Grasshopper abundance and offtake increase after prescribed fire in semi-arid grassland

Introduction

As globally ubiquitous herbivores, grasshoppers (Orthoptera: Acrididae) contribute to ecosystem function around the world. Grasshoppers are particularly important in open ecosystems–rangeland biomes such as grasslands and savannas in which plant communities are regulated by interactive disturbances including fire and herbivory (Bond 2021).

Historically, interest in grasshoppers has generally increased with their local density, as grasshopper outbreaks and locust swarms have wrought economic damage for centuries (Cease et al. 2015). While such outbreaks were long considered to be primarily driven by environmental conditions beyond human control, research has described close interactions between land management and grasshopper dynamics (Le Gall et al. 2019). Although the utility of this broader understanding of grasshoppers and human land use has mostly been realised within the context of pest control (Branson et al. 2006), grasshoppers also contribute to nutrient cycling and plant community composition (Belovsky and Slade 2000; Meyer et al. 2002; Zhang et al. 2011; Kietzka et al. 2021).

Because the nutritive value of vegetation in open ecosystems often varies depending on the time since it last burned, fire likely also affects grasshoppers by modulating their food resources. Perennial, fire-adapted plants resprout using energy stored in organs protected from heat damage, and post-fire plant tissue is typically higher in crude protein and lower in structural carbohydrates than the mature or senescent tissue that was consumed by the fire (McGranahan and Wonkka 2021). Thus, despite overall lower plant biomass on account of the fire, grasshopper abundance on recently-burned areas is often higher than unburned areas, especially for graminivorous (grass-eating) species (Meyer et al. 2002). More broadly, post-disturbance succession and plant nutritive value have been identified as important drivers of grasshopper abundance (Fartmann et al. 2012; Schirmel et al. 2019). Yet explicit examination of the relationships between time-since-fire, plant nutritive value, and grasshopper abundance have not been conducted.

We measured grasshopper abundance and forage consumption, along with grass protein content, in a replicated experiment that created a time-since-fire gradient in temperate grassland. We predicted that more recently burned plots would have both higher protein content, especially in leaves, and greater grasshopper abundance. As such, we predicted a greater degree of vegetation removal by grasshoppers from recently-burned plots, as determined by comparing aboveground plant biomass against that from within grasshopper exclosures.

Materials and methods

Results

Overall, aboveground plant biomass was lower outside of exclosures in both fire treatments (64 ± 4% less in fall burn plots and 55 ± 9% less in spring burn plots), but did not differ between exclosures and accessible unburned plots (1 ± 8%). Biomass removal by grasshoppers accounted for statistically-significantly lower biomass outside of grasshopper exclosures in both fall and spring burns (t = −7.4, P < 0.001 and t = −5.8, P < 0.001, respectively; Fig. 1). Aboveground biomass was not different between grasshopper exclosures and areas accessible to grasshoppers in unburned plots (t = −0.16, P > 0.05). Offtake was significantly lower in unburned plots than plots burned in both the previous fall and spring (P < 0.01 and P = 0.01, respectively). Over the course of the study period, daily offtake rates averaged 3.9 ± 0.5 kg ha⁻¹ day⁻¹ in fall burns and 2.4 ± 1.3 kg ha⁻¹ day⁻¹ in spring burns, while offtake in unburned plots was not different from 0 (−0.4 ± 1.9 kg ha⁻¹ day⁻¹).

Fig. 1.

Biomass removal by grasshoppers in control (shaded, unenclosed) frames relative to paired exclosures in plots with three different fire treatments. Standing crop (kg ha⁻¹) was determined by clipping at the end of the 4-week study period and differences attributable to grasshopper removal from control frames are expressed as a percentage of total standing crop in paired exclosures.

Crude protein content of western wheatgrass tillers varied among the fire treatments (t = 57, P < 0.001; Fig. 2). Crude protein content in fall and spring burns averaged 6.4% ± 0.2 s.e. and did not differ from one another (P > 0.05). But crude protein content in unburned plots, which included a substantial amount of senesced material from previous growing seasons was lower than in both fall and spring burns plots (t = −2.7, P < 0.001 and t = −3.1, P < 0.001, respectively).

Fig. 2.

Mean protein content of western wheatgrass Pascopyrum smithii sampled from three burn treatments as a percentage of total dry matter. Orange circles indicate the protein content of leaf blades; blue triangles are stems (including leaf sheaths).

Across all samples, crude protein content did not vary among leaves and stems (t = 2.7, P > 0.05). Despite a trend towards higher crude protein in leaf tissue in unburned plots (Fig. 2), the pattern was not influential enough to create a significant fire treatment × organ interaction (t = 2.1, P > 0.05).

Grasshopper abundance was similar across plots at the beginning of the study period (early July) but increased significantly over the next month in fall and spring burn plots (τ = 0.29, P < 0.01 and τ = 0.62, P < 0.001; Fig. 3a). Grasshopper abundance remained constant over the study period in unburned plots (τ = 0.039, P > 0.05). While grasshopper abundance increased in both burn treatments, the rate of increase was approximately three times greater in plots that had been most recently burned in the spring than those that had been burned in the previous fall (Fig. 3b), which represented more than a four-fold increase in density from less than 3–12 grasshoppers m⁻² (Fig. 3a).

Fig. 3.

(a) Observed grasshopper counts per square metre. Orange circles indicate data taken from fall burn treatments, blue triangles from spring burn treatments, and red squares from unburned (control) plots. (b) Data from Kendall’s Tau statistic, which assessed the observed count trendline consistency over time. Tau values were compared against the null hypothesis that there was no trend (slope = 0). 95% confidence intervals show the possible variance in slope for the data over time. Most grasshoppers observed were the migratory grasshopper Melanoplus sanguinipes.

Discussion

Interactions between fire and herbivores drive plant and nutrient dynamics and affect ecosystem service delivery in open ecosystems. Grasshoppers are widely seen as pests in competition with economically-valuable livestock for herbaceous primary productivity (Zhang et al. 2019). Hewitt and Onsager (1983) estimated grasshoppers consume nearly USD400 million (USD1.7 billion, adjusted for inflation) worth of livestock forage per year in the western United States. Fire interacts with grasshoppers via direct and indirect effects, which are variable among species depending on their biology (e.g. Vermeire et al. 2004). In our study of indirect grasshopper responses to fire, grasshoppers removed over half of available aboveground plant biomass in burned plots but had no detectable effect in unburned plots, consistent with other ecosystems in which grasshoppers select burned vegetation (Stein et al. 1992; Lopes and Vasconcelos 2011).

Time since last fire drives spatial and temporal variability in the distribution of plant nutritional quality, which can be a strong determinant of grasshopper abundance and distribution (Joern et al. 2012; White 2012; Ozment et al. 2021). While the effect on grasshopper responses of differing phenological stages when burning occurs have not been specifically evaluated, plant tissues that resprout after fire generally have higher protein content than their mature counterparts on account of having a lower proportion of structural carbohydrates (McGranahan and Wonkka 2021). At the stand level, fire removes low quality, senesced material from previous seasons' growth, allowing high quality regrowth to dominate the sward. This elevated protein content in burned areas can be maintained over longer periods by repeated grazing (Wanchuk et al. 2021), even during drought (Spiess et al. 2020). Higher post-fire nutritional quality attracts herbivores, whose repeated defoliation maintain the high nutritional quality (Archibald et al. 2005; Sensenig et al. 2010; Allred et al. 2011). Previous work shows that the dominant grasshopper species in our plots, M. sanguinipes, prefers current year’s growth and standing dead material makes up only a small proportion of its diet (Anderson and Wright 1952; Mulkern et al. 1962). We suggest that in our study, burned plots with overall higher crude protein content as a result of higher proportions of green, resprouting tissue likely attracted M. sanguinipes.

Other sources of variability likely modulate the ubiquity of the fire–crude protein–grasshopper interaction. Firstly, grasshopper community composition can vary, with variable effects on food resources. While we observed M. sanguinipes to be the dominant species in our single-season study, other work from our research station has shown variability in dominant species. For example, Branson and Vermeire (2016) reported Ageneotettix deorum and Opeia obscura were the two most common species, while Branson and Haferkamp (2014) reported dominance by Phoetaliotes nebrascensis. Further research ought to determine how general the attraction to burned areas is among rangeland grasshopper species; previous fire ecology work has focused on direct effects of heating during fire, not indirect effects of post-fire resource alteration.

Secondly, crude protein is but one component of herbivore nutrition and is therefore not the sole determinant of use, despite being commonly used as a proxy for forage nutritive quality, more broadly. Certainly, diet selection among M. sanguinipes populations varies considerably, especially within temperate regions (Fielding and Defoliart 2008). At the same time, M. sanguinipes responds positively to plant nitrogen content (Branson 2003) and demonstrates higher physiological performance under high-protein diets and their relative diet selection tends to mirror that of what is available on the landscape (Zembrzuski et al. 2021). Furthermore, the pattern of fire and forage extends from crude protein to a broad suite of forage quality components including mineral content (Wanchuk 2022). Thus, it is reasonable to infer that crude protein content likely had a substantial contribution to the correlation between grasshopper density and time-since-fire reported here.

Thirdly, fire altered both vegetation nutritional status and structure in our study plots, making it difficult to parse the relative effect of either. For example, Schirmel et al. (2019) identified vegetation structure as the primary driver of grassland Orthoptera community composition, but found strong evidence for plant nutritive quality as driver as well. Fortunately, Ozment et al. (2021) provided some insight into the relative effects of vegetation structure and nutrient content: not only did they find grasshoppers are generally attracted to high-nutrient grazed areas, but they found the attraction weakened when the nutritional contrast between grazing lawns and surrounding areas lessened during drought. But the structural difference remained, which supports nutritional quality as an important driver.

Regardless of the mechanism, recently burned plots clearly attracted more grasshoppers and subsequently had more aboveground biomass removal than unburned plots, which has potential implications for management.

In terms of fire management, we suggest restoring fire regimes with frequent, spatially-discrete prescribed fire might leverage the direct, negative effects of heating to reduce overall grasshopper populations against the potential nutritional gains post-fire vegetation provides for individual grasshoppers that survive or migrate into recently-burned areas. Improved survival and reproduction resulting from nutrient enhancement in burned vegetation (Branson 2003) could intensify competition between livestock and grasshoppers in burned areas. In contrast, benefits of nutritive enhancement could be offset by negative fire effects, as fire alone can result in short-term reductions in grasshopper abundance by up to 75% (Branson and Vermeire 2016). Direct effects of fire include adult and nymphal mortality (Bock and Bock 1991), and egg mortality due to soil heating (Vermeire et al. 2004; Branson and Vermeire 2013, 2016). Thus, even despite positive nutritional benefits, fire could remain a sustainable low-cost alternative to conventional (chemical) control of economically-damaging grasshopper outbreaks (Branson et al. 2006), which are expensive, unreliable, and have off-target effects on non-pest species (Joern 2000).

Data availability

Data and R script used herein are available under a U.S. Public Domain license at the USDA Ag Data Commons (doi.org/10.15482/USDA.ADC/1528475).

Conflicts of interest

The authors declare that they have no conflicts of interest.

Declaration of funding

NGH received salary support from the USDA-ARS Plains Area co-funded internship with matching funds from LARRL and NPARL.