Assessing the potential impact of grasshopper outbreaks on Patagonian wetlands through mathematical modelling

Abstract

Herbivorous insects occasionally produce population outbreaks that can alter the availability of food resources for other animals and cause economical losses. In the Patagonian steppe, wetlands are important ecosystems due to their environmental and ecological functions. Within these ecosystems, there is a wide diversity of phytophagous insects, among which two species of orthoptera are predominant: Dichroplus elongatus (usually considered a pest) and D. vittigerum. These species are native to Argentina and commonly feed on grasses and herbaceous plants present in wetlands and crops. To evaluate the consequences of grasshopper population outbreaks on wetlands, we conducted an interdisciplinary study that included field and laboratory experiments, along with the development of a mathematical model. We determined the plant cover of the most representative species included in the diet of grasshoppers in a specific Patagonian wetland and performed feeding experiments to determine their consumption rate and preferences. We employed this information to develop a spatially explicit stochastic model based on individuals. This model demonstrates that the potential impact of these species depends on both their densities and the wetland's vegetal biomass. Our results enabled us to define pest thresholds for various realistic scenarios. Conducting such studies is crucial for developing early warning strategies and promoting the conservation and management of natural environments.

Keywords: Dichoroplus sp.; Acrididae pests; Insect herbivory; Spatially explicit individual based model.

Fig. 1

Schematic representation of the main components of the model. Components marked with an asterisk indicate that their values, distribution or behaviour were derived from the data presented in this work or from other bibliographic references (as indicated in the paper). The black-bordered boxes enclose the parameters that were varied in the simulations presented in this work and that gave rise to the scenarios proposed.

Fig. 2

Temporal evolution of the state of a wetland for different densities of D. elongatus. (A) Percentage of patches with at least one resource. (B) Percentage of biomass of the wetland. The wetland is in a moderate condition, with an initial vegetal biomass of 450 g/m². In both figures, the value 100% indicates the state of the wetland on day 1 (before the introduction of the grasshoppers). In all the cases, the density of D. vittigerum is 1 indiv/m².

Fig. 3

Temporal evolution of the resources in a wetland in a moderate condition. (A) Percentage of patches with resources. (B) Percentage of biomass for each plant in the wetland. Parameters: D. vittigerum = 1 indiv/m², D. elongatus = 20 indiv/m², Initial vegetal biomass = 450 g/m². In both figures, the value 100% indicates the state of the wetland on day 1 (just before the grasshoppers invasion).

Fig. 4

Degradation of a wetland. Percentage of patch and biomass degradation after 150 days for several densities of D. elongatus and three wetland conditions: (A) good, corresponding to 600 g/m² of initial vegetal biomass, (B) moderate, with 450 g/m², and (C) poor, with 210 g/m². Blue curves indicate the percentage of patches that lose all their resources. Red ones indicate the percentage of biomass reduction. Note that the scale on the vertical axis is different for each curve (as indicated by the corresponding coloured horizontal arrows). The three scenarios for the density of D. elongatus are separated by vertical dashed lines. The case presented in Fig. 3 is indicated here with black squares.

Fig. 5

Pest threshold as a function of initial vegetal biomass. The threshold is defined as the value above which degradation exceeds 50% of the initial wetland biomass. Each point is the average over 10 realisations of the dynamics (standard deviation shown as error bars). The three wetland condition scenarios (labelled A–C) are separated by dashed lines.