Evolutionary drivers of the hump-shaped latitudinal gradient of benthic polychaete species richness along the Southeastern Pacific coast

Abstract

Latitudinal diversity gradients (LDG) and their explanatory factors are among the most challenging topics in macroecology and biogeography. Despite of its apparent generality, a growing body of evidence shows that ‘anomalous’ LDG (i.e., inverse or hump-shaped trends) are common among marine organisms along the Southeastern Pacific (SEP) coast. Here, we evaluate the shape of the LDG of marine benthic polychaetes and its underlying causes using a dataset of 643 species inhabiting the continental shelf (<200 m depth), using latitudinal bands with a spatial resolution of 0.5°, along the SEP (3–56° S). The explanatory value of six oceanographic (Sea Surface Temperature (SST), SST range, salinity, salinity range, primary productivity and shelf area), and one macroecological proxy (median latitudinal range of species) were assessed using a random forest model. The taxonomic structure was used to estimate the degree of niche conservatism of predictor variables and to estimate latitudinal trends in phylogenetic diversity, based on three indices (phylogenetic richness (PDSES), mean pairwise distance (MPDSES), and variation of pairwise distances (VPD)). The LDG exhibits a hump-shaped trend, with a maximum peak of species richness at ca. 42° S, declining towards northern and southern areas of SEP. The latitudinal pattern was also evident in local samples controlled by sampling effort. The random forest model had a high accuracy (pseudo-r2 = 0.95) and showed that the LDG could be explained by four variables (median latitudinal range, SST, salinity, and SST range), yet the functional relationship between species richness and these predictors was variable. A significant degree of phylogenetic conservatism was detected for the median latitudinal range and SST. PDSES increased toward the southern region, whereas VPD showed the opposite trend, both statistically significant. MPDSES has the same trend as PDSES, but it is not significant. Our results reinforce the idea that the south Chile fjord area, particularly the Chiloé region, was likely the evolutionary source of new species of marine polychaetes along SEP, creating a hotspot of diversity. Therefore, in the same way as the canonical LDG shows a decline in diversity while moving away from the tropics; on this case the decline occurs while moving away from Chiloé Island. These results, coupled with a strong phylogenetic signal of the main predictor variables suggest that processes operating mainly at evolutionary timescales govern the LDG.

Filling gaps on the diversity and biogeography of Chilean millipedes (Myriapoda: Diplopoda)

“…Many macro- (e.g. Villagrán and Hinojosa 1997; Samaniego and Marquet 2009; Moreno et al 2021) and microorganisms (i.e. protists; Fernández et al 2016; Campello-Nunes et al 2022) also exhibit a bell-shaped latitudinal diversity gradient in Chile.…”

“…Driven by these historical contingencies many northern and southern mesophile taxa became extinct or migrated to the temperate climate of central Chile, where they subsequently diversified (Villagrán and Hinojosa 1997). Today, the diversity of most taxa peaks in mid-latitudes and then decreases towards low and high latitudes (Villagrán and Hinojosa 1997; Samaniego and Marquet 2009; Fernández et al 2015; Moreno et al 2021; Campello-Nunes et al 2022).…”

The latitudinal gradient of functional diversity of Miocene marine mollusks from Chile

“…R 2 = coefficient of determination; SE = standard error of the parameter; z-value: z statistic; p = probability of observing the statistic or larger by chance. diversity pattern (Valdovinos et al 2003, Rivadeneira et al 2011, Moreno et al 2021). However, those fjords were formed by the growth and retreat of glacial ice sheets during the Pleistocene, after the Miocene (Glasser et al 2008).…”

“…In some regions and for some taxa, the latitudinal gradient of biodiversity is inverse, or hump‐shaped and even bimodal (Navarrete et al 2014, Rivadeneira and Poore 2020, Yasuhara et al 2020, Chaudhary et al 2021, Moreno et al 2021, Yasuhara and Deutsch 2022). Examples of inverse biodiversity gradients come from freshwater arthropods, pinnipeds, sponges, bryozoans, anthozoans, polychaetes and Chilean marine mollusks (Valdovinos et al 2003, Krug et al 2007, Kaschner et al 2011, Morinière et al 2016, Beaugrand et al 2018).…”

Filling gaps in the diversity and biogeography of Chilean millipedes (Myriapoda: Diplopoda)

“…Driven by these historical contingencies many northern and southern mesophilic taxa became extinct or migrated to the temperate climate of central Chile, where they subsequently diversified (Villagrán and Hinojosa 1997). Today, the diversity of most taxa peaks in mid-latitudes and then decreases toward low and high latitudes (Villagrán and Hinojosa 1997;Samaniego and Marquet 2009;Fernández et al 2015;Moreno et al 2021;Campello-Nunes et al 2022).…”