Turkey Population Growth in California

Wild turkeys did not inhabit California until they were brought over by humans in the 1900’s. In northern California, the subspecies of turkey, Rio Grande, took off and the population saw a dramatic increase over time. Recently, however, studies are minimal and outdated relating to modern day turkey populations.

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It is important to monitor the wild turkey population over time consistently in order to make other claims relating to their impact on California’s natural environment, crop fields, and urban areas. The data shows that there is a steady increase in population over time since the 1970’s. This could be due to many factors, such as optimal food supply and quality nesting grounds, both of which are environmental factors. Although the data shows a steady increase, the data could also be flawed due to lack of supporting results that other studies would provide. Also relating to inconsistency, the procedures taken place to count the turkey population may have been inconsistent across the different locations. A solution to this would be to map a consistent procedure, collect data more consistently across all locations, and more frequently.

Key words: population growth, non-native species, Rio Grande, transect, trend

Introduction:

The wild turkey population is a non-native species to California. In recent years, however, their presence seems to be more prominent than ever. Biologists are questioning whether or not their inhabitants in California is causing ecological damage within California’s native ecosystems. The wild turkey species, the Rio Grande, originally inhabited dry grasslands in Texas, Oklahoma, Kansas, and northeastern Mexico (Garner et al. 2004). As a result of deforestation, their natural habitat was significantly lossed and they were at risk of going extinct in 1800s. In 1877, Turkeys were introduced into every state except Alaska to make sure they would not go extinct (Dickson 1992). The Rio Grande and other subspecies were released into Santa Clara County, but only the Rio Grande population was able to withstand the introduction into this new ecosystem.

In order to understand how the turkey population may be affecting California’s natural ecosystems and urban areas, it is important to understand their population trend over time. This research will be discussing the methodologies used to gather population data, the finalized data collected, how that data can be used to describe population trend over time, and how this research can be useful for further applications.

According to The Journal of Wildlife Management, written in 2008, the wild turkey population size depends on their nesting failure and their food availability. So, depending on environmental factors, the population can either stabilize, decrease, and increase significantly.

The amount of research done on the turkey population is very minimal, with only four studies being done before 2009, and even those are now outdated. It is important to conduct annual research on the turkey population in order to determine if the population is consistent with scientific predictions, which is why this research topic is important.

In general, scientists use transects, which are long straight narrow sections within a given area to make observations. In this case, they would be used to count how many turkeys are found within the transect in order to estimate the population size in a larger area. There are many other methods to conduct this type of research as well. This study will go more in depth into the various methods used to collect data on the wild turkey population, the methodologies used to collect data for this specific experiment and the mathematical approaches that were taken to analyze the given set of data.

Methodology:

To better understand the data retrieved in Riva Madan’s research, it is important to look at their methodology to collecting the data. In Riva Madan’s article, they discuss the population change of turkeys in California from 1972 to 2013. One of the most important sources used was the North American Breeding Bird Survey (BBS). This survey is done every year around mid-May to early July. “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). Madan included in their research 76 of the 80 routes from the year 1972 to 2013.

Another factor included in the gathering of data was taking hunting into account. Hunting data from the California Department of Fish and Wildlife (CDFW) was used. The CDFW collects their data by mailing survey forms to hunters. Around four percent of the hunters respond every year. Even though the data of these mail surveys range from 1949-2010, there was only a significant change in the turkey population starting around 2000. Researcher Riva Madan therefore discarded data from anytime before 2000.

Another approach used to gather accurate data was for Madan to obtain data from five different land covers which include: forest, urban, grassland, agriculture, and shrubland. They used buffer distances of 1, 5, and 10 km to get a diverse range of habitats at different sizes. Lastly, they used temperature and precipitation raster images from Oregon State University PRISM Climate Group. They averaged the temperature and precipitation within 10km of each route. This was to see if temperature or precipitation affected the turkey population in any way.

Results:

Following a generalized linear model, it can be concluded that there was a 10.09 percent increase of the wild turkey population in California from 1972 to 2013. Looking at more specific intervals in the blue maps pictured above, it can be seen the some of the biggest increases in the population happened from 1984 to 1989 and 1990 and 1995.

For land cover, most wild turkeys were found in the forest land cover type. For the 1 and 10 km buffers, turkeys were commonly found in the same land cover type but it was different for the 5 km buffer area. The 5 km buffers prefered grassland instead of forest. How this was calculated was by using a linear regression to find the overall change in land cover type for each buffer area.

Hunting changed the turkey population the most with coefficient values of 0.79 and 0.84. These positive coefficient values mean that the wild turkey population increased in areas with more hunting. Some factors that were not significant to the turkey population were agriculture, forest, and grassland. These were all inversely proportional to the wild turkey population.

Year is the second most determinant factor in the wild turkey population. As the years go on, it can be seen in a linear regression that the turkey population has indeed increased. There is still more research that can be done in further examining the cause as to why the turkey population is on the rise. It is conclusive in this study that the overall wild turkey population in California is increasing and is predicted to continue increasing.

Discussion:

Overall, the data shows that the Rio Grande turkey population in California has been on the rise since 2013. On average around California, the population has increased by ten percent each year since 1972, which is consistent with scientific predictions. There are many reasons the turkey population can increase or decrease. For example, deforestation forest turkey populations to move elsewhere, specifically into urban areas. This would explain why you can see turkeys everywhere in Santa Clara County and on the campus of UC Santa Cruz. The turkey can also decrease because of hunting activity. Most importantly, however, is that California provides are perfect habitat for the Rio Grande subspecies as it has an abundance of food availability as well as good nesting grounds. Because of this, it seems reasonable that the turkey population will exhibit an upward trend. It is seen that this trend does not only account for a centralized location in California, but rather the entire state. Although in some locations the population is exhibiting downward trend, the vast majority of locations across California are increasing. We know this by comparing previous data collections from years before to current population data.

Although this data does prove that the turkey population is on the rise, it would be more credible with other studies to back it up. However, turkey population research is very limited and hard to find. Only four papers relating to this subject have been published since 2009, making this topic relatively outdated. Having more papers relating to population would increase the accuracy of the study overall.

Another problem with this study is that is relies on the accuracy and consistency of secondary data. More specifically, not every location in California is surveyed consistently, making it hard to compare populations in various locations. It has also been proven that surveys collected near roads will be smaller on average than other locations because a turkey flock is less detectable and therefore underestimated (Butler, 2007). It is also notable that USGS, U.S. Geological Survey does not recommend combining data sets due to variation in the collection of data. Because of the lack of modern day research, it is still unclear whether the turkey population rise has a detrimental impact on the ecosystem. It is also unclear how the wild turkeys predators impact the population, if at all.

In future research, it would be helpful to determine how the turkeys are impacting the ecosystem in areas that showed large population numbers. To develop accuracy within current research, it would also be beneficial to create consistent data collecting procedures as well as consistents dates that collection takes place across all of California. That way, the average of all the data will be more representative of the population trend, assuming that it follows a normal distribution.

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