The impact of nesting cormorants on plant and arthropod diversity

Abstract

Seabirds can strongly affect several major factors correlated with species diversity by concentrating marine nutrients on their nesting islands and by physically 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. Nesting cormorants negatively affected plant species richness and vegetation cover and that changed plant species composition. The effect of nesting cormorants on island arthropods varied between feeding groups and sampling methods. Most orders did not change in abundance or species richness but some, such as coleopterans and spiders changed in species composition. Herbivorous coleopterans were generally negatively affected by cormorants whereas fungivorous species and scavengers were generally positively affected. In structural equation modeling we found that the effect of cormorants was sometimes direct, such as on scavengers, but many effects on island consumers were mediated by changes in vegetation caused by cormorant presence. Overall, arthropod communities were highly dissimilar between cormorant and reference islands, and we therefore conclude that nesting cormorants not only affect the diversity of their nesting islands but also of the archipelago as a whole. The total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g. scavenging and fungivorous coleopterans).

The complexity of ecological impacts induced by great cormorants

“…The abundance and diversity of herbivorous insects is reduced while fungivorous, xylophagous species and scavengers are usually promoted (Kolb et al, 2012). Fish regurgitated by cormorants provide an additional source of food.…”

“…Over time cormorants, especially in dense colonies, increase N and P content in soil layers to extreme extents (Ligęza & Smal, 2003;Hobara et al, 2005;Kolb et al, 2012;Rush et al, 2013;Litaor et al, 2014;Klimaszyk et al, 2015b). However, even a small cormorant flock can induce a short-term rise in their concentrations, usually restricted to period the birds are present (Klimaszyk et al, 2015b).…”

“…The total number of plants, including herbs and shrubs, may decrease up to as much as 70%, while forest-specific floral communities may disappear altogether. Instead, nitrophilous species such as Sambucus nigra L. overgrow the area due to high N deposition from bird droppings (Ishida, 1996;Kolb et al, 2012;Klimaszyk et al, 2015b). Their dense development may decrease light availability within the floor zone and thereby limit the growth of herbaceous species (Ishida, 1996;Hofmeister et al, 2009;Klimaszyk et al, 2015c) which grow only within shrub gaps and are usually represented by highly nitrophilous grasses such as Calamagrostis epigejos L., Poa nemoralis L. and Poa trivialis L. (Goc et al, 2005).…”

Expansion of great cormorant colony immediately increased isotopic enrichment in small mammals

“…However, the impact of such colonies on plant and animal species is insufficiently investigated (see Bostrom et al, 2012;Kolb et al, 2012). 15…”

“…Cormorant faeces may cover up to 5 80% of vegetation, with as many as 70 % of plant species disappearing in the established colonies, the rest being dominated by nitrophilous plants, such as elder (Sambucus nigra), common nettle (Urtica dioica), woodland groundsel (Senecio sylvaticus) and greater celandine (Chelidonium majus) (Goc et al, 2005;Klimaszyk et al, 2015). The abundance and diversity of herbivorous insects also decreases, while other arthropod groups may be abundant (Kolb et al, 2012).…”