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The impact of cormorant nesting colonies on plants and arthropods
Stockholm University, Faculty of Science, Department of Botany. (Plant ecology)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seabirds concentrate large amounts of marine nutrients on their nesting islands. This nutrient input can have large consequences for island food webs and community structure. The high nutrient load may also cause runoff into surrounding waters and affect marine communities. In my thesis, I studied the effect of cormorant nesting colonies on the stoichiometry, abundance, species richness, and species composition of plants, algae, and invertebrates on land and in costal waters and investigated if differences in the elemental composition or homeostasis can explain differences in the numerical response among invertebrate groups. δ15N analysis indicated that ornithogenic nitrogen provided a significant nitrogen source for plants and arthropods on nesting islands and around high nest density islands also for brackish algae and invertebrates. Furthermore, nutrient runoff created a potential feed-back loop to spiders via chironomids. Cormorant nutrient input changed island vegetation and increased plant P and N content and epiphytic algae:Fucus ratio, but decreased plant species richness and vegetation cover. Invertebrates responded indirectly to these qualitative and quantitative changes in their food source and habitat, but also directly to cormorant subsidies. However not all taxonomic and feeding groups were affected and responses were both positive and negative. Differences in the numerical response among taxonomic groups could not be explained by differences in the level of homeostasis, since, generally, all invertebrates were strongly homeostatic. Similarly, consumer nutrient content was a poor predictor for displayed responses. I conclude that cormorant colonies have strong impacts on island vegetation and some consumer groups. However, even if they can decrease the species richness of some organism groups on their nesting islands, they increase the habitat heterogeneity in an archipelago and thus may increase the regional species diversity.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2010. , 80 p.
Keyword [en]
seabirds, cormorants, islands, nitrogen, phosphorus, plants, arthropods, δ15N, Baltic Sea, ecological stoichiometry, species richness, species composition, numeric response
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
URN: urn:nbn:se:su:diva-43165ISBN: 978-91-7447-147-2 (print)OAI: oai:DiVA.org:su-43165DiVA: diva2:354359
Public defence
2010-11-05, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Accepted. Paper 4: Manuscript.Available from: 2010-10-14 Created: 2010-10-01 Last updated: 2010-10-18Bibliographically approved
List of papers
1. The impact of cormorants on plant–arthropod food webs on their nesting islands
Open this publication in new window or tab >>The impact of cormorants on plant–arthropod food webs on their nesting islands
2010 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 13, no 3, 353-366 p.Article in journal (Refereed) Published
Abstract [en]

This study investigated the effects of cormorant colonies on plant–arthropod island food webs, the consequences of nutrient-rich runoff on marine communities, and feedback loops from marine to terrestrial ecosystems. Terrestrial plant responses were as expected, with the highest plant biomass on islands with low nest density and the highest nitrogen (N) content on islands with high nest density. In contrast to our hypothesis, we found no uniform density response across guilds. Among herbivores, the variable responses may depend on the relative importance of plant quality or quantity. As expected, nutrient-rich runoff entered water bodies surrounding cormorant nesting islands, but only at high nest density, and increased the density of emerging insects. This created a potential feed-back loop to spiders (major terrestrial predators), where stable isotope analyses suggested great use of chironomids. Contrary to our expectation, this potential feed-back did not result in the highest spider density on islands with a high cormorant nest density. Web spiders showed no changes in density on active cormorant islands, and lycosids were actually less abundant on active cormorant islands compared to reference islands. The variable response of spiders despite increased dipteran densities, and also in other consumer groups, may be due to direct negative effects of cormorants on soil chemistry, vegetation cover, and other density regulating forces (for example, top–down forces) not studied here. This study highlights the importance of including processes in the surrounding marine ecosystem to understand the impacts of seabirds on the food web structures of their nesting islands. 

Keyword
seabird islands, food webs, vegetation, chironomids, stable isotopes, nitrogen
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-43292 (URN)10.1007/s10021-010-9323-8 (DOI)000277283600001 ()
Available from: 2010-10-06 Created: 2010-10-06 Last updated: 2017-12-12Bibliographically approved
2. The impact of nesting cormorants on plant and arthropod diversity
Open this publication in new window or tab >>The impact of nesting cormorants on plant and arthropod diversity
(English)Article in journal (Refereed) Submitted
Abstract [en]

Seabirds can strongly affect major factors correlated with species diversity – primary productivity, heterogeneity, and disturbance – on their nesting islands through the concentration of marine nutrients and physical disturbing island vegetation. In this study, we investigated the effects of nesting cormorants on the abundance, species richness, and composition of plants and arthropods (Coleoptera, Heteroptera, Araneae, and Chironomidae) on islands in Stockholm archipelago, Sweden. We found that cormorants had a negative effect on plant species richness and vegetation cover and that they changed plant species composition. Plant biomass showed no linear correlation with nest density when considering all islands studied, but was negatively correlated with nest density when considering only cormorant islands. The effect of nesting cormorants on island arthropods varied across feeding groups and sampling methods. Coleopterans and cursorial spiders responded with shifts in species richness and composition, and several coleopteran feeding groups and chironomids also changed in abundance. The abundance and species richness of saprophagous and fungivorous coleopterans were higher on active cormorant islands than on reference islands, while the abundance and species richness of herbivorous coleopterans and the species richness of cursorial spiders were negatively correlated with nest density. In structural equation modeling we found that some feeding groups were directly affected by nest density, but that many of the effects of seabirds on island consumers were mediated by changes in vegetation. We conclude that nesting cormorants affect the diversity of their nesting islands and the archipelago as a whole. Although cormorant colonies can decrease the species diversity of plants and some invertebrate groups on their nesting islands, the total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g., saprophagous and fungivorous coleopterans).

Keyword
seabirds, islands, species richness, species composition, heterogeneity, feeding groups
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-43243 (URN)
Available from: 2010-10-06 Created: 2010-10-05 Last updated: 2010-10-06Bibliographically approved
3. Effects of seabird nesting colonies on algae and aquatic invertebrates in coastal waters
Open this publication in new window or tab >>Effects of seabird nesting colonies on algae and aquatic invertebrates in coastal waters
(English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599Article in journal (Refereed) Accepted
Abstract [en]

Seabirds concentrate marine nutrients, from large marine areas, on their nesting islands. The high nutrient load may cause runoff into surrounding waters and affect marine communities in similar ways as reported from marine fertilization experiments. In order to test if cormorant colonies affect algae and invertebrates in surrounding coastal waters, we collected Fucus vesiculosus fronds, its epiphytic algae and associated invertebrate fauna near abandoned and active cormorant nesting islands and reference islands without nesting cormorants in the Stockholm archipelago in the northern Baltic Proper, Sweden. First, we showed, with δ15N analyses, that ornithogenic nitrogen provided a significant nitrogen source for algae and invertebrate consumers near islands with high nest density. Second, the nitrogen and phosphorus content of algae near active cormorant islands with high nest density was elevated and epiphytic algae increased relative to Fucus. Third, 3 of 5 invertebrate taxa (Jaera albifrons, Gammarus spp. and Chironomidae) showed increased biomasses near islands with high nest density, but contrary to earlier fertilization studies only J. albifrons increased in abundance compared to reference islands. We conclude that runoff from seabird colonies has a profound effect on primary producers and some consumers in the surrounding water, but only if the colonies exceed a certain nest density. Thus, seabirds not only affect marine communities via top-town forces as commonly assumed, but also via bottom-up forces by concentrating nutrients around their nesting islands. Consequently, seabird islands can be seen as natural fertilization experiments and give important insights in the effects of eutrophication of marine systems.

Keyword
nitrogen, stable isotopes, bottom-up, eutrophication, Baltic Sea, cormorants
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-43246 (URN)
Available from: 2010-10-06 Created: 2010-10-05 Last updated: 2017-12-12Bibliographically approved
4. Ecological stoichiometry and homeostasis of plants and invertebrates on and nearby heavily fertilized cormorant nesting islands
Open this publication in new window or tab >>Ecological stoichiometry and homeostasis of plants and invertebrates on and nearby heavily fertilized cormorant nesting islands
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Ecological stoichiometry has generalized the fundamental role of individual nutrient demand in all ecological processes and interactions. It implies that the elemental composition (C:N:P) of a consumer relative to the C:N:P of its resource determines its growth rate and thus secondary productivity. A central, but recently questioned, principle of ecological stoichiometry is the assumption that heterotrophs, in contrast to autotrophs, keep their elemental composition strongly homeostatic. Since neither the relationship between consumer C:N:P and its numeric response to changes in resource C:N:P nor the C:N:P homeostasis of arthropods have been extensively studied for arthropods, we used a natural gradient of N and P loads, in form of seabird and non-seabird islands, to investigate the stoichiometry and homeostasis of primary producers and invertebrates, both on and around islands. We then looked for causal relationships between stoichiometry and the level of homeostasis of a taxonomic group, respectively, and observed numerical responses to seabird fertilization. We found in accordance to principal theories that invertebrates, generally, strongly regulated their stoichiometry while autotrophs were stoichiometrically plastic. Thus, we found no causal relationship between consumer homeostasis and displayed numeric responses. Furthermore, we found only weak support for the hypothesis that the C:N:P of a taxa determines its numeric response to increased resource nutrient content (lepidopteran larvae had high P:C and high abundance on P-rich cormorant islands). We conclude that other species traits than nutrient content mainly determine the success of a taxa in a certain environment. Additionally, due to the strong effects of different level of homeostasis on ecological interactions, food web dynamics and nutrient cycles, we underline the need of further studies on the homeostasis of arthropods.

Keyword
N:C ratio, P:C ratio, heterotrophs, numeric response, Galerucella
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-43245 (URN)
Available from: 2010-10-06 Created: 2010-10-05 Last updated: 2010-10-06Bibliographically approved

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