Abstract

Over 100 species of "short-horned" bugs (Insecta: Homoptera; Auchenorrhyncha) in Canada are very localized. The majority of these are leafhoppers which (excepting the wind-dispersing "microleafhoppers" subfamily Typhlocybinae) are mostly slow dispersers associated with particular specialized habitats or very limited numbers of host plants and, therefore, potentially threatened by habitat destruction. Suites of endemic species constituting characteristic faunal assemblies can be used to identify habitats of particular significance, and when these habitats are limited in size the potential for total destruction is high. Leafhopper assemblies indicate the need for habitat preservation in four parts of Canada not usually considered as endangered habitats: (1) bogs of Newfoundland; (2) LaCloche islands in Lake Huron; (3) interlake grasslands of Manitoba, (4) Seton Lake Valley, west of Lillooet, British Columbia.

Sommaire

Plus de 100 espèces de « criquets » du Canada (Insecte : Homoptera; Auchenorrhyncha) sont très localisés. Il s'agit en majorité de cicadelles qui (à l'exception des micro-cicadelles, sous-famille des Typhlocybinae) sont essentiellement des insectes de dispersion lente associés à des habitats spécialisés ou à un nombre très limité de plantes hôtes; ils sont donc vraisemblablement menacés par la destruction des habitats. Les familles d'espèces endémiques qui constituent des ensembles de spécimens fauniques caractéristiques peuvent servir à recenser les habitats qui revêtent une importance particulière et, lorsque la superficie de ces habitats est limitée, le risque de destruction totale est élevé. Le recensement des familles de cicadelles souligne la nécessité de préserver ces habitats dans quatre régions du Canada qui ne sont pas considérées généralement comme des habitats menacés : 1) marais de Terre-Neuve; 2) îles LaCloche sur le lac Huron; 3) prairies de la région des lacs du Manitoba; et 4) vallée du lac Seton à l'ouest de Lillooet, en Colombie-Britannique. 

Introduction

Canada is a huge land mass with extensive areas of seemingly uniform biota. Only 21 of the 52 ecological regions of North America are represented in Canada compared with 24 in the much smaller area of Mexico (CCEA, draft document). On a finer scale, however, the picture is much more complex. There are at least 45 vegetation regions (Atlas of Canada 1974) and when soils and topography are considered, there are 217 ecoregions divided into over 1000 ecodistricts, with literally thousands of possible subdivisions. How to represent these with a network of ecological preserves is a daunting task. Even the job of deciding on priorities among sites available for conservation is a massive undertaking.

This paper introduces a neglected analytical tool that can be used efficiently to help determine which ecodistricts have unique or at least rare ecosystems in need of preservation. This tool is the fauna of leafhoppers (Fig. 1), the insect family Cicadellidae.

Leafhoppers as environmental indicators

Insects make good environmental indicators of even small sites. Their faunal assemblies are apparently little influenced by patch size. Furthermore, distance from other such sites is often not a significant factor provided that the community has been in continuous existence since human activities began to fragment landscapes. Populations can survive for years on few plants and thus persist even in very small, isolated sites. For example, five prairie-endemic species of leafhoppers were taken on a 10-m2 square patch of grassland behind a warehouse in the city of Winnipeg, Manitoba; this species richness is equivalent to that of an average managed prairie site of over 10 hectares (Hamilton 1996). Likewise, the same study found 25 prairie-endemic species (the highest number recorded for any eastern prairie site) along a railway grade within the village of Grosse Isle, Manitoba far from any other prairie remnant.

Why should you have to study minute insects like leafhoppers? Various other groups of larger, better known insects have been used as a source of information to categorize ecological areas, or for monitoring environmental changes, or assessing the quality of site preservation. Ground beetles (Carabidae) are especially good indicators of microhabitat conditions (Ball and Currie 1979) and have been used in formulating hypotheses about prehistorical conditions (e.g. Kavanaugh 1979). Butterflies are useful indicators of forb community types (e.g. Swengel and Swengel 1997). Leafhoppers make a good «fit» in environmental studies along with ground beetles and butterflies, because they feed mainly on other plants (trees, grasses, and sedges) and appear little influenced by microhabitat. Leafhoppers are well represented in both forested and grassland areas; in fact, they are one of the few insect families with numerous grassland-endemic species (Ross 1970).

Leafhoppers are highly suitable for use as indicators of ecological areas, being diverse enough to be found in many different ecosystems, individually numerous, and easily sampled. The very slow dispersal rate of many species is also an important consideration in their usefulness as environmental indicators.

Diversity.  Leafhoppers are the most common and diverse family of the «short-horned» bugs (Homoptera: Auchenorrhyncha), a group of sap-sucking insects that include cicadas (Cicadidae), spittlebugs (Cercopidae), treehoppers (Membracidae) and planthoppers (Fulgoroidea). There are 1,500 species of «short-horned» bugs known to occur in Canada (unpublished checklist, 1997), which I estimate to represent about 85% of the total fauna; of these, 1200 species are leafhoppers of which perhaps 95% of the species are now known. The fauna is, therefore, diverse enough to provide meaningful differences throughout the large number of Canadian ecodistricts. It is sparse only in the far north where the leafhopper fauna falls rapidly in high boreal areas to just 15 species by treeline (Hamilton 1997). Most other «short-horned» bugs show similar or even steeper declines northward; for example, only one cicada, two treehoppers (Beirne 1961), and a single spittlebug (Hamilton 1982) are found in the most temperate part of the Northwest Territories. Delphacid planthoppers have a sizeable northern Canadian fauna of 30 species (Wilson 1997) but this is only a fifth of the number of leafhopper species in the same area.

Abundance. Leafhoppers can attain astounding numbers without apparent damage to their host plants. Samples from ungrazed temperate-zone grasslands based on vacuum collecting (which does not pick up nymphal spittlebugs or cicadas) yielded nearly 1000 individuals per 8 m2 in July (Morris 1971). Even neglecting probable sampling error, this shows that «short-horned» bug populations can rise to well over 1 million individuals per hectare in midsummer. Artifically concentrated populations of leafhoppers disperse rapidly to about 28/m2 (Andrzejewska 1961) or 280,000 per hectare but, in doing so, probably do not displace other bugs.

Sampling. About a third of all insects sampled by suction traps in grasslands may be leafhoppers and delphacid planthoppers, and these may be represented in nearly equal proportions (Heikinheimo and Raatikainen 1962). These insects are collected differentially by sweep nets as planthoppers tend to live lower on the plant and are less likely to be caught. Leafhoppers on the other hand are caught readily in this manner and are sometimes most reliably sampled this way. They are more easily collected using pan traps although the sampling time is greatly extended. Flight intercept traps are usually ineffective in collecting most leafhoppers. Light trapping is productive although selective as not all species come to light, but leafhoppers usually are able to avoid falling into such traps. A suction device must be included (as in a New Jersey light trap) or specimens must be hand-picked by aspirator from a sheet behind a light source.

Dispersal. Leafhoppers and their relatives are jumping insects with powerful hind legs. They disperse largely by running and jumping, but they also disperse by flight even when most of the population are short-winged («brachypterous») and flightless. They have been known to migrate over thousands of kilometers (Medler 1962; Cheng et al. 1979; Ghauri 1983) when aided by strong winds. Yet many species have very restricted distributions: over 90 species of leafhoppers plus 35 planthoppers, 8 treehoppers, 7 spittlebugs, and 4 cicadas are known from only very small areas of Canada (unpublished checklist, 1997). This seeming contradiction apparently reflects the diversity of life styles found in these insects.

Most migratory bugs are light-bodied insects not more than 4 mm long that are easily carried by air currents. They usually show modifications for flight: their wings are usually more than four times as long as wide, and (in planthoppers) their eyes are very large compared to the width of the head. These migratory insects apparently include most «microleafhoppers» (subfamily Typhlocybinae) and many of the common delphacid planthoppers. Thus, in the following discussion, the term «hoppers» is used to denote mainly non-migratory bugs, the «short-horned» bugs exclusive of both Delphacidae and Typhlocybinae.

A great number of species of «hoppers» fly, but only few individuals of most species are found in flight intercept traps. The main exception to this rule is the genus Xestocephalus which are believed to be ant-guest insects; apparently adults fly actively near ground level in search of ant nests. Traps more than 1 m above the ground collect few «hoppers,» mainly long-winged species of Macrosteles (Waloff 1973), at least some of which are known to be migratory (Chiykowski and Chapman 1965). Otherwise, tree canopy species are more commonly collected in such traps than species from low vegetation, as the usual flight path of «hoppers» is obliquely downwards. The exception seems to be sexually immature individuals (Waloff 1973); possibly these actively disperse over short distances to prevent inbreeding. By the time females become gravid, they usually lose the power of flight.

The rate at which «hopper» populations spread is best observed in species imported by human activity. The ranges of such «exotics» expand at rates between 10 and 100 km/year (Hamilton 1983). These figures may be taken to be upward extremes for «hoppers» as introduced species are often the most aggressive ones, and their habitats are usually linked through transportation corridors that typically have introduced floras suitable for these non-native insects. Native species or ones with fragmented habitats appear to spread at much slower rates. Northern leafhoppers often do not occupy the entire width of the boreal forest zone, and half the arctic leafhoppers that were restricted to Alaska and the Yukon during the ice age show even slower rates of migration (Hamilton 1997): 20% reached Hudson Bay after the boreal forest did, thus travelling less than 1 km/year, and 30% never even crossed the 10-km-wide Mackenzie valley (Fig. 2). Only one arctic species out of 24 has been able to invade islands across major water channels.

Thus, the majority of leafhoppers are slow dispersers associated with particular specialized habitats or very limited numbers of host plants and are potentially threatened by habitat destruction.

Habitats of significance

Only a small fraction of the Canadian ecodistricts have been intensively sampled for leafhoppers. It is therefore premature to give an accounting of comparative leafhopper faunas. However, preliminary sampling on selected parts of Canada thought most likely to have interesting faunas has turned up some unexpected habitat restrictions. When these habitats are limited in size the potential for total destruction is high and the need for conservation should become a priority.

Thirty-two leafhopper species are known from only one ecodistrict, and nowhere else in the world (Table 1). This partly reflects lack of collecting elsewhere, or lack of life-history knowledge needed to sample effectively for these insects. Some exceptions are notable:

(A)    Species restricted by ecology. A large proportion of leafhopper species are monophagous or oligophagous, feeding either on just a single plant species or on closely related species even in speciose groups of plants such as willows (Salix).  Some of their host species were once widespread and abundant but are now found only in small, isolated stands or as scattered individuals incapable of providing a reliable food source for leafhoppers. An example is an undescribed species of Flexamia that feeds only on mat muhly, Muhlenbergia richardsonis (Trin.) Rydb., a prairie grass that is rare in eastern Canada. The leafhopper is known only from a single alkaline fen in Michigan, one of the few such sites where this grass occurs in sufficient numbers to support its leafhopper host.

Another case is Rosenus decurvus Hamilton & Ross (1975) which occurs in tremendous numbers on wheatgrass (Agropyron sp.) growing on south-facing bluffs along the Peace River in Ecodistrict 591. This apparently isolated grassland is maintained in this northerly location by the local buildup of heat on sun-warmed slopes. Sampling on similar sites further north has failed to find additional populations of this species.

(B)    Species restricted by geography. Mountaintop species are, in effect, on islands in a sea of inhospitable territory. One such leafhopper, Psammotettix beirnei Greene (1971) occurs on two adjacent mountains in Ecodistrict 985; its sister species is known only from Mount Washington and adjacent peaks in New Hampshire.
An endemic spittlebug (Philaenarcys sp.nov.) has been found on the unglaciated Magdalen Islands in the Gulf of St. Lawrence (Ecodistrict 539). Due to the very isolated situation of these islands, it is unlikely that the spittlebug will be found on mainland coastal sites. An endemic species of grasshopper is also found there (Vickery and Kevan 1985, p. 395).

(C)    Unique species co-existing. When more than one species of «hopper» is found in the same ecodistrict and nowhere else in the world, this ecodistrict is probably something special. The only such ecodistricts known to date are 521 (Cape Breton Highlands), where Colladonus balius Hamilton coexists with Cribrus micmac Hamilton (Hamilton and Langor 1987), and in the adjacent lowlands (Ecodistrict 522) where Idiocerus cabbottii Hamilton (1985) coexists with a typhlocybine leafhopper Typhlocyba hollandi Hamilton (Hamilton and Langor 1987).

Suites of endemic species, even if not unique to one ecodistrict, constitute characteristic faunal assemblies that can be used to identify habitats of particular significance. Such leafhopper assemblies have been found in four parts of Canada not usually considered as endangered habitats.

(1) Bogs of Newfoundland have been very inadequately sampled, but the little we know has yielded unexpected riches (Hamilton and Langor 1987). These include two endemic leafhopper species from unique sites, Cosmotettix unica Hamilton and Typhlocyba unicorn Hamilton, plus two widespread endemic taxa, Oncopsis speciosa Hamilton and O. minor terranovae Hamilton that feed on birches including the Newfoundland and Labrador-endemic dwarf birch Betula michauxii Spach (Fig. 3). At present no correlations with ecodistricts are possible.

(2) The LaCloche Islands near Manitoulin Island in Lake Huron have an extensive limestone plain or «alvar» similar to those extending from Belleville to Kingston, Ontario. Unlike the more southerly alvars, this northern alvar has a suite of 11 widely disjunct prairie leafhoppers (Hamilton 1994). These leafhoppers occur only on a tiny corner of Ecodistrict 411 (Sudbury) but some species are also found on Manitoulin Island itself and on suitable sites on the Bruce Peninsula (Ecodistrict 550).

(3) Interlake grasslands of Manitoba in Ecodistrict 846 (Lundar), are usually considered a mere extension of the tall-grass prairie (Ecodistricts 849, 852, 853). However, its prairie-endemic leafhopper fauna is the richest in northeastern North America (Hamilton 1996) and includes three undescribed species apparently unique to this area: one each in Attenuipyga, Flexamia and Macrosteles. The first of these (Fig. 1) has been found in most of the Interlake sites sampled; its sister species is extremely rare, ranging from southern Wisconsin to Kansas.

(4) Seton Lake valley west of Lillooet, British Columbia (Ecodistrict 1002) is home to a number of highly disjunct arid-zone species not represented in the much richer arid-adapted fauna of the Okanagan. The most surprising of these is the large, black and orange cicada Okanagana ornata (Van Duzee), a Californian species that is also known from Mount Hood in Oregon. Other local disjuncts include the leafhopper Colladonus aureolus (Van Duzee) and the planthoppers Pissonotus rubrilatus Morgan and Beamer (Delphacidae) and Oliarus coconinus Ball (Cixiidae). Two other disjuct planthopper species occur in the Lower Fraser Valley (Ecodistricts 1002 and 1005): Oeclidius brickellus Ball and Oliarus beirnei Meade & Kramer. Most of these species are characteristic of the southwestern U.S.A. and have scattered populations throughout the western parts of Oregon and Washington (Fig. 4A). The pattern of these disjunct populations strongly suggests that Ecodistrict 1002 received its distinctive faunal elements during some postglacial period when the coastal valleys were drier than at present, allowing northward migration of Californian species (Fig. 4B).

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Figure 1. An undescribed species of leafhopper, genus Attenuipyga (inset: male)

Figure 2. A "natural experiment" performed by glaciation

Figure 3. Distribution of a Newfoundland-endemic dwarf birch, Betula michauxii Spach

Figure 4. Origin of the distinctive fauna of Seton Lake valley

Table 1. Non-typhlocybine leafhoppers restricted to a single ecodistrict