Sea Slugs—“Rare in Space and Time”—But Not Always

Julie Schubert,Stephen D A Smith

doi:10.3390/d12110423

Abstract

The term “rare in space and time” is often used to typify the spatial and temporal patterns of occurrence of heterobranch sea slugs. However, “rare” in this context has not been clearly defined. In an attempt to provide more insight into the concept of rarity in sea slug assemblages, we analysed abundance data from 209 individual surveys conducted over a 5-year period in a subtropical estuary and a 7-year period on a shallow coastal reef, on the Sunshine Coast, Qld, Australia. Using an ‘intuitive’ method (<10 individuals recorded over the study), and the ‘quartile’ method we assessed numerical rarity (number of individuals of a species seen over the study period) and temporal rarity (frequency of observation). We also assessed numerical rarity using octaves based on log2 abundance bins. The quartile method did not effectively capture either measure of rarity. The octave method, however, fitted closely to subjective classifications of abundance and defined a similar number of species as rare when compared to the intuitive method. Using the octave method, 66% of species in both the estuary and on the reef, were considered as rare. Consequently, we recommend the octave method to allocate abundance classifications. To address the poor fit for temporal classifications based on quartiles, we propose the following as a working model for wider testing: rare ≤25% of surveys; uncommon 26−50%, common 51−75%; and abundant >75%.

Highlights

Ecologists have long recognised that many species within a community are represented by very few individuals (e.g., [1]) and highlighted the importance of these species in terms of potential extinction risk and, more recently, for broader ecosystem function (e.g., [2])
The objectives of this study were to: (i) describe patterns of abundance for sea slug assemblages from 2 sites in subtropical eastern Australia; (ii) assess patterns of numerical and temporal rarity using the quartile method [4], quartiles based on bins of abundance on a log2 scale [15], and the abundance scale applied to assessments at Heron Island [8]; and, (iii) conduct preliminary exploration of associations among water temperature, species richness and the appearance of rare
Using long-term data sets that provide much-needed, additional ecological data on sea slug assemblages in a subtropical location, this study explored the distribution of abundance among species to see if commonly-used models are useful for describing rarity

Summary

Introduction

Ecologists have long recognised that many species within a community are represented by very few individuals (e.g., [1]) and highlighted the importance of these species in terms of potential extinction risk and, more recently, for broader ecosystem function (e.g., [2]). 7 forms of rarity, which reflected combinations of differences across 3 descriptive variables—geographic range, habitat specificity and local population size. Species with both narrow geographic range and strong habitat specificity typified the “classic” concept of rarity, but different combinations of restrictions across each of these 3 variables may result in a species being considered rare. Heterobranch sea slugs (Mollusca: Gastropoda) are often referred to as being “rare in space and time” [6,7,8] and represent an interesting taxon for exploring the broad use of commonly-used rarity metrics. Rarity is a relative concept, and an intuitive

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