Description
In ecology, edge effects refer to the changes in population or community structures that occur at the boundary of two habitats.[1]:780 Areas with small habitat fragments exhibit especially pronounced edge effects that may extend throughout the patch. As the edge effects increase, the boundary habitat will allow for greater biodiversity.
QUANTIFYING THE EDGE EFFECTS ASSOCIATED WITH PREDATOR REMOVAL BLOCKS ON THE NESTING SUCCESS OF UPLAND DUCKS IN NORTH DAKOTA
A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Master of Science in The School of Renewable Natural Resources
by Margaret Jean Kuhn B.S., University of Vermont, 2003 August 2007
Dedication This thesis is dedicated to my parents, Bryant and Jean Kuhn, who have, and will continue to, support and encourage me in everything I do. I love you both.
ii
Acknowledgments I would like to thank my advisors, Frank Rohwer and Elizabeth Loos, for taking me on as a graduate student and providing valuable assistance and guidance over the past two years. Thank you also to my committee members, Michael Chamberlain and James Geaghan, for their valuable editorial and statistical advice. Thank you to Terry Shaffer for his statistical advice as well. This project would not have been possible had it not been for the financial support of the Delta Waterfowl Foundation. I am indebted to Matthew Pieron for the tremendous amount of help he provided during every aspect of this project. Thank you, Matt. Thank you also to all the other graduate students with whom I worked over the past two years. Thank you to Kevin Wlock for collecting my scent-station data. Without all the hardworking assistants this project would not have been possible, thank you for working long hours and never complaining. This project would not have been possible had it not been for the cooperation of North Dakota landowners. Thank you also to the U.S. Fish and Wildlife Service for allowing me to search for nests on their WPAs. Thank you to my parents for their unwavering love and support. Last but not least, thank you to Ryan Darr, for his love and for supporting my decision to pursue graduate studies, even if it meant we would be far apart for several years.
iii
Table of Contents Dedication ................................................................................................................................... ii Acknowledgments....................................................................................................................... iii List of Tables............................................................................................................................... v List of Figures ............................................................................................................................. vi Abstract ....................................................................................................................................... vii Introduction ................................................................................................................................. 1 Study Site and Study Design....................................................................................................... 5 Nest Success.................................................................................................................... 5 Predator-Scent Stations ................................................................................................... 6 Statistical Analyses ..................................................................................................................... 8 Nest Success.................................................................................................................... 8 Predator-Scent Stations ................................................................................................... 8 Results ......................................................................................................................................... 10 Nest Success.................................................................................................................... 10 Predator-Scent Stations ................................................................................................... 11 Discussion ................................................................................................................................... 25 Nest Success.................................................................................................................... 25 Predator-Scent Stations ................................................................................................... 29 Conclusion................................................................................................................................... 31 Literature Cited ........................................................................................................................... 32 Vita .............................................................................................................................................. 37
iv
List of Tables 1. Research conducted on different sized predator trapped blocks and its effect on nest success………………………………………………………………………….………3 2. Model selection criteria for all twenty-one logistic-exposure candidate models of daily survival rates inside trapped blocks, 2005-2006…………………………………...12 3. Model-averaged estimates of regression coefficients for inside trapped blocks, 2005-2006…………………………………………………………………………………...13 4. Model selection criteria for all nineteen logistic-exposure candidate models of daily survival rates outside trapped blocks, 2005-2006…………………………….………14 5. Model-averaged estimates of regression coefficients for outside trapped blocks, 2005-2006…………………………………………………………………………………...15 6. Number of scent-station visits per 0.8 kilometer increment for each predator species, inside and outside trapped blocks, 2006…………………………………………………………...16 7. Analysis of covariance of predator scent-station visitation rates by distance from the edge of a trapped block, 2006…………………………………………………..………19 8. Logistic regression analysis of predator scent-station visitation rates inside versus outside trapped blocks, 2006………………………………………………………………..………20
v
List of Figures 1. Possible shapes of the hypothesized nest success decline as you move further from a trapped block edge…………………………………………………………………………………...4 2. Mean scent-station visitation rates by distance for all predator species combined, 2006...... 21 3. Mean scent-station visitation rates by distance for badger, 2006 .......................................... 21 4. Mean scent-station visitation rates by distance for coyote, 2006........................................... 22 5. Mean scent-station visitation rates by distance for red fox, 2006.......................................... 22 6. Mean scent-station visitation rates by distance for Franklin’s ground squirrel, 2006 ........... 23 7. Mean scent-station visitation rates by distance for mink, 2006 ............................................. 23 8. Mean scent-station visitation rates by distance for raccoon, 2006......................................... 24 9. Mean scent-station visitation rates by distance for striped skunk, 2006................................ 24 10. Logistic-exposure daily survival rates by distance for individual nests outside trapped blocks, 2005 .......................................................................................................................... 27 11. Logistic-exposure daily survival rates by distance for individual nests outside trapped blocks, 2006 .......................................................................................................................... 27 12. Logistic-exposure daily survival rates by distance for individual nests inside trapped blocks, 2005 .......................................................................................................................... 28 13. Logistic-exposure daily survival rates by distance for individual nests inside trapped blocks, 2006 .......................................................................................................................... 28
vi
Abstract Much evidence suggests that nest success is one of the key drivers of duck production. Accordingly, for the past thirty years, waterfowl managers have focused their efforts on increasing nest success. One way to increase nest success is through predator trapping. Previous studies have shown that predator trapping increases nest success on different sized trapped blocks. This study attempted to answer the question: does trapping affect nest success on areas directly adjacent to trapped block boundaries? I hypothesized that predator abundance outside trapped blocks would be reduced. I predicted that nest success would decline with distance from the boundary while predator abundance would increase with distance from the boundary. This study was conducted in the Drift Prairie section of the Prairie Pothole Region in northeast North Dakota during the summers of 2005 and 2006. A total of 3,231 nests were found inside of six trapped blocks and a total of 2,006 nests were found outside of five trapped blocks. Daily survival rates were estimated using Shaffer’s logistic-exposure model and then related to distance from the center (for inside) or distance from the edge (for outside) of a trapped block, trapped block, field within a trapped block, and all interactions. Model fit was assessed using Akaike’s information criteria as adjusted for small sample size. The most important variables for explaining variation in daily survival rates of nests, both inside and outside trapped blocks, were year, and field within a trapped block. Distance appeared to have a negligible effect on daily survival rates for nests inside and outside trapped blocks. Mean daily survival rates were higher inside trapped blocks. Trapping, therefore, did not appear to increase daily survival rates outside trapped blocks. Predator scent-stations were used in 2006 to obtain an index of predator activity. Distance from a trapped block edge did not affect visitation rates for any individual predator species or for all species combined, both inside and outside trapped blocks. There were,
vii
however, significantly higher visitation rates inside versus outside trapped blocks for raccoons and for all species combined.
viii
Introduction The Prairie Pothole Region (PPR) of the United States and Canada is the primary breeding ground for most North American waterfowl (Bellrose 1980). Much evidence suggests that nest success is one of the key drivers of duck production in the PPR; however, nest success levels are often below the 15-20% necessary to maintain population levels (Klett et al. 1988, Kantrud 1993, Greenwood et al. 1995, Beauchamp et al. 1996, Sovada et al. 2000, Garrettson and Rohwer 2001, Hoekman et al. 2002, Drever et al. 2004). There are many factors that contribute to nest failure, but predation by mammals, especially red fox (Vulpes vulpes), raccoon (Procyon lotor), and striped skunk (Mephitis mephitis), is the primary cause (Greenwood 1986, Klett et al. 1988, Johnson et al. 1989, Sargeant et al. 1993, Greenwood et al. 1995). For the past thirty years, managers have focused their efforts on increasing nest success through habitat improvements (Rohwer et al. 2004). Efforts at habitat restoration have ranged from planting more grass and acquiring more nesting habitat to constructing nesting islands and other safe nesting sites (Duebbert and Lokemoen 1976, Giroux 1981, Duebbert et al. 1983). More direct efforts to reduce nest predation include the fencing of nesting cover to exclude predators (Lokemoen et al. 1982, Greenwood et al. 1990, Lokemoen and Woodward 1993). Unfortunately, most of the aforementioned techniques have proven less effective than hoped, prohibitively expensive, or not applicable to many areas. Nesting success has been suggested to be dependent upon habitat patch size, with larger patch sizes having higher success (Greenwood et al. 1995, Sovada et al. 2000, Reynolds et al. 2001). Habitat restoration, through the planting of grass, however, is expensive and nest success has been suggested to have a linear relationship with the amount of cover available (i.e. more cover equals higher success; Reynolds et al. 2001). Creating more cover is something that is difficult in the farmland matrix
1
of the PPR (Rohwer et al. 2004). Predator fences were initially thought to work well until it was discovered that they delayed the exit of ducklings and elevated duckling mortality (Peitz and Krapu 1994, Trottier et al. 1994). To mediate this problem, managers have added brood exits or opened fences into water. This solution speeds brood exit but also allows predators to enter the fenced area. Lethal predator management has also been used to increase nest success. Early predator reduction efforts relied on poisons, which are now illegal for widespread use (Balser et al. 1968, Lynch 1972, Duebbert and Kantrud 1974, Duebbert and Lokemoen 1980). In 1993, the Delta Waterfowl Foundation initiated research to examine the efficiency of predator reduction through legal trapping. The initial study, conducted by Pam Garrettson from 1994 to 1996, compared nest success on nine, 41.5 km2 trapped blocks with nine, 41.5 km2 untrapped blocks. Trapping dramatically increased nest success, with trapped blocks experiencing nearly twice the nest success of untrapped blocks (42% vs. 23% respectively; Garrettson and Rohwer 2001). Other studies with different sized trapped blocks have shown similar results: trapping predators can substantially increase nest success (see Table 1). In addition, a study conducted by Aaron Pearse showed that trapping also increased duckling survival (Pearse and Ratti 2004). Trapping predators on prime duck nesting habitat has proven to be a viable management option for increasing nest success. Nothing is known, however, about what happens to nest success on areas directly adjacent to trapped block boundaries. As a result, this study attempted to answer the following question: does trapping affect nest success on areas directly adjacent to trapped block edges? I hypothesized that predator abundance outside trapped blocks would be reduced. I predicted that nest success would decline with distance from the boundary while
2
Table 1. Research conducted on different sized predator trapped blocks and its effect on nest success. Researcher Date Years of Study Size of Trapped Block Experimental vs. Control Nest Success (%) 2 0.61 - 3.01 km 14 vs. 6 Sargeant et al. 1995 4 Mense Hoff Garrettson and Rohwer Chodachek and Chamberlain Lester Oligschlaeger 1996 1999 2001 2006 unpublished unpublished 2 2 3 2 2 2 41.5 km 93.2 km 41.5 km 2.6 km
2
57 vs. 29 36 vs. 15 42 vs. 23 53 vs. 29 48 vs. 19 49 vs. 23
2
2
2
41.5 km 93.2 km
2
2
3
predator abundance would increase with distance from the boundary. I could not predict the shape of the decline, but I expected it to have a rather smooth shape due to reduced predator numbers (See Figure 1).
Figure 1. Possible shapes of the hypothesized nest success decline as you move further from a trapped block edge.
4
Study Site and Study Design Nest Success
This study was conducted during the summers of 2005 and 2006 in the Drift Prairie section of the PPR in northeast North Dakota. Nest success of all upland nesting duck species was examined, but the primary focus was on the five most common species: blue-winged teal (Anas discors), gadwall (A. strepera), mallard (A. platyrhynchos), northern shoveler (A.
clypeata), and northern pintail (A. acuta; Bellrose 1980, Johnson and Grier 1988). For both
years, 32.4 ha fields of grass were searched for nests both inside of six trapped blocks and outside of five trapped blocks. In the 2006 season, one trapped block studied in 2005 was replaced with a new trapped block. Outside the trapped blocks, fields were searched in 0.8 km increments up to 4.8 km from the edge of the trapped block. Landowner permission often limited the number of fields that were available for nest searching. Owners of suitable fields were contacted for permission to search for nests. Fields were then randomly chosen only from those areas on which permission was granted. If there was no 32.4 ha field of grass available to search at any particular distance interval, then an additional 32.4 ha field at the same distance interval on another trapped block was selected. The number of 32.4 ha fields, both inside and outside trapped blocks, was kept as even as possible to prevent some trapped blocks from having disproportionate representation in the data. Nest searching started in early May and continued until mid-July in both years. Nests were located by dragging a 50 m chain between two all terrain vehicles (Klett et al. 1986). Outside the trapped blocks, fields were searched at least twice per season. Inside the blocks, fields were searched three times per season. Nests were marked with a numbered wooden stake 10 m north of the nest and with a metal rod (3.2 mm diameter and 0.9 m length) at the nest bowl.
5
The following was recorded for each nest: species, date found, dates checked, clutch size, incubation stage (determined by candling of at least two eggs; Weller 1956) GPS coordinates, hatch date if successful, failure date if not successful, and cause of failure. Nests were checked approximately every eight days until fate was determined (Klett et al. 1986). A nest was considered successful if at least one egg hatched (Klett et al. 1986).
Predator Scent-Stations
During the 2006 field season, scent-stations were used to obtain an index of predator activity both inside and outside trapped blocks. Stations consisted of a 1 m in diameter circle of sand and mineral oil mixture (1 liter of oil to 22 liters of sand). The sand and mineral oil mixture served as a tracking medium and one sardine placed in the center of the sand served as a lure (Linhart and Knowlton 1975, Roughton and Sweeny 1982, Conner et al. 1983, Nottingham et al. 1989, Travaini et al. 1996, Sargeant et al. 1998, Sargeant et al. 2003). Scent-stations were located at the edge of unpaved roads as close to the vegetation as possible. Inside the trapped blocks, stations were set up in 0.8 km increments, starting at the center of the block and progressing toward the perimeter of the block in all cardinal directions. Adjacent stations were put on alternate sides of the road. Outside the trapped blocks, stations were put out in 0.8 km increments up to 4.8 km from the block edge in all cardinal directions. Lines were alternately started at either the block edge or 0.8 km from the block edge. Scent-stations were set up in the afternoon on days with no rain in the forecast and checked once the following morning for tracks. All tracks were identified but only nest predators were included in the analysis. Predators used in the analysis included the coyote (Canis latrans), raccoon, red fox, striped skunk, badger (Taxidea taxus), mink (Mustela vison),
6
and Franklin’s ground squirrel (Spermophilus franklinii; Sargeant 1972, Fritzell 1978, Greenwood 1981, Sargeant et al. 1984, Arnold and Fritzell 1987, Choromanski-Norris et al. 1989, Sargeant et al. 1993, Sovada et al. 1995, Greenwood et al. 1999).
7
Statistical Analyses Nest Success
Daily survival rates for individual nests, both inside and outside trapped blocks, were estimated using Shaffer’s logistic-exposure model (Shaffer 2004) and PROC GENMOD (SAS Institute 1999). Daily survival rates were related to four explanatory variables including, distance from center or edge of a trapped block, trapped block, field within a trapped block, year, and their interactions. Model fit was assessed using Akaike’s information criterion as adjusted for small sample size (AICc; Burnham and Anderson 2002). Daily survival rates were modeled separately for nests found inside and outside trapped blocks. Twenty-one candidate models were selected to model daily survival rates inside trapped blocks, including a constant survival model. The interaction between the variables field within a trapped block and distance was omitted due to unusual patterns in the data that prevented the model likelihood from being maximized. Unusual patterns in data tend to occur in complex candidate models or in studies in which sample size is not under strict control (T. L. Shaffer pers. comm.). Nineteen candidate models were selected to model daily survival rates outside trapped blocks, including a constant survival model. For analyses outside trapped blocks all interactions with the variable field within a trapped block were omitted due to unusual patterns in the data that prevented the model likelihood from maximizing.
Predator Scent-Stations
An Analysis of Covariance (PROC LOGISTIC, SAS Institute 1999) was used to test for a distance (from edge of a trapped block) effect on scent-station visitation rates inside and outside trapped blocks. Visitation rates were related to the following variables: distance from the edge of a trapped block (continuous), inside or outside a trapped block (categorical with two levels),
8
trapped block (categorical with five levels), and all interactions. Separate analyses were done for each predator species and for all predators combined. A Logistic Regression (PROC LOGISTIC, SAS Institute 1999) was used to test for a difference between scent-station visitation rates inside and outside trapped blocks. Presence at a scent-station was related to the categorical variable inside or outside of a trapped block. Separate analyses were done for each predator species and for all predators combined.
9
Results Nest Success
A total of 3,231 nests were found in both years inside the trapped blocks. Of those, 117 were destroyed by investigators or farm equipment and thus were not suitable for use in the analysis. A total of 2,006 nests were found in both years outside the trapped blocks. Of those, 54 were not suitable for use in the analysis. Of the 21 candidate models for inside trapped blocks, the three most supported had ?AICC values 0.182. The remaining models had ?AICC values >28 and Akaike weights of zero (Table 2). The most supported model, with a ?AICC value of zero and an Akaike weight of 0.497, included the variables field within trapped block, year, distance, and the interaction of field within trapped block and year. The second most supported model, with a ?AICC value of 0.87 and an Akaike weight of 0.321, deleted only the distance variable. The third most supported model, with a ?AICC value of 2.006 and an Akaike weight of 0.182, contained the distance variable and added an interaction of distance and year. Model-averaged regression coefficients for inside trapped blocks supported the above results (Table 3). Outside trapped blocks, the three most supported models, of the 19 total candidates, had ?AICC values 0.177. The remaining models had ?AICC values >10.9 and Akaike weights
In ecology, edge effects refer to the changes in population or community structures that occur at the boundary of two habitats.[1]:780 Areas with small habitat fragments exhibit especially pronounced edge effects that may extend throughout the patch. As the edge effects increase, the boundary habitat will allow for greater biodiversity.
QUANTIFYING THE EDGE EFFECTS ASSOCIATED WITH PREDATOR REMOVAL BLOCKS ON THE NESTING SUCCESS OF UPLAND DUCKS IN NORTH DAKOTA
A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Master of Science in The School of Renewable Natural Resources
by Margaret Jean Kuhn B.S., University of Vermont, 2003 August 2007
Dedication This thesis is dedicated to my parents, Bryant and Jean Kuhn, who have, and will continue to, support and encourage me in everything I do. I love you both.
ii
Acknowledgments I would like to thank my advisors, Frank Rohwer and Elizabeth Loos, for taking me on as a graduate student and providing valuable assistance and guidance over the past two years. Thank you also to my committee members, Michael Chamberlain and James Geaghan, for their valuable editorial and statistical advice. Thank you to Terry Shaffer for his statistical advice as well. This project would not have been possible had it not been for the financial support of the Delta Waterfowl Foundation. I am indebted to Matthew Pieron for the tremendous amount of help he provided during every aspect of this project. Thank you, Matt. Thank you also to all the other graduate students with whom I worked over the past two years. Thank you to Kevin Wlock for collecting my scent-station data. Without all the hardworking assistants this project would not have been possible, thank you for working long hours and never complaining. This project would not have been possible had it not been for the cooperation of North Dakota landowners. Thank you also to the U.S. Fish and Wildlife Service for allowing me to search for nests on their WPAs. Thank you to my parents for their unwavering love and support. Last but not least, thank you to Ryan Darr, for his love and for supporting my decision to pursue graduate studies, even if it meant we would be far apart for several years.
iii
Table of Contents Dedication ................................................................................................................................... ii Acknowledgments....................................................................................................................... iii List of Tables............................................................................................................................... v List of Figures ............................................................................................................................. vi Abstract ....................................................................................................................................... vii Introduction ................................................................................................................................. 1 Study Site and Study Design....................................................................................................... 5 Nest Success.................................................................................................................... 5 Predator-Scent Stations ................................................................................................... 6 Statistical Analyses ..................................................................................................................... 8 Nest Success.................................................................................................................... 8 Predator-Scent Stations ................................................................................................... 8 Results ......................................................................................................................................... 10 Nest Success.................................................................................................................... 10 Predator-Scent Stations ................................................................................................... 11 Discussion ................................................................................................................................... 25 Nest Success.................................................................................................................... 25 Predator-Scent Stations ................................................................................................... 29 Conclusion................................................................................................................................... 31 Literature Cited ........................................................................................................................... 32 Vita .............................................................................................................................................. 37
iv
List of Tables 1. Research conducted on different sized predator trapped blocks and its effect on nest success………………………………………………………………………….………3 2. Model selection criteria for all twenty-one logistic-exposure candidate models of daily survival rates inside trapped blocks, 2005-2006…………………………………...12 3. Model-averaged estimates of regression coefficients for inside trapped blocks, 2005-2006…………………………………………………………………………………...13 4. Model selection criteria for all nineteen logistic-exposure candidate models of daily survival rates outside trapped blocks, 2005-2006…………………………….………14 5. Model-averaged estimates of regression coefficients for outside trapped blocks, 2005-2006…………………………………………………………………………………...15 6. Number of scent-station visits per 0.8 kilometer increment for each predator species, inside and outside trapped blocks, 2006…………………………………………………………...16 7. Analysis of covariance of predator scent-station visitation rates by distance from the edge of a trapped block, 2006…………………………………………………..………19 8. Logistic regression analysis of predator scent-station visitation rates inside versus outside trapped blocks, 2006………………………………………………………………..………20
v
List of Figures 1. Possible shapes of the hypothesized nest success decline as you move further from a trapped block edge…………………………………………………………………………………...4 2. Mean scent-station visitation rates by distance for all predator species combined, 2006...... 21 3. Mean scent-station visitation rates by distance for badger, 2006 .......................................... 21 4. Mean scent-station visitation rates by distance for coyote, 2006........................................... 22 5. Mean scent-station visitation rates by distance for red fox, 2006.......................................... 22 6. Mean scent-station visitation rates by distance for Franklin’s ground squirrel, 2006 ........... 23 7. Mean scent-station visitation rates by distance for mink, 2006 ............................................. 23 8. Mean scent-station visitation rates by distance for raccoon, 2006......................................... 24 9. Mean scent-station visitation rates by distance for striped skunk, 2006................................ 24 10. Logistic-exposure daily survival rates by distance for individual nests outside trapped blocks, 2005 .......................................................................................................................... 27 11. Logistic-exposure daily survival rates by distance for individual nests outside trapped blocks, 2006 .......................................................................................................................... 27 12. Logistic-exposure daily survival rates by distance for individual nests inside trapped blocks, 2005 .......................................................................................................................... 28 13. Logistic-exposure daily survival rates by distance for individual nests inside trapped blocks, 2006 .......................................................................................................................... 28
vi
Abstract Much evidence suggests that nest success is one of the key drivers of duck production. Accordingly, for the past thirty years, waterfowl managers have focused their efforts on increasing nest success. One way to increase nest success is through predator trapping. Previous studies have shown that predator trapping increases nest success on different sized trapped blocks. This study attempted to answer the question: does trapping affect nest success on areas directly adjacent to trapped block boundaries? I hypothesized that predator abundance outside trapped blocks would be reduced. I predicted that nest success would decline with distance from the boundary while predator abundance would increase with distance from the boundary. This study was conducted in the Drift Prairie section of the Prairie Pothole Region in northeast North Dakota during the summers of 2005 and 2006. A total of 3,231 nests were found inside of six trapped blocks and a total of 2,006 nests were found outside of five trapped blocks. Daily survival rates were estimated using Shaffer’s logistic-exposure model and then related to distance from the center (for inside) or distance from the edge (for outside) of a trapped block, trapped block, field within a trapped block, and all interactions. Model fit was assessed using Akaike’s information criteria as adjusted for small sample size. The most important variables for explaining variation in daily survival rates of nests, both inside and outside trapped blocks, were year, and field within a trapped block. Distance appeared to have a negligible effect on daily survival rates for nests inside and outside trapped blocks. Mean daily survival rates were higher inside trapped blocks. Trapping, therefore, did not appear to increase daily survival rates outside trapped blocks. Predator scent-stations were used in 2006 to obtain an index of predator activity. Distance from a trapped block edge did not affect visitation rates for any individual predator species or for all species combined, both inside and outside trapped blocks. There were,
vii
however, significantly higher visitation rates inside versus outside trapped blocks for raccoons and for all species combined.
viii
Introduction The Prairie Pothole Region (PPR) of the United States and Canada is the primary breeding ground for most North American waterfowl (Bellrose 1980). Much evidence suggests that nest success is one of the key drivers of duck production in the PPR; however, nest success levels are often below the 15-20% necessary to maintain population levels (Klett et al. 1988, Kantrud 1993, Greenwood et al. 1995, Beauchamp et al. 1996, Sovada et al. 2000, Garrettson and Rohwer 2001, Hoekman et al. 2002, Drever et al. 2004). There are many factors that contribute to nest failure, but predation by mammals, especially red fox (Vulpes vulpes), raccoon (Procyon lotor), and striped skunk (Mephitis mephitis), is the primary cause (Greenwood 1986, Klett et al. 1988, Johnson et al. 1989, Sargeant et al. 1993, Greenwood et al. 1995). For the past thirty years, managers have focused their efforts on increasing nest success through habitat improvements (Rohwer et al. 2004). Efforts at habitat restoration have ranged from planting more grass and acquiring more nesting habitat to constructing nesting islands and other safe nesting sites (Duebbert and Lokemoen 1976, Giroux 1981, Duebbert et al. 1983). More direct efforts to reduce nest predation include the fencing of nesting cover to exclude predators (Lokemoen et al. 1982, Greenwood et al. 1990, Lokemoen and Woodward 1993). Unfortunately, most of the aforementioned techniques have proven less effective than hoped, prohibitively expensive, or not applicable to many areas. Nesting success has been suggested to be dependent upon habitat patch size, with larger patch sizes having higher success (Greenwood et al. 1995, Sovada et al. 2000, Reynolds et al. 2001). Habitat restoration, through the planting of grass, however, is expensive and nest success has been suggested to have a linear relationship with the amount of cover available (i.e. more cover equals higher success; Reynolds et al. 2001). Creating more cover is something that is difficult in the farmland matrix
1
of the PPR (Rohwer et al. 2004). Predator fences were initially thought to work well until it was discovered that they delayed the exit of ducklings and elevated duckling mortality (Peitz and Krapu 1994, Trottier et al. 1994). To mediate this problem, managers have added brood exits or opened fences into water. This solution speeds brood exit but also allows predators to enter the fenced area. Lethal predator management has also been used to increase nest success. Early predator reduction efforts relied on poisons, which are now illegal for widespread use (Balser et al. 1968, Lynch 1972, Duebbert and Kantrud 1974, Duebbert and Lokemoen 1980). In 1993, the Delta Waterfowl Foundation initiated research to examine the efficiency of predator reduction through legal trapping. The initial study, conducted by Pam Garrettson from 1994 to 1996, compared nest success on nine, 41.5 km2 trapped blocks with nine, 41.5 km2 untrapped blocks. Trapping dramatically increased nest success, with trapped blocks experiencing nearly twice the nest success of untrapped blocks (42% vs. 23% respectively; Garrettson and Rohwer 2001). Other studies with different sized trapped blocks have shown similar results: trapping predators can substantially increase nest success (see Table 1). In addition, a study conducted by Aaron Pearse showed that trapping also increased duckling survival (Pearse and Ratti 2004). Trapping predators on prime duck nesting habitat has proven to be a viable management option for increasing nest success. Nothing is known, however, about what happens to nest success on areas directly adjacent to trapped block boundaries. As a result, this study attempted to answer the following question: does trapping affect nest success on areas directly adjacent to trapped block edges? I hypothesized that predator abundance outside trapped blocks would be reduced. I predicted that nest success would decline with distance from the boundary while
2
Table 1. Research conducted on different sized predator trapped blocks and its effect on nest success. Researcher Date Years of Study Size of Trapped Block Experimental vs. Control Nest Success (%) 2 0.61 - 3.01 km 14 vs. 6 Sargeant et al. 1995 4 Mense Hoff Garrettson and Rohwer Chodachek and Chamberlain Lester Oligschlaeger 1996 1999 2001 2006 unpublished unpublished 2 2 3 2 2 2 41.5 km 93.2 km 41.5 km 2.6 km
2
57 vs. 29 36 vs. 15 42 vs. 23 53 vs. 29 48 vs. 19 49 vs. 23
2
2
2
41.5 km 93.2 km
2
2
3
predator abundance would increase with distance from the boundary. I could not predict the shape of the decline, but I expected it to have a rather smooth shape due to reduced predator numbers (See Figure 1).
Figure 1. Possible shapes of the hypothesized nest success decline as you move further from a trapped block edge.
4
Study Site and Study Design Nest Success
This study was conducted during the summers of 2005 and 2006 in the Drift Prairie section of the PPR in northeast North Dakota. Nest success of all upland nesting duck species was examined, but the primary focus was on the five most common species: blue-winged teal (Anas discors), gadwall (A. strepera), mallard (A. platyrhynchos), northern shoveler (A.
clypeata), and northern pintail (A. acuta; Bellrose 1980, Johnson and Grier 1988). For both
years, 32.4 ha fields of grass were searched for nests both inside of six trapped blocks and outside of five trapped blocks. In the 2006 season, one trapped block studied in 2005 was replaced with a new trapped block. Outside the trapped blocks, fields were searched in 0.8 km increments up to 4.8 km from the edge of the trapped block. Landowner permission often limited the number of fields that were available for nest searching. Owners of suitable fields were contacted for permission to search for nests. Fields were then randomly chosen only from those areas on which permission was granted. If there was no 32.4 ha field of grass available to search at any particular distance interval, then an additional 32.4 ha field at the same distance interval on another trapped block was selected. The number of 32.4 ha fields, both inside and outside trapped blocks, was kept as even as possible to prevent some trapped blocks from having disproportionate representation in the data. Nest searching started in early May and continued until mid-July in both years. Nests were located by dragging a 50 m chain between two all terrain vehicles (Klett et al. 1986). Outside the trapped blocks, fields were searched at least twice per season. Inside the blocks, fields were searched three times per season. Nests were marked with a numbered wooden stake 10 m north of the nest and with a metal rod (3.2 mm diameter and 0.9 m length) at the nest bowl.
5
The following was recorded for each nest: species, date found, dates checked, clutch size, incubation stage (determined by candling of at least two eggs; Weller 1956) GPS coordinates, hatch date if successful, failure date if not successful, and cause of failure. Nests were checked approximately every eight days until fate was determined (Klett et al. 1986). A nest was considered successful if at least one egg hatched (Klett et al. 1986).
Predator Scent-Stations
During the 2006 field season, scent-stations were used to obtain an index of predator activity both inside and outside trapped blocks. Stations consisted of a 1 m in diameter circle of sand and mineral oil mixture (1 liter of oil to 22 liters of sand). The sand and mineral oil mixture served as a tracking medium and one sardine placed in the center of the sand served as a lure (Linhart and Knowlton 1975, Roughton and Sweeny 1982, Conner et al. 1983, Nottingham et al. 1989, Travaini et al. 1996, Sargeant et al. 1998, Sargeant et al. 2003). Scent-stations were located at the edge of unpaved roads as close to the vegetation as possible. Inside the trapped blocks, stations were set up in 0.8 km increments, starting at the center of the block and progressing toward the perimeter of the block in all cardinal directions. Adjacent stations were put on alternate sides of the road. Outside the trapped blocks, stations were put out in 0.8 km increments up to 4.8 km from the block edge in all cardinal directions. Lines were alternately started at either the block edge or 0.8 km from the block edge. Scent-stations were set up in the afternoon on days with no rain in the forecast and checked once the following morning for tracks. All tracks were identified but only nest predators were included in the analysis. Predators used in the analysis included the coyote (Canis latrans), raccoon, red fox, striped skunk, badger (Taxidea taxus), mink (Mustela vison),
6
and Franklin’s ground squirrel (Spermophilus franklinii; Sargeant 1972, Fritzell 1978, Greenwood 1981, Sargeant et al. 1984, Arnold and Fritzell 1987, Choromanski-Norris et al. 1989, Sargeant et al. 1993, Sovada et al. 1995, Greenwood et al. 1999).
7
Statistical Analyses Nest Success
Daily survival rates for individual nests, both inside and outside trapped blocks, were estimated using Shaffer’s logistic-exposure model (Shaffer 2004) and PROC GENMOD (SAS Institute 1999). Daily survival rates were related to four explanatory variables including, distance from center or edge of a trapped block, trapped block, field within a trapped block, year, and their interactions. Model fit was assessed using Akaike’s information criterion as adjusted for small sample size (AICc; Burnham and Anderson 2002). Daily survival rates were modeled separately for nests found inside and outside trapped blocks. Twenty-one candidate models were selected to model daily survival rates inside trapped blocks, including a constant survival model. The interaction between the variables field within a trapped block and distance was omitted due to unusual patterns in the data that prevented the model likelihood from being maximized. Unusual patterns in data tend to occur in complex candidate models or in studies in which sample size is not under strict control (T. L. Shaffer pers. comm.). Nineteen candidate models were selected to model daily survival rates outside trapped blocks, including a constant survival model. For analyses outside trapped blocks all interactions with the variable field within a trapped block were omitted due to unusual patterns in the data that prevented the model likelihood from maximizing.
Predator Scent-Stations
An Analysis of Covariance (PROC LOGISTIC, SAS Institute 1999) was used to test for a distance (from edge of a trapped block) effect on scent-station visitation rates inside and outside trapped blocks. Visitation rates were related to the following variables: distance from the edge of a trapped block (continuous), inside or outside a trapped block (categorical with two levels),
8
trapped block (categorical with five levels), and all interactions. Separate analyses were done for each predator species and for all predators combined. A Logistic Regression (PROC LOGISTIC, SAS Institute 1999) was used to test for a difference between scent-station visitation rates inside and outside trapped blocks. Presence at a scent-station was related to the categorical variable inside or outside of a trapped block. Separate analyses were done for each predator species and for all predators combined.
9
Results Nest Success
A total of 3,231 nests were found in both years inside the trapped blocks. Of those, 117 were destroyed by investigators or farm equipment and thus were not suitable for use in the analysis. A total of 2,006 nests were found in both years outside the trapped blocks. Of those, 54 were not suitable for use in the analysis. Of the 21 candidate models for inside trapped blocks, the three most supported had ?AICC values 0.182. The remaining models had ?AICC values >28 and Akaike weights of zero (Table 2). The most supported model, with a ?AICC value of zero and an Akaike weight of 0.497, included the variables field within trapped block, year, distance, and the interaction of field within trapped block and year. The second most supported model, with a ?AICC value of 0.87 and an Akaike weight of 0.321, deleted only the distance variable. The third most supported model, with a ?AICC value of 2.006 and an Akaike weight of 0.182, contained the distance variable and added an interaction of distance and year. Model-averaged regression coefficients for inside trapped blocks supported the above results (Table 3). Outside trapped blocks, the three most supported models, of the 19 total candidates, had ?AICC values 0.177. The remaining models had ?AICC values >10.9 and Akaike weights