This blog post is an adaptation of a talk presented at the 2016 annual meeting of the Minnesota Chapter of The Wildlife Society held in Mankato, Minnesota. The topic of insect conservation on lands managed via the use of prescribed fire is complex, and this is my attempt to briefly cover a few important aspects to consider. The original presentation filled a 25 minute time slot during the conference, but this topic could easily take up several days of lecture and discussion. The PowerPoint presentation file can be made available upon request.
Below you will find slides presented during the conference, and a brief discussion under each.
This presentation covers a brief introduction to insect classification, basic biology, life-history characteristics that influence species' response to disturbance, and some prescribed fire considerations.
What is an insect?
A few terms to clarify include the following:
- Invertebrates - Includes insects and other arthropods, but also includes snails, mussels, and worms. The latter lack an exoskeleton.
- Arthropods - Includes crabs, spiders, ticks, millipedes, insects, etc. Have an exoskeleton.
- Insects - Beetles, flies, bees, etc. Excludes spiders, mites, etc.
The basic insect body plan includes a segmented body comprised of a head, thorax, and abdomen, three pairs of legs, and antennae.
In general insects follow one of three life-cycle strategies. Species that lack metamorphosis (ametabolous) start as an egg, and emerge as immatures that superficially resemble miniature adults. They grow and eventually become sexually mature adults. One of the more familiar examples of this is the silverfish.
Species with incomplete metamorphosis (hemimetabolous or paurometabolous) go from an egg, to a nymph, and then become an adult. Nymphs somewhat resemble adults, but lack adult structures (e.g., wings). Nymphs and adults often occupy similar habitats and feed on similar plants (if herbivorous) or animal prey. Nymphs do not pupate, but emerge (eclose) from their last molt as a fully formed adult. The remaining exoskeleton is referred to as an exsuvia.
Insects with complete metamorphosis (holometabolous) go from an egg to a larva, then turn into a pupa from which they emerge (eclose) as an adult. Many insect groups have specialized names for their larval stage: butterflies and moths (caterpillars); flies (maggots); and beetles (grubs) to name a few. Often larvae of insects with complete metamorphosis utilize different habitats and/or food resources than adults. Most insects undergo complete metamorphosis.
Looking closer at the two most common life-cycle strategies, it is important to note that insect growth occurs during the nymph / larva stages. Within these stages, there are multiple 'sub-stages' called instars. The number of instars varies by species; the ever popular Monarch Butterfly (Danaus plexippus) has five instars.
Following this period of growth, the final nymph instar molts and emerges as an adult, whereas the final larval instar forms a pupa. During pupation the larva liquefies and rearranges to become a completely different looking critter (e.g., caterpillar to a butterfly). Checkout this short video of a Monarch caterpillar becoming a pupa (also called chrysalis) and emerging as an adult.
Not all insects are created equal.
Despite insects generally following one of three broad life-cycle strategies, there is a lot of variation in life-history characteristics. This variation largely determines how individual species respond to disturbance, including prescribed fire. The table featured in the slide above includes several important characteristics, most notably the number of generations per year, reproductive output, dispersal ability, and overwintering location.
Examples of tolerance.
The above slide includes a couple examples of tolerant, moderately tolerant, and relatively intolerant species of butterfly to prescribed fire. Examples on the left, the Fiery Skipper (Hylephila phyleus) and Monarch (Danaus plexippus), are among the more tolerant. In both cases these insects are migratory (e.g., overwinter outside of Minnesota), and have two or more generations per year. Dormant and early season fires have little effect on these migratory species because they are not around to be effected. In addition, having two or more generations per year allow populations to more quickly recolonize following disturbance.
Examples in the middle that fall into the moderately tolerant category include the Regal Fritillary (Speyeria idalia) and the Karner Blue (Plebejus melissa samuelis) - both species of conservation concern. The Regal Fritillary has only a single generation per year, but also has a relatively long adult lifespan, good dispersal ability, and the ability to lay hundreds to thousands of eggs. The Karner Blue on the other hand has two generations per year, but produces relatively few eggs and has a relatively short adult lifespan. In both of these cases, their mix of life-history characteristics (some favoring a positive response, some favoring a negative response) make them moderately tolerant to disturbance.
Last, but certainly on the least, are the intolerant species on the right. In this case the Leonard's Skipper (Hesperia leonardus) and the Arogos Skipper (Atrytone arogos). Both of these species have a single generation per year, have a short adult lifespan (week), and overwinter on stems or blades of prairie grasses. Because these species overwinter aboveground on blades of grass, and because they are egg, larva, or pupa for > 11.5 months of the year, they are extremely susceptible to disturbance - even if a dormant season fire. Prescribed burning through occupied sites for these species results in a catastrophic loss of individuals - making a neighboring unaffected source population necessary to sustain these species long term.
Note that the federally endangered Poweshiek Skipperling (Oarisma poweshiek) also overwinters and pupates aboveground.
Take-home Message: Not All Insects Are Created Equal
It is important to pay attention to focal species, and their life-history characteristics, when evaluating scientific publications on insects and fire effects.
Hopefully it goes without saying that conserving insects and other invertebrates is important, but here are a couple figures if you need some convincing. Looking at Minnesota's list of endangered, threatened, and special concern species (2013), just over half of all animal species listed are invertebrates. Add that the status of many groups of invertebrate is unknown - and therefore remain unlisted - the true number of imperiled insects is likely higher than represented here (e.g., lost ladybugs).
Another way to evaluate insect and other invertebrate importance in the ecosystem is to look at species diversity relative to other groups of organisms. Depicted in the pyramid diagram above you see that invertebrate diversity makes up the base of the pyramid - the foundation if you will. The next level is the plants. Traditionally this is where most land managers have focused - maintaining and restoring native plant communities. Next up are vertebrates... This is where the majority of wildlife research evaluating the effects of land-management activities has occurred. Most of that effort has focused on grassland birds despite representing only a tiny fraction of biodiversity in these systems.
Recently Grodsky et al. (2015) reviewed articles published in prominent wildlife journals and found that less than 2.5% included invertebrates, with less than 1% having inverts as focal species. This highlights the need for wildlifers to better incorporate invertebrates into their studies.
It should come as no surprise that habitat loss is the greatest threat to insect conservation, but there is a growing body of evidence that land-management activities on already protected public and private reserves may be a leading cause of decline on these lands. Fortunately these activities can be easily controlled by land managers.
Best Management Practices
The remaining slides will focus on best management practices (BMPs) for using prescribed fire in grassland systems, but many of these recommendations carry over into woodland / forest systems as well. These recommendations are also applicable to other types of disturbance used to manage lands (e.g., grazing, mowing, haying, etc.).
Important Aspects to Consider
- Frequency - Disturbance interval, or in the following examples, prescribed fire return interval (FRI).
- Spatial Arrangement - How management units are arranged, spatially, over a property.
- Temporal Arrangement - How prescribed fire is applied through time.
- Size - Overall size of prescribed burns.
Prescribed Fire Frequency
Many Minnesota prairies are burned every 3, 4, or 5 years in an attempt to maximize benefits to fire-dependent native plant communities; typically under the assumption that if the native-plant community in conserved, so too are the species of wildlife that depend on those communities. However, there is a growing body of research showing that frequent fire may decimate wildlife populations, and at a few sites fire has even been linked to the extirpation of state-listed insects in Minnesota.
Prescribed fire can influence both insect species richness and abundance, and for many species with life-history characteristics on the right side of the table (thinking back to Slide 5), there is a positive correlation between year since fire and richness and/or abundance. Because of this, land managers must manage beyond native-plant communities to conserve wildlife populations - including pollinators.
- Where possible, use less destructive tools to meet management objectives (e.g., mechanical removal of woody species) to allow additional years of rest. Four to five years of rest is recommended at minimum. Many species may require greater than five years of rest between burns depending on the spatial, temporal, and overall size of burn units (continue reading below).
- In dry barrens and prairies in Minnesota, Plains Pocket Gophers (Geomys bursarius) provide soil disturbance and allow sites to remain in good condition with some woody removal required for > 20 years. If these ecosystem engineers are absent from a site that this ecologically suitable, managers could consider experimenting with reintroductions to add soil disturbance and reduce woody encroachment. Contact me if wanting to see Minnesota examples of A-ranked native plant communities that have been fire free for decades.
- Avoid burning on schedules. Instead use schedules to plan site visits to evaluate need, and defer prescribed fire where possible in favor of spot treating problem areas.
- Monitoring is a must. On sites with imperiled species known or suspected of being negatively impacted by fire, valuating fire effects on imperiled species populations is critically important. If you cannot spare the resources to monitor populations of federally or state-listed species, you should not be burning.
In addition to fire frequency, the spatial arrangement of burn units is also critically important. Above is an approximation of a real, relatively small, conservation property where frequent prescribed fire is used. At present, the property is divided into three burn units (top graphic), and note that the left-most unit contains primarily restored prairie, the center unit contains all the sedge meadow and sand blowouts on the site, and the right-most unit contains the majority of the oak savanna on the site. Of particular concern here is that some habitat types completely occur within a single management unit. If a fire-intolerant species that is present, and is dependent on sedge meadows, it could be completely eliminated from the site with one or a few burns on the single burn unit.
To reduce the probability of eliminating a fire-intolerant species from the site, it is important to avoid including all of a particular habitat in a single burn unit. An alternative in this scenario would be to further divide the site into six units (bottom graphic).
- Divide sites into as many burn units as is feasible.
- Split remnant habitats into multiple burn units (even if small). Even a small 1/2 acre of remnant habitat may harbor populations of imperiled insects.
- Avoid having all of a particular habitat in a single burn unit.
- Leave natural fire skips, and purposely create skips. May be done by altering ignition practices or by adding burn-breaks within a larger burn unit.
- Consider adding fire-free management units (refugia) to increase likelihood of conserving fire-intolerant species.
Temporal Arrangement - Part 1
Building upon fire frequency and spatial arrangement, we must also consider the temporal arrangement of when fire is applied to a property. The biggest thing to consider here is to avoid burning adjacent units consecutively. In the example in the slide above, you see what not to do. Burning adjacent units reduces the ability of poor-dispersing species to recolonize a previously burned unit. This slowly reduces species abundance and may push populations through population bottlenecks and increase issues related to small population sizes.
Temporal Arrangement - Part 2
A better alternative is to use the small burn units recommended in the Spatial Arrangement slide, and to spread burning out temporally so that adjacent burn units are not burned in consecutive years. It may be necessary to include fire-free years to accomplish this. To increase the likelihood of conserving fire-intolerant species, it may be wise to include a fire-free management unit (refugia) where less destructive tools will be used (e.g., mechanical removal of woody species, spot spraying of herbicide, etc.). Refugium / refugia should be placed in areas with known species occurrences and/or where host plants occur.
In the example above, the yellow numbers represent the year fire is applied.
Management Unit Size
Often best management practices (BMPs) recommend burning no more than 1/3 to 1/2 of a site in a single burn. As we have already discussed above, burning that much of an area is probably not wise, but those of us fortunate enough to work on large sites (thousands of acres) must also consider overall burn unit size. Though a manager working on a 10,000 acres property may only burn 10% of the site, that equates to 1,000 acres burned. These large units may take too long to recolonize by species with limited dispersal ability, low reproductive output, and/or short adult lifespans (units are often burned again before population rebound from the previous burn 3, 4, or 5 years earlier). Again, this reduces species abundance and may push populations through population bottlenecks and increase issues related to small population sizes.
Note: Many species of fire-intolerant insect (e.g., Poweshiek Skipperling, Arogos Skipper, Leonard's Skipper) are already so rare at many sties that the additional mortality associated with prescribed fire may push them over the edge.
- Avoid burning large units (large is defined as > 100 acres). This is especially important at sites with poor-dispersing rare species (e.g., many skippers, leafhoppers, and snails).
- Squared units may take longer to recolonize than thin units - there is more core area and less edge.
- If large units are required, include multiple unburned areas (refugia) within burn units. Refugia should be placed in remnant habitats where possible and/or in areas with known rare species occurrences. There is no magic formula for how large of refugium is needed within these burn units, but unburned areas as small as one acre may be enough for some species if properly placed (pers. obs.).
Wildlifer Call to Action
Wildlife biologists, land managers, foresters, and others can play an important role in reversing the adverse impacts of land management activities on imperiled insect populations. Above are a set of challenges I encourage folks to seriously consider.
Please remember... In our highly fragmented landscape, it is important to consider the impact of land-management practices on invertebrate species. Management activities, especially prescribed burning, should be planned to reduce significant impacts to rare invertebrate species, allow for refugia habitat, connect isolated parcels, and increase habitat diversity. Simply managing for native floristic components within a property will not guarantee the conservation of native wildlife. If planning assistance is needed for a particular species or site, consult with your Nongame Wildlife Specialist(s).
Thank you for considering invertebrates in your management activities!
The Field Ecology Blog is meant to be informational and thought provoking. Wherever possible, I provide links to supporting external resources. Views expressed here do not represent the views of my employer(s).
Frequently Asked Questions
Q. Didn't these species evolve with fire, why the issues now?
A. Habitat loss and fragmentation. While true that these species evolved to depend on fire-dependent plant communities, they evolved on large landscapes of millions of acres. Populations would have been large and widespread, and would have been able to recolonize from patchy wildfires. Now species occur on 'islands' of fragmented habitat, and natural recolonization is improbable in many areas.
Q. Should we just stop using fire altogether?
A. No. Prescribed fire is an important tool in the land-manager toolbox. However, in addition to following the guidance above, land managers should use less destructive tools, even if more costly, when possible.
Q. Other tools cost too much, how do you justify the extra expense.
A. Putting the intrinsic value of species aside, many of these fire-intolerant species are state or federally listed, and using fire results in their "take." If considering the costs of recovering species, the cost of using other tools is cheap. Recovery of federally endangered species often runs into the millions.
Web: Minnesota Rare Species Guide (MNDNR)
Wallner, A. M., Molano-Flores, B., & Dietrich, C. H. 2012. The influence of fire on Illinois hill prairie Auchenorrhyncha (Insecta: Hemiptera) diversity and integrity. Journal of Insect Conservation, 16:433-445.