Invasion of the Beavers

Guest post by John Cherkas

Fifty years ago, Dr. Walter Howard presented his thoughts on invasive mammals at a symposium on colonizing (invasive) species, which was later turned into the volume "The Genetics of Colonizing Species." He speculated on the nature of predator-prey interactions, population growth limits and habitat disruptions. His ideas still resonate, but how well do they match up with a certain invasive mammal today.

May I bring your attention to some invasive beavers? Our national creature has been making quite a mess is the Southern most reaches of the Western Hemisphere. In the 1940s, Argentina was seeking economic improvements and imported beavers, mink and muskrat to Tierra del Fuego in an attempt to establish a fur trade. That fur trade didn't turn out as expected. Within a few years, beavers had colonized the entire island and were soon crossing channels to reach other Chilean islands, including Cape Horn, a UNESCO Biosphere Reserve.

Angry beaver -roar!

The ecological effects have been pretty well researched recently by Dr. Christopher B. Anderson. In seeing if beavers behave differently in their new habitat than back home, he’s been finding a few differences in the environment and beavers here. One of the most obvious changes is that the beaver colonies are at least twice as dense in Cape Horn. Is this for lack of predators or an abundance of food? So far, I couldn’t say, but I’d lean toward the latter. The Cape Horn forests are entirely southern beeches, which provide ample resources for the beavers’ engineering projects.

But how disruptive have beavers been to the environment: and environment that has no animal that makes such a massive environmental impact as the beaver. Howard suggested that an animal moving into a habitat where its niche doesn’t exist would have wider impacts than one who’s niche does exist. It’s fairly clear that the beaver’s landscaping projects is not something that other animals (except humans) partake in.

In the beaver situation this ecological disruption holds true. The floral assemblage in Cape Horn has never had to deal with beaver-like behaviour. The beavers foraging and building habits prevent forest regrowth, and provide a pathway for other plants to invade. It seems this beaver introduction might be a good example of invasional meltdown. The Chilean archipelago is home to quite a few invasive species already, and this synergistic effect is definitely concerning.

All the beaver-induced worries come with a grave concern for the natural environment. Cape Horn is referred to as pristine quite a bit by Anderson. Is this the place to have a deep political, socio-economic discussion about “pristine” environments? No, not today; you’ll have to read elsewhere for that. Cape Horn is certainly already at risk from invasive species. Beavers have a tremendous impact on the ecological structure of streams and forests. I am certainly one to wonder whether the eradication effort can be truly successful and both removing the beavers and reversing the environmental changes.

I surely hope that the environmental disruption can be reversed. Unfortunately we cannot look back to Howard to speculate on what happens when we remove an alien species. Just fifty years ago, species invasions were seen as a great research opportunity, not something to be extensively managed or eradicated.

Further Reading:

C.B. Anderson et al. (2006), The effects of invasive North American beavers on riparian plant communities in Cape Horn, Chile. Do exotic beavers engineer differently in sub-Antarctic ecosystems? Biological Conservation, 128: 467-474.

doi:10.1016/j.biocon.2005.10.011

C. Choi, (2008) Tierra del Fuego: the beavers must die. Nature 453: 968. doi:10.1038/453968a

Going natural: biological control of insect pests

*Guest post by Sheena Fry

Damage caused by agriculture pests is one of the most important factors of crop yield reduction (Cramer, 1967; Oerke et al., 1994) and can cause billions of dollars worth of damage each year (e.g. in Brazil, insect pests cause up to US$ 17.7 billon year^-1 of damage, Oliveira et al., 2014). Due to its economic impact, controlling pest populations is a priority for agricultural scientists. Chemical control is the primary method of pest management due to its relatively low costs and high effectiveness (Cooper and Dobson, 2007). Despite the widespread use of chemical controls, the health and environmental risks associated with their use are well known (Pimentel et al. 1992; Pimentel, 2005). The risks associated with pesticide use, as well as the evolution of pesticide resistance, has lead to a surge in interest in the use of biological control for pest management over the past 50 years.

The most important decision to be made in a biological control program is which biological control agent to use against a pest. Success rates for biological control of insects are low, with only 24-35% resulting in the establishment of the introduced species (Hall and Ehler 1979, van Lentern, 1983) and only 16% resulting in complete control of pest species (Hall et al., 1980). What determines the success of colonization and establishment is a key question in biological control research and must be answered in order to make predictions about establishment and success of introduced species. In 1965, Debach attempted to identify characteristics of successful colonizers but found that neither success nor failure could be explained by the presence or absence of a common characteristic. Over the past 50 years, several attempts have been made to list characteristics of successful invaders (e.g. Murdoch et al., 1985) and while they seem logical, there are too many exceptions for them to be used as a reliable indicator of a species’ potential to colonize and establish in a new area. DeBach saw “no possibility of predicting the fate of a purposely colonized imported entomophagous insect” and at present it remains an elusive goal (Fischbein and Corley, 2015).

Paul Debach 1914-1992

The environmental and health risks associated with chemical controls of insects (see references above) are not an issue when using biological controls. In addition to this, successfully established biological control species will be able to maintain stable populations without the need for additional investment by humans (unlike chemical controls, which must be applied each season). Despite the obvious benefits of biological control, there are also risks associated with the use of insects in biological control, such as the risk to non-targeted species (Simberloff and Stiling, 1996) or host switching. In order to make decisions about biological control we need to understand the evolution of introduced species in new environments, which can increase the efficiency of biological control (through post-colonization adaptation) or can increase the risk to non-targeted species. “The Genetics of Colonizing Species” (1965) brought together evolutionary biologists and ecologists (theoretical and applied) to discuss the evolution of introduced species. In DeBach’s chapter, he focused on colonizing entomophagous insects and, using biological control case studies, looked at the relative influence of pre- and post colonization adaptation, a key question in evolutionary biology. One such case study was the introduction of a parasitoid wasp (Comperiella bifasciata Howard, Figure 1), which was introduced to control a citrus pest, the California red scale (Aonidiella aurantii Maskell). The parasitoid wasp was released throughout southern California but initially was only able to establish at one location. It slowly spread and increased in abundance and, by 1957 was found at various locations throughout southern California. DeBach interpreted the poor initial establishment of the parasite followed by intense colonization as an indication that genetic adaptation had occurred.

Figure 1. A female parasitic wasp (Comperiella bifasciata Howard) infesting a California red scale (Aonidiella aurantii Maskell), from Forester et al. (1995).

Fifty years have passed since the publication of “The Genetics of Colonizing Species” (1965) and understanding the relative effects of pre- and post-colonization adaptation has remained an important issue. Phillips and colleagues (2008) examined the relative effects of genetic drift and selection in the frequencies of two asexually reproducing, genetically distinct parasitoid biotypes. This South American parasitoid wasp (Micrictonus hyperidae Loan, Figure 2) was introduced as a biological control for a pasture pest (Listronotus bonariensis Kuschel, Figure 2) in New Zealand in 1992. Phillips and colleagues recorded the relative frequencies of each biotype over a 10-year period and found that changes in biotype frequency were consistent with strong directional selection, favouring one of the parasitoid biotypes. This resulted in parasitoid populations being better adapted to New Zealand conditions than those originally released. 

Figure 2. A female parasitic wasp (Micrictonus hyperidae Loan, right) infesting a South American weevil (Listronotus bonariensis Kuschel, left). © Copyright AgResearch

There have been significant advance in the tools (statistical and molecular) available for the study of post-colonization success and adaptation since the publication of “The Genetics of Colonizing Species” (1965). These tools allow for better understanding of the post-colonization process of introduced species but, despite these advances, there has been little progress towards being able to predict the success of introduced species.

References:

Baker, H. G., & Stebbins, G. L. (Eds.). (1965), The Genetics of Colonizing Species. New

York: Academic Press.

Cooper, J., & Dobson, H. (2007). The benefits of pesticides to mankind and the

environment. Crop Protection, 26, 1337-1348.

Cramer, H. H. (1967). Plant protection and world crop production. Pflanzenschutz Nachr,

20, 1-524.

DeBach, P. (1965). Some biological and ecological phenomena associated with

colonizing entomophagous insects. In H.G. Baker, and G.L Stebbins (Eds.).

The Genetics of Colonizing Species. New York: Academic Press.

Fischbein, D., & Corley, J. C. (2015). Classical biological control of an invasive forest pest:

a worldwide perspective of the management of Sirex noctilio using the parasitoid Ibalia leucospoides (Hymenoptera: Ibaliidae). Bulletin of Entomological Research, 105, 1-12.

Forster, L. D., Luck, R. F., & Grafton-Cardwell, E. E. (1995). Life stages of California red

scale and its parasitoids- University of California. Dividion of Agriculture and Natural Resources. Publication N^o21529.

Hall, R. W., & Ehler, L. E. (1979). Rate of establishment of natural enemies in classical

biological control. Bulletin of the Entomological Society of America, 25, 280-282.

Hall, R. W., Ehler, L. E., and Bisarbi-Ershadi, B. (1980). Rate of success in classical

biological control of arthropods. Bulletin of the Entomological Society of America, 26, 111-114.

Oerke, E. C., Dehne, H. W., Schonbeck, F., & Weber, A. (1994). Crop Production and Crop

Protection- Estimated Losses in Major Food and Cash Crop. Elsevier Science: Amsterdam.

Oliveira, C. M., Auad, A. M., Mendes, S. M., & Frizzas, M. R. (2014). Crop losses and the

economic impact of insect pests on Brazilian agriculture. Crop Protection, 56, 50-54.

Murdoch, W, W., Chesson, J., Chesson, P. L. (1985). Biological control in theory and in

practice. The American Naturalist, 125, 344-366.

Phillips, C. B., Baird, D. B., Lline, I. I., McNeill, M. R., Proffitt, J. R., Goldson, S. L., & Kean,

J. M. (2008). Journal of Applied Ecology, 45, 948-956.

Pimentel, D. (2005). Environmental and economical costs of the application of pesticides

primarily in the United States. Environment, Development, and Sustainability, 7, 229-252.

Pimentel, D., Acquay, H., Biltonen, M., Rice, P., Silva, M., Nelson, J., Lipner, V., Giordano,

S., Horowitz, A., & D’Amore, M. (1992). BioScience, 42, 750-760.

Simberloff, D., & Stiling, P. (1996). How risky is biological control? Ecology, 77, 1965-

1974.

van Lentern, J. C. (1983). The potential of entomophagous parasite for pest control.

Agriculture, Ecosystems, and Environment, 10, 143-158.