BALTIC SHORELANDS FACING CLIMATE CHANGE. Alma Strandmark

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3 BALTIC SHORELANDS FACING CLIMATE CHANGE Alma Strandmark

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5 Baltic shorelands facing climate change Alma Strandmark

6 Alma Strandmark, Stockholm University 2017 Cover: Shoreland by Gustaf Almqvist ISBN print ISBN PDF Printed in Sweden by US-AB, Stockholm 2017 Distributor: Department of Ecology, Environment and Plant sciences, Stockholm University

7 To my family

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9 Abstract This thesis provides new insight concerning drivers behind differences in arthropod diversity and abundance in Baltic shore ecosystems and how the arthropod communities might be affected when the conditions in the Baltic Sea are altered due to climate change. The focus has been on climate related changes that are unique for coastal ecosystems, especially sea level rise and changes in the inflow of marine nutrients. As sea levels rise, features in coastal landscapes will be altered, islands and habitats will be flooded and diminished, and structural connectivity within the island landscape will therefore change. This thesis shows that arthropod diversity within the two arthropod groups, spiders and beetles, increases with island size but also that diversity is positively influenced by a high number of islands in the surroundings. A changed distribution and occurrence of marine species, due to climate change or eutrophication, can also affect terrestrial organisms on the shore. In the Baltic Sea the new conditions following climate change will decrease the prevalence of bladder-wrack and benefit filamentous algae. Algal deposits on shores reflect the marine species composition and a decreased prevalence of bladder-wrack in the Baltic Sea will also be visible on the shores. This thesis shows that a lower proportion of bladder-wrack in the algal deposits would decrease the diversity and abundance of arthropods in these deposits. Changes in the marine environment may also affect the inflow of insects with aquatic life stages and terrestrial adult stages. On Baltic shores, prey species with aquatic life stages, especially chironomids, constitute a large proportion of the diet of the terrestrial predatory group, wolf spiders. In freshwater system, the inflow of chironomids is known to decrease at high water temperatures if this is true in the Baltic Sea prey availability for wolf spiders would decrease. This thesis supports the importance of chironomids as a prey for coastal wolf spiders, but also shows that the diet varies over season with dominance of terrestrial prey in early summer shifting to a dominance of marine prey in late summer and autumn. This seasonal variation is primarily due to a gradual increase in the consumption of chironomids over season. Climate change has the potential to alter the biogeographical conditions in coastal landscapes as well as the density and quality of marine nutrient inflow. Sea level rise will diminish and flood islands and this thesis shows that

10 a moderate sea level rise of 0.5 meters would make the total number of islands in the outer part of Stockholm archipelago decrease with about 25 %. Sea level rise could thus have consequences for arthropod diversity in Baltic shore meadows in the near future. The combined effects of sea level rise and changed prevalence of marine species in the Baltic Sea will affect the abundance and diversity of arthropods substantially. The abundance and diversity of spiders and beetles will decrease on shores that today have a high occurrence of bladder-wrack and prey availability for coastal predators might decrease due to a decreased inflow of chironomids. Changes in the arthropod communities could have consequences also further up in the food chain, such as for shore birds feeding on these arthropods.

11 Swedish abstract Syftet med min avhandling har varit att öka kunskapen om Östersjöns strandekosystem och de leddjur som lever där, samt vilka effekter kommande klimatförändringar kommer att ha i dessa ekotoner. Jag har fokuserat på de klimatrelaterade problem som är unika just för strandekosystem, så som höjda havsnivåer och förändrat inflöde av marina näringsämnen till stränderna. När havsnivåerna stiger förändras det kustnära landskapet, öar och strandekosystem kommer antingen minska i storlek eller helt försvinna under vattenytan och konnektiviteten mellan habitat och öar kommer att förändras. Jag visar i min avhandling att artdiversiteten hos spindlar och skalbaggar har ett positivt samband med östorlek men även att ett stort antal öar i omgivningen ökar diversiteten. Terrestra växter och djur som lever i strandzonen kommer också att påverkas av förändringar i utbredning och abundans av marina arter. Enligt rådande klimatmodeller kommer Östersjön att få lägre salthalt, högre vattentemperatur och ökad övergödning vilket i sin tur skulle innebära att blåstången troligen kommer att begränsas både i sin utbredning och täckningsgrad medan fintrådiga alger i stället kommer att gynnas av de nya förhållandena. Jag visar i min avhandling att både artdiversitet och abundans av strandlevande artropoder är högre i blåstångsvallar än i algbäddar som utgörs av fintrådiga alger. En annan typ av näringsinflöde från marina till terrestra miljöer som kan komma att påverkas av ett förändrat klimat är inflödet av insekter med akvatiska livsstadier. Chironomider (fjädermyggor) utgör en stor del av detta inflöde till Östersjöns stränder och de utgör en betydande del av födan för strandlevande predatorer så som vargspindlar. Studier i sötvatten har visat att inflödet av chironomider minskar markant vid höga vattentemperaturer. Om de chironomider som lever i Östersjöns reagerar på ett liknande sätt på höga vattentemperaturer skulle abundansen av en viktig bytesgrupp för vargspindlar minska kraftigt. Mina resultat styrker att chironomider utgör en viktig del i kustnära vargspindlars diet, men jag visar också på en säsongsvariation i spindlarnas födoval. Terrestra bytesdjur dominerar under försommaren medan den marina dominansen är störst under sensommar och tidig höst. Denna säsongsvariation beror främst på en succesivt ökande dominans av chironomider i födan. Klimatförändringen kommer att medföra nya förutsättningar för kustlevande organismer både genom att landskapet förändras och genom förändringar i

12 inflödet av marin näring till terrestra strandekosystem. Höjda havsnivåer kommer att medföra att öar och habitat försvinner och att de minskar i storlek. Jag visar i min avhandling att en relativt liten höjning i havsnivån på 0,5 m skulle minska antalet öar i Stockholms mellan- och ytterskärgård med 25%. Dessa förändringar i landskapet skulle troligen medföra en markant förändring av artdiversiteten hos leddjur eftersom diversiteten påverkas både av östorleken och antalet öar i omgivningen. Kombinerat med effekterna av höjda havsnivåer kommer framtida förändringar av arters utbredning och abundans i Östersjön att ha stor påverkan på strandlevande terrestra organismer. Diversiteten och abundansen av skalbaggar och spindlar kommer att minska på stränder med mindre blåstång och bytestätheten för kustanknutna predatorer kan komma att minska på grund av ett minskat inflöde av chironomider. Minskad diversitet och abundans av leddjur kan också få konsekvenser högre upp i födoväven exempelvis för insektsätande strandfåglar. Min avhandling bidrar med ökad kunskap om vad som styr diversiteten och abundansen av leddjur i Östersjöns strandekosystem och hur de som lever där kan komma att påverkas av vad ett förändrat klimat innebär.

13 List of papers This thesis is based on the following four papers referred to in the text by their Roman numerals: Paper I: Strandmark A, Bring A, Cousins SA, Destouni G, Kautsky H, Kolb G, de la Torre-Castro M, Hambäck PA Climate change effects on the Baltic Sea borderland between land and sea. Ambio 44: Paper II: Strandmark A, Aggemyr E, Cousins SA, Hambäck PA. (Manuscript). Drivers behind local and regional arthropod diversity in naturally fragmented landscapes Paper III: Strandmark A, Vicente R, Hambäck PA. (Manuscript).The structure of marine deposits affects arthropod communities in southern Baltic Shore ecosystems Paper IV: Strandmark A, Verschut V, Esparza Salas R, Hambäck PA. (Manuscript). Seasonally varying marine influences on the coastal ecosystem detected through molecular gut analysis but not through stable isotope analysis My contribution to the papers: (I) in charge of the writing and a large part of the writing. (II) half of the fieldwork, all statistical analyses and writing. (III) experimental design and field work, all of the statistical analyses and writing. (IV) Experimental design and field work, laboratory work and preparation for stable isotope analysis, contributed to the writing. The published paper is reprinted with the kind permission of the publishers.

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15 Contents Abbreviations... xiv Introduction The Baltic Sea area Baltic shorelands Climate change in Baltic shorelands Sea level rise Changes in species distribution and prevalence of marine species Aims of the thesis Methods Study areas Study species Processes determining species composition and diversity in Baltic shorelands Seasonal variability in the marine component in the diet of terrestrial predators Climate change impact in Baltic shorelands Main findings and discussion Processes determining species composition and diversity in Baltic shorelands Seasonal variability in the marine component in the diet of terrestrial predators Climate change impact in Baltic shorelands Concluding remarks References Svensk sammanfattning Tack... 39

16 Abbreviations Arthropod: an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages (paired appendages). The arthropods included in this thesis are spiders and beetles. α-diversity: the species diversity in sites or habitats at a local scale. β-diversity: the difference in species composition between sites, here also estimated as the ratio between regional (γ) and local (α) species diversity. Ecotone: a transition area between two biomes where two communities meet and integrate. γ-diversity: the total species diversity in a landscape or a study system, also called regional diversity. Isostatic rebound: the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period. Prevalence: the fact of something happening or being present. Stable isotope: a non-radioactive isotope that is used as a biological tracer, in biology typically in food web analysis.

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18 Introduction The Baltic Sea area The Baltic Sea is a semi-enclosed, brackish water body with only a narrow canal, the Danish straits, connecting it with the North Sea. There are pronounced latitudinal gradients in temperature and salinity with close to freshwater conditions in the northern parts where the winters are long, cold and ice rich (Rabalais et al. 2009). In the southern parts of the Sea, the salinity is higher and the winters are mild and usually ice free. The present brackish state is very young and was preceded by different stages of marine and freshwater conditions following the last ice age years ago (Björck 1995). The Baltic Sea is relatively species poor and hosts very few brackish water specialists. The more marine conditions in the south favour species with marine origin which today are prevented to spread further north by the low salinity. The northern parts host many freshwater species which instead are unable to spread to the southern parts of the Sea since the salinity there is too high (Haatela 1974; Bergström and Bergström 1999). The drainage basin of the Baltic Sea is shared between 13 countries and in 2002, 84 million people lived here (Hannerz and Destouni 2006). The relatively small, semi-enclosed Sea is thus utilized by many people and the anthropogenic pressure has been high for a long time. As a consequence, the Sea is highly eutrophic, heavily polluted and the fish stocks are depleted (Lane and Jensen 1996). Baltic shorelands Since the last ice age 10,000 years ago, the Baltic Sea area has been under the influence of isostatic rebound as the land and the sea floor are rising after being pressed down by the ice (Ekman 1996). The isostatic rebound is not constant along the Baltic coast. The largest uplift is found in the northern parts, the Bothnian Sea, with an annual uplift of close to 1 cm, while the most southern coast is instead sinking (Jerling 1999). A history of glaciation, together with different magnitudes of land uplift and latitudinal differences in processes like ice scouring has created a heterogeneous coastline. The coastline in the southern parts of the Baltic Sea is typically flat with extensive sandy and stony beaches and shore meadows. In the central parts of the 16

19 Sea, we find the large archipelagos. The landscape is fragmented and characterized by rocky islands with small patches of forests, shrubs and grasslands. The Baltic Sea has about islands of which most are found in the central parts. In the northern part of the Sea the coastline varies between steep rocks with forest cover and flat shores with low vegetation and characteristic marks from ice scouring. Figure 1a,b and c shows the variation in coastal habitats along the Baltic coast. Figure 1 a. A typical coastal landscape in the southern part of Sweden, from the island of Öland (photo: Gundula Kolb). Figure 1 b. A typical coastal landscape from the central parts of the Baltic Sea with small habitats interspersed between rocky areas and water (Stockholm archipelago). 17

20 Figure 1 c. A typical coastal landscape from the northern part of the Baltic coast with low vegetation and visible marks of Ice scouring (photo: Peter Hambäck). Climate change in Baltic shorelands The focus of this thesis is how altered conditions in the marine environment due to climate change also influence terrestrial ecosystems and organisms in the vicinity. Sea level rise The first and obvious climate change related problem for shore ecosystems and their related organisms all over the globe is sea level rise. Global sea level rise is at present about 3 mm year -1 (Church and White 2011) but the rate is constantly increasing after doubling over the last two decades (Wiens 2016) and it is expected to accelerate even more in the near future (Grinsted et al. 2015). The effects of sea level rise will be intensified due to the projected larger fluctuations in sea levels, causing flooding into areas that currently do not experience any such effects. More frequent and long lasting flooding can have large consequences for coastal organisms and ecosystems, especially if the flooding happens during parts of the year when organism are extra sensitive, e.g. during the germination of coastal plants (Jerling 18

21 1999). The effects of sea level rise will be more severe for plant species with a short life span such as most herbaceous plants. As coastal ecosystems are flooded, these species will need to establish new populations in new areas further up the shore. Because forests are often bordering shore ecosystems with herbivorous vegetation and because the trees have a long life span, short lived species will have trouble establishing populations further up the shore where the slowly dying forests can persist for a long time. In the Baltic Sea area, the isostatic rebound is to some degree compensating for the sea level rise. The line where the current land uplift and the sea level rise are equal lies just south of Stockholm but with escalating sea level rise it will move further north (Strandmark et al. 2014). Due to compensation by isostatic rebound, sea level rise has been suggested to only increase with 80 % of the global mean in the Baltic Sea (Grinsted 2015). Isostatic rebound is not equal across the Baltic area and in the southern Baltic Sea, the combination of a sinking coastline and a sea level rise is already causing problems with erosion of coastal habitats. Erosion is a large problem related to sea level rise and particularly in areas where the shore consists of fine material. More rocky areas, like the archipelagos in the central Baltic Sea will be faced primarily with other problems as sea levels rise, such as habitat loss and changed connectivity between habitats. Baltic archipelagos consist of a habitat matrix dominated by water and bare rock. The small patches of grasslands, forests and shrubs are inhabited by insects, birds and small mammals. Species diversity within these isolated habitats is influenced by different ecological and biogeographical conditions such as island and habitat size and connectivity at different spatial scales. Rising sea levels will both diminish islands and habitats and change the connectivity between island and habitats and thus indirectly influence species distribution. Changes in species distribution and prevalence of marine species Shore ecosystems in general are highly influenced by an inflow of marine nutrients into the terrestrial environment (Polis and Hurd 1995). This inflow is especially important in shorelands where the terrestrial primary production is low, such as on sandy beaches. Nutrients reach the shores mainly in three ways; guano of sea birds (Kolb et al. 2010), marine plant material being flushed onto the shores, and insects with aquatic larvae that enter the terrestrial food chain as adults (Polis and Hurd 1995). As climate change alters the marine conditions and species composition in the Sea, the type of marine nutrients reaching the shores will also be altered. The effects of sea bird guano on terrestrial diversity are not included in my thesis, but have been the focus of earlier studies concerning the link between land and sea (Kolb 2010), and will not be further discussed. On the Baltic Sea shores, predatory arthropods like wolf spiders consume a large proportion of insects with aquatic larvae, especially chironomids 19

22 (Mellbrand and Hambäck 2010). If the abundance of chironomids would change due to altered conditions within the Baltic Sea then the predation pressure on other coastal prey species may also be influenced. I am not aware of any studies concerning climate effects on insects with aquatic larvae in the Baltic Sea, but in freshwater systems higher water temperatures have been shown to increase the body size within this arthropod group mostly caused by a decreased inflow of chironomids (Jonsson et al. 2015). Marine plant material that float ashore provide nutrients both for terrestrial plants and arthropods. Since the types of plant material that reach the shores reflect the species composition in marine plant communities, changes affecting the marine ecosystem will also be visible in terrestrial environments. However, while there are many studies concerning arthropod communities in wrack (Colombini et al. 2000; Dugan et al. 2003; Defeo et al. 2009) only a few deal with how the arthropod communities are influenced by the species composition of the inflowing marine plant material (Colombini et al. 2000). The largest marine deposits on Baltic shores consist of bladder-wrack (Fucus vesiculosus) and other Fucus-species, but many large algal beds, predominantly in the southern Baltic Sea, also consist of different species of filamentous algae. The difference between algal deposits consisting of bladder-wrack and filamentous algae is obvious to anyone visiting these shores. The structure within a Fucus deposit is coarse and complex while the filamentous algae produce a compact algal bed with little structural complexity. In the marine ecosystem, Fucus-species are key elements providing protection and food for a large number of associated organisms (Bäck et al. 1992; Wikström and Kautsky 2007), but future projections for the Baltic Sea include a drop in salinity which will push the distribution limit of many marine species like Fucus further south. Climate change further has the potential to reduce the biomass of Fucus due to a combination of higher water temperatures, higher CO 2 concentrations and a projected increased eutrophication (Werner et al. 2016). Filamentous, fast-growing algae instead benefit from the projected future conditions within the Baltic Sea (Bergström et al. 2003) and have the potential to further reduce the prevalence of Fucus by competing for space and by growing on Fucus. Moreover, a projected increased spread of invasive species like Gracilaria vermiculophylla, which have been shown to have several negative effects for the Fucus populations, might also cause a decrease in Fucus biomass (Weinberger et al. 2008). Thus, the large Fucus deposits on Baltic shores will likely be scarcer in the future and compact algal beds of filamentous algae will instead reach a higher prevalence. 20

23 Aims of the thesis The overall aim of this thesis was to investigate how shore ecosystems and the organisms living there will be influenced by altered conditions in the Baltic Sea due to climate change. To reach this general aim, the thesis also includes specific aims that provide insight about the processes and biogeographical conditions that are important to determine species composition and diversity of shore organisms today. The aims can be summarized as: 1. To conceptualize the current knowledge about the connection between land and sea in the Baltic Sea area and the unique influence that climate change will have in this borderland (paper I). 2. To increase the knowledge about the processes determining species composition and diversity in Baltic shore ecosystems, and especially their arthropod communities (paper II and III). 3. To increase the knowledge about the link between land and sea concerning the inflow of marine prey to terrestrial predators and the seasonal variability in the marine component in the diet of terrestrial predators (paper IV). 4. To investigate how a changed species composition and decreased prevalence of bladder-wrack in the Baltic Sea influences the diversity and species composition of shore-living arthropods (paper III). 5. To investigate how sea level rise will impact the Baltic Sea shorelands and the diversity (paper II) and prevalence of arthropods living there. 21

24 Methods Study areas The included papers concern a range of different shore ecosystems in the Baltic Sea area; the focus areas for the different papers are visualized in figure 2. The focal area for papers I and II was the Stockholm archipelago. The Stockholm archipelago is the second largest archipelago in the Baltic Sea encompassing over islands of different sizes, from large populated islands with forests and farmland, to small islets with little or no vegetation. Paper I is a concept paper which touches on climate change impacts on the whole Baltic Sea area, but with a main focus on the Stockholm archipelago. Paper II concerns one particular type of shore habitat within the habitat matrix of the archipelago, the shore meadow, defined as an area directly by or very close to the water line covered with herbaceous vegetation. Within this habitat, I focussed on the diversity of arthropods in 140 meadows distributed among 13 island landscapes in the outer part of Stockholm archipelago. The study area for paper IV was the island of Öland off the southern part of the Swedish coast. The sites for this study were spread across the eastern and western coastlines of the island to get a large spatial spread of the data collection. The study area for paper III also included some of the same sites on Öland, but this study also included corresponding sites with algal deposits in the southernmost part of Sweden, the county of Skåne. The sites for paper III were chosen based on the presence of algal deposits. 22

25 Figure 2. Map showing the study sites and focus areas of paper II, III and IV (map provided by Elsa Aggemyr). Paper III only included sites on Öland, while paper III also included the Skåne-sites. 23

26 Study species All included studies, except paper I which is a concept paper, focus on arthropods, primarily on the arthropod groups spiders (paper II, III and IV) and beetles (II and III). Spiders and beetles were chosen as study organism because they are common in coastal environments, because their feeding behaviour and habitat requirements are relatively well known and because they are comparatively easier to identify, compared to other common arthropod groups such as Diptera. In papers II and III, the two arthropod groups are divided into different subgroups based on functional diversity. Functional diversity was included since organisms have diverse habitat requirements and feeding strategies and thus inclusion of different functional groups can provide deeper knowledge about the species composition and ecosystem function. In paper II, spiders are divided into actively hunting spiders and web spiders whereas beetles are divided into the three subgroups herbivore beetles, predatory beetles and other beetles. The group actively hunting spiders is to the largest part composed of species from the family Lycosidae (wolf spiders), while the group web spiders predominantly consists of different species of Linyphiidae (sheet weavers). Paper III includes many of the same functional groups as paper II; spiders are divided into the groups actively hunting spiders and web spiders, but since the web spiders only contained species of sheet weavers, the group is instead named linyphid spiders. For functional diversity of beetles, I used the same groups in paper III as in paper II but in paper III I also included two additional subgroups; habitat specialists and habitat generalists in relation to coastal habitats. Habitat specialists imply species that are more or less only occurring in coastal areas while habitat generalists also occur in inland areas. Paper IV concerns the diet of spiders within the subgroup wolf spiders. Processes determining species composition and diversity in Baltic shorelands In paper II, I investigated the ecological and biogeographical predictors that explain differences in arthropod diversity at different spatial scales. To include spatial scale in ecological studies has large advantages since correlations with environmental predictors such as area-dependency are strongly connected to spatial scale (Crist et al. 2003, Gering et al. 2003). I used the concepts α-, β-, and γ-diversity, where α-diversity is the local, within-site diversity, γ-diversity is the regional diversity (the total diversity for all sampled sites on an island) and β-diversity is the divergence between the sampled species pools within or between islands. In the studies, γ-diversity was expressed as the total number of species found, while β-diversity was calcu- 24

27 lated both as γ/α and as Bray-Curtis similarity index where the latter express the ecological similarity between two species pools. Finally, α-diversity was expressed both as the number of species found at a site and as Shannon- Weavers and Simpsons diversity indices, where both indices are based on the relative abundance of different species. As predictive variables for α-, β-, and γ-diversity, I included island and habitat size, distances to potential source areas for colonization, local site conditions and information about the surrounding island landscape. The Bray-Curtis similarity index was tested against island size and the distance between the included habitats on each island, to determine if the size of the island or the isolation of the habitats on the island has the main influence on diversity. In paper III, I also investigated differences in the diversity and abundance of arthropods but now in algal deposits on southern Baltic shores. One of the main aims was to investigate if a changed prevalence of bladder-wrack in the Baltic Sea ecosystem might also influence diversity and species composition of terrestrial organisms associated with algal deposits. As predictive variables for diversity, I included the compactness of the deposit, the proportion of different types of marine plant material, the surrounding vegetation and the prey abundance. I defined diversity as the number of species present at the site (also commonly expressed as species richness) and the abundance as the number of individuals present at a site. Since sampling effort was uneven among sites, diversity was calculated with rarefaction analysis using the statistical software EstimateS version (Colwell 2013). Abundance was calculated as the number of individuals found divided by the number of active traps per site. Seasonal variability in the marine component in the diet of terrestrial predators In paper IV, I used a combination of stable isotope analysis and molecular gut content analysis to trace the seasonal variation in the diet of wolf spiders in coastal ecosystems over a full season. We aimed at revealing seasonal patterns of the use of insects with aquatic larvae as food for wolf spiders. Since spiders have external digestion, ordinary morphological gut content analysis cannot be performed (Foelix 1996), but the two methods used in the study provide information to the same end. Stable isotope analysis measures the ratios of different isotopes within a solid, liquid or gas. The enrichment of isotopes such as 18 O, 13 C, 15 N, 34 S and hydrogen (D/H) all provide information about food web dynamics and origin of organisms (Hobson 1999). Stable isotopes are commonly used in many scientific disciplines, e.g., by archaeologists to measure the age of the historic food sources of humans and animals (Schoeninger and Moore 1992) and by biologists to trace food webs 25

28 and nutrient flows within ecosystems as well as the trophic rank of different organisms (Post 2002). Stable isotopes can also be used to trace if the main food source of an organism has its origin in a marine or a terrestrial environment (Chisholm 1982), where especially the isotopes 13 C, 15 N and 34 S are relative more enriched in marine compared to terrestrial food-webs (Hobson 1999). Stable isotopes provide a cost effective way to get information about food webs, but the taxonomic resolution is poor and offers no information regarding the prey species identity. Molecular gut content analyses on the other hand have good taxonomic resolution and can provide the exact species identity of many groups of prey (e.g, Pompanon et al. 2012, Wirta et al. 2015). However, molecular gut content analyses are more work-intensive and costly, and do not provide the same information concerning the quantitative importance of different components within the diet as stable isotopes do. The two methods thus complement each other and together offer a good alternative to ordinary gut content analyses when such analyses cannot be performed. Climate change impact in Baltic shorelands One of the long-term objectives in paper II was to understand how arthropod diversity in Baltic shore ecosystems might be influenced by sea level rise. The approach in the paper was to investigate the environmental predictors driving arthropod diversity today and based on this knowledge it is possible to project how the arthropod communities might react when these environments change due to sea level rise. To investigate the effects on arthropod diversity, I wanted to quantify future the changes in the island landscape and therefor I projected sea level rise in a GIS (not included in any paper) in collaboration with another PhD-student, Elsa Aggemyr. We projected changes in the island landscape following different magnitudes of sea level rise (0.5, 1, 1.5, 2, 2.5 and 3 m). The projections included the outer part of Stockholm archipelago (defined by the marked area outside the line in Stockholm archipelago Figure 2). Since arthropod diversity in this part of the archipelago was connected mainly to island size and the total number of surrounding islands (paper II) we used mean island area, total island size and total number of islands in the area today and then we calculated changes in these landscape parameters following different magnitudes of sea level rise. A digital elevation model (DEM) with a resolution of 20 metres was built in GIS from which a polygon file was created to calculate island area. The DEM was also used to model sea level rise by subtracting values of the expected rise (0.5, 1, 1.5, 2, 2.5 and 3 m) using a raster calculator in GIS. The resulting DEM represents the island landscape after corresponding sea level rise, and is used to calculate the new island area, total island area and number of islands, for each sea level rise. 26

29 Main findings and discussion Processes determining species composition and diversity in Baltic shorelands In paper II, I found that the strongest predictor of diversity of both spiders and beetles in the Stockholm archipelago was island size. Island size had a positive relationship both to within-site diversity and to the diversity at the island level. The analysis of Bray-Curtis similarity between species pools indicated that the main reason for the increased -diversity was not the size of the island per se but rather that the study habitats on the larger islands were farther apart, which in turn increased the -diversity. Habitats more isolated from each other had larger differences in the species composition, probably due to lower exchange of species among the habitats within the island. However, the longer distances and the more different species pools on larger islands do not explain the higher within-site diversity on larger islands. A large -diversity could explain also the higher local diversity at larger islands as the availability of different species that potentially could establish is higher in the landscape. However, it is also possible that the local conditions within habitats are different depending on island size and larger islands might provide more suitable conditions for many species, through some aspect that was over-looked in my study. Regional diversity was also influenced by the total number of islands in the surrounding landscape, which in turn could be attributed to a difference in edge effects. An island landscape with a large number of islands will provide the islands in the central part of the island landscape with shelter from weather and wind while the islands at the edge will be more exposed. In island landscapes with very few islands there will be no central islands and thus all islands will be at the edge. The variability in local conditions on different islands will thus potentially be larger in a landscape with many islands ranging from sheltered conditions in the central parts and more exposed conditions at the edges. A large variability in local conditions can support species with a larger variance in habitat requirements and thus increase regional diversity. In paper III, I further found that the species diversity and abundance of arthropods varied depending on the type of marine plant material reaching the shores. The most important factor influencing all included arthropod groups was the proportion of bladder-wrack (Fucus) within the deposits. 27

30 Fucus has been shown to be a preferred food source for algal-feeding arthropods over other marine plant material (Lastra et al. 2008) and it is well known that Fucus beds are rich in detritivores (Backlund 1945). Fucus deposits reside for a long time due to very slow decomposition compared to other marine plant material (Smith and Forman 1984) and thus detritivores and algal-feeding herbivores might have developed special links to the sustainable source of energy that stranded Fucus provides. My studies on diet analysis of spiders (paper IV) also show that algal associated detritivores are important as prey, at least for wolf spiders. Large populations of detritivores may potentially support a large population of predators and increase the total diversity and abundance of arthropods. The predatory species in marine deposits with Fucus are not only more diverse but the mean body size is also larger, mainly consisting of ground hunting wolf spiders and large predatory beetles while the more compact deposits have a high abundance of small linyphid spiders. It is possible that small sized species are better able to find sheltering structures within these compact algal deposits, but an alternative hypothesis is that the preferred prey of these spiders are more abundant in the algal beds with filamentous algae. Seasonal variability in the marine component in the diet of terrestrial predators It has been suggested in earlier studies that shore dwelling wolf spiders, by feeding to a large part on insects with aquatic larvae, is an arthropod group with a close connection to the marine system (Mellbrand and Hambäck 2010). In paper IV, we also find that wolf spiders to a large part depend on prey species with aquatic larvae, such as chironomids, but the analyses also showed that the diet has large seasonal variations. The large seasonal variation in the diet of wolf spiders indicates an opportunistic feeding behaviour, with wolf spiders seemingly feeding on the prey species that are most common at the moment. However, it should be noted that we lack data on the seasonal availability of the different prey species. Climate change impact in Baltic shorelands Paper I summarizes the current knowledge about Baltic shore ecosystems and how they might be influenced by climate change. One of the conclusions of this paper was that there is a lack of knowledge concerning the multitude of drivers behind the differences in diversity and abundance of coastal arthropods, and this lack reduces our capacity to develop detailed predictions 28

31 on their responses to climate change. This thesis provides some new input to this research field which can make projections about climate change in this borderland more reliable. In paper II, I show that the main important predictors of arthropod diversity in Stockholm archipelago is island size and the number of islands in the vicinity. As sea level rise, these two landscape parameters will change as islands are both flooded and disappear and diminished. The results from the GIS models (figure 4.) showed that the mean island size will increase from 0.31 km 2 to 0.38 km 2 if sea levels increase with 0.5 meters. This change will occur because the large number of very small islands will disappear following such a relatively moderate sea level rise. The total number of islands in the study area will decrease with about 25 % from just above 12,000 to about 9,000 islands, if sea levels rise with 0.5 meters. For the highest sea level rise scenario (3 meters), the number of islands will decrease less fast and to about 8,000, but instead the mean island area would decrease slightly in this scenario and be about 0.35 km 2. The declining decrease in the number of islands at the higher sea level rise scenarios is probably related both to the current size distribution of islands and to the fact that islands will split into several islands in the same way that they have earlier grown together with land uplift. The total land area would decrease constantly as sea levels rise. The current area of 374 km 2 would decrease to 352 km 2 at a sea level rise of 0.5 meters and to 280 km 2 at a 3 meters sea level rise. The main features influencing arthropod diversity in the focus area were island size and the number of islands in the surrounding landscape (paper II) a decrease in island area in combination with the large loss in number of islands, even for a moderate sea level rise of 0.5 meters, could thus have large consequences for both regional and local diversity of arthropods in Baltic shore meadows in the near future. The shore meadow habitats in the Stockholm archipelago will further suffer from coastal squeeze when sea levels rise, since the typical shore meadow in the area is either surrounded by rocks or bordered by a forest which hinder the establishment further inland for coastal species. As sea levels rise, plants and arthropods associated to shore habitats will have to rely on the creation of new shore meadows. As trees have a long life span they will probably withstand for a long time and thus occupy many coastal habitats in which more short lived plant species could have established. The creation of new shore meadows can thus take longer than the life span of many species connected to shore meadow habitats. The severity of the projected sea level rise is of course related to the speed of the process. A rapid change in sea level will probably have larger consequences than a more moderate and constant increase. Projections are that the rise will be relatively constant but escalating (Grinsted et al. 2015). However, larger fluctuations in water levels are also projected for the Baltic Sea area and together with a constant sea level rise this will cause flooding 29

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33 of areas that are unaffected by flooding at present, causing the zonation of shore species to move even further inland, intensifying the coastal squeeze. Apart from projecting changes in terrestrial diversity due to sea level rise this thesis also provide evidence that a change in the species distribution and prevalence of species in the Baltic Sea has the potential to influence terrestrial diversity. The results from paper III indicate that algal deposits with large proportions of Fucus have higher abundance and diversity of arthropods within most included arthropod groups. The results also indicate a difference in body size of arthropod species depending on the composition, as Fucus rich deposits are inhabited by larger predatory species. As Fucus deposits in the future likely will be replaced by compact deposits of filamentous species in many coastal areas due to changed species composition in the Sea, arthropod communities will thus have lower diversity and abundance but also be inhabited by species of smaller size. Shorebirds typically prefer arthropod species of large size as prey (Buchanan et al. 2006), and a reduced arthropod abundance in combination with an arthropod community dominated by species of small body size can thus have consequences further up in the terrestrial food web. Further, if the inflow of chironomids would decrease due to warmer water temperatures as suggested by studies in freshwater systems (Jonsson et al. 2015), the diet of coastal predators like wolf spiders would change towards other prey species. The proportion of chironomids in the diet of wolf spiders is largest in late summer and early fall (August-September) which corresponds to the large hatching during this period. The predation pressure on terrestrial prey species is thus highly correlated to the inflow of marine prey and if there would be a large decrease in this inflow predation pressure on terrestrial arthropods should increase. However, since chironomids constitute such a large part of the diet of wolf spiders it is also possible that the predators would decrease due to shortage of food. If so, the predation pressure for terrestrial prey organisms might remain the same or even decrease. Seasonal variability is often overlooked in ecological studies but can be of large importance when making conclusions about food web dynamics and how these are influenced by different changes in the environment. 31

34 Concluding remarks To conclude, this thesis provides new insight concerning drivers behind arthropod diversity and abundance in Baltic shore ecosystems and how the arthropod communities might be affected when the conditions in the Baltic Sea are altered due to climate change. I provide knowledge concerning the biogeographical and environmental conditions that influence diversity and abundance of arthropods in several different types of Baltic shore ecosystems. Knowledge about the present connection between species diversity and abundance to the environment is essential to be able to project how future alterations in the environmental conditions, such as climate change, will influence different species. The thesis further explores how the landscape and environment will change due to climate change and how these altered conditions influence the coastal arthropod communities. I also provide some insight about the inconsistence of ecological connections, something that should be considered when we discuss how environmental changes impact food-webs and ecosystems. Shore ecosystems are complex in the way that they are influenced both by the marine and the terrestrial environments. Scientific studies on the other hand are usually concerning only one of the two elements. Referring to the main conclusions of paper I, I want to stress the importance of an ecosystem approach in scientific studies, of coastal areas without the common separation of terrestrial and marine ecosystems. 32

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