Ottoe skipper (Hesperia ottoe) COSEWIC assessment and status report: chapter 9
Conversion of habitat to non-grassland
Since the 1850s, over 99% of native prairie habitat has been ploughed, converted to agricultural row crops or hay fields, or degraded by overgrazing (Samson and Knopf 1994). Agricultural habitats are completely unsuitable for the survival of H. ottoe. Many mixed-grass and sand prairie remnants have probably survived because poor soils (sandy) or steep terrain make them unsuitable for row-crop agriculture. However, most mixed-grass prairie sites west of Shilo have been recently converted to potatoe fields or are currently being mined for gravel.
Grazing
Mixed-grass- and sand-prairie specialist butterflies, such as H. ottoe and H. dacotae, appear to be very susceptible to the effects of overgrazing (McCabe and Post 1977, Royer and Marrone 1992, Royer and Royer 1998, Swengel 1998a, Swengel and Swengel 1999), which reduces or eliminates critical adult nectar resources and removes forage for larvae. Trampling by cattle may kill larvae and cause soil compaction. These factors make overgrazed habitats unsuitable for the skipper (McCabe and Post 1977). Dana (1997) further observed that exotic grasses, such as P. pratensis and B. inermis, become the major or dominate species, and native species richness and diversity declines.
Although overgrazing can potentially eliminate populations of H. ottoe, grazing is not always detrimental (Dana 1991, Swengel 1998b). Light rotational grazing can be beneficial by creating or maintaining areas of mixed-grass vegetation structure which is preferred by this skipper (Dana 1991). Swengel (1998b) observed that a lightly grazed prairie in Wisconsin had relatively high richness of prairie specialist butterflies, especially compared to burned or idle sites.
Haying
Haying may either be detrimental or beneficial to H. ottoe populations depending on when in the season it is done. If mowing is done before or during the adult flight period, the critical adult nectar sources are eliminated and exotic grasses such as P. pratensis are favoured (McCabe 1981, Dana 1997). This situation can cause the elimination of H. ottoe from the prairie. Haying later in the summer when H. ottoe larvae are feeding within the aerial leaf shelters may also be detrimental to the skipper. In contrast, late-season (late September into October) mowing when the larvae have entered their buried shelters at the soil surface (Dana 1991) reduces the adverse effects created by mowing early and may even be beneficial to H. ottoe and other prairie specialists (McCabe 1981, Skadsen 1997, Swengel and Swengel 1999). Mowing prairies and removing the cuttings helps to maintain the vegetation structure by preventing or delaying succession to woody plants and reducing the accumulation of litter on the soil.
Controlled burning and wildfires
Wildfires were an important element for sustaining the flora and fauna of native prairies prior to their destruction (Bragg 1995). Now, prescribed or controlled burns are often used by managers to maintain the native grassland structure and floral complexes. These burns differ from wildfires in that remnant prairies are often burned far more frequently (sometimes once every three years), more thoroughly (sometimes border to border), and at times during the season when wildfires would not normally occur (Orwig and Schlicht 1999). Although prescribed burns may be beneficial for maintaining the prairie flora and certain insect species, they may be devastating to other species of insects (Swengel 2001). Prescribed burning of isolated prairies can cause local extirpations of some insect species, especially habitat specialists such as H. ottoe, H. dacotae and Oarisma poweshiek (Parker) (McCabe 1981, Schlicht and Saunders 1994, Swengel 1996, 1998b, 2001, Orwig and Schlicht 1999). Prior to the destruction of the prairies, burns were patchy, which allowed re-colonization of burned sites by skippers from adjacent unburned areas (Swengel 1998b). Now, there are often no source populations available for re-colonization once a population has been locally extirpated. In Minnesota prairies, significantly lower abundances of H. ottoe and other habitat specialists were observed at sites that had been burned than at sites that had been hayed (Swengel and Swengel 1999, Swengel 1996, 1998b). Dana (1991) suggested that rotational, controlled, early spring burning might benefit H. ottoe by increasing nectar plant density and reducing high levels of litter that might negatively impact the development of immature stages. It was suggested that early spring burning would have less impact on the larvae than late spring, summer or fall burning, because the larvae would still be within shelters at or below the soil surface. However, two to four years after early spring burning, the abundances of H. ottoe, H. dacotae, and several other habitat specialist butterflies were still lower than pre-burn abundances on several Minnesota reserves (Swengel 1996), showing that early spring burning is a major threat to these species.
Currently, controlled burns are not being done in Spruce Woods Provincial Park. However, wildfires started in the Shilo Military Base to the west of the park periodically burn into the park and could negatively impact the H. ottoe and other insect species should large areas of the park burn.
Succession
Prairies that are protected from all activities, such as grazing, mowing or prescribed burns, can become unsuitable for H. ottoe because of the growth of woody shrubs and taller grasses, accumulation of litter, reduction in nectar sources, and invasion of exotic plants, such as B. inermis (McCabe 1981, Dana 1991, Dana 1997).
It appears that some form of disturbance is required for the persistence of prairie habitat that is appropriate for this skipper. In view of the detrimental effects of prescribed burning, probably the best solution for preventing succession is mowing in the late summer or in the fall. McCabe (1981) suggests that the optimal time for mowing is in October. Mowing at this time has no apparent negative impact on the tall-grass prairie flora or fauna. The Hook and Bullet Refuge in Minnesota has been maintained in this way for over 50 years (McCabe 1981). However, because of high topographic relief, haying may not be feasible on many of the mixed and sand-prairies where H. ottoe occurs. Small controlled rotational burns early in the spring may be the only solution for maintaining the flora and fauna at these sites. Prior to the colonization of the prairies by Europeans, most prairie habitats were maintained by periodic grazing by bison and occasional prairie fires. Since much of the habitat was suitable for H. ottoe, adults were able to re-colonize adjacent suitable habitats when forced to leave areas made temporarily unsuitable by grazing or fires. The suitable habitats that now remain are too widely separated to allow for re-colonization, and must be maintained by artificial means (McCabe 1981).
Exotic species
Exotic plants, such as Leafy Spurge (Euphorbia esula L.), Kentucky Blue Grass (P. pratensis) , and Smooth Brome (B. inermis), are significant threats to native prairie habitats in North America. Once these exotic species invade a site, they can out-compete and replace the native plants required for the survival of H. ottoe and other prairie specialists. Hesperia dacotae was probably eliminated from at least one site in North Dakota as a direct result of the invasion of E. esula (Royer and Royer 1997). Because of early senescence, P. pratensis and B. inermis are unsuitable for the larvae of H. ottoe (Dana 1991). Grasslands that become dominated by these species cannot support this skipper. Chemical control of E. esula often eliminates the nectar sources required by this skipper and may have caused the extirpation of H. dacotae from several sites in North Dakota (Royer and Marrone 1992). Euphorbia esula was abundant on mixed-grass prairies near Aweme and is invading Spruce Woods Provincial Park. Nectar sources were eliminated in 2004 near the Criddle Estate (Aweme) in Manitoba as a direct result of chemical control of E. esula.
Habitat fragmentation
Originally, H. ottoe probably existed as essentially a single population throughout much of the almost continuous mixed-grass prairies in the north- and south-central plains of North America. Now it occurs as a series of isolated populations throughout much of its former range (NatureServe 2003). In Canada, only one population may still exist, and none of the known Canadian sites is closer than 150 km to population centres in the United States. The closest population in North Dakota is considered imperiled (S1) (NatureServe 2003). There is only a very remote probability that any one of the Canadian population centres where the species formerly occurred could be re-founded by natural dispersal after being eliminated. Small isolated populations of this species are also at a greater risk of becoming extirpated by unusual weather events or other accidental events (Hanski et al. 1996).
Collection of natural history specimens
Collection of natural history specimens may pose a threat to this species because population numbers are very low (assuming the species is still present in Canada). However, skippers are generally not as popular with most collectors as are other, more showy species of butterflies. In Manitoba, it is currently illegal to collect specimens of H. ottoe without a scientific permit.
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