Friday, 19 October 2007

Belligerent newcomers and shrinking wings - Carabid Ecology on Maui

To make use of an allusion from literature, things are rotten in the Hawaiian Islands. Like many places, they are being overrun by Argentine ant (Linepithema humile) (Hymenoptera: Formicidae). Coupled with this, they also have a carabid beetle, Trechus obtusus that is running amok over the island of Maui. Stuck in the middle are a number of endemic carabids in the genus Mecyclothorax.

James Liebherr and Paul Krushelnycky have published a paper detailing the plight of the Mecyclothorax over the past seven years, and it makes for interesting reading. In a nutshell: Trechus obtusus was first found on the island of Maui in 1999 (sounds familiar…) and increased dramatically, such that a 4m2 collection of leaf litter in 2001 had 77 beetles in it. The numbers of Mecyclothorax since that time have had a statistically significant decline since then, while T. obtusus numbers remain approximately the same. The presence of ants is also an important predictor in whether or not Mecyclothorax is present in an area or not. The numbers of Mecyclothorax are significantly lower in ant-infested areas, while the abundance T. obtusus is not significantly affected by Argentine ants. It looks grim for Mecyclothorax, but the authors do not seem to express any pessimism about the future of the beetles.

Another interesting aspect about the system is how Trechus obtusus is losing its wings. T. obtusus has two forms - a fully winged (macropterous) form that can fly around, and a short winged (brachypterous) form whose wings reach halfway down the abdomen and is flightless. There has been a very rapid change in the proportion of wing forms in populations of T. obtusus on Maui. In one site, 0% of beetles collected in 2001 had short wings. Four years later, in 2005 15% of beetles collected were brachypterous. At another site the proportions have increased from 0% brachyptery to 18% in three years. This change has a number of possible consequences. It is hoped that the decrease in numbers of flighted individuals will help prevent T. obtusus from spreading to other Hawaiian Islands. However, this does not prevent it from invading through human transportation, which is the most likely way it got to Maui in the first place. Second, the dimorphism of T. obtusus will no doubt help it spread around Maui to a much greater extent, and form more stable populations, than the endemic Mecyclothorax which are exclusively brachypterous.

Wing dimorphism is considered to be determined by a single allele. Individuals with full wings are thought to be homozygous recessive, while shorter wings can be heterozygous or homozygous dominant. While the authors suggest that all of the first beetles to arrive were macropterous (i.e. homozygous recessive), this would seem to indicate that there would be no alleles for brachyptery in the population. The situation here probably suggests that wing size in carabids is slightly more complicated than classical Mendelian genetics, and that more than one allele influences the size of wings in Trechus obtusus.

The influence of Argentine ant on the populations of carabid beetles is also pretty scary. Argentine ant is one of the world's worst invasive ant species and it is established in Hawaii, New Zealand and Australia. It is not found elsewhere in the Pacific. It is one of the species on the Pacific Ant Prevention Plan and it is hoped that it doesn't get established. The carabids elsewhere in the Pacific are not well known, and their response to Argentine ant invasion would probably be similar to the situation in Hawaii.

The globalisation of biodiversity has had a huge impact on native ecosystems. It looks set to continue, despite advances in biosecurity policy and techniques. It looks gloomy, and to an extent it is. However, nature has survived this long, and will continue to get by, though in a slightly more impoverished state. Another aspect of invasions that I haven't looked up yet is the beneficial side, where new organisms enrich and add to the indigenous ecosystem. Does it happen? I don't know, and it's not something you hear about. Maybe an idea for someone?

Liebherr, JK; Krushelnycky PD. 2007.
Unfortunate encounters? Novel interactions of native Mecyclothorax, alien Trechus obtusus (Coleoptera: Carabidae), and Argentine ant (Linepithema humile, Hymenoptera: Formicidae) acress a Hawaiian landscape.
Journal of Insect Conservation 11:61-73

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