Browsing by Author "Stears, Keenan."

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Key factors driving the foraging ecology of Oribi : fear, cattle and the quality and quantity of food.
(2015) Stears, Keenan.; Shrader, Adrian Morgan.
Body size and digestive physiology are two factors that influence herbivores foraging behaviour, habitat use and potential to compete with other animals. The aim of this study was to determine how seasonal changes in grass quality, interactions with cattle, and perceived predation risk influenced oribi (Ourebia ourebi) foraging and landscape use. Oribi are one of the smallest pure grazing ruminants. As a result, they should select high quality vegetation. In line with this, I found that throughout the study oribi fed selectively over multiple spatial scales ranging from plant parts to habitats. By focussing on green grass within these different scales, oribi were able to maintain their crude protein intake needed for maintenance. Throughout South Africa, oribi frequently interact with cattle. Due to their differences in body size and nutritional requirements, there should be sufficient resource partitioning to avoid competition. However, I found that the nature of the interspecific interactions (i.e. competition or facilitation) between these species depended on season and cattle stocking rates. During the wet season, cattle facilitated oribi by providing high quality regrowth. However, at high stocking rates, cattle indirectly competed with oribi during the dry season via the impacts of their wet season grazing. Specifically, intense wet season grazing by cattle reduced the availability of high quality grass for oribi in the dry season. This was not the case at low and intermediate stocking rates. Differences in food availability and predation risk across habitats can influence how herbivores utilise landscapes. I found that predation risk greatly affected oribi foraging behaviour, with oribi preferring to feed in safer rather than riskier habitats. However, when food availability increased in these risky habitats, oribi increased their risk-taking behaviour at both small- and large-scales within these risky areas. Ultimately, this suggests that oribi trade-off between predation risk and food availability. Finally, to link my results to the management and conservation of this vulnerable antelope, I applied the knowledge I gained from the above research to a case study.
Population dynamics and relocation success of the oribi antelope (Ourebia ourebi) in KwaZulu-Natal, South Africa.
(2015) Patel, Tamanna.; Shrader, Adrian Morgan.; Stears, Keenan.; Little, Ian Tchagra.
In South Africa, oribi (Ourebia ourebi) antelope are listed as vulnerable. The lack of understanding of their population dynamics makes it difficult for oribi conservation. To address these gaps, I used the Oribi Working Groups’ long-term survey database to determine 1) the trends (increasing, decreasing, stable) in oribi populations across KwaZulu-Natal, 2) the spatial distribution of these trends across the province, and 3) the factors influencing these trends. The overall oribi population trend for KwaZulu-Natal was linked with the number of survey returns submitted. This highlights the importance of landowners submitting consistent returns, resulting in more accurate population estimates. The majority of oribi populations across the province had decreasing population trends. I found that initial population size and the amount of suitable habitat available significantly affected oribi population growth rates. These growth rates increased when the availability of suitable habitat increased. In addition, grazing regime influenced growth rates. However, the variance observed was high, signifying that there may be other factors that are also responsible for driving these growth rates. Dog hunting was non-significant, however, because it is illegal, it was difficult to accurately measure its effect on oribi populations and thus should not be dismissed as a potential threat. Relocating oribi has been used as a conservation tool over the past 16 years. However, the success of these relocations has been poorly documented. To address this, I determined 1) the success rate of previous oribi relocations in KwaZulu-Natal, 2) the factors driving the success/failure of the relocations, and 3) whether relocation is a successful tool for the conservation of oribi in South Africa. I found a relocation success rate of only 10% (N = 1). Moreover, I found that following basic relocation criteria (e.g. the removal of threats (such as predators) or long-term post-release monitoring) was important in assuring relocation success. In all instances where relocations failed, key criteria were not considered prior to the relocation. This was in contrast to the one successful relocation, where all the criteria were considered and followed. Similar to my first study, I found that oribi population size, the availability of suitable habitat, and stocking rates of other large herbivores influenced growth rates, and ultimately, relocation success. Moreover, I found a significant interaction between suitable habitat available and stocking rates and their influence on population growth rates. Ultimately, this study highlights key factors that must be considered in any conservation or management decisions for oribi. In addition, prior to a relocation, landowners need to follow the basic criteria for successful relocations.
Scrounging herbivores use both patch quality and dominance status of patch holders when deciding which patch to join.
(2011) Stears, Keenan.; Shrader, Adrian Morgan.; Kerley, Graham I. H.
One of the major costs of group living is increased competition due to social information. Social information allows foragers to gain information about the location and the quality of food patches from observing other group members. Ultimately, this allows social foragers to use resources more efficiently. However, the distribution and quality of food varies both spatially and temporally and social information may result in aggregations of foragers around favourable food patches when they are available. This results in individuals of various dominance ranks aggregating around food resources and competing for food. In order to understand how dominance and patch quality interact to determine foraging behaviour, various foraging models have been created. In particular, producer-scrounger social foraging models are used to understand foraging behaviour and patch choice under competitive conditions. In producer scrounger games, individuals can either find their own food patches (produce) or join other individuals at food patches (scrounge). This study focused on how the combination of patch holder dominance status and patch quality interacts to influence patch joining decisions by scroungers. According to producer-scrounger models scroungers only join patches held by subordinate individuals. However, I found that a scrounger will join any patch as long as the patch holder is not within the top five most ranked individuals in the herd. However, as patch quality increased, fewer of the top ranked patch holders were avoided at each patch quality. This suggests that foraging is a trade-off between the costs of an aggressive interaction and the benefits gained from each patch. Behavioural titrations found that the initial density of food at a patch needs to be 2.3 times greater for a scrounger to feed from the next dominant patch holder. At high patch qualities there was a threshold point where patch quality became the driving force behind patch joining decisions and no patch holders were avoided, no matter their dominance status. Scroungers that fed from the top ranked patch holders had the same intake rate when compared with feeding with subordinate patch holders. This could be due to an increased level of vigilance by the scrounger in order to avoid an aggressive interaction with the dominant patch holder. However, scrounger’s intake rate increased when they fed from dominant patch holders that were not the top ranked herd members. This suggests that only the top ranked herd members are aggressive enough to prevent scrounging attempts. A novel discovery of this study was that although the goats formed a linear dominance hierarchy, they did not forage in accordance with it, with lower ranking individuals avoiding dominant patch holders. This suggest that in social herbivores dominance hierarchies are not maintained to determine who has priority access to food. This study shows that even large differences in dominance are not sufficient enough to prevent scrounging decisions and only the top ranked patch holders have the ability to prevent joining attempts.