New parasites and predators follow the introduction of two fish species to a subarctic lake: implications for food-web structure and functioning

Abstract

Introduced species can alter the topology of food webs. For instance, an introduction can aid the arrival of free-living consumers using the new species as a resource, while new parasites may also arrive with the introduced species. Food-web responses to species additions can thus be far more complex than anticipated. In a subarctic pelagic food web with free-living and parasitic species, two fish species (arctic charr Salvelinus alpinus and three-spined stickleback Gasterosteus aculeatus) have known histories as deliberate introductions. The effects of these introductions on the food web were explored by comparing the current pelagic web with a heuristic reconstruction of the pre-introduction web. Extinctions caused by these introductions could not be evaluated by this approach. The introduced fish species have become important hubs in the trophic network, interacting with numerous parasites, predators and prey. In particular, five parasite species and four predatory bird species depend on the two introduced species as obligate trophic resources in the pelagic web and could therefore not have been present in the pre-introduction network. The presence of the two introduced fish species and the arrival of their associated parasites and predators increased biodiversity, mean trophic level, linkage density, and nestedness; altering both the network structure and functioning of the pelagic web. Parasites, in particular trophically transmitted species, had a prominent role in the network alterations that followed the introductions.

Fig. 1

Pelagic food web, including parasites, of the subarctic Lake Takvatn a before and b after the introductions of arctic charr (Salvelinus alpinus) and three-spined stickleback (Gasterosteus aculeatus). The two introduced fish species are indicated by blue boxes, and the additional bird and parasite species in green and orange boxes, respectively. Predator–prey links are indicated by black lines whereas parasite–host and predator–parasite links are indicated by red lines. Nodes include: Phytoplankton: Asterionella formosa Ph1, Cyclotella comensis Ph2, Cyclotella kützingiana Ph3, Stephanodiscus medius Ph4, Chrysomona Ph5, Ceratium hirundinella Ph6, Gymnodinium helveticum Ph7, Elakatothrix genevensis Ph8. Rotifers: Asplanchna priodonta Ro1, Keratella cochlearis Ro2, Kellicottia longispina Ro3, Polyarthra sp. Ro4, Synchaeta sp. Ro5, Ascomorpha sp. Ro6, Conochilus unicornis Ro7, Filinia gr. longiseta-terminalis Ro8. Cladocerans: Polyphemus pediculus Cl1, Holopedium gibberum Cl2, Daphnia galeata Cl3, Bosmina longispina Cl4, Bosmina longirostris Cl5. Copepods: Cyclops scutifer Co1, Eudiaptomus graciloides Co2, Acanthocyclops gigas Co3, Heterocope appendiculata Co4. Fishes: brown trout Salmo trutta Fi1, arctic charr S. alpinus Fi2, three-spined stickleback G. aculeatus Fi3). Birds: common gull Larus canus Bi1, arctic tern Sterna paradisaea Bi2, red-breasted merganser Mergus serrator Bi3, long-tailed duck Clangula hyemalis Bi4, arctic loon Gavia arctica Bi5, red-throated loon Gavia stellata Bi6, common scoter Melanitta nigra Bi7, tufted duck Aythya fuligula Bi8, goldeneye Bucephala clangula Bi9. Parasites: Diphyllobothrium dendriticum PA1, Diphyllobothrium ditremum PA2, Eubothrium crassum PA3, Eubothrium salvelini PA4, Proteocephalus sp. PA5, Schistocephalus solidus PA6, Philonema oncorhynchi PA7, Salmincola edwardsii PA8, Gyrodactylus arcuatus PA10, Saprolegnia PA9, fungi on crustacean zooplankton PA11, Rotiferophthora PA12, Chytridiomycetes PA13. Trophically transmitted parasite species are indicated in bold

Fig. 2

Frequency distributions of links in the Takvatn pelagic food web before (open bars) and after (shaded bars) the fish introductions; parasites are included

Fig. 3

Vulnerability to natural enemies (a statistic of topological food webs representing the number of natural enemy species that feed on a particular species) at different trophic levels of the Takvatn pelagic food web a before and b after the fish introductions