# The spatial extent and the dispersal strategy of species shape the occupancy frequency distribution of stream insect assemblages

## TL;DR

This study shows how the spatial scale and dispersal abilities of stream insects affect their occupancy patterns in natural streams.

## Contribution

The study reveals how spatial extent and dispersal strategies interact to shape occupancy frequency distributions in stream insect assemblages.

## Key findings

- OFD patterns shifted from bimodal to unimodal as spatial extent increased for active dispersers.
- Dispersal strategies influenced the strength of support for OFD patterns across all spatial extents.
- Unique metacommunity processes underlie assemblage organization depending on spatial extent and dispersal strategy.

## Abstract

Several theoretical models have been proposed as the underlying mechanisms behind occupancy frequency distribution (OFD) patterns. For instance, the metapopulation dynamic model predicts bimodal OFD pattern indicating the dominance of dispersal processes in structuring the assemblages, while the niche‐based model predicts unimodal right‐skewed OFD pattern, and thus assemblages are driven mostly by niche processes. However, it is well known that the observed OFD pattern reflects the interplay of several other factors (e.g. habitat heterogeneity, species specificity and sampling protocol parameters). It follows that the individual contribution of each factor to the OFD pattern is rather complicated to explore. Our main objective was to examine the role of the spatial extent of the sampling and the dispersal strategies of species in shaping OFD pattern. For this, we collected samples of stream insect assemblages inhabiting near‐natural streams in the Pannon Ecoregion. We formed groups of species representing contrasting dispersal strategies (referred to as dispersal groups). Applying a computer program algorithm, we produced samples with different spatial extent. We found that with increasing spatial extent, the OFD pattern changed from bimodal to unimodal for active dispersers. Insect groups with different dispersal strategies differed in the strength of support for OFD patterns within all spatial extent. Furthermore, the strength of support for OFD patterns varied across dispersal groups differently as the spatial extent increased. Our results reflected underlying changes in mechanisms structuring assemblages along an increasing spatial extent. We also assumed that the stream insect dispersal strategy influences the relative role of dispersal and niche processes particularly as spatial extent increases from stream reaches to the extent of adjacent valleys. We could define spatial extents and dispersal strategies within which unique metacommunity processes could underlie the organisation of assemblages.

We examined the role of spatial extent of sampling and dispersal strategies of stream insect species in shaping occupancy frequency distribution (OFD) in near‐natural streams in the Pannon Ecoregion. With increasing spatial extent, the OFD pattern changed from bimodal to unimodal for active dispersers. Insect groups with different dispersal strategies differed in the strength of support for OFD patterns within all spatial extent. We could define spatial extents and dispersal strategies within which unique metacommunity processes could underlie the organisation of assemblages.