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The effect of hunting on non-target prey

Blog written by Jessica L. Mohlman & James A. Martin. Read the full paper here.

Predator-prey dynamics have long fascinated ecologists because of their effects on community ecology and behavior. These topics have even spilt over into the entertainment industry with famous cartoons such as “Looney Toons” depicting such relationships. As one of the main archnemeses of Bugs Bunny, Elmer Fudd’s mission was to try and hunt the trickster hare. However, Bugs Bunny was often able to outsmart Elmer Fudd and escape his hunting tactics. Not all prey are as calm and collected around predators as Bugs Bunny. In fact, most predators provoke fear in prey resulting in them changing their behavior. Human hunters, such as Elmer Fudd, may even provoke a more profound threat to prey than natural predators.

            While Bugs Bunny may have seemed calm and collected around Elmer Fudd, he was still altering his habitat use and increasing his alertness as a way to reduce his risk of predation by the hunter. Actively attempting to avoid direct predation, such as the pursuit of a hunter, can have negative indirect effects of its own, such reduced foraging effort, greater loss of energy, and increased vulnerability to other predators.

            Historically, ecologists have studied the direct lethal effects (e.g. prey death) and indirect nonlethal effects (e.g. reduced foraging) of human hunting on prey. However, what about the effects of hunting on prey that are not the target of predation? While Bugs Bunny was the target of Elmer Fudd’s pursuits, the hunter often found himself effecting other non-target characters, such as Daffy Duck, instead.

When it comes to hunting, why would we expect that the effects would be confined to only the species being directly pursued? What about other species that share the same habitat as the target prey, such as Daffy Duck? Additionally, are these indirect effects on non-target prey greater if the prey is also a game species?

 Within our study, we investigated such effects through examining the indirect effects of hunting Eastern Cottontails (Sylvilagus floridanus; hereafter, rabbit) on Northern Bobwhite (Colinus virginianus; hereafter, bobwhite). These species were ideal to answer our question of “how does hunting one species effect another?” as both of these species are commonly hunted within the United States and share similar hunting seasons and habitat. Additionally, both species are hunted on foot with the use of hunting dogs.

We were particularly interested in the possible effect of rabbit hunting on bobwhite because of its designation as a species of conservation priority. Due to the fact that bobwhites are not being actively pursued during rabbit hunts, increased perceived risk of predation by bobwhite may provoke anti-predator behavior. We hypothesized that bobwhite would respond to signals during rabbit hunts as threats and would be unable to discern that they were not the target of predation by the hunters. We tested this hypothesis by subjecting bobwhites to varying levels of rabbit hunting intensity and predicted that their behavior would change across this gradient. Specifically, we predicted that bobwhites may try to reduce this perceived risk by decreasing their movement under greater rabbit hunting pressure or increasing their movement as a result of more frequent encounters with hunters. Moreover, we hypothesized bobwhite would remain closer to escape cover (e.g. scrub/shrubs) and hardwoods where there was hunting pressure.

Rabbit hunting treatments were established on a Wildlife Management Area (WMA) in the state of Georgia. The treatments were assigned across the study site in a randomized complete block design (i.e., three levels of rabbit hunting and two spatial blocks). The treatments included 0 days of hunting per week (“No Rabbit Hunting”), 3 days of hunting per week (“Reduced Hunting”), and 5 days of hunting per week (“Reference”), with duplicates of each treatment.

Prior to hunting seasons, we captured bobwhite and equipped them with radio-tags to allow us to track their movements using telemetry. We tracked coveys (e.g. groups) of bobwhite during both rabbit and bobwhite hunting days across the varying rabbit hunting treatments. While being tracked, locations were taken of the bobwhite coveys every 30 minutes to determine fine-scale movement patterns. The telemetry data were analyzed to determine specific movement patterns such as step-length (i.e., distance moved every 30 minutes), path straightness, and trajectory distance (i.e., total distance moved in a day). To determine bobwhite use of hardwood and scrub/shrub habitats, we measured the distance of each individual location to the nearest example of that habitat feature.  

We found that bobwhite adjusted their movements to the spatial patterns of rabbit hunters to mediate the perceived predation risk. In areas in which rabbit hunting occurred, bobwhite decreased their overall movement, regardless of if hunting occurred for 3 or 5 days. Additionally, bobwhite increased their use of scrub/shrub and hardwoods where rabbit hunting occurred. Nevertheless, the full extent of the effect of rabbit hunting on bobwhite still needs to be investigated. Heightened antipredator behavior through decreased movement may assist with bobwhite predator avoidance by taking advantage of their cryptic coloration in the landscape. However, decreased movement and increased use of poor habitats, such as hardwoods, may also have negative effects as a result of reduced foraging time or increased susceptibility to other predators. Additionally, heightened use of antipredator behavior may reduce survival by means of increased energy use to avoid the perceived predation. While the full extent of rabbit hunting on bobwhite still needs to be investigated, we have shown that there is indeed an effect on behavior. This is critical for management and conservation efforts, as it reinforces the fact that species do not live independently of one another and that hunting and management activities targeted at one species may have negative nonlethal effects on other species.

When, where and why do white storks die?

Blog written by Yachang Cheng & Andrea Flack. Read the full paper here.

Migration is a fascinating and extraordinary phenomenon that has evolved as a response to seasonally changing environments. But despite its benefits, migration is also costly in terms of time, energy, and many other unpredictable risks. On top of that, life in the Anthropocene, the current epoch in which humans have become a global geophysical force, challenges many ancient evolutionary adaptations, including migration.

Recently we have observed that birds are changing their migratory behavior by flying along different routes at different times, or even completely stopping their migration. Humans can have massive and rapid impacts on the life and surroundings of animals, forcing them to react quickly to the changing world in order to survive. Some individuals might be better suited to adapt to human-modified landscapes, causing differences in individual fitness (productivity and survival) and subsequently influencing the demography of the entire species.

The white stork is one of the most symbolic examples of avian migration, and, with it, migration research.  Thanks to storks, people discovered in 1822 that some birds overwinter on the African continent by observing a stork in Europe that had an African arrow stuck in its neck. Inspired by that discovery, researchers started to put individually numbered leg bands on storks leading to an extensive knowledge of their migration routes and overwintering grounds in Sub-Saharan Africa up to 4000km away from home.

Human influences have, however, affected the storks’ migratory behaviour drastically during the last three decades. In particular, birds that breed in Western Europe and migrate across the Strait of Gibraltar to overwinter in the Sahel region of Western Africa have altered their winter destination. Between 1970 and 1980, very few individuals overwintered in Europe, but since the late 1980s, the number of storks staying north of the Sahara increased dramatically (Archaux et.al.2004). This was mainly caused by additional food sources such as organic rubbish in open landfills, the invasive crayfish (Procambarus clarkii), and warmer temperatures caused by climate change.

Flocks of white storks included one of a tagged stork at a landfill in Spain. Photo credit: Max Planck Institute of Animal Behavior.

An interesting question that arises is whether the storks that shortened their migratory distance have a higher fitness. While it has been shown that a shorter migration distance can bring breeding benefits to the adults (Gilbert et.al.2016), the effect on survival has not been explored. In contrast to adults, white stork juveniles have a very low survival rate; only 30% of the juveniles survive their first year (compared to 80% of the adults). This also means that the selection pressure on juvenile survival is much higher. So are juveniles with shorter migratory distance surviving better? What other individual traits are linked to their mortality risks, e.g. sex, hatching ranks, fledging time or migration departure time?

Since 2013, we have attached more than three hundred GPS-accelerometer loggers to juvenile white storks across their entire distribution range (i.e. from Spain to Uzbekistan). We explored the differences in their migration patterns (Flack et.al. 2016), and their social behaviour while travelling to the South (Flack et al. 2018). These long-term data now allowed us to also explore the link between migration strategy and survival in juveniles from Germany and Austria.

Two tagged white stork chicks on their nest. Photo credit: Max Planck Institute of Animal Behavior

The attached loggers provide us with various information on the individual, maybe comparable to a smart watch or wristband for humans. They also allowed us to identify death events, for example, when the movement recorder (accelerometer) showed flat lines, or the GPS positions were all  clustered in one place for a long time.  And often, thanks to many wonderful and dedicated collaborators along the migration routes, we located the dead individual in the field allowing us to confirm the death and explore its causes.

Feathers of tagged storks and a smashed tag. Photo credit: Max Planck Institute of Animal Behavior

A majority of mortality events occured during the migration period. Our results showed that longer migratory distance increased the mortality risk during migration. Most of our storks stayed in Europe (55.26%) or North Africa (28.95%) for their first winter. These survived better than their conspecifics who overwintered more traditionally in Sub-Saharan Africa. Also, birds that fledged late and moved less actively had higher mortality risks. Our findings suggest that overwintering in Europe may, in fact, benefit juvenile survival, causing a change in the migration strategy of the Western European white stork population.

The long-term consequences of these changes in migration strategy are still unforeseeable and require further monitoring, especially in the light of the coming closure of open landfills in Europe. Will white storks regain their long-distance migratory behaviour and adjust their strategy again? Exploring white stork survival is essential when it comes to understanding the demography of the entire population, and it also helps conservation mangers to develop action plans to deal with current rapidly global changes. Most importantly, it informs us how humans are alterering selection forces and with it migratory behavior.

Since we believe that scientific knowledge can be gained by improving data accessibility, we published our complete data set in the Movebank Data Repository (https://www.datarepository.movebank.org/).  

Reference

Archaux F., G. Balança G, P. Y. Henry, G. Zapata. 2004. Wintering of white storks in Mediterranean France. Waterbirds 27(4): 441-446.

Flack, A., M. Nagy, W. Fiedler, I. D. Couzin, and M. Wikelski. 2018. From local collective behavior to global migratory patterns in white storks. Science 360:911–914.

Flack, A., W. Fiedler, J. Blas, I. Pokrovsky, M. Kaatz, M. Mitropolsky, K. Aghababyan, I. Fakriadis, E. Makrigianni, L. Jerzak, H. Azafzaf, C. Feltrup-Azafzaf, S. Rotics, T. M. Mokotjomela, R. Nathan, and M. Wikelski. 2016. Costs of migratory decisions: A comparison across eight white stork populations. Science Advances 2:e1500931–e1500931.

Gilbert, N. I., R. A. Correia, J. P. Silva, C. Pacheco, I. Catry, P. W. Atkinson, J. A. Gill, and A. M. A. Franco. 2016. Are white storks addicted to junk food? Impacts of landfill use on the movement and behaviour of resident white storks (Ciconia ciconia) from a partially migratory population. Movement Ecology 4:7.

The hunger games: Do fledged chicks beg honestly?

Blog written by Kayla Davis. Read the full paper here.

Imagine you are a newly fledged tern chick, bright-eyed and ready to take on the skies with your freshly grown feathers. There’s a problem though; catching fish is hard, and you always seem to be hungry because you are still growing! Luckily, your dad (or mom, if you were hatched second) is by your side at the breeding colony and during pre-migratory staging to make sure you stay healthy and strong by feeding you tasty sand eels. However, as with most growing youths, the relationship between parents and offspring changes as offspring grow and gain independence. Do you try to maximize parental care from your dad or mom by begging dishonestly? Or are you an honest tern chick that only begs when you need to be fed?

Honest signaling theory seeks to explain animal communication and interactions by describing signals as “honest”, meaning that the sender displays a reliable signal, or “dishonest”, meaning that the sender gives false information to the receiver of the signal. Theory states that costs imposed on creating signals should function to maintain honest signaling mechanisms. However, in the case of begging, especially for nearly adult-sized fledglings, this behavior may not be particularly costly. There is a long history of research on begging behavior as an honest signaling mechanism in birds, but most of this work has focused on nestlings. Parent-offspring communication is expected to change as offspring gain the functional independence needed to survive on their own, and the costs and benefits of behaving honestly or dishonestly are also likely to change during this time.

In our study, we conducted behavioral observations of roseate tern fledglings during the post-breeding, pre-migratory staging period at Cape Cod National Seashore, Massachusetts, USA to investigate the honesty of parent-offspring interactions during the postfledging period. Roseate terns are unique in that most of the northwest Atlantic breeding population departs the breeding colonies (Nova Scotia, CA to Connecticut, USA) after chicks have fledged to stage for several weeks on beaches and islands around Cape Cod, Massachusetts. While there, roseate tern fledglings continue to depend on their care-giving parent for food as they continue to grow and build fat reserves for fall migration to South America. This unique staging strategy gave us the opportunity to observe post-fledging parent-offspring interactions, including lots of begging behavior.


Roseate tern chick banded with a plastic field-readable leg band. Colony managers across the entire northwest Atlantic breeding range banded tern chicks with uniquely-identifiable leg bands.

We located tern flocks at the Cape Cod staging grounds and conducted focal sampling of uniquely-marked roseate tern fledglings to quantify begging behavior as it related to date of the staging season and time of day. We expected tern fledglings to gain independence from their parents and improve their fishing skills over the course of the staging season, so we predicted that begging behavior would decrease with date of the staging season if begging was an honest signal of need. We also predicted that begging would increase with time of day if begging was an honest signal of need because roseate terns do not fish during the night, and dusk would be the last time fledgling terns could be fed before nightfall. Thus, we expected fledgling need, and therefore begging behavior, to be highest at the end of the day before a night-time fast.

We also were interested in identifying whether young terns begged at non-parents. In colonially breeding species like roseate terns, offspring may deceitfully beg at non-parents to try to receive extra-parental care. Based on previous work on parent-offspring recognition and discrimination, we expected that non-parents would be able to discriminate their offspring from the offspring of others and would therefore not be fooled by deceitful begging. Thus, we predicted that the lack of benefits to be gained from begging deceitfully at non-parents would result in honesty of parent-offspring interactions.


Juvenile roseate tern begging at an adult. Photo credit: David Hollie.

Our predictions about begging as an honest signaling mechanism were upheld. Roseate tern fledglings begged at their parents more than non-parents, but they did not always beg at true parents. Recent conceptual studies have shown that partial honesty, particularly if the signal is low cost to produce, may be an evolutionary stable strategy. It is likely that begging is a low-cost behavior for tern fledglings, so begging at non-parents may have more potential benefits than costs, even if non-parents are rarely fooled and begging at them often does not result in feeding. The relative lack of benefits to be gained from begging at non-parents has likely resulted in mostly honest communication between tern fledglings and adults; however, the low cost of this behavior may keep deceitful begging present at low frequency because the possible benefits of extra-parental care outweigh the low cost of deceitful begging.


Relationship between begging behavior and date of staging season. As the staging season progressed, juvenile roseate terns begged at parents less frequently, but they continued to beg throughout the staging season.

Begging behavior increased with time of day as would be expected if fledgling needs were highest before nightfall and further supports our finding of honest communication between parents and offspring. We also found that begging behavior decreased throughout the staging period. However, tern fledglings continued to beg at their parents even at the end of the staging period, albeit at reduced frequency relative to the beginning of the staging season. This may be evidence to suggest that parental care continues past the staging period into migration and possibly the wintering period. If this is true, begging behavior may function as more than a signal to indicate need; it could also function to reinforce the parent-offspring bond prior to migratory departure from staging areas.

Essential waters: young bull sharks in Fiji’s largest riverine system

Written by Kerstin Glaus. Read the full article here.

The bull shark (Figure 1) is one of the few sharks that can freely swim between fresh, -and saltwater environments. Although the bull shark occurs pantropically, there is a large knowledge gap in their distribution and habitat use patterns of neonate, young-of-the-year (YOY) and juvenile bull sharks between regions. So far, such information has been gathered primarily in the northern Gulf of Mexico, in Florida and on the east coast of Australia. According to previous studies, young age classes of bull sharks occupy environmentally heterogeneous habitat and age-associated habitat transitions have been documented with YOY bull sharks occupying locations with lower mean salinities than juveniles, while sub-adults were more abundant in nearshore marine areas. Within coastal environments, juvenile bull sharks reportedly have an affinity for mesohaline areas. However, even within the same species, such habitat requirements can differ between and across regions and alter due to changing environments. To date, distribution and habitat use patterns of young bull sharks, are largely lacking from historically data-poor regions such as the South Pacific.

South Pacific Islands are among the least known or understood regions in the world. This lack of knowledge is highlighted by the fact that two aggregation sites for the young of several shark species have been discovered in just the last three years. Fiji is the South Pacific’s economic center. The archipelago consists of more than 330 islands, but the vast majority of the population inhabits the two main islands Viti Levu and Vanua Levu. Located off Viti Levu’s south coast, adult bull sharks can be studied year-round in the Shark Reef Marine Reserve (SRMR), the country’s first national marine park. Contrastingly, the exact location of essential habitats for young bull sharks and associated environmental parameters are either only preliminary investigated or virtually unknown. As a result, we have little data about essential fish habitats (EFH) for young age classes of bull sharks in Fiji.

Figure 1: A bull shark (Carcharhinus leucas) photographed in Fiji’s Shark Reef Marine Reserve. Copyright Valerie Taylor

To bridge this knowledge gap, a team of four researchers started a two-year vessel based survey in the Rewa River to the east, the Navua River to the south and the Sigatoka River to the west of Viti Levu (Figure 2). We aimed to confirm the occurrence of young bull sharks in several riverine systems, to determine their distribution and abundance in the rivers, and to collect environmental parameters at capture sites.

Our surveys usually started at low tide and typically lasted between two to six hours per day depending on weather conditions. We placed captured bull sharks in an on-board tank filled with river water (Figure 3). The following parameters were recorded for each individual caught: total straight length (Figure 4), umbilical scar condition (open, semi-healed, healed), and sex. Also, captured bull sharks were tagged with an internal Passive Integrated Transponder below the first dorsal fin for individual identification (Figure 3) prior to release (Figure 5,6). In addition, using a water quality meter, surface and bottom water temperature, dissolved oxygen and salinity were recorded at the respective sampling locations in the Rewa and Sigatoka Rivers at the beginning and end of each fishing survey.

Figure 2: The Rewa, Sigatoka and Navua Rivers in southern Viti Levu. Dashed inlets denote the stretches that were sampled
Figure 3: A neonate bull shark captured in the Rewa River, placed in an on-board tank filled with river water. The specimen is tagged with an internal Passive Integrated Transponder below the first dorsal fin for individual identification
Figure 4: A neonate bull shark captured in the Sigatoka River and placed on a board for total straight length measurement.
Figure 5, 6: Bull sharks are released after length measurement, sex determination, PIT tagging and assessment of the umbilical scar conditions. The whole procedure does not take more than 40-70 seconds.

After more than two years of extensive sampling, we captured 159 neonate and YOY bull sharks in the Rewa River and are now able to show that the Rewa River may be a hot-spot for the study of neonate bull sharks in Fiji. The study covers the first multi-year assessment of young bull shark’s occurrence and distribution across several rivers in a Pacific Island Country. Also, we examined and compared environmental conditions of two rivers, showing that the environmental profile with the highest bull shark abundance in the Rewa River typically was oligohaline and that young bull sharks are more likely to occur in the Rewa than in the Sigatoka River.

The poor knowledge of population trends in bull sharks in this unique upwelling region, together with habitat alterations and an increasing local demand for shark products for domestic consumption may lead to a potential decline of some age-classes of different elasmobranch species that may go unnoticed. Our data helps us to learn more about the bull shark’s distribution and abundance, information that is essential for studies of the species life-cycle. These new insights can provide a foundation for the urgently required assessments of essential shark habitats within the South Pacific.

Vertebrate invasions don’t seem to conform to the norm

Blog written by Marcus Lashley. Read the full article here.

Biological invasions are one of the biggest threats to biodiversity globally. Because of the pervasive threat invasions pose, understanding basic principles of invasion and how those invasions affect biodiversity is of primary interest to ecologist and conservationist. A fundamental scale-dependent relationship has been observed repeatedly across plant and invertebrate invasions. That is, invasion commonly suppresses biodiversity at small or local scales but as the spatial scale considered increases, biodiversity is either not affected or facilitated at large scales. This could happen for several reasons but most commonly, invaders regulate the dominance of a native species which can release other species in the same community ultimately resulting in the scale-dependent pattern. The scale-dependent pattern has been generalized to include vertebrate invasions but because they are often more difficult to study, to our knowledge the scale-dependence hypothesis has not been explicitly tested in a vertebrate system.

There are three basic requirements to test the hypothesis: 1) a vertebrate invasion, 2) an estimate of biodiversity with and without the vertebrate invader, and 3) an estimate of biodiversity with and without the invader across spatial scales. In the United States, one of the most problematic nonnative vertebrates is the feral pig (Sus scrofa). Feral pigs are problematic because of their foraging behavior and their taste for agricultural crops. Interestingly, in agroecosystems, land clearing often results in fragmentation of forests leaving relatively isolated forest fragments similar in composition but varying in size. Feral pigs use those forest fragments for cover while not foraging on surrounding crop fields. Thus, an agroecosystem where feral pigs have invaded may provide all three basic needs to test the scale dependence hypothesis if forest fragments have a predictable species-area relationship.

Feral pig (Sus scrofa)

We established camera traps in 36 forest fragments ranging across four orders of magnitude in area to estimate species richness of forest fragments. With camera traps, we knew many species would not be detected, however, it is common to take a subset of biodiversity in these types of studies given it is nearly impossible to tally all species present. In all, we detected 41 species, including feral pigs in 11 of the forest fragments. We took the 25 forest fragments without feral pigs and ran a simple linear model to determine that the number of species detected was very well predicted by the area of the forest fragment. The number of species detected increased as the forest fragment area increased. In other words, based on the area of a forest fragment, we can accurately predict how many species should be detected. Using that species-area relationship as a basis for how many species should be detected in a forest fragment of a given area, we then used a similar model to evaluate the number of species detected in forest fragments where feral pigs had invaded. If feral pigs are negatively affecting species richness in a scale dependent pattern, we should expect the predicted species richness across scale to have two basic properties as it relates to uninvaded fragments: 1) the y-intercept of the line should be lower than in the uninvaded estimate and 2) the slopes of the lines should not be parallel. Indeed, in forest fragments where feral pigs had invaded the y-intercept was 26% lower indicating we detected 26% fewer species than should have been expected given the fragment area. The slopes of the two lines were parallel indicating this reduced species richness occurred across the range in forest fragment size when pigs had invaded.  Collectively, those results do not support the scale dependence hypothesis in this vertebrate invasion calling into question how generalizable the scale dependence hypothesis is to vertebrate invasions.

Log–Log relationship between species richness and forest fragment area in the Mississippi Alluvial Valley invaded (solid line and solid points) and absent (broken line and hollow points) of feral swine.

In this study, we did not establish causation, so it would not be appropriate to assume that feral pigs cause the reduction in biodiversity. However, our observations are similar to other causative studies which show invasions reduce diversity by 19-27%. Moreover, feral pigs are competitors and predators of many of the native wildlife in this ecosystem, so it is plausible that they do suppress biodiversity through those mechanisms. Ultimately, more research is needed to determine if feral pigs are causing this reduction in biodiversity and to determine if the lack of scale dependence applies to other vertebrate invasions.

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