American Society of Naturalists

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“Patch size as a niche dimension: aquatic insects behaviorally partition enemy-free space across gradients of patch size”

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William J. Resetarits Jr., Matthew R. Pintar, Jason R. Bohenek, and Tyler M. Breech (Dec 2019)

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Patch size as a niche dimension changes our view of patch size variation in supporting local and regional diversity

Many dytiscid beetles like <i>Thermonectus nigrofasciatus</i> (left) prefer<br/><br/>smaller habitats, while aquatic true bugs like <i>Notonecta irrorata</i> (right)<br/><br/>prefer larger habitats, spatially sorting species into different habitats<br/><br/>based on patch size.<br />(Credit: Matthew R. Pintar)
Many dytiscid beetles like Thermonectus nigrofasciatus (left) prefer smaller habitats, while aquatic true bugs like Notonecta irrorata (right) prefer larger habitats, spatially sorting species into different habitats based on patch size.
(Credit: Matthew R. Pintar)

One of the truisms in Nature is that habitat patches in all types of systems – terrestrial, aquatic, marine – vary in size, whether they are host plants for plant-eating insects, prairie remnants for grassland birds, coral reefs for marine fish, or ponds for aquatic insects. In all these systems, both average population size and number of species tend to increase with patch size. This relationship is partly attributable to reduced extinction rates in larger patches, higher passive colonization rates as a result of larger “targets,” and possibly increased cue intensity resulting in easier detection. When considering habitat selection, however, organisms may conceivably display active preferences for patches of different size. Thus, patch size could function as another component of patch quality, along with factors like predation risk and resource level. We have a strong, consistent pattern (increasing diversity with patch area) that may arise from a variety of processes, requiring an experimental approach to disentangle the ecological drivers.

Resetarits et al. manipulated both patch quality (fish presence/absence) and patch size, and found that, in contrast to expectations of no preference or overall preference for large patches, insect species showed strong and highly variable active preferences for patch size. Most insect species avoided fish, but differentially colonized fishless patches of different size, resulting in different colonist species composition and relative abundance across the size gradient. Thus, for colonizing aquatic insects, patch size functions as a niche dimension that generates spatial segregation and reduces the potential for intense species interactions (both competition and predation) within the insect assemblage. Establishing patch size as a niche dimension changes how we view the role of patch size variation in supporting conservation of local and regional diversity, as well as the importance of preserving patch size variation as a driver of diversity.


Abstract

Positive correlation of species richness with area is ubiquitous in nature, but the processes driving that relationship, and those constraining typical patterns, remain elusive. Patch size variation is pervasive in natural systems, and thus it is critical to understand how variation in patch size, as well as its potential interaction with factors like predation and isolation, affect community assembly. We crossed patch quality (fish presence/absence) with patch size to examine effects of quality, size, and their interaction on colonization by aquatic insects. Overall, beetles favored small, fishless patches, but individual species sorted across patch size, while hemipterans aggregated into large, fishless patches, producing sorting between Coleoptera and Hemiptera. Both size and predation risk generated significant variation in community structure and diversity. Patch size preferences for the 14 most abundant species, and pre-eminence of species turnover in patterns of beta-diversity, reinforce patch size as a driver of regional species sorting via habitat selection. Species sorting at the immigration stage plays a critical role in community assembly. Identifying patch size as a component of perceived quality establishes patch size as a critical niche dimension, and alters our view of its role in assembly dynamics, and the maintenance of local and regional diversity.