The Effect of Land Management on Aquatic Invertebrates
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DEPARTMENT OF BIOLOGICAL AND ENVIRONMENTAL SCIENCES THE EFFECT OF LAND MANAGEMENT ON AQUATIC INVERTEBRATES Christopher Magnusson Degree project for Master of Science (120 hec) with a major in Biology BIO797 Conservation Biology 60 hec Second cycle Semester/year: Autumn 2019 Supervisor: Johan Höjesjö, Department of Biological and Environmental Science Examiner: Karin Hårding, Department of Biological and Environmental Science Table of contents Abstract 2 Sammanfattning 2 Introduction 3 Background 3 Focus of the study 4 Aim 4 Material and methods 4 Materials 4 Methods 5 Indexes and selection 6 Statistics 6 Results 7 Novel finds 10 Discussion 10 Conclusion 12 Acknowledgements 12 Reference list 12 Appendix 1. Populärvetenskaplig sammanfattning 15 Appendix 2 16 Appendix 3. 17 Photo on front page: Wetland in municipality of Mariestad Photographer: Christopher Magnusson 1 Abstract Wetlands have an important role in the agriculture and can, among other things, elevate the ground water table, clear run-off water from nutrients and mitigate the effects of droughts and floods. They also serve as important hot spots for biodiversity in the landscape. Some insects and molluscs favour grazed shores while others need the shadow that a shoreline covered by canopies offer. This study investigates what type of shores aquatic invertebrates’ favour: covered, grazed or a mixture of both. In total 29 wetlands in the region of Västra Götaland was examined, divided into four categories: (1) grazed and covered, (2) grazed not covered, (3) covered not grazed and (4) no covering or grazing. Macroinvertebrates were collected by netting in the water and most specimens in the samples were identified to either species or genus. Among the 9 556 specimens found in the study 68 genera and two subfamilies were identified. Shannon’s diversity index, ASPT-index, total number of individuals as well as numbers Ephemeroptera, Trichoptera, Asellus, Corixidae and Coenagrionidae were compared between the different categories. ASPT-index seemed to be higher in wetlands both grazed and covered. One wetland had significantly higher drainage area compared to the others. It was therefore removed from the analysis and then there was a difference in the average amount of Ephemeroptera species in favour for grazed ones. Furthermore, 13 wetlands were more distinct and when comparing them a trend was found showing higher values of Shannon and numbers of Trichoptera in covered wetlands. An added value of this study is increased knowledge on the distribution of species in small wetlands, many of them underreported. Future studies should include additional factors such as bottom substrate, water chemistry, abundance and species composition of aquatic macrophytes that can also be important in determining species composition. Keywords: Wetland, invertebrates, grazing, covered shorelines, freshwater Sammanfattning Våtmarker är viktiga i jordbrukslandskapet, de påverkar bland annat grundvattennivåerna, renar avrinningsvatten från näringsämnen och minskar effekterna av både torka och översvämningar. De är också viktiga som reservoarer för biodiversitet. Vissa djur behöver betade stränder, medan andra istället vill ha dem skuggade av buskar eller träd. Denna studie syftar till att undersöka vilken strandtyp som akvatiska evertebrater föredrar: skuggade, betade eller en blandning av dessa. Totalt undersöktes 29 små våtmarker i Västra Götaland, uppdelade på fyra kategorier: (1) betad och skuggad, (2) betad, ej skuggad, (3) skuggad, ej betad och (4) varken betad eller skuggad. Våtmarkerna provtogs genom vattenhåvning av makroinvertebrater. Efteråt bestämdes de insamlade individerna till art eller släkte så långt det var möjligt. Bland de 9 556 individer som hittades i studien identifierades 68 släkten och två underfamiljer. Diversitetsindex, ASPT-index, totala antalet individer och antalet av Ephemeroptera, Trichoptera, Corixidae och Coenagrionidae jämfördes mellan de fyra kategorierna. ASPT-värdet verkar bli högre i våtmark med både betning och skuggning. En skillnad syntes i antal Ephemeroptera-arter, där det fanns fler i betade våtmarker. Detta var dock endast om en våtmark med mycket större tillrinningsområde än de övriga räknades bort. Utöver detta valdes 13 stycken våtmarker ut med tydligare kategorisering. När dessa undersöktes fanns en trend mot högre biodiversitet och mer Trichoptera i skuggade våtmarker. En viktig del av studien var att öka kunskapen om utbredningen av arter som trivs i små våtmarker, många underrapporterade. Framtida studier bör inkludera ytterligare andra aspekter som bottenförhållanden, vattenkemi, täckning och artsammansättning av makrofyter vilka kan vara korrelerade till biodiversitet av makroeverebrater. En metod att kategorisera småvattens betydelse för den regionala biodiversiteten kan vara värdefull för naturvården och vid inrättande av biotopskydd. Nyckelord: Våtmarker, invertebrater, bete, skuggade stränder, sötvatten 2 Introduction Background Small wetlands have important functions and provide ecosystem services. This study focusses on the biodiversity associated with wetlands. For example, several species of birds use small wetlands during either breeding season, migration or both and need a network of water bodies to prosper (Johnson 2001). Amphibians depend on small wetlands to reproduce and benefit from small isolated ones (Semlitsch & Bodie 1998). The number of insects has declined recently (Hallmann et al. 2017) and many species of insects benefit from water bodies and are dependent on them in one or more stages of their life cycle (Batzer & Wissinger 1996). Insects also constitute prey for e.g. water fowl and have an important role in the wetland ecosystem. Grazed shorelines are important to many species, both plants and animals. Waders, such as the Lapwing (Vanellus vanellus) need the open area that grazing animals provide and benefit from shores free from trees where crows (Corvus corone) and other predators may perch (Berg et al. 1992, Żmihorski et al. 2018). Grazing cattle also provide an increasing amount of organic matter that can serve as food for invertebrates (Scrimgeour & Kendall 2003). This is more important in waters with open shores and especially in running waters. They might however also increase the nutrient load in the wetland. This is an especial concern in small bodies of water. It is therefore important to not overgraze the wetlands. A moderate amount of cattle might improve the biodiversity by creating microhabitats by their trampling and, when allowed to venture out into the water, a blue border (Strand 2019). The blue border is a border of open water between the shoreline and the reed bed. This area can be very high in biodiversity. Grazing is a management method best used with care however. Overgrazing might, as mentioned before, increase nutrient load and also increase turbidity. In some environments grazing might even be detrimental to the biodiversity (Marty 2005). In Sweden, shorelines covered by trees and shrubs is mostly advised against when creating new wetlands and when managing already existing ones (Strand 2019). They might however have an important role as some caddis fly lay their eggs on leaves hanging over water and the shredder feeding group can use fallen branches and leaves as a food source (Dosskey et al. 1997). Trees may also provide shelter for some species and as shade prohibiting the temperatures in the water from rising too much or too fast. Covered shorelines and grazing are not mutually exclusive. With few grazing animals, shrubs and trees might have time to grow big enough to avoid getting eaten. There is also the possibility to manage an open shoreline without animals by cutting down shrubs and mowing grass surrounding the wetlands by manpower. Small wetlands also have a few more important functions and are especially important in agricultural areas (O’Geen et al. 2010). They elevate the ground water table, especially if they are isolated from other bigger sources of water (Mitsch & Gosselink 2000) and prevent both flooding and drought by storing water in the landscape. Wetlands purifies water from nutrients such as nitrogen and phosphorous, especially if they are designed to optimize retention of them (Vymazal 2007). Nitrogen is a bit complicated to remove since the removal processes needs both aerobic and anaerobic environments. Phosphorus is best removed by deep ponds, where the molecules can combine with iron (III) and deposit on the bottom, combined with plant uptake (Vymazal 2007). The emission of the strong greenhouse gas nitrous oxide (N2O) can, according to a new study, be reduced by small wetlands (Webb et al. 2019). The potential to store carbon is also an important function of wetlands, but they can also work as a carbon source (Kayranli et al. 2010). They also provide important refuges for different species of plants and animals that lives on farmland (Ruggiero et al. 2008). They can, however, easily be affected by the neighbouring activities such as fertilising. It is therefore important to have a buffer strip close to the wetland that is not cultivated or fertilised (Dosskey et al. 1997). 3 Focus of the study Macroinvertebrates in freshwater can be divided into four different functional feeding groups (Allan & Castillo 2007). The first group are called grazers, which feeds on algal growth on substrates like tree branches, stones and macrophytes. They include, among others, many different kinds of snails and mayflies of the genus Cloeon. The second group, shredders, break