Snow depth drives habitat selection by overwintering birds in built-up areas, farmlands and forests

Aim Abundances of animals vary according to species-specific habitat selection, but habitats are undergoing rapid change in response to anthropogenic alterations of land use and climate. The long-term decline of snowfall is one of the most dramatic abiotic changes in boreal regions, with potential to alter species communities and shape future ecosystems. However, the effects of snow cover on habitat-specific abundances remain unclear for many taxa. Here we explore whether long-term declines in snow cover affect the abundances of overwintering birds. Taxon Fifty bird species. Location Finland, Northern Europe. Methods We used generalized linear mixed models to analyse citizen-led monitoring data from 196 transects over a 32-year period to assess whether abundances of birds have changed in built-up areas, farmlands and forests, and whether these covary with warming temperatures and decreasing snow. We then explored if changes in abundance can be explained by body mass, migration strategy or feeding guilds of the species. Results Over the study period, the abundance of overwintering birds increased. This increase was most pronounced in farmlands (69.6%), where abundances were positively associated with decreasing snow depth. On the other hand, while abundances in built-up habitats (19.5%) decreased over the study period, they increased in periods of high snow depths. Finally, we found that the short-distance migration strategy explains changes in bird abundances with snow. In farmlands, ground feeding birds and heavier birds also show a positive trends in abundance with decreasing snow depths. Main conclusions Local snow conditions are driving habitat selection of birds in the winter; birds in farmlands were most responsive to a decrease in snow depth. Changing snow depths can affect bird movements across habitats in the winter, but also influence migratory patterns and range shifts of species.Peer reviewe

Range shifts of overwintering birds depend on habitat type, snow conditions and habitat specialization

Climatic warming is forcing species to shift their ranges poleward, which has been demonstrated for many taxa globally. Yet, the influence of habitat types on within- and among-species variations of distribution shifts has rarely been studied, especially during the non-breeding season. Here, we investigated habitat-specific shift distances of northern range margins and directions of the distribution center based on long-term data of overwintering birds in Finland. Specifically, we explored influences of habitat type, species' snow depth tolerance, species' climatic niche and habitat specialization on range shifts during the past 40 years in 81 bird species. Birds overwintering in arable land shifted more clearly toward north compared to birds of the same species in rural and forest habitats, while the northern range margin shift distances did not significantly differ among the habitat types. Range shifts were more linked with the species' snow depth tolerance rather than species' climatic niche. Snow depth tolerance of species was negatively associated with the eastward shift direction across all habitats, while we found habitat-specific patterns with snow depth for northward shift directions and northern margin shift distances. Species with stronger habitat specializations shifted more strongly toward north as compared to generalist species, whereas the climatic niche of bird species only marginally correlated with range shifts, so that cold-dwelling species shifted longer distances and more clearly eastward. Our study reveals habitat-specific patterns linked to snow conditions for overwintering boreal birds and highlights the importance of habitat availability and preference in climate driven range shifts.Peer reviewe

Native fauna interact differently with native and alien trees in a tropical megacity

The negative effects of invasive alien plant species on natural ecosystems are well known. However, in rapidly growing cities, alien plants can provide native fauna with resources otherwise lost due to the biotic homogenization, which is common to urban ecosystems. Interactions of native fauna with alien flora have thus far focused largely on invertebrate pollinators in temperate cities in the northern hemisphere. Cities in tropical areas, however, are larger and are growing more rapidly, and host a variety of vertebrate pollinators. Understanding how birds and mammals interact with native and alien flora in these megacities could improve management of urban ecosystems in highly biodiverse regions while limiting invasion potential. Therefore, here we investigate whether native diurnal birds and mammals interact differ-ently with native versus alien trees in Bengaluru, India where historical planting has led to an abundance of alien tree species. We find that tree origin alone was not an important predictor for bird species richness and abundance, but taller native trees with large floral display sizes were more species rich than alien trees of similar floral displays. As expected from their shared evolutionary history, nectarivorous birds fed from native trees more often in a manner that could facilitate pollination, but engaged in nectar theft more often with alien trees. Squirrels (the mammal ob-served most frequently to interact with flowers) were more likely, however, to depredate flowers of native trees. Our results suggest alien trees can be an important resource for fauna in expanding urban areas, and that nectar theft by birds could reduce the seed set of alien trees.Peer reviewe

Alien Species, Warming Climates, Growing Cities, and the Birds that live with them.

In the Anthropocene, it is difficult to find natural systems that are not directly or indirectly affected by the actions of humans. Species with which we share our planet have to adapt to the changes humans introduce to survive. Human actions not only change abiotic components of the environment but also biotic components, such as the introduction or removal of species from habitats, resulting in altered ecosystem functioning. Even though we now understand the changes needed to curb the ill effects of human actions on natural environments, we are still progressively crossing thresholds set to protect the environment from anthropogenic effects. To prevent loss of biodiversity and maintain healthy ecosystems in the future, we need to do research where most of these disturbances are introduced acutely. In this thesis, I use birds as a study taxon to investigate how biodiversity is reacting to anthropogenic changes introduced on local and global scales. I consider two mechanisms of change operating at different scales, anthropogenic climate change and urbanisation. In chapters I and II, I investigate the effects of decreasing snow depth, due to anthropogenic climate change, on the abundances and range shifts of overwintering birds, respectively. I especially focus on the role of interactions between habitats and changing environmental variables in explaining the shuffling of overwintering bird communities. While shifts in the distributions of organisms and their abundances are well studied, in chapters III and IV, I focus on biotic interactions of organisms in the face of urbanisation and climate change, which are less explored. In particular, I consider ecosystems at a city level and explore the interactions of birds with native and alien plants in two very different cities facing similar problems: Helsinki, Finland and Bengaluru, India. Like many other cities of the world, Helsinki and Bangalore host several alien plant species, including, but not limited to, invasive species. Helsinki, at 60° N, faces the brunt of the consequences of a warming climate and the associated changes in weather. Here, frugivorous birds can act as vectors for alien plants to spread into novel habitats, amplified by change in phenologies brought on by warmer autumns. In Bengaluru, with rapid urbanization, avian pollinators in the city are quickly losing habitat and food resources. Here, alien plants could replace the native species that were historically present, allowing biodiversity to be maintained in the city. I show here how decreased snow depth due to the warming climate has resulted in an increase in the abundance of overwintering birds in Finland and how this also results in birds moving further poleward. More importantly, however, in chapters I and II, I find that birds are responding to climatic factors differently in different habitats, with the largest changes and shifts taking place on farmlands. Additionally, while a majority of the literature on the effects of warming climates focuses on the effects of temperature, here I show that snow cover could be playing an equally, if not more, important role in shaping winter bird communities. In chapter III, in Helsinki, I found that more birds are feeding on native fruit, which are more abundant in streetscapes despite there being more species of alien plants in the same habitats. However, there is no difference between native and alien species in the number of seeds dispersed. Finally, in chapter IV, I show that the alien trees in Bengaluru are an important resource for fauna in the city and they are unlikely to spread outside the city due to pollination by birds, as avian pollinators were more likely to engage in nectar theft from alien trees. The results of chapter III and IV show that alien species can provide resources to urban fauna with varying levels concerning the risk of dispersal. However, the species that have been seen to escape in small numbers need special attention from future research. The results of my thesis highlight the importance of studying the interactive effect of land use and climate change on global and local scales to aid in finding solutions that ensure we maintain healthy ecosystems in the future.Antroposeenikaudella ihmistoiminnan vaikutukset ulottuvat lähes kaikkiin ekosysteemeihin, ja lajien täytyy sopeutua ihmisten aiheuttamiin muutoksiin selviytyäkseen. Torjuaksemme nopeasti etenevää luonnon monimuotoisuuden katoamista ja ylläpitääksemme terveitä ekosysteemejä, tutkimusta on kohdistettava osa-alueisiin, joihin toimintamme seuraukset kohdistuvat akuuteimmin. Väitöskirjassani tutkin kuinka biodiversiteetti reagoi ihmisen aiheuttamiin muutoksiin globaalilla ja paikallisella tasolla käyttäen lintuja tutkimuskohteena. Keskityn kahteen mekanismiin, jotka toimivat eri tasoilla: ihmisen aiheuttamaan ilmastonmuutokseen ja urbanisaatioon. Kappaleissa I ja II tutkin ihmisen aiheuttaman ilmastonmuutoksen myötä vähenevän lumensyvyyden vaikutuksia talvehtivien lintujen runsauteen ja levinneisyyden muutoksiin. Keskityn habitaattien ja muuttuvien ympäristötekijöiden vuorovaikutuksiin talvehtivien lintuyhteisöjen muutoksia selittävänä tekijänä. Kappaleissa III ja IV paneudun harvemmin tutkittuihin elollisten organismien välisiin vuorovaikutuksiin urbanisaation ja ilmastonmuutoksen aiheuttamien paineiden alaisuudessa. Keskityn erityisesti kaupunkeihin ekosysteemeinä ja tutkin lintujen vuorovaikutuksia paikallisten ja vierasperäisten kasvilajien välillä Helsingissä (Suomi) ja Bengalurussa (Intia). Molemmissa kaupungeissa kasvaa useita vierasperäisiä kasvilajeja, joista osa on haitallisia vieraslajeja. 60° N sijaitseva Helsinki kohtaa lämpenevän ilmaston aiheuttamia muutoksia; kaupungissa hedelmiä syövät linnut voivat auttaa vieraslajeja leviämään uusiin elinympäristöihin lämpenevien syksyjen tehostamina. Bengalurussa vauhdikas urbanisaatio puolestaan vähentää pölyttävien lintujen elinympäristöjä ja ruokaresursseja; täällä vieraslajit saattavat korvata paikalliset lajit mahdollistaen monimuotoisuuden säilymisen kaupungissa. Osoitan väitöskirjassani, kuinka ilmastonmuutoksen myötä vähentynyt lumensyvyys on kasvattanut talvehtivien lintujen määrää Suomessa ja lisännyt lintujen pohjoisempaa levinneisyyttä. Ennen kaikkea osoitan kappaleissa I ja II muuttuvien ilmastotekijöiden vaikutuksen lintuihin olevan elinympäristöstä riippuvaa: suurimmat muutokset runsaudessa ja levinneisyydessä tapahtuvat viljelysmailla. Löydän myös todisteita lumipeitteen merkittävästä vaikutuksesta talvehtivien lintujen yhteisöihin lämpötilan vaikutusten ohella. Kappaleessa III esitän useampien lintujen käyttävän paikallisten lajien hedelmiä ravintonaan suhteessa vieraslajien hedelmiin Helsingissä. Kaupunkialueella esiintyy paikallisia hedelmiä enemmän, mutta vieraslajien lajirikkaus on runsaampaa. Huolimatta eroista ruokailussa, levitettyjen siementen määrissä ei näy eroja paikallisten lajien ja vieraslajien välillä. Kappaleessa IV esitän vierasperäisten puulajien toimivan tärkeänä resurssina paikallisille lajeilla Bengalurussa. Lajien riski levitä lintujen pölytyksen myötä kaupungin ulkopuolelle on pieni, sillä pölyttävät linnut varastavat mettä todennäköisemmin vierasperäisten lajien kukista. Yhdessä kappaleiden III ja IV tulokset osoittavat vieraslajien voivan tarjota resursseja eläinlajeille urbaaneissa ympäristöissä lajien levinneisyyden uhan vaihdellessa. Pienissäkin määrin leviämään onnistuneisiin vieraslajeihin on syytä perehtyä tulevissa tutkimuksissa. Väitöskirjani tulokset korostavat maankäytön ja ilmastonmuutoksen vuorovaikutusten tutkimuksen tärkeyttä eri mittakaavoissa, jotta voimme löytää ratkaisuja terveiden ekosysteemien säilyttämiseen tulevaisuudessa