Alien plants alter the growth form ratio and structure of Australian grasslands

Greg R. Guerin, Irene Martín‐Forés, Samantha E.M. Munroe, Ben Sparrow & Andrew J. Lowe

Mokota Conservation Park, a remnant grassland in the mid-north region of South Australia (approximately 160 km north of Adelaide). Photo by Greg Guerin.

Loads of evidence suggests particular invasive weed species wreak havoc in native plant communities by out-competing, or rendering the local environment unsuitable for, natives. Even so, emerging case studies have revealed overall positive relationships between native and weed diversity, notably in Mediterranean Biome grasslands. If we break this down a little, however, would we discover that natives most similar to weeds are being displaced? Using information from a regional survey of remnant grasslands in South Australia, we found that native herbs and grasses actually increased when there were more weed species in a habitat patch, despite most of the weeds in the study system also being herbs or grasses. The effect remained strong when soil type, regional climate and seasonal rainfall (factors that influence native and weed growth and diversity) were factored in.

The result seems counter-intuitive. We know many weeds impact native systems and expect ecologically similar species to compete. The explanation may lie partly in subtle differences between introduced and native species, leading to separation of resources. For example, introduced herbs tended to have higher leaf mass but lower seed mass – characteristics promoting rapid growth and dispersal – compared to native herbs. Introduced herbs and grasses were also more likely to be annual rather than perennial, a key difference in growth period and life history strategy.

Small native shrubs and low, multi-stemmed eucalypt trees known as ‘mallees’ were less frequent in habitats where weeds were diverse or abundant. The pattern was stronger in terms of proportional ground cover, although not as important as the positive association of weeds with native herbs and grasses. While weeds may suppress regeneration from seed in some woody species, our study did not resolve the mechanisms involved.

To interpret our finding that native herbs and grasses respond positively to weeds, we broke the analysis down by individual weed species. This approach seeks patterning within data and is not a definitive experiment that directly tests the impacts of species. Rather, we compared the diversity of natives to the abundance of each weed, while controlling for factors that influence both, such as environmental differences. While a set of species had strong negative associations with native diversity, many weeds had minimal relationships to natives and some had strong positive relationships. This suggests only a subset of weeds impact natives and require management – where abundant, native diversity is heavily reduced.

While we factored land-use into our analysis (only ‘native’ grasslands were surveyed), we were unable to score disturbance such as grazing by managed livestock or wild species such as kangaroos or rabbits. Part of the observed relationships between weeds and natives may be due to shared responses to disturbance intensity, rather than direct cause and effect. Even so, our data show an overall positive coexistence between weeds and natives. Legacy regional surveys provide important information beyond their original purpose of biological inventory. Long-term monitoring of species in their environmental context would provide further insights into the relationships between season, disturbance and native/weed diversity.

This is a plain language summary for the paper of Guerin et al. published in the Applied Vegetation Science. This post was provided by Greg Guerin, who is a Research Fellow at the University of Adelaide, Australia.