Confirmatory path analysis in a generalized multilevel context

Shipley, Bill

doi:10.1890/08-1034.1

Cited by 848 publications

(964 citation statements)

References 9 publications

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“…This multivariate technique is useful for testing a priori defined models and quantifying the relative importance of explanatory variables. SEM can also test whether a given effect is direct (e.g., IAP cover influences native plant diversity) or indirect (e.g., flow regime influences IAP cover, which influences native plant diversity) (Shipley 2009;Lefcheck 2016). We developed a conceptual model (meta-model) detailing potential cause-effect relationships based on biological relevance in the literature or logical arguments to guide the modelling process (Fig.…”

Section: Structural Equation Modellingmentioning

confidence: 99%

“…Upon model validation a significant missing path was identified, the effect of mean annual flood frequency on the diversity of propagules, and added to the SEM. Fisher's C [Shipley's test of directed separation; (Shipley 2009)] was used to evaluate SEM fit, where higher P values ([ 0.05) indicate that the data supports the model (i.e., H 0 = no difference between the data and the hypothesised paths). However, it should be noted that alternative models could also support the data, and for one response variable (BCI spring v summer Yr2), two alternative SEMs were constructed based on a priori hypotheses, and the best model in this case was selected using AICc.…”

Section: Structural Equation Modellingmentioning

confidence: 99%

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Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

2017

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Riparian zones are formed by interactions between fluvio-geomorphological processes, such as sediment deposition, and biota, such as vegetation. Establishment of invasive alien plant (IAP) species along rivers may influence vegetation dynamics, evidenced as higher seasonal or inter-annual fluctuations in native plant diversity when IAP cover is high. This could impact the overall functioning of riparian ecosystems. Conversely, fine sediment deposited in riparian zones after floods may replenish propagule banks, thus supporting recruitment of native species. The interactive effects of invasion and fine sediment deposition have hitherto, however, been ignored. Vegetation surveys across rivers varying in flow regime were carried out over 2 years to assess changes in community composition and diversity. Artificial turf mats were used to quantify over-winter sediment deposition. The viable propagule bank in soil and freshly deposited sediment was then quantified by germination trials. Structural Equation Models were used to assess causal pathways between environmental variables, IAPs and native vegetation. Greater variation in flow increased the cover of IAPs along riverbanks. An increased in high flow events and sediment deposition were positively associated with the diversity of propagules deposited. However, greater diversity of propagules did not result in a more diverse plant community at invaded sites, as greater cover of IAPs in summer reduced native plant diversity. Seasonal turnover in the above-ground vegetation was also accentuated at previously invaded sites, suggesting that a legacy of increased competition in previous years, not recent sediment deposition, drives above-ground vegetation structure at invaded sites. The interaction between fluvial disturbance via sediment deposition and invasion pressure is of growing importance in the management of riparian habitats. Our results suggest that invasion can uncouple the processes that contribute to resilience in dynamic habitats making already invaded habitats vulnerable to further invasions.

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Section: Structural Equation Modellingmentioning

confidence: 99%

Section: Structural Equation Modellingmentioning

confidence: 99%

Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

2017

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“…We tested the hypothesis that abiotic conditions affected Trientalis performance via their relationships with biotic conditions using structural equation models (Grace, 2006) and mediation tests (d-separation tests, Shipley, 2009;Clough, 2012). With the structural equation models we were able to test for potential direct and indirect associations between broad categories of abiotic conditions (soil and topographic microclimate) and plant performance, while with the mediation tests we were able to select the combination of biotic conditions that explained a significant proportion of the effect of abiotic variables on plant performance.…”

Section: Discussionmentioning

confidence: 99%

“…'mediating' effects (defined in Grace et al, 2012), were examined with mediation tests (d-separation tests, Shipley, 2009;Clough, 2012). We first considered which abiotic variable (serpentine soils or elevation) was significantly related to each biotic condition (overstorey shading, litter, understorey cover, mycorrhizal or septate fungal colonization) and whether these biotic conditions were significantly associated with plant variables (presence, abundance or reproduction).…”

Section: Discussionmentioning

confidence: 99%

Shading and litter mediate the effects of soil fertility on the performance of an understorey herb

Copeland

Harrison

2016

Ann Bot

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Background and Aims Soil fertility and topographic microclimate are common determinants of plant species distributions. However, biotic conditions also vary along these abiotic gradients, and may mediate their effects on plants. In this study, we investigated whether soils and topographic microclimate acted directly on the performance of a focal understorey plant, or indirectly via changing biotic conditions.Methods We examined direct and indirect relationships between abiotic variables (soil fertility and topographic microclimate) and biotic factors (overstorey and understorey cover, litter depth and mycorrhizal colonization) and the occurrence, density and flowering of a common understorey herb, Trientalis latifolia, in the Klamath-Siskiyou Mountains, Oregon, USA.Results We found that the positive effects of soil fertility on Trientalis occurrence were mediated by greater overstorey shading and deeper litter. However, we did not find any effects of topographic microclimate on Trientalis distribution that were mediated by the biotic variables we measured. The predictive success of Trientalis species distribution models with soils and topographic microclimate increased by 12 % with the addition of the biotic variables.Conclusions Our results reinforce the idea that species distributions are the outcome of interrelated abiotic gradients and biotic interactions, and suggest that biotic conditions, such as overstorey density, should be included in species distribution models if data are available.

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“…From full models of litter size and juvenile survival, we selected a baseline model for these two reproductive traits by retaining only the confounding variables with statistically significant effects on a given reproductive trait (see electronic supplementary material, section B). To test the fit of the data to the indirect scenario (scenario 2), we compared the C value associated with scenario 2 to a x 2 -distribution with 2k degrees of freedom (where k is the number of pairs of variables in the graph that are not associated in the focal model) [27]. We rejected scenario 2 when the p-value was below 0.05 [27].…”

Section: (E) Sample Sizesmentioning

confidence: 99%

Early and adult social environments have independent effects on individual fitness in a social vertebrate

Berger¹,

Lemaître²,

Allainé³

et al. 2015

Proc. R. Soc. B.

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Evidence that the social environment at critical stages of life-history shapes individual trajectories is accumulating. Previous studies have identified either current or delayed effects of social environments on fitness components, but no study has yet analysed fitness consequences of social environments at different life stages simultaneously. To fill the gap, we use an extensive dataset collected during a 24-year intensive monitoring of a population of Alpine marmots (Marmota marmota), a long-lived social rodent. We test whether the number of helpers in early life and over the dominance tenure length has an impact on litter size at weaning, juvenile survival, longevity and lifetime reproductive success (LRS) of dominant females. Dominant females, who were born into a group containing many helpers and experiencing a high number of accumulated helpers over dominance tenure length showed an increased LRS through an increased longevity. We provide evidence that in a wild vertebrate, both early and adult social environments influence individual fitness, acting additionally and independently. These findings demonstrate that helpers have both short-and long-term effects on dominant female Alpine marmots and that the social environment at the time of birth can play a key role in shaping individual fitness in social vertebrates.

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Confirmatory path analysis in a generalized multilevel context

Cited by 848 publications

References 9 publications

Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

Shading and litter mediate the effects of soil fertility on the performance of an understorey herb

Early and adult social environments have independent effects on individual fitness in a social vertebrate

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