Population dynamics of an alpine grasshopper (Orthoptera) community over 30 years and the effects of climate warming and grazing

Abstract

Long-term studies on the population dynamics of grasshoppers are rare, and are even missing completely for the alpine regions of Central Europe. Here the effects of climate warming and the recent changes in land use, like abandonment of pasturing, are believed to be serious. Our study on an alpine grasshopper community over 30 years allows, for the first time, the long-term description and analysis of these processes in the northern part of the Alps. In the years 1991–2020 the distribution and abundance of grasshoppers (Orthoptera) was surveyed annually with a standardized method on fixed plots in the Hohe Tauern National Park (Salzburg, Central Alps, Austria) in elevations between 2070 and 2440 m. During the study period five Orthoptera species Melanoplus frigidus, Miramella alpina, Omocestus viridulus, Gomphocerus sibiricus and Pseudochorthippus parallelus were present. This species spectrum did not change during the 30 years of study. The number of species per plot and year varied between 0 and 4, this value increased significantly during the 30 years. The yearly fluctuation rate of the abundance was very large, especially for G. sibiricus, its maximum abundance reached 40 times the minimum value. All species—with the exception of the boreo-alpine M. frigidus—increased their range at the study site slowly but continuously. A significant increase in abundance was only detected in O. viridulus. During the study the mean temperature in the vegetation period increased significantly by 1.6 °C. Snow cover in June at the hatching of the nymphs varied greatly but did not change during the 30 years. The closest relationship between weather and abundance was found for G. sibiricus, whose occurrence is significantly determined by the temperature in June. But the increase in temperature did not lead to higher abundance during the study period. With increasing temperature, the abundance of M. frigidus was positively affected by cooling weather like higher precipitation in May and high snow cover in June. M. alpina was favored by late thaw and showed reduced densities in years with high temperature in summer. Climate warming favored the ubiquitous species P. parallelus, which enlarged its distribution continuously into higher elevations, and O. viridulus increasing its abundance. The abandonment of grazing had no negative effect on species composition and abundance in the long term. There was even a tendency for increased abundance, especially in M. alpina. Cyclical fluctuations in the abundance of certain species prove that eggs hibernate more than one winter under these alpine conditions. G. sibiricus exhibits a biennial, M. alpina even a triennial life-cycle, the latter is maybe also the case in M. frigidus. In contrast to this, P. parallelus seems to be mainly univoltine. Climate warming, which is more pronounced in the study site than in Central Europe in general, favors mainly widespread generalist species, while the most characteristic species of alpine habitats M. frigidus does not profit from this development. The negative effects of warming on M. alpina has so far been compensated by the abandonment of grazing, which has probably led to a more suitable vegetation structure. The proof of bi- and triannual life-cycles in the studied populations of alpine grasshoppers increases the knowledge of their potential plasticity. These strategies seem to be an important prerequisite to colonize such extreme habitats and to overcome occasional events of harsh weather conditions.

Implications for insect conservation

Our results show that the long-term influence of climate warming in alpine habitats leads to different reactions of species, favoring generalists that expand their range and increase their densities, while specialized alpine species do not profit or even show negative reactions to warmer and dryer conditions. This highlights the potential extinction risk of range-restricted species living at high elevations. The abandonment of grazing with heavy cattle, which was common in the study site at the beginning, did not lead to significant changes in the grasshopper-community. Conventional grazing is not necessary to preserve the biodiversity of these communities at such high elevations above the forest line. The study also exhibited only slow changes in the grasshopper-populations masked by very pronounced fluctuations in abundance, which underlines the importance of long-term studies to understand the dynamics in these extreme habitats and elaborate conservation measures.

Appendix

See Tables 1, 2, 3, 4, 5, 6, 7, 8 and 9.

Table 1 Description of the study plots

Table 2 Date of the surveys at the different plots in the Piffkar during this study

Table 3 Numbers of recorded grasshoppers (imagines and nymphs) during the study in the Piffkar at the seven plots (á 100 m²) surveyed every year from 1991 to 2020

Table 4 Numbers of recorded grasshoppers (imagines and nymphs) during the study in the Piffkar at plot 1 (100 m²) surveyed every year from 1991 to 1995 and 2015 to 2020

Table 5 Numbers of recorded larval instars and adults on plot 6 during the non-standardized survey in July

Table 6 Parameters of population dynamics of the grasshoppers in the Piffkar during 1991–2020

Table 7 Significant correlations between abundance of grasshopper species in the Piffkar with other species and climatic parameters (Spearman rank order correlation r_s, *p < 0.05, **p < 0.01)

Table 8 Trends in abundance and distribution (= number of occupied transects) of the species in the Piffkar for the whole period and the three decades (Spearman rank order correlation r_s; *p < 0.05, **p < 0.01)

Table 9 Mean abundances (individuals/100 m²) of grasshoppers at plots with grazing until 1998 (a) compared to those grazed until 1989 (b), differentiated by the three decades