Learning from the past to predict the future: an historical analysis of grass invasions in northern Australia

Abstract

An important focus of biosecurity is anticipating future risks, but time lags between introduction, naturalisation, and (ultimately) impact mean that future risks can be strongly influenced by history. We conduct a comprehensive historical analysis of tropical grasses (n = 155) that have naturalised in Australia since European settlement (1788) to determine what factors shaped historical patterns of naturalisation and future risks, including for the 21 species that cause serious negative impacts. Most naturalised species were from the Old World (78 %), were introduced for use in pasture (64.5 %), were first recorded prior to 1940 (84.5 %) and naturalised before 1980 (90.3 %). Patterns for high-impact species were similar, with all being first recorded in Australia by 1940, and only seven naturalised since then-five intentionally introduced as pasture species. Counter to expectations, we found no evidence for increased naturalisation with increasing trade, including for species introduced unintentionally for which the link was expected to be strongest. New pathways have not emerged since the 1930s despite substantial shifts in trading patterns. Furthermore, introduction and naturalisation rates are now at or approaching historically low levels. Three reasons were identified: (1) the often long lag phase between introduction and reported naturalisation means naturalisation rates reflect historical trends in introduction rates; (2) important introduction pathways are not directly related to trade volume and globalisation; and (3) that species pools may become depleted. The last of these appears to be the case for the most important pathway for tropical grasses, i.e. the intentional introduction of useful pasture species. Assuming that new pathways don't arise that might result in increased naturalisation rates, and that current at-border biosecurity practices remain in place, we conclude that most future high-impact tropical grass species are already present in Australia. Our results highlight the need to continually test underlying assumptions regarding future naturalisation rates of high-impact invasive species, as 123Biol Invasions (2015( ) 17:565-579 DOI 10.1007 conclusions have important implications for how best to manage future biosecurity risks.

The changing patterns of plant naturalization in Australia

“…Regardless, the number of known new 'weed' species naturalizing in Australia has steadily declined since the period 1890-1920 ( Fig. 9), similar to the findings of both Murray & Phillips (2012) and van Klinken et al (2015).…”

“…9). The substantial deliberate introduction of pasture species into northern Australia (Lonsdale, 1994;Cook & Dias, 2006;van Klinken et al, 2015) was also evident through the ongoing naturalization of species used for animal foods (Fig. 9).…”

“…Notwithstanding the literature reviews of Groves (1986), Michael (1994), Scott (2000), Gallagher & Leishman (2015), no comprehensive analysis of the naturalized flora of Australia has been published. Previous analyses have focussed on temporal (Groves, 1998), geopolitical (Everist, 1960;Specht, 1972Specht, , 1981Kloot, 1986Kloot, , 1987aRozefelds et al, 1999;Phillips et al, 2010b), taxonomic (Lonsdale, 1994;van Klinken et al, 2015) or weed status (Phillips et al, 2010a;Murray & Phillips, 2012) subsets of the Australian flora or specific questions such as introduction purpose (Virtue et al, 2004;Cook & Dias, 2006) or introduction success (Diez et al, 2009).…”

Unassisted invasions: understanding and responding to Australia’s high-impact environmental grass weeds

“…The rates of naturalisations are not increasing in Australia for the flora overall (Dodd et al 2015), and both importation ('colonisation pressure') and naturalisation rates are approaching historical lows at least for tropical grasses (van Klinken et al 2015a). It therefore appears that existing quarantine restrictions are effective, provided no new entry pathways arise, and existing regulations and efforts are not weakened.…”

Changing Fire Regimes in Tropical and Subtropical Australia

“…Where: w is measured in t/ha and r is measured in km/h Clements and Henzell 2010;van Klinken et al 2015). Guinea grass is not a prohibited or restricted invasive plant under the Queensland Biosecurity Act 2014 and it is used extensively as a fodder grass (State of Queensland 2016).…”