Factors contributing to the spatial variation of stream communities in boreal and tropical regions

Freshwaters harbour disproportionally high biodiversity in relation to their area. Yet, they are among the most threatened ecosystems on Earth due to the increasing anthropogenic pressures. The ongoing climate change and Holocene extinctions give rise to an increasing need for conservation efforts in order to avoid future species losses. Successful conservation attempts require a comprehensive understanding how species and communities are distributed in space and time. There are notable gaps in the knowledge of diversity gradients of hyperdiverse microbes and invertebrates, and this knowledge gap is more pronounced in the tropics. This thesis seeks to 1) investigate the effects of local environmental, catchment characteristics, spatial and climatic factors on stream communities across sites and catchments; 2) examine benthic diatom diversity patterns and the underlying factors within and among streams and stream orders; 3) examine the effect of environmental heterogeneity on benthic diatom beta diversity and; 4) compare the diversity patterns of stream diatoms and insects between boreal and tropical regions. To accomplish these aims, microbial and insect communities were collected from boreal and tropical regions and the underlying causes of community spatial variation were investigated using advanced statistical methods. The thesis demonstrated that stream communities are driven by a range of factors acting on multiple spatial scales. Water chemistry, stream physical variables, biotic interactions, land use, spatial and climatic factors contributed to the variation in stream community composition and taxonomic richness. Diatom community composition exhibited significant within- and among-stream variation at intermediate spatial scales, which has relevance for biomonitoring using diatoms. Headwater streams exhibited higher beta diversity and harboured regionally unique diatom communities, which encourages the conservation of headwater streams. It was further demonstrated that environmental heterogeneity promotes diatom beta diversity, which emphasizes the role of habitat heterogeneity in sustaining diverse communities. Somewhat surprisingly, diatom species richness was not higher in the tropics than in the boreal study region, and a notable number of diatom species were found from both regions. This implies that diatoms may not follow the traditional global latitudinal diversity gradient and further suggests that some diatom species exhibit global distributions. Insect genus richness was slightly higher in the tropics than in the boreal study region, whereas insect abundance was significantly higher in the boreal than the tropical region. The large within-region variation in insect genus richness and abundance may be more strongly driven by factors operating at regional scale than by the region itself, further suggesting that streams exhibit uniqueness and do not fit well into predefined categories based purely on latitude. In summary, this thesis increases knowledge of the underlying variables affecting stream community variation. Further, biomonitoring and conservational efforts may benefit from the identified factors contributing to regional stream diversity. Finally, this thesis increases knowledge and understanding of the similarities and differences of stream communities across regions.Makeiden vesien kokonaispinta-alaan suhteutettuna niiden eliöstö on hyvin monimuotoinen. Väestönkasvun makeisiin vesiin kohdistama paine asettaa ne maapallon uhatuimpien ekosysteemien joukkoon. Ilmastonmuutos ja kuudes joukkosukupuutto korostavat suojelutoimien lisäämisen tarvetta tulevien sukupuuttojen välttämiseksi. Onnistuneet suojelutoimet vaativat kattavaa ymmärrystä lajien ja yhteisöjen maantieteellisestä levinneisyydestä. Mikroskooppisisten eliöiden ja selkärangattomien levinneisyyteen liittyvä tutkimus on ollut vähäistä verrattuna suurempiin eliöihin ja erityisen vähän näihin eliöihin kohdistuvia tutkimuksia on tehty tropiikissa. Tämän väitöskirja pyrkii 1) selvittämään paikallisten ympäristötekijöiden, maankäytön, spatiaalisten- ja ilmastotekijöiden vaikutusta virtavesien eliöyhteisöihin purojen sisällä ja niiden välillä; 2) tutkimaan pohjan piilevien monimuotoisuuden säännönmukaisuuksia ja niihin vaikuttavia tekijöitä erityyppisissä ja -kokoisissa virtavesissä; 3) tutkimaan ympäristötekijöiden heterogeenisyyden vaikutusta pohjan piilevien beta-diversiteettiin ja; 4) vertailemaan virtavesien pohjan piilevien ja selkärangattomien pohjaeläinten monimuotoisuuden säännönmukaisuuksia boreaalisilla ja trooppisilla alueilla. Näitä tavoitteita varten boreaaliselta ja trooppiselta alueelta kerättiin kattava mikroeliö- ja vesihyönteisaineisto, jonka jälkeen näiden yhteisöjen spatiaalisen vaihtelun taustalla olevia syitä tutkittiin monipuolisilla tilastotieteellisillä menetelmillä. Tämä väitöskirja osoitti, että virtavesiyhteisöihin vaikuttaa suuri määrä eri mittakaavoilla toimivia tekijöitä. Vesikemia, virtaveden fysikaaliset ominaisuudet, bioottiset vuorovaikutukset, maankäyttö sekä spatiaaliset- ja ilmastotekijät vaikuttivat virtavesien yhteisörakenteeseen ja lajirunsauteen. Piilevien yhteisörakenteessa havaittiin suurta vaihtelua purojen sisällä ja välillä, mikä tulee ottaa huomioon piileviä käyttävissä biomonitorointiohjelmissa. Etenkin latvapurojen piileväyhteisöjen lajisto oli hyvin ainutlaatuista ja piilevien beta-diversiteetti oli latvapuroissa suurempaa, kuin suurissa joissa. Tämä tulos korostaa latvapurojen suojelun merkitystä. Ympäristötekijöiden heterogeenisyys kasvatti piilevien beta-diversiteettiä, mikä puolestaan korostaa elinympäristön heterogeenisyyden merkitystä yhteisöjen monimuotoisuudelle. Piilevien lajirunsaus ei ollut trooppisella alueella boreaalista aluetta suurempi ja suuri määrä piilevälajeja löydettiin molemmilta alueilta. Tämä tulos viittaa siihen, ettei piilevien lajimäärä välttämättä noudata perinteistä leveysasteen mukaista gradienttia, jonka mukaan lajimäärä kasvaisi navoilta päiväntasaajalle siirryttäessä. Lisäksi tulos tukee näkemystä, jonka mukaan joidenkin piilevälajien levinneisyys on maailmanlaajuinen. Hyönteissukujen määrä oli jonkin verran suurempi tropiikissa kuin boreaalisella alueella, kun taas yksilömäärä oli huomattavasti suurempi boreaalisella alueella. Alueen sisäinen vaihtelu hyönteissuku- ja yksilömäärissä saattaa aiheutua enemmän alueellisista ympäristötekijöistä kuin alueesta itsestään, mikä viittaa siihen, että virtavedet ovat ominaispiirteiltään ainutlaatuisia, eivätkä välttämättä sovi leveysasteen mukaisiin keinotekoisiin ryhmiin. Väitöskirja lisäsi tietämystä virtavesien eliöyhteisöjen vaihteluun vaikuttavista tekijöistä. Alueelliset biomonitorointi- ja suojelutoimet saattavat hyötyä tunnistetuista virtavesien monimuotoisuuteen vaikuttavista päätekijöistä. Väitöskirja myös edisti virtavesiyhteisöjen alueiden välisten erojen ja samankaltaisuuksien ymmärtämystä

Tropical stream diatom communities – the importance of headwater streams for regional diversity

Peer reviewe

Does catchment geodiversity foster stream biodiversity?

Context One approach to maintain the resilience of biotic communities is to protect the variability of abiotic characteristics of Earth's surface, i.e. geodiversity. In terrestrial environments, the relationship between geodiversity and biodiversity is well recognized. In streams, the abiotic properties of upstream catchments influence stream communities, but the relationships between catchment geodiversity and aquatic biodiversity have not been previously tested. Objectives The aim was to compare the effects of local environmental and catchment variables on stream biodiversity. We specifically explored the usefulness of catchment geodiversity in explaining the species richness on stream macroinvertebrate, diatom and bacterial communities. Methods We used 3 geodiversity variables, 2 land use variables and 4 local habitat variables to examine species richness variation across 88 stream sites in western Finland. We used boosted regression trees to explore the effects of geodiversity and other variables on biodiversity. Results We detected a clear effect of catchment geodiversity on species richness, although the traditional local habitat and land use variables were the strongest predictors. Especially soil-type richness appeared as an important factor for species richness. While variables related to stream size were the most important for macroinvertebrate richness and partly for bacterial richness, the importance of water chemistry and land use for diatom richness was notable. Conclusions In addition to traditional environmental variables, geodiversity may affect species richness variation in streams, for example through changes in water chemistry. Geodiversity information could be used as a proxy for predicting stream species richness and offers a supplementary tool for conservation efforts.peerReviewe

Subtropical streams harbour higher genus richness and lower abundance of insects compared to boreal streams, but scale matters

Aim: Biological diversity typically varies between climatically different regions, and regions closer to the equator often support higher numbers of taxa than those closer to the poles. However, these trends have been assessed for a few organism groups, and the existing studies have rarely been based on extensive identical surveys in different climatic regions. Location: We conducted standardized surveys of wadeable streams in a boreal (western Finland) and a subtropical (south-eastern Brazil) region, sampling insects identically from 100 streams in each region and measuring the same environmental variables in both regions. Taxon: Aquatic insects. Methods: Comparisons were made at the scales of local stream sites, drainage basins and entire regions. We standardized the spatial extent of the study areas by resampling regional richness based on subsets of sites with similar extents. We examined differences in genus richness and assemblage abundance patterns between the regions using graphical and statistical modelling approaches. Results: We found that while genus accumulation and rank-abundance curves were relatively similar at the regional scale between Finland and Brazil, regional genus richness was higher in the latter but regional abundance much higher in the former region. These regional patterns for richness and abundance were reproduced by basin and local genus richness that were higher in Brazil than in Finland, and assemblage abundance that was much higher in Finland than in Brazil. The magnitude of the difference in genus richness between Brazil and Finland tended to increase from local through basin to regional scales. Main conclusions: Our findings suggest that factors related to evolutionary diversification might explain differences in genus richness between these two climatically different regions, whereas higher nutrient concentrations of stream waters might explain the higher abundance of insects in Finland than in Brazil.Peer reviewe

Beta diversity of stream insects differs between boreal and subtropical regions, but land use does not generally cause biotic homogenization

Previous studies have found mixed results regarding the relationship between beta diversity and latitude. In addition, by influencing local environmental heterogeneity, land use may modify spatial taxonomic and functional variability among communities causing biotic differentiation or homogenization. We tested 1) whether taxonomic and functional beta diversities among streams within watersheds differ between subtropical and boreal regions and 2) whether land use is related to taxonomic and functional beta diversities in both regions. We sampled aquatic insects in 100 subtropical (Brazil) and 100 boreal (Finland) streams across a wide gradient of land use, including agriculture and exotic planted, secondary, and native forests. We calculated beta diversity at the watershed scale (among 5 streams in each watershed). We found higher taxonomic beta diversity among subtropical than among boreal streams, whereas functional beta diversity was similar between the 2 regions. Total land use was positively correlated with taxonomic and functional beta diversity among subtropical streams, while local environmental heterogeneity was positively correlated with beta diversity among boreal streams. We suggest that different types and intensities of land use may increase among-stream heterogeneity, promoting distinct insect assemblage compositions among streams. Our findings also suggest that beta diversity patterns and their underlying determinants are highly context dependent

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

[EN] Understanding the roles of deterministic and stochastic processes in community assembly is essential for gaining insights into the biogeographical patterns of biodiversity. However, the way community assembly processes operate is still not fully understood, especially in oceanic islands. In this study, we examine the importance of assembly processes in shaping diatom communities in islands and continents, while also investigating the influence of climate and local water chemistry variables on species distributions. Location Global. Taxon Stream benthic diatoms. Methods We used diatom datasets from five continents and 19 islands and applied beta diversity analyses with a null model approach and hierarchical joint species distribution modelling. To facilitate comparisons with continents, we used continental area equivalents (CAEs), which represent continental subsets with comparable areas and the same number of study sites as their corresponding islands counterparts. Results We found that homogeneous selection (i.e., communities being more similar than the random expectation) was the dominant assembly process within islands whereas stochastic processes tended to be more important within continents. In addition, assembly processes were influenced by study scale and island isolation. Climatic variables showed a greater influence on species distribution than local factors. However, in islands, local environmental variables had a greater impact on the distributions of unique taxa as opposed to non-unique taxa. Main Conclusions We observed that the assembly processes of diatom communities were complex and influenced by a combination of deterministic and stochastic forces, which varied across spatial scales. In islands, there was no universal pattern of assembly processes, given that their influence depends on abiotic conditions such as area, isolation, and environmental heterogeneity. In addition, the sensitivity of species occurring uniquely in islands to local environmental variables suggests that they are perhaps less vulnerable to climatic changes but may be more influenced by changes in local physicochemistrySIFor financial support, the authors thank the Academy of Finland (grant nr. 346812 to JS); the Institut Francais de Finlande; the Embassy of France to Finland; the French Ministry of Education and Higher Education; Finnish Society of Sciences and Letters. J.J. Wang was further supported by the National Natural Science Foundation of China (91851117, 41871048), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-DQC043), and The National Key Research and Development Program of China (2019YFA0607100

Distance decay 2.0-A global synthesis of taxonomic and functional turnover in ecological communities

Aim: Understanding the variation in community composition and species abundances (i.e., beta-diversity) is at the heart of community ecology. A common approach to examine beta-diversity is to evaluate directional variation in community composition by measuring the decay in the similarity among pairs of communities along spatial or environmental distance. We provide the first global synthesis of taxonomic and functional distance decay along spatial and environmental distance by analysing 148 datasets comprising different types of organisms and environments. Location: Global. Time period: 1990 to present. Major taxa studied: From diatoms to mammals. Method: We measured the strength of the decay using ranked Mantel tests (Mantel r) and the rate of distance decay as the slope of an exponential fit using generalized linear models. We used null models to test whether functional similarity decays faster or slower than expected given the taxonomic decay along the spatial and environmental distance. We also unveiled the factors driving the rate of decay across the datasets, including latitude, spatial extent, realm and organismal features. Results: Taxonomic distance decay was stronger than functional distance decay along both spatial and environmental distance. Functional distance decay was random given the taxonomic distance decay. The rate of taxonomic and functional spatial distance decay was fastest in the datasets from mid-latitudes. Overall, datasets covering larger spatial extents showed a lower rate of decay along spatial distance but a higher rate of decay along environmental distance. Marine ecosystems had the slowest rate of decay along environmental distances. Main conclusions: In general, taxonomic distance decay is a useful tool for biogeographical research because it reflects dispersal-related factors in addition to species responses to climatic and environmental variables. Moreover, functional distance decay might be a cost-effective option for investigating community changes in heterogeneous environments

Community size can affect the signals of ecological drift and niche selection on biodiversity

Ecological drift can override the effects of deterministic niche selection on small populations and drive the assembly of some ecological communities. We tested this hypothesis with a unique data set sampled identically in 200 streams in two regions (tropical Brazil and boreal Finland) that differ in macroinvertebrate community size by fivefold. Null models allowed us to estimate the magnitude to which beta-diversity deviates from the expectation under a random assembly process while taking differences in richness and relative abundance into account, i.e., beta-deviation. We found that both abundance- and incidence-based beta-diversity was negatively related to community size only in Brazil. Also, beta-diversity of small tropical communities was closer to stochastic expectations compared with beta-diversity of large communities. We suggest that ecological drift may drive variation in some small communities by changing the expected outcome of niche selection, increasing the chances of species with low abundance and narrow distribution to occur in some communities. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities. These environmental pressures might make smaller communities more vulnerable to novel conditions and render community dynamics more unpredictable. Incorporation of community size into ecological models should provide conceptual and applied insights into a better understanding of the processes driving biodiversity.peerReviewe

Local environment and space drive multiple facets of stream macroinvertebrate beta diversity

Abstract Aim: Understanding variation in biodiversity typically requires consideration of factors operating at different spatial scales. Recently, ecologists and biogeographers have recognized the need of analysing ecological communities in the light of multiple facets including not only species‐level information but also functional and phylogenetic approaches to improve our understanding of the relative contribution of processes shaping biodiversity. Here, our aim was to disentangle the relative importance of environmental variables measured at multiple levels (i.e., local, catchment, climate, and spatial variables) influencing variation in macroinvertebrate beta diversity facets (i.e., species, traits, and phylogeny) and their components (i.e., replacement and abundance difference) in boreal streams. Taxon: Aquatic macroinvertebrates Location: Western Finland Methods: A total of 105 streams were sampled in western Finland, encompassing a geographical extent over 500 km. We analysed variation in the different beta diversity facets and components using distance‐based redundancy analysis and associated variation partitioning procedures. We modelled spatial structures using distance‐based Moran eigenvector maps. Results: We found that the relative influence of explanatory variables on each diversity facet and component revealed relatively similar patterns. Our main finding was that local environmental and spatial variables generally contributed most to the total explained variability in all facets and components of beta diversity, whereas catchment and climate variables explained less variation in the beta diversity facets at the spatial scale considered in this study. Main conclusions: Different facets of beta diversity were mainly influenced by local environmental variables and spatial structuring, likely acting through deterministic and stochastic pathways respectively. Identifying the ecological variables and mechanisms that drive variation in beta diversity may be used to guide the conservation and restoration efforts for biodiversity under global change