Abstract

TEMPORAL VARIATION IN RANGELAND GRASSHOPPER (ORTHOPTERA: ACRIDIDAE) COMMUNITIES IN THE STEPPE REGION OF MONTANA, USA

WILLIAMP. KEMP USDA Agricultural Research Service, Rangeland Insect Laboratory, Bozeman, Montana, USA 59717-036

Abstract

Can. Enr. 124: 437450 (1992)

A study was conducted to evaluate changes in rangeland grasshopper communities over a 5-year period in the Agropyron spicatum (Pursh) Scribn. and Smith and Bouteloua gracilis (H.B.K.) Lag. provinces of the steppe region of Montana, USA. Results showed that it was possible to categorize years into outbreak, non-outbreak, and transitional based on rangeland grasshopper intensity. Nearly twice as many species were observed in outbreak versus non-outbreak years. Of the 57 total grasshopper species collected over the entire study period, 16 species were found only during outbreak years and only two were found exclusively during non-outbreak years. Of the remaining 39 species collected during outbreak and non-outbreak years, 27 species showed no significant differences in the percentage of the community that they represented and 11 species showed significant increases. The only species that made "p p~oportionately less of the community as densities declined from outbreak to non-outbreak was Melanoplus sanguinipes (F.). Although M. sanguinipes, Ageneotettix deorum (Scudder), and Aulocara elliotti (Thomas) were the three top-ranked species in both outbreak and nonoutbreak years, M. sanguinipes contributed most to overall shifts in grasshopper intensity. Results support the hypothesis that grasshopper communities overall are sensitive to temporal changes in resources, even though responses of individual species differed. Kemp, W.P. 1992. Variation temporelle au sein des communaut6s de criquets (Orthoptera: Acrididae) des zones de pLturage dans la kgion des steppes du Montana, 8.-U.Can. Enr. 124: 437450.

Un programme d'ttude de 5 ans a t t t entrepris dans le but d'tvaluer les fluctuations des communautts de criquets dans les paturages, dans les zones B Agropyron spicatum (Pursh) Scribn. et Smith et B Bouteloua gracilis (H.B.K.) Lag., dans la dgion des steppes du Montana, E.-U. Les rtsultats ont dtmontrt qu'il ttait possible de classifier les annks en annks tpidtmiques, anntes non tpidtmiques et annks de transition d'ap&s la densitt des criquets dans les zones de paturage. M s de deux fois plus d'espkces ont t t t observks au cows des anntes tpidtmiques qu'au cours des anntes non tpidtmiques. Des 57 e s e e s de criquets dcoltks au total durant la @node d'Ctude, 16 n'ont kt6 rencontrks qu'au cours des tpidtmies et seulement deux ont kt6 rtcolt&s exclusivement au cours des anntes non tpidtmiques. Parmi les 39 autres espkes rkolt&s tant au cours des anntes tpidtmiqu& que des anntes non tpidtmiques, 27 repdsentaient le m&mepourcentage de leur population au cours des deux types d'anntes, alors que 11 de ces esptkes reprtsentaient une proportion significativement plus grande de leur population au cours des anntes d'tpidtmies. La seule espi?ce qui composait une moins grande proportion de la communaut6 i mesure que les densitts diminuaient, des anntes d'tpidtmies aux anntes non tpidCmiques, ttait Melanoplus sanguinipes (F.). Bien que M. sanguinipes, Ageneotettix deorum (Scudder) et Aulocara elliotti (Thomas) se soient avtrks les trois espkces les plus abondantes B la fois au cours des annks d'CpidCmies et au cours des anntes non Cpidtmiques, M. sanguinipes est I'es@e qui a contribuC le plus aux fluctuations globales de la densitt des communaut6s de criquets. Les rtsultats corroborent I'hypothhse selon laquelle les communautts de criquets sont de fason gCnCrale sensibles aux fluctuations temporelles des ressources, m8me si les fluctuations des difftrentes e s e e s ne sont pas toujours les mCmes. [Traduit par la ddaction]

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THE CANADIAN ENTOMOLOGIST

MayIJune 1992

Introduction Grasshoppers (Orthoptera: Acrididae) are the dominant native herbivores throughout much of the steppe region of North America. Although it is recognized that grasshopper communities exhibit large temporal oscillations in abundance (Gage and Mukerji 1977; Joern and Pruess 1986; Kemp 1987a), little effort has been directed toward an understanding of the variation in natural communities as they move from outbreak to non-outbreak densities. At present, we know very little about the actual processes that control grasshopper communities in space and time. Much of the literature concerned with temporal grasshopper fluctuations is based on a correlative approach between some indicator of general grasshopper prevalence and indicators of the environment such as climate (e.g. Capinera and Horton 1989; Fielding and Brusven 1990). However, survey data used in many previous studies of grasshopper temporal variation are single samples of adult density per site per year, and species composition information is unavailable. Adult surveys are annual snapshots of general grasshopper density at an unspecified point in their seasonal phenology and usually provide no information on the relative prevalence of differing species over time. Thus, contrasts made between any two widely disparate geographical locations, relative to differences in "grasshopper cycles," are likely to be comparing two completely different insect communities. Although important, the degree of temporal constancy of entire insect herbivore assemblages has not received much attention. From studies conducted in insect herbivore systems other than grasshoppers, species ranks appear to be relatively predictable from year to year, in spite of fluctuations in densities among the communities as a whole (Connell and Sousa 1983; Lawton 1984; Strong et al. 1984). Perhaps the most detailed study of temporal constancy in grasshoppers, to date, was conducted at two sites in Nebraska over differing annual intervals (Joern and Pruess 1986). Although densities and relative abundances of about 45 species at the two sites varied, Joern and Pruess (1986) found that species composition based on ranks suggested significant structure when three or more grasshopper species were considered in analyses. Further, population variability appeared to be intermediate among the range of insect communities considered thus far in the literature (as above). Although Joern and Pruess (1986) found that the highest densities were roughly 25 times greater than the lowest, mean densities in general were low (approximately 0.5-2.3 grasshoppers per square metre) relative to observations in other parts of the steppe region in North America. Thus, it was not possible to assess changes in grasshopper communities that resulted in temporal density shifts from outbreak to non-outbreak. Two other investigations that monitored grasshopper assemblages over time were conducted by Capinera and Thompson (1987) on shortgrass prairie in northeastern Colorado and by Pfadt (1977) on shortgrass prairie in southeastern Wyoming. Although two dominant species studied by Capinera and Thompson (1987), Opeia obscura (Thomas) and Melanoplus gladstoni Scudder, showed fluctuations with time, mean densities were generally low (0.5-4.8 grasshoppers per square metre) over the 4-6 years of investigation, and it is unclear whether such differences were significant. Further, the increasing populations followed by Pfadt (1977) were all based on sites that were treated chemically some time during the study, and results must be viewed with the understanding that communities were drastically disturbed. Because the recognition of pattern necessarily precedes the construction of hypotheses regarding process (Eberhardt and Thomas 1991), I established a study to assess temporal variation for rangeland grasshopper communities. The first objective was to determine whether it was possible to identify outbreak versus non-outbreak years in terms of general (statewide) grasshopper density trends. Second, I was interested in determining whether all species contributed equally to grasshopper densities each year (i.e. species fluctuate in unison). Although this study focused on the patterns of temporal variation in rangeland

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FIG. 1. Sentinel sites used for collection of rangeland grasshopper data 1986-1990, Montana, USA. Chester and Red Lodge sites were added to original 10 in 1987.

grasshopper communities, results support the hypothesis that rangeland grasshoppers are sensitive to resource availability in space and time.

Materials and Methods Sentinel Sites. In 1986, 10 sentinel sites were established throughout Montana, USA, to monitor rangeland grasshopper density, phenology, and species composition over a wide geographic region. The number of sentinel sites was expanded to 12 in 1987 and thereafter remained constant through 1990 (Fig. 1). Sites were located over a range of long-term plant phenological zones (see Kemp 1987b, fig. 24.6, p. 365) in an attempt to include the range of climatic conditions characteristicof sites within either the Agropyron spicatum (Pursh) Scribn. and Smith or Bouteloua gracilis (H.B.K.) Lag. provinces in the steppe regjon of Montana (Daubenmire 1978). Four of the sentinel sites (Broadus, Jordan, Great Falls, and Glasgow) were moved locally (less then 2 km away on similar vegetation) in 1987 to ensure long-term access. In view of the large geographic scope of this study (Fig. I), the relocation distance of these four sites was trivial. Weather permitting, sites were visited on a weekly basis from just prior to hatch through the nymphal period each year. Thereafter, sites were visited bi-weekly through the remainder of each season (until adults were no longer detected or until weather precluded further sampling). Sweep net samples [each sweep consisting of an arc of 180" through the vegetation (Evans et al. 1983; Evans 1988)l were collected at all sites each year during the interval between mid-April and October. The number of net sweeps at a site in 1986-1987 was variable, though at least 100 grasshoppers were collected at each sample date. This was standardized to 100 net sweeps at each sample date in 1988-1 990, but still resulted in large samples. Grasshoppers collected via sweep net were placed in plastic bags, put on ice, and returned to the laboratory for identification to species and determination of development stages. Additionally, at each sample date, community density estimates were obtained by censusing 40, 0. I-rn' rings following the methods developed by Onsager and Henry (1977).

THE WWADIAN ENTOMOLOGIST

Year FIG.2. Mean rangeland grasshopper intensity ? 2 SE by year for sample sites (1986, N = 10; 1987-1990, N = 12) in Montana, USA.

Analyses. To assess differences in overall population levels for each site and year, density data from weekly ring counts were used. For a given site and year the weekly density counts (per square metre) were plotted as a function of time [Julian Date (JD), 0-3651. Next, the trapezoid method of integration was used in a computer program to compute the area under the density curve from 1 June (JD 152) through 31 August (JD 243) for each site and year. This provided a standard, annual estimate of overall grasshopper intensity (grasshopper days, GD) for all sites and years. This method has been used previously when

Year FIG.3. Mean adult rangeland grasshopper density

2 2 SE by year for the steppe region of Montana 1948-1990 (N = 23, 201).

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Table 1. Rangeland grasshopper species richness during outbreak and non-outbreakyears, Montana, USA 19861990

Y m

Classification

Number of site-vears

1986-1987 1989-1990

Outbreak Non-outbreak

22 24

d (SE) number of soecies collected

Sig. diff. between years within classification (a= 0.051?

22.1 (0.7) 11.8 (1.1)

No No

considering yearly grasshopper feeding intensity (Hewitt and Onsager 1982) and is superior to single yearly samples for the purpose of characterizing relative differences in densities over the course of a season. Mean GDs + 2SE were plotted by year and compared with yearly regional adult survey trends to determine whether similar trends were evident and to determine whether it was possible to distinguish between years of outbreak versus non-outbreak densities among all sites sampled. Standard nonparametric statistics (SAS Inst., Inc. 1988) were used to test the hypothesis (HI) that there was no difference among years in mean grasshopper intensity (GD). In all cases a = 0.05. The next hypotheses tested were that regional estimates of species richness were constant through time (H2), and that relative-percentages of grasshopper species were constant through time (H3). For each site, the weekly sweep net collections were pooled and used to compute yearly species richness and relative species proportions. Standard nonparametric statistics (SAS Inst., Inc. 1988) were used to test both hypotheses. Two additional hypotheses tested were that the five top-ranked species (based on rankings during outbreak years) contributed equally to overall levels of grasshopper intensity during outbreak and non-outbreak years (H4) and that intensity of the five top-ranked species was constant with time (H5). TOtest both of these hypotheses, species percentages obtained from the pooled weekly sweep net collections each year were multiplied by the measure of annual grasshopper intensity (GD). This permitted the evaluation of the contribution of individual species to the overall mean intensity. Standard nonparametric analyses (SAS Inst., Inc. 1988) were used in testing both hypotheses. Finally, because rangeland grasshopper species distribution data were collected over an extensive geographic range of site conditions during the 5-year study, it was also possible to examine species distribution characteristics (Hanski 1982; Brown 1984; Kolasa 1989). Standard regression analyses (SAS Inst., Inc. 1988) were used to examine the relationship between the log mean abundance (sweep net data) and the number of sites over which each species was collected (Hanski 1982; Brown 1984; Kolasa 1989). This was done to test the hypothesis (H6) that there was no relationship between log mean abundance and distribution [expressed as the number of site-years regionally (statewide) for each species].

Results Temporal Constancy of Intensity - General. To begin, it was necessary to establish whether there were significant differences among years in overall grasshopper intensity (Hl). Examination of mean values by year (Fig. 2) revealed that 1986-1987 had higher grasshopper intensities than 1989-1990, with 1988 apparently a transition year (KruskalWallis test, X2= 56.4, df=4, P