A third reason for the success of invasive species

A recent paper by Colautti et al. (2014) stated that the success of invasive species can be explained by two views.

1. “intrinsic factors make some species inherently good invaders”
2. “species become invasive as a result of extrinsic ecological and genetic influences such as release from natural enemies, hybridization or other novel ecological and evolutionary interactions”

This is not an unusual view of invasion biology, but one that is based on a static view of the world. This philosophy views habitats as fixed entities and the invaders being imposed upon them. There is a third reason for the success of invasive species, that new habitat has been create for them. Take for example the simple case of oak gall wasps Andricus aries, A. corruptrix, A. grossulariae, A. liginocolus, A. lucidus and A. quercuscalicis which have all invaded northern Europe in the past 60 years. They all have a complex alternating life cycle requiring two oak species Quercus cerris and Q. robur (Ozaki et al., 2006). However, only Q. robur is native to northern Europe. Q. cerris was introduced to northern Europe in the 18th century and was first found in the wild in the mid-19th century. It has, since then, continually been planted and has naturalised in many places. Thus, mankind created a habitat for these Andricus species where both oaks lived side-by-side and after a short lag the wasps came and occupied it. There is no reason to think these wasps are particularly good invaders, though some have spread quicker than others. Nor have they managed to escape their parasitoids (Ellis, 2005). It is not known if some evolutionary process aided their spread, but why invoke such a mechanism when a more parsimonious one exists?
Another, much less obscure, example is roadside halophytes. In recent years in the UK, roadside halophytes were amongst plants with the largest increase in range (Groom, 2013). Some plants such as Danish scurvy-grass (Cochlearia danica), reflexed saltmarsh grass (Puccinellia distans) and lesser sea-spurrey (Spergularia marina) have increased the quickest, but many different native saltmarsh plants can also be found by roadsides somewhere in Britain. There is no reason to think that halophytes are better invaders than other plants, nor is there any reason to believe that they have escaped natural enemies, as these are native plants. It is unlikely that all of them have undergone evolutionary changes or that there are “novel ecological and evolutionary interactions”. The main thing that has changed is the creation of large amounts of interconnected saline habitat at the side of roads.
Many other examples exist, C4 grasses are spreading in northern Europe due to changes in farming practises (Hoste & Verloove, 2001). Chasmophytes such as ivy-leaved toadflax (Cymbalaria muralis), buddleia (Buddleja davidii) and wall-rue (Asplenium ruta-muraria)  are increasing in cities because of the large number of walls on which they can grow. In numerous cases of invasion the creation of new habitat precedes the invasion. Climate change, atmospheric nitrogen deposition, pollution, forestry, agricultural and cultural changes all alter habitats, potentially priming these species for invasion. If a new habitat is being created one can be fairly sure that some organism will come along and occupy it. I do not discount the other two reasons for invasions, but if we ignore mankind's role in priming the environment for invasion we misunderstand the invasion process of many species.

Cited Literature

1. Colautti, R.I., Parker, J.D., Cadotte, M.W., Pyšek, P., Brown, C.S., Sax, D.F. and Richardson, D.M. (2014) Quantifying the invasiveness of species. In: Capdevila-Argüelles L, Zilletti B (Eds) Proceedings of 7th NEOBIOTA conference, Pontevedra, Spain. NeoBiota 21: 7–27 doi:10.3897/neobiota.21.5310
2. Ellis, H.A. (2005) Observations of the Agamic (Knopper Gall) of Andricus quercuscalicis and the associated inquilines and parasitoids in Northumberland. Cecidology 20: 12–27
3. Groom Q.J. (2013) Some poleward movement of British native vascular plants is occurring, but the fingerprint of climate change is not evident. PeerJ 1:e77 http://dx.doi.org/10.7717/peerj.77
4. Hoste, I. & Verloove, F. (2001) De opgang van C4-grassen (Poaceae, Paniceae) in de snel evoluerende onkruidvegetaties in maïsakkers tussen Brugge en Gent (Vlaanderen, België). Dumortiera 78: 2–11
5. Ozaki, K., Yukawa, J., Ohgushi, T. and Price, P.W. (2006) Galling Arthropods and Their Associates. Springer-Verlag, Tokyo

Knopper gall on oak (Quercus robur) induced by Andricus quercuscalicis (gall wasp) by Bj.schoenmakers

This work by Quentin Groom is licensed under a Creative Commons Attribution 3.0 Unported License.