Enemy release at range edges: do invasive species escape their herbivores as they expand into new areas?
Cane toads had been found to escape their parasites at the range-front in northern Australia, which could have contributed to the evolution of longer legs and greater dispersal distance… and faster spread. A weed species expanding into uncharted territory could likewise temporarily “hide” from its herbivore and pathogen enemies that are present deeper within its range in enemy-free space (particularly species-specialist enemies which could be more nasty). However, whether invasive plants experience this effect has never been tested.
Instead of spending resources to making defences, such as toxic chemicals and being a toughie (i.e. tougher, denser leaves), range-edge plants can spend more on growth, reproduction and dispersal. In this way the range-edge populations can achieve higher fitness, and be even more invasive which could lead to faster range expansion — potentially leading to a self-reinforcing cycle of creation of enemy-free space and high invasiveness.
We tested this idea by measuring and comparing enemy attack on range-edge (Located south of Nowra) and centre plants (Located north Sydney). This was done over two 1-month long snapshots in spring and autumn (when herbivores are most active). These invasive species were used: Cinnamomum camphora, Lantana camara, Ligustrum sinense, Senna pendula.
To measure enemy attack, we tagged perfect/near-perfect mature leaves with small twist wires, and revisited the same leaves after 1 month (once in spring and once in autumn). Leaf area missing or damaged was visually measured (% leaf damaged). We also measured leaf toughness.
Four native plant species were used to control for differences that are due to local environmental conditions (e.g. general background herbivory levels, climate conditions).
% leaf damaged did not differ significantly among range-edge and centre plants. There was a trend (p = 0.1) for lower herbivory in Lantana at the range-edge, where leaf toughness was also lower.
Leaves were generally tougher at the range-edge location for most species (both invasive and native). These seem to reflect local conditions.
The results were not consistent with our hypothesis of enemy release at the range-edge. In this study we have included damage from generalist herbivores as well as those from specialist enemies. Future studies can focus on particular enemy-host pairings.
(2016) Journal of Plant Ecology 9(5):636–647**
**Please refer to the original publication for more information. Available at (free): https://academic.oup.com/jpe/article/9/5/636/2222594/Enemy-release-at-range-edges-do-invasive-species