Department of Physics, Chemistry and Biology (IFM)

Effects of warming on food web metrics

The results show that there were changes in the predator and prey communities between the cold and warm years. In the predator community for instance, species like Boreogadus saida, Pleuronectes quadrituberculatus and Hippoglossoides elassodon were only present in cold years. The first two mentioned predator species are arctic species (cold water species), so their absences during the warm years strongly indicate that they are vulnerable or may have relocated during warm years which is in correlation with other studies, which  review that warming affects their feeding behavior and habitats preferences (Mueter and Litzow 2008). Hippoglossoides elassodon which is a subarctic species (Mueter and Litzow 2008) and it’s distribution and spawn processes have often been seen in deeper and cold water near the margin of the continental shelf (Bailey, Abookire, and Duffy-Anderson 2008; Bailey and Picquelle 2002) which explains its absences in the warm years. 

In the prey community, species in family Pholidae were only recorded in cold years while Salmonidae and Hexagrammidae were only recorded in warm years. The presences of family Salmonidae in warm years, but not cold, indicates that the warmer environmental conditions are favorable for their survival and growth. This observation supports the findings by Yasumiishi et al (2016) , who did a study in the EBS and concluded that salmon species that belong to family Salmonidae, particularly the juvenile salmon increased their ranges and abundances during warm years.  Similarly the presences of the family Hexagrammidae in warm years indicates that warming provides suitable environmental conditions for their survival as they are subarctic species (warm water species) (Mueter and Litzow 2008). 

Furthermore, the results show that a lower average generality and  average vulnerability were recorded in the warm years than in the cold years. This suggests that warming affects a number of species interactions. Also that some species become more of specialists than generalists in warmer environmental conditions, as other studies show, that due to climate warming, generalists are expected to respond faster and shift to other habitats with optimum environmental conditions suitable for their survival and ability to exploit available resources (Hollowed et al., 2013; Mueter, and Litzow, 2008),  and also because of the limited interactions between the incoming and resident species (Beaugrand et al., 2015). However, there was so much differences between  the average generality and average vulnerability within each  period, and also the average vulnerability measured in both periods was very low. This is due to the dataset used, since the dataset only considered the 13 key predators and their respective prey species, therefore interactions for many other predators that preyed on these prey species were not considered hence resulting in the lower average vulnerability and huge difference between the two average generality and average vulnerability within each period.

Effects of warming on groups of ecological equivalence

Here I assessed any potential changes in the group structure in the EBS ecosystem, I analyzed variations in the identified groups of ecological equivalence in the cold and warm years in EBS food web. The results show that approximately 30% of the species changed groups between the cold years and the warm years. This resulted in changes in group structure as seen in Figure 4. The group turnover was higher among groups that comprised of prey species, with the highest turnover recorded in the pelagic community. This indicates that warming can potentially change the availability of some prey species. This is in line with reviews of  Livingston et al., (2004), who highlights that warming affects food webs by changing the distribution and abundance of prey species found within the pelagic community. The most interesting change recorded between the two temperature regimes was seen in the turnover for some flatfish and roundfish predator species, which belonged to a group comprising of predator species during the cold years and changed to a group that comprised of prey species during the warm years. This suggests that the species in flatfish and roundfish guilds (here particularly the Flathead sole, Alaska plaice and Artic cod) had more the role of prey species during warm years. The reason for this observation can be that warming affects the behavior of flatfish (the Flathead sole, Alaska plaice) and causes the species shift their distributions to deeper and cooler water column that has favorable climatic conditions, seek refuge and also able to search for food (McConnaughey and Smith 2011). The round fish (the arctic cod in particular) are affected by warming at all stages of their life cycle, especially at egg development and also at early larval stage as highlighted  by Koenker et al (2018). This may lead to effects on the population and availability of adult species.

The fish guilds and prey communities used in this study have been widely used in several studies to examine biomass trends in relation to changes in climate conditions in marine ecosystems. For instance, the flatfish is used because its biomass accounts for a large portion of the fish community in the EBS ecosystem (Hurst 2016) and the roundfish makes up  the largest biomass of demersal aggregated community in the Bering sea at large (Hoff 2006). The benthic prey community comprises of zooplankton and epifauna which are the primary producer and also primary source of energy in the EBS ecosystem. The pelagic prey community on the other hand comprises of foraging species, juvenile flatfish and infauna which makes up the middle energy source (Aydin et al., 2002).The two fish guilds and prey communities describe the status and trends of the fish and invertebrates (mostly the preys) in the Eastern Bering Sea (Aydin and Mueter 2007). Therefore, any changes in their distribution, abundance and feeding patterns are critical to understand how the ecosystem may change as well. Hence, in this study the small notable changes in abundance of the species in the two fish guilds and prey communities suggests potential changes in the functioning in EBS ecosystem due to changes in climate conditions. 

Closely related species are more likely to have similar traits, for instance similar habitat preference and feeding mode compared to distant ones (Eklöf et al. 2011). Hence, I analyzed the taxonomic composition within the groups to evaluate if there was a signal from species taxonomy in the group structure in the EBS ecosystem. Generally, the result in Figure 5shows quite large similarities in phyla distribution in the different groups between the two regimes. However, there was some notable variation in phyla composition as well as diversity in some phyla in the groups between the two regimes. For instance, phyla Echinodermata was only present in group 2 during cold years while in the warm years it was present in group 1 and 2. Also phyla Cnidaria was only present in group 2 in the warm years while during the cold years it was divided into two groups (0 and 2). These changes together with changes in the species diversity in respective phyla e.g. mollusks and chordates, strongly suggest that the species move or shift habitats during the warming years. This may lead to changes in species diversity as well as food web structure, which eventually may lead to changes in predator-prey interactions and later result to trophic cascade effects. This is so because of changes in the functioning in some groups in the food web may influence the dynamics on other groups (Worm and Myers 2003). 

Sander and colleuges (2015) found that group model gives a slightly more refined picture of a complex network structure. They further highlight that the model identifies ecologically equivalent species, which can be useful in identifying which species plays unique roles in the community. In addition, Baskerville et al (2011) found that the group model grouped species in groups clearly connected to habitat types in the Serengeti ecosystem. As such they showed that the group structure reflects the flow of energy up the food web from different spatial locations affecting the functionality in the system. Therefore, the changed group structure between the cold and warm regimes in my study of the Bering Sea could indicate that the species and their feeding relationships play unique roles for the energy flow depending on the climate conditions. Therefore, I concluded that any changes in temperature can potentially give changes in predator-prey interaction and as such, changes in functional groups in the food web.

In conclusion I can say that in overall there were changes in species composition in both predator and prey communities due to the fact that some key groundfish (predators) and invertebrates (preys) are affected by warming conditions. Also, there were variations in (functional) group structure between the cold and warm years, suggesting that some species may perform different functions during the cold and warm years. Since the changes in the groups structure may indicate the species change their functional role in the food web. Furthermore, the species that changed groups between the cold and warm years suggest changes in prey species availability, habitat preferences by some predator species as well as changes in the prey preferences. Therefore, all these changes directly lead to alternations in some of the predator-prey interactions which influences the full structure and potentially functioning of the EBS ecosystem at large.