Wolves and Wolf-­‐Birds in a Snowy Climate The Effects of the Wolves and Snowpack on Ravens Zach Gregory 350 14 300 12 250 10 200 8 150 6 100 4 50 2 0 My study area was located Yellowstone Na9onal Park and adjacent Gardner, MT. Raven counts inside the park consisted of breeding bird survey (BBS) counts and raven counts at wolf-­‐killed carcasses. Both counts were performed on a single day in March. BBS counts (n = 86) were taken at 0.5 mile intervals along Grand Loop Rd and NE Entrance Rd in the Northern Range for 3 minute periods each. Wolf-­‐killed carcass counts were performed via airplane or ground observa9on and were maximum es9mates at each site. Raven counts outside the park consisted of counts performed at random loca9ons throughout Gardiner, MT, and counts taken outside of town at the human-­‐hunted bison carcass disposal area. Wolf counts were obtained by USNPS biologists in a focused winter study throughout the month of March. Snowpack depth was gathered from the Mammoth Hot Springs weather sta9on on the Na9onal Clima9c Data Center’s Online Database. Addi9onal wolf metrics such as wolf-­‐killed carcass biomass, carcass count, and number of wolf packs were also gathered by USNPS biologists in March. BBS counts were tested for their reliability in a viewshed analysis study that showed no correla9on between raven count and viewshed obstruc9on due to topographical features. 2010 First, and foremost, I would like to thank my professor, John Marzluff, for his help and oversight throughout the dura9on of my project. I would also like to thank several Yellowstone biologists—Doug Smith, Dan Stahler, and Mao Metz—for providing me with most of the data that was used in my research. Lastly, I would like to thank all of the students from ESRM 459 for their contribu9ons in collec9ng data. Ravens Outside Park Wolf Count Snowpack (inches) Ravens Out Ravens In Stable Greater Fluctua9on 2011 2012 2013 2014 2015 Year Figure 1. Graph shows fluctua9on of variables by year. Result 2 Ravens Within the Park Ravens In 0.525 0.143 -­‐0.276 0.275 0.553 0.099 -­‐0.225 0.314 -­‐0.186 0.362 Snowpack Figure 2. Blue arrows represent zero order correla9on between either wolf count or snowpack and ravens inside the park. Purple arrows represent zero order correla9on between wolf count and snowpack. Green arrows represent par9al correla9on controlling for snowpack and red arrows represent par9al correla9on controlling for wolf count. Values in black show correla9on coefficient and values in red show 1-­‐tailed p-­‐value. Ravens inside the park appear to be most strongly and posi9vely correlated to wolf count, though it is not sta9s9cally significant. Snowpack does not appear to have a significant impact on raven counts within the park, but the weak correla9on does show that an increase in snowpack decreases the amount of ravens. Par9al correla9ons did not strengthen correla9ons between either variable. Snowpack also shows a nega9ve correla9on with wolf count, though it is not sta9s9cally significant. Result 3 Ravens Outside the Park Ravens Out -­‐0.809 0.025 -­‐0.6 0.077 Wolf Count Acknowledgements Ravens Inside Park Ravens inside the park (blue) and wolf count (green) have a coefficient of varia9on of 0.38 and 0.24, respec9vely. Ravens outside the park (red) and snowpack (purple) exhibit more variability with values of 0.71 and 0.99, respec9vely. Discussion 0 2009 Wolf Count Methods Inches Yellowstone Na9onal Park is home to both gray wolves (Canis lupus) and common ravens (Corvus corax). Common ravens, also known as wolf-­‐
birds, are a highly-­‐intelligent, generalist bird species found across the northern hemisphere. As a result of their advanced intelligence, ravens are able obtain food through a variety of strategies. Research reveals that ravens will follow wolves in an effort to scavenge whatever kills a pack may provide as well as implemen9ng similar behaviors in response to human ac9vity (Stahler, Heinrich & Smith 2002). Ravens respond to carrion le` by hunters outside of park boundaries and will supplement their diet with food found in nearby towns (Wilmers, et al. 2003). It is also known that there is an op9mal group size for foraging ravens and that subordinate birds may not have adequate scavenging opportunity when group size is too great (Marzluff & Heinrich 1991). Gray wolves were reintroduced to Yellowstone in 1995 a`er a nearly 70 year absence. As a keystone species within their ecosystem, wolves facilitate the distribu9on of carrion for scavenging species such as the common raven (Wilmers, Stahler, et al. 2003). My objec9ve was to discover how wolves and other factors influence raven abundance in the Greater Yellowstone Ecosystem. I hypothesized that the reintroduc9on of wolves, and their subsequent increase in popula9on, would result in a posi9ve effect on raven abundance within the park and that increased snowpack would result in increased raven abundance within the park because in snowy condi9ons wolves are known to make more frequent kills (Vuce9ch, et al. 2012). Fluctua7on of Variables Over Time Number of Individuals Introduc9on
Result 1 Data Variability 0.857 0.007 -­‐0.225 0.314 0.926 0.004 Snowpack Figure 3. Blue arrows represent zero order correla9on between either wolf count or snowpack and ravens outside the park. Purple arrows represent zero order correla9on between wolf count and snowpack. Green arrows represent par9al correla9on controlling for snowpack and red arrows represent par9al correla9on controlling for wolf count. Values in black show correla9on coefficient and values in red show 1-­‐tailed p-­‐value. Ravens outside of the park are influenced strongly by snowpack. As snowpack increases, the amount of ravens found outside the park increases. Wolf count is nega9vely correlated with raven counts outside of the park, though it is not quite sta9s9cally significant, and as their number increases, ravens outside the park decreases. When controlling for the opposite variable, both wolf count and snowpack became more 9ghtly correlated to raven count. Wolf count especially strengthened with regard to raven count, and became sta9s9cally significant, when controlling for the effects of snow. •  Raven abundance within the park and wolf count are rela9vely stable while raven abundance outside the park and snowpack are more variable. •  Raven abundance within the park is most responsive to wolf count and is less influenced by snowpack. •  Raven abundance outside the park is most responsive to snowpack, but is also influenced by wolf count. Since the wolf popula9on within the park is rela9vely stable, it is difficult to observe no9ceable shi`s in the raven popula9on, which could be why the raven popula9on within the park is much less variable than the popula9on outside the park. With that being said, it appears that there is a stable popula9on of ravens that remain in the park and rely mostly on scavenging opportuni9es created through wolf ac9vity. There also appears to be a large number of ravens that move between the park and human areas depending on the depth of snowpack and amount of wolves in the park. I suggest that when snowpack increases, many ravens seek out the easier, less energe9cally intensive op9on of finding food in town or at the bison carcass disposal site instead of figh9ng for their share of food at a wolf kill. These are likely less dominant birds that have a difficult 9me obtaining their share of carrion within the park over dominant individuals. However, as wolves increase within the park, the effects of snowpack on ravens is tempered as there is likely more carrion available. My study suffers from a small sample (7 years of wolf and raven counts). Con9nuing to study and record yearly data could poten9ally strengthen any correla9ons found in the data. It may also be beneficial to tag and track ravens found inside and outside of the park to monitor what loca9ons they move between and how much 9me is spent at each place. It would also be worth no9ng whether birds at human areas are typically less dominant than those found inside the park to confirm my suspicion that subordinate birds choose to scavenge in town rather than fight for a share of carrion. References Marzluff, J.M. & Heinrich, B. (1991) Foraging by common ravens in the presence and absence of territory holders: an experimental analysis of social foraging." Animal Behaviour, 42.5, 755-­‐70. Stahler, D.R., Heinrich, B. & Smith, D.W. (2002) Common ravens, Corvus corax, preferen9ally associate with gray wolves, Canis lupus, as a foraging strategy in winter. Animal Behavior, 64, 283–290. Vuce9ch, J.A., Huntzinger, B.A., Peterson, R.O., Vuce9ch, L.M., Hammill, J.H. & Beyer, J.E. (2012). Intra-­‐seasonal varia9on in wolf Canis lupus kill rates. Wildlife Biology, 18(3), 235-­‐245. Wilmers, C.C., Crabtree, R.L., Smith, D.W., Murphy, K.M. & Getz, W.M. (2003) Trophic facilita9on by introduced top predators: grey wolf subsidies to scavengers in Yellowstone Na9onal Park. Journal of Animal Ecology, 72, 909-­‐916. Wilmers, C.C., Stahler, D.R., Crabtree, R.L., Smith, D.W. & Getz, W.M. (2003) Resource dispersion and consumer dominance: scavenging at wolf-­‐ and hunter-­‐killed carcasses in Greater Yellowstone, USA. Ecology Leoers, 6, 996-­‐1003.