DANCING WITH FIRE: OAK BARRENS/SAVANNA
PATCH DYNAMICS, MANAGEMENT, AND
THE KARNER BLUE BUTTERFLY
John A. Shuey
The recent listing of the Karner Blue Butterfly (Lycaeides melissa samuelis Nabokov) as an endangered species has increased interest in managing and restoring populations of this charismatic invertebrate. The Karner Blue and other lepidopteran species are rapidly becoming popular symbols for restoring and conserving the savanna ecosystems which occur on well drained sand deposits. In the Midwest, the habitat types which support the Karner Blue are generally characterized as oak barrens/savanna communities.
Each oak barrens/savanna ecosystem owes its existence to the unique interplay between past biogeographic events, edaphic conditions, and past and present disturbance regimes. The same dynamic processes which produced the unique botanical communities also produced a highly specialized community of invertebrates adapted to this regime. The importance of oak barrens/savanna habitats to invertebrates is illustrated by the Lepidoptera, a species rich group which includes the butterflies and moths. In Ohio (the only midwestern state with a completed state-wide survey of all Lepidoptera species), the Oak Openings, Ohio's only oak barrens/savanna community, supports the largest assemblage of imperiled butterflies and moths in the state. For example, five species of imperiled butterflies and 17 species of owlet moths (Noctuiidae) occur in the Oak Openings, representing approximately 4% and 3% respectively, of the resident species in Ohio (Shuey et al. 1987a, 1987b; Metzler and Lucas 1990; Iftner et al. 1992; Rings et al. 1992). The maintenance of this ecosystem is obviously vital for the preservation of lepidopteran biodiversity as well as for other lesser known plants and animals.
The decline of oak barrens/savanna lepidopteran communities can be attributed to several factors, but habitat loss, the disruption of ecosystem level processes and patch dynamics, and the collapse of metapopulation dynamics of many species are generally the primary contributors.
Outright habitat destruction is generally the most easily implicated factor contributing to the decline of most imperiled invertebrate species (Hafernik 1992) and the Karner Blue is no exception. To be locally persistent, populations of the Karner Blue require relatively large stands of the hostplant, blue lupine (Lupinus perennis L.). Habitats which support the butterfly are generally open, sunny habitats with scattered trees and shrubs, but which are dominated by grasses and other herbaceous species growing in well drained, sandy soils; i.e., healthy oak barrens/savanna communities. Like virtually all habitats in the midwest, oak barrens/savanna loss can be attributed to several factors ranging from outright destruction to more subtle secondary impacts such as the encouragement of forest growth in areas of urban encroachment.
Closely related to the impact of habitat destruction is the elimination of ecosystem level processes. Oak barrens/savanna communities are among the most dynamic in the Midwest and the open habitats which support the Karner Blue were originally maintained by a steady procession of wildfires, which killed woody invasive plants while favoring fire-adapted dune and savanna communities. Without fire disturbance, shade tolerant and fire sensitive species increase in density, and open barrens and savanna species decline.
Functional oak barrens/savanna communities are in a constant, dynamic flux. Succession pushes the community towards an association characterized by fire intolerant woody and shade tolerant herbaceous species, while fire disturbance realigns the community towards one of fire tolerant and shade intolerant species. The original patch dynamics of these communities was in constant flux, and individual sites supported communities that reflected recent disturbance history. Although fire was likely a yearly occurrence within oak barrens/savanna ecosystems, the spatial distribution of the fire was probably less predictable. Thus, these communities were composed of a constantly changing patch-work of habitats, reflecting the hit or miss nature of recent wildfires. Interdispersed through this patch-work were the recently disturbed habitats which supported populations of the Karner Blue.
Unfortunately, modern culture has traditionally abhorred wildfires because of the perceived destructive nature of fire. Thus, oak barrens/savanna ecosystems which are near urbanized areas are subject to routine/reflexive fire suppression. Similarly, state and national forests routinely suppress wildfires occurring on their lands. With few positive attributes to associate with wildfire, active ecosystem management still remains controversial to the general public in many areas. Society deprives these ecosystems of the very force that created them, a predictable and frequent fire disturbance regime.
Without the influence of a disturbance regime, oak barrens/savanna communities have succumbed to other community types. The impact of fire suppression on these communities has been as great or greater than outright habitat destruction in most areas. For example, these community types are critically endangered and the Karner Blue is extirpated from Ohio's Oak Openings, despite the 'preservation' of over 9,000 acres by state, local and private organizations. Most of the habitats in the Oak Openings which once may have supported oak barrens/savanna have converted to young oak forest.
The plants and animals that together form oak barrens/savanna communities are obviously adapted to the ecosystem level processes which originally structured these communities. To be regionally persistent in this dynamic ecosystem type, invertebrates such as insects must cope with both the ecosystem patch dynamics as well as the drivers behind the patch dynamics. In simple terms, insect populations must shift locations as quality habitats become available/ unavailable and they must be able to survive wildfire, either directly or indirectly.
While healthy metapopulations of the Karner Blue may seem to occupy entire oak barrens/savanna ecosystems, individual sub-populations are usually highly localized and are often isolated from neighboring populations by 'barriers' of unsuitable habitat. These isolated sub-populations are extremely vulnerable to extinction from both community succession and ecosystem disturbance regimes. Optimal Karner Blue habitat is best characterized as open, oak barrens/savanna habitat; that is, Karner Blues use habitats recently disturbed by fire. In the absence of fire, these communities shift towards species adapted to more overgrown, shade tolerant conditions, and habitat quality for the Karner Blue decreases. Wildfire 'resets' habitat suitability to favorable conditions for the butterfly. To persist, populations of this butterfly must be adapted to the patch dynamics of the oak barrens/ savanna community, which is highly dynamic and based upon frequent and regular disturbance by wild fire.
Unfortunately, the Karner Blue is not adapted to directly survive fire. The mechanism which is so critical for creating and maintaining habitat for this species, also kills all life stages of the butterfly and often results in localized extinction of the butterfly. Recently burned habitats must be colonized or recolonized by individuals immigrating from nearby or adjacent habitats. Confounding this issue is the limited dispersal abilities of the Karner Blue. Givnish et al (1988) estimate that maximum dispersal distance for colonization of unoccupied habitats is approximately 0.5 miles.
Because most oak barrens/savanna communities are suffering from the effects of fire suppression, optimal Karner Blue habitats are generally limited in size and widely dispersed. This combination of reduced optimal habitat patch size combined with increased distance between optimal habitat patches has destroyed the metapopulation dynamics of the Karner Blue. For example, suitable but unoccupied habitats may not have a nearby Karner Blue source population from which colonization is possible. Likewise, occupied habitats often must be recolonized following fires; recolonization has become less likely as the distance separating occupied habitats increases. In effect, the rate of localized population extinction has been accelerated by declining habitat suitability and size, while the odds of new colonization events have declined as optimal habitats become increasingly fragmented due to succession and alteration. This disruption of metapopulation dynamics is currently causing the downward spiral of several metapopulations of the Karner Blue, even as we proceed with regional attempts to restore these ecosystems.
For the Karner Blue, the interplay between habitat, habitat distribution and patch dynamics, metapopulation dynamics and metapopulation persistence is complex. This is best illustrated by the historical distribution of the butterfly itself. The ecosystems known to support metapopulations of this butterfly are generally large, measured in the tens-of-thousands of acres. Smaller sand barren/oak savanna complexes are less likely to have supported Karner Blues in historic times. This is probably a reflection of the element of chance involved with the interplay between the ecosystem processes which maintain habitat quality and the metapopulation dynamics of the species. The larger the ecosystem, the better are the odds that all the pieces fall together and populations persist. Smaller ecosystems may have provided fewer opportunities for population persistence, and Karner Blue populations did not persist to historic times. As Givnish et al (1988) poetically state, persistent populations exist as a "flickering mosaic of Karner Blue populations, with some going extinct in a given area as others are being founded on sites recently burnt by colonists from sites burnt a somewhat longer time ago." In smaller ecosystems, these populations may simply flicker out.
However, given that almost every oak barrens/savanna community in the Midwest must now be actively managed to persist, much of the element of chance can be removed from Karner Blue management. With intensive management, carefully planned burn units and suitable habitats, Karner Blue populations should be manageable on preserves as small as 200 acres. At this scale, management would have to be almost mechanical, with approximately 10-15% of the land-base burned annually, and the burn units configured to provide adequate dispersal opportunities for Karner Blues. Larger management areas could be managed less mechanically, but would still require carefully planned management activities. Small management units could be used to establish 'core' populations within larger ecosystem management areas, from which dispersing butterflies could become more widely established. Finally, to protect against catastrophic disaster, several independent sets of Karner Blue populations should be maintained in each oak barrens/savanna ecosystem. Because of the flammable nature of the ecosystems, true wildfires which consume thousands of acres at one time are a reality. Because individual Karner Blue populations may succumb to such an event, independent 'core' populations should be dispersed through the ecosystem to ensure that single catastrophic events cannot eliminate entire metapopulations.
Preserve managers and stewards must struggle to re-establish the processes which created the barrens and savanna ecosystems they manage. If the Karner Blue is to survive, we must literally take it back to the big dance, where metapopulations swirled with patch dynamics to the music of fire. By managing remnant barrens and savanna communities in light of large-scale ecosystems processes, it should be possible to preserve not only the Karner Blue, but the untold other inconspicuous life-forms adapted to these ever changing ecosystems.
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