Wild and prescribed fires often result in heterogeneous mixtures of severity, and subsequent habitats, across the landscape (Forman and Boerner 1981; He et al. 2019; Hiers et al. 2020). Therefore, studying biodiversity while disregarding a fire’s severity could lead to misleading or incomplete results. This is likely one of the reasons why there has been little consensus across the broader pyroentomology literature regarding insect responses to fire (Mason et al. 2021).
Our goal was to demonstrate that by methodically quantifying fire severity via satellite imagery and stratifying our sampling across this classification, we would show differences for surface-active ground beetle communities, particularly in the NJ Pinelands National Reserve, the northern portion of the Atlantic Coastal Plain. We predicted that ground beetle taxonomic and functional trait community compositions would be different when comparing control, moderate, and high fire severity sites, and these hypotheses have been supported. Our data show that there were pairwise differences in both ground beetle taxonomic and functional community composition, specifically for our control and moderate severity fire and moderate and high severity fire sites. Even with some taxonomic and functional community composition changes, our results indirectly support the “habitat-heterogeneity hypothesis” by showing that different fire severity sites lead to more/higher diversity of ground beetle community compositions (Beal‑Neves et al. 2020; Cramer and Willig 2002; Whittaker 1972). Additionally, we report on representative species for each of our control and fire severity taxonomic communities along with unique species that did not occur at the other sites (e.g., Syntomus americanus at control; Calosoma calidum at moderate severity; Cicindela patruela at high severity). We also found that functional traits that characterized our unburned community were slow, brachypterous species that were not favored by human activity with contrasting traits of macropterous and gregarious species that favored human activity characterizing both our fire sites. The trait associated with moderate severity fire is being a climbing species, and traits associated with high severity fires are being diurnal, small and medium-sized species. Based on this research, we see no detrimental effects of high prescribed fire severity in the PNR if the management goal is to increase ground beetle biodiversity in the form of taxonomic and functional community composition.
Moderate severity fire:
Moderate severity fires showed the most differences in ground beetle community composition when doing pairwise comparisons between our study sites. Taxonomically and functionally, our moderate severity site showed differences with both the unburned and the high severity site. Since the moderate fire severity site did show a unique taxonomic community composition for ground beetles, it is not surprising that the functional trait community composition would likewise be unique. The direct biotic association between the study species and their functional traits can often explain similar responses when studying both taxonomic and functional approaches (Gagic et al. 2015; Pakeman and Stockan 2014; Wong et al. 2019).
We mostly attribute the moderate severity ground beetle community composition to the unique forest structure post-fire. The Differenced Normalized Burn Ratio (dNBR) accounts for changes in vegetation cover following a disturbance event (Miller et al. 2009; Miller and Thode 2007) and is the metric we used to measure fire severity. Unburned pine forests, particularly in the PNR, that have not been burned over a long period of time can create a near climax and stable community that is homogeneous with established, competitively successful plant and animal species (Ammer 2016; Forman 1998; Molles, Jr. 2013). Similarly, high-severity fires can completely consume all of the forest’s above-ground biomass (i.e., canopy, understory, leaf litter) and expose the mineral soil (Clark et al. 2020; Galbraith et al. 2019; Skowronski et al. 2020), leaving another homogeneous community (“structural simplification”) (Radea and Arianoutsou 2012) without much niche diversity.
In contrast, moderate severity fires have the ability to create a unique vegetation community with characteristics similar to both low and high severity fires, e.g., consumed litter layer and understory (i.e. high severity) and a canopy that is still mostly intact (i.e., low severity) (Keeley 2009, Malmström 2010, Ryan 2002). In other words, sites with moderate severity fires include micro-habitats where species from unburned and high severity sites can co-exist. Moderate severity fires have even been described as “restarting the community, but without total destruction”, therefore creating a landscape that is “best” for insect diversity (Lazarina et al. 2019). Additionally, Ponisio et al. 2016 found that early and late colonizer species were characteristic of their moderate severity site which included a diversity of niches from soil and canopy structure. Specifically for ground beetles, there are opportunistic species that prefer open areas, while unburned forests will contain forest specialist species that prefer thicker vegetation and higher humidity (Gongalsky 2003, Gongalsky 2008; Fernandez and Costas 2004; Zdzioch 2003). A moderate severity fire has enough niches for both kinds of ground beetles to coexist which result in a unique community composition, even if it's potentially for the short term. Thus, we believe that the inclusion of microhabitats and niche diversity from the two extremes that naturally comes with moderate fire severity strongly influences the ground beetle taxonomic and functional community compositions.
The other possible contribution to the unique moderate severity fire community is the distance away from the unburned and high severity community. Distance (i.e., scale) is both the solution and issue in ecological studies, especially when considering spatial autocorrelation (Legendre 1993; Negret et al. 2020). The analyses we used, NMDS and PERMANOVA, do not inherently account for spatial relationships. Though all three of our study sites were upland pine-oak forest habitats before the prescribed fires, the moderate severity fire is approximately three kilometers away from our control site, while the high severity site is approximately one-and-a-half kilometers away from the control site. Even though a one-and-a-half-kilometer distance might not be a relatively long distance, there is very little information about how far ground beetles can disperse via flying and/or running (Den Boer 1970; Den Boer 1990; Frampton et al. 1995). Additionally, the ground beetle community at our moderate severity site might have been different from the start, as suggested by a much higher abundance of Galerita janus. Ideally, having more fire severity and control sites would have helped to give more clarity to the actual fire severity effects on ground beetle communities. Unfortunately, being able to get adequate replication of study sites, especially for landscape-scale events such as fires, are difficult to achieve, hence treatment effects are often tested at the trap level vs overall treatment (Davies and Gray 2015; van Mantgem et al. 2001).
Unburned and high severity sites:
Distance might also explain how the unburned and high fire severity ground beetle taxonomic and functional community compositions were not dissimilar from each other since these sites were the closest in proximity to one another out of all three sites. Therefore, insects from the unburned site could rapidly recolonize into the high severity burn site much easier and faster than the moderate fire severity site. This high resilience of insects is likely reflected in their evolutionary history through functional traits, especially if they occur in fire-adapted ecosystems like the PNR (He at al. 2019; New 2014; Wong et al. 2019). However, we still believe that there would have been a difference between the unburned and high fire severity sites, not only because they were nearly dissimilar from each other (taxonomically: p = 0.075), but mostly because they are essentially two extremes on opposite ends of a fire severity gradient; a site that has not been burned in approximately 70 years with an established forest canopy, understory and decades of accumulated leaf litter and a site that lost all of its vegetation cover and biomass (Supplemental Data 8). Additionally, previous pyroentomology studies have demonstrated that different forest structures after fires can change invertebrate community compositions. Samu et al. 2010, showed that spider assemblages tracked vegetation structure. They provided evidence that some spider species prefer higher vegetation strata in their unburned area, while other species that prefer lower strata in the burned area. Galbraith et al. 2019 reported that high severity fires with more open canopies had 20 times more individuals and 11 times more species of bees compared to lower fire severity sites. Additionally, Kwon et al. 2013b reported that butterfly species compositions changed between their burned and unburned forest sites during the year of the fire due to a “vegetation change” from tree species to more shrubs and grasses. Thus, we agree with the “dynamic vegetation hypothesis” in which species’ post-fire responses are driven by vegetation structure (Nimmo et al. 2014), which is mostly affected by fire severity.
Predictors (covariates):
The seasonal phenology of ground beetles plays a critical role in contributing to ground beetle community composition. This is often correlated with the activity period of local climate, looking for mates and shelter, and foraging for food (Fernandez and Costas 2004; Larochelle and Lariviere 2003; Radea and Arianoutsou 2012). Thus, it is not a surprise that seasonal phenology explained at least a third of the variation in our study as a predictor of our ground beetle community composition (taxonomically: 0.324; functionally; 0.379). However, it is important to note that the significant seasonal phenology contribution might be higher than its true effect because we analyzed our data by trap (i.e., samples collected every other week throughout the season) instead of by field site to account for our study’s sampling design. This is not unusual in fire ecology studies since fires tend to be single events and therefore cannot have a larger sample size without running the risk of pseudoreplication (Davies and Gray 2015; Hurlbert 1984; van Mantgem et al. 2001). Under a replicated sampling design with independent prescribed fires of varying severity, we anticipate that the fire severity predictor would have a larger effect on our ground beetle taxonomic and functional community composition. Post-fire effects from different fire severities not only affect vegetation structure but also ambient and ground temperatures and soil nitrogen and pH (Iba'nez et al. 2022; Keeley 2009) which can directly and indirectly affect insect biodiversity and behavior (Mason et al. 2021; New 2014).
As for surface active ground beetle community composition by year, we found it interesting that the taxonomic composition was different between 2017 and 2018, but functional composition was not dissimilar. As previously mentioned, functional traits directly associated to specific species are often tied together and similar patterns can emerge when comparing both taxonomic and functional approaches. However, this was not the case when we investigated year as a predictor for ground beetle communities. This result is likely a relic of our analyses since we combined different ground beetle community compositions together from sites. Regardless, we indirectly have supported that ground beetle functional community composition was not driven by taxonomic community composition when considering temporal variation. We suspect that in the following years, the taxonomic community composition will get back to being more stable as indicated by the 2018 ellipse being smaller than the 2017 ellipse. The additional residual variation could be explained by a combination of other variables that are not directly associated with fire severity, such as interspecific competition and behavior and phenotypic plasticity, and/or stochastic noise (Mitchell et al. 2021; Ramakers et al. 2020; Shoemaker et al. 2020).
Taxonomic observations:
We noticed abundance changes for each of the three most commonly collected ground beetle species by site and/or year. Our data shows that Pasimachus depressus is likely a species that prefers a stable community based on its decline in abundance between our unburned and moderate and high severity burn sites in 2017 (UnBurn: 88, ModSev: 26, and HighSev: 7). Larochelle and Lariviere 2003 note that Pasimachus depressus’ habitat is sparse vegetation where they can burrow under dead leaves and wood. Consequently, the higher the fire severity, the more forest leaf litter and dead wood would get consumed. These resources are vital for surface active ground beetles as they are dependent on abiotic (e.g. humidity and temperature) and biotic (e.g. shelter and food supply) conditions (Apigian et al. 2006; Gongalsky 2008; Koivula et al. 1999). Former studies supported this by showing ground beetle abundance increasing in plots with added leaf litter and more coarse woody debris (Cobb et al. 2007; Koivula et al. 1999). Fortunately, this species is one of the most common ground beetles in the PNR (Boyd 1991), and its population tripled in abundance from 2017 to 2018 at our high severity site showing resilience to recover after fire. As for Polyderis laeva, we only collected a total of 6 individuals across all our sites in 2017 (UnBurn: 4, ModSev: 2, and HighSev: 0), but in 2018 we collected a total of 61 individuals across all our sites with the high severity site having the highest abundance (UnBurn: 17, ModSev: 16, and HighSev: 28). This observation can either be due to a “boom” year for the species and/or the species was attracted to the prescribed fire and recolonized into our study sites in 2018. Since Polyderis laeva is a frequent flier that prefers open ground (Larochelle and Lariviere 2003), individuals were likely on the way to the fire sites to take advantage of the newly opened landscape. With reference to Galerita janus, there seemed to be a population at the moderate fire severity site with 40 individuals occurring there post-prescribed fire in 2017 and then only having 10 individuals in 2018. As mentioned previously, the relative abundance of Galerita janus is proportionally higher than the other species we collected at this site which likely influenced our analyses. Regardless, this was the only gregarious species that we collected with enough individuals that can support that this species was actually gregarious. Interestingly, the joint cooperation between NJCF and the American Entomological Society that began in 2006 to survey insects at FPP (Mason 2015) has never collected Galerita janus. The only Galerita species previously collected at FPP has been G. bicolor (Drury, 1773), which we did not capture at all. This perhaps indicates that Galerita janus is a deep forest species while G. bicolor occurs more along forest edges and roadsides where entomologists are more biased to collect (Kadmon et al. 2004; Oliveira et al. 2016), despite Smith 1909 noting that they can occur together with G. bicolor being “more rare”.
Another species of note that we collected is Cicindela patruela, which is a rare and declining tiger beetle only known from the mid-Atlantic coastal plain that benefits from habitat management such as prescribed fires (Mawdsley 2007; Schlesinger and Novak 2011). Our data supports this hypothesis as Cicindela patruela was only found at our high prescribed fire severity site. In contrast, the tiger beetle Cicindela unipunctata which has been reported to forage under leaf litter along the forest floor (Knisley and Schultz 1997) likely does not prefer prescribed fires. Our data supports this with 6 individuals collected at our unburned site, 2 at our moderate fire severity site, and 0 at our high fire severity sites. The two ground beetles, Scaphinotus sp. (c.f., elevatus) and Apenes sinuata, that we only collected one individual of, has been noted as being “rare” in New Jersey (Smith 1909). Since the soils in the PNR have high acidity levels, low levels of magnesium, and a lack of shell building calcium, there are relatively very few shelled animals (Mollusca) in the region, thus a lack of food resources for Scaphinotus spp. (Boyd 2008; McCormick 1970). Lastly, we found one individual of Calosoma calidum which has been noted to be “the most abundant and generally distributed species” out of the “caterpillar hunters” (Calosoma spp.) (Smith 1909). However, we feel this species should be considered rare in the PNR, not only from the lack of collecting it in this study, but also due to the lack of collected specimens and observations in the region in recent decades.
Functional traits:
Wing morphology was a strong indicator in both the burned and control sites, as macropterous species were associated at both burn sites and brachypterous species were found at our unburned site. Our results support Samu et al. 2010 and Barber et al. 2017 who also found macropterous species were linked to their fire sites (wildlife and prescribed fire, respectively). However, our results were likely dependent on the brachypterous Pasimachus depressus since this species represented 57% of the total ground beetles collected, and 71% of Pasimachus depressus specimens were collected at our unburned site. Another large influence for wing morphology could be that our moderate severity site was dominated by the macropterous Galerita janus, and our high severity site had an influx of the also macropterous Polyderis laeva in 2018, but with no specimens collected in 2017. Thus, our data is in agreement with many studies suggesting that the high-dispersal capabilities of macropterous species are early colonizers to the newly burned and disturbed landscapes (Holliday 1991; Koltz et al. 2018; New 2014; Ribera et al. 2001); Polyderis laeva is a great example of that. In contrast, our data support former studies that suggest brachypterous species, such as Pasimachus depressus, are more frequent in undisturbed habitats and rare in early successional habitats (Barber et al. 2017; Bargmann et al. 2016; Holliday 1991; Ribera et al. 2001). It is important to note though that the majority of ground beetles we collected were strictly macropterous anyway and only five species were strictly brachypterous, three of which were actually more common at our burned sites (i.e., Dicaelus elongatus, Carabus sylvosus, Carabus vinctus). Due to this, our interpretation is that the functional community composition turnover from brachypterous to macropterous traits might not just be an influx of macropterous species, but also the loss of brachypterous species that directly and indirectly perish from fires and cannot rapidly colonize the new burned habitat.
Ground beetles that were favored by human activity was another strong indicator of our study sites. Since humans constantly alter natural landscapes via urbanization (Varet et al. 2013) we felt this trait would be a reliable indicator for general disturbance preference. Our results show that species with preference for human activities are indicative of both our disturbed (burned) sites, while species without this preference were indicative of our undisturbed (unburned) site. The changes that fire disturbances create generally provide new and/or additional resources and preferred habitat preferences to disturbance specialists (Koltz et al. 2018; New 2014). These resources can be more dead wood that provide shelter, food, and breeding microhabitats, along with lower humidity and increased temperature that specialist and generalist beetles can readily take advantage of (Gandhi et al. 2008; Hammond et al. 2018; Wikars 2002). In contrast though, non-disturbance species might benefit from a late successional community due to more leaf litter and a closed canopy (Cobb et al. 2007; Niwa and Peck 2002; Oliver et al. 2000). Regardless, it seems that our species pool of ground beetles that are favored by human activity also benefit from fires; suggesting that this functional trait could be used to infer species responses to other types of disturbance (e.g. grazing, thinning). It should be noted though, that the ten species we collected that were favored by humans are also all macropterous species. Therefore, it is important to consider and test if disturbance specialist ground beetles are actually using post fire environment resources or are simply dispersing through.
We found two other surface-active ground beetle functional traits that can be indicators of overall “fire” since they occurred in both moderate and high severity burn sites. The first trait is being gregarious, and 12 of the 23 species we collected were considered gregarious. This can be beneficial or detrimental from “fires” for ground beetles. As a benefit, this trait could mean quick re-establishment after fires by hiding out in large numbers together in refugia from fires (Brennan et al. 2011; Iglay et al. 2012; New 2014). As a detriment though, it could mean that populations get locally extirpated if they are all relatively close together and vulnerable to a fire. However, we suspect this trait might be a byproduct of other traits that directly contribute to our fire sites. As two examples, the gregarious species Galerita janus and Polyderis laeva were two of the three most dominant species we collected throughout 2017 and 2018 and likely influenced our analysis. Specifically, for Galerita janus, 40 individuals were caught at our moderate severity site in 2017 and only 10 found at the site in 2018, suggesting that many individuals survived the fire, but cannot survive the post-fire environment. Thus, this species did not re-establish itself in 2018, despite being gregarious. Furthermore, for Polyderis laeva, we collected 0 specimens at our high severity site and 2 specimens at our moderate severity site in 2017, but in 2018 we collected 28 at our high severity site and 16 at our moderate severity site. This suggests that Polyderis laeva were more common at our burned sites in 2018, likely because they flew in (being macropterous), and also just so happen to be gregarious.
Spring and summer breeding species was the second trait indicative of our fire sites. In contrast, only summer breeding species and not spring, was a trait indicative of our unburned site. However, these traits should be explored further, mostly because 11 out of the 23 species we collected were considered unknown for breeding season (Supplemental Data 4). Learning what these unknowns are would give us a much better sense of how spring and summer breeders are affected by fires and strengthen or weaken our initial result. Nevertheless, the breeding season pattern is a functional trait that still represents the ground beetle activity period which fires can greatly influence (Holliday 1991). With more support from future pyroentomology studies, the timing of prescribed fires could be better guided, particularly when this trait can be associated with uncommon/rare species (e.g., Cicindela patruela, Calosoma calidum for NJ).
Being attracted to light was the third trait that was represented for moderate severity fires, yet we still suspect this trait could also be a general fire trait and be included with high severity fires. Insects that are attracted to lights are known to be able to disperse relatively farther distances than species that are not attracted to light (Firebaugh and Haynes 2016; Perkin et al. 2014). Thus, with a more open forest structure at moderate (and high) fire severity sites, light can travel farther distances and essentially attract insects that are attracted to light. It is not surprising that 12 of the 16 species we collected that were known to be attracted to light were also macropterous, reflecting how the two traits are connected and potentially have co-evolved together.
We found that small and medium sized individuals were indicative of our high fire severity site. In our case, we measured body size as length, which has been shown to be a functionally important indicator of higher dispersal ability for beetles (Fountain-Jones et al. 2015; Koivula 2011; Ribera et al. 2001). Thus, it is not surprising that another functional trait connected to dispersal ability was found at our high fire severity site which has a more open canopy allowing smaller dispersers to take advantage of the new habitat. Additionally, the absence of larger species of ground beetles can be due to the absence of suitable prey for them, such as lepidopterous caterpillars (Holliday 1991). As for moderate severity fires, the indicative trait was being a climber. For the species that were strictly climbers, 8 out of the 9 were found at our burned sites. However, this result was more likely influenced by the climber Galerita janus with 86% of the specimens collected at our moderate severity site. Climbing as a functional trait has been understudied in pyroentomology, but there is some evidence of “vertical dispersal” up trees to escape fires (Dell et al. 2017). Lastly, regarding our unburned site, we found that one of the indicative traits was slow speed. This is another trait that ties in with dispersal power which can limit a species’ ability to escape fires, resulting in direct or indirect mortality and a slower rate to recolonize unburned habitats (Barton et al. 2011; Fountain-Jones et al. 2015). We only collected two species that were considered slow for our analysis. The first was Pasimachus depressus, which our result undoubtedly was weighted towards due to its high abundance in our unburned site. The second species was Stenolophus comma (Fabricius, 1775), represented by one individual that was also collected at our unburned site.
Activity period and diet are two other traits groups that we feel are important to mention. For activity periods in beetles, diurnal and nocturnal traits are often linked to sensory mechanisms that are related to eye size and structure (Fountain-Jones et al. 2015). Larger eyes (i.e., more ommatidia and surface area) usually indicate diurnal species of more open habitats, while nocturnal species have smaller eyes and are associated with more complex habitats (Stranger-Hall et al. 2018; Talarico et al. 2011; Tocco et al. 2019). Because of these traits, we expected that diurnal species would be indicative of our more open fire sites and nocturnal for the more closed canopies of our unburned sites. Our results do show some signals that this might be supported. As for diet, we could not support or reject either Barber et al. 2017, Bargmann et al. 2016, and Samu et al. 2010 because we had no species that were phytophagous and granivorous. However, predators seem to make up the center of our burned functional compositions, while also being associated with our unburned community, while omnivores could potentially be influencing the moderate severity composition. In general, though, Koltz et al. 2018 notes that species with general feeding habits are the most likely to benefit from post-fire resources.
Conservation Management Implications:
Conservation management programs can be impeded by the lack of knowledge, understanding, and most importantly, evidence of the potential positive and negative effects of that management strategy. This can be the case with implementing prescribed fires which have the ability to cause direct mortality to organisms which potentially can reduce species richness and abundance (Carbone et al. 2019; He et al. 2019; Mason et al. 2021). In our case, prescribed fires were used to reduce hazardous fuel loads in the New Jersey Pinelands National Reserve. This allowed us to methodically quantify prescribed fire severity via satellite imagery and to study the fire severity effects on ground beetle communities. We specifically provide evidence that show ground beetle taxonomic and functional community compositions at moderate fire severities are different from both unburned and high severity fire sites. Thus is the importance of quantifying fire severity.
Knowing that there can be differences between two fire severities shows the value and need to quantify them. If no differences were found, future studies could continually generalize “fire” because different fire severity effects would more or less be the same. We encourage future pyroentomology studies to account for fire severity and also suggest caution when interpreting results that do not quantify fire severity. Pyroenomology studies that generalize “fire” will likely attribute their results and explanations to the overall “fire” effect. This can be misleading since their results could be more dependent on the specific fire severity site and/or mix of fire severity sites they sampled, which could represent different biodiversity metrics (e.g., community composition) of their study taxa. Additionally, the pyroentomology studies we found that mention/included fire severity all showed differences on their study taxa between fire severity sites. In other words, we have found no evidence that shows different fire severity sites would result in the same effect for invertebrates. By quantifying fire severity, we can more accurately compare pyroentomology studies to help solve the conflicting results in this developing field.
We also report on the “winners and losers'' of the ground beetle species we sampled after prescribed fires. Specific species that we feel should be investigated further are Polyderis laeva, Galerita janus, Cincindela patruela, Calosoma calidum, Scaphinotus sp, and Apenes sinuata, not only because they had interesting responses to prescribed fire severity, but also because the latter four species could potentially be considered rare or uncommon in the PNR. We also report that wing morphology and being favored by humans are strong indicators of how ground beetles respond to prescribed fires. These traits could potentially be universal fire traits for other taxonomic groups and should also be investigated further. We encourage more studies that focus on the natural history and ecology of ground beetles and their functional traits particularly after prescribed fires of differing severities. This ultimately will help conservation managers make more informed decisions and accurate predictions about prescribed fire effects on biodiversity in our native landscapes. We encourage future work on prescribed fire severity for all insect taxa. More of these studies will undoubtedly develop and reveal various biodiversity metric responses for different taxa in different environments, adding to our overall base knowledge of pyroentomology.