The effects of density, habitat properties, and traits on habitat selection

Background/Question/Methods

Regular patterning in community structure is often found across heterogeneous systems. While community ecologists have paid much attention to the role of inter-specific trait variation for species distributions, investigating the importance of environmental filtering versus competition, the influence of intra-specific trait variation for differential distribution is still not known, intra-specific variation often masked by inter-specific differences. Questions also remain as to whether differential habitat preferences are a function of past or present inter-specific competition (i.e., exclusion vs. adaptation) or species functional abilities. Streams are ideal systems to test ideas regarding habitat selection as their microhabitats (pool, run and riffles) present marked differences in depth, velocity and substrate compositions. Habitat selection is known to play an important role for species distribution and preferential use of particular microhabitat types, with non-preferred areas increasingly occupied as density thresholds are met within preferred. The purpose of this study was to investigate the role of intra-specific phenotypic variation for (1) differential microhabitat use across densities and (2) the importance of specific habitat properties for microhabitat preferences. Four density trials and three microhabitat manipulations were conducted within an artificial stream using four stream-fish species: creek chub and Johnny darter (pool), mottled sculpin and blacknose dace (riffle).

Results/Conclusions

As expected, species preferentially used the sections in which they were found in nature and, with the exception of creek chub, individuals increasingly used non-preferred sections as density within the system increased. Intra-specific variation played a large role in the differential use of microhabitats across density trials, phenotypic differences between individuals in different habitat sections largest at lower densities and decreasing as density increased. While across all habitat manipulations individuals within different microhabitat sections again housed significantly different morphologies, the importance of specific habitat properties for preferred section use differed; generally substrate the most important factor for the use of riffles and velocity for the use of pools. Intra-specific phenotypic variation, then, plays a large role in the distribution of individuals within heterogeneous systems and is an important structuring factor for habitat selection with regards to density effects and the influence of specific habitat properties. Although often overshadowed by inter-specific differences, intra-specific variation should gain more focus and explicit examination within future studies of community structure, with advances made towards the integration of trait-mediated inter and intra-specific processes, a potentially important consideration for management within increasingly fragmented systems.