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Background

Background

Alpine scree wetas (Deinacridaconnectens) live above the tree line in the South Island of New Zealand. The alpine scree weta issexually dimorphic with females being larger than males. Alpine wetas are known to consume fleshy fruit and disperse the fruit seeds after digestion. The effectiveness of this mutualistic dispersal is under debate. The scree weta’s ability to disperse seeds was tested with the consumption of mountain snowberries (Gaultheria depressaEricaceae), which produce fleshy fruit (Average seed number ~ 279.67± 12.64 SE). There is much debate over whether the alpine weta is a seed disperser or a seed predator.

Seed dispersal effectiveness increases with body size in New Zealand alpine scree weta (Deinacridaconnectens)By Hannah Larsen and Kevin C. Burns

Aim

To determine whether Alpine scree weta (Deinacridaconnectens) are seed dispersers (Mutualistic) or seed predators of mountain snowberries (Gaultheria depressaEricaceae) in New Zealand.

Hypothesis

Alpine scree weta will act as mutualistic seed dispersers (for the mountain snowberry) when large (e.g. adults) and act more as a seed predators when small (e.g. juveniles).

Methods

15 individuals were captured on scree slopes above the tree line in the Nelson lakes National Park on Mt Robert at ~ 1450m above sea level. Upon capture the individuals were sexed and then measured for weight and length. Individuals were then placed in containers and transported back to the laboratory at the base of the mountain.

Field Observations

Any scat that could be identified as coming from a scree weta in the field was gathered and analyzed for the amount of snowberry seeds that had passed through the gut. Captured wetas were subjected to a diet of carrot for 3 days to allow all snowberry seeds to pass through the gut before experimentation.

Consumption

All captured wetas were given 3 randomly selected snowberries and the resulting consumption of the berries was monitored over 48hours. Each individual weta was monitored every 4 hours for the degree of fruit consumption. Every berry was categorized based on amount consumed by the weta (> 90%, 50 – 90% and 1 – 50% consumed).

Seed Passage

All captured wetas were given a small block of cheese (~ 3mm in diameter) containing a known number of snowberry seeds. After ingestion, the subsequent scat was analyzed to determine the amount of viable seeds. To determine the viability of the seeds, 20 were randomly selected and were subjected to a staining procedure with 2,3,5-triphenyl tetrazolium chloride (for detailed description of technique read de Vega et al., 2011).

Movement

Captured individuals were placed in an arena and their movements were monitored for 30 seconds. Distances travelled were measured with a flexible tape measure and were averaged over 3 consecutive nights between 3 - 4am.


Background

Results

ConclusionWeta show ontogenetic shifts from seed predators at a small size (e.g. young) to mutualistic seed dispersers of the mountain snowberryat the larger adult sizes. There was much variation among the individuals in effectiveness of dispersal results suggesting an increase in effectiveness of dispersal with increased size.

  • High correlation of body size to weight (R2=0.95 P< 0.001)

  • Total of 1945 intact seeds were found in the scat of captured wetas. A high variation among individuals was seen.

  • Field scat collection: 40 individual scat found, highly variable in numbers of seeds found in scat, 20 scat held < 50 seeds.

  • A) Consumption: Overall average of 2 snowberries consumed per night (increased with size of individuals R2=0.834 P < 0.001)

  • B) Passage: Average of 31.95% ± 7.73 SE passed through gut intact (increased with size R2=0.609 P < 0.001)

  • C) Seed dispersal vs. body length: Positive correlation R2=0.594 P < 0.001, Average 251.5± 107.8 SE seeds per night.

  • Distance moved: Increased for large wetas (R2=0.598 P < 0.001)

Fig. 1. Relationships between the body size of 15 scree weta and (A) the average number of snowberries consumed per night, (B) proportion of consumed seeds that were passed intact, and (C) extrapolated numbers of seeds dispersed by scree weta per night (nightly fruit consumption rates ¥ average number of seeds per fruit ¥ proportion of seeds passed intact).

Fig. 2. Relationship between scree weta body size and the average distance (m) weta moved per minute under laboratory conditions.

  • Reference and recommended readings

  • de Vega, C., M. Arista, P. L. Ortiz, C. M. Herrera, and S. Talavera. 2011. Endozoochory by beetles: a novel seed dispersal mechanism. Annals of Botany 107: 629-637.

  • King, P., L. Milicich, and K. Burns. 2011. Body size determines rates of seed dispersal by giant king crickets. PopulEcol 53: 73-80.

  • Wyman, T. E., S. A. Trewick, M. Morgan-Richards, and A. D. L. Noble. 2011. Mutualism or opportunism? Tree fuchsia (Fuchsia excorticata) and tree weta (Hemideina) interactions. Austral Ecology 36: 261-268.

Speed Paper Synopsis

By Nicholas Scott