Predicting Direct and Indirect Effects: An Integrated Approach Using Experiments and Path Analysis

Abstract

Determining the strengths of interactions among species in natural communities presents a major challenge to ecology. Using an approach combining experimental perturbations and path analysis, I examined the mechanisms by which birds directly and indirectly affected other members of an intertidal community, evaluated alternative causal hypotheses, and predicted whether interactions among the unmanipulated species would be strong or weak. Comparing treatments with t tests indicated that excluding bird predators with cages caused increases in Pollicipes polymerus, and declines in Nucella spp., Mytilus californianus, and Semibalanus cariosus. However, these conclusions provided no insight into the underlying mechanisms causing the differences. Path analysis permitted insight into the causal mechanisms by making a variety of predictions about the strength of direct interactions: (a) Bird predation negatively affects Pollicipes, but not Nucella, Leptasterias, or Mytilus; (b) Pollicipes reduces Semibalanus and Mytilus abundance because of space competition; (c) Mytilus reduces Semibalanus cover through competition for space; and (d) as prey species, Semibalanus and Pollicipes enhance Nucella density, but Nucella predation does not have important effects on Semibalanus of Pollicipes. Based on the estimated strength of direct interactions, the importance of indirect effects among species could also be predicted. In experiments manipulating Nucella, Pollicipes, Semibalanus, and birds independently of one another, I tested 11 of the interactions predicted by the path analysis; all were supported. Path analysis in conjunction with limited experiments may provide an efficient means to predict important direct and indirect interactions among unmanipulated species within ecological communities.

Environmental correlates of avian diversity in lowland Panama rain forests

“…Structural equation modelling (or path analysis) is a powerful tool for such analyses and is being increasingly used when ecologists want to quantify indirect effects (Mitchell, 1992;Wootton, 1994;Petraitis et al, 1996;Calme & Desrochers, 1999). This method has been successfully used for modelling drivers of species richness in other species groups and habitats (Grace & Pugesek, 1997;Stevens, 2004).…”

“…This method has been successfully used for modelling drivers of species richness in other species groups and habitats (Grace & Pugesek, 1997;Stevens, 2004). Its specific contribution is to provide a way to understand the individual contribution of each variable within complex systems and to evaluate the relative strengths of direct and indirect interactions among unmanipulated variables (Wootton, 1994;Shipley, 2000). A typical multiple regression model was presented among the path diagrams (model 5) and performed poorly relative to more detailed causal links hypothesized by biogeographical theory.…”

Functional genetics of intraspecific ecological interactions in Arabidopsis thaliana

“…We use path analysis to test for these functional relationships, because it can be used to describe and test hypotheses about directional relationships between variables. Path analysis has been used successfully to examine ecological [26,27] and evolutionary [28,29] relationships and fits well with the interacting phenotypes framework, which is built conceptually on path models [23]. However, in complex systems, with many traits spanning multiple individuals, there are so many possible relationships between traits expressed in interacting individuals that it is useful to look for relationships within individuals before examining any causal relationships between them owing to interactions.…”

Temporal dynamics of a simple community with intraguild predation: an experimental test

“…Although much theoretical development in ecology has treated the stability and dynamics of predation and interspecific competition separately (e.g., May 1973, Tilman 1982, 1988, Kot 2001, Murdoch et al 2003, it is clear that in nature both processes occur simultaneously, often with strong interaction strengths (e.g., Wooton 1994, Schmitz et al 2000, Hampton et al 2006. Theory that explores the implications of food web complexity for community dynamics has generally been confined to analyses of stability and species persistence (e.g., May 1973, Martinez et al 2006, Allesina and Pascual 2008, and only rarely has the nature of the underlying temporal dynamics been considered (Fussmann and Heber 2002).…”