Botanic Gardens Trust, Sydney, Australia

Plant interactions - between native and exotic plants

Of the 200 or so plant species in the woodland, about 35% are naturalised exotic species. Our conservation management aim is to improve conditions for the native species component of the woodland over the long-term. Detrimental impacts by exotic species on native species are to be reduced, and ideally all exotic species should be eradicated. However it is unrealistic to expect that all exotics can be removed, and priority should be given to those which clearly impact on the long-term viability of native species.

The most threatening exotic species are large perennials that outcompete natives by shading e.g. the shrub African Olive *Olea europaea subsp. cuspidata, or vigorous growth e.g. Chilean Needle Grass *Nasella neesiana, St Johns Wort *Hypericum perforatum and Heliotrope *Heliotropium amplexicaule. These, like most native species, can survive fire and drought particularly well.

These threatening species however, make up only a small proportion of the total exotic flora.

A greater proportion of the exotic species are short-lived species (see Lifespans), that are able to respond and grow in changed environmental conditions by way of soil-stored seedbanks or wind-dispersed seeds from adjacent sites. In contrast a greater proportion of the native species have relatively stable long-lived above-ground populations.

Following the cessation of grazing and mowing in 1988 the majority of exotic species have generally decreased in abundance in the woodland. For example, comparing numbers of species present in 1988 and 15 years later (2003) at five undisturbed sites (Table ), we found that:

  • Similar proportions of both native and exotic species first appeared or increased in abundance over the period (45%)
  • a greater proportion of exotic species (53%) than native species (41%) decreased in frequency or disappeared.
  • Frequencies for only 2% of the exotic species were unchanged at the end of the period, compared with 15% of the native species.

Table: Number and percentage of species showing changes in frequency (increased, stable or decreased) after 15 years in the 10 ha Conservation Research Woodland , the Australian Botanic Garden

Comparison of 1988 and 2003 data from five undisturbed woodland sites. 

No. of native species % No. of exotic species %
Greatly increased, freq>50% 14 21 10 22
Increased, freq<50% 3 4 3 7
Stable 10 15 1 2
Decreased freq <50% 5 7 6 13
Greatly decreased freq>50% 13 19 4 9
New appearance by 2003 13 19 7 16
Disappeared by 2003 10 15 14 31
Total 68 45  
         

Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.

Ecology of Cumberland Plain Woodland

Olea europea subsp cuspidata fruit
Fruit of *Olea europaea subsp. cuspidata

Nassella neesiana plant
*Nassella neesiana

Hypericum perforatum flower
*Hypericum perforatum