Linköping University | Department of Physics, Chemistry and Biology Bachelors thesis, 16 hp | Educational Program: Biology Spring term 2021 | LITH-IFM-G-EX—21/4001--SE
Population dynamics of the
horned grebe in constructed
wetlands in Östergötland
Johanna Pellnor
Examiner, Lars Westerberg Supervisor, Karl-Olof Bergman
Contents
1 Abstract ... 1
2 Background ... 1
3 Materials and methods ... 3
3.1 The species ... 3 3.2 The wetlands ... 4 3.2.1 Rosenkällasjön ... 4 3.2.2 Edhaga ... 5 3.2.3 Frökärret ... 5 3.2.4 Ekängssjön ... 5 3.2.5 Ängelska kanalen ... 6 3.2.6 Ullaberg ... 6
3.3 Field work: counting individuals ... 6
3.4 Data analysis ... 6
4 Results ... 7
4.1 Data analysis ... 7
4.1.1 Population dynamics ... 7
4.1.2 Effect of wetland age on breeding success ... 11
4.2 Habitat preference ... 12
5 Discussion ... 14
5.1 Negative effects from the presence of pike ... 14
5.2 Other fish species cause competition for food ... 15
5.3 Macrophyte abundance affects the ability to forage ... 17
9 References ... 20 Appendix ... 24
1 Abstract
The population size of the horned grebe, Podiceps auritus, is declining in most of the world due to loss of wetlands, deteriorating water quality and establishment of predatory fish such as pike, Esox lucius, in former fish free wetlands. The horned grebe is now globally classified as vulnerable. In this study, data on population dynamics of the horned grebe in six created wetlands in Linköpings kommun was examined together with field work carried out in three of the wetlands. The results indicate that the number of pairs and juveniles of horned grebe crash six to eight years after the wetland is created and does not recover if there is fish present. The pairs and juveniles of horned grebe decreased significantly with the increasing age of the wetland if there was pike present in the wetland. Reduction fishing and drainage of the water in two of the wetlands inhabited pike showed a small improvement in population size of the horned grebe, but the numbers ultimately declined after a few years. Reduction fishing of common roach in one wetland showed a continuing improvement in the population size of horned grebe. Other factors that affected the horned grebe negatively, was an abundance of macrophytes such as Canadian pondweed, Elodea canadensis, that makes foraging harder. Keywords: Biodiversity, Canadian pondweed, Common roach, Horned grebe, Pike, Population dynamics, Wetland
2 Background
The human impact on the environment has increased dramatically over the past hundred years. The higher demands for food, timber, fiber, fresh water, and fuel are causing dramatic changes in our terrestrial and aquatic ecosystems, leading to lower water quality, increased pollution and loss of important biotopes and biodiversity (Millennium Ecosystem Assessment, 2005). Wetlands are a type of biotope that is currently decreasing due to drainage for agriculture and infrastructure development (Silva et al., 2007). In south-western Sweden, 67 % of the wetlands has disappeared over the last 50 years due to drainage, and most of Europe is showing the same alarming loss of wetlands (Silva et al., 2007). Wetlands are important for many different ecosystem services, such as nutrient retention, water purification, being carbon sinks and providing habitat to different organisms. The current loss of wetlands in Sweden is a big threat to the organisms that inhabit that type of biotope, especially birds (Silva et al., 2007). However,
areas for several bird species, and a decrease in habitats such as wetlands will be followed by a decrease in biodiversity (Silva et al., 2007). One example of a bird species that inhabit wetlands in Sweden is the horned grebe.
The horned or Slavonian grebe, Podiceps auritus, is a small freshwater lake-bird (Cramp & Simmons, 1977). The species migrates to coastal wintering areas in the end of summer (Sonntag et al, 2009) and returns to their breeding grounds in March and onwards (Summers & Mavor, 1995). The horned grebe inhabits small freshwater wetlands during breeding season, both created and natural ones and their diet consists of smaller fish in the winter but can switch to aquatic macroinvertebrates during breeding (Stedman, 2000). In Sweden, the horned grebe was classified as vulnerable (VU) in 2000 and 2005, near threatened (NT) in 2010 and least concern (LC) in 2015 and 2020 in the Swedish Red list (Artdatabanken, 2021). The number of nesting pairs of horned grebe in Sweden decreased with an estimated 54 % from 1972 until 1996 (Douhan, 1998), but has increased and is now at a more stable population size like the one in 1972 (Norevik, 2014). The horned grebe is one of 12 rare species chosen by Linköpings kommun that needs extra protection and measures to be preserved in the county (Linköpings kommun, 2020). The trend of the horned grebe in Sweden deviates from the global trend and BirdLife International has classified the species as vulnerable due to its rapid decline in many other countries (2021). Several factors can be the reason for the decrease of horned grebe in the world. Examples are habitat loss due to drainage of wetlands, or habitat degradation due to predatory fish such as pike (Esox lucius) that compete for the same food and prey on the chicks, changes in water quality (Crooke et al. 1993) and an abundance of macrophytes (Summers et.al. 2011). The impact of these factors can increase with the age of the wetland due to the establishment of pike and invasive plants such as Canadian pondweed (Elodea canadensis).
Rosenkällasjön is an example of a created wetland, located in Linköping, Sweden, and was created in 2004 (Jörneskog, personal communication, April 12, 2021). The wetland is a popular place for bird watching and has over the years established itself as an important place for recreation and nature conservation. Over the years, the wetland has been infested with Canadian pondweed, the pike population has increased, and the horned grebe started to disappear from the wetland. Several measures such as drainage and pike fishing has been taken since the establishment of the wetland to make it more inhabitable to several organisms, for example different bird species. Other created wetlands in the area have faced the same problems, and
Linköpings kommun is taking measures like those in Rosenkällasjön to recreate their former environment (Jörneskog, personal communication, April 12, 2021).
The aim of this report is to study how factors in created wetlands, such as the increasing age of the wetland, presence of fish and macrophytes, affect the population dynamics of the horned grebe over time and discuss some measures that can help the population increase. The data on the horned grebe will be compared to different events in wetlands such as drainage of the water and reduction fishing. The hypotheses are that a newly created wetland will have a larger population size of horned grebe, and the numbers will decline with the increasing age of the wetland as the impact of the different factors increases on the horned grebe. It is also expected to find an increase in population size a few years after drainage of the wetland and reduction fishing, as a result of less competition for food and easier foraging.
3 Materials and methods 3.1 The species
The horned grebe, Podiceps auritus, returns to its nesting grounds in Sweden in March and onward. The species is relatively small, with a short straight beak, red–brown neck, long body, flat head, black cheeks, red eyes, and yellow tuft over their eyes (Svensson et al., 2009) (Fig. 1). In summer, the species is unmistakable due to their particular looks and can be spotted in shallow wetlands rich in vegetation. The horned grebe can be mistaken for the black-necked grebe (Podiceps nigricollis) in spring, but the black-necked grebe has a yellow tuft under their eyes and a rounder head that can be used to tell the two species apart. The horned grebe feeds on smaller fish or aquatic macroinvertebrates during breeding depending on what is available in the wetland (Stedman, 2000). This is because the pair of grebes is restricted to the wetlands during breeding and must rely on the food resources within it to feed
themselves and their nestlings. The horned grebe builds partially floating nests from the wetland’s emergent vegetation primarily (Fjeldså, 2004), but also use some submerged plants and the leaves of riparian trees and bushes that they collect from the bottom of the wetland
Figure 1. Adult horned grebe in breeding plumage. By Mark Medcalf - Red
EyeUploaded by snowmanradio, CC BY 2.0, https://commons.wikimedia.org/w/index.php?c urid=15791447
hatched, but cannot forage on their own, thermoregulate fully or avoid predators the first few days after hatching (Fjeldså, 2004).
3.2 The wetlands
The six wetlands are all located south of Linköping, Sweden (Fig. 2).
Figure 2. Map over the six wetlands in Linköping, Sweden. Sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community
3.2.1 Rosenkällasjön
Rosenkällasjön was created in 2004, has an area of 45 hectare and has been drained and fished to reduce the pike population and amount of Canadian pondweed in 2012, 2015 and 2019. The two first instances of draining and reduction fishing resulted in a dramatic decrease of pike in the wetland, but the attempt failed in 2019 and the pike population was reduced with an unknown result. It was estimated that the pike population was reduced by 70 % in 2012 and 95 % in 2015, with total population sizes of 5095 in 2012 and 1725 in 2015. In 2019, a total of 2863 pikes were caught and removed from the lake, but the proportion of the catch was not
known. The pikes ranged in size between 100 to 700 mm in 2012, 120 to 480 mm in 2015 and 150 to 900 mm in 2015 (Jörneskog, personal communication, April 23, 2021). Canadian pondweed has been classified as abundant from year 2007 to 2020, and the total coverage of the macrophyte is large in the wetland (Gustafsson, 2020a).
3.2.2 Edhaga
Edhaga wetland was created in 2008 with an area of 4,5 hectare and was drained and fished to reduce pike in the same years as Rosenkällasjön, 2012 and 2015. It was estimated that the pike population was reduced by 99 % in both 2012 and 2015, with total population sizes of 718 in 2012 and 484 in 2015. The pikes ranged in size between 120 to 510 mm in 2012 and was not measured at all in 2015 (Jörneskog, personal communication, April 23, 2021). Edhaga is connected to Rosenkällasjön through a narrow passage.
3.2.3 Frökärret
Frökärret was created in 2010 and has an area of 9 hectares. The wetland was drained and fished to reduce the common roach (Rutilus rutilus) population and the amount of Canadian pondweed in the fall of 2014. An estimate of 5000 common roach individuals was caught and transferred, and the lake was drained to reduce the amount of Canadian pondweed by the winters bottom freezing. The coverage of Canadian pondweed in the wetland was estimated to 8 % in 2015, 87 % in 2018, 0 % in 2019 and 31 % in 2020 (Gustafsson, 2020b).
3.2.4 Ekängssjön
Ekängssjön was created in 2012 and has a total area of 20 hectares together with Ullstämmasjön that it is connected to. Ekängssjön and Ullstämmasjön will be treated collectively in this report, and only be called Ekängssjön. The wetland is fish free and shallow with moderately amounts of vegetation, such as reeds, around the edges. About five years ago, around 50 dead individuals of European perch (Perca fluviatilis) floated up to the surface after the winter. This is most likely due to an illegal release of perch in the wetland for hobby fishing, but all the individuals died in the winter due to lack of oxygen, and the wetland has been fish free since (Jörneskog, personal communication, May 3, 2021).
3.2.5 Ängelska kanalen
Ängelska kanalen was created in 2015 and has an area of 5 hectares. The area has historically been a wetland but was drained to create meadow land and agricultural fields. The wetland is fish free and shallow (Jörneskog, personal communication, May 3, 2021).
3.2.6 Ullaberg
Ullaberg wetland was created in 2020 and has a total area of 20 hectares together with Mossens wetland that it is connected to. It is one of the newest created wetlands in Linköping, Sweden. The wetland is fish free with many trees and bushes in the water, both fallen and standing, and the edges of the wetland have a moderately amount of vegetation (Jörneskog, personal communication, May 3, 2021).
3.3 Field work: counting individuals
The wetlands that were visited during the field work was Rosenkällasjön, Ekängssjön and Ullaberg due to their different ages: one older, one a bit younger and one very young. The field work was carried out in the morning, between 5.30 and 8.00, one day a week for each wetland for three weeks. The wetlands have several locations were the counting takes place and the coordinates for these was provided by ecologist Anders Jörneskog. The counting in Rosenkällasjön took place in the three bird towers around the wetland and on Rödberget. There were three bird towers around Ekängssjön where the birds are counted, and a location on the eastern part of the wetland where one can get a good view of that part of the area. Ullaberg does not have bird towers, so the counting took place in locations around the wetland where one had a good view of the area. The wetlands were divided into zones for each location, so no individual was counted twice. Each location was visited for 20–25 minutes each, where the individuals was counted in two “sweeps” with the binoculars and the maximum number of individuals seen at one occasion was recorded. The coordinates for the bird’s placement in the lake was noted in Google maps on a mobile phone.
3.4 Data analysis
The data from earlier years was provided by ecologist Anders Jörneskog. The data on the horned grebe in the wetlands has been collected since 2004 by Anders, where the number of nesting pairs and juveniles was noted. It is also noted for each year the highest number of individuals in one single count, called max momentarily. The numbers of nesting pairs,
juveniles and max momentarily was transferred into R (R core team, 2021). The number of juveniles was divided by the number of pairs, and the number of pairs was divided by the max momentarily for each year. Statistical tests in form of three linear regressions was performed in R to find out if the population dynamics of the horned grebe was affected negatively by the increasing age of the wetlands. One regression compared the number of pairs and the age of the wetlands, the second the number of juveniles and the age of the wetland, and the third one the number of juveniles and the number of pairs in the wetlands. The significance level was set at 0,05 (5 %) with two degrees of freedom. The coordinates collected for the locations of the birds in the wetlands from the field work was transferred to ArcGIS. The maps were created with the locations where the individuals were spotted, and from the locations a density plot was created to show habitat preferences for the horned grebe in the wetlands. Maps throughout this study were created using ArcGIS® software by Esri.
4 Results
4.1 Data analysis
4.1.1 Population dynamics
The pairs, juveniles and max momentarily of horned grebe in Rosenkällasjön increased from 2004 to 2009 to 20 pairs and over 60 individuals (Fig. 3). The numbers crashed in 2009 and 2010 and has not yet recovered. The other wetlands are showing a similar pattern with a higher number of horned grebe the first years after the creation of the wetland, to then decrease. Some of the wetlands have a small population of the horned grebe after the decline and have not recovered but two of them, Ekängssjön and Frökärret, is starting to increase the years after.
Figure 3. The fluctuation of the horned grebe in the six wetlands visualized over the years. The quantity of pairs (A), juveniles (B), and max momentarily (C) is compared.
All the wetlands except for Ängelska kanalen and Ullaberg indicates a crash in pairs when the wetland is around six to eight years old (Fig. 4). This is most visible in Rosenkällasjön six years
0 5 10 15 20 2005 2010 2015 2020 Year Quantity Wetlands Edhaga Ekängssjön Frökärret Rosenkällasjön Ullaberg Ängelska kanalen
A. Pairs of horned grebe
0 10 20 30 2005 2010 2015 2020 Year Quantity Wetlands Edhaga Ekängssjön Frökärret Rosenkällasjön Ullaberg Ängelska kanalen
B. Juveniles of horned grebe
0 20 40 60 2005 2010 2015 2020 Year Quantity Wetlands Edhaga Ekängssjön Frökärret Rosenkällasjön Ullaberg Ängelska kanalen
after its creation, when the pairs drop from 20 to 2 in just one year. Wetlands younger than six years, Ängelska kanalen and Ullaberg, doesn’t have enough data to show such a pattern in the pairs yet. The number of juveniles is showing approximately the same pattern, with a decrease around six to eight years after the wetland was created. The only wetland that is showing a bigger increase in both pairs and juveniles after its crash is Ekängssjön, while Frökärret is showing a smaller increase of both after the crash.
Figure 4. The population dynamics of the horned grebe in the six wetlands visualized with the same start year. The quantity of pairs (A) and juveniles (B) in the wetlands is compared.
Table 1 shows the mean number of juveniles per pairs in the wetlands. The general pattern for the wetlands is to have a higher mean number of juveniles close to the creation of the wetlands, to then decrease a few years after. Two of the wetlands, Rosenkällasjön and Edhaga, stays at that smaller mean number up to present time and Frökärret, Ekängssjön and Ängelska kanalen shows an increase in the quota later. Ullaberg only has one value due to its young age.
0 5 10 15 20 5 10 15 Years Quantity Wetlands Edhaga Ekängssjön Frökärret Rosenkällasjön Ullaberg Ängelska kanalen
A. Pairs of horned grebe with same start year
0 10 20 30 5 10 15 Years Quantity Wetlands Edhaga Ekängssjön Frökärret Rosenkällasjön Ullaberg Ängelska kanalen
Table 1. The mean number of juveniles per pair in the six wetlands over the years. The orange indicates the years the wetland was drained, and reduction fished.
The mean number of pairs in the wetlands is seen in table 2. The same pattern as in table 1 can be seen, with a higher number of pairs close to the creation of the wetlands and a decrease after a few years. The two wetlands that is staying at a lower mean number up to present time is the same as in table 1 and so is the three wetlands that is showing an increase later on.
Year Rosenkällasjön Edhaga Frökärret Ekängssjön Ängelska kanalen Ullaberg
2004 1 2005 2,63 2006 3,25 2007 0,76 2008 1,65 0,33 2009 0,2 0,25 2010 1,5 0,4 0,57 2011 1 0,33 1 2 2012 0 0 1,5 1,45 2013 0 0 1 0,63 2014 - - - 1,4 2015 - - - 0,9 2016 1,5 - 3 0,75 2 2017 0 0 0,76 2 1 2018 0 - 0 2,2 -2019 1 0 0,75 1,13 0 2020 - - 1,2 1,85 1 2,5
Table 2. The mean number of pairs that formed from the max momentarily in the six wetlands over the years. The orange indicates the years the wetland was drained, and reduction fished.
4.1.2 Effect of wetland age on breeding success
Number of pairs decreased significantly with wetland age for Rosenkällasjön (Regression model global test: F(1,15)=7,55; p=0,015) and Edhaga (F(1,11)=9,25; p=0,011). Frökärret (F(1,9)=0,17; p=0,68), Ekängssjön (F(1,8)=1,57; p=0,25) and Ängelska kanalen (F(1,3)=0; p=1) did not show a similar decrease in pairs with an increasing age of the wetland. Ullaberg only has data from one year due to its young age, so no regression could be carried out for that wetland.
Number of juveniles decreased significantly with wetland age for Rosenkällasjön (F(1,15)=9,43; p=0,007) and Edhaga (F(1,11)=12,1; p=0,005). Frökärret (F(1,9)=0,13; p=0,72), Ekängssjön (F(1,8)=1,47; p=0,26) and Ängelska kanalen (F(1,3)=1,42; p=0,32) did not show a similar decrease in juveniles with an increasing age of the wetland. No test was carried out for Ullaberg due to its young age.
The number of juveniles increased with and increasing number of pairs in Rosenkällasjön (F(1,15)=11,91; p=0,0035), Edhaga (F(1,11)=43,53; p=0,000038), Frökärret (F(1,9)=12,59; p=0062)
Year Rosenkällasjön Edhaga Frökärret Ekängssjön Ängelska kanalen Ullaberg
2004 1 2005 1,33 2006 0,84 2007 0,73 2008 0,7 1 2009 0,63 1 2010 0,06 1 1 2011 0,06 0,86 0,8 1,18 2012 0,12 0,4 0,4 0,71 2013 0,75 0,33 0,4 0,41 2014 0 0 0 0,63 2015 0 0 0 0,87 2016 0,8 0 0,76 0,84 1 2017 0,57 0,8 0,86 0,5 1 2018 0,4 - 0,5 0,77 0 2019 0,5 0,8 0,73 0,71 0,76 2020 0 - 0,91 0,98 1 0,8
4.2 Habitat preference
A total of three individuals was spotted in Rosenkällasjön during the field work, thus a map was not created because of the small amount of retrieved data over the three weeks. Maps were created over the distribution of the horned grebe in Ekängssjön and Ullaberg, where they were seen in larger quantities (Fig. 5 and 6).
Figure 5. The distribution of horned grebe in Ekängssjön over three weeks in spring, 2021. A darker, more red color indicates more individuals. Sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.
A total of 66 individuals of horned grebe was seen in Ekängssjön during the three weeks of field work. The western part of the wetland had a larger amount of macrophytes, the middle had clear water and the eastern part also had clear, open water with two bigger islands of vegetation. The horned grebe showed a preference for areas with less aquatic vegetation and were mostly foraging during the observations.
Figure 6. The distribution of horned grebe in Ullaberg over three weeks in spring, 2021. A darker, more red color indicates more individuals. Sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.
A total of 29 individuals of horned grebe was seen in Ullaberg during the three weeks of field work. This is an increase from 2020 where a total of 5 individuals were seen. Most individuals were seen foraging in areas with less aquatic vegetation, just as in Ekängssjön.
5 Discussion
The result from this study indicates that the horned grebe decreased considerably in wetlands inhabited by predatory fish and contains a large amount of macrophytes. This corresponds to the result of from Summers et.al. (2011) and Zimmer et.al (2001) where the studies showed that the amount of macrophytes and presence of fish in wetlands affect the population dynamics of the horned grebe. Fish can greatly decrease the abundance of aquatic invertebrates in the wetland, increase competition for food and pose a threat to juveniles, while an abundance of macrophytes in the open water makes it harder to forage.
5.1 Negative effects from the presence of pike
Other studies (Summers et.al. 2009; Summers et.al 2011; Wagner, 1997) has showed that the diet of some fish species, for example pike, roach, and perch, is overlapping with the diet of birds. The horned grebe is one of them. Out of the six wetlands in this report, two are inhabited by the fish species pike and one by the common roach. Pike is established in Rosenkällasjön and Edhaga, while Frökärret is inhabited by the common roach. Pike is a piscivore but can also eat smaller aquatic invertebrates depending on the size of the pike and the size of the prey (Craig, 2008). The number of pairs and juveniles of horned grebe decreased with the increasing age of the wetland in Rosenkällasjön and Edhaga, the only wetlands inhabited with pike. The other wetlands did not show the same pattern. This indicates that the presence of pike in wetlands have a negative effect on the population’s survival and reproduction of the horned grebe, making foraging harder for the pairs and posing a threat to the juveniles.
Before 2012, no measures had been taken to decrease the number of pike in Rosenkällasjön, and the total population size was estimated to 5095 individuals in 2012. After the reduction fishing of pike in 2012 and 2015, a slight increase in pairs is seen (Fig. 3) as well as an increase in mean number of pairs to total number of individuals (Table 2). Edhaga did not show an increase in pairs after 2012, but after 2015 there was a small increase. This could be because of the decreased pike populations in the wetland and less food competition, but the number of juveniles was not increasing in the same way. Even though the wetlands pike populations were reduced after the reduction fishing, the pike re-established themselves fast in both Rosenkällasjön and Edhaga, and pike once again reached a large population size a few years after. This is most likely the reason for the declines of the horned grebe a few years after the reduction fishing as the mean number of juveniles per pair in both Rosenkällasjön and Edhaga
goes down. The number of pairs and juveniles in Rosenkällasjön was low in 2010, but the max momentarily was very high with over 60 individuals the same year. This could be because the horned grebe arrived at the wetland but realized there was a big competition for food because of the large pike population, did not form pairs and ultimately decided to move to other wetlands. Rosenkällasjön later went down to zero pairs for several years after this event.
Pike is also known to prey on the juveniles of horned grebe. The pike can grow between 60 to 90 cm in 5 years depending on factors such as food availability and competition in the wetland (Van Engel, 1940). In 2009, five years had passed since Rosenkällasjön was created. This means that some pike individuals potentially could have grown to between 60 to 90 cm, and those individuals could prey on the juveniles of horned grebe when they are newly hatched and has left the nest. This could explain Rosenkällasjöns decrease in juveniles in 2007 if some pike individuals had the chance to grow large and prey on the chicks. In 2014, seven years had passed since Edhaga was created and is showing a similar decrease in juveniles as Rosenkällasjön. Summers et.al (2009) found that the loss of juveniles of horned grebe was 5,9 times higher in waters where pike was established due to predation, and this could explain the decrease in juveniles in both wetlands.
5.2 Other fish species cause competition for food
As mentioned above, the common roach is another fish species that can create competition for food in wetlands for the horned grebe. The common roach diet consists of organisms from the taxonomic classes Gastropoda, Bivalvia and Insecta depending on what is available in the habitat (Lappalainen et.al. 2001). The insects in the common roach diet are chironomids and Trichoptera. This is overlapping with the diet of the horned grebe because they also feed on these aquatic invertebrates. A study by Brooks et.al. (2012) showed a correlation between the abundance of chironomids and annual productivity of horned grebe. Frökärret was inhabited by the common roach up to 2014. The diet of the horned grebe preferably consists of fish in winter, and a study by Piersma (1988) showed that up to 95 % of the diet in the winter consists of smelt (Osmerus eperlanus) and around 5 % of European perch (Perca fluviatilis). The study found no evidence of common roach being a part of the diet of the horned grebe. During summer, the main fish in their diet is sticklebacks (Gasterosteidae sp), but also minnows (Phoxinus
does not feed on the common roach, but both species feed on chironomids and other insects, there could be a competition for food between the bird and the fish. The horned grebe and the fish populations will compete for the food in the wetlands, making it harder for the grebe to collect enough food for themselves and the juveniles. Frökärret is not showing the same decrease in pairs and juveniles as Rosenkällasjön and Edhaga, and this could be because of reduction fishing in 2014. About 5000 common roach individuals were caught in Frökärret 2014 and transferred, and no more reduction fishing has been implemented in the wetland since. This could potentially be the reason for the decline of the horned grebe in Frökärret up until 2014, when the common roach population was increasing dramatically. After the reduction fishing in 2014, the horned grebe is showing an upward trend in both pairs and juveniles.
Four of the wetlands show a similar trend with a crash in number of pairs and juveniles six to eight years after its creation, but the only recovering wetlands is Ekängssjön which is fish free, and Frökärret that is showing a smaller increase in both pairs and juveniles after the reduction fishing. The other wetlands ultimately stay at a lower or non-existing number of horned grebe after the crash. Ekängssjön is showing a similar pattern in the number of horned grebe the first seven years to those who are inhabited by fish. The difference between Ekängssjön and the wetlands inhabited by fish is the big increase in both pairs and juveniles the years after its crash in 2017. One thing that is known, is that about five years ago around 50 dead individuals of European perch floated up to the surface after the winter. In a study by Rask (1986) it was discovered that perch that ranges in size between 11-13 cm or bigger feed on larvae of Odonata, Trichoptera and Chironomidae in early summer, and later switched to other invertebrates. This is much like the diet of the horned grebe. Although it has been showed that a few percentages of the diet of the horned grebe are made up by perch (Piersma, 1988), the length of the perch that is eaten by horned grebe ranges between 4-7 cm. If the perch that was placed illegally in the wetland had grown larger than that, it would not be consumed by the horned grebe due to its size, but instead create some potential competition for food. This incident matches with the decline in pairs and juveniles of horned grebe in 2017 and could potentially explain the crash that is seen in the wetland. However, 50 individuals of perch are not a very large population, and other unknown factors could be the reason for the decline.
More pairs of horned grebe should give more juveniles, and less pairs should give less juveniles. This pattern is visible in all the wetlands most years, but Rosenkällasjön is showing a big
deviation in Rosenkällasjön in 2007 could once again be due to the pike and their predation on the chicks. The pairs either didn’t mate due to the competition for food or couldn’t feed their juveniles and they died due to starvation. A similar deviation in the number of juveniles is seen in Ekängssjön in 2013, 2015 and 2016. As seen in table 1, the number of juveniles per pair in Ekängssjön is the lowest those years with 0,63, 0,9 and 0,75 juveniles per pair respectively. This decline in juveniles somewhat matches with the discovery of the illegally released perch, and this could explain the decline.
5.3 Macrophyte abundance affects the ability to forage
The horned grebe prefers wetlands with clear open water and more vegetation close to the edges (Summers et.al. 2011). The clear open water makes it easier to forage, and the vegetation close to the edges provides important nesting material. An abundance of macrophytes in the open water in wetlands inhabited by horned grebe will affect their ability to forage, making it harder to feed themselves and their juveniles (Jörneskog & Molin, 2015). The results in this study indicates that the horned grebe prefers foraging in the areas with clear, open water in the wetlands, and is seen more sparsely in areas with more macrophytes or trees. Rosenkällasjön was drained the same years as the reductions fishing to reduce the amount of Canadian pondweed, but the plant was classified as abundant in the wetland between 2004 and 2020 and could still be a problem for the grebe. There was a small increase in pairs of horned grebes after the draining, but this could also be because of the reduced amount of macrophytes after the draining of the wetland and not just the reduced pike population. The amount of Canadian pondweed in Frökärret has fluctuated considerably over the years with coverage ranging from 0 to 87 % (Gustafsson, 2020). The coverage of the macrophyte in 2018 was 87 %, and this could explain the dip in both pairs and juveniles that year. The coverage of Canadian pondweed in 2019 and 2020 was 0 and 31 %, and this could explain the increasing numbers of pairs and juveniles those years. Macrophytes is also important for the pike’s survival in wetlands, and they need macrophyte cover that ranges from 35 to 70 % (Casselman & Lewis, 1996). Macrophytes is abundant in Rosenkällasjön (Gustafsson, 2020), which also makes it a suitable habitat for the pike. During the field work, it was seen that both Ekängssjön and Ullaberg has clear, open waters which makes them good habitats for the horned grebe. There is no available information about Ängelska kanalens aquatic vegetation, which makes it hard to come to any
was created in 2015 and is fish free, but it is not showing the same upward trend in horned grebe as Ullaberg, which is only one year old.
A total of three individuals of horned grebe was spotted in Rosenkällasjön during the field work. This is a very low number considering the wetlands size and former status for the horned grebe. Ekängssjön is still showing a significant number of horned grebe and matches the criteria for a preferred wetland. Ullaberg is showing a big increase of the horned grebe from 2020 to 2021. The wetland is very young, and the horned grebe is still discovering the wetland, but is showing a positive trend for the species. The field work in Ullaberg provided new data that can be used in the future to study the horned grebe in Linköping.
In conclusion, the factor that impacts the population dynamics of the horned grebe the most in created wetlands in Linköping is predatory fish, such as pike, in the wetland. Other fish species with similar diets, such as common roach and European perch, can also pose a threat to the population of the horned grebe because of the increased competition for food. The abundance of macrophytes in the wetland also seems to affect the horned grebe, but not in the same extent as the presence of fish. The result confirms the hypothesis that wetlands inhabited by fish and a large coverage of macrophytes is unsuitable for the horned grebe. Right now, Ekängssjön and Ullaberg are the best nesting sites out of the six wetlands, with an upward trend in both pairs and juveniles.
6 Conservation measures
Measures that can be taken to benefit the horned grebe is creation of new, fish free wetlands with more consistent reduction fishing in case of establishment of pike or other fish species. If there is an abundance of macrophytes in the open water, drainage of the wetland to remove the macrophytes such as Canadian pondweed can be done to clear up the water and help the horned grebe forage. However, the presence of sedges, willows and birches close to land is important for the species ability to build nests (Summers et.al. 2011). It is therefore appropriate to keep the existing vegetation near the edges or create more sedge-beds, and plant willows and birch around the wetland to help the horned grebe build good, protected nests. If possible, introducing more organisms that is in the diet of the horned grebe in the wetland could help increase the population. By introducing sticklebacks or minnows in the wetland, the horned grebe will get more food for themselves and their juveniles. Ängelska kanalen is fish free, just as Ekängssjön and Ullaberg, but is not showing the same numbers of horned grebe. By implementing some
measures, like removing macrophytes from the open water, create more sedge-beds, plant trees near the edges and planting fish that the horned grebe eats, the wetland can become a more suitable nesting site for the species.
7 Societal and ethical considerations
The conservation and restoration of the horned grebe and their habitats in Sweden and the rest of the world is important for many societal aspects. The biodiversity in the environment must be preserved to minimize the risk of disrupting food webs and ecosystem services. It is also important to preserve the species and restore their habitats to reach several of the 17 sustainability goals from the United Nations, such as clean water and sanitation, climate action and life on land. Wetlands are very important because of their ability to purify water, keep nutrients from reaching our oceans and act as carbon sinks. Restoration of this habitat can help us halt climate change and provide key habitats to endangered and sensitive species. Taking necessary measures to conserve the horned grebe increases the biodiversity in the wetlands and stabilizes the food webs. It is also important from a recreational and ethical point of view to conserve our bird species. The diversity in bird species will enhance the nature experience and it is not ethical to cause harm to these species and not take the necessary measures to conserve them. Linköpings kommun has decided to take a big responsibility for the horned grebe, and eleven more species who are not the focus of this study and take measures to preserve it in the county. Studies such as this can also help raise awareness of our more vulnerable bird species and provide new, important data of the species distribution and population dynamics in Sweden or other countries.
From an ethical view, the study of the population dynamics of the horned grebe in different wetlands were non-invasive. The grebe can be monitored from far away with minimal influence on their wellbeing and activities, but the results from a study can greatly increase the knowledge around the bird and lead to more measures to preserve it. To get a greater understanding of the species reproductive success, studies could be a little bit more invasive. One could for example count and measure the eggs of the horned grebe during nesting, and this could potentially disturb the birds. Studies like this are however important to get an even greater understanding of the horned grebe and their dynamics and should always be done professionally with minimal
8 Acknowledgements
I would like to thank the following people for helping me finalize my bachelors’ theses: My supervisor Karl-Olof Bergman has provided great support and aid during the semester, his involvement and knowledge has made this thesis possible, and I am very grateful for his help. Ecologist Anders Jörneskog from Linköpings kommun has provided indispensable data and information about the horned grebe and the wetlands.
Student Joakim Mathiasson aided me when I created the maps over the wetlands in ArcGIS, his help was truly indispensable for my work.
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Appendix
Table 3. Raw data over the number of pairs of horned grebe in the wetlands over the years.
Table 4. Raw data over the number of juveniles of horned grebe in the wetlands over the years.
Year Rosenkällasjön Edhaga Frökärret Ekängssjön Ängelska kanalen Ullaberg
2004 4 2005 8 2006 8 2007 15 2008 20 3 2009 20 4 2010 2 5 7 2011 1 3 5 10 2012 1 1 4 11 2013 3 1 2 8 2014 0 0 0 5 2015 0 0 0 10 2016 2 0 1 8 1 2017 2 2 6 1 1 2018 2 0 2 10 0 2019 1 2 4 15 1 2020 0 0 5 20 1 2
Year Rosenkällasjön Edhaga Frökärret Ekängssjön Ängelska kanalen Ullaberg
2004 4 2005 21 2006 26 2007 10 2008 33 1 2009 4 1 2010 3 2 4 2011 1 1 5 20 2012 0 0 6 16 2013 0 0 2 5 2014 0 0 0 7 2015 0 0 0 9 2016 3 0 3 6 2 2017 0 0 4 2 1 2018 0 0 0 22 0 2019 1 0 3 17 0 2020 0 0 6 37 1 5
Table 5. Raw data over the max momentarily number of horned grebes in the wetlands over the years.
Year Rosenkällasjön Edhaga Frökärret Ekängssjön Ängelska kanalen Ullaberg
2004 8 2005 12 2006 19 2007 41 2008 57 2009 63 8 2010 62 10 14 2011 31 7 13 17 2012 17 5 20 31 2013 8 6 10 34 2014 8 5 7 16 2015 2 3 2 23 2016 5 1 3 19 2 2017 7 5 14 4 2 2018 10 0 8 26 2 2019 4 5 11 42 3 2020 1 0 11 41 2 5
