Daniel Polakow

Predicting invasion dynamics of four alien Pinus species in a highly fragmented semi-arid shrubland in South Africa

This study explored the determinants of spread of four alien Pinus species and the ability of models to predict invasion dynamics in a complex fragmented landscape. The role of environmental factors, natural and anthropogenic disturbance in relation to invasion history was assessed for different stages in the invasion process using a Geographic Information System. Pines escaped from plantations over the past 30 years and spread into the natural semi-arid shrubland (renosterveld). The pattern of spread was compared with a simulated random distribution using two different techniques, a standard logistic regression, and a new recursive modelling approach (Formal Inference-based Recursive Modelling; FIRM). FIRM analysis improved the accuracy of predictions and revealed interactive effects of variables hidden by the logistic regression analysis. More than 80% of isolated pine individuals were found in 20% of the habitat classified as suitable by the models. Soil pH was the most important predictor for the distribution of isolated trees, whereas the establishment of dense pine stands was largely determined by fire history. Differences in invasive behaviour could be explained by species attributes such as limited dispersal for P. canariensis, and better drought-tolerance for P. halepensis. Sixty-five percent of the current pine distribution was accurately predicted by the spatial distribution of the first trees to have invaded. Such models could be used to predict potential spread of invasive plants and gain a better understanding of the main factors driving the invasion process. However, the spread of invasive species in fragmented landscapes, strongly modified by human activities, is very complicated, and the spread remains difficult to predict in the long term. The dynamics of invasion are discussed in relation to changes in land use and disturbance regime.

Modelling fire-return interval T: Stochasticity and censoring in the two-parameter Weibull model

Abstract Biologists are heavily reliant on the characterisation and description of disturbance phenomena and disturbance regimes. This dependency is well illustrated within research on fire recurrence and models of fire-frequency. It is well understood that the process of fire-return is subject to stochastic variation. However, a deterministic paradigm underlies many contemporary studies of fire-frequency to the possible detriment of robust ecological description. It is also commonplace for data on historical fire processes to contain some degree of either partial or missing data. Yet the parameter estimators of the prevailing two-parameter Weibull model of fire-recurrence are rarely seen to account for incomplete data structuring. The consequences of ignoring the structure of incomplete data will likely lead to misrepresentation of predicted fire frequencies and trends. In this paper, we introduce methods for incorporating parameter stochasticity into the two-parameter Weibull model under different degrees of data censoring. Both the explicit handling of censored fire observations and the intrusion of variability about the average predicted patterns in inter-fire interval recurrence are obligatory to any modelling that admits an effective ecological and evolutionary interpretation. These points are well illustrated through a worked example on modelling the fire-return interval regime within the Cape of Good Hope Nature Reserve, South Africa. Here, the observed process of fire recurrence is seen to include a substantial stochastic component about the trends predicted from the two-parameter Weibull model. The implications of a stochastic fire-regime interpretation to both management practises and to the evolutionary understanding of fynbos vegetation are discussed.

Ecotourism ventures: rags or riches?

Abstract The diminishing wilderness areas remaining in southern Africa offer opportunities to establish and market ecotourism destinations. It would thus be useful to gain insight into what conditions make certain ventures viable. With the aid of a model and using data from Ongoye Forest in South Africa, the long-term financial viability of three operational ecotourism scenarios is assessed: upmarket lodges, middle-of-the-range chalets, and campsites. The resulting “preferred” scenario depends on a specific weighted interest in favor of the community, the environment, or the investor. The economic realities of southern Africa suggest that without government support, it may be risky to invest extensive initial capital into the larger scale ecotourism projects proposed.

Numerical recipes for disaster: changing hazard and the stand-origin-map

The quantitative study of predictable temporal characteristics of disturbance phenomena is of tremendous consequence for both an effective understanding of biological process and for successful implementation of sustainable practices. This reliance is well demonstrated in both pure (e.g. biodiversity research) and applied (e.g. forestry sciences) research into patterns of fire-recurrence and its consequent interpretation and management implications. Much of fire-frequency prediction is dependent on the data manifest in a stand-origin-map. In this paper, we review the statistical methods for fire-frequency estimation from a stand-origin-map, where fire-recurrence is governed by an underlying probabilistic process that changes with time since last occurrence (i.e. non-constant hazard). We argue that both patterns of fire-recurrence that change with time are likely to be commonplace biological phenomena and that the conventionally prescribed methods for non-constant hazard fire-recurrence estimation from a stand-origin-map are erroneous. Even modified maximum-likelihood techniques that account for different degrees of data censoring are likely to be severely restrained in their capacity for rigorous estimation. We propose a novel formulation whereby the two-parameter Weibull distribution can be fitted retrospectively to the data in a stand-origin-map. This formulation is explored through an example from the Canadian Rockies.

How many independent bets are there

The benefits of portfolio diversification are a central tenet implicit in modern financial theory and practice. Linked to diversification is the notion of breadth. Breadth is correctly thought of as the number of independent bets available to an investor. Conventionally, applications using breadth frequently assume only the number of separate bets. There may be a large discrepancy between these two interpretations. We utilise a simple singular-value decomposition and the Kaiser–Guttman stopping criterion to select the integer-valued effective dimensionality of the correlation matrix of returns. In an emerging market such as South Africa, we document an estimated breadth that is considerably lower than anticipated. This lack of diversification may be because of market concentration, exposure to the global commodity cycle and local currency volatility. We discuss some practical extensions to a more statistically correct interpretation of market breadth and its theoretical implications for both global and domestic investors.

The functional significance of tibial displays in the damselflyPlatycypha caligata (Selys) (Idonata: Chlorocyphidae)

Animal communication repertoires typically include visual displays that incorporate color and accentuated morphological structures (Hailman, 1977; Sebok, 1977; Burtt, 1979; HaUiday and Slater, 1983). However, experimental verifications of the functional significance of visual displays are infrequent, probably because of the difficulties associated with direct manipulation of the structures involved in signaling (but see Smith, 1972; Rohwer, 1977; Slater, 1983). The tibiae of the first pair of legs of male Platycypha caligata damselflies are laterally expanded with white anterior and red posterior surfaces (Pinhey, 1951). These structures are used to generate visual communication signals that are central to courtship and territoriality (Robertson, 1982a,b). Platycypha caligata males intercept and attempt to court females (that traverse their territories) by circling them while laterally waggling (sensu Robertson, 1982a) their white tibial surfaces. Successful courtship culminates in females landing in the territory and allowing tandem formation [see Robertson (1982a) for a detailed description]. Resident males defend their territories by chasing off intruders and engaging in display interactions. Displays are initiated when intruders land in occupied territories. Displays comprise a brief lateral waggle of the white tibial surfaces, followed by complex flight maneuvers and a sequence of flash displays, in which both males raise their tibiae to reveal the red surfaces (Robertson 1982a). Although the use of visual signals in damselfly courtship and territorial defense is documented (Jacobs, 1955; Waage, 1973; Williams, 1977; Robertson and Paterson, 1981; Robertson, 1982b), this study is the first to investigate

The global currency tango — The relationship between the carry trade, emerging/commodity currencies and risk

ABSTRACT This study has foci on the global drivers of currencies and their relationship to economic jurisdiction in the presence of global risk appetite. We focus on a comprehensive basket of global currencies, deriving three statistically motivated currency market factors. We cluster on these factor loadings and find three currency groupings – Developed/European, Emerging/Commodity as well as an Asian currency cluster. Constructing ‘risk states’ based on the VIX Volatility Index, we find that Developed/European and Emerging/Commodity cluster currencies are better explained by the first three principal components when in the ‘Low’ and ‘High’ risk states respectively. As previously corroborated in the literature, we find evidence that the second principal component is a ‘carry trade factor’. We detail that the impact of this factor on currencies in the Emerging/Commodity cluster is heightened (for both positive and negative changes) by ‘High’ risk states.

How disruptive can Smart Beta be to the South African Active Fund Management Fraternity

ABSTRACT This research paper explores the theme of Smart Beta and the current state of inefficiency of active fund fee structures in South African domestic equity unit trusts. The emerging understanding is that many of the latent sources of value-add (or alpha) of active fund managers are currently accessible in cheaper form via Smart Beta products. Smart Beta products use mechanical and automated rules to establish exposures to tradeable instruments that emulate many of the understood and replicable themes in current active asset management. We sample 91 well-known general equity funds along with nine local Smart Beta funds and demonstrate how disruptive Smart Beta products could be to the fee structures of many of these active funds. We do this by mapping the reproducible elements of the active return of these funds to fungible Smart Beta factors. We conclude with five broad predictions around the active fund management industry in South Africa. Additionally, we focus on how active managers might prudently align themselves with an understanding of what aspects of their value-add are not replicable in order to persist and thrive.