Katydid (Orchelimum gladiator) movement responses to grassland vegetation structure

Background/Question/Methods

Plant community heterogeneity is presumed to impact associated insects. Vegetation structure could affect insect movement into and out of plant patches via impacts to insect individuals’ propensity to move, move distance, and turn angles, all of which affect the likelihood that an insect will stay versus disperse away from a focal patch. For example, if a vegetation type induces low movement propensity, short movement distances, or turn angles causing tortuous movement paths, insects would be more likely to remain in a patch instead of moving through. To assess effects of vegetation type on movement we tracked katydids (Orchelimum gladiator) through an array of 1 x 1 m planted plots (2 m mown aisle) in the SPaCE (Species Pattern and Community Ecology) experiment, focusing on movement responses into and out of plots planted with a grass (Elymus canadensis, E. trachycaulus, or Schizachyrium scoparium) and stiff goldenrod (Solidago rigida). Forty-eight katydids were released and their locations and vegetation type (grass, goldenrod, aisle) were recorded every 30 seconds by placing pin flags displaced a constant direction and distance to avoid disturbing the katydids. The vegetation type at each marked location was recorded and movement trails were mapped and then analyzed.

Results/Conclusions

Katydids moved more frequently in grass patches (proportion of 30 second time steps moving [95% confidence interval]: 0.23 [0.21, 0.26]) and in mown aisles (0.21 [0.16, 0.27]) than in goldenrod (0.15 [0.12, 0.18]). Turns were concentrated in the forward direction in grass patches (mean turn angle [95% confidence interval] 4.5 [-23.5, 32.5]), where oblique in goldenrod patches (-43.9 [-104.9, 39.9]) and concentrated in the backward direction in mown aisles (-150.4 [-293.7, -3.2]). Vegetation type had no detectable effect on distance moved during a 30 second time step (Kruskal Wallis test 0.3588, df=2, p=0.8358), or the concentration parameter (larger values indicate more concentrated turn angles) for a von Mises distribution (parameter [95% confidence interval] for grass 0.42 [0.25, 0.64], for goldenrod 0.30 [0.08, 0.67] and for weeds 0.23 [0.06, 0.81]). At least some components of katydid movement behavior were influenced by the patch of vegetation they were in. These movement responses to vegetation types are likely to impact katydid retention on vegetation patches (more likely to move through grass patches and more likely to stay in goldenrod patches) and point toward the importance of understanding insect movement responses to plant identity and pattern when restoring grasslands.