| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 450 March 14, 2012 | aceska@telus.net | Victoria, B.C. |
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Photo Plates: http://bomi.ou.edu/ben/450/ben-goose-impacts.pdf
Prior to the arrival of the first Europeans on Vancouver Island, the native, breeding goose was the Vancouver Canada Goose (Branta canadensis fulva). Evidence suggests it was likely not numerous as a breeding bird and probably nested no further south than the Great Central Lake area. Other native races of the Canada Goose also staged on the island during spring and autumn migrations and overwintered at coastal areas such as Port Hardy, Parksville, and Victoria.
The status of this goose remained unchanged until the late 1920s and early 1930s when some early introductions-likely of Branta canadensis moffitti from the Okanagan were made at Elk Lake, near Victoria. The introduced birds eventually established a small breeding population that subsequently increased to perhaps several hundred birds ranging north to the Duncan region. By the late 1960s, this small population had begun to expand from its two central areas, i.e., Elk and Quamichan lakes.
In the early 1970s, further introductions of hundreds of individuals of non-native, hybrid subspecies occurred, which has led to at least 15,000 resident Canada Geese now living on the east coast of Vancouver Island. The majority of these geese tend to be present year round and today they are causing significant damage to both human-modified landscapes and native habitats. The historical distribution of Canada Geese and a history of their introductions and spread on Vancouver Island are more fully discussed in Dawe & Stewart (2010).
In 1982, Eric White and I had described aspects of the vegetation ecology of the Little Qualicum River estuary (Dawe & White 1982). At that time, the estuary had a healthy and productive brackish marsh with significant Carex lyngbyei beds growing alongside the dendritic channels. The Carex-channel edge community ranked high in above ground biomass at over 1,700 g.m2 dry mass. Extensive Deschampsia cespitosa flats covered the mid-elevations of the marsh. At that time, I had less than 20 records of Canada Geese using the estuary with only 1 record of a solitary bird during the nesting season.
In 1984, the first Canada Goose nesting on the estuary was documented and by the mid-1990s, goose use of the estuary had increased significantly. Impacts to the vegetation, particularly in the Carex-channel edge community were becoming evident. By the early 2000s, the goose impacts had escalated to the point where we thought it important to document the changes that were occurring.
In 2005, Sean Boyd, Ron Buechert, Andy Stewart and I gathered comparative data using the field methods used in Dawe and White (1982) and in a recent paper we document the significant, negative impacts to the native marsh vegetation of the Little Qualicum River estuary, strongly inferring that resident Canada Geese are the principal cause (Dawe et al. 2011).
We found that 24 of 56 marsh species showed significant changes in either frequency or mean cover from their 1982 values; of those, 14 increased in frequency and/or mean cover while 10 decreased. Decreasing species had a significantly higher proportion of known Canada Goose dietary items while increasing species had a higher proportion of species with high salt tolerance.
Goose grubbing and grazing behaviour had the greatest impact on the Carex-channel edge community (Figure 1), changing over 10,000 m2 of it to an alternative stable state of primarily bare substrate dominated by ruderal species such as Spergularia canadensis and Glaux maritima. The complete loss of a large portion of this community suggests that the detrital food web of the estuary is losing at least 17 tonnes of above ground dry mass every year. In addition, at least another 5 tonnes dry mass per year are being lost from impacts to the Deschampsia-flats community.
In addition, the increasing halophytic vegetation suggests a concurrent change in the salinity regime of the estuary, perhaps through a reduction of the freshwater hydraulic head, which is allowing salt water intrusion to occur. This may be caused by increasing water withdrawal from the aquifer to meet the needs of a growing human population in the area. This change in the salinity regime may also be working synergistically with the geese to increase the effect of some of the impacts; however, the geese appear to be the primary agent of change.
Cursory examinations of other estuaries on the east coast of Vancouver Island, including Nanoose-Bonell creeks (Figure 2), Englishman River and Campbell River estuaries, suggest that these systems are experiencing similar negative impacts from increasing numbers of resident geese.
In Dawe et al. (2011) we discuss the implications of these changes and possible management options to mitigate them. We also encourage wildlife managers to assess all Vancouver Island estuaries where resident Canada Geese occur to determine the extent of their impacts, if any. Ultimately, if the marshes are going to be restored to their former productivity, the numbers of geese using them must be brought below the current carrying capacity of each system. This could involve a management strategy to establish a humane program that would significantly reduce or eliminate local resident goose populations. Even then, marsh recovery could take decades, especially where conditions such as hypersalinity, loss of organic matter, or significant soil compaction have taken place. Concurrent with the goose control, a marsh rehabilitation program likely involving seeding, planting, and fencing of key areas (e.g., the former Carex-channel edge communities) would also have to be implemented.
The amount of detrital loss the geese are causing to the estuarine food web, affects the higher trophic levels of the system, resulting in what has been called an "apparent trophic cascade." The geese are affecting a myriad of other organisms dependent on that food web. The sooner restorative actions begin, the more likely the marshes can successfully be returned to their former areal extent and productivity.
Photo Plates: http://bomi.ou.edu/ben/450/ben-goose-impacts.pdf
Figure 1. North dendritic channel of the Little Qualicum River estuarine marsh, 29 August 1980 (top) and 18 August 2005 (bottom). The tall form of Carex lyngbyei, adjacent to the channel, has been eliminated from much of the Carex-channel edge community by resident Canada Geese. Note the substrate erosion that has also occurred by 2005, a result of goose grubbing activities. Photos: Neil K. Dawe
Figure 2. The central delta of the Nanoose-Bonell estuary, end of August 1980 (top) and 23 August 2005 (bottom). Resident Canada Geese have eliminated the tall form of Carex lyngbyei from the edge of a small ponding area (bottom). Behind the Carex (top) is the Salicornia-Distichlis community; Distichlis spicata is the light green vegetation, also obvious in the lower right corner (top). By 2005, Distichlis spicata had been heavily grazed by the geese resulting in a much reduced frequency on the central delta (compare the lower right corners of the images). Photos: Neil K. Dawe
Photo Plates: http://bomi.ou.edu/ben/450/ben-garry-oak.pdf
In British Columbia, we use the term Garry oak ecosystem to include a wide range of meadows and open woodlands in the Georgia Basin that usually include Garry oak (aka Oregon white oak, Quercus garryana). Though Garry oak itself extends from BC to California, oak-prairie ecosystems with a similar composition to those in BC extend only as far south as the Puget Trough and Willamette Valley. In BC, there is a common perception that Garry oak and the associated meadow and woodland ecosystems are confined to the driest sites. Oak-associated wetlands are an often overlooked component of the landscape on southern Vancouver Island. While much Garry oak ecosystem related literature in BC includes discussion of vernal pools, there is little mention of true wetlands. Vernal pools, in the strict sense, are shallow depressions on impermeable substrates that fill with rainwater in the winter and experience extreme drought in the summer (Keely & Zedler 1998). The stresses associated with the cycles of flooding and drought exclude many perennial plants, and instead vernal pools support an assortment of extremely stress-tolerant annual species. In contrast, the topic of this article is wetlands that form on permeable substrates, are often influenced by ground water or large drainage basins, have a less pronounced summer drought, and are vegetated mainly by robust perennial species. Many people can think of a place or two where they see Garry oak growing in very wet habitats, but these sites are often dismissed as somehow anomalous. Are these wet sites anomalous? This question can be answered by looking elsewhere within the Willamette Valley - Puget Trough - Georgia Basin ecoregion.
In western Washington, there is good historic evidence that many oak-associated wetlands were destroyed early in the agricultural settlement period (Easterly et al. 2005). Today, in the South Puget Sound, a few examples of seasonally flooded prairie remain in the flood plains of creeks and in places where the prairies are underlain by clay. In addition, some of Washington's remaining populations of Oregon Spotted Frog (Rana pretiosa) and Western Pond Turtle (Actinemys marmorata) breed in wetlands surrounded by Garry oak (Hayes et al. 1999, Watson et al. 2000). Further south, in Oregon's Willamette Valley, a high proportion of the historic prairies were seasonally hydric (Christy & Alverson 2011) and wetland prairies today are better preserved than their upland counterparts. Within the Willamette wet prairie landscape, a wide range of habitats can be found, including both annual-dominated vernal pools and wetlands dominated by perennials. The occurrence of oak-associated wetlands throughout our ecoregion suggests that these habitats are not anomalous, but are a normal part of the continuum of the oak/prairie landscape.
Given the scale of the historical conversion of both wetlands and deep-soiled oak parkland, oak-associated wetlands were probably much more common in the pre-contact landscape of Vancouver Island than they are today. We know that Euro-Canadians deliberately targeted wetlands for agriculture; in fact, the governments of the day actually advised settlers that a drained wetland provided the best soil for farming (Pemberton 1860). We also know that more than 99% of the deep-soil oak woodland that existed in the Victoria area in 1800 has been destroyed (Lea 2006). Where these two habitat types overlapped, the losses must have been severe.
It is still possible to find many traces of oak-associated wetlands around the Victoria area. To date, I have noticed about twenty such sites where either Garry oak or Camassia species can be found in wetland or riparian situations. There are probably many others. Most sites that I have observed are degraded to the point of supporting just a few individuals of a few native species. However, there are sites that appear relatively intact and provide an opportunity to study the flora of this forgotten habitat type.
Oak-associated wetlands in Victoria vary greatly from one to the next, but there are a few species that are usually present. The most frequent of these are Carex exsiccata (Figure 1), C. obnupta, C. unilateralis, Eleocharis palustris, and Veronica scutellata. Other common species include Mentha arvensis, Nuphar polysepala, Persicaria amphibia, Potentilla anserina s.l., and Sium suave. The margins of these wetlands (when they are not dominated by Phalaris arundinacea) may support species more typical of vernal pools, such as Plagiobothrys scouleri and Gratiola ebracteata. The transitional areas immediately surrounding the wetlands may contain Deschampsia cespitosa (Figure 2), Hordeum brachyantherum, and/or Ranunculus orthorhynchus.
Garry oak-associated wetlands and the surrounding transitional areas are also important habitats for species at risk. Examples of the many species that are strongly associated with this kind of wetland, either in BC or further south, include Epilobium densiflorum, E. torreyi, Navarretia intertexta, Plagiobothrys figuratus, Ranunculus alismifolius, and Ranunculus lobbii. (Note: Epilobium torreyi and Ranunculus lobbii Are considered extinct in BC). Paying more attention to our remaining oak-associated wetlands may be an important step towards the recovery of these species.
These observations provide the barest outline, and are not meant to be authoritative. I know that there are other oak-associated wetlands in the Gulf Islands, the Cowichan Valley, and the Nanaimo area. It is hoped that others will continue to document the distribution and composition of these wetlands and share their own findings.
Photo Plates: http://bomi.ou.edu/ben/450/ben-garry-oak.pdf
Figure 1. Wetland in Langford, showing Carex exsiccata in the foreground and Quercus garryana in the background.
Figure 2. Deschampsia cespitosa growing beside a wetland in Metchosin.
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