Results of the linear discriminant analysis based on climatic variables of the 95 sites agreed with the initial grouping of sites into the five different zones (NE, NW, W, SE, SW), suggesting a sound classification of the sites with a minimum error of 12.75% (11 sample sites) (Fig. 2). The climatic variables that mainly contributed to the discrimination of zones in the first canonical axis were BIO 01 (2.83) and BIO 05 (-2.19) (Table 1). The sites in the NW had an average performance concerning these variables, while the sites in the W displayed an opposite behavior to those in the NE and SE. In the second discriminant axis, the variables that contribute the most to the classification were BIO 05 (0.73) and BIO 15 (-0.64). In this axis, the NE was positively associated with the average annual temperature, while the SE and SW were associated with greater precipitation seasonality.
Throughout the entire study, a total of 85.956 grasshoppers were collected, belonging to 50 species in three families (Acrididae, Ommexechidae, Romaleidae). Acrididae was the most diverse family with 41 species (82%) followed by Romaleidae with 8 (16%) and Ommexechidae with one (2%). Within Acrididae, the subfamily Melanoplinae was the most abundant and species-rich with 19 species, Gomphocerinae with twelve, and Acridinae with five. Copiocerinae and Leptysminae had two species each, and Oedipodinae had only one (Table 2). The species richness registered in each zone varied between 29 in the NE and 42 in the W (Tables 2 and 3). According to the ACE estimator, the proportion of rare species in the total species of each zone was higher in the sites located in the NE (16 of the 29 collected species had less than 10 individuals: 55.17%) (Table 3). In the NW, rare species represented 44.1%, while in the W, SW, and SE they represented 39.1%, 33%, and 28.5%, respectively.
The sampled completeness profiles (Fig. 3) showed that for Q=0, the estimated sample completeness for the NW zone was 69%, indicating a higher proportion (31%) of rare species (singletons and doubletons) that was not detected. In contrast, the NE had a lower percentage of species not detected at 15%, while the W and SW zones had 5%, and the SE had 3%. Curves for diversity orders Q=1 and Q=2 stabilized, and the sample completeness profile reached 100%, meaning that the asymptotic diversity estimates for these two indexes worked satisfactorily to infer true diversities and nearly all abundant and highly abundant species had been found for each assemblage.
The rarefaction and extrapolation curves based on the individuals’ number suggest that the stabilization of Q0 occurred at approximately 2000 individuals in each zone. There was a significant difference in species richness between the SE and the other sampling zones when the same number of individuals was considered. However, when Shannon diversity (Q1) was taken into account, the NE, SW, and W zones exhibited significantly higher diversity and richness than the NW and SE zones (Fig. 4). In terms of Simpson diversity (Q2), only the NW zone had significantly lower diversity compared to the other zones (see Fig. 4). When standardized coverage of 99.9% was applied, the species richness, Shannon diversity, and Simpson diversity results between the four zones were consistent with the rarefaction curves by individuals, with the NW zone exhibiting the lowest diversity. These differences were statistically significant as indicated by non-overlapping 95% confidence bands.
Regarding the presence of species in the different zones, the PCA revealed that the first three components explained 94.2% of the total variation, the first plane alone accounting for 75% of the total variation (Fig. 5). The three planes were used to observe the species performance. Species with higher abundance according to zone were: Aleuas lineatus, Amblitropidia australis, Dichroplus elongatus, Scotussa lemniscata, and Ronderosia bergii in the NE, D. elongatus, R. bergii, Baeacris pseudopunctulata, and Staurorhectus longicornis in the NW, Borellia bruneri, Borellia pallida, Covasacris pallidinota, Dichroplus maculipennis, Dichroplus pratensis, and Parorphula graminea in the SE, D. pratensis, Dichroplus vittatus, Neopedies bruneri, Rammathocerus pictus, Baeacris punctulatus, and B. pseudopunctulata in the W, and D. elongatus, D. pratensis, Leiotettix pulcher, S. longicornis, and B. pallida in the SW. Dichroplus elongatus and D. pratensis occurred in all of the sampling areas.
Of the 50 collected species, 25 were recorded in at least three of the plant communities sampled while the remaining 25 were found only in one or two. The highest number (50 species) was collected in GG followed by 33 in DG, 23 in both HG and C, and 17 in P. Crops exhibited the lowest number of individuals, representing only 2.51% of the total number collected while GG had the highest number, representing 51.87%. The correspondence analysis revealed a significant relationship between species and grassland, accounting for over 88% of the variance (Fig. 6) and showing how species were associated among the various plant communities. The HG contributed the most to the first dimension, accounting for 73.9% of the variation followed by the GG (21.3%) whereas P contributed the most to the second dimension, accounting for 79% of the variation (Table 4). Concerning species abundance by plant community, B. bruneri (10 in Table 5), B. pallida (11), C. pallidinota (14), D. maculipennis (19), and P. graminae (34) were more abundant in the HG (Table 5). In P, S. lemniscata (40 in Table 5) was the most abundant species. Dichroplus elongatus (17) was the most frequently found species, and it was present in all environments, with the highest abundance in GG (56.32%) and DG (24.69%). Dichroplus pratensis (21) was also one of the most abundant species, mainly found in the GG.
In the Beta diversity analysis carried out to measure the species composition differences over time in each zone, it was observed that in SW, W, and NE, the species turnover (βsim and βjtu) was the key component that showed differentiation between years. Specifically, in the SW, the highest values were observed between the periods 1990-95/2001-05 (50% βsim, 66% βjtu) (Table 6). In the NE, the highest values of the species turnover β component were recorded between1996-2000 and the rest of the periods, and in 2001-05, 2006-10, and >2010. In the W, between 45% and 68.9% of differentiation was observed over all considered periods. In the SE, the Sorenson and Jaccard turnover indexes had lower values, and the highest diversity component was species loss, with values between 31.7% and 35.5% βjne observed between 1996/2000 and 2006/2010, and 1996/2000 with >2010. Additionally, between 2001/05 and 2006/2010, 40.7% βjne was registered. In the NW, beta diversity between periods was affected by both the turnover component and the loss of species (Table 6).
In each zone, there were between 5 and 7 more abundant species that appear across the analyzed periods. Besides, it was observed that communities were structured with few dominant species, the three or four most abundant generally representing more than 50% of the community (Table 7). Moreover, some of the most frequent species were also the most abundant. While there were some abundant species recorded in the early years and then decreased or did not appear, there were others that showed up in later periods that were not initially collected. Baeacris punctulata decreased its abundance throughout the study in the SW, NW, W, and SE, and it was only found in one study period in the NE. Other species that decreased their abundance over the years were A. gracilis, B. bruneri, D. maculipennis, Orphulella punctata and P. graminae in the SW, A. australis in the NW, Parorphula graminae and Zoniopoda tarsata in the W, A. australis, B. bruneri, Laplatacris dispar, and Sinipta dalmani in the NE,and Tucayaca gracilis and S. dalmani in the SE. On the other hand, species such as R. bergii and R. forcipata increased their abundance towards the end of the study in the SW, NW, and W. Also, in the different zones, species that appear in only one or two sampling periods were recorded, such as D. vittatus in the SW; B. bruneri, Cocitotettix argentina, P. graminae, and Scotussa darreguei in the NW; O. punctata, S. daguerrei, Chromacris speciosa, Xileus laevipes, L. dispar in the W; P. graminae, Syllinula variablis, T. gracilis, L. pulcher, N. bruneri, O. punctata in the NE;and C. argentina, A. gracilis, Aleuas vitticolis, R. pictus, Trimerotropis pallidipennis in the SE (Fig. 7 and 8).