Connor Wood, Seasonal Project Officer and PhD Student with the University of the West of Scotland, January 2025

Introduction

We know that invasive non-native species are a major threat to biodiversity, but the danger they pose to our ecosystems can often be hard to see. Picture a sewer pumping sludge into a river or a forest being cleared and replaced with concrete—those are obvious, visible threats. But when an ecosystem is taken over by a non-native species, the damage can be much harder to recognize. Unless you can identify which species are native and which are non-native and invasive, the problem might not stand out. An understanding of why these invaders are a threat requires knowledge of their subtle differences from similar native species and how these differences can disrupt the balance of the ecosystem.

At first glance, some invasive non-native species don’t seem all that different from their closely related native counterparts. Take the invasive giant hogweed (Heracleum mantegazzianum) and the native common hogweed (Heracleum sphondylium) — sure, the invader stands much taller than its British relative, but they appear to function quite similarly overall. Or consider the Eurasian otter (Lutra lutra) and American mink (Neovison vison): the otter may be noticeably larger, but both are semi-aquatic predators from the weasel family, swimming through the same rivers. So, why is one a valued part of our ecosystem and the other a serious biodiversity threat?

The answer isn’t just blind favouritism for native species. The danger posed by invasive species is very real, and it often comes down to small, crucial details – differences in biological traits (for example, size or fecundity) and how these traits shape their interactions with other species.

This is the focus of my ongoing PhD research, where I compare the traits of a native freshwater shrimp (Gammarus pulex) with two closely related invasive species (the killer- and demon shrimp, Dikerogammarus species) to understand how these differences play out. In this blog, I’ll focus on the traits that give invasive species like giant hogweed and American mink their destructive edge over native species.

Case Study 1: Giant Hogweed vs Common Hogweed

Giant hogweed, a member of the carrot family from the Caucasus Mountains, is closely related to common hogweed which is native to the UK. In fact, they’re so closely related they’re even capable of producing the occasional hybrid offspring [14]. Despite these similarities, their ecological roles differ dramatically. Here are some of the key differences between them which can explain why giant hogweed is considered a high-impact invader.

Growth and Light Competition
Giant hogweed can grow up to 5 meters tall, dwarfing the smaller common hogweed which grows to a maximum of only 2 metres. The leaves of giant hogweed are similarly oversized, spanning 1.5 to 3 metres in width compared to the much smaller 55cm length leaves of the common hogweed. This size, when combined with rapid vegetative growth, allows giant hogweed to form dense leaf canopies that block out sunlight for smaller native plants. Competition for light is one of the major drivers of plant community composition [21], and the giant hogweed has a clear advantage in this contest.

Seed production and Emergence
High reproductive output is one of the most commonly reoccurring traits in successful and high impact invasive species [17]. Looking at the hogweed species, we can see a major difference between the two. Both species take around three years to reach the flowering stage. However, the reproductive potential of giant hogweed far outstrips its native counterpart. While common hogweed produces about 850 seeds per flowering season and usually flowers two to three times during its lifespan [16], giant hogweed can produce anywhere from 20,000 to a staggering 100,000 seeds in a single season. These seeds also germinate more frequently and emerge earlier in the season, giving the giant hogweed a big head start on outgrowing the competition [15]. What’s more, the seeds can remain viable in the soil for up to 15 years.

The reason for this difference lies in the giant hogweeds evolutionary history – the high seed production is a vital adaptation to survive the extreme cold and nutrient poor soil of the mountains in its native range. This reproductive advantage means that once giant hogweed takes hold, it is difficult to eradicate, allowing it to spread rapidly and persistently.

Toxic sap
Giant hogweed is notorious for its highly toxic sap, which can cause painful, long-lasting burns due to a chemical compound called furocoumarin. While common hogweed also contains furocoumarin and can cause skin irritation, its effects are far milder because the chemical is present in significantly lower quantities. Whether giant hogweed’s toxic sap provides an ecological advantage over common hogweed or other native plants remains unclear—it may deter some herbivores [20], but our sheep trials have shown that certain livestock are unaffected. However, the difference in impact on human health is undeniable, with giant hogweed posing a far greater threat.

Insects and Pollination
Common hogweed is a vital part of the local insect food web, attracting a broad variety of pollinators such as bumblebees, butterflies, hoverflies, and beetles. These pollinators play an important role in maintaining biodiversity. Giant hogweed, on the other hand, attracts a much narrower range of insects. Studies have shown that areas dominated by giant hogweed experience a decrease in pollinator diversity [13], which can have ripple effects throughout the entire ecosystem. The loss of specialist insect species in areas overtaken by giant hogweed is particularly concerning, as it impacts the broader insect community and the health of native plants that rely on these pollinators [19].

Summary
Giant hogweed’s size, reproductive capacity and highly toxic defence strategies have made it a highly successful and destructive invader. As they overtake and dominate plant communities, the abundance and diversity of both native plant species and specialist insect species declines in turn. Without this native biodiversity and the ecosystem services it provides – pollination, nutrient cycling and habitat creation – the ecosystem suffers as a whole.

Case Study 2: American Mink vs. Native Otter

Background
The American mink and the European otter are both top predators, members of the weasel family and share similar wetland and river habitats across the UK. Despite these overlaps, these two species have sharply different ecological impacts. The American mink poses a significant threat to native species, particularly causing dramatic declines in water vole and ground-nesting bird populations [1], whereas the otter is often regarded a keystone species crucial in maintaining ecosystem health. These are some of the trait differences that make mink a danger when compared to the otter.

Size Differences and Competitive Advantage
One key difference between the two species is size. Quite the opposite from the giant- and common hogweed, in this case the native otter is the larger animal. Otters can grow up to 130 cm in length, while mink are much smaller, ranging from 42 to 65 cm. Given the significant size difference, otters would be expected to hold a dominant competitive advantage over mink, enabling them to essentially bully them out of their territories or away from their food sources [4] [12]. Unfortunately, this is frequently not the case – many sites in the UK show a coexistence of otters and mink rather than competitive exclusion (we’ll discuss why later). All to say, the negative environmental impacts of American mink aren’t seen through direct competitive interaction with otters, rather through the direct predatory impacts on a variety of prey species.

Impact on Naïve Prey
The small build of the mink actually contributes to one of their most devastating impacts – their predation on the UK’s endangered water voles [23]. The mink is small and agile enough to invade the burrows of the voles, chasing them into their dens through the underwater entrances that deter most of the voles native predators. Water voles, like many other native species, are considered “naïve prey” for the mink [22] – they have not evolved natural defences or behaviours to evade this newly introduced predator, making them easy targets. This inability to respond to new threats can have dramatic consequences for populations already weakened by habitat loss and other pressures [18]. What’s more, the aggressive prey drive of the mink leads them to kill more than they consume, which has led to the eradication of entire local vole populations.

Diet, Habitat, and Adaptability
While otters tend to have a more specialized diet focused on fish, mink are opportunistic generalist predators [6] [9]. They can hunt a wide variety of prey, including birds, amphibians, fish, rabbits, rodents, crayfish, domestic fowl and more, giving them a significant adaptive edge over otters. This dietary flexibility allows mink to thrive in diverse habitats and conditions, while otters are more limited in their prey choices. The otter needs a river with a good fish population, whereas the mink can find food just about anywhere. The specialized fish diet of the otter also makes them more susceptible to aquatic pollutants [7] [8] – if a river becomes contaminated with toxic chemicals, so too does the otter’s food source, while mink can sustain themselves on terrestrial prey and avoid the contaminant.

Reproduction and Fecundity
The American mink exhibits an “r-selected” or “high output” reproductive strategy, which is typical among invasive species and contributes to their rapid population growth. Effectively this means the mink has higher fecundity (reproductive rate) compared to otters, so they can produce more offspring in a shorter time frame. While both species reproduce once a year, mink typically produce 4–6 kits per litter, compared to the otter’s 1–3 cubs. Additionally, mink can breed as early as one year old, while otters take 2–3 years to reach sexual maturity. Juvenile mortality is high for both species – as high as 50% before reaching maturity for both mink and otters, by some estimates [10] – and under these conditions the r-selected / high output reproductive strategy of the mink proves advantageous.

As well as the high fecundity, mink have a number of traits which make their reproduction adaptable to changing conditions. They’ve been shown to adjust their litter size and breeding times in response to changing population density [2] or prey availability [5]. They also have a rather unique capability for polyamorous breeding, where female mink can sire a single litter from multiple different males she has mated with [3]. These traits make the mink a particularly resilient invader as they can easily respond to changing conditions and “bounce back” after population control efforts.

Summary
The comparison between the American mink and the Eurasian otter gives an insight into the mink’s invasive success in the UK. Their high fecundity, adaptability, and aggressive predation make them a serious threat to native ecosystems. While both species face challenges from habitat loss and pollution, mink’s ability to thrive under these conditions compounds the pressure on vulnerable species like otters and water voles. Efforts to control mink populations must be sustained and strategic to protect native wildlife from further declines.

References

[1] Fraser, E.J., Harrington, L.A., Macdonald, D.W. and Lambin, X. (2018). Control of an invasive species: the American mink in Great Britain. Oxford Scholarship Online. Oxford University Press. doi:https://doi.org/10.1093/oso/9780198759805.003.0016.

‌[2] Melero, Y., Robinson, E. and Lambin, X. (2015). Density- and age-dependent reproduction partially compensates culling efforts of invasive non-native American mink. Biological Invasions, 17(9), pp.2645–2657. doi:https://doi.org/10.1007/s10530-015-0902-7.

[3] Thom, M. D., & Bagniewska, J. (2015). Biology, ecology, and reproduction of American mink Neovison vison on lowland farmland. Wildlife conservation on farmland: conflict in the countryside, 2, 126.

‌[4] Macdonald, D.W. and Harrington, L.A. (2003). The American mink: The triumph and tragedy of adaptation out of context. New Zealand Journal of Zoology, 30(4), pp.421–441. doi:https://doi.org/10.1080/03014223.2003.9518350.

‌[5] García-Díaz, P., & Lizana, M. (2013). Reproductive aspects of American minks (Neovison vison) in central Spain: Testing the effects of prey availability. Mammalian Biology, 78(2), 111-117.

[6] Bonesi, L., & W. Macdonald, D. (2004). Differential habitat use promotes sustainable coexistence between the specialist otter and the generalist mink. Oikos, 106(3), 509-519.

[7] Mason, C.F. and MacDonald, S.M. (1989). Acidification and other (Lutra lutra) distribution in Scotland. Water, Air, and Soil Pollution, 43(3-4), pp.365–374. doi:https://doi.org/10.1007/bf00279202.

‌[8] Androulakakis, A., Alygizakis, N., Gkotsis, G., Nika, M.-C., Nikolopoulou, V., Bizani, E., Chadwick, E., Cincinelli, A., Claßen, D., Danielsson, S., Dekker, R.W.R.J., Duke, G., Glowacka, N., Jansman, H.A.H., Krone, O., Martellini, T., Movalli, P., Persson, S., Roos, A. and O’Rourke, E. (2022). Determination of 56 per- and polyfluoroalkyl substances in top predators and their prey from Northern Europe by LC-MS/MS. Chemosphere, 287, p.131775. doi:https://doi.org/10.1016/j.chemosphere.2021.131775.

‌[9] Wise, M.H., Linn, I.J. and Kennedy, C.R. (2009). A comparison of the feeding biology of Mink Mustela vison and otter Lutra lutra. Journal of Zoology, 195(2), pp.181–213. doi:https://doi.org/10.1111/j.1469-7998.1981.tb03458.x.

‌[10] Harrington, L.A., Birks, J., Chanin, P. and Tansley, D. (2020). Current status of American mink Neovison vison in Great Britain: a review of the evidence for a population decline. Mammal Review, 50(2), pp.157–169. doi:https://doi.org/10.1111/mam.12184.

[11] Bradshaw, A. V., & Slater, F. M. (2002). A postmortem study of otters (Lutra lutra) in England and Wales. Bristol, UK: Environment Agency.

[12] Erlinge, S. (1972). Interspecific relations between otter Lutra lutra and mink Mustella vison in Sweden. Oikos, 327-335.

[13] Zych, M. (2007). On flower visitors and true pollinators: the case of protandrous Heracleum sphondylium L.(Apiaceae). Plant Systematics and Evolution, 263, 159-179.

[14] Stewart, F., & Grace, J. (1984). An experimental study of hybridization between Heracleum mantegazzianum Somm. & Levier and H. sphondylium L. subsp. sphondylium (Umbelliferae). Watsonia, 15, 73-83.

‌[15] Pyšek, P., Cock, M. J., Nentwig, W., & Ravn, H. P. (2007). Master of all traits: can we successfully fight giant hogweed?. In Ecology and management of giant hogweed (Heracleum mantegazzianum) (pp. 297-312). Wallingford UK: CABI.

[16] Roberts, H. A. (1979). Periodicity of seedling emergence and seed survival in some Umbelliferae. Journal of Applied Ecology, 195-201.

[17] Jelbert, K., Stott, I., McDonald, R. A., & Hodgson, D. (2015). Invasiveness of plants is predicted by size and fecundity in the native range. Ecology and Evolution, 5(10), 1933-1943.

[18] Doherty, T. S., Dickman, C. R., Nimmo, D. G., & Ritchie, E. G. (2015). Multiple threats, or multiplying the threats? Interactions between invasive predators and other ecological disturbances. Biological Conservation, 190, 60-68.

[19] Bogusch, P., Vojtová, T., & Hadrava, J. (2023). High abundance but low diversity of floral visitors on invasive Heracleum mantegazzianum (Apiaceae). NeoBiota, 86, 193-207.

[20] Buttenschøn, R. M., & Nielsen, C. (2007). Control of Heracleum mantegazzianum by grazing. In Ecology and management of giant hogweed (Heracleum mantegazzianum) (pp. 240-254). Wallingford UK: CABI.

[21] Gioria, M., & Osborne, B. A. (2014). Resource competition in plant invasions: emerging patterns and research needs. Frontiers in Plant Science, 5, 501.

[22] Anton, A., Geraldi, N. R., Ricciardi, A., & Dick, J. T. (2020). Global determinants of prey naiveté to exotic predators. Proceedings of the Royal Society B, 287(1928), 20192978. [23] Aars, J., Lambin, X., Denny, R., & Griffin, A. C. (2001, August). Water vole in the Scottish uplands: distribution patterns of disturbed and pristine populations ahead and behind the American mink invasion front. In Animal Conservation forum (Vol. 4, No. 3, pp. 187-194). Cambridge University Press.