Birds vs. Clams: Assessing the impacts of South Island pied oystercatcher predation on Toheroa at Ripiro Beach, New Zealand
Vallyon, L.-M. D. (2020). Birds vs. Clams: Assessing the impacts of South Island pied oystercatcher predation on Toheroa at Ripiro Beach, New Zealand (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13647
Permanent Research Commons link: https://hdl.handle.net/10289/13647
Managing the recovery of interacting species, such as predator and prey, is one of the most challenging factors of an ecosystem-based conservation approach. The actions taken to protect one species may be in conflict with the actions necessary to protect another. For example, the recovery of a predator in an ecosystem can lead to a significant conservation conflict between the protected predator and its protected prey. In these instances, research is required to determine whether a perceived conflict is in fact happening in order to inform proper management decisions. In New Zealand, a conservation conflict exists between an endemic, recovering shorebird, the South Island pied oystercatcher (Haematopus ostralegus finschi), and an endemic surf clam, the toheroa (Paphies ventricosa) at Ripiro Beach, Northland. The toheroa was overharvested until the populations collapsed, with commercial and recreational harvesting bans put in place by the 1970s. The toheroa has continued to decline, and the cause is currently unknown. Oystercatcher predation has been implicated as the reason toheroa populations have not recovered. There have even been calls to cull the birds to protect the clams. Currently, there is no information that would facilitate conservation management in determining the appropriate action to take. My thesis aimed to investigate the claims surrounding this perceived interspecific conflict. It sought to examine the predator-prey interactions between the two species by answering the following three questions: 1) what are the spatio-temporal associations between oystercatchers and toheroa, 2) what is the composition of the oystercatcher’s diet, and 3) what size toheroa are the birds taking? Bird surveys were conducted to examine the distribution of oystercatchers monthly from March 2019 to February 2020. Oystercatcher foraging behaviour was observed to collect information on prey type and prey location. Feeding holes left by oystercatcher bills at predation sites were examined to determine oystercatcher predation success. A population survey was conducted on toheroa beds exposed to oystercatcher predation versus control beds to assess if there were differences in the density and size structure between toheroa populations. Results did not support the hypothesis that the oystercatchers are the cause behind the limited recovery of the toheroa, but they did indicate that the birds may be having localised impacts. There was limited overlap with toheroa, as the birds were found predominantly at the southern half of Ripiro Beach and were only associated with two major toheroa beds. Observations found that bivalves are a significant food resource for oystercatchers and the birds have a high predation success rate. There was a significant difference in the density and size structure between toheroa populations in beds that were oystercatcher predation sites and those at no-predation sites. However, even in the areas with the most intense oystercatcher predation, toheroa have persisted over time. While there potentially is a small local impact, overall, the South Island pied oystercatcher is not responsible for the continuing decline of the toheroa across the whole of Ripiro Beach and are not causing devastation as claimed. The perceived conservation conflict between the two endemic species is likely incorrect. This research demonstrates the importance of acquiring information on interacting species prior to management action to protect one of those species. Information presented in this thesis can be used to support conservation managers in making informed decisions to protect both the toheroa and the South Island pied oystercatcher.
The University of Waikato
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- Masters Degree Theses