Short Communication |
Corresponding author: David H. Branson ( dave.branson@ars.usda.gov ) Academic editor: Corinna S. Bazelet
© 2017 David H. Branson.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Branson DH (2017) Grasshopper species composition shifts following a severe rangeland grasshopper outbreak. Journal of Orthoptera Research 26(1): 7-10. https://doi.org/10.3897/jor.26.14542
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Little is known about how grasshopper species abundances shift during and following severe outbreaks, as sampling efforts usually end when outbreaks subside. Grasshopper densities, species composition and vegetation have infrequently been sampled during and after a severe outbreak in the western U.S., which is needed to better understand the cause of outbreaks and population declines. In this study, grasshopper densities, species composition and vegetation were monitored at a northern mixed rangeland site from 1999 to 2003 where densities reached 130 per m2 during a severe outbreak. Phoetaliotes nebrascensis (Acrididae: Melanoplinae) comprised 79% of the outbreak in 2000, but declined to 3% by 2003. The dramatic shifts in proportional and actual abundance of P. nebrascensis over a 5 year period illustrate that species dominance can change rapidly, even for a highly dominant outbreak species. The difficulty of fully understanding factors causing shifts in grasshopper populations is illustrated by population declines in all species observed in 2002 and 2003. The data can help predict the intensity and decline of outbreaks and points to the critical importance of long term simultaneous monitoring of grasshopper densities, species composition and vegetation for outbreak prediction.
Phoetaliotes , Melanoplinae , rangeland, Acrididae, prairie, prairie
Grasshoppers are often the dominant herbivore in western U.S. grasslands (
Grasshopper sampling occurred in a large livestock exclosure at the USDA, Agricultural Research Service, Fort Keogh Livestock and Range Research Lab located near Miles City, Montana, U.S., from 1999 through 2003. Cattle were the only mammal excluded from the site and insects were not controlled. The site consisted of mixed grass prairie, with Western Wheatgrass (Pascopyrum smithii) initially comprising over 90% of vegetation (
Grasshopper density was estimated by counting grasshoppers flushing from within eight, 0.1 m2 wire rings by tapping in the ring (
Vegetation was sampled in mid and late summer from 1999 to 2001, but only during mid-summer in 2002 and 2003. Plots were clipped after randomly tossing five to ten 0.1 m2 rings on rangeland. Green vegetation was separated by grasses and forbs, dried, weighed, and ground. Percentage total nitrogen content of grass was assessed using a dry combustion C/N analyzer and used as an index of plant quality.
Grasshopper densities at sites >2 km from the study site ranged from 8 to 17 per m2 in 1997 and 13 to 31 per m2 in 1998 (Branson unpublished data). At the study site, peak grasshopper densities in 1999 were 30.9 per m2 (Table
Proportional species composition shifted during and following the grasshopper outbreak, with species composition initially dominated by P. nebrascensis (Fig.
Densities of both A. deorum and M. sanguinipes doubled from 1999 to 2000 (Table
The large positive and negative shifts in the dominant species P. nebrascensis from 1999 to 2001 matched patterns of late season food availability. Above average August precipitation in 1998 and 1999 led to elevated grass nitrogen content (
There is a paucity of data where concurrent grasshopper density and species composition sampling combined with vegetation sampling occurred during and after a grasshopper outbreak. The dramatic shifts in proportional and actual abundance of P. nebrascensis over a 5 year period illustrate that species dominance can change rapidly, even for a species that was highly dominant in a severe outbreak. Precipitation timing is important, as peak plant biomass production in this system is driven by spring and early summer moisture (
A. Peak total and common species grasshopper densities from 1999 to 2003. B. Green grass biomass (g/m2) in late June or early July (Mid-summer) in all years and late August or early September (Late-summer) from 1999 to 2001. C. Grass percent nitrogen content in mid-summer and late summer. * Indicates missing data.
A. Grasshopper density (#/m2) | 1999 | 2000 | 2001 | 2002 | 2003 |
---|---|---|---|---|---|
M. sanguinipes | 6.52 | 13.67 | 5.86 | 2.33 | 1.97 |
P. nebrascensis | 17.03 | 100.31 | 3.05 | 0.48 | 0.15 |
A. deorum | 3.68 | 7.36 | 7.08 | 1.19 | 1.13 |
Total grasshopper density | 30.90 | 130.00 | 21.00 | 4.75 | 5.15 |
B. Grass biomass (g/m2) | |||||
Mid-summer | 148.4 | 53.0 | 123.4 | 66.8 | 98.4 |
Late-summer | 52.3 | 4.2 | 104.0 | * | * |
C. % Nitrogen content | |||||
Mid-summer | 0.94 | 1.48 | 1.55 | 1.25 | 1.07 |
Late summer | 1.84 | 0.82 | 0.71 | * | * |