Empirical approaches to quantifying interaction intensity: Competition andfacilitation along productivity gradients

Goldberg, DE Rajaniemi, T Gurevitch, J Stewart-Oaten, A

De. Goldberg et al., Empirical approaches to quantifying interaction intensity: Competition andfacilitation along productivity gradients, ECOLOGY, 80(4), 1999, pp. 1118-1131

87

INGLESE

Article

Environment/Ecology

ECOLOGY

0012-9658 → ACNP

80

4

1999

1118 - 1131

ISI

0012-9658(199906)80:4<1118:EATQII>2.0.ZU;2-N

Quantitative synthesis across studies requires consistent measures of effec t size among studies. In community ecology, these measures of effect size w ill often be some measure of the strength of interactions between taxa. How ever, indices of interaction strength vary greatly among both theoretical a nd empirical studies, and the connection between hypotheses about interacti on strength and the metrics that are used to test these hypotheses are ofte n not explicit. We describe criteria for choosing appropriate metrics and m ethods for comparing them among studies at three stages of designing a meta -analysis to test hypotheses about variation in interaction intensity: (1) the choice of response variable; (2) how effect size is calculated using th e response in two treatments; and (3) whether there is a consistent quantit ative effect across all taxa and systems studied or only qualitatively simi lar effects within each taxon-system combination. The consequences of diffe rent choices at each of these stages are illustrated with a meta-analysis t o examine the relationship between competition/facilitation intensity and p roductivity in plants. The analysis used a database of 296 cases in 14 stud ies. The results were unexpected and largely inconsistent with existing theory: competition intensity often significantly declined (rather than increased) with productivity, and facilitation was sometimes restricted to more produc tive (rather than less productive) sites. However, there was considerable v ariation in the pattern among response variables and measures of effect siz e. For example, on average, competitive effects on final biomass and surviv al decreased with standing crop, but competitive effects on growth rate did not. On the other hand, facilitative interactions were more common at low standing crop for final biomass and growth rate, but more common at high st anding crop for survival. Results were more likely to be significant using the log response ratio (In[removal/control]) as the effect size than using the relative competition intensity ([removal - control]/removal), although the trends for these conceptually similar indices did not differ. When all studies were grouped in a single meta-regression of interaction intensity o n standing crop to test quantitative similarity among studies, survival sho wed the dearest negative relationship. However, when the same regressions w ere done for each unique combination of taxon and site within each study to test for qualitative similarity among studies, the slopes averaged over st udies tended to be negative for biomass and growth rate, but not different from zero for survival. These results are subject to a number of caveats be cause of the limitations of the available data-most notably, the extension of effects of interactions on individual growth or survival to effects on p opulation distribution and abundance or community structure is highly probl ematic. Nevertheless, the fact that none of the meta-analyses demonstrated a significant positive relationship between competition and standing crop b ut that we frequently found negative relationships is an important pattern that has not been apparent from qualitative surveys of individual studies, and it demonstrates the potential power of meta-analysis in ecology. We con clude with recommendations to overcome some of the limitations of the curre ntly available data and meta-analytical procedures.