Newsletter of the Biological Survey of Canada (Terrestrial Arthropods)

Volume 16 No. 2, Fall 1997

Arthropods of Alvar Habitats

Patrice Bouchard and Terry A. Wheeler

Department of Natural Resource Sciences, McGill University, Macdonald Campus, Ste-Anne-de-Bellevue, QC, H9X 3V9

The carabid beetle Pterostichus novus

Introduction

Until quite recently, the initial response of most entomologists to the mention of alvar habitats was “what’s an alvar?”. As a result of increased research, and increased publicity in recent years, alvars and their fascinating arthropod faunas are now becoming more familiar to biologists. Alvars are naturally open areas of thin soil lying over flat limestone or dolostone. The vegetation is generally sparse and is usually dominated by shrubs and native herbaceous vegetation. Trees seldom grow in these habitats because of the restricted soil available and drought conditions during the growing season. When trees are present, they can be found in the deeper and wider cracks in the bedrock where soil has accumulated over time.

Alvar communities are found in the Baltic region on islands in southwestern Sweden, in Estonia and in a small area in western Russia (Rosen 1995); in fact, the word alvar has long been applied in the Baltic Sea Region and was first used in North America in the late 1960’s to define a similar type of habitat found only around the Great Lakes (Catling and Brownell 1995).

The limestone on which most of the North American alvars can be found was deposited about 450 million years ago and overlies the granite and quartzite of the Precambrian shield. This limestone was denuded by glaciation (Morton and Venn 1984) and the areas were subsequently maintained as natural openings by a variety of environmental factors including the lack of soil, fires, grazing by large herbivores and a pattern of flood-drought-flood experienced during different seasons. The number of alvar sites known in the Great Lakes region is estimated to be between 250 and 300, concentrated in southern Ontario, but also found in New York, Michigan, Vermont and Ohio (Catling and Brownell 1995). Six main alvar regions are recognised in Ontario: Manitoulin Island, the Bruce Peninsula, the Carden Plains, the Smith Falls Plains, the Napanee Plains and Western Lake Erie (Pelee Island).

Within each of the geographical regions, up to six different types of alvars can be found, based on the percentage of herb and shrub cover, the percentage of exposed bedrock and the percentage of tree cover (Table 1).

Table 1 - Alvar community types

open: <10% trees savanna: 10-15% trees
>50% exposed bedrock

<50% herbs

<50% shrubs

pavement alvar pavement savanna
<50% exposed bedrock

>50% herbs

usually <25% shrubs

grassland alvar grassland savanna
<50% exposed bedrock

<50% herbs

>25% shrubs

shrubland alvar shrubland savanna

Two types (grassland and pavement alvars) have been classified as globally imperiled and imperiled in Ontario by The Nature Conservancy and most savanna alvars have been classified as critically imperiled globally by the same organization (Catling and Brownell 1995). Plants from northern, western and southern regions have been observed to coexist together due to the unusual conditions (geology, hydrology, microclimate, etc.) found in alvar communities. These floral associations are thought to have originated from the presence of a tundra-like habitat that existed near the continental ice front during the last glaciation (Catling and Brownell 1995). A small number of endemic species such as the Lakeside Daisy (Hymenoxys herbacea) and the Dwarf Lake Iris (Iris lacustris) are now restricted to alvar sites in the Great Lakes Region.

Alvar habitats have been getting more and more publicity in recent years, due in part to the pristine state of many of the sites, making them refuges for many native species of plants and animals. Another reason for the recent interest in alvars is the fact that they are under constant threat of destruction from human activities such as quarrying, urban development, timber operations and garbage deposition. Because of these threats, conservation organizations such as The Nature Conservancy and the Federation of Ontario Naturalists have been putting emphasis on research on alvars in North America in order to make sound conservation decisions. Basic research on the diversity and ecology of alvar flora and fauna is an integral part of this decision-making process.

Alvar arthropods

Most of the existing information on alvar arthropods has been obtained through intensive research which started about six decades ago in Sweden, mainly on the islands of Oland and Gotland. A major study of alvar arthropods on Oland between 1977 and 1979 revealed a number of biogeographically important insect species (many of them found nowhere else in Northern Europe) and it was concluded that the conservation of the natural vegetation against non-natural stresses was essential to preserve the great biodiversity of arthropods (summary in Entomologisk Tidskrift, vol 104, 1983).

In the Great Lakes region, the study of alvar arthropods has been patchy with no major comprehensive studies. Some observations on rare, disjunct or restricted species of Lepidoptera were made by Brunton (1986), Catling (1977) and Catling and Brownell (1995). One species of carabid beetle, Chlaenius purpuricollis Randall, is very rare and restricted in eastern Canada; a population disjunct from western populations is present in the Burnt Lands alvar near Ottawa (H. Goulet, pers. comm.). The sawfly Blennogeneris spissipes (Cresson) (Hymenoptera: Tenthredinidae) is another western disjunct species that can be found in the Ottawa valley alvars and near Marmora but nowhere else in Ontario (H. Goulet, pers. comm.). The Great Lakes alvars also have a number of disjunct and rare species of leafhoppers that are usually associated with western prairie vegetation (Hamilton 1994, 1995).

In 1996, as part of the International Alvar Conservation Initiative (funded and coordinated by The Nature Conservancy and the regional Natural Heritage Information Centers in the U.S.A. and Ontario), we began a study of alvar arthopods in southern Ontario. This project is the first full-season, multitaxon study of arthropods in North American alvars. The overall objective of the International Alvar Conservation Initiative is to characterize the biodiversity and ecology of alvars for conservation purposes. The first specific objective for our study is to conduct a preliminary assessment of the species richness in alvar habitats for different target groups of insects (see below for groups studied to date). The next phase of this objective will concentrate on a variety of Diptera families. The second objective of our study is to compile data on the status and distribution of significant insect species (i.e. rare species, endemic species, disjunct populations, etc.). Such knowledge can be used to assess the importance of alvars for Canadian and North American biodiversity and to contribute to future conservation strategies. Finally, comparison between different alvar types will be conducted to determine if any differences exist in the richness or abundance of the species found within each type.

In the first season of the study (mid-May to mid-September, 1996) insects were collected using a variety of standard techniques (yellow pan traps, pitfall traps, flight intercepts, malaise traps, light traps, sweeping, hand collecting) in four different types of alvars on Manitoulin Island, Ontario.

The first site was within the Misery Bay Provincial Park and is referred to as a pavement alvar. It is characterized by the presence of flat dolostone (mostly bare or covered by mosses and lichens), a few grass patches growing in cracks and a few shrubs. The second site is referred to as a shrubland alvar because roughly 65% of the area is covered by shrubs such as the common juniper (Juniper communis) and the remaining 35% is almost entirely open limestone bedrock. The third site sampled is classified as a bur-oak grassland savanna alvar, which is characterized by the presence of about 65% of grasses and sedges, bur-oak trees (Quercus macrocarpa) and a few bare rock openings. The fourth site is a grassland alvar where herbaceous vegetation covers close to 95% of the area and no trees are present. Unlike the first two types of alvars, the bur-oak grassland savanna and the grassland alvars have a thicker layer of soil sometimes reaching 20 - 25 cm.

Species abundance and species richness

A list of more than 300 species from the following groups was gathered from roughly 10,000 specimens identified: ground beetles (Coleoptera: Carabidae), leafhoppers and allies (Homoptera: Auchenorrhyncha), sawflies (Hymenoptera: Symphyta) and butterflies (Lepidoptera: Papilionoidea). Data from a local amateur lepidopterist also yielded a list of some 128 species of noctuid moths from the same alvar sites (Lepidoptera: Noctuidae).

The insects were more abundant overall in the bur-oak and grassland alvars and this was due to the denser vegetation found on these sites. However, the number of species found on the pavement alvar was almost identical to the number of species from the bur-oak and grassland alvars even though only about half as many specimens were found there. The shrubland alvar was consistently less diverse in all of the groups.

A surprisingly high number of species identified were interesting from the point of view of conservation, including rare species found in large numbers in alvars, species restricted to alvars, disjunct populations, and range extensions. The two carabid beetles Chlaenius p. purpuricollis and Pterostichus novus Straneo are good examples of species restricted to alvars in the east and found in impressive numbers on certain alvar types (these species are probably the best alvar indicators found to date). The phenology of these two species (as well as other rare species found in large numbers) was determined for the first time in this study. A first manuscript on the ecological, phenological and morphological characteristics of the two closely related species Pterostichus novus and P. coracinus (Newman) has been submitted for review and a more extensive manuscript on the carabid fauna of Manitoulin Island alvars is being completed.

Other interesting findings included 11 western prairie endemic species of Auchennorhyncha which are mostly restricted to alvars in the east. For example, outside its usual western prairie range the species Mocuellus americanus Emeljanov has been recorded only from Manitoulin Island and Great LaCloche Island. It is thought that more than 15 leafhoppers and allies are likewise restricted to alvar habitats in eastern North America. A few species of sawfly were found to be at their extreme southern or northern limits in Manitoulin Island alvars. Similar distribution patterns were also found in the species of Lepidoptera recorded.

The insect fauna recorded in 1996 showed very low percentages of European species in the various sites, which is similar to data for the flora. The presence of the introduced ground beetle Pterostichus melanarius (Illiger), for example, revealed an agricultural past for two of the sites observed (these sites were later confirmed to have been used as pasture fields more than seven years ago).

The dominant species of carabids and leafhoppers (the two groups combined contained about 8000 specimens) were identified and compared between the different sites. Similarities among dominant species based on quantitative indices of similarity led us to group the bur-oak and grassland alvars together, with the pavement and shrubland alvars comprising a second group. There are, however, some very distinct differences in the species composition between different types of alvar which suggests that there are many types of alvar insect communities.

Future plans

Six more alvar sites will be sampled by the first author in 1997: two sites on the Bruce Peninsula, two on the Carden Plains and two in eastern Ontario. Pristine sites will be chosen when possible in order to build a collection of alvar insects which could later be used for monitoring and comparison. Two collaborators on this project (H. Goulet and K.G.A. Hamilton, both of Agriculture and Agri-Food Canada) will sample insects in other potential and known alvar sites throughout Ontario and Quebec in an attempt to learn more about the insect fauna of those habitats.

The results to date are encouraging; we have identified more species of interest than we initially predicted and the overall species diversity is higher than anticipated. Although much of the present research comprises the M.Sc. project of the first author, we are planning to continue and expand this alvar project after the completion of that degree. The Lyman Entomological Museum at McGill University now houses a comprehensive reference collection of alvar insects (including large numbers of unsorted trap residues) and present funding for the project will continue until 1999. The current collaborators in this study are committed to further work and we hope that additional research involving collaborators on other arthropod groups can be put into place.

References

Brunton, D.F. 1986. A life science inventory of the Burnt Lands. Ontario Ministry of Natural Resources. 118 pp. + maps.

Catling, P.M. and V.R. Brownell. 1995. A review of the alvars of the Great Lakes region: distribution, composition, biogeography and protection. Canadian Field-Naturalist 109: 143-171.

Catling, P.M. 1977. On the occurrence of Oarisma garita (Reakirt) (Lepidoptera: Hesperiidae) in Manitoulin District, Ontario. Great Lakes Entomologist 10:59-63.

Hamilton, K.G.A. 1994. Leafhopper evidence for origins of Northeastern relict prairies (Insecta: Homoptera: Cicadellidae). Proceedings of the Thirteenth North American Prairie Conference. Preney Print and Litho Inc., Windsor. pp. 61-70.

Hamilton, K.G.A. 1995. Evaluation of leafhoppers and their relatives (Insecta: Homoptera: Auchenorrhycha) as indicators of prairie reserve quality. Proceedings of the Fourteenth North American Prairie Conference. Preney Print and Litho Inc., Windsor. pp.211-226.

Morton, J.K. and J.M. Venn. 1984. The flora of Manitoulin Island and adjacent islands of Lake Huron, Georgian Bay and the North Channel. Second Revised Edition. University of Waterloo, Waterloo. 106 pp.

Rosen, E. 1995. Periodic droughts and long-term dynamics of alvar grassland vegetation on Oland, Sweden. Folia Geobotanica et Phytotoxonomica, Praha 30:131-140.

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