Individual study: A literature review of insect responses to fire, compared to other conservation managements of open habitat
Swengel A.B. (2001) A literature review of insect responses to fire, compared to other conservation managements of open habitat. Biodiversity and Conservation, 10, 1141-1169
The "ecosystem approach" to conservation, which emphasizes natural (ecological and evolutionary) processes over individual species, is frequently advocated today. But concerns have been raised about negative impacts for localized or rare insects attributed to ecosystem management with fire. However, others present observations of particular species or sites as corroboration that insects specialized to live in open habitats asserted to be fire-dependent will likewise be fire-adapted. A factor contributing to this controversy is that studies of entomological responses to ecosystem management techniques have used a variety of analytical approaches, comparisons (i.e., types of managements and sites compared), and taxonomic scales (i.e., level of insect identification).
This paper contains a critical review and synthesis of 172 articles in the international scientific literature concerning the following issues:
1) types of management comparisons - more vs. less recent burning; single wildfire vs. rotational fire management; burning vs. idling (long-term non-management), and burning vs. other plausible and feasible conservation managements (e.g., grazing, haying, mowing);
2) spatial and temporal scales of treated and untreated areas;
3) types of insect groupings - all members of a higher taxon (e.g., family, order) vs. individual, or groups of, species categorized by habitat affinity, dispersal tendency, or conservation concern;
4) utility of theories about ecological processes for explaining insect patterns and responses, compared to the utility of life history traits and habitat specificity of the taxa.
More vs. less recent burning: Many insect groups decline markedly immediately after fire, with the magnitude of reduction related to the degree of exposure to the flames and mobility of the insect. Niche diversity is lower in recently burned habitat, and the rate of insect increase following fire also relates to the species' ability to gain access to the regrowing vegetation. Postburn flora can be quite attractive to some recolonizing insects, possibly to some degree as a result of fire-caused insect mortality which provides plants with short term release from insect herbivory. As a long-term strategy for conservation of open-habitat insects, the broadcast killing of grassland insects by recurrent fires so that the lushly regrowing vegetation will attract recolonizing grassland insects should be examined with caution. The reduction of generalist and pest insects by fire will likely be short-term, but the (inadvertent) reduction of more habitat-restricted species by the same fires may be much longer term, or even permanent.
Single wildfire vs. rotational fire management: When insect responses to a single wildfire are more favorable than to rotational fire management, this can be understood as functions of access and time to repopulate, since these wildfires occurred in a context of long-unburned adjacent habitat also occupied by the species. Another factor may be the relatively higher suitability of habitat after wildfire (more likely to be stand-replacing, and therefore canopy-reducing) than rotational fire management (which, to stay controllable, may be restricted more to the herb layer).
Burning vs. idling: At the scale of higher taxa (family; order), the literature indicates minimal differences between burning and idling. Studies of individual species showed varied and sometimes inconsistent, responses. Thus, based on studies of overall insect faunas, it is difficult either to justify the investment in fire management or substantiate harm from it. Results become only somewhat more conclusive when examining particular species of conservation concern. Even then, though, differences between idling and burning are often relatively minor compared to other types of management compatible with the continued existence of the floristic community.
Burning vs. haying/mowing: Insect declines may follow immediately after mowing, but are usually of lesser degree and shorter duration than after a fire of comparable timing and size. Season and scale of cutting may affect how much and which species showed positive or negative responses. Cut areas offer the vegetational structure and composition preferred by some insects, but cutting, or cutting at certain scales, seasons, or frequencies, is also unfavorable for some species.
Grazing: Heavy grazing results in niche and assemblage simplification. Nonetheless, some invertebrates prefer the short turfs and bare ground resulting from heavier grazing. Other species vary in whether they peak in abundance and diversity in intermediate, light, or no grazing. In comparisons of mowing/haying and grazing regimes of similar compatibility with maintenance of the same habitat types, responses of particular species and species groups varied as to whether they had a preference for one or the other.
Theory vs. observation: Mortality during treatment, stress post-treatment in a more simplified habitat, suitability of regrowing vegetation, and ability to repopulate appear equally useful for explicating the effects of fire as well as other managements such as haying, mowing, and grazing on insect species. Another basis for predicting insect response to fire (and other managements) assumes that the most habitat restricted species should be most adapted to (or dependent on) the ecological forces thought to be prevalent in that ecosystem. But life history traits that may afford some protection from fire mortality (or other managements) are not ecosystem specific but rather taxonomically associated. Thus, insect responses to fire (and other managements) can be interpreted on the basis of biological mechanisms and traits that do not assort by ecosystem type.
Conclusions: This literature review suggests the importance of retaining considerable spatiotemporal variation among sites of the same ecosystem type in the frequency of fire and other natural events, such as grazing, and other management interventions, such as mowing and cutting. Reducing this variation leads to greater uniformity (simplification) of niches and therefore species among sites of the same habitat type. Optimal site management for an open habitat patch can only be determined based on the efficacy of fire or other management to produce a specified species composition and abundance.
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