Nesting cormorants and temporal changes in Island habitat

Abstract

Double‐crested cormorant (Phalacrocorax auritus) populations have increased greatly across North America. The interior North America subpopulation is the largest with many birds nesting on the Laurentian Great Lakes. Lake Erie supports a large number of breeding pairs that nests primarily on islands in the western basin of the lake. These islands also harbor many rare plant species constituting some of the last vestiges of Carolinian plant communities in Canada. Nesting cormorants can adversely affect the plant communities on the islands on which they nest. Annual ground censuses were conducted from 1979 to 2011 to assess temporal changes in the density of nesting cormorants on 3 islands in western Lake Erie. We used aerial photographs taken over a maximum 16‐year span to assess changes in forest cover on these island ecosystems. We observed declines in forest cover on all 3 islands ranging from 47% to 85%. Trends among islands differed reflecting differences in cormorant colonization histories and the degree to which cormorants were managed, thereby influencing nest densities. Islands without cormorant management had cormorant nest densities ranging from approximately 300–500 nests/ha and forests declined continuously through time. On the island where cormorants were culled, nest densities were lower (approx. 200 nests/ha) and forest decline stabilized. © 2014 The Wildlife Society.

The complexity of ecological impacts induced by great cormorants

“…The population of P. c. lucidus was estimated at 40,000 individuals in Coastal West Africa, 15,000 in Coastal South Africa and 500,000 in Central and Eastern Africa (Delany et al, 2007;van Roomen et al, 2014), while the population of P. c. hanedae was estimated at 100,000 individuals (Brazil, 2009). In North America, the population of P. carbo is stable unlike the doublecrested cormorant Phalacrocorax auritus Lesson, whose number is rising (Hebert et al, 2014).…”

Double‐Crested Cormorants: Too Much of a Good Thing?

“…Cormorants have caused extensive damage to vegetation where they nest due to excessive guano, associated soil chemistry changes, and physical destruction (Hebert et al 2014;Ayers et al 2015;Lafferty et al 2016; Figure 5). These impacts can be relatively rapid (tree mortality in 3-10 years).…”

“…Changes in soil have been shown to affect plant species composition, resulting in reduced number of species and opportunity for exotic, invasive plants (Ayers et al 2015), as well as impacts to arthropod communities (Craig et al 2012). Hebert et al (2014) documented significant damage to trees on forested islands associated with cormorant nesting and that vegetation damage can negatively impact co-nesting avian species that are obligate tree nesters. These impacts are not relegated to the cormorant's northern breeding grounds, as Lafferty et al (2016) documented significant impacts to trees and soils on southern breeding colonies.…”

“…Overall, there has been a sustained positive veg- There are a few other examples of management of cormorants to protect insular habitats, primarily in the Great Lakes. Hebert et al (2014) documented decline in forest cover on three islands in western Lake Erie, Ontario, Canada, ranging from 47% to 85%. Cormorant control to reduce numbers, including culling, on one of the islands (Middle Sister Island) reduced cormorant nesting densities, and vegetation has begun to recover, whereas vegetation on the other two islands without management has continued to decline (Hebert et al 2014).…”

“…Hebert et al (2014) documented decline in forest cover on three islands in western Lake Erie, Ontario, Canada, ranging from 47% to 85%. Cormorant control to reduce numbers, including culling, on one of the islands (Middle Sister Island) reduced cormorant nesting densities, and vegetation has begun to recover, whereas vegetation on the other two islands without management has continued to decline (Hebert et al 2014). Management of cormorants at Tommy Thompson Park, Toronto, Canada, by the Toronto and Region Conservation Authority (TRCA) was undertaken in 2008 on four island peninsulas within the park due to severe declines in forested habitat and conesting species (TRCA 2016).…”