Quantifying nonadditive selection caused by indirect ecological effects

terHorst, Casey P.; Lau, Jennifer A.; Cooper, Idelle A.; Keller, Kane R.; Rosa, Raffica J. La; Royer, Anne M.; Schultheis, Elizabeth H.; Suwa, Tomomi; Conner, Jeffrey K.

doi:10.1890/14-0619.1

Cited by 52 publications

(65 citation statements)

References 49 publications

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“…Because most species interact with a broad range of other species, studying pairwise species interactions in isolation does not provide a complete picture of the dynamics of a community (Strauss et al ., ; Walsh, ; terHorst et al ., ; Mayfield & Stouffer, ). Indirect ecological effects, which occur when a third species alters the direct interaction between two other species (Strauss, ; Wootton, ), can change the direction and strength of selection on a trait and generate nonadditive selection (reviewed in Walsh, ; terHorst et al ., ).…”

Section: Introductionmentioning

confidence: 99%

Ecological sorting and character displacement contribute to the structure of communities ofClarkiaspecies

Eisen

Geber

2018

J of Evolutionary Biology

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Despite long-standing interest in the evolutionary ecology of plants that share pollinators, few studies have explored how these interactions may affect communities during both community assembly (ecological sorting) and through ongoing, in situ evolution (character displacement), and how the effects of these interactions may change with community context. To determine if communities display patterns consistent with ecological sorting, we assessed the frequency of co-occurrence of four species of Clarkia in the southern Sierra foothills (Kern County, CA, USA). To investigate potential character displacement, we measured pollination-related traits on plants grown in a greenhouse common garden from seed collected in communities with one, two or four Clarkia species. Among the four species of Clarkia in this region, the two species that are often found in multi-species communities also co-occur with one another more frequently than expected under a null model. This pattern is consistent with ecological sorting, although further investigation is needed to determine the role of pollinators in shaping community assembly. Patterns of trait variation in a common garden suggest that these two species have diverged in floral traits and converged in flowering time where they co-occur, which is consistent with character displacement. Trait variation across community types also suggests that the process and outcome of character displacement may vary with community context. Because community context appears to affect both the direction and magnitude of character displacement, change in more species-rich communities may not be predictable from patterns of change in simpler communities.

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Section: Introductionmentioning

confidence: 99%

Ecological sorting and character displacement contribute to the structure of communities ofClarkiaspecies

Eisen

Geber

2018

J of Evolutionary Biology

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“…We appreciate the opportunity to reply to Bolstad's (2017) comment on our paper, "Quantifying nonadditive selection caused by indirect ecological effects" (terHorst et al 2015). We respectfully disagree with Bolstad's argument that our method does not properly quantify nonadditive selection in response to indirect ecological effects, as it certainly does for many of the biological scenarios we envision.…”

Section: Commentsmentioning

confidence: 89%

Quantifying nonadditive selection caused by indirect ecological effects: Reply

terHorst

Lau

Conner

2017

Ecology

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CommentsWe appreciate the opportunity to reply to Bolstad's (2017) comment on our paper, "Quantifying nonadditive selection caused by indirect ecological effects" (terHorst et al. 2015). We respectfully disagree with Bolstad's argument that our method does not properly quantify nonadditive selection in response to indirect ecological effects, as it certainly does for many of the biological scenarios we envision. However, as with all tests related to ecological indirect effects, the appropriateness of our approach depends on the underlying model that best describes the multispecies interaction. Specifically, the null model we propose applies only to fitness effects of multispecies interactions that are best modeled with additive rather than multiplicative models. The lntransformations Bolstad proposes are appropriate for constructing null models of nonadditive selection when the underlying species interactions are best modeled multiplicatively. The use of additive vs. multiplicative models has a long history in the study of indirect ecological effects (Case and Bender 1981, Billick and Case 1994, Wootton 1994b, which we revisit briefly here and extend to measures of selection. Specifically, we clarify our goals and approach and address Bolstad's two major criticisms of our paper: (1) that we incorrectly infer indirect ecological effects from our measure of the non-additivity of selection, and (2) that log fitness (in contrast to fitness relativized across treatments as we propose) is more appropriate for quantifying selection in response to multiple species over varying timescales. We explain our reasoning and use a simple model to demonstrate when our null model is appropriate and when Bolstad's log fitness model is most appropriate for identifying nonadditive selection in multispecies communities.First, to clarify the goals of our original paper, we provided a method to detect and estimate the strength of nonadditive selection, which occurs when selection on a trait imposed by one selective agent is altered in the presence of a second selective agent. Our goal was not to use evidence for nonadditive selection to infer indirect ecological effects; ecologists have many simpler methods available for detecting indirect ecological effects (Strauss 1991, Wootton 1994a, Menge 1995 that do not require the large sample sizes and additional trait measurements needed to understand natural selection. Most studies estimate the consequences of indirect ecological effects for species abundances, rather than fitness or selection (Miller and terHorst 2012), and rarely measure traits. Indirect ecological effects do not necessarily result in non-additivity of selection, and nonadditive selection can result from processes other than indirect ecological effects. For example, an indirect ecological effect that alters fitness will not necessarily also result in nonadditivity of selection on the traits under consideration. Rather, for any species to alter selection on another it must alter the covariance between relative fitness and a ...

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“…A recent discussion about the ecological causes and mechanisms of nonadditive selection through indirect ecological effects illustrate some of the conceptual problems we have raised in this article (ter Horst et al 2015;2017;Bolstad 2017). Consider the case where one species (e.g., an herbivore) exerts selection on a trait of another species (e.g., plant defense).…”

Section: Discussionmentioning

confidence: 99%

“…Consider the case where one species (e.g., an herbivore) exerts selection on a trait of another species (e.g., plant defense). Key to testing this hypothesis is comparison of the strength of selection on a focal species when both manipulated species are present to a null model of additive selection constructed using selection estimates obtained from each species alone (ter Horst et al 2015). Or do indirect ecological effects (e.g., interactions between herbivores) lead to nonadditive selection, and thus "diffuse" coevolution (Inouye and Stinchcombe 2001)?…”

Section: Discussionmentioning

confidence: 99%

“…Does the exerted selection remain constant when another species (e.g., a second herbivore) is present? ter Horst et al (2015) argued that in such an experiment, only nonadditive effects that result from changes in the absolute fitness-phenotype relationship should qualify as indirect ecological effects on selection; interactive effects arising from a second-species effect on focal mean fitness are irrelevant, as such effects do not to represent a physical interaction between the two herbivore species (ter Horst et al 2017). Key to testing this hypothesis is comparison of the strength of selection on a focal species when both manipulated species are present to a null model of additive selection constructed using selection estimates obtained from each species alone (ter Horst et al 2015).…”

Section: Discussionmentioning

confidence: 99%

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On the standardization of fitness and traits in comparative studies of phenotypic selection

Lisle

Svensson

2017

Evolution

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Comparisons of the strength and form of phenotypic selection among groups provide a powerful approach for testing adaptive hypotheses. A central and largely unaddressed issue is how fitness and phenotypes are standardized in such studies; standardization across or within groups can qualitatively change conclusions whenever mean fitness differs between groups. We briefly reviewed recent relevant literature, and found that selection studies vary widely in their scale of standardization, but few investigators motivated their rationale for chosen standardization approaches. Here, we propose that the scale at which fitness should be relativized should reflect whether selection is likely to be hard or soft; that is, the scale at which populations (or hypothetical populations in the case of a contrived experiment) are regulated. We argue that many comparative studies of selection are implicitly or explicitly focused on soft selection (i.e., frequency and density-dependent selection). In such studies, relative fitness should preferably be calculated using within-group means, although this approach is taken only occasionally. Related difficulties arise for the standardization of phenotypes. The appropriate scale at which standardization should take place depends on whether groups are considered to be fixed or random. We emphasize that the scale of standardization is a critical decision in empirical studies of selection that should always warrant explicit justification.

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Quantifying nonadditive selection caused by indirect ecological effects

Cited by 52 publications

References 49 publications

Ecological sorting and character displacement contribute to the structure of communities ofClarkiaspecies

Ecological sorting and character displacement contribute to the structure of communities ofClarkiaspecies

Quantifying nonadditive selection caused by indirect ecological effects: Reply

On the standardization of fitness and traits in comparative studies of phenotypic selection

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