The impact of grey heron Ardea cinerea and great cormorant Phalacrocorax carb., nesting in colonies, on the plant communities and soils of Latvia's lake islands | Kolonijās ligzdojošo zivju gārņu (Ardea cinerea) un jūraskraukļu (Phalacrocorax carbo) ietekme uz Latvijas ezera salu augu valsti un augsnēm

In Latvia grey heron (Ardea cinerea) and great cormorant (Phalacrocorax carbo) are nesting in colonies in trees next to water bodies. According to the literature grey heron has nested in the territory of present-day Latvia since the 18th century, whereas great cormorant as invasive and very aggressive species have emerged here only about 30 years ago. The colonies of grey heron and great cormorant are frequent on the islands of Latvia’s lakes. Because of the small size of lake islands and sensitive ecosystems found there, the impact of these birds is fairly strong. The given study summarizes the data how the presence of grey heron and great cormorant has affected the vegetation and soil chemical composition on two lake islands in Latgale (eastern Latvia): the Pildas Island in Lake Pilda and the Siena Island in Lake Ežezers. Hypotheses for the future development of woodlands affected by the studied bird species are advanced. For the study carried out in 2006-2007 on each island two observation plots (each 400 m2) were set up, one representing a natural habitat (background), the other – a site with trees comprising more than 10 nests (contaminated site). For the observation plots the occurrence of plant species for each layer was determined visually. Soil acidity (pHKCl), hydrolytic soil acidity, exchange base, humus content, and total nitrogen content were determined for the topsoil (2-7 cm deep). The amount of chemical elements (Ca, Mg, K, Na, Fe, Mn, Ni, Zn, Cu, Cd, Pb) was determined by using the atomic absorption spectrometer Aanalyst 200, but P – by colorimetricy. Prolonged supply of bird droppings on the soil surface increases substrate acidity and reduces the amount of exchange cations. Normally in broadleaved forests the ground cover is slightly acidic (pH 5.8-6.1) while under the trees with nests it is acidic (pH 3.6-4.0). The difference between the exchange base for a background and a bird contaminated substrate is 4.0-4.7 times, but the difference for the total nitrogen content is 1.4-2.0 times. There are also various results between the content of some chemical elements in the ground cover of the background and contaminated sites. Compared to the background site (Pildas Island), the amount of P in the contaminated site is even 48 times higher, Zn 6.4 and K 2.9 times higher. For Mg and Mn this difference is less: 2.8 and 1.8 times, respectively. The islands of Pildas and Siena are covered by broadleaved forests of lime (Tilia cordata) and oak (Quercus robur) with a slight admixture of spruce (Picea abies) and pine (Pinus sylvestris). Because of bird droppings the ground right under the trees with nests is completely stripped of vegetation. On less impacted places there are clumps of monodominant nitrophilous plants as Urtica dioica, Anthryscus sylvestris, Aegopodium podagraria, Sambucus racemosa. Substrate acidification, disproportions of N and especially of P under the trees with nests affect over a wider area the forest stand composition and the transformation of plant communities. Colonizing an island both grey heron and great cormorant make nests first of all in pines and spruces, and to a lesser extent in lime trees and oaks. Due to changes in substrate chemical properties spruce trees decay at first, while pines and broadleaves are more resistant against the excessive amount of bird droppings. The birds nesting in colonies in trees is an extreme natural disturbance in woodlands, which promotes formation of glades taken over by high-growing herbaceous (Urtica dioica, Rubus idaeus, Pteridium aquilinum et al.). Later the glades may overgrow by trees tolerant to acid soils as birch (Betula pendula) and oak (Quercus robur). Nowadays the migration of great cormorant, similarly to grey heron in the 19th and 20th centuries, occurs along the Baltic coast and the Baltic Ridge, rich with lakes. The migration habits of the both species are associated not only by biological factors (overpopulation, feed availability) but also by the climate changes during the 19th and 20th century.