Eastern prairies and oak savannas lose prairie-dependent plant species without frequent or even annual burning (see Bowles and Jones, 2013; Alstad et al., 2016; Alstad and Damschen, 2016; Ladwig et al., 2018), and with those plant species, they lose all the specialists consumers that depend on them. Clonal woody species increase more rapidly with infrequent burning (less often than every third year) than they do very frequent burning (Ratajczak et al., 2016). Moreover, effects of prescribed fire frequency are dominant over those of precipitation change in determining woody encroachment in tallgrass prairie (Brunsell et al., 2017); this is not surprising, given the the historical occurrence of savannas and even areas of open grasslands in much of the wet eastern US prior to significant European presence, even in the Shenendoah Valley of Virginia (Omer Stewart, Forgotten Fires). Fire can override changes in precipitation. The use of frequent fire is necessary to maintain prairies and should serve as valuable tool in helping prairies and associated communities remain resistant to and resilient from climate change.
None of this should surprise people that visit remnant prairies and associated communities in the eastern tallgrass prairie region. Prairies with long histories of frequent burning are in far better shape than those not frequently burned or those with on again, off again management. Especially where there is much woody vegetation on the surrounding landscape, prairies burned less often than every 2-3 years are being rapidly degraded or lost. Places like Chiwaukee Prairie in far southeast Wisconsin are deteriorating and the hard work cutting and treating brush is wasted more and more as burns remain too infrequent to hold the line as the brush as pushed back. Frequently burned sites like Black Earth Rettenmund Prairie thrive, despite their small size and the abundance of woody vegetation on the surrounding landscape, which was all formerly prairie, oak savanna, and oak woodland.
This brings me to topic refugia that are left unburned for the roughly 40% of prairie insect taxa that experience at least brief population reductions with prescribed fire (Panzer 2002) [I will set aside the question of whether population reductions are universally bad, as we know native plant species get out of balance on poorly managed prairies, so it would be surprising if insects didn’t. We have floristic quality assessment to account for this, but not invertebrate quality assessment]. Refugia are necessary, because most remnant prairies are small, so insects that likely depended on recolonization from refugia naturally present at larger scales prior to the loss the prairie landscape need assistance to lower the risk of extirpation resulting from the very necessary use of frequent fire.
These refugia need to move. Without moving them, the prairie within them will degraded as described above and host plants will be lost within refugia, and if that happens, it stands to reason that insects will favor areas outside of refugia where their food plants grow with greater vigor and abundance, and where they are more vulnerable to fire. Let us take regal fritillary as an example. It requires any of several species of violets that occur on prairies. Refugia for that species should be sited where good populations of violets are present. However, infrequent or no fire leads to the loss of violets, because violets are low-growing and smothered by accumulated litter (Henderson et al. 2018), so permanent, unburned refugia would probably be counter-productive for regal fritillary, host violets, and prairie conservation. This will be true for many, if not most prairie plant species that serve as host plants for rare, prairie-dependent insects. For this reason, I believe the most prudent approach is to place refugia where high quality, prairie-dependent plant species occur, which are the most likely to host rare, prairie-dependent insects, and move footprints to adjacent portions of the area of high quality vegetation from year to year. The healthier and more abundant the vegetation and host plant populations become, the more options for refugia there will be.
I’ll add as an aside, that many dry-mesic to dry prairies, sand prairies, and many savannas and oak woodlands have lower, sparser vegetation with intervening areas between vegetation covered by cryptobiotic crusts of mosses, lichens, and cyanobacteria. However, litter does accumulate over time such that fires burn more intensely in areas left unburned for several years (so long as they aren’t overtaken by buckthorn or honeysuckle) than where fire is frequent or annual (patches of crust are often left unsinged); fire intensity (Byram’s equation) is the product of fuel heat of combustion, rate of fire spread, and quantity of fuel consumed in the flame front. Unburned or infrequently burned sites, unless they are hyper-xeric and very sparse, actually lose the crusts, which need light, and frequently burned sites tend to have them. This is just a personal observation, but it should be a consideration, because it surely affects the environment invertebrates face. We often consider the effects of single fire events versus the long term effects of how a particular fire frequency structures (sensu lato) the community. The short-term effect of a fire where none has occurred for four years may be very different from that where fire occurred the year before. Studies of insect responses to fire must consider host plant responses and view short term responses in light of the long-term consequences of fire frequency for the community as a whole.