ABSTRACT
Recombinant ecosystems comprise novel plant and animal associations that have been induced or created by people deliberately, inadvertently or indirectly. They are generally made up of various mixes of indigenous and exotic species, but they may also involve associations of indigenous species alone, never before seen in nature, for example plant signatures (Robinson 1993), native landscape garden designs and pictorial meadows (Dunnett and Hitchmough 2004), indigenous feature species introduced to areas beyond their natural range, or back-filled ‘gaps’ created by local extinctions. Hobbs et al. (2006, 2009) propose that ‘novel ecosystems’ (‘synthetic vegetation’ in Bridgewater 1990; ‘no-analog communities’ of Williams and Jackson 2007; ‘hybrid ecosystems’ in Mulcock and Trigger 2008) result from ‘human action, introduction of species, and environmental change’. Disturbance is normal in urban environments, and urban survivors (Wittig 2004; Van der Veken et al. 2004) are pre-adapted to high levels of change. As time goes on, species are added, deleted and re-sorted and vegetation converges. But active manipulation and moulding of landscapes is a major influence in the evolution of recombinant ecosystems. In the most extreme case, gardens of only alien plants or horticultural varieties (Acar et al. 2007) would qualify, although surely some indigenous microbe lurks in every urban or rural habitat. Milton (2003) refers to ‘emergent ecosystems’ as those where new or different, maybe synergistic functional relationships occur. In New Zealand (NZ) exotic animals compete with or eat honey-eating bellbirds, yet the latter prosper from new sources of nectar supplied by plantings of Australian Eucalyptus and proteas. On the other hand, loss of large seed dispersers may diminish regeneration of keystone plant species (Norton 2009). With climate change there are emerging consequences of diseases, pests and weeds extending poleward; and increasing drought stress in urban environments may make species more susceptible to disease and deterioration. For instance, English oak and other introduced trees in Christchurch, NZ are succumbing to ‘old age’ at 100 to 150 years old, long before their natural span. Ironically, an emerging fashion is to replace English with American red oaks, somewhat undermining the city’s claim to Englishness. In many ways, NZ is an unhappy experiment that demonstrates an almost infinite array of permutations and combinations of species mixes from all over the temperate and subtropical
worlds. To many this is desirable enrichment, but it is devastating to the highly endemic biota (Wilson 2004; Meurk and Swaffield 2000). The NZ flora comprises about 2500 indigenous vascular plant species, over 80 per cent of which are regarded as endemic. However, since European colonisation from the 1840s onwards, over 25 000 plants have been introduced into the country. Most thrive only in sheltered cultivation, as house plants, or in the ‘winterless’ north, but are often biding their time before transitioning to the wild, perhaps on the back of warming climate (Chapin and Starfield 1997). About one tenth have already become naturalised and each year four more enter the wild (Esler and Astridge 1987). So, what is new? Species have been coming and going throughout geological history (McGlone 2006), adjusting and forming novel associations continually. This accelerated during periods of landscape disruption due to tectonics or glaciations, with human effects in NZ being felt from about a millennium ago. It is thus a moving feast and every point in space and time is novel or emergent in some sense. The issue now is biodiversity conservation (Zerbe et al. 2004; Erfurt Declaration 2008) and the simple reality that anthropogenic, novel ecosystems generally involve displacement of local species or genes by widespread, generalist species (Van der Veken et al. 2004). The resulting globalisation and reduction in biodiversity and natural patterns, especially where the indigenous biota is highly endemic, is calamitous. In NZ, unlike continental countries with highly competitive floras, the introductions do not stay put. Nevertheless, there is valid interest in the theoretical ecology of colonisation, assembly and the novel pact invaders make with indigenous species, providing they do survive and coexist. The niche windows of native species in novel ecosystems are informative and must be better understood for their continued coexistence.
