Mapping functional similarity of predators on the basis of trait similarities

Theoretical and empirical studies in community ecology often simplify their study system by lumping together species on the basis of trait similarities ( e. g., their taxonomy, resource or microhabitat usage) and then assume species sharing similar traits are functionally similar. To date, no study has directly tested whether species more similar with respect to any of these traits are more similar in their functional effects on population or ecosystem processes. In this study, we examined the association between traits and functional effects of six different aquatic predatory vertebrates. We demonstrated that functional similarity across multiple response variables was not correlated with trait similarity, but differences in trait values were associated with significantly different effects on individual response variables. The exact relationship between species traits and functional effect of predators, however, was different for each response variable. Using traits to predict functional similarity among species may be useful when considering individual response variables, but only if it is known which traits have the greatest influence on the response variable of interest. It is doubtful that any one scheme will predict the functional similarity of species across a diverse array of response variables because each response will likely be strongly influenced by different traits

Experimental venue and estimation of interaction strength: Comment

While experiments are vital for understanding how ecological systems operate, different philosophies exist concerning how experiments should be conducted (e.g., Petranka 1989, Dunham and Beaupre 1998, Resetarits and Fauth 1998, Skelly and Kiesecker 2001; also see the special features in Herpetologica [1989; 45:111– 128] and Ecology [1996; 77:663–705, see Dahler and Strong 1996]). Recently, Skelly (2002) asked how experimental venue (i.e., cattle tanks set up as mesocosms in a field setting vs. screened enclosures placed into natural ponds) influences competitive interactions between two species of larval anurans (Pseudacris crucifer and Rana sylvatica) and how results from the two venues match a standard of realism. He observed that density affected competitive interactions among larval anurans in mesocosms but not in enclosures and concluded that enclosures were more realistic because the observed size of tadpoles measured in the field was more similar to the size of tadpoles predicted by the enclosure experiment than by the mesocosm experiment. Although an empirical examination of venue is valid, we believe that this study has serious flaws and claims differences between venues that erroneously devalue the use of mesocosms. Our goal is to reinterpret the results from Skelly (2002) in light of its design, point out methodological/statistical issues associated with his study, and argue that both venues can make meaningful contributions to the field of ecology if they are designed correctly with regard to the questions being asked and the specific population of interest

Predation and Competition Within an Assemblage of Larval News (Triturus)

The impact of crested newts (Triturus cristatus) on the smaller-bodied palmate and smooth newts (T. helveticus and T. vulgaris) was studied during the larval stages using a combination of field and laboratory experiments. In pond enclosures T. cristatus larvae had no effect on the two smaller species over the first four weeks of development. By eight weeks, however, T. cristatus had achieved a size advantage which enabled it to eliminate T. helveticus and severely reduce T. vulgaris by predation. In laboratory trials under food-limited conditions, T. helveticus and T. vulgaris were slightly smaller when raised with T. cristatus, suggesting that this predatory effect was complemented by interspecific competition during early development. Predation of the smaller species started when T. cristatus reached a threshold size of c. 27 mm. No reciprocal effects on T cristatus growth or survival were observed. Although T. cristatus may be a significant predator of congeneric species in natural ponds, other factors, such as differences in microhabitat selection, higher-order predator-prey interactions, and the occasional desiccation of pond habitats may facilitate coexistence between the species