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Page 95 ï~~2005 THE MICHIGAN BOTANIST 95 ECOLOGY AND FLORISTICS OF KNIFE ISLAND, A GULL AND CORMORANT ROOKERY ON LAKE SUPERIOR, NEAR TWO HARBORS, LAKE COUNTY, MINNESOTA. Derek Anderson, Jessica Brandt, Lynn Wright, and Donald Davidson Department of Biology and John Thomson Herbarium University of Wisconsin- Superior Superior, WI 54880 email@example.com firstname.lastname@example.org ABSTRACT Certain aspects of the ecology and floristics of Knife Island, near Two Harbors, Lake County, Minnesota, a small Herring Gull and Double-crested Cormorant rookery on the north shore of Lake Superior, were studied. In 1971, 63 species of vascular plants and 23 species of bryophytes were collected, including one moss newly recorded in Minnesota. In 2004, the number of vascular plants found had declined to 39. In 1971, high levels of soil organic matter and low pH levels were found. Similarly, in 2004, high levels of organic matter, with a low pH were observed. These levels appear to reflect the intensive use of the island by Herring Gulls and cormorants. Thus, it is concluded that the major factor controlling the development of vegetation on Knife Island is the Herring Gull and cormorant populations. INTRODUCTION Knife Island is a small, rocky island approximately 300 by 800 feet in size, located just off the north shore of Lake Superior at the Village of Knife River, near Two Harbors, Lake County, Minnesota. Geologically, Knife Island is an extension of the Stoney Point Sill Diabase (a kind of igneous rock) which extends along the north shore just south of the island, then emerges on the surface as Knife Island (Schwartz 1949; John W. Green, personal communication). The island is of particular interest because historically it is a rookery for a large number of Herring Gulls (Larus argentatus) and, more recently, Double-crested Cormorants (Phalacrocorax auritus). The birds use the island extensively during the early summer (Minnesota DNR Colonial Bird Nest Surveys). Hofslund (1952, 1959) stated that the island supports at least 250 breeding birds per year; he banded some 500 young in one year. He further noted that most of the nests are located on bare rock on the mainland side of the island. Hofslund (personal communication) stated that the island is one of the only large gull breeding locations in the western part of the Great Lakes. In the late 1970s and early 1980s the island contained on average 250-600 active Herring Gull nests, with a high of 759 occupied nests in 1984. The cormorants were sporadically observed and not monitored until recently (Minnesota DNR Colonial Bird Nest Surveys). In the summer of 2004 (Duluth News-Tribune, June 8) Janet Green, noted local ornithologist, observed more than 100 cormorants on Knife Island in late evening. Drilling et. al. (Minnesota DNR Colonial Bird Nest Surveys) observed
Page 96 ï~~96 THE MICHIGAN BOTANIST Vol. 44 twenty-six active nests with both feathered young, and newly flying young in mid July of 2004. It was also noted that the vegetation in the vicinity of the cormorant nests on Knife Island has been destroyed as a result of their nesting (Figure 1). Control efforts were instituted on the cormorants and thirty-eight cormorant FIGURE 1. A dead conifer on Knife Island, Lake County, Minnesota with cormorant nests (Photograph by Derek Anderson, 4 Sep 2004). There was no vegetation growing within 3-4 meters of the tree base. The red-berried elder (Sambucus racemosa L., Caprifoliaceae) that grew nearest to this dead conifer had leaves and branches that were coated with the white excrement that "rained" down from the birds above.
Page 97 ï~~2005 THE MICHIGAN BOTANIST 97 nests were destroyed at the beginning of the control effort, which ran from 27 April through 26 May 2004. This is the first time the Department of Natural Resources has made any effort to control cormorants in Minnesota. It was made possible by a recent change in federal law. After the thirty-eight cormorant nests were destroyed, between twenty and thirty nests were rebuilt on the island. There is little published information on what effect birds and their excrement have on vegetation; however, some information was found on Great Blue Heron rookeries. Fahey (1968) noted dead and dying red and white pine trees in the Great Blue Heron (Ardea herodias) colony at Kirk Lake, Minnesota, while Hanlon (1956) noted that shrub underbrush was killed by excrement in the Heron Island Rookery he studied, also in Minnesota. D. W. Davidson (unpublished) noted large numbers of dead red pine trees and restricted development of ground vegetation in a Great Blue Heron rookery on Basswood Lake in southern Ontario, Canada. In this rookery Urtica dioica, which is also found on Knife Island (hdble 1), was observed to be abundant under the nests of the Great Blue Herons. One other small island on the north shore (Beaver Island, which forms part of the Cleveland Cliffs Harbor at Silver Bay, Minnesota) has been studied by Lakela (1948). She noted birch-conifer forest developed on the island, which had a flora typical of the adjacent mainland. She also noted a Herring Gull colony on Beaver Island, but did not discuss the possible impact of nesting activities on the flora and vegetation of the island. METHODS The 2004 species list was compiled from the survey of Donald W. Davidson, Jessica Brandt, Derek S. Anderson, and Lynn Wright, and by the collections of small vegetative specimens. The species collected were determined by Welby Smith, Botanist, Department of Natural Resources, Ecological Services, St. Paul, Minnesota, in December of 2004. Eighteen species were determined by sight through reconnaissance by the authors. Soils were collected from two locations in the forested interior of the island and used to form a composite. The soil was analyzed by the University of Wisconsin Soil and Plant Analysis Lab, Verona, Wisconsin. Collecting trips for this study were made in the summer of 1968 and the fall of 1969 for the 1971 paper, and a collecting trip for the 2004 paper was made on 4 September 2004. Vascular species were collected on all trips while bryophytes were collected in 1969 only. In 2004, a few bryophytes were observed on Knife Island, but none were collected, as the necessary expertise to identify them was not available. Voucher specimens of all plants cited are located in the John Thomson Herbarium at the University of Wisconsin-Superior (SUWS). Some duplicate bryophytes were deposited in the University of Minnesota Herbarium (MIN). Conard (1956) and Crum et. al. (1965) were used for bryophyte identification in 1971. Gleason and Cronquist (1991) was used for the identification of vascular plants. National Geographic (2002) was used for the Latin nomenclature of the birds. RESULTS AND OBSERVATIONS There are three distinct areas on the island: 1) the gently sloping lakeside, which is essentially bare rock exposed to periodic intense wave action, 2) the mainland side, characterized by a vertical rock cliff approximately one meter
Page 98 ï~~98 THE MICHIGAN BOTANIST Vol. 44 TABLE 1. Knife Island Vascular Plant List, 2004 The names, both Latin and common, follow those described in Gleason and Cronquist's Manual of Vascular Plants of Northeastern United States and Adjacent Canada, 2nd Edition. Abies balsamea (L.) Mill.; balsam fir Acer spicatum Lam.; mountain maple Achillea millefolium L.; yarrow Betula papyrifera Marsh.; paper birch, white birch Campanula rotundifolia L.; harebell Cerastium vulgatum L. [syn. Cerastiumfontanum Baum. emend. Jalas]; mouse-ear chickweed Chenopodium album L.; lamb's quarters, pigweed Cinna latifolia (Trevir. ex Gopp.) Griseb.; drooping woodreed Collinsonia canadensis L.; northern horse-balm, stone-root Cornus sericea L. [syn. Cornus stolonifera Michx.]; red osier-dogwood Dryopteris carthusiana (Vill.) H.P. Fuchs; toothed wood fern Euthamia graminifolia (L.) Nutt.; common flat-topped goldenrod Galeopsis tetrahit L.; hemp-nettle Hordeum jubatum L.; foxtail barley Juncus tenuis Willd.; path rush Lonicera canadensis Marsh.; fly honeysuckle Matricaria matricarioides (Less.) Porter; pineapple weed Medicago lupulina L.; black medick Phalaris arundinacea L.; reed canary grass Picea glauca (Moench) Voss; white spruce Plantago major L.; common plantain Poa annua L.; speargrass Poa compressa L.; Canada bluegrass Polygonum arenastrum Boreau.; dooryard knotweed Polygonum aviculare L.; knotweed Populus tremuloides Michx.; quaking aspen Potentilla norvegica L.; strawberry weed Pteridium aquilinum (L.) Kuhn.; bracken fern Ribes sp.; gooseberry Rosa acicularis Lindl.; bristly rose Rubus idaeus L.; red raspberry Rubus sp.; bramble Sambucus racemosa L. [syn Sambucus pubens Michx.]; red-berried elder Sorbus americana Marsh.; American mountain-ash Tanacetum vulgare L.; common tansy Taraxacum officinale Weber ex Wiggers; dandelion Taxus canadensis Marsh.; yew Urtica dioica L.; nettle, stinging nettle Viola sororia Willd.; dooryard violet high at the northeast end to three meters high at the southwest end, and 3) a small forest of mountain ash (Sorbus americana) in the middle of the island. The shore side and much of the small forest are modified by gull and cormorant activities. White excrement covers much of the rock surfaces, and dead birds are common in crevices (Figure ). The white color of excrement on the rocks is readily visible from the Knife Island Harbor (nearly half a mile away). The interior of the small forest was mostly bare and trampled in 1971; in contrast, by 2004 it was a tangle of Sambucus pubens and was barely penetrable. In 1971, the small forest was almost entirely mountain ash (Sorbus ameri
Page 99 ï~~2005 THE MICHIGAN BOTANIST 99 2005 THE MICHIGAN BOTANIST 99 FIGURE 2. The rocky, southeast-facing shore of Knife Island, Lake County, Minnesota, with the white bird excrement clearly visible (Photograph by Derek Anderson, 4 Sep 2004). The dominant species on the island are red-berried elder (Sambucus racemosa L., Caprifoliaceae) and mountain ash (Sorbus americana Marsh., Rosaceae). cana), with a few paper birch (Betula papyrifera), and one white spruce (Picea glauca). Several large dead firs (Abies balsamea) were also observed, and an extensive growth of yew (Taxus canadensis) was the only undergrowth of particular importance in the forest. In 2004, the large spruce was dead, while one small sapling was found alive. A few small fir saplings were also found, and the clones of Taxus canadensis had decreased dramatically in size. The green alder (Alnus crispa) and ninebark (Physocarpus opulifolius), shrubs common in rocky crevices on the entire north shore, also were present on Knife Island in 1971; however, in 2004 the two species were not observed. In 1971, 63 species of vascular plants were collected. They included six trees, twelve shrubs, thirty forbs, thirteen grasses, and two ferns (Table 2). In addition to the vascular plants, 23 species of bryophytes (Table 3) were collected. Most of the bryophyte species (11) were collected in the forest, seven on the shore side, and five on more or less exposed, bare soil. One of the species of moss, Plagiothecium latebricola, was new to Minnesota in 1971. It is a small, and relatively rare species, which has been reported from Newfoundland, Nova Scotia, New Jersey, Ontario, and Wisconsin. In 2004, 39 species of vascular plants were collected. They included four trees, seven shrubs, eighteen forbs, four grasses, and
Page 100 ï~~100 THE MICHIGAN BOTANIST Vol. 44 TABLE 2. Knife Island Vascular Plant List, 1971 The names, both Latin and common, follow those described in Gleason and Cronquist's Manual of Vascular Plants of Northeastern United States and Adjacent Canada, 2nd Edition. The nomenclature of this list has been updated and names that were used in 1971 are included in brackets. Abies balsamea (L.) Mill.; balsam fir Acer spicatum Lam.; mountain maple Achillea millefolium L.; yarrow Actaea rubra (Ait.) Willd.; red baneberry Agrostis gigantea Roth. [syn. Agrostis alba L. var. alba]; redtop Alnus viridis (Vill.) Lam. [syn. Alnus crispa (Ait.) Pursh]; green or mountain alder Aster ciliolatus Lindl.; northern heart-leaved aster Aster lanceolatus Willd. [syn. Aster simplex Willd.]; eastern-lined aster Beckmannia syzigachne (Steud.) Fern.; American sloughgrass Betula papyrifera Marsh.; paper birch, white birch Calamagrostis canadensis (Michx.) Beauv. var. canadensis; bluejoint Campanula rotundifolia L.; harebell Capsella bursa-pastoris (L.) Medic.; shepherd's purse Cardamine pensylvanica Muhl.; Pennsylvania bitter-cress Cinna latifolia (Trevir. ex Gopp.) Griseb.; drooping woodreed Cirsium arvense (L.) Scop.; Canada thistle Cirsium vulgare (Savi) Tenore; bull thistle Clintonia borealis (Ait.) Raf.; bead lily Cornus sericea L. [syn. Cornus stolonifera Michx.]; red osier-dogwood Deschampsia caespitosa (L.) Beauv.; tufted hairgrass Diervilla lonicera Mill.; bush honeysuckle Dryopteris carthusiana (Vill.) H. P. Fuchs [syn. Dryopteris spinulosa (Mull.) Watt.]; toothed wood fern Elymus trachycaulus (Link) Gould ex Shinners [syn. Agropyron trachycaulum (Link) Malte var. glaucum Malte]; slender wheat grass Epilobium angustifolium L.; fireweed, great willow-herb Erysimum cheiranthoides L.; wormseed-mustard Galeopsis tetrahit L.; hemp-nettle Glyceria grandis S. Watson ex A. Gray; American mannagrass Hordeum jubatum L.; foxtail barley Impatiens capensis Meerb.; orange touch-me-not, jewel-weed Lonicera canadensis Marsh.; fly honeysuckle Matricaria matricarioides (Less.) Porter; pineapple weed Physocarpus opulifolius (L.) Maxim.; ninebark Picea glauca (Moench) Voss; white spruce Plantago major L.; common plantain Poa annua L.; speargrass Poa compressa L.; Canada bluegrass Poa glauca Vahl.; bluegrass Poa interior Rydb.; inland bluegrass Poa palustris L.; fowl meadow-grass Polygonum aviculare L.; knotweed Polygonum cilinode Michx.; fringed bindweed Populus tremuloides Michx.; quaking aspen Potentilla norvegica L.; strawberry weed Prunus pensylvanica L.; pin-cherry Ranunculus acris L.; tall buttercup Ribes glandulosum Grauer; skunk-currant Ribes oxyacanthoides L.; northern gooseberry Rorippa palustris (L.) Besser [syn. Rorippa islandica (Oeder) Borbas]; common yellow-cress Rosa acicularis Lindl.; bristly rose Rosa blanda Ait.; smooth rose
Page 101 ï~~2005 THE MICHIGAN BOTANIST 101 TABLE 2. (Continued) Rubus idaeus L. [syn. Rubus strigosus Michx.]; red raspberry Rubus parviflorus Nutt.; thimbleberry Rubus setosus Bigel.; bristly blackberry Sambucus racemosa L. [syn. Sambucus pubens Michx.]; red-berried elder Sorbus americana Marsh.; American mountain-ash Tanacetum vulgare L.; common tansy Taxus canadensis Marsh.; yew Trifolium repens L.; white clover Urtica dioica L.; nettle, stinging nettle Viola sororia Willd.; dooryard violet two ferns (Table 1). The number of species in 2004 had declined dramatically from 1971. In 1971, the soil was very shallow in the forest under the mountain ash trees. In 2004, the soil profiles were thicker and rich, reflecting the heavy load of bird excrement that rains down on the forest ecosystem. In 1971, the pH of the four samples taken from various locations on the island ranged from 3.6 to 4.2. Very high levels of soil organic matter, phosphate, and potassium were present (Table 4). In 2004, the soil samples came from the forest interior. The soil was also laden with a rich excrement layer. The pH was 4.1, organic matter was 50% (high), total nitrogen was high, phosphorus and potassium were very high, and calcium and magnesium were low (Table 5). TABLE 3. Knife Island Bryophyte List, 1971; names and their authors are given in accord with the current "Index of Mosses Database," W3MOST, at the Missouri Botanical Garden website. Amblystegium juratzkanum Schimp. Brachythecium digastrum Muill. Hal. ex Kindb. Brachythecium plumosum (Hedw.) Schimp. Brachythecium reflexum (Starke) Schimp. Brachythecium rutabulum (Hedw.) Schimp. Bryum argenteum Hedw. Ceratodon purpureus (Hedw.) Brid. Funaria hygrometrica Hedw. Grimmia alpicola Hedw. var. rivularis (Brid.) Wahlenb. Haplocladium microphyllum (Hedw.) Broth. Heterophyllium haldanianum (Grev.) M. Fleisch. Homomallium adnatum (Hedw.) Broth. Hygroamblystegium tenax (Hedw.) Jenn. Hygrohypnum luridum (Hedw.) Jenn. Leptodictyum trichopodium (Schultz) Warnst. Leskea gracilescens Hedw. Lophocolea heterophylla (Schrad.) Dumort. Mnium punctatum Hedw. Orthotrichum anomalum Hedw. Plagiothecium latebricola Schimp.-New State Record Pogonatum alpinum (Hedw.) Rohl. Pohlia nutans (Hedw.) Lindb. Pylaisia polyantha (Hedw.) Schimp.
Page 102 ï~~102 THE MICHIGAN BOTANIST Vol. 44 TABLE 4. Knife Island Soil Analysis, 1971 Organic Matter P K Location pH Tons/Acre Pounds/Acre Pounds/Acre Forest Interior 3.6 > 150 110 180 Edge of Island 4.0 > 150 400 475 Edge of Island 3.9 > 150 230 440 Open, Trampled Area 4.2 > 150 400 600 TABLE 5. Knife Island Soil Analysis, 2004 Organic P K Ca Mg Matter Total Pounds/ Pounds/ Pounds/ Pounds pH % N% Acre Acre Acre Acre Composite 4.1 50.3 1.83 298 1,748 832 218 Interpretation Low High High Very High Very High Low Low DISCUSSION It appears that the major factor influencing the flora and vegetation of Knife Island is the Herring Gull and cormorant populations. The large number of resident birds (particularly with the arrival of the nesting cormorants), the nesting activities, the trampling of the soil and vegetation, and the large amount of excrement tend to restrict the development of several plant species. Plants are found primarily in crevices on the shore side of the island and in the forest interior, where there is less bird activity. In 2004, the interior was nearly an impenetrable thicket of elderberry, and as the four authors walked the island searching for flora, it was very difficult to penetrate the thickets of elderberry and mountain ash without breaking many of the tree and shrub branches. Both the mountain ash and the elderberry are very robust and vigorous. The branches of elderberry are, in many cases, one inch in diameter. The mountain ash trees are quite robust and tall; many of them are over thirty feet tall. The mountain ash and elderberry are apparently doing very well as a result of the fertilizing effect of the droppings from the gulls and cormorants. The changes in the plant cover and soils have been quite significant since Bernard et al. visited the island in 1971. Significantly fewer species are present now (39 versus 63 in 1971). In addition, there remains only one large tree on the island, mountain ash, as the large white spruce and balsam fir are dead or nearly dead. Seedlings of woody and herbaceous species are subject to damage and possible extirpation from the island. The soil data also illustrate the profound influence of the bird populations on this island. The low pH values and the very high organic matter, phosphorus, and potassium levels probably inhibit growth of some of the plant species. The higher values of phosphorus and potassium were found where bird activity was greatest in 1971. Hofslund also noted (personal communication) that colonial
Page 103 ï~~2005 THE MICHIGAN BOTANIST 103 birds associated with large bodies of water are almost always nesting in areas that are not associated with heavy vegetation. The soil samples of 2004 were taken in the late fall after gull and cormorant activity had lessened. It is possible that soil nutrient values then are lower than what would be found during nesting time, since rainfall certainly flushes nutrients off the island into Lake Superior. Notably lacking from the flora of Knife Island were many species typical of northern forest conditions, which were found by Lakela on Beaver Island, thus confirming the idea that the flora of Knife Island is of a more "weedy" nature. Totally lacking from Knife Island were the following species found on Beaver Island, and all typical of northern coniferous forests: Lycopodium (five species), Thuja occidentalis, Cornus canadensis, Polypodium virginianum, Woodsia ilvensis, Aster macrophyllus, Linnaea borealis, Vaccinium (three species), Pyrola (two species), Aralia nudicaulis, Osmunda (two species), and Maianthemum canadense. The shrub stratum on the two islands was also considerably different. On Beaver Island Lakela noted that it was "well developed." Knife Island shrubs were found to be clumped and scattered in 1971 and 2004. The herbaceous situation seemed to be similar; there is a well-developed herb stratum on Beaver Island and poor one on Knife Island. Most of the species on Knife Island, whether reported from Beaver Island or not, were weedy species from the mainland. The conditions of the gull rookery and cormorant nesting on Knife Island are somewhat similar to those found in the rookeries of the Great Blue Heron. Trees were killed in the heron rookeries (Fahey 1968; Davidson, unpublished observations), and understory vegetation is drastically modified. However, the gulls have been documented for many decades on the island, and as a result, naturally influence the habitat in which they live, and only time will tell what further changes will occur in the flora of the island. ACKNOWLEDGMENTS Thanks are extended to Stephen Dahl for providing transportation to Knife Island. LITERATURE CITED Bernard, John M., Donald W. Davidson, & Rudy G. Koch. 1971. Ecology and floristics of Knife Island, a gull rookery on Lake Superior. Journal of The Minnesota Academy of Science 37(2 & 3): 101-103. Conrad, Henry S. 1956. How to Know the Mosses and Liverworts. Win. C. Brown Co., Dubuque, Iowa. 226 pp. Crum, H., W. C. Steere, & L. E. Anderson. 1965. A list of the mosses of North America. The Bryologist 68: 377-416. Duluth News Tribune, June 8, 2004. Duluth, Minnesota. Fahey, Patricia L. 1968. Great Blue Herons. The Loon 40: 37-40. Field Guide to the Birds of North America. 2002, 4th edition. National Geographic Society, Washington, D. C. 480 pp. Gleason, Henry A. & Arthur Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. 2nd Edition, 7th printing. New York Botanical Garden. 993 pp. Hanlon, Robert W. 1956. Heron Island - General Shields Lake. The Flicker 28(4): 130-132. Hofslund, P. B. 1952. Census of Knife Island. The Flicker 24: 162-163.
Page 104 ï~~104 THE MICHIGAN BOTANIST Vol. 44 Hofsulnd, P. B. 1959. Fall migration of Herring Gulls from Knife Island, Minnesota. Bird Banding 30: 104-114. Lakela, Olga. 1948. Ferns and flowering plants of Beaver Island. Bulletin of the Torrey Botanical Club 75(3): 265-271. Minnesota Department of Natural Resources. Ecological Services Division. Unpublished colonial bird nest counts data. 1941-2004. Schwartz, George M. 1949. The Geology of the Duluth Metropolitan Area. Minnesota Geological Survey volume #33. University of Minnesota Press, 136 pp.