Reduced predation risk for melanistic pygmy grasshoppers in post‐fire environments

Abstract

The existence of melanistic (black) color forms in many species represents interesting model systems that have played important roles for our understanding of selective processes, evolution of adaptations, and the maintenance of variation. A recent study reported on rapid evolutionary shifts in frequencies of the melanistic forms in replicated populations of Tetrix subulata pygmy grasshoppers; the incidence of the melanistic form was higher in recently burned areas with backgrounds blackened by fire than in nonburned areas, and it declined over time in postfire environments. Here, we tested the hypothesis that the frequency shifts of the black color variant were driven, at least in part, by changes in the selective regime imposed by visual predators. To study detectability of the melanistic form, we presented human “predators” with images of black grasshoppers and samples of the natural habitat on computer screens. We demonstrate that the protective value of black coloration differs between burnt and nonburnt environments and gradually increases in habitats that have been more blackened by fire. These findings support the notion that a black color pattern provides improved protection from visually oriented predators against blackened backgrounds and implicate camouflage and predation as important drivers of fire melanism in pygmy grasshoppers.

Detection experiments with humans implicate visual predation as a driver of colour polymorphism dynamics in pygmy grasshoppers

“…Our present finding that the protective value of the black morph increases with increasing proportion burnt substrate in the visual background corroborates the results of a previous experiment that used a similar approach [28], and is consistent with the demonstration that the spatiotemporal variation in the incidence of the melanistic morph in natural T. subulata pygmy grasshopper populations is correlated with habitat modifications associated with succession in post-fire environments. Specifically, very high proportions of melanistic individuals in sooty environments the first year after fire are followed by rapid declines associated with recovery of vegetation and the gradual disappearance of burnt substrates [7].…”

“…Participants were asked to search for the grasshopper image on the computer screen and to use the mouse to point and click on it. As in our previous study [28], we recorded for each prey colour morph and visual background, the number of presented prey images that were detected (correct), time to detection, the number of times that a participant failed to detect the image within 60 seconds (wrong), and the number of times the participant clicked somewhere on the screen where there was no grasshopper image (wrong).…”

“…One model system in which the role of predation for the dynamics of colour polymorphism has been previously investigated is the pygmy grasshopper Tetrix subulata (Orthoptera: Tetrigidae) [28,29]. These are small (up to 15-mm body length, average dry body mass ca 0.07 g), diurnal, ground dwelling insects that inhabit different habitat types in biomes ranging from tropical rainforests to arctic regions of Europe, Asia and North America [7,30,31].…”

“…To this end, we present human ‘predators’ with images of black, grey or striped female grasshoppers on computer screens against photographic samples of either greenish (unburnt), intermediate (ca 50% burnt) or completely burnt natural pygmy grasshopper habitats [28]. We estimate rates of detection and examine whether differences in relative protective value among morphs are consistent, or whether they change across habitat types.…”

On the role of sex differences for evolution in heterogeneous and changing fitness landscapes: insights from pygmy grasshoppers

“…The relative frequencies of colour morphs vary among populations and change over time within populations according to spatio-temporal variation in environmental conditions [38,41–43] (Y. Yildirim, J. Tinnert, A. Forsman 2018, unpublished manuscript). This includes ‘fire melanism’ manifest as rapid evolutionary shifts in the incidence of the black form driven by oscillating selection in post-fire environments [41,43,44] (figure 2). Populations in stable environments are also less colour morph diverse than in disturbed environments [38] (Y. Yildirim, J. Tinnert, A. Forsman 2018, unpublished manuscript), possibly owing to purifying or stabilizing selection.…”