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Anthropogenic impact on predator guilds and ecosystem processes: apex predator extinctions, land use and climate change Marianne Pasanen Mortensen Department of Zoology Stockholm University 2013 Anthropogenic impact on predator guilds and ecosystem processes: apex predator extinctions, land use and climate change Doctoral Dissertation 2013 Marianne Pasanen Mortensen Department of Zoology Stockholm University S-106 91 Stockholm, Sweden ©Marianne Pasanen Mortensen ISBN 978-91-7447-860-0 Cover photo by Marianne Haage Printed in Sweden by US-AB, Stockholm, Sweden Distributor: Department of Zoology, Stockholm University In memory of my father LIST OF PAPERS I. Pasanen-Mortensen, M., Pyykönen, M. & Elmhagen, B. (2009) Where lynx prevail, foxes will fail – limitation of a mesopredator in Eurasia. Global Ecology and Biogeography. 22, 868-877. II. Pasanen-Mortensen, M. & Elmhagen, B. One continent, two ecosystem states? Land cover effects on mesopredator abundance in the presence and absence of apex predators. Submitted manuscript. III. Khalil, H., Pasanen-Mortensen, M. & Elmhagen, B. The relationship between wolverine and larger predators, lynx and wolf, in a historical ecosystem context. Conditionally accepted manuscript. IV. Pasanen-Mortensen, M., Elmhagen, B., Lindén, H., Andrén, H., Bergström, R., Wallgren, M., van det Velde, Y. & Cousins, S. A. O. Can recolonizing apex predators reclaim their ecosystem function? Impact of land use and climate change on intraguild interactions. Manuscript. Copyright paper I – John Wiley & Sons ABSTRACT Humans affect ecosystems by changing species compositions, landscape and climate. This thesis aims to increase our understanding of anthropogenic effects on mesopredator abundance due to changes in apex predator status, landscape and climate. I show that in Eurasia the abundance of a mesopredator, the red fox (Vulpes vulpes), is limited top-down by the Eurasian lynx (Lynx lynx) and bottom-up by winter severity. However, where lynx has been eradicated, fox abundance is instead related to bottom-factors such as summer temperature, productivity and cropland (paper I, II). Fox abundance was highest when croplands constituted 25% of the landscape (paper II). I also project red fox abundance in Sweden over the past 200 years and in future scenarios in relation to lynx density, land use and climate change. The projected fox abundance was highest in 1920, when lynx was eradicated and the proportion of cropland was 22%. In 2010, when lynx had recolonised, the projected fox abundance was lower than in 1920, but higher than in 1830. Future scenarios indicated that lynx abundance must increase in respond to climate change to keep fox at the same density as today. The results suggest a mesopredator release when lynx was eradicated, boosted by land use and climate change, and that changes in bottom-up factors can modify the relative strength of top-down factors (paper IV). From 1846-1922, lynx, wolverine (Gulo gulo) and grey wolf (Canis lupus) declined in Scandinavia due to persecution; however I show that the change in wolverine abundance was positively related to the changes in lynx and wolf abundance. This indicates that wolverine is subsidized by carrions from lynx and wolf kills rather than limited top-down by them (paper III). This thesis illustrates how mesopredator abundance is determined by a combination of top-down and bottom-up processes, and how anthropogenic impacts not only can change the structures of predator guilds, but also may modify top-down processes through changes in bottom-up factors. Keywords: Mesopredators, apex predators, top-down, bottom-up, interspecific killing, red fox, Eurasian lynx, grey wolf, wolverine, productivity, winter severity, cropland. ECOSYSTEM PROCESSES Bottom-up and top-down processes An ecosystem is an entity of biotic and abiotic factors that are interrelated (Tansley 1935). Theory predicts that within an ecosystem, the number of trophic levels, and the biomass on each trophic level, is determined by bottom-up and top-down processes (Hairston, Smith & Slobodkin 1960; Fretwell 1977; Oksanen et al. 1981). The fundamental bottom-up factor is the potential productivity of the ecosystem - the maximum primary gross productivity (i.e. the rate at which photosynthesis occur) that can be generated by the ecosystem (Oksanen et al. 1981). If the potential productivity is low, only plants persists in the ecosystem (Oksanen et al. 1981; Oksanen & Oksanen 2000). However, if the potential productivity is high, plant biomass is high, and herbivores can also persist. The higher the potential productivity is, the higher the herbivore biomass will be. Concurrently, as herbivores consume the surplus of plants, they limit plant biomass top-down and keep it at a constant level regardless of the potential productivity. Herbivore biomass, however, increases with potential productivity. There is thus an interaction of bottom-up and top-down processes, which limits plants as well as herbivores (Oksanen et al. 1981; Oksanen & Oksanen 2000). However, many ecosystems are productive enough to also support a third trophic level - predators. The predators consume the surplus of herbivores and predator biomass is thus limited bottom-up, increasing with potential productivity, while predators simultaneously limit herbivore biomass top-down and keep herbivores at a constant level. By limiting herbivores, predators also have an indirect impact on plants, releasing them from top-down limitation (Oksanen et al. 1981; Oksanen & Oksanen 2000). Thus, bottom-up and top down processes works in concert in an ecosystem, determining the biomass at different trophic levels (Fretwell 1977; Oksanen et al. 1981; Hunter & Price 1992; Power 1992; Oksanen & Oksanen 2000). Predator interactions Species within the same trophic level can be grouped into guilds, based on their food resources and methods of foraging. As species within a guild share food resources, competition between them can arise (Root 1967; Simberloff & Dayan 1991). Competition over shared resources can be exploitative or through interference. Exploitative competition is an indirect process where the use of a resource by one species limits the access of the resource for the other species, while interference competition implies direct confrontations, such as fighting (Park 1962). Within predator guilds, interference competition can have a lethal outcome, and is termed interspecific killing or intraguild killing (Palomares & Caro 1999). Intraguild predation implies that some energy is gained from the competitor by consuming it (Polis, Myers & Holt 1989). However, among mammalian predators consumption is not always involved (Palomares & Caro 1999; Donadio & Buskirk 2006), why the term interspecific killing might be more useful in many cases. Relative body size of the predators is of major importance for interspecific killing among mammals (Palomares & Caro 1999; Donadio & Buskirk 2006). Although packs of predators can kill solitary predators that are larger than the individuals of the pack, and adults of smaller species can kill juveniles of larger species, large predatory species generally kills smaller ones (Palomares & Caro 1999). But as the difference in body size between species increases, dietary overlap decreases, and the benefit for the larger predator to kill the smaller one probably decreases. At the other hand, as the difference in body size decreases, the risk of being injured probably increases also for the larger species. Most interspecific killing therefore occur when the larger predator weighs 2-5.4 times more than the smaller predator (Donadio & Buskirk 2006). 1 Apex predators and mesopredators Apex predators are often considered to be large predators and mesopredators to be midsized predators. However, the terms apex predator and mesopredator are context dependent, and related to the trophic function and to the size of other predators that are present in the ecosystem (Prugh et al. 2009; Ritchie & Johnson 2009). Apex predators hold the top position in the food web, and limit other species within the ecosystem, both its prey species by predation, and smaller predators by interspecific killing (Elmhagen et al. 2010; Estes et al. 2011). Apex predators are large in relation to other predators in the ecosystem, but can be small in relation to apex predators in other ecosystems. If a larger predator is introduced, the apex predator can be submerged to the role of mesopredator (Roemer, Donlan & Courchamp 2002). Also, several apex predators can exist within the same ecosystem (Ritchie et al. 2012). This can occur when diets and habitat use of the predators differs (Schmidt et al. 2009). Mesopredators holds an intermediate position in the food web, where they are limited topdown by apex predators (Ritchie & Johnson 2009; Elmhagen et al. 2010). They seem to avoid habitats utilized by apex predators, or adapt activity pattern to avoid encounters with apex predators (Fedriani, Palomares & Delibes 1999; Thompson & Gese 2007; Bischof et al. 2014). This indirect effect of apex predator on mesopredators may add to the direct limiting effect of killing (Ritchie & Johnson 2009; Roemer, Gompper & Valkenburgh 2009). A species that is a mesopredator in one ecosystem can be apex predator in another ecosystem if larger predators are absent (Ritchie & Johnson 2009; Roemer, Gompper & Valkenburgh 2009). Further, there can be several mesopredators within the same ecosystem which interact with each other through interspecific killing (Ritchie & Johnson 2009; Levi & Wilmers 2011; Ritchie et al. 2012; Bischof et al. 2014). ANTHROPOGENIC IMPACT Apex predator extinctions and cascading effects A trophic cascade occurs when a higher trophic level has an indirect effect on a lower trophic level, mediated by a trophic level in-between (Carpenter, Kitchell & Hodgson 1985; Estes et al. 1998; Gilman et al. 2010). Since apex predators limit herbivores as well as mesopredators, they can initiate trophic cascades along two routes in an ecosystem. In one route they limit the herbivores that they prey on, and thereby indirectly affect the abundance of plants on which the herbivores feed. In the other route they limit mesopredators and thereby indirectly affect the abundance of smaller herbivores, i.e. the prey of mesopredators, and the plants that the small herbivores feed on (Elmhagen et al. 2010). Therefore, extinctions and reduction of apex predators can have extensive cascading effects on the ecosystem, and there are many examples of this from terrestrial as well as aquatic ecosystems all over the world (Estes et al. 2011; Ripple et al. 2014). Loss of apex predators can cause a mesopredator release, i.e. an increase of mesopredator abundance as mesopredators are no longer limited top-down (Soulé et al. 1988). This can result in higher rates of predation in smaller prey species (Berger, Gese & Berger 2008), decline of prey populations (Palomares et al. 1995) and even extinction of prey species (Soulé et al. 1988). Apex predators, even if introduced, can also limit introduced mesopredators which otherwise can have negative effect on native wildlife (Johnson, Isaac & Fisher 2007; Rayner et al. 2007). When herbivores are released from predation they can reach higher densities as well as utilize habitats that earlier were avoided due to predation risk (Creel et al. 2 2005). This can have negative effects on vegetation and on other species that are related to the vegetation (Ripple & Beschta 2004). Because of their impact on ecosystems, apex predators have sometimes been suggested to be keystone species (Sergio et al. 2008), i.e. species that are important in structuring ecological communities (Paine 1969). However, the effect of apex predators is context dependent and can be affected by factors such as carrying capacity and food web structure, and they do therefore not always function as keystone species (Sergio et al. 2008). For example, trophic cascades can be relaxed in ecosystems with many predators (Finke & Denno 2004; Bruno & O'Connor 2005). Land use Homo sapiens is an ecosystem engineer, i.e. an organism that can maintain or alter a habitat or even create new habitats (Jones, Lawton & Shachak 1994; Laland, Odling-Smee & Feldman 2001). Humans have for example transformed 40% of the Earth´s surface into croplands and pastures, which are used for food production. Large areas of forests have therefore been lost globally, while many of the remaining forests are fertilized, drained or managed in other ways to increase the wood biomass (Foley et al. 2005). Changes from natural land cover to anthropogenic land cover have globally resulted in a decrease of net primary production. However, land use does not always decrease productivity - intensive agriculture can increase the net primary productivity of the natural land cover (Haberl et al. 2007). Another consequence of anthropogenic land use is that habitats are lost or split up into smaller areas more or less isolated from each other, a process known as fragmentation, which impacts biodiversity as well as the abundance and distribution of species (Fahrig 2003). This is expected to have negative effects on species that for example are habitat specialists or less mobile (Ewers & Didham 2006). Fragmentation can also have a negative effect on apex predators and cause mesopredator release, with following decrease and extinction of local prey populations (Crooks & Soule 1999). Habitat generalist may even gain from fragmentation, as they use different habitats in the landscape (Andrén 1992; Andrén, Delin & Seiler 1997). Nevertheless, generalist mesopredators that benefit from anthropogenic land use changes can limit generalist prey species that also are gained by anthropogenic land use (Eagan et al. 2011). Further, apex predator persecution can be related to land use and lead to eradication of apex predators and mesopredator release (Elmhagen & Rushton 2007). Climate While land use change is expected to have the largest global impact on biodiversity over the coming 100 years, climate change is right behind and is expected to have particularly high impact at high latitudes (Chapin III et al. 2000). The global warming over the past century seem to be caused by a combination of anthropogenic and natural factors (Stott et al. 2000). Over this period, temperate species have increased in abundance and expanded their distribution range, while polar species have decreased in abundance (Parmesan & Yohe 2003). Earlier climate changes in the history of Earth altered the distribution and abundance of species, which led to new communities and new biotic interactions. Experiments and models indicate that this may happen due to present-day climate change as well, resulting in for example new predator-prey relationships (Blois et al. 2013). Over the period of 1982-1999, when a noteworthy climate change took place, the global net primary productivity (i.e. the energy produced by plants that can be used by higher trophic levels) increased with 6%. However, there were regional differences; net primary productivity increased in 25% of the vegetated areas of the Earth and decreased in 7 % of the vegetated areas (Nemani et al. 2003). Primary production can be related to the impact of apex predators 3 on herbivores and mesopredators, and to the strength of tropic cascades (Elmhagen & Rushton 2007; Melis et al. 2009; Ritchie et al. 2012; Ripple et al. 2014). Therefore, even if community composition in an ecosystem is not altered by climate change, the strength of interactions and relative abundance of species may change with warming. MESOPREDATOR AND APEX PREDATOR INTERACTIONS IN EURASIA The red fox, Vulpes vulpes, is the most widespread canid in the world, with a distribution range covering the entire northern hemisphere, except from Iceland, the islands of the Arctic Ocean, parts of Siberia and extreme desserts. It has also been introduced to Australia and New Zeeland (Macdonald & Reynolds 2008). The red fox is a medium sized omnivore, feeding on fruit, invertebrates, mammals and birds. They kill prey up to about 3.5 kg, but scavenge on carcasses of larger mammals (Macdonald & Reynolds 2004). Rodents, especially voles, constitute a large part of the diet in many areas of Europe (Sequeira 1980; Lindström 1989; Jȩdrzejewski & Jȩdrzejewska 1992; Panek & Bresiński 2002; Sidorovich, Sidorovich & Izotova 2006; Dell'Arte et al. 2007). The red fox can limit game species such as grouse, hare and roe deer (Lindström et al. 1994; Jarnemo & Liberg 2005). When roe dear density is high, the red fox seem to specialize on roe deer fawns, while at low roe deer density they function as a generalist (Panzacchi et al. 2008). The red fox also interfere with smaller predators. Small mustelids are being killed or predated on by the fox (Lindström et al. 1995; Dell'Arte et al. 2007; Carlsson et al. 2010). The red fox also have extended its distribution range in for example the Scandinavian mountains, which has a negative impact on the already threatened arctic fox population, as it kills and outcompete the smaller arctic fox (Tannerfeldt, Elmhagen & Angerbjörn 2002). It has also been suggested that red fox limits the southern distribution range of arctic fox (Hersteinsson & MacDonald 1992). In many ecosystems, the red fox is a mesopredator which interfere with and is limited by larger predators belonging to the family Felidae (Fedriani, Palomares & Delibes 1999; Helldin, Liberg & Glöersen 2006; Elmhagen & Rushton 2007; Elmhagen et al. 2010) and Canidae (e.g. Sargeant & Allen 1989; Johnson & VanDerWal 2009; Letnic et al. 2011; Levi & Wilmers 2011). The Eurasian lynx is a large predator native to the Nordic countries, Russian boreal forest belt and central Asia (Nowell & Jackson 1996; Breitenmoser et al. 2008). While historically present in large parts of Europe, a combination of deforestation, reduction of ungulate populations and persecution, the Eurasian lynx is today lost from southern Europe (Breitenmoser et al. 2008). The most important prey are small ungulates, but the Eurasian lynx take smaller prey such as hares where ungulates are scarce or absent (Nowell & Jackson 1996; Breitenmoser et al. 2008). It is well documented that the Eurasian lynx kills red foxes without eating them, an indication that lynx kills foxes to decrease competition (Sunde, Overskaug & Kvam 1999; Helldin, Liberg & Glöersen 2006). In areas with limited access of prey, lynx however predates on foxes (Mattisson et al. 2011a). It has been suggested that the red fox is limited by lynx, but that the impact depends on the bioclimatic productivity. Where lynx abundance is high relative to ecosystem productivity, the red fox is limited by lynx and fox abundance does not respond to differences in productivity. However, where lynx abundance is low relative to productivity, red fox abundance increase with productivity (Elmhagen et al. 2010). There are two larger canids that occur sympatric with the red fox in Eurasia - the golden jackal (Canis aureus) and the grey wolf (Canis lupus). The diets of golden jackal and red fox overlap to a high extent (Lanszki, Heltai & Szabó 2006), which increases the risk for interference competition (Donadio & Buskirk 2006). As the wolf mainly feed on large 4 ungulates (Mech & Boitani 2004), the diet overlap between fox and wolf is smaller. However, little is known about interference between the red fox and these two predators in Eurasia. Remains of red foxes in golden jackal scat have been reported a few times. However, it is not known whether these foxes were killed by jackals or eaten from carrions (Lanszki, Heltai & Szabó 2006). Neither does it exclude that jackals kills foxes, as they do not necessarily consume them if they kill them. In Sweden, historical decline and eradication of Eurasian lynx (Lynx lynx) and grey wolf caused a mesopredator release of red fox (Elmhagen & Rushton 2007). Also, the dingo (Canis lupus dingo) seems to limit red fox abundance in Australia (Johnson & VanDerWal 2009; Letnic et al. 2011). However, in North America, the grey wolf (C. lupus) has an indirect positive effect on red fox, by limiting coyotes (Levi & Wilmers 2011). It is therefore not clear whether the grey wolf and the golden jackal are apex predators in relation the red fox in Eurasia. The wolverine, Gulo gulo, is a large opportunistic carnivore scavenger, preying on for example ungulates, hares, ground squirrels, birds, voles and lemmings (Pasitschniak-Arts & M. 1995). It is also an opportunistic scavenger, utilizing carrions from prey killed by Eurasian lynx and grey wolf (Dijk et al. 2008; Mattisson et al. 2011a; Mattisson et al. 2011b; Koskela et al. 2013). However, in North America, grey wolves have been reported to kill wolverines (White et al. 2002; Krebs et al. 2004). It is therefore uncertain what net effect the grey wolf has on wolverines. OBJECTIVES The objectives of my thesis were to investigate how mesopredator abundance is limited by bottom-up and top-down factors, and to explore mesopredator abundance in relation to anthropogenic induced changes in apex predator status, land use and climate. Therefore, in the first paper (Paper I) we investigated red fox density at a continental scale in relation to three apex predators - the Eurasian lynx, the grey wolf, and the golden jackal - and in relation to different bottom-up factors - winter severity, summer temperature, primary production, tree cover and human density (Fig. 1). We also investigated differences in bottom-up impact in the presence and absence of apex predators. In the second paper (Paper II) we explored the effect of land use on red fox abundance in the presence and absence of the Eurasian lynx. In the third paper (Paper III) we investigated whether wolverine was positively or negatively related to the historical decline and eradication of Eurasian lynx and grey wolf in Sweden and Norway, and in relation to relative snow depth, land use and human density of study sites. In the fourth paper (Paper IV) we projected red fox abundance in south-central Sweden in the past, present and future, to explore how anthropogenic changes in apex predator status (Eurasian lynx), land use and climate impact mesopredator abundance, and to explore how changes in bottom-up factors affect apex predator impact on mesopredators. 5 Fig. 1. Network of top-down and bottom-up factors that are assumed to directly or indirectly impact red fox abundance in Eurasia. METHODS Studies at continental scale For the studies of ecosystem processes at the continental scale (Paper I, II), we collected published data on red fox density from 111 different sites in Europe and northern Eurasia (Fig. 2). We assessed presence-absence of potential apex predators - Eurasian lynx (Paper I, II), grey wolf (Paper I) and golden jackal (Paper I) - from each paper or report and from published descriptions of distribution range of each apex predator at the period when each red fox survey was conducted. As the golden jackal only was present at two of the sites, we grouped it together with grey wolf as “Canis”. We used remote sensing data from online databases in geographic information system to assess bottom-up factors in a buffer zone of 10 km (314 km²) around each red fox site (Paper I, II). We used a yearly mean of the fraction of photosynthetically active radiation (FPAR) to measure net primary productivity (Yang et al. 2006) in each buffer zone (Paper I, II). As FPAR measures the ratio of radiation at 0.4–0.7 absorbed by the vegetation, it reflects the energy absorption capacity of vegetation (Myneni et al. 2002). To assess winter severity in each buffer zone, we used information on snow depth and mean temperatures for DecemberFebruary. These variables were strongly correlated, so we reduced multicollinearity by a principal component analysis. The first principal component was used as a measure of winter severity in the analyses (Paper I, II). Mean temperatures for June-August were used to represent summer temperatures (Paper I, II). We also assessed human density (Paper I, II), tree cover (Paper I) and the proportion of cropland (anthropogenic open habitat), grassland (semi-natural and natural open habitats) and a gradient of openness (anthropogenic, seminatural and natural open habitats) (Paper II) in each buffer zone. 6 Fig. 2. Red fox study sites and density of red fox in presence (black dots) and absence (white dots) of Eurasian lynx. Studies at regional and local scale To investigate whether there was a positive or negative relationship between wolverine and the two larger predators Eurasian lynx and grey wolf, we used historical hunting bags from Norwegian and Swedish counties in 1846-1922 (Paper III). As data on bottom-up variables were not available for this time period, we used present-day remote sensing data as a proxy. We could do this as we did not investigate the change over time in bottom-up variables, but only the relative conditions between counties, which we assumed to be constant over time. We included April snow depth, proportion of cropland and productivity (FPAR) as bottom-up variables, all assessed from remote sensing data. To project historical (1830 and 1920), present (2010) and future (2050) red fox abundance and analyse how changes in land use and climate impact top-down processes, we implemented a two-step model building (Paper IV, Fig. 3). In the first step we analysed the impact of Eurasian lynx, winter temperatures (December-February) and land use (cropland and other open habitats) on red fox density in present-day Finland and south-central Sweden, in 106 squares of 50x50 km. We used data on species abundance from wildlife triangles and remote sensing data to assess the bottom-up factors within each square. In the second step we created models for lynx density, land use and winter temperatures in 1830, 1920 and 2010 in the Norrström Drainage Bassin in south-central Sweden. We used historical hunting bags and present-day data from authority lynx surveys to model historical and present-day lynx density. To model land use we used cadastral maps from 1897-1901 together with soil and bedrock maps. Present day land use was assessed from remote sensing data. We used temperature stations from 1980-2000 together with information on elevation, latitude and distance to the sea to model winter temperatures. We then used the model derived from the first step and the data derived from the second step to project red fox abundance in Norrström Drainage Bassin in south-central Sweden. For future scenarios with different lynx density we assumed a continued global warming. 7 Fig. 3. The two steps to develop a model of red fox abundance in Norrström Drainage Bassin. In the first step (left graph) the effects of top-down and bottom-up variables on red fox abundance was analysed. In the second step (right graph) top-down and bottom-up variables for Norrström Drainage Bassin in 1830, 1920 and 2010 were modeled. Data on present-day land cover was assessed from remote sensing data. Information from these two steps was used to project red fox abundance at the different time steps, as well as for future scenarios. RESULTS AND DISCUSSION Although earlier studies have investigated relationships between mesopredators abundance and either top-down or bottom-up factors on a continental scale (Bartoń & Zalewski 2007; Letnic et al. 2011), no studies until now have investigated the combined effect of bottom-up and top-down factors on mesopredators at large scale. We showed that there was a combined effect of top-down and bottom-up factors on red fox abundance at a continental scale (Paper I, Fig. 4). Winter severity had a negative effect on red fox abundance, and our results therefore supports theory that red fox is limited by winter climate (Hersteinsson & MacDonald 1992; Bartoń & Zalewski 2007). However, also presence of Eurasian lynx had direct negative impacts on red fox density (Paper I). Furthermore, lynx mediated a negative effect of winter severity and tree cover on red fox, indicating that lynx are mainly present in forested areas with harsh winters. There was, however, no direct effect of tree cover on red fox abundance. An important finding was also that the presence of canids - mainly grey wolf but also golden jackal - was not related to red fox abundance. This suggests that the grey wolf does not have a significant effect on red fox abundance at a continental scale, while the result is more uncertain for the golden jackal due to the small sample size of this species (Paper I). As Eurasian lynx, but not the canids had a significant impact on red fox, we investigated the impact of bottom-up factors only in the presence and absence of lynx (Paper I, II). We found that within the distribution range of lynx, winter severity was the only bottom-up factor significantly limiting red fox abundance. However, outside the distribution range of lynx, red fox abundance was positively related to summer temperature, human density and yearly productivity (Paper I). Also, outside the distribution range of lynx, red fox abundance was 8 related cropland, indicated by a humpbacked shaped relationship (Paper II). Red fox reached the highest abundance when Fig. 4. Factors that affect red fox abundance in Eurasia. There was a direct negative effect of Eurasian lynx and winter severity, as well as an indirect negative effect of winter winter severity and tree cover, mediated through lynx. The canid species (grey wolf and golden jackal) affect red fox abundance. Single-headed arrows represent directed relationships and double-headed curved arrows represent correlated errors. Paths significant at the 0.05 level are indicated by solid arrows, and insignificant paths are indicated by dotted arrows. The path coefficients are standardized, residual errors are given for each variable, and d.e. = deviance explained. cropland covered about 25% of the landscape. Red fox abundance thus seems to be boosted by anthropogenic land use in absence of lynx. Our result is consistent with a study from Finland, showing that the highest red fox abundance is reached when 20-20% of the landscape is covered by cropland. This probably provides foxes with high access to prey in fields but also access to breeding habitats in forest (Kurki et al. 1998). We also found a negative effect of grassland outside the distribution range of lynx, and we suggest that this is caused by low rodent density in grasslands (Paper II). In Norway and Sweden, the decline of wolverine, Eurasian lynx and grey wolf was ultimately a result of anthropogenic persecution; however, these predators did not decline at the same rate (Paper III). When accounting for variation in land use and snow depth, a positive relationship between wolverine and lynx remained. Our model indicated that slow rate of decrease in lynx also led to a slow rate of decrease in wolverine. As lynx was negatively affected by land use, there was also a mediated negative effect of land use on wolverine. Further, year to year fluctuations of wolverine was positively related to year to year fluctuations in grey wolf. These results support theory that wolverine is positively affected by grey wolf and lynx, as these provides carrions for the wolverine (Dijk et al. 2008; Mattisson et al. 2011b). The Eurasian lynx and the grey wolf therefore do not function as apex predators on wolverines in this ecosystem, under the circumstances investigated here (Paper III). In a world that is changing fast due to anthropogenic impact on land use and climate, it is important to understand how changes in bottom-up processes modify top-down processes. Our projection of mesopredator abundance over time can contribute to the understanding of this, as well as give an insight in historical ecosystem composition (Paper IV). Our models for Norrström Drainage Basin suggested that in 1830, the proportion of cropland was lower than today, winters were colder and lynx density was high. The projected red fox abundance was therefore lower than today. In 1920, red fox abundance reached high density, as lynx were 9 eradicated and croplands covered 22% of the landscape (Paper IV), which is optimal for red fox (Paper II). In 2010, lynx had recolonised the study area and red fox density was lower than in 1920, but higher than in 1830 (Paper IV). Future scenarios in which the winter temperature was 1.5°C higher than today, suggested that if red fox abundance is to be kept at the same level as today, the lynx population should be twice as large as today. If lynx density is kept as today, red fox abundance will increase. This illustrates that as bottom-up factors changes in an area, the impact of top-down processes is also modified (Paper IV). CONCLUSIONS Altogether, the results in this thesis are in line with theory of bottom-up and top-down impact on mesopredators (Crooks & Soule 1999; Elmhagen et al. 2010). When apex predators are present, they limit mesopredators top-down and diminish mesopredator respond to bottom-up factors. However, when functional apex predators are absent, mesopredators are released from top-down control and increase in abundance as a response to bottom-up factors. This implies that the red fox function as a mesopredator in presence of lynx, and as an apex predator where the lynx has been eradicated (Paper I, II). Furthermore, the Eurasian lynx and the grey wolf does not function as apex predators in relation to the wolverine, but rather subsidize the wolverine through increased food access (Paper III). The results thus support theory that predators have different trophic functions in different contexts. Finally, an important insight from this thesis is that as anthropogenic impact on bottom-up factors, e.g. land use and climate, alters ecosystems towards a state with higher carrying capacity for mesopredators, the relative impact of apex predators is weakened. To maintain the same relative impact of apex predators on mesopredators and prevent increased mesopredator abundance, apex predator abundance should increase (Paper IV). Whether apex predators can increase in abundance or not depends on anthropogenic apex predator control, as well as on carrying capacity, i.e. on natural conditions as well as direct and indirect anthropogenic impact on prey species. 10 REFERENCES Andrén, H. (1992) Corvid density and nest predation in relation to forest fragmentation: A landscape perspective. Ecology, 73, 794-804. Andrén, H., Delin, A. & Seiler, A. (1997) Population response to landscape changes depends on specialization to different landscape elements. Oikos, 80, 193-196. Bartoń, K.A. & Zalewski, A. (2007) Winter severity limits red fox populations in Eurasia. Global Ecology and Biogeography, 16, 281-289. Berger, K.M., Gese, E.M. & Berger, J. (2008) Indirect effects and traditional trophic cascades: a test involving wolves, coyotes, and pronghorn. 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Yang, W., Shabanov, N.V., Huang, D., Wang, W., Dickinson, R.E., Nemani, R.R., Knyazikhin, Y. & Myneni, R.B. (2006) Analysis of leaf area index products from combination of MODIS Terra and Aqua data. Remote Sensing of Environment, 104, 297-312. 15 SAMMANFATTNING Människan har en stor inverkan på jorden och dess ekosystem genom att förändra landskapssammansättning och klimat. Dessa förändringar kan få följder för den biologiska mångfalden och för interaktioner mellan olika arter. Dessutom har utrotning av stora rovdjur i många av världens ekosystem fått konsekvenser för ekosystemens struktur. Ett ekosystem kan delas upp i en näringspyramid med olika trofinivåer, där växter utgör den lägsta trofinivån, växtätare den mellersta trofinivån och rovdjur den högsta trofinivån. Enligt ekologisk teori så påverkas förekomsten av olika arter i ett ekosystem genom processer nedifrån, d.v.s. tillgång till föda, och uppifrån, genom predation. Vidare så kan konkurrens mellan arter inom samma trofinivå ha en begränsande effekt, då större rovdjur kan döda mindre rovdjur för att minska konkurrensen. Större rovdjur i ett ekosystem fungerar därigenom som toppredatorer, medan de mindre fungerar som mesopredatorer. När stora rovdjur utrotas eller minskar kraftigt i antal kan det således få följder för förekomsten av arter på lägre trofinivåer genom en trofisk kaskad, och detta kan ske på två olika sätt: Dels genom att stora växtätare kan öka i antal och därmed begränsa växtbiomassan, och dels genom att mesopredatorer kan öka i antal och begränsa antalet mindre växtätare, vilket vidare kan påverka biomassan av vissa växter. Därmed kan minskning och utrotning av stora rovdjur förändra interaktioner mellan organismer och påverka den biologiska mångfalden. Denna avhandling undersöker hur människans förändring av toppredatorers förekomst, markanvändning och klimat påverkar antalet mesopredatorer i vissa områden. Rödräven är en vanlig mesopredator som finns spridd över en stor del av norra halvklotet. Vi undersökte hur antalet rävar på olika platser i Eurasien (Europa och norra Asien) påverkas uppifrån av de större predatorerna eurasiskt lodjur, varg och guldschakal, samt nerifrån av ekosystemproduktivitet, landskapssammansättning, klimat och människotäthet. Vi fann att vinterns hårdhet och närvaro av lodjur avgjorde täthet av räv i Eurasiaen men också att i de områden där lodjur är utrotade så påverkas rävarna nerifrån av bl.a. sommartemperatur, produktivitet och åkermark (artikel I, II). De högsta tätheterna av räv i frånvaro av lo nåddes där 25 % av landskapet utgjordes av åkermark (artikel II). Vi studerade också effekten av lodjur och varg på järv i norra Skandinavien genom att analysera jaktstatistik från 1800- och 1900-talet för dessa tre arter. Järven är en predator som även nyttjar kadaver från byten som fällts av andra rovdjur. Vi fann att även om nedgången av dessa tre arter berodde på mänsklig förföljelse, så minskade antalet järvar snabbare där antalet lodjur minskade snabbt. Dessutom var järven positivt associerad med fluktuationer i antalet vargar. Detta indikerar att lodjur och varg inte begränsar järv uppifrån genom att fungera som toppredatorer i förhållande till järv, utan istället påverkar järv nedifrån genom att öka födotillgången (artikel III). Det är inte bara antalet rovdjur som har förändrats i Skandinavien de senaste 200 åren, utan även markanvändningen och klimatet. Vi gjorde därför en projektion av antalet rödrävar i Mälardalen 1830, 1920 och 2010 utifrån dessa förändringar samt utforskade hur förändringarna i landskap och klimat förändrade den effekt som lodjur har på rävar. Antalet rävar var som högst 1920, när lodjuret hade utrotats och andelen jordbruksmark hade ökat till 22 %, samtidigt som vinterklimatet hade blivit mildare. År 2010 fanns lodjur åter i ekosystemet och antalet rävar var färre än 1920 men fler än 1830 p.g.a. mildare vinterklimat 2010. Våra resultat tyder vidare på att förändringarna i klimat och markanvändning, där andelen jordbruksmark har ökat till dryga 20 % av landskapet, har förändrat den relativa effekten av lodjur, vilket innebär att det behövs fler lodjur idag än för 200 år sedan för att begränsa antalet rävar till en viss nivå. Framtida scenarion med ytterligare förändring av klimatet antyder att det om 50 år skulle behövas en dubbelt så stor lodjurspopulation som idag för att behålla antalet rävar på samma nivå som idag (IV). 16 Avhandlingen illustrerar hur förekomsten av mesopredatorer styrs av olika faktorer, både uppifrån och nedifrån, och hur antropogen förändring inte bara kan förändra relativa tätheter av predatorer men också modifiera den effekt som toppredatorer har på mesopredatorer. 17 ACKNOWLEDGEMENTS A very big thank you goes to my supervisor Bodil Elmhagen for being engaged, always having enthusiasm and a solution for every problem; I have learned a lot from you. I am also grateful to my co-supervisor Sara Cousins for guidance over the years. I am thankful to all co-authors for your contributions to the papers, and to Ranga Myneni and Arindam Samanta for contributing with productivity data. Also Anders Angerbjörn and Ulrika Alm Bergvall in my evaluation group have been a good source of comments and advices. I have appreciated the company of my roommates over the years, Alexandra, Jessica, Houshuai, Alyssa and Christina, as well as meetings with the Arctic fox group and help and advices from other colleagues at Zootis. Lina, thanks for the cakes. Marianne, thanks for the good times and for the cover photo of this thesis. Thanks to my friends for cheering me. Frida and Anna-Lena, I am glad to have you as my friends. To my family, thank you so much for your support over these years, you are invaluable. Mother, thank you for your patience and care, I look forward to see you more often. Lene, thank you for being there when I need it, always prepared to listen. Hanne, thanks for your calls, they are worth a lot. Josefine, thanks for making my day every time we meet, I look forward to share more great and fun moments with you. 18