Declines in both redundant and trace species characterize the latitudinal diversity gradient in tintinnid ciliates

Abstract

The latitudinal diversity gradient is a well-known biogeographic pattern. However, rarely considered is how a cline in species richness may be reflected in the characteristics of species assemblages. Fewer species may equal fewer distinct ecological types, or declines in redundancy (species functionally similar to one another) or fewer trace species, those occurring in very low concentrations. We focused on tintinnid ciliates of the microzooplankton in which the ciliate cell is housed inside a species-specific lorica or shell. The size of lorica oral aperture, the lorica oral diameter (LOD), is correlated with a preferred prey size and maximum growth rate. Consequently, species of a distinct LOD are distinct in key ecologic characteristics, whereas those of a similar LOD are functionally similar or redundant species. We sampled from East Sea/Sea of Japan to the High Arctic Sea. We determined abundance distributions of biological species and also ecological types by grouping species in LOD size-classes, sets of ecologically similar species. In lower latitudes there are more trace species, more size-classes and the dominant species are accompanied by many apparently ecologically similar species, presumably able to replace the dominant species, at least with regard to the size of prey exploited. Such redundancy appears to decline markedly with latitude in assemblages of tintinnid ciliates. Furthermore, the relatively small species pools of the northern high latitude assemblages suggest a low capacity to adapt to changing conditions.

Community Structure of Tintinnid Ciliates of the Microzooplankton in the South West Pacific Ocean: Comparison of a High Primary Productivity with a Typical Oligotrophic Site

“…This pattern was been reported for assemblages in both the Southern (Santoferrara and Alder ) and the Northern Hemisphere (Dolan et al. ). While the patterns appear coherent among similar systems, little is known with regard to temporal variability over short time‐scales or responses to transient changes in the environment.…”

“…For both species and LOD size-classes, we constructed hypothetical log-rank abundance curves that could fit the data using parameters of the particular assemblage. We constructed curves for three different models of community organization: geometric series, log-series, and log-normal, as in several previous studies (Raybaud et al 2009;Claessens et al 2010;Doherty et al 2010;Dolan et al 2007Dolan et al , 2009Dolan et al , 2013Dolan et al , 2016Dolan and Stoeck 2011).…”

Tintinnid ciliates of the marine microzooplankton in Arctic Seas: a compilation and analysis of species records

“…This is in contrast to findings of little ecological redundancy from our recent study which was based on samples from a transect of stations across the western Arctic to 82°N (Dolan et al 2016). The discrepancy likely reflects the fact the overall species pool described here sums a large number of samplings conducted over a long period of time.…”

“…Sampling and sampling analysis followed methods described in detail in Dolan et al 2013Dolan et al , 2016. Briefly, for most stations a 20 µm mesh plankton net was towed from approximately 100 m to the surface.…”

“…Recently, we examined the latitudinal pattern in morphological diversity as well as species diversity in tintinnids along a Northern Hemisphere transect from the Sea of Japan/East Sea to the High Arctic sampled in late summer of 2012, the year of record low sea ice (Dolan et al 2016). Both species richness and morphological diversity (as numbers of distinct oral diameters) declined dramatically to low levels in the High Arctic.…”