One Introduced Species Helping Another
Possum pellet showing seed wasp emergence holes.
Fruit-infesting insects that survive passage through the digestive tract of fruit-eating (frugivorous) vertebrate species are not that common, and there is debate as to whether these plant–insect–disperser ‘triads’ have co-evolved or not.
Most of the recorded ‘triad’ cases are in captive conditions involving birds and mammals, with only few cases reported in the wild. However, in the course of their work in Central Otago, Landcare Research scientists Carlos Rouco and Grant Norbury came across an unusual example of a plant–insect–disperser triad in the wild involving sweet briar (Rosa rubiginosa), a rose seed wasp (Megastigmus aculeatus), and possums − all introduced species in New Zealand.
Sweet briar is an extremely invasive wild rose species affecting dry areas of South Island hill country. Infested land has a greatly reduced stock carrying capacity. This prickly plant was first recorded growing as a garden plant in 1835 and by 1900 had been classified a noxious weed. The seed wasp was probably introduced to New Zealand at the same time as sweet briar. While the wasp can be commonly found attacking sweet briar here, infestation rates are too low to have any noticeable impact on the plant. Biological control for sweet briar was explored in the 1960s but the project was terminated, without releasing any control agents. There were a number of reasons for abandoning the project, including the challenge of finding suitable agents that would not harm the large number of horticulturally important related plants, and the logistics involved in undertaking the necessary safety testing to prove this.
Sweet briar blooms from November to January and the bright red rose hips last from February–March through until August–September. Adult female seed wasps lay eggs inside the hips when the fruit begins development just after petal-fall (late February–May). The wasps overwinter as mature larvae within the seed, and emerge as adults in January–February. Other researchers had suggested that the wasp may survive gut passage through small- to medium-sized mammals, but this had not been recorded in the wild.
As possums in dryland areas are known to like eating rose hips, Carlos and Grant decided to collect possum faecal pellets to assess emergence rates of adult wasps from seeds in the pellets, and compare this with emergence rates from uneaten seeds. “As it takes about 50 hours for the seeds to pass in one end of a possum and out the other we expected this would compromise the viability of the seeds and hence the survival of the wasps,” explained Carlos. By February, 146 adult wasps had emerged from 700 possum pellets that had been collected. A high proportion (88%) of pellets contained rose seeds and 19% of pellets were infested by wasps. By comparison, 42% of unconsumed rose hips were infested by wasps. Nearly 7% of seeds within these fruits were infested, which was significantly higher than in seeds in possum pellets (4.7%). However, contrary to expectations, survival of adult wasps in unconsumed fruit was no higher than in seeds that passed intact through the possums’ digestive system (85%).
Because the wasp has very limited flight capacity, some researchers believed birds eating infested seeds played a vital role in dispersal. In New Zealand, blackbirds are the most widely distributed avian seed disperser but they mainly disperse seeds over short distances of 50–100 m, and only occasionally over longer distances. By comparison, possums in the dryland study area have home ranges up to 54 ha and would therefore potentially spread the wasp much further. However, because of the low prevalence of seed infestation in both fruits and possum pellets, Carlos and Grant believe the wasps are unlikely to reduce the spread of sweet briar.
“Because all three players involved in this triad are introduced species, their interdependence could not have co-evolved; instead the relationship is simply a fortuitous one derived from the wasps’ pre-existing ability to conceal themselves inside seeds and survive vertebrate ingestion and gut passage,” concluded Grant.
Rouco C, Norbury G 2013. An introduced species helping another: dispersal of a rose seed infesting wasp by a marsupial in New Zealand. Biological invasions 15(8): 1649-1652. http://dx.doi.org/10.1007/s10530-013-0415-1