Effects of Double‐Crested Cormorants (Phalacrocorax auritus Less.) on Island Vegetation, Seedbank, and Soil Chemistry: Evaluating Island Restoration Potential

Abstract

The unique plant community on Middle Island, Lake Erie, Canada, has been greatly modified by double-crested cormorants (Phalacrocorax auritus Less.) whose population has increased enormously in the last two decades. The aims of this study were to assess the impact of cormorants on island tree canopy, understorey vegetation, soil seedbank, and soil chemistry. The ultimate objective was to assess the resilience of the vegetation community for recovery should cormorant nest densities decrease significantly. Forty-three point stations were established in a grid system covering the entire island. The herbaceous and woody vegetation was surveyed over 2 years and tree crown damage was assessed at each point station. In 2008, soil samples were collected for both chemical analysis and a seedbank inventory. Vegetation and seedbank species richness were impoverished compared to the vegetation surveyed prior to cormorant colonization. Cormorants affected not only the tree canopy where they nested but also the understorey vegetation. Exotic plant species were very common in the standing vegetation and constituted the bulk of the abundance in the seedbank. However, there was little relationship between aboveground vegetation and seedbank composition. Cormorants appeared to have little influence on seedbank richness, abundance, and composition. Notably, several species of conservation interest were found in the aboveground vegetation and in the seedbank providing a positive sign for future efforts to restore island plant communities.

Black spots for aquatic and terrestrial ecosystems: impact of a perennial cormorant colony on the environment

“…During the early phases of colonization, the constant increase in nutrient content can temporarily promote biomass production and number of plant species due to the higher bioavailability of N and P (Żółkoś and Meissner, 2008). However, at some point, the amount of nutrients exceeds the tolerable level for most species and biodiversity can significantly decrease (Boutin et al, 2011). This is mostly due to altered root absorption capacities in a highly fertile habitat that leads to reduced growth.…”

“…The effect of these birds on plant communities is increasingly being investigated (Hebert et al, 2005;Ellis et al, 2006;Breuning-Madsen et al, 2008;Żółkoś and Meissner, 2008;Boutin et al, 2011). During the early phases of colonization, the constant increase in nutrient content can temporarily promote biomass production and number of plant species due to the higher bioavailability of N and P (Żółkoś and Meissner, 2008).…”

The complexity of ecological impacts induced by great cormorants

“…This may even be reflected in an increase in the number of plant species ( _ Zółkoś & Meissner, 2008). However, at some point the chemical properties of soils begin to greatly exceed tolerable levels and plant biodiversity significantly decreases (Boutin et al, 2011). This is mostly due to altered root absorption capacities in a highly fertile habitat and as a result growth is subsequently reduced.…”

Nesting cormorants and temporal changes in Island habitat

“…Previous studies have documented specific impacts of nesting cormorants on forest stands, plant communities, and soil chemistry (Hebert et al , Duffe , Parks Canada Agency , Boutin et al , Rush et al ). However, this is the first to assess whole‐island forest change across several islands in the Lake Erie archipelago.…”

“…On Lake Erie, breeding cormorant populations increased from 174 individuals in 1979 to 32,564 birds in 2011 (Canadian Wildlife Service [CWS], unpublished data). Expanding cormorant populations have gained attention on the Laurentian Great Lakes because of perceived impacts on fish (Wires et al , Burnett et al , Lantry et al , Stapanian and Bur , Hebert and Morrison , Ridgway et al ), other birds (Cuthbert et al , Weseloh et al , Somers et al ), and their breeding habitats (Hebert et al , Duffe , Boutin et al , Rush et al , McGrath and Murphy ).…”