1,721,065 research outputs found
The use of ringing data in the study of climatic influences on common passerines
No full textTo understand the potential impact of forecasted increases in climatic variability we need to determine the impact of climatic stochasticity on demographic rates. This thesis used available long-term ringing data collected by volunteers, augmented by data from research projects, to investigate the influence of climatic variation on survival of 10 common passerines in southern Africa. Through sheer numbers common species are fundamental to ecosystem functioning. Migratory species are subject to climatic stochasticity in breeding and wintering grounds, and during migration. In a population of African Reed Warblers Acrocephalus baeticatus (an azonal wetland specialist) a capture-mark-recapture model correlated higher temperature in the breeding grounds with higher adult survival (1998-2010), but - contrary to expectations - not wetter winters. A spatial analysis using a multi-state model in a Bayesian framework did not link survival in populations across southern Africa to environmental seasonality. However, as hypothesised, migratory populations appeared to survive better than sedentary populations. Increased climatic variation could synchronize survival of species assemblages and colonies in meta-populations. I investigated a 3-species assemblage in climatically stable fynbos (2000-2007) and a 4-species assemblage in more seasonal wetland (1999-2013) with a hierarchical model, run in WinBUGS, with a temporal, synchronous (common) and asynchronous (species-specific) component. Comparison of models with and without climatic covariates quantified the impact of climatic stochasticity as a synchronizing and desynchronizing agent. As expected, the wetland assemblage exhibited more synchronous and asynchronous variation in survival than the fynbos assemblage, but the analysis did not find evidence of climatic forcing. Demographic rates of a population of 25 colonies of a Sociable Weaver Philetairus socius meta-population in savanna near Kimberley did not correlate with climatic indices during 1993-2014. Age-specific survival and fecundity of the largest colony were influenced by climatic variation reinforcing earlier inference that colonies respond differently to environmental stochasticity. The integrated population model using count, ringing, and productivity data enabled the first estimation of annual fecundity, juvenile survival and recruitment. The volunteer data yielded the first estimates of adult survival of two African endemics and estimates of a second population for three other species. A review of volunteer ringing resulted in recommendations to improve its use from a demographic perspective
Effects of protected areas and climate change on the occupancy dynamics of common bird species in South Africa
Full text linkProtected areas are tracts of land set aside primarily for the conservation of biodiversity and natural habitats. They are intended to mitigate biodiversity loss caused by land-use change worldwide. Climate change has been shown to disrupt species' natural distributions and patterns, and poses a significant threat to global biodiversity. The goals of this thesis are to address these important issues, and understand how protected areas and climate change affect the range dynamics of common, resident bird species in South Africa. Common species were used because they have been shown to drive important ecosystem patterns, and a decline in abundance and diversity of common species can indicate drastic declines in ecosystem integrity. This thesis comprises four data chapters; in the first three I model the occupancy dynamics of 200 common, resident bird species in South Africa to gain an understanding of how the proportion of protected areas within a landscape affects common species. For the last data chapter, I examined the effects of protected areas and a changing climate on the range dynamics of Cape Rock-jumper (Chaetops frenatus), a species endemic to the southwestern part of South Africa and whose population is declining rapidly in response to climate change. I modelled its occupancy dynamics in relation to climate, vegetation, and protected area. Overall, my key findings show bird abundances vary widely as a function of protected areas, but on average, bird abundances are higher in regions with a higher proportion of protected areas, compared to regions with a lower proportion. I found that the conservation ability of protected areas was influenced by the type of land-use found in the surrounding landscape. For example, the extent of agricultural land in proximity to a protected area significantly increased the mean abundance of birds in that protected area, whilst the average abundance of most species was not affected by the extent of urban area near protected area. On average, species preferentially colonized and persisted within landscapes with a higher proportion of protected area, compared to landscapes with a lower proportion of protected area. However, protected areas were not able to slow the extinction rate for all species, and the average extinction rate for some groups of species actually increased as the extend of protected areas within a landscape increased. Furthermore, Cape Rock-jumper also preferentially occupied regions with higher proportions of protected area. Despite this, Cape Rock-jumper’s range is predicted to shrink considerably in response to a hotter and mildly drier climate forecast for the region. As a result, Cape Rock-jumper will likely be of conservation concern as the climate over its range continues to change. I conclude that, in general, protected areas are effective at conserving common bird species over a heterogeneous landscape in South Africa, and should be prioritised as key conservation strategies in the future. I further conclude that climate change will be a concern to an endemic species, and to biodiversity in general. This will likely place extra stress on the importance of protected areas to mitigate responses of species to climate change
Multistage density dependence in an amphibian
Full text linkDensity dependence is the major process keeping the sizes of natural populations within bounds. In organisms with complex life cycles, the stage at which density dependence occurs and whether it occurs in one or several life stages have important consequences for the dynamics of their populations. I manipulated density of pool frogs (Rana lessonae) during the aquatic larval and the terrestrial juvenile stages and examined the effect on growth and survival until 1year of age. High larval density, but not high juvenile density, led to smaller size at this age. Both larval and juvenile density led to reduced growth during the early juvenile stage, but the effect of the larval density appeared stronger than the effect of juvenile density. No density dependence in survival could be found. My results suggest that density dependence in both the larval and the terrestrial juvenile stage may play important roles in the regulation and dynamics of amphibian population
Bird's response to seasonality: investigating the range dynamics of birds through dynamic occupancy models
Full text linkBirds can respond to seasonal environmental fluctuations through migration. Among different migratory species and populations, there is varying sensitivity to different seasonal environmental cues, thus leading to different seasonal range dynamics and migration strategies. In this study, I used dynamic occupancy models on the South African Bird Atlas Project 2 (SABAP 2) data to investigate the seasonal range dynamics of a selection of five migratory and six nomadic bird species in western South Africa. In the context of this study, seasonal range dynamics are defined as when a species arrives and departs a given area. I used dynamic occupancy models to estimate monthly changes in occupancy between 2014 and 2018. I modelled local extinction (departure) and colonisation (arrival) as a function of changes and anomalies in average monthly rainfall, temperature and vegetation. Among the obligate migrants, the best performing models indicated that colonization and extinction parameters were mostly driven by seasonal changes in average rainfall, temperature and vegetation. In nomadic species, colonization was driven by seasonal changes in average rainfall, temperature and vegetation, while extinction was driven by anomalies in rainfall, temperature and vegetation. The models successfully captured the generally known seasonal pattern in occupancy (arrival and departure) of all five obligate migrants, while no regular seasonal fluctuation in occupancy was evident among the nomadic species. Over the four-year period, no species showed a strong shift in seasonal range dynamics in both groups. However, I do suspect that the same methodology done at a finer spatial and temporal scale may reveal changes in seasonal range dynamics of some species. This study demonstrates that dynamic occupancy modelling using citizen science data is a viable methodology for investigating seasonal range dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Does supplemental feeding affect the viability of translocated populations? The example of the hihi
No full textTranslocations are commonly used conservation actions that aim at establishing new, self-sustaining populations of threatened species. However, many translocated populations are not self-sustaining but managed through supplemental feeding from the onset. Often, the decision to start managing is ad hoc, but managers will eventually have to make decisions for the future, for example, stop intervening, continue as it is or change the quantity of food provided. Such a decision requires managers to quantify the importance of supplemental feeding in determining the performance and population dynamics of translocated populations, information that is rarely available in the published literature. Using the hihi as a case study, we examined the importance of supplemental feeding for the viability of a translocated population in New Zealand. We found that supplemental feeding positively affected the survival and abundance of translocated adult hihi but also found evidence of negative density dependence on recruitment. We present two stochastic population models that project the hihi population under different management scenarios, quantitatively assessing the impact supplemental feeding has had on the population. Our results illustrate how important long-term targeted monitoring is for robust decision making about adaptive management. © 2012 The Zoological Society of London
Calling ecology of micro frogs (Microbatrachella capensis): a case study using acoustic spatial capture recapture
Full text linkAmphibians worldwide are declining, increasing the demand for monitoring populations of many threatened amphibians, including the South African Critically Endangered micro frog (Microbatrachella capensis). I attempted to improve ongoing monitoring efforts by determining the calling ecology of the micro frog population on the Cape Flats. I used acoustic spatial capture recapture to calculate call density and identify the main factors that determine periods of maximum frog calls. Increased calling behaviour was found early in the season and in response to rainfall. In addition, micro frogs were found to call more at night than during the day. This suggests that future monitoring of this species should occur at night, after rain, and early in the winter breeding season. From the call density estimates, I calculated the size of the micro frog population on the Cape Flats to be about 200 adult frogs when an equal sex ratio of adult males and females is assumed. Future monitoring and conservation efforts should take into consideration this baseline population estimate and keep track of any demographic trends in the population
Geographic range dynamics of South Africa's bird species
Full text linkA key issue in species conservation is a knowledge of the geographic ranges of species, and how these are changing through time. For birds there is a special opportunity to undertake studies of range changes, making use of the data collected by the First and Second Southern African Bird Atlas Projects (SABAP1 and SABAP2), which are separated in time by about two decades. In this thesis, I first describe the strengths and the weaknesses of the databases collected by these two citizen science projects, and therefore discuss the limitations placed on the analyses. We then undertake two sets of analyses, one focused on species, and one focused on areas. I show that, across all species, the Family to which the species belongs is an explanatory variable which explains approximately 45% of range expansion or contraction of a species. Diet and mass are also significant explanatory variables. For the analyses by areas, we demonstrate that the general encroachment of shrubs and trees in the savanna biome appears to have had a profound impact on the occurrence and abundance of a large suit of bird species, with the small insectivores and frugivores showing the largest increases
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