high rates of SLA
turnover leaf characteristics drastically change through the releve.
Study Areas:
𝑙𝑒𝑎𝑓 𝑑𝑟𝑦 𝑚𝑎𝑠𝑠 (𝑔)
• The change in species composition, or species beta diversity (βspecies), was calculated as the Jaccard distance
between each of the subplots within a community and averaged per releve. The change in SLA values, or SLA
beta diversity (βSLA), was calculated between each of the subplots within a community and averaged per releve.
• Although trait values are useful in characterizing communities, plants demonstrate plasticity.
Future studies should take into consideration the ability of plants to change, and the degree to
which they change, their physiology and morphology to suit their environment when
measuring traits.
• Our results confirm that both the neutral theory and niche theory are valid and that
assembly processes for a given community lie somewhere along a gradient between
the two. The new question is under what circumstances is one promoted over the other,
and why? Environmental stressors as well as variation within the environment have
been shown to influence the position of a community along this gradient.
References
• Data were analyzed using the lavaan package for structural equation modeling in R 2.14.1. The model was
evaluated through a chi squared test (p > 0.05 means there is little evidence to suggest that the covariance
structure of the data significantly differs from the model) as well as a standardized root mean square residual
(values below 0.08 indicate a good model fit). The standardized path coefficients were calculated.
1. Bascompte, J., Jordano, P., Melian, C. J. and Olesen, J. M. 2003. The nested assembly of plant-animal mutualistic networks. Proceedings of the National Academy of Sciences of the United States of America 100.
2. Bell, G. 2005. The co-distribution of species in relation to the neutral theory of community ecology. - Ecology 86.
3. Chase, J. M. 2010. Stochastic community assembly causes higher biodiversity in more productive environments. - Science 328.
4. Chesson, P. 2000. Mechanisms of maintenance of species diversity. - Annual Review of Ecology and Systematics 31.
5. Clements, F. E. and Clements, E. S. 1928. Plant succession and indicators. - Plant succession and indicators.
6. Cody, M. L. and Diamond, J. M. 1975. ecology and evolution of communities. - Ecology and Evolution of Communities.
One of several wetland sites. Most
sites contained Carex, Polygonum,
Sagitaria, Scirpus, and
Sparganium
8. Connor, E. F. and Simberloff, D. 1979. The assembly of species communities - chance or competition. - Ecology 60.
9. Costello, E. K., Lauber, C. L., Hamady, M., Fierer, N., Gordon, J. I. and Knight, R. 2009. Bacterial Community Variation in Human
Body Habitats Across Space and Time. - Science 326.
UWEC Differential tuition
10. Cornwell, W. K. and Ackerly D. D. 2009. Community assembly and shifts in the distribution of trait values across environmental
gradient in costal California. Ecological Monographs 79.
11. Diaz, S., Cabido, M. and Casanoves, F. 1998. Plant functional traits and environmental filters at a regional scale. - Journal of
Vegetation Science 9.
Blugold Commitment
12. Diaz-Zorita, M., Grove, J. H., Murdock, L., Herbeck, J. and Perfect, E. 2004. Soil structural disturbance effects on crop yields and soil
properties in a no-till production system. - Agronomy Journal 96.
13. Fukami, T. and Nakajima, M. 2011. Community assembly: alternative stable states or alternative transient states? - Ecology Letters 14.
14. Gibson, C. W. D., Brown, V. K., Losito, L. and McGavin, G. C. 1992. The response of invertebrate assemblies to grazing. - Ecography
15.
15. Gleason, H. A. 1939. The individualistic concept of the plant association. - Amer Midland Nat 21.
subplot
study
.site
Funding sources:
7. Cole, B. J. 1983. Assembly of mangrove ant communities - patterns of geographical-distribution. - Journal of Animal Ecology 52.
established
…community
type
releve
Acknowledgements
Office of Research and Sponsored
Programs
16. Horner-Devine, M. C. and Bohannan, B. J. M. 2006. Phylogenetic clustering and overdispersion in bacterial communities. - Ecology
87.
17. Horner-Devine, M. C., Silver, J. M., Leibold, M. A., Bohannan, B. J. M., Colwell, R. K., Fuhrman, J. A., Green, J. L., Kuske, C. R.,
Martiny, J. B. H., Muyzer, G., Ovreas, L., Reysenbach, A.-L. and Smith, V. H. 2007. A comparison of taxon co-occurrence patterns for
macro- and microorganisms. - Ecology 88.
1 m2
18. Hubbell, S. P. 2005. Neutral theory in community ecology and the hypothesis of functional equivalence. – Functional. - Ecology 19.
19. Hutchinson, G. E. 1961. The Paradox of the Plankton. The American Naturalist 95.
Field and Technical support:
20. Jumpponen, A. 2003. Soil fungal community assembly in a primary successional glacier forefront ecosystem as inferred from rDNA
sequence analyses. - New Phytologist 158.
21. Poff, N. L. and Allan, J. D. 1995. Functional-organization of stream fish assemblages in relation to hydrological variability. - Ecology 76.
3m
22. Ricklefs, R. E. 2004. A comprehensive framework for global patterns in biodiversity. - Ecology Letters 7.
Ruth Cronje
23. Tilman, D. 1999. The ecological consequences of changes in biodiversity: A search for general principles. - Ecology 80.
One of several prairie sites. Most sites contained
grasses such as Andropogon and Poa well as
forbs such as Solidago, Monarda, and Achillea.
24. Weiher, E. and Keddy, P. A. 1995. Assembly rules, null models, and trait dispersion - new questions front old patterns. - Oikos 74.
25. Weiher, E., Clarke, G. D. P. and Keddy, P. A. 1998. Community assembly rules, morphological dispersion, and the coexistence of plant
species. - Oikos 81.
26.Weiher, E., Freund, D., Bunton, T., Stefanski, A., Lee, T. and Bentivenga, S. 2011. Advances, challenges and a developing synthesis
of ecological community assembly theory. - Philosophical Transactions of the Royal Society B-Biological Sciences 366: 2403-2413.
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