Habitat selection determines abundance, richness and species composition of beetles in aquatic communities

Distribution and abundance patterns at the community and metacommunity scale can result from two distinct mechanisms. Random dispersal followed by non-random, site-specific mortality (species sorting) is the dominant paradigm in community ecology, while habitat selection provides an alternative, largely unexplored, mechanism with different demographic consequences. Rather than differential mortality, habitat selection involves redistribution of individuals among habitat patches based on perceived rather than realized fitness, with perceptions driven by past selection. In particular, habitat preferences based on species composition can create distinct patterns of positive and negative covariance among species, generating more complex linkages among communities than with random dispersal models. In our experiments, the mere presence of predatory fishes, in the absence of any mortality, reduced abundance and species richness of aquatic beetles by up to 80% in comparison with the results from fishless controls. Beetle species' shared habitat preferences generated distinct patterns of species richness, species composition and total abundance, matching large-scale field patterns previously ascribed to random dispersal and differential mortality. Our results indicate that landscape-level patterns of distribution and species diversity can be driven to a large extent by habitat selection behaviour, a critical, but largely overlooked, mechanism of community and metacommunity assembly.<br/

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

Oviposition behavior partitions aquatic landscapes along predation and nutrient gradients

That individuals attempt to minimize the ratio of mortality risk/growth rate (μ/g) when foraging within individual habitat patches is well established. Do species partition among spatially discrete communities embedded in complex landscapes in a similar manner? We investigated how 3 ovipositing species (2 Hyla treefrogs and a hydrophilid beetle, Tropisternus lateralis) responded to simultaneous gradients of nutrients and predation risk. Species partitioned our experimental metacommunity primarily by reducing oviposition with fish. Tropisternus positively responded to increased nutrients, but the effect decreased with increasing risk, as predicted by μ/g theory. Use of fish habitats by Tropisternus was unrelated to breeding intensity. In contrast, Hyla showed no nutrient response but oviposited with fish only on nights with high breeding activity. Behavioral responses to the spatial distribution of resources and risk among discrete patches generated substantial variation in habitat-specific colonization rates, which has been identified as a primary mechanism generating both community and metacommunity structure. Copyright 2008, Oxford University Press.

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

Functional diversity within a morphologically conservative genus of predators: implications for functional equivalence and redundancy in ecological communities

1. The idea that sets of species may have similar effects on population, community or ecosystem processes is a prevalent theme in many areas of ecology, especially in the context of biodiversity and ecosystem function. If indeed species are functionally equivalent, limiting similarity suggests that it should be closely related, morphologically similar species using similar resources in a similar manner.2. We assayed the functional equivalence of three congeneric, morphologically similar predatory fish species (genus Enneacanthus). Functional equivalence was evaluated using aspects of both effects of fish on a variety of prey responses and the growth responses of the fish themselves as a measure of energy consumption. Fish were matched by initial size to control for effects of body size. A strict definition of functional equivalence based on niche theory was used to delineate it from the alternative of functional diversity.3. Based on observed effects on larval anurans, only a single species pair could roughly be judged functionally equivalent, but these two species showed the greatest differences in growth rate and, hence, metabolic demand. Using the criterion of relative yield total, again, only a single pair could roughly be judged equivalent, however, members of this alternative species pair were dramatically different in their effects on larval anurans. Thus, as previously shown for a more diverse set of species, grouping of species by similarity in effects depends upon the specific response variable. 4. Overall range of effects produced on a variety of response variables was surprising, given the similarity in morphology and autecology, strong phylogenetic affinity, and the fact that neither predator size nor growth explained significant variation. Each species appears to be interacting with the environment in a different manner, either as a consequence of differences in metabolic demand or differences in preferences or efficiency with regard to prey types.5. Observed responses are consistent with the predictions of niche theory and support an alternative explanation for observed relationships between diversity and ecosystem function. Our work suggests that functional equivalence may be uncommon, difficult to predict a priori, and that functional diversity, not functional equivalence, may underlie observed diversity–ecosystem function relationships