Predictors of Wild Turkey (Meleagris gallopavo) Population Change in California from 1972 to Riva T. Madan ABSTRACT

Transcription

1 Predictors of Wild Turkey (Meleagris gallopavo) Population Change in California from 1972 to 2013 Riva T. Madan ABSTRACT Wild turkeys never lived in California until humans began introducing them around The success of this introduced species was variable at first, but in the 1990s there was a noticeable increase in their population and range. The reasons behind what drove these population changes are unknown, especially with only a few studies on wild turkeys in California. With Breeding Bird Survey, National Land Cover Database, PRISM Climate Group, and California Department of Fish and Wildlife data, I used a zero-inflated poisson mixed model to determine the influence of climate, translocations, hunting, and land cover on wild turkey population change. I ran a separate model for three land cover distances. I calculated population change statewide and at individual routes using a generalized linear model. Wild turkey populations increased 10% per year statewide from 1972 to Hunting was the most influential predictor of population change showing that populations are increasing even as hunting increases. Land cover was a significant predictor, but the effects of different land cover types varied with the scale looked at. Urban land cover was positively related to population change only at 1 and 10 km distances. At the 5 km distance, forest, grassland, and agriculture were inversely related to population change. This study suggests increasing urban land cover, which may increase food supply, can increase wild turkey populations. However, the results also suggest that other factors that are not included in this model have large influences on wild turkey population change. KEYWORDS introduced species, Breeding Bird Survey, zero inflated poisson mixed model, land cover change, ArcGIS 1

2 INTRODUCTION Humans often accidentally or intentionally transport species to new locations beyond their natural range. If the species successfully establishes itself, the species becomes an introduced species. At this point their populations may grow very large and spread over a large area. If the introduced species has both expanded in range and is causing ecological impacts, the species is considered invasive (Duncan et al. 2003). Large populations of introduced species can also be destructive to human activities, making the species become a pest (Baldwin et al. 2012). Wild turkeys (Meleagris gallopavo) are an example of an introduced species in California that has expanded in range and increased in population. Although they are potentially a pest, wild turkeys are not considered invasive due to lack of research on its ecological impacts. The wild turkey never originally lived in California. Is native range is east of the Mississippi River, in the southwest United States (Arizona, New Mexico, and Texas), and Mexico (Gardner et al. 2004). In the late 1800s, wild turkey populations in their native range declined as a result of deforestation and hunting (Dickson 1992; Barret and Kucera 2005). As a reaction to the fear of turkeys going extinct, turkeys were introduced into every state except Alaska (Dickson 1992). The first record of wild turkey introduction in California was in 1877, when private ranchers introduced the wild turkey on Santa Cruz Island. The next recorded introduction was in 1908 when the California Department of Fish and Wildlife (CDFW) began releasing turkeys for hunting purposes. Since then the CDFW has released several thousands of turkeys throughout California, but many of these introduction attempts failed (Gardner et al. 2004). However, the CDFW noticed an increase in wild turkey population size and range in the 1990s. As a response, they stopped releasing turkeys in 1999 (Gardner et al. 2004). The reasons behind the recent spread and growth of wild turkey populations in California are not well understood, but past studies of wild turkeys in their native range provide knowledge about what effects their population growth. Nesting failure and food availability are two main factors that determine wild turkey population growth. These two factors largely depend on habitat type (Spohr et al. 2004; Lehman et al. 2008; Fuller et al. 2013). The primary reason for nest failure is from predation and the chance of nest predation depends on the density of vegetation. In areas with somewhat dense vegetation, turkeys are able to hide their nests from predators well and therefore reduce nest predation. Open grasslands would have a high amount of predation due to little nest obscurity. However, if the 2

3 vegetation is too dense, such as a dense shrubland, it reduces the ability of a turkey to flee if a predator arrives, thereby increasing predation (Keegan and Crawford 1997; Lehman et al. 2008; Fuller et al. 2013). Urban and agricultural areas have been shown to increase food availability. Wild turkeys are often reliant on agricultural grains and human feeding to maintain large populations when other food sources are low (Burger 1954; Barrett and Kucera 2005). Although urban areas may provide additional food, habitat fragmentation from urbanization has resulted in increased nest predation (Hogrefe et al. 1998; Sphor et al. 2004). Contrastingly, habitat fragmentation between forests, open areas and agriculture supports large turkey populations (Glennon and Porter 1999). A majority of these studies on factors affecting population growth took place in several states in the wild turkey s native range, but not in California. There have only been four studies (Burger 1954; Gardner et al. 2004; Barret and Kucera 2005; Wengert et al. 2009) on wild turkeys in California and three of the four studies are outdated by at least a decade. Land use changes increasing habitat fragmentation, urbanization and agriculture bring wild turkeys closer to humans and make them more likely to become pests. Wild turkeys were shown to damage twenty-three crops, with corn being the most often damaged crop (Tefft et al. 2005). Additionally, residents of suburban areas have complained about large numbers of turkeys being a nuisance on their property (Barrett and Kucera 2005). However, no study has analyzed whether these land use types have encroached on wild turkey habitat or whether wild turkeys have moved from their original release sites to these land use types. Besides anecdotal and observational information, there has not been any studies looking at how the population and range of wild turkeys has changed in California and the possible factors driving the changes. To better understand wild turkeys in California and as a step towards determining if they are pests or invasive species, this study maps and analyzes the changes in population and range to explain the reasons behind the recent spread and growth of wild turkey populations. Climate and human involvement, such as hunting and translocation, have often influenced population changes of species, but I hypothesize that land cover, specifically agriculture and urbanization, had the greatest influence in increasing wild turkey populations in California. To see whether wild turkey populations are actually increasing, I quantified how wild turkey population changed and mapped how range changed in California from 1968 to To determine what factors could have led to the changes in population, I ran a model to determine the influence of land cover, climate, and human involvement on wild turkey population changes. 3

4 Study system METHODS The sites I looked at were predetermined by the USGS Patuxent Wildlife Research Center s North American Breeding Bird Survey that carries out bird surveys to determine presence and count of many bird species, one of which being the wild turkey. Wild turkeys are generalists; they can live in many different types of habitats, such as forests, shrublands, grasslands, woodlands and around agriculture and urban areas (Dickson 1992). Each study site is surrounded by a combination of these land types. Percentages of each land type: grassland, shrubland, forest, agriculture, and urban, are given for each site with wild turkeys present in Appendix A. Data sources I obtained georeferenced wild turkey population data from the North American Breeding Bird Survey (BBS) to determine population change. The BBS conducts point count surveys along 40 km long predetermined, non-random roadsides called routes (Figure 1). Citizens skilled in avian identification conduct surveys between mid-may and early July. Beginning a half hour before sunrise, the surveys, conducted by one observer, take 4 to 5 hours to complete. Point counts are taken every 0.8 km along the route, for a total of 50 stops. The point count is conducted by recording every bird seen or heard within a 400 m radius over a 3 minute timespan (Link and Sauer 2002; Sauer and Link 2011). The USGS Patuxent Wildlife Research Center has been conducting the BBS since 1966, but surveys in California began in I only included data beginning in 1972 because I originally planned on using LANDSAT satellite imagery that began in 1972 for land cover type, but the classification was too inaccurate to use. Only one detection of a wild turkey in 1969 was left out from my analysis. Although the BBS is conducted annually, not every route is surveyed each year. Turkeys have been observed in 80 routes in California between 1972 and I only included 76 routes because there was no geospatial information on the path of these four routes. 4

5 Figure 1: Routes with and without wild turkeys present I obtained wild turkey translocation and hunting data in California from the California Department of Fish and Wildlife (CDFW). Translocation data included translocation within California and between California and another state. Translocation data is from 1959 to 1999 with the year 1998 missing. For hunting data, CDFW calculated the amount of turkeys hunted by extrapolating the results from surveys. Mail survey forms were sent out to randomly selected hunters until approximately 4% of the hunters returned the survey. CDFW has not documented the specific methodology of the extrapolation. Data spans from 1949 to 2010, with the year 2009 missing. However, there was still hunting in 2009 and after Instead of leaving zero values indicating no hunting, which would be incorrect, I used the same data from 2010 for years 2009 to This was reasonable given that the counties where wild turkeys were hunted had very little variation since Both of these datasets are only specific to county, but routes are at a smaller scale and sometimes span two counties. Therefore it would be incorrect to directly assign each route a specific value for this data. To account for this inaccuracy, I only included hunting and translocation as binary data to show whether it occurred in the county the route in. I assigned a 5

6 county to each route based on which county the majority of the route was located in. For a few routes that were about evenly split between counties, I combined both of the county s data. To obtain land cover data, I used USGS National Land Cover Database (NLCD) for 1992, 2001, 2006 and I only looked at five land cover classes: forest, urban, grassland, agriculture and shrubland. Because these datasets contain more than just five land cover classes, I combined similar land cover classes together. For example, I included both evergreen forest, mixed forest, and deciduous forest under the forest land cover type. I calculated percentage of land cover within three buffer distances from the route path: 1, 5 and 10 km. Wild turkey flock home range varies from 1.5 to 10 km 2, therefore I will use multiple distances to account for the various home range sizes as well as to see if habitat has different effects at different scales (Zeiner et al. 1990; Gardner et al. 2004; Barrett and Kucera 2005). For years that didn t have a given NLCD, I used the same land cover values from the next available dataset with the assumption that the land cover didn t change very much between these years. Years prior to and including 1992 had land cover values from the 1992 dataset, years 1993 to 2001 had values from the 2001 dataset, years 2002 to 2006 had values from the 2006 dataset and years 2007 to 2013 had values from the 2011 dataset. For climate data, I used temperature and precipitation raster images from Oregon State University PRISM Climate Group. To obtain average temperature and precipitation for each route, I used ArcGIS v10.2 (ERSI 2014) to average the temperature and precipitation within 10 km from each route path. I only used one buffer distance for climate variables because it is unlikely that the climate will vary significantly under 10km from the route. I squared the average temperature and precipitation because animals tend to have a quadratic relationship to temperature and precipitation rather than linear. Data Analysis To look at range change, I used kriging, an interpolation technique, in ArcGIS v10.2 to determine wild turkeys range in California for 6 year intervals. To quantify population change of wild turkeys at each route and California overall from 1972 to 2013, I used a generalized linear model (GLM) in R (R Core Team 2014) v3.1.2 using the glm package. I did not run a GLM on routes that only contained one non-zero data point and I excluded the results of routes where the generalized linear model didn t converge due to lack of data. To see the effect of different 6

7 predictors on population change, I used a zero-inflated poisson mixed model in R v3.1.2 using the glmmadmb package on all 77 routes. I included the percentage of land cover type, temperature, precipitation, the occurrence of wild turkeys translocation and hunting as covariates. Route was included as a random effect to account for differences between sites that cannot be explained by the covariates. I standardized all values, excluding binary and percent values, by subtracting the mean and dividing by the standard deviation to get more comparable covariate estimates. I ran a separate zero-inflated model for each of the 3 land cover buffer distances. RESULTS Population and Range Change Wild turkeys throughout California increased in population 10.09% (±0.279%) from 1972 to 2013, according to the generalized linear model (p < ). Different areas in California experienced various amounts of population change (Figure 2). Majority of routes experienced less than a 25% increase in population. More specifically, 31 out of the 61 routes included for this analysis had an increase between 9% and 20% Route 416 near Meadow Valley in Plumas county experienced the greatest population change of 108% per year, but was statistically insignificant (p = ). Route 210 in Mendocino County and route 12 in Sutter County had increases of 88% and 67%, respectively, but were also not statistically significant. Route 172, located east of Berkeley in Contra Costa County, had the greatest statistically significant increase of 39% per year (p < 0.001). Route 422 located south of Yosemite National Park had the next highest significant increase of 30% per year (p < 0.01). Route 415 near Crescent Mills in Plumas County and route 202 in Sonoma County had a significant increase of about 28% and 27%, respectively (p < 0.5). Four routes had a decrease in wild turkey population, but only one route had a significant decrease in population. This route, route 409 located between Burnt Ranch and Hyampom in Trinity County, decreased in population of 81% per year (p < 0.05). The percent population change per year, standard errors and p-values for each route is given in Appendix B. Range expansion can be seen through a significant (p < ) increase of 0.266% (±0.035%) per year in the proportion of wild turkey detected on BBS routes from 1969 to 2013 (Figure 3). This is further supported by kriging maps of six year intervals which show wild turkey 7

8 range, indicated in blue, to be increasing (Figure 4). Based on the maps, the largest expansion was between 1984 to 1989 interval and the 1990 to 1995 interval. After 1990, the population, indicated by the darkness of blue, increased more than the range changed. From 1972 to 1989, turkeys increased 24.8% (±4.56%) per year. However, this increase was actually insignificant (p = 0.489). In the following period, 1990 to 2013, turkey populations increased by 91.3% (±4.56%) per year (p < ). Given that there was no significant increase in population before 1990 and that the kriging maps show very little change in population and range, I decided to only include years 1990 to 2013 when running the zero-inflated poisson model. Additionally, because the land cover data only begins in 1992 and all years prior to 1992 would have the same land cover value, the result of the model would be more accurate after removing the years before Figure 2: Geographic distribution of population change. Percent population change, calculated by the generalized linear model, of wild turkeys from 1972 to Routes with significant population change are indicated by a black dot. 8

9 Percentage Riva T. Madan Wild Turkey Population Change Spring Year Figure 3: Wild turkey presence. Proportion of Breeding Bird Survey routes in California where wild turkeys were detected from 1972 to Figure 4: Wild turkey range expansion. I mapped the change in wild turkey range and population from 1972 to 2013 by averaging BBS counts for six year intervals for each route and then used kriging (in ArcGIS). 9

10 Predictors of population change According to the zero-inflated poisson mixed model for years 1990 to 2013, the most significant predictor of wild turkey population change for all buffer distances was hunting (p < 0.01) (Table 1). The estimate for hunting was 0.8 for 1 km buffers, 0.74 for 5 km and 0.73 for 10 km. Year was the second most significant predictor than hunting, but still very influential with estimates of 0.74 for 1 km, 0.78 for 5 km and 0.75 for 10 km. All other factors were much less influential predictors with estimates of less than 0.1. Precipitation, temperature, translocation and grassland were not significant for any of the buffer distances. The 95% confidence intervals included zero, making the effect these covariates neither positive nor negative. Different factors were significant and had various influences on population change at each buffer distances. For the 1 km buffer, the percent of urban land cover was significant (p < 0.01) with an estimate of This indicates that areas with increasing urban land cover will experience increasing population sizes, given that all other factors are held constant. Similarly, for the 10 km buffer, urban land cover was also significant (p < 0.05) and positively related to population change (estimate of 0.047). For the 5 km buffer, forest, shrubland and agricultural land cover were significant covariates, but urban land cover was not (p = 0.078). Forest, shrubland and agriculture were all inversely related to population change. Forest, with an estimate of was more of an influential predictor than shrubland and agriculture, both with an estimate of A negative estimate indicates that areas that are decreasing with this land cover type have an increasing wild turkey population. Table 1: Parameter values for the 3 buffer distances. Summary of each parameter s influence on wild turkey populations according to the zero-inflated mixed model. Significant p-values are indicated with *. Parameter Estimate Standard Error 95% CI p-value 1 km buffer Intercept Year <2e-16 * Precipitation

11 Temperature Translocation Hunting * Urban * Forest Grassland Shrubland Agriculture Random effect: Variance Standard Deviation Route AIC: km buffer Intercept Year <2e-16 * Precipitation Temperature Translocation Hunting * Urban Forest * Grassland Shrubland * Agriculture * Random effect: Variance Standard Deviation AIC: 3576 Route km buffer Intercept Year <2e-16 * Precipitation Temperature Translocation Hunting * 11

12 Urban * Forest Grassland Shrubland Agriculture Random effect: Variance Standard Deviation Route AIC: 3753 Land Cover Most wild turkeys were found on routes with majority of forest land cover type. Second was grassland and then shrubland. For 1 km and 10 km buffers, wild turkeys were common in land cover types in that same order, but for 5 km the routes with more turkeys tended to be majority grassland, instead of forest, and followed by shrubland instead of grassland. To get a perspective of how land cover is changing and predict how wild turkeys should respond, I used linear regression to calculate the overall change in each land cover type at each buffer distance. Urban and shrubland both significantly increased for all buffer distances. Forest and grassland significantly decreased for only the 5 km buffer. Agriculture had no significant changes, but showed decreases in land cover except for the 10 km, which showed a very small increase (0.002). All the estimates for agriculture were very small, compared to forest, urban, grassland and shrub land cover types changes that ranged from 20% to over 40%. 12

13 Number of Turkeys Riva T. Madan Wild Turkey Population Change Spring km 5 km 10 km Forest Grassland Shrub Urban Agriculture Land Cover Type Figure 5: Wild turkey habitat preference. The number of turkeys found in each land cover type based on the route s majority land cover type. Table 2: Change in land cover. Using linear regression, I calculated the change for each land cover type at each buffer distance using values from 1992, 2001, 2006 and Significant p-values are indicated with *. Land Cover Buffer Estimate Std. Error p-value Distance Urban 1 km * 5 km E-06 * 10 km E-08 * Shrub 1 km * 5 km * 10 km * Forest 1 km km * 10 km Grassland 1 km km * 10 km Agriculture 1 km km km

14 DISCUSSION Wild turkeys have significantly increased in population by 10% per year since 1972, which is consistent with anecdotes and observations of turkey increases. As hypothesized, urban land cover had a significant factor influencing population change, and climate and translocation were not significant. Hunting, which I did not hypothesize to be significant, was the most influential factor. Additionally, I expected there to be similar results at each buffer distance or if there was a difference, it would be the 10 km distance because it could be too large of an area to accurately predict turkey population change. Although I expected agriculture to be significant, it was surprising that agriculture, forest and grassland were all inversely related to population change, but only at the 5 km distance. Urban land cover was only significant at 1 and 10 km distances. These differences likely indicate that habitat effects wild turkeys differently at the local and landscape scale. Given that the results of the model indicate year as the second most influential factor, the factors I included in the model are probably not enough to explain why wild turkey populations have increased. Population and Range Change Wild turkey population increase is not centralized in one specific region, but the rate of increase is similar throughout California. Besides Plumas County, all routes with over 20% increase in population were in different counties. The fairly large increase of 28% per year for route 202 in Sonoma County is consistent with Barret and Kucera's research in Annadel State park located less than 10 km from the route (2005). The wild turkey populations grew rapidly in Sonoma because there were no limiting factors for reproduction, such as lack of food. The food supply was actually increased due to humans intentionally and unintentionally feeding turkeys high quality commercial feed. High food quality can then lead to increased reproduction. With a high nutrition diet, female turkeys tend to lay more eggs and lay eggs earlier in the breeding season, allowing for attempts at nesting if one fails (Dickson 1992). Adding to that, wild turkeys already have a large reproductive capacity. They start reproducing at the age of one and each year they can produce a clutch size, the number of eggs laid in a single nesting, between 10 and 12. And if their first nesting fails, they often nest a second time (Lehmen et al. 2008). Route 172, with the largest significant 14

15 increase of 39% per year, had the largest amount of urban area for the 5 and 10 km distances and fourth largest amount at 1 km. Route 172 probably had similar reasons for population increase as route 202 in Sonoma county. The large amount of urban area increases the likelihood of wild turkeys getting additional food from humans. All routes increased in urban land cover. However, urban land cover did not seem to increase by such a large percent where it seemed to be encroaching on natural turkey habitat. Wild turkeys were probably either introduced near urban areas or spread there due to the availability of increased food supply and quality. Therefore, it is more likely that turkeys moved into areas with humans, rather than humans moving into areas with turkeys. Surprisingly, many turkeys are not found in routes with a majority of agriculture or urban land cover, especially considering urban land cover was a significant influence on population change (Figure 5). This is probably because CDFW didn t introduce them into areas that are majority urban or agriculture, but some turkeys spread to these locations. Route 409, which declined in population, also increased in urban land cover and was not very different from other routes in terms of land cover composition and change. I suspect that the change at this route is not representative of the change in that area. However, routes in that area did have much smaller population changes of less than 5% increase per year. This is likely do to later introductions by CDFW in this area according to translocation data (Gardner et al. 2004). Insignificant results and non-convergent GLMs from calculating population change at each route is probably due to lack of data. Not every route was surveyed every year and some routes were surveyed as few as 8 to 10 years. Although the results were insignificant or the GLMS did not converge, the trends they show, usually of an increasing population, are probably close to what is actually happening. This is likely because translocation and hunting data often indicated turkeys in counties that the BBS did not detect them. The BBS data is just lacking data to statistically show the trend and is probably underestimating wild turkey populations. Wild turkeys probably also have a larger range than what is shown in kriging maps based of BBS data (Figure 4). Wild turkeys are found more frequently on routes in northern California, with the exception of route 106 in southern California near San Diego. The majority of routes where turkeys were found are primarily in the foothills and mountains of California. This distribution resembles the distribution of CDFW s wild turkey release sites from 1959 to 1999 (Gardner et al. 2004). This makes it unclear whether wild turkeys are found in this pattern because CDFW released them in 15

16 these locations, turkeys have a particular habitat preference, or both. CDFW may have also released them there because studies suggested that these areas are suitable habitat. Consistent with studies of wild turkey habitat preference, turkeys seem to be found in places that are primarily forest, followed by grassland, shrubland or a mixture of these three (Figure 5) (Dickson 1992; Gardner et al. 2004). This consistency shows that wild turkeys have similar preferences and characteristics regardless of the specific location they are studied. This allows us to reliably make inferences about wild turkeys in other states they have not been studied in. Predictors of population change Hunting is the most influential predictor of wild turkey populations, with coefficient values ranging from 0.79 to This positive value indicates that wild turkey populations increased with more hunting. However, it is more likely that relationship is reversed. From looking at the data, the amount of hunting seemed to increase as a response to an increasing wild turkey population. The years that turkeys show up in counties and the year hunting starts in those counties often coincides. For example, in San Diego county turkeys did not show up until 1990 to 1995 and similarly hunting did not start until This was not a result of hunting data being inputted as occurrence data in binary rather than number of turkeys actually hunted, which could show an increase in counties hunted but not an increase in numbers hunted. However, the amount of turkeys hunted in California usually increased every year according to CDFW Game Take Survey Reports. Therefore, this result is actually showing that although hunting is increasing, wild turkey populations are increasing at an even greater rate. The implications of this could be huge, especially if CDFW expects wild turkey populations to stay under control or stop increasing from hunting alone. As predicted, land cover type was an influential predictor of wild turkey population change. The percent of urban area was the only significant land cover factor at both the 1 and 10 km distances. In Sonoma County, urbanization is probably increasing food supply. Additional food sources support larger populations than a habitat would naturally allow (Burger 1954; Barrett and Kucera 2005; Glennon and Porter 1999). In winter and times when vegetation does not grow well, food is scarce, but if wild turkeys are not reliant on natural food sources, the food scarcity won t have any effect on them. Contradictorily, urbanization may increase predation for wild turkeys due 16

17 to the habitat fragmentation it creates and therefore reduce populations (Hogrefe et al. 1998; Sphor et al. 2004). It is likely that this is not a huge influence in many places in California because turkey predator populations are probably low. Looking at a 5 km scale, forest had a negative influence on population change. Forest is one of the wild turkey's primary habitat and also is the most preferred habitat in California, therefore it seems unreasonable that wild turkeys would increase due to a reduction in forest cover (Dickson 1992) (Figure 5). Instead, given that forests are decreasing in general, it is more likely that the negative coefficient could be a coincidental result of the two trends occurring at the same time (Table 2). Similar to the result with hunting, although decreasing forest cover is correlated with an increasing population, one is not the cause of another. This suggests that another factor could be influencing wild turkey population increase. The same could be said of agriculture which decreased overall as well. For shrubland, however, the reason for the inverse relationship with population change could be because dense shrubland is bad habitat for turkeys because it can increase predation as well as limits the mobility of them (Keegan and Crawford 1997; Gardner et al. 2004; Lehman et al. 2008; Fuller et al. 2013). But this is uncertain given that a large amount of turkeys resides on routes with majority shrubland (Figure 5). The effects of land cover type at different scales are surprisingly different and contradictory. The reasons that different factors are significant at different scales is not clear. Looking at the land cover, it seems these results are attributed to land cover percent values because the 1 km and 10 km distances often have very similar values that are different from the 5 km distance. These results show that wild turkeys are effected by habitat at different scales, both local and landscape. The different results could be also indicating that wild turkeys have a specific home range size, making two of the three distances inaccurate. If turkeys have a particular home range, one would expect the results for that distance to best explain the population change. However, the home range of wild turkeys specifically in California is unknown, therefore it is hard to say which of the three distances is probably the most accurate. Also, based on other studies, the home range has a huge range from 1.5 km 2 to 10 km 2 scales (Zeiner et al. 1990; Gardner et al. 2004; Barrett and Kucera 2005). Additionally, it is complicated to compare wild turkeys response to habitat at different scales because turkeys have a huge capacity for range expansion. Hens disperse up to 13 kilometers to find a nest site and individuals can fly up to 64 kilometers in a week (Barrett and Kucera 2005). Habitat fragmentation and edge effects, which were not taken into account in my 17

18 model may also be a cause of these odd results. Looking at the land cover at each routes, many of them did have similar amounts of several land cover types, often about 20%. Wild turkeys are often associated with edges and fragmentation and can have large populations in these landscapes (Glennon and Porter 1999). Temperature and precipitation did not significantly influence population change. It is reasonable that temperature would not be influential because wild turkeys are habitat generalists. In addition, they have a large range that spans the entire United States and successfully survive in various climates (Dickson 1992). Increased precipitation, on the other hand, has been associated with lower populations due to increased predation. Predation increases in wet weather because the wild turkey s scent is more detectable under wet conditions (Palmer et al. 1993, Roberts et al. 1995, Roberts and Porter 1998). However, other studies have shown that the effect of precipitation is more complex (Lehman et al. 2008; Fuller et al. 2013). In addition to the strength of their scent, the probability of predation also depends on the type and density of vegetation cover. A large amount of precipitation over at least a short duration, such as a few days, can increase vegetation density and therefore reduce predation because more vegetation cover decreases detectability from predators. The effect of precipitation is varied such that it could either increase or decrease populations depending on the vegetation type in the area and the amount and duration of precipitation. The possibility of both positive and negative effects could be a reason why the 95% confidence interval for precipitation spanned zero, indicating neither a clear positive nor negative influence on population change. Consequently, precipitation was not a significant predictor of population change. Limitations The biggest limitation of this study is that it relies solely on the accuracy and consistency of secondary data. Breeding Bird Survey data has a specific methodology and is much more consistent than other citizen survey data, such as the Christmas Bird Count. However, it still contains inconsistencies, most notably that not every route is surveyed every year. There is also surveyor error due to their varying experience in observing and identifying birds. In addition, the route locations I used for mapping and obtaining land cover and climate data have low accuracy. According to BBS, if part of the route was not on the map they were digitizing they estimated the 18

19 route s location. Start and end points of the routes were also estimated, which often made the digitized routes longer than the actual routes surveyed. Also, parts of some routes were not mapped out in the shapefile due to traffic noise or overlap with other routes. The routes BBS uses may also cause inaccuracies because they follow a road. Road-based surveys are usually biased because animals are often reluctant to go near a road. Although this hasn t been shown with wild turkeys, it has been shown that a turkey flock is less detectable when near roads and populations are therefore underestimated (Butler et al. 2007). Also, since BBS routes are located on secondary roads, which are primarily in rural areas, trends in more urban areas are hard to detect (Butcher et al. 2014). Additionally, limited number of years for land cover datasets probably lead to less accurate results. The USGS does not advise the 1992 dataset to be combined with the other datasets due to different methods in creating it. I still combined the data because the land cover classes had similar descriptions and did not look too different visually. However, this could still have led to inaccuracies. For example, urban area seems much smaller for all 1992 years. This could be due to the different methods used or due to an actual increase in urban area. The only alternative to the NLCD would have been classifying satellite imagery, but it is not easy when looking at a large area, such as California, and it may not have been any more accurate. Future Research Although vegetation influences the amount of wild turkey predation, the direct effect of predator population was not taken into account in this study. Adding a predator population as another covariate to the model in this study will distinguish whether habitat s effect on population is due more to its ability to reduce or increase predation or to other factors, such as food availability. From this study and the lack of research on wild turkeys in California, the question of whether an increase in wild turkey populations has a significant ecological impact remains unanswered. Like many introduced species, large wild turkey populations have the potential to impact food web dynamics. Invasive and introduced species, including wild turkeys, can influence native species populations through both apparent and direct competition. Apparent competition occurs when growing populations cause their predator population to increase leading to a potential 19

20 decline in population of another species that their predators prey on. Direct competition occurs when a species has to compete for habitat and food resources with a similar species that shares its range. Research into other ground-nesting birds, such as the mountain quail and California quail, would be useful to see if they are experiencing direct or apparent competition from wild turkeys because they have similar predators and diet. The maps of wild turkey population growth can also be used to find localized areas for further research. Areas with the largest population increase can be studied to see if there are any detrimental effects, either ecological impacts or as pests, because the effects would be most noticeable in these areas. Conclusion The wild turkey has been introduced into every state except Alaska as well as several other countries, and in general they have been successful in establishing populations. Besides the wild turkey, there are many species, usually invasive or introduced, that are spreading their range over huge areas. These species tend to be generalists and often do well in urban or agricultural environments, such as the wild turkeys and Eurasian collared doves (Fujasaki et al. 2010). As urbanization and agriculture increases all over the globe, these species have more areas that they can expand in successfully. These land cover changes are creating homogenous landscapes that promote non-native species, the loss of native species and therefore less biodiversity (McKinney 2002). AWKNOWLEDGEMENTS I would like to greatly thank Steve Beissinger for telling me about the Breeding Bird Survey and giving me the suggestion to go about answering my thesis questions with a modelling approach. My thesis would have been very different if I wasn t able to discuss my initial research question with him. He also suggested my mentor, Sarah Maclean, who I am very grateful for helping me with all the little details and problems I ran into, especially discussing analysis approaches. I would like to acknowledge Reginald Barret, who provided me with literature about wild turkeys and who to contact in the California Department of Fish and Wildlife. From the California Department of Fish and Wildlife, I would like to thank Scott 20

21 Gardner and Levi Sousa for providing data and answering my questions on wild turkey management. I would also like to Anne Murray for help with the organization and writing of my thesis. Finally, thank you to Jenny Sholar, Kaela Shiigi and Erik Schmiett for your feedback as well as the rest of my peers for the support through these semesters. REFERENCES Baldwin, R. A., T. P. Salmon, R. H. Schmidt, and R. M. Timm Wildlife pests of California agriculture: Regional variability and subsequent impacts on management. Crop Protection 46: Barrett, R.H. and T.E. Kucera The Wild Turkey in Sonoma County State Parks. California Department of Parks and Recreation. Burger, G. V Wild Turkeys in Central Coastal California. The Condor 56: Butcher, A. J., B. A. Collier, N. J. Silvy, and J. A. Roberson Spatial and temporal patterns of range expansion of white-winged doves in the USA from 1979 to Journal of Biogeography 41: Butler, M. J., W. B. Ballard, M. C. Wallace, and S. J. Demaso Road-Based Surveys for Estimating Wild Turkey Density in the Texas Rolling Plains. Journal of Wildlife Management 71: Dickson, J. G The wild turkey: biology and management. Stackpole Books, Mechanicsburg, PA. Duncan, R. P., T. M. Blackburn, and D. Sol The ecology of bird introductions. Annual Review of Ecology, Evolution, and Systematics 34: Fujisaki, I., E. V. Pearlstine, and F. J. Mazzotti The rapid spread of invasive Eurasian Collared Doves Streptopelia decaocto in the continental USA follows human-altered habitats. Ibis 152: Fuller, A. K., S. M. Spohr, D. J. Harrison, and F. A. Servello Nest survival of wild turkeys Meleagris gallopavo silvestris in a mixed-use landscape: influences at nest-site and patch scales. Wildlife Biology 19: Gardner, S., T. Blankinship, J. Decker Strategic Plan for Wild Turkey Management. California Department of Fish and Game. Wildlife Programs Branch. Sacramento, CA. Glennon, M. J., and W. F. Porter Using satellite imagery to assess landscape-scale habitat for wild turkeys. Wildlife Society Bulletin 27:

22 Hogrefe, T. C., R. H. Yahner, and N. H. Piergallini Depredation of artificial ground nests in a suburban versus a rural landscape. Journal of the Pennsylvania Academy of Science 72:3 6. Keegan, T. W., and J. A. Crawford Brood-rearing habitat use by Rio Grande wild turkeys in Oregon. Great Basin Naturalist 57: Lehman, C. P., M. A. Rumble, L. D. Flake, and D. J. Thompson Merriam's turkey nest survival and factors affecting nest predation by mammals. The Journal of Wildlife Management 72: McKinney, M. L Urbanization as a major cause of biotic homogenization. Biological Conservation 127: Spohr, S. M., F. A. Servello, D. J. Harrison, and D. W. May Survival and reproduction of female wild turkeys in a suburban environment. Northeastern Naturalist 11: Sauer, J. R., and W. A. Link A hierarchical analysis of population change with application to Cerulean Warblers. Ecology 83: Sauer, J. R., and W. A. Link Analysis of the North American breeding bird survey using hierarchical models. The Auk 128: Tefft, B. C., M. A. Gregonis, and R. E. Eriksen Assessment of crop depredation by wild turkeys in the United States and Ontario, Canada. Wildlife Society Bulletin 33: Wengert, G. M., M. W. Gabriel, R. L. Mathis, and T. Hughes Food Habits of Wild Turkeys in National Forests of Northern California and Central Oregon. Western Birds 40: Zeiner, D.C., W.F. Laudenslayer, Jr., K.E. Mayer, and M. White California's Wildlife. Vol. II. California Department of Fish and Game, Sacramento, California. 22

23 APPENDIX A Table A1: Percent land cover at each route for a 1km buffer. These results were calculated using tabulate area function in ArcMap. 1 km Route Year Urban Forest Grassland Shrub Agriculture

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