Prey Biomass and Availability as Factors Influencing the Distribution of Pueo

Laura R. Luther*, Chad J. Wilhite, Alba I. Rípodas-Melero, Javier Cotin, Kristen Harmon, Karen Steensma and Melissa R. Price. Department of Natural Resources and Environmental Management, College of Tropical Agriculture and Human Resources, University of Hawai‘i at Mānoa. lrluther@hawaii.edu, www.pueoproject.com

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Introduction, Research Q's & Hypotheses
  • Raptors are limited by prey availability. Many models of predator-prey relationships assume foraging behavior is directly related to prey density (1); however, habitat characteristics may impact the effort required of raptors to detect and capture prey, thus impacting their distribution (2,3).
  • The Hawaiian Short-eared Owl, or Pueo (Asio flammeus sandwichensis) is a subspecies of the globally distributed Short-eared Owl and the only native raptor present across the Hawaiian Islands (7).
  • Pueo are more generalist in their diet (Figure 1) and use a wider range of habitats than continental owls; however, it it is unclear how prey biomass and habitat characteristics impact Pueo distribution  (Figure 2)(4,5,6).
  • Considered an ancestral guardian and once common across the Islands, the Pueo is currently state-listed as endangered on O‘ahu, with declines attributed to habitat fragmentation and nest predation (9). 
  • The objective of this study is to examine factors that impact Pueo distribution across the island of Maui.
Figure 1. Pueo pellet content on Hawai‘i, O‘ahu, Lānaʻi  and Kaua‘i (4).

Research Questions & Hypotheses:

  • Q1: Is Pueo occupancy driven by prey biomass or habitat structure? 
  • H1: Pueo distribution will be more highly correlated with vegetation structure, than with prey biomass.
  • Q2: How does habitat structure impact prey biomass?
  • H2: Prey biomass will be lower in habitats with shorter, less dense vegetation.
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Biomass Pyramid & Methods
Figure 2. Potential biomass pyramid. 
Figure 3. Random-stratified site selection, Maui island, HI.  

Prey species surveys

  • Rodent surveys. 50 Sherman traps covering 5 x 125 m area using mark and recapture techniques.
  • Invertebrate surveys. Insect sweep net in 10 x 10 m area.
  • Avian surveys. Audio/visual 1 hour before sunset to 30 minutes after, recording all species within 200 m.
  • Habitat characteristics (vegetation height and cover) collected for each point.

Data Analysis 

  • Pearson's t-test to determine the relationship between Pueo presence and (a) total prey biomass, (b) vegetation height.
  • One-way ANOVA to determine relationship between prey biomass and habitat-type. 
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Preliminary Results
  • Pueo were observed in habitats that contained low vegetation height (avg 37 cm), and medium vegetation cover (avg=80%).
  • Total prey biomass was not significantly different (p=0.32) between locations with and without detections of Pueo.
  • Vegetation height (p=0.28) and vegetation cover (p=0.31) were not significantly different between surveys with and without detections of Pueo.
  • Prey biomass was significantly different among habitat types (p=0.03). Pueo were observed in shrubland and pasture habitats which contained low prey biomass.
  • Owl presence in relation to elevation was significant (p=0.09). Barn Owls (Tyto alba) were observed at lower elevations (average = 390 m), while Pueo were observed at higher elevation sites (average= 1400 m).

 

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Conclusion & Next Steps

Conclusion:

  • Pueo were only observed in pasture and shrubland habitats which contained low prey biomass, low vegetation height, and medium vegetation cover, thus habitat structure may play a greater role in predicting Pueo presence than prey biomass. 
  • Anecdotal observations suggest competition with Barn Owls may influence Pueo distribution considering they were observed at different elevations.
  • More data is necessary to explore these relationships. 

Next Steps:

  • Pueo presence and prey biomass across habitat types by season.
  • Pueo presence in relation to individual prey types.
  • Barn Owl presence in relation to prey biomass and vegetation characteristics.
  • More complex models to predict Pueo occupancy.
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References
  1. Korpimaki, E. 1984. “Population dynamics of birds of prey in relation to fluctuations in small mammal populations in western Finland.” Ann. Zool. Fenn. 21: 287-293. 
  2. Bechard, M. J. 1982. Effect of vegetative cover on foraging site selection by Swainson's Hawk. Condor, 153-159. 
  3. Preston, C. R. 1990. Distribution of raptor foraging in relation to prey biomass and habitat structure. Condor, 107-112.  
  4. Mostello, C. S. and S. Conant. 2016. Diets of native and introduced apex predators in Hawai‘i. Pacific Conservation Biology 24:25-34. 
  5. Cotin, J., A. Siddiqi, J. Misaki and M. R. Price. 2018. The pueo project final report, April 2017 – March 2018. Hawai‘i Division of Forestry and Wildlife, Honolulu, HI, USA. 
  6. East Maui Watershed Partnership personal communication.  
  7. Booms, T. L., G. L. Holroyd, M. A. Gahbauer, H. E. Trefry, D. A. Wiggins, D. W. Holt, J. A. Johnson, S. B. Lewis, M. D. Larson, K. L. Keyes  and S. Swengel. 2014. Assessing the status and conservation of the short-eared owl in North America. The Journal of Wildlife  Management 78(5):772-778.  
  8. Work, W. M., and J. Hale. 1996. Causes of owl mortality in Hawaii, 1992 to 1994. Journal of Wildlife Disease 32:266-273. 
  9. Hawai’i Department of Land and Natural Resources [HDLNR]. 2005. DLNR fact sheet: Pueo or Hawaiian short-eared owl Asio flammeus  sandwichensis. Hawaii’s Comprehensive Wildlife Conservation Strategy, Honolulu, USA. 
  10. Tomich, P. Q. 1962. Notes on the Barn Owl in Hawaii. 'Elepaio 23: 16-17.
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Acknowledgements: I would like to thank our partnerships through the Western Sustainable Agriculture Research Association (WSARE) for making this project possible. I thank Haleakalā Ranch, Haleakalā National Park, 'Ulupalakua Ranch, Kaonoulu Ranch, Mahi Pono and Hawai‘i Department of Land and Natural Resources for access to their land and support of the project. I thank Hawai‘i Wildlife Ecology Lab members Chad Wilhite, Kristen Harmon, Alba Rípodas, Derek Risch, and Javier Cotin for their assistance. I would also like to thank Tom Kualii and Darcy Fiero for their photographs of Pueo.