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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. 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