Chapter 5 General Discussion
5.5 Implications and Recommendations for Further Research
5.5.1
Low Population Survival
In common with the system studied here, many invasive aliens display large spatial and temporal fluctuations in distribution (Pysek and Hulme 2005) and are frequently associated with stochastic disturbance and human activities (e.g. Stapanian et al. 1998, Chytry et al. 2005, Alexander et al. 2009, Pysek et al. 2010). Despite this, persistence of alien plant populations is rarely considered (but see Wade et al. 1997, Pysek et al. 2001, Pergl et al. 2012).
Neglecting to quantify and account for the persistence of alien plant populations may result in overestimation of alien abundance and distribution and miss opportunities for control or eradication. Cumulative survey and herbarium records are often used as the basis for assessments of species distributions in plant atlases and the invasion literature (Petrik et al. 2010). Failing to account for those populations which have become extinct can result in significant overestimates of a species’ distribution. Further studies are required to determine the rates of persistence of alien plant populations and to verify current assessments of abundance and distribution after accounting for populations which may have failed to persist. These studies are likely to be particularly pertinent for alien plants which are associated with frequent, widespread or high density anthropogenic seed dispersal, in particular species grown as crops, pasture, and ornamentals (Hodkinson and Thompson 1997). As with Brassica in this system, such plants may often have short-lived populations and rely on anthropogenic seed rain to maintain their presence in the landscape. In this case, cumulative records would tend to overestimate their abundance and distribution.
Where alien plants are adventive rather than naturalised, controlling or eliminating the propagule supply offers an effective method of control that may avoid many of the difficulties in attempting to control established populations (Hobbs and Humphries 1995). In my study system it seems likely that
reduce the range or abundance of even fully naturalised species. Further studies which aim to identify adventive alien species that could be managed by better control of propagule sources would likely bring benefits such as reduced costs and greater efficiency of control; species which could be fruitfully investigated include those which are intensively used by humans and produce numerous seeds, such as grasses and seed crops.
5.5.2
Intraspecific Variation
At present most research on biological invasions and efforts to identify potentially harmful aliens through risk assessment protocols uses species as the unit of observation (e.g. Mack 1996, Reichard and Hamilton 1997, Pheloung 1999, Pheloung et al. 1999, Williamson 1999, Kolar and Lodge 2001, Daehler et al. 2004, Callaway and Maron 2006, Theoharides and Dukes 2007, Catford et al. 2009, van Kleunen et al. 2010, Blackburn et al. 2011, Gurevitch et al. 2011, Pysek et al. 2012). My results, in conjunction with a growing body of work that suggests there may be significant intraspecific differences in invasiveness (Saltonstall 2002, Kinter and Mack 2004, Vellend et al. 2010, Merrill et al. 2012, Hierro et al. 2013), raises the possibility that some of our current assessments of invasiveness may in fact be measures of the average invasiveness of a sample of subspecific taxa, or the invasiveness of one or a few subspecific taxa that have been the subject of study. Unknown genotypes developed through breeding, hybridisation or sourced from novel populations may pose an invasion risk different to that at which the species is currently assessed.
At present it is unclear how common large variation in traits and performance within species is, or how often it translates to variation in invasion outcomes. Studies across a broad range of taxa are required; experimental studies which partition performance among taxonomic levels (e.g. Chapters 2 and 3), as well as reciprocal common garden experiments (e.g. Hierro et al. 2013) and genetic studies (e.g. Saltonstall 2002, Merrill et al. 2012) may all be useful approaches. Invasive genera which contain substantial variation within species may be fruitful targets for further research, these include, but are not limited to, Acacia (Kriticos et al. 2003, Wardill et al. 2005), Pinus (Wheeler and Guries 1982, Sykes 2001), Phragmites (Belzile et al. 2010, Lambertini et al. 2012, Guo et al. 2013), Chrysanthemoides
(Weiss 1984), and Bromus (Kinter and Mack 2004, Chambers et al. 2007, Merrill et al. 2012). Future research should aim to identify factors (e.g. a long history of breeding, multiple uses by humans for the same species, widely varying morphology, wide geographic distributions, or a propensity to hybridise or form polyploids) which could be used to identify genera and species that are likely to contain substantial intraspecific variation in traits or invasiveness, and could be targeted for more intensive risk assessment of intraspecific taxa.
My finding that there can be significant variation in the performance of alien plants within species, and that this may result in significant variation in invasiveness, has important implications in the area of weed risk assessment (WRA). A species which has been assessed as low risk may actually contain high risk subspecific taxa. Managers and policymakers should consider the costs and benefits of modifying WRA protocols to screen for intraspecific variation in traits linked to invasiveness. My results suggest that seed parameters (in particular viability, but also others such as seed mass) may be important characteristics influencing variation in invasiveness within short-lived herbaceous species. Such data are simple and inexpensive to measure and could be incorporated into existing WRAs with comparative ease. Despite being widely considered to be a strong control on the naturalisation success of aliens (Pheloung et al. 1999, Thuiller et al. 2005, Hayes and Barry 2008), in this system at least, climatic match between source and recipient region were non-informative in predicting performance. It seems that climatic match may be a higher order ecological filter on establishment than other factors such as bare ground for establishment (Chapters 2 & 4, Miller and Schemske 1990, Miller 1995, Diepenbrock 2000) and post-emergence disturbance (Eager et al. 2013). Climatic match between source and recipient region may not be a consistent predictor of the invasiveness of alien plants due to the overwhelming influence of lower order ecological filters.
Further work is required to identify which parameters drive differences in performance and invasiveness within species; I identified seed performance as a driver of some of the variation in performance among species (Chapter 3), but even having accounted for these there was still much unexplained variation in performance within species. Common garden experiments paired with glasshouse studies to quantify the traits of the taxa used (e.g. Kempel et al. 2013) could be a powerful tool which could be used to identify key traits which drive variation in invasiveness within species, and these traits could then be incorporated into WRA protocols. Substantial intraspecific variation has been documented in parameters linked invasiveness such as seed viability (Chapter 3), seed mass (Baker 1965, Hendrix and Sun 1989, Rejmanek et al. 2005, Voller et al. 2012), fecundity (Miri 2007, Mason et al. 2008), tolerance of physical stress (Rapacz 1999, Qasim et al. 2003, Chahal et al. 2013, Gabler and Siemann 2013), growth rate (Williams and Hill 1986, Hofmann et al. 2000, Grotkopp et al. 2010, Pan et al. 2012), and ploidy (Marhold and Lihova 2006, Munzbergova 2006, Halverson et al. 2008, Elling et al. 2009). Investigating these parameters would be a good start towards identifying which traits drive variation in performance and invasiveness within species. It is important to note, however, that even at the species level research has been unable to identify a
used to assess differences in invasion risk within species. From a WRA perspective, applying the precautionary principle would suggest that the same restrictions should apply to the importation of new plant varieties, subspecies and genotypes as are applied to the importation of new plant species. In jurisdictions where a “white list” approach is applied, this would mean all new taxa, whether new species or new intraspecific taxa of an existing species, would require risk assessment prior to introduction (Csurhes et al. 2006, Simberloff 2006, Invasive_Species_Council 2009), and this may require a combination of WRA protocols using trait and biogeographic data (e.g. Pheloung et al. 1999) and field trials (e.g. Davis et al. 2011).