Non-native species are a problem in the environment when they establish new populations and disperse – i.e. become invasive. A highly invasive, global invader of freshwaters is the common carp, a fish capable of reaching weights of over 30 kg as well as producing a large number of offspring, and is proving to be an ecological and economic pest wherever it goes. They are a highly popular species for catch-and-release recreational angling (see Figure 1), as well as being an important species in aquaculture – but this has resulted in their invasion of all continents except Antarctica. They have been invading British freshwaters since at least the Middle Ages and perhaps as early as the Roman Times.
Invasive carp are well known as ecological engineering species, altering the physical habitats of their invaded freshwaters through their aggressive benthic foraging. However, what is less known is the extent of their competitive interactions with native fish species, especially those that have populations which are already threatened with issues such as habitat loss – like the crucian carp, a relatively diminutive fish in the same family as common carp and with similar benthic feeding habits and functional morphology.
Although generating understandings of the interactions of invasive and alien species can be achieved through field studies alone, a major issue with many field studies on invasive species is that they tend to have high context-dependency. For example, while studies provide interesting and useful information, that information is often only insightful to the study site in question due to, for example, issues such as a lack of data prior to the invasion, limited knowledge on the introduction event, the use of one-off sampling events, and a lack of control in the environmental conditions. In combination, this makes it very difficult to draw strong conclusions beyond the study site and species in question. To overcome these issues, in our study we completed two sets of experiments that would help us understand the competitive interactions of the invasive species (common carp) versus the threatened native species (crucian carp) under relatively controlled conditions to understand the processes that might be producing the patterns in our field data.
The first of these experiments was a set of comparative functional response experiments completed in tank aquaria. As both species shoal, we used the fish in conspecific pairs and at a water temperature that the fish typically experience in Southern Britain during summer (17 oC). We exposed the pairs of fish to a range of prey densities for fixed time intervals to determine their feeding rates. The results showed common carp had much higher feeding rates than crucian carp, suggesting they would monopolise food resources when they are together.
This experiment complemented a much larger and longer experiment completed between 2016 and 2019 in a set of three ponds that were drained prior to the experiment (so they started with no fish) and were then seeded with 100 fish of similar sizes into each pond – one with only crucian carp, one with only common carp and one with 50 of each. Although we could not replicate these treatments, we could follow the feeding interactions of the fish across the experiment through the ecological application of stable isotope analysis. We revealed that when only one species was present in a pond, the extent of the food resources each species consumed – their trophic niches – were similar. When they were together, however, the trophic niches of both species were much larger and were very different from each other (they had ‘divergent niches’). These results indicated that their interactions resulted in them having to feed on a much greater range of prey items than when they were separate, with common carp also having to alter their diet – despite being the superior competitor.
These experimental findings were then used to help us interpret the patterns in our field data from four wild ponds where the two species were present together. In all ponds, their trophic niches were also strongly diverged from each other, as per the experimental ponds, and where the comparative functional response data suggested this was driven by the strong competition pressure from common carp.
The use of the two experiments enabled us to identify that the highly invasive species – common carp – is a strong competitor and one that the threatened native species – crucian carp – finds it difficult to compete with. As common carp become more prevalent across the world’s freshwaters, the outcome for fish species already under threat, such as crucian carp, do not look favourable.