Not established in North America, including the Great Lakes Pontogammarus robustoides has a high probability of establishment if introduced to the Great Lakes (Confidence level: High).
Pontogammarus robustoides is a non-specific feeder (Bij de Vaate et al. 2002), consuming detritus, sediments, unicellular green algae, and filamentous green algae, as well as oligochaetes and chironomids (Berezina et al. 2005). It has an invasion history of outcompeting a variety of other macroinvertebrate species in Europe, including the North American native amphipod Gammarus lacustris (see Impacts). When abundant, P. robustoides significantly contributes to the diets of various fish species (Bubinas 1979; Arbaciauskas and Gumuliauskaite 2007), but the extent to which this predation will have an effect on potential populations in the Great Lakes is unknown.
The climatic conditions of the native (Ponto-Caspian) and introduced (Baltic) ranges of P. robustoides are very similar to those of the Great Lakes. Pontogammarus robustoides overwinters in its native range (Ponto-Caspian) and introduced areas (Baltic Sea and major river basins) (Bacela and Konopacka 2005). This species is able to survive in moderately wide temperature and salinity ranges (see Ecology) encompassing conditions occurring in the Great Lakes. Its preferred ionic concentrations may limit the distribution of P. robustoides to the four lower Lakes (Michigan, Huron, Erie, Ontario), as spring chloride concentrations in Lake Superior are regularly very low (< 2 mg/L) (U.S. EPA 2012). Increased salinization as an effect of climate change is likely to be beneficial to the establishment of P. robustoides, as this species occurs naturally in brackish water (Dobrzycka-Krahel and Surowiec 2011). This species is likely to benefit from shorter ice cover duration.
Arbaciauskas (2002) hypothesized that oxygen concentration is the principal limiting factor in determining the survival and sustainability of populations of Ponto-Caspian amphipods, but P. robustoides has greater tolerance to low oxygen these other amphipod invaders in the Baltic Sea (Dedyu 1980). As oxygen content in higher latitude eutrophic waters that are ice covered for a substantial portion of the year is often reduced (Arbaciauskas 2002, 2005), P. robustoides tends to occur in eutrophic waters only when ice cover is infrequent (Grabowski and Bacela 2005); ice covered eutrophic waters may kill off established populations (Arbaciauskas and Gumuliauskaite 2007). Anoxic conditions, as present in the central basin of Lake Erie (U.S. EPA 2012), may prevent P. robustoides from establishing in some regions of the Great Lakes. However, populations of P. robustoides in Lithuanian lakes have adapted to the lentic eutrophic waters that experience long-term hypoxia under winter ice cover and short-term hypoxia during summer months (Šidagyte and Arbaciauskas 2016), and a similar process could occur in the Great Lakes.
Disturbance factors such as eutrophication, macroalgal blooms, and oxygen deficiency may facilitate the success of this species when introduced to a new environment, as competition for resources with established species becomes reduced (Berezina et al. 2007). This species prefers to inhabit lentic as opposed to lotic environments due to its inability to disperse against flow (Jazdzewski et al. 2002; Berezina and Panov 2003; Bacela and Konopacka 2005; Grudule et al. 2007), making the majority of the Great Lakes basin suitable habitat for establishment with respect to water movement. A high diversity of available habitats in a colonized area may facilitate survival and establishment of populations of P. robustoides by reducing niche overlap between various size classes (Czarnecka et al. 2010).
Pontogammarus robustoides is a highly fecund species among Ponto-Caspian invaders in Europe and one of Poland’s most fecund amphipods, producing more eggs than other species of similar size (Bij de Vaate et al. 2002; Bacela and Konopacka 2005). This species undergoes rapid maturity, producing three generations per year with short (4–5 week) generation times (Bacela and Konopacka 2005). The autumn generation typically overwinters, but in thermally polluted waters (e.g., hydroelectric cooling water discharge), this species may reproduce year round; therefore, warming waters as a result of climate change could be beneficial to its invasion success.
The invasion pattern of P. robustoides across Europe was very similar in magnitude and rate to another European mass invader, Dikerogammarus villosus, both of which have spread across the entire European continent in roughly 50 years (Bij de Vaate et al. 2002). The spread of P. robustoides follows “jump” dispersal patterns, suggesting the involvement of anthropogenic factors in its expansion (Arbaciauskas and Gumuliauskaite 2007). Once introduced, it has quickly become established throughout many major European water bodies, reaching densities of 2700–6000 individuals/m2 in some locations (Berezina et al. 2005; Arbaciauskas and Gumuliauskaite 2007; Politi et al. 2021). In Lake Ladoga, Russia it contributed 48–99.5% of the total biomass of all amphipods (Barbashova et al. 2021). Among gammarids, this species is a strong competitor for food and habitat, but is considered an inferior competitor to Dikerogammarus villosus (Jermacz et al. 2015a) which is also not currently in the Great Lakes. However, despite their competition, D. villosous may aid in the dispersal and establishment of P. robustoides to some degree if both are introduced to the Great Lakes. Competitive interaction of P. robustoides and D. villous enhanced the dispersal of P. robustoides, which has a higher mobility (Kobak et al. 2016). Native and non-native fish in the Great Lakes are also likely to prey on Pontogammarus robustoides, but the extent to which its establishment will be impeded is unclear. In a laboratory experiment, P. robustoides used chemoreception to detect predation risk of both sympatric and allopatric fish predators, thus is is likely to recognise and evade allopatric predators in newly invaded areas, increasing establishment success. Further, under threat of predation, P. robustoides congregates with D. villous to avoid predation when normally it is displaced by D. villous (Jermacz et al. 2017).