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1

Metrics of predation: perils of predator-prey ratios

100%
Bowyer R. T., Kie J. G., Person D. K., Monteith K. L.
Acta Theriologica
|
2013
|
tom 58
|
nr 4
We developed an original modeling approach using program Stella® to investigate the usefulness of predator–prey ratios (PPRs) for interpreting top-down and bottom-up forcing on moose Alces alces. We included density-dependent feedbacks for the moose population, allowed K to vary based on amount and quality of available forage for moose, integrated effects of compensatory mortality, and added time lags in wolves Canis lupus tracking the moose population. Modeling scenarios we developed included bottom-up and top-down regulation as predetermined outcomes. We then evaluated whether PPRs would reflect the various combinations of trajectories of predator and prey populations under top-down versus bottom-up regulation. The resulting patterns of PPRs were impossible to disentangle from one another, and did not provide reliable insights into whether top-down or bottom-forcing occurred, especially over short time spans where critical decisions related to management of moose and wolves might be necessary. Only under top-down regulation did PPRs reflect the degree of predation experienced by moose, but in that instance, knowledge of top-down regulation must be known a priori to correctly interpret PPRs. Potential problems with interpreting PPRs include their double-variable nature, which resulted in the failure to reflect patterns of increase and decrease for predators and prey. We suggest that confidence intervals for PPRs be calculated from a binomial, similar to that proposed for sex and age ratios, which should help discourage the inappropriate use of this metric. We caution that the temptation to use PPRs often is irresistible, but their reliability is highly questionable. We provide an alternative method to using PPRs or other predation metrics for determining whether top-down or bottom-up forcing is occurring by adopting an approach based on the physical condition and life-history characteristics of prey.
2

Geospatial analysis of giant liver flukes among moose: effects of white-tailed deer

100%
Peterson W. J., Lankester M. W., Kie J. G., Bowyer R. T.
Acta Theriologica
|
2013
|
tom 58
|
nr 4
Natural infections of giant liver flukes (Fascioloides magna) occur primarily in cervids and bovids. In northeastern North America, a common definitive host for giant liver flukes is the white-tailed deer (Odocoileus virginianus). Giant liver flukes cannot reproduce in moose (Alces alces) and eventually die, but only after causing extensive tissue damage in the liver. We used data on the occurrence of giant liver flukes in adult moose collected between 1972 and 2000 from northeastern Minnesota, USA. These data were recorded by 93 km2 sampling units (square grid of 9.66 km on each side). Sample sizes varied between 0 and 45 adult moose examined per sampling unit. We fitted a second-order global polynomial model to adjust for trends in the occurrence of flukes across the study area, modeled the de-trended data using a circular semi-variogram model, and finally kriged our data, arriving at a predicted response surface for the occurrence of liver flukes in moose. Correlational analyses indicated that the occurrence of liver flukes in moose was influenced more by the density of white-tailed deer based on rates of hunter harvest (r = 0.54) than was the proportion of wetland habitats (r = 0.25). Ordinary least-squares multiple regression (R adj = 0.29, AICc = 795.3) documented a strong relationship between the occurrence of liver flukes in moose and population density of white-tailed deer (p < 0.001) but a weaker relationship for wetland habitats (p = 0.16). A geographically weighted multiple regression produced a stronger relationship (R adj = 0.60, AICc = 765.7). Disease maps, as we developed here, are a useful geospatial tool that has relevance for understanding disease processes in moose that may be extended to other mammals.
3

Population density of Dall's sheep in Alaska: effects of predator harvest?

88%
Mitchell C. D., Chaney R., Aho K., Kie J. G., Bowyer R. T.
Acta Theriologica
|
2015
|
tom 60
|
nr 1
We measured population abundance and density of Dall’s sheep (Ovis dalli) before and during gray wolf (Canis lupus) and coyote (Canis latrans) harvest for 3 years (1998–2000) on two similar and adjacent study sites with treatment (canid harvests) and reference (no canid harvest) in interior Alaska, USA. Between 1998 and 1999, density of Dall’s sheep in the treatment area increased by 0.746 ± 0.163 sheep/km2. Between 1999 and 2000, after a winter with above-normal, crusted snow, density of sheep decreased by 2.1 ± 0.14 sheep/km2. Because of this large decline, sheep density on the treatment area decreased by 1.3 ± 0.08 sheep/km2 over the entire monitoring period. Sheep in the reference area showed no significant change in density between 1998 and 2000. We hypothesize that Dall’s sheep in the treatment area initially benefited from the harvest of wolves and coyotes via reduced predation and predation risk and that resultant high sheep densities coupled with severe winter weather caused a dramatic decline in Dall’s sheep between 1999 and 2000. Thus, this study illustrates the potential consequences of interactions between density-dependent and density-independent factors for the conservation of large mammals, especially those residing at high latitudes and altitudes. These results should serve as a cautionary tale for those wishing to increase ungulate numbers via predator control without regard to other ecological factors, such as the proximity of the prey population to ecological carrying capacity (K).
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