Stream diatom biodiversity in islands and continents—A global perspective on effects of area, isolation and environment

Abstract

Aim The species–area relationship (SAR) is one of the most distinctive biogeographic patterns, but global comparisons of the SARs between island and mainland are lacking for microbial taxa. Here, we explore whether the form of the SAR and the drivers of species richness, including area, environmental heterogeneity, climate and physico‐chemistry, differ between islands and similarly sized areas on mainland, referred to as continental area equivalents (CAEs). Location Global. Taxon Stream benthic diatoms. Methods We generated CAEs on six continental datasets and examined the SARs of CAEs and islands (ISAR). Then, we compared CAEs and islands in terms of total richness and richness of different ecological guilds. We tested the factors contributing to richness in islands and CAEs with regressions. We used structural equation models to determine the effects of area versus environmental heterogeneity, climate and local conditions on species richness. Results We found a non‐significant ISAR, but a significant positive SAR in CAEs. Richness in islands was related to productivity. Richness in CAEs was mainly dependent on area and climate, but not directly on environmental heterogeneity. Species richness within guilds exhibited inconsistent relationships with island isolation and area. Main conclusions Ecological and evolutionary processes shaping diatom island biogeography do not depend on area at the worldwide scale probably due to the presence of distinct species pool across islands. Conversely, area was an important driver of diatom richness in continents, and this effect could be attributed to dispersal. Continents had greater richness than islands, but this was a consequence of differences in environmental conditions such as specific island climatic conditions. We stress the need for more island data on benthic diatoms, particularly from archipelagos, to better understand the biogeography of this most speciose group of algae.

Revisiting global diversity and biogeography of freshwater diatoms: New insights from molecular data

“…Teittinen and Soininen (2015) on the other hand did not find such relation in Finish spring diatoms. Finally, a recent study (Jamoneau et al, 2022) also found a higher diversity of stream diatoms in continents compared to islands. The authors explain this difference by environmental characteristics of the islands (e.g.…”

“…This is a hypothesis that we cannot explore here because of the lack of environmental data. It should nevertheless be noted that the diversity studied by Jamoneau et al (2022) is a cumulative richness over 15 sites (i.e. a regional gamma diversity integrating space), whereas we focus here on alpha diversity (average richness per site).…”

“…Over the last decades, the biogeography of diatoms has been intensively discussed and fueled by numerous results obtained at local and regional scales, and more and more results based on the compilation of combined large‐scale data have been published (e.g. Jamoneau et al, 2022; Soininen et al, 2016). Still, technical limitations hindered scientists from fully understanding geographical patterns at larger scales and their explanation.…”

Stream diatom community assembly processes in islands and continents: A global perspective

“…These data consist of benthic (epilithic and epiphytic) diatoms collected from a total of 19 islands and six continental areas covering 5440 sites in total (Table 1). The procedures for sampling, counting, and identifying diatoms are described in detail in Jamoneau et al (2022). While sampling aimed to comprehensively cover entire islands, this was not feasible in certain cases due to the absence of suitable stream sites.…”

“…In addition, the island data were subsampled by pulling 15 samples at random except for Possession Island, Hawaii and Kauai, which were not randomized due to their smaller sample size. Out of the total number of 833 CAEs generated by Jamoneau et al (2022), in this study we use only 448 CAEs as this was the number corresponding to the continental-island pair data from America, Europe, and Africa datasets (note that for China and Pacific datasets, there were no CAEs data as these datasets exclusively comprised continental and island data, respectively). In addition, area of island and CAEs, and island isolation data were extracted from Jamoneau et al (2022).…”