Single-factor Explanations Research Articles

Countering Democratic Decay Judicially: Is Resistance Futile?

Abstract With democratic decay advancing around the globe, scholars deeply disagree about courts’ ability to protect democracy. I argue that this divergence is influenced by conceptual and theoretical deficiencies of the debate. The existing approaches to judicial responses to democratic decay often do not pay sufficient attention to the differences among courts and the environments in which they operate. To tackle these issues, this article introduces the judicial countering capacity frame as an analytical tool to guide further empirical analysis. The framework recognizes courts’ agency but emphasizes the constraints and resources stemming from the environment in which a court operates. Moving away from single-factor explanations, I make a case for a multicausal understanding of judicial countering of democratic decay. I argue that the extent of a court’s countering capacity rests on three pillars: institutional design, the level of political pluralism, and political culture towards courts. The judicial countering capacity frame invites an empirically oriented agenda focusing on micro-mechanisms of judicial countering, their effectiveness, structural preconditions, and the plausibility of available judicial strategies. The article demonstrates the new frame on case studies of Colombia, South Africa and Poland.

Geography is not destiny: A quantitative test of Diamond's axis of orientation hypothesis.

Jared Diamond suggested that the unique East-West orientation of Eurasia facilitated the spread of cultural innovations and gave it substantial political, technological and military advantages over other continental regions. This controversial hypothesis assumes that innovations can spread more easily across similar habitats, and that environments tend to be more homogeneous at similar latitudes. The resulting prediction is that Eurasia is home to environmentally homogenous corridors that enable fast cultural transmission. Despite indirect evidence supporting Diamond's influential hypothesis, quantitative tests of its underlying assumptions are currently lacking. Here we address this critical gap by leveraging ecological, cultural and linguistic datasets at a global scale. Our analyses show that although societies that share similar ecologies are more likely to share cultural traits, the Eurasian continent is not significantly more ecologically homogeneous than other continental regions. Our findings highlight the perils of single factor explanations and remind us that even the most compelling ideas must be thoroughly tested to gain a solid understanding of the complex history of our species.

The Broad Reach of Multivariable Thinking

Simple explanations are very often inadequate and can encourage faulty inferences. We examined college students’ explanations regarding illegal immigration to determine the prevalence of single-factor explanations. The form of students’ explanations was predicted by their responses on a simple three-item forced-choice multivariable causal reasoning task in which they selected the strongest evidence against a causal claim. In a further qualitative investigation of explanations by a sample of community adults, we identified positive features among those who scored high on this multivariable causal reasoning task. We consider limitations of single-factor reasoning and means of encouraging more comprehensive explanations to support claims.

Using individual-based modelling to investigate a pluralistic explanation for the prevalence of sexual reproduction in animal species

Attempts to resolve the paradox of sexual reproduction in animal species have typically involved single factor explanations, including but not limited to sexual species being better able to cope with fluctuating resources, being better able to evade pathogens and predators and sex facilitating the elimination of deleterious alleles. Only a handful of studies have investigated two potentially interacting factors, which most commonly involve the ability to evade pathogens and predators and mutation load; only two simulation studies by the same authors have examined the potential interaction of 3 factors.. Our study is the first three-factor study to examine the potential interaction between purifying selection, the ability to avoid predators and the ability to cope with fluctuating resources. Therefore, we addressed a number of open eco-evolutionary questions in our study: First, what is the effect of these three factors on the prevalence of sexual reproduction in animals? Second, do these factors act integratively, antagonistically or independently in determining the prevalence of sexual reproduction The ‘controls’ were our three prior studies where we investigated each of these factors individually. Our aim was to determine if the effect sizes in our individual studies were integratively enhanced by the interaction with the remaining two factors. What we found is that purifying selection purging deleterious alleles in sexual species was integratively enhanced by the two additional factors of increased predation and fluctuating resources. On the other hand, the effect of purifying selection and increased predation had an antagonistic effect on the levels of sexual reproduction and asexual reproduction in facultative species where there were fluctuating resources. Finally, the presence of an intermediate intensity deleterious allele and fluctuating resources had no discernable effect on the extinction levels of sexual and asexual species where there is increased predation. These results are not surprising, as there is no a priori reason to assume that multiple factors will have a greater impact on mode of reproduction than any of these factors acting independently. Our results suggest that multiple factors can act intergratively, antagonistically and non-reductively all at the same time, which is a significant contribution to the open question in the biological literature regarding the effect of multiple factors on the prevalence of sexual reproduction.

Interpreting global variations in the toll of COVID-19: The case for context and nuance in hypothesis generation and testing.

As of January 2022, the COVID-19 pandemic was on-going, affecting populations worldwide. The potential risks of the Omicron variant (and future variants) still remain an area of active investigation. Thus, the ultimate human toll of SARS-CoV-2, and, by extension, the variations in that toll among diverse populations, remain unresolved. Nonetheless, an extensive literature on causal factors in the observed patterns of COVID-19 morbidity and cause-specific mortality has emerged-particularly at the aggregate level of analysis. This article explores potential pitfalls in the attribution of COVID outcomes to specific factors in isolation by examining a diverse set of potential factors and their interactions. We sourced published data to establish a global database of COVID-19 outcomes for 68 countries and augmented these with an array of potential explanatory covariates from a diverse set of sources. We sought population-level aggregate factors from both health- and (traditionally) non-health domains, including: (a) Population biomarkers (b) Demographics and infrastructure (c) Socioeconomics (d) Policy responses at the country-level. We analyzed these data using (OLS) regression and more flexible non-parametric methods such as recursive partitioning, that are useful in examining both potential joint factor contributions to variations in pandemic outcomes, and the identification of possible interactions among covariates across these domains. Using the national obesity rates of 68 countries as an illustrative predictor covariate of COVID-19 outcomes, we observed marked inconsistencies in apparent outcomes by population. Importantly, we also documented important variations in outcomes, based on interactions of health factors with covariates in other domains that are traditionally not related to biomarkers. Finally, our results suggest that single-factor explanations of population-level COVID-19 outcomes (e.g., obesity vs. cause-specific mortality) appear to be confounded substantially by other factors. Our methods and findings suggest that a full understanding of the toll of the COVID-19 pandemic, as would be central to preparing for similar future events, requires analysis within and among diverse variable domains, and within and among diverse populations. While this may seem apparent, the bulk of the recent literature on the pandemic has focused on one or a few of these drivers in isolation. Hypothesis generation and testing related to pandemic outcomes will benefit from accommodating the nuance of covariate interactions, in an epidemiologic context. Finally, our results add to the literature on the ecological fallacy: the attempt to infer individual drivers and outcomes from the study of population-level aggregates.

Occupational Skill Premia around the World: New Data, Patterns and Drivers

Firms hire workers to undertake tasks and activities associated with particular occupations, which makes occupations a fundamental unit in economic analyses of the labor market. Using a unique dataset on pay in identically defined occupations in developing and advanced countries, we find that in most countries occupational skill premia narrowed substantially from the 1950s to the 1980s, then widened through the 2000s, creating a U-shaped pattern of change. The narrowing was due in part to the huge worldwide increase in the supply of educated workers. The subsequent widening was due in part to the weakening of trade unions and a shift in demand to more skilled workers associated with rising trade. The data indicate that supply, demand, and institutional forces are all drivers of occupational skill premia, ruling out simple single factor explanations of change. The paper concludes with a call for improving the collection of occupational wage data to understand future changes in the world of work.

The second demographic transition, 1986\u20132020: sub-replacement fertility and rising cohabitation\u2014a global update

The article considers the evolution of two “Second Demographic Tradition” (SDT) core characteristics: fertility postponement and the rise of cohabitation, with particular attention being given to the first two decades of the new century. It can be considered as the sequel to the concise overview of the SDT published earlier in the US Proceedings of the National Academy (PNAS) (Lesthaeghe, 2014).In the first section, three optimistic views concerning the evolution of fertility are considered: (i) rises due to the end of postponement, (ii) rises connected with advancing human development and (iii) rises associated with advancing gender equality. The focus in this section is mainly but not exclusively on the European experience and its large degree of variation in fertility patterns. It is argued that these three optimistic predictions of sustained fertility rises are mainly based on observations prior to 2010, with too much weight being given to four Nordic countries and too little to other Western European countries with very similar fertility levels. However, these expectations have been thwarted during the second decade, even in the presence of advances in human development and/or gender equality. Hence, the original SDT prediction of 1986 of sustained sub-replacement fertility still holds after 35 years. We expect this to continue during the third decade as well. Furthermore, single-factor explanations are not likely to do justice to far more intricate situations that are responsive to varying structural and ideational influences.In the second section, the evolution of cohabitation is traced in Europe, the USA and Canada, the Latin American countries, three East Asian populations and selected sub-Saharan cases. At the onset, cohabitation can start either from a SDT basis among the better educated or among the poorer classes following a pattern of disadvantage (POD). It is argued that the feature of cohabitation spreads rapidly among all social classes and across all education groups and that in the process of increasing cohabitation, the POD versus SDT argument loses its significance. On a global scale, the rise in cohabitation is contingent on two dimensions: (i) contrasting historical patterns of kinship organisation, including the position of women, and (ii) further advances of the “ethics revolution”, indicating the growing dominance of individual autonomy over traditional societal norms. As a result, no breakthroughs in the near future are expected in countries with a Muslim or Hindu tradition in which no such major cultural shifts have occurred so far.

The interaction of multiple drivers of intra-organizational change in ministerial administrations: A study of three decades of structural reforms in the Austrian Ministry of Agriculture

Administrative reform policies cutting across several sectors are commonplace in the public sector. However, reform policies do not necessarily result in organizational change. This article examines intra-organizational change within the Austrian Ministry of Agriculture in a longitudinal case study covering a period of three decades, which allows us to study short-term and long term-effects of administrative reforms. Whereas existing research mainly uses single-factor explanations for inter- and intra-organizational change, this article emphasizes the interplay of various drivers of organizational change within government organizations. In analytical terms, we draw on the multiple streams framework to study intra-organizational decision-making which is embedded in government-wide administrative reform policies. We find that reform leads to intra-organizational change when a political entrepreneur is able to couple solutions and problems in a decision window, which may happen decades after the initial reform attempts, underscoring that short-term reform failure may turn into success in a long-term perspective.

Intra-individual variation in performance on novel variants of similar tasks influences single factor explanations of general cognitive processes.

Intra-individual variation in performance within and across cognitive domains may confound interpretations of both domain-general and domain-specific abilities. Such variation is rarely considered in animal test batteries. We investigate individual consistency in performance by presenting pheasant chicks (n = 31), raised under standardized conditions, with nine different cognitive tasks. Among these tasks were two replicated novel variants of colour learning and colour reversal problems, tests of positional learning and memory, as well as two different tasks that captured multiple putative measures of inhibitory control and motor-related performance. These task variants were also used to compare subjects' performance on alternative test batteries comprised of different task combinations. Subjects’ performance improved with experience, yet we found relatively little consistency in their performance, both within similar tasks using different paradigms and across different tasks. Parallel analysis revealed non-significant factors when all nine tasks were included in a principal axis factor analysis. However, when different combinations of six of the nine tasks were included in principal axis factoring, 14 of 84 combinations revealed significant main factors, explaining between 28 and 35% of the variance in task performance. While comparable findings have been suggested to reflect domain-general intelligence in other species, we found no evidence to suggest that a single factor encompassed a diverse range of cognitive abilities in pheasants. Instead, we reveal how single factor explanations of cognitive processes can be influenced by test battery composition and intra-individual variation in performance across tasks. Our findings highlight the importance of conducting multiple tests within specific domains to ensure robust cognitive measures are obtained.

Between Rhetoric and Action: The Politics, Processes and Practice of the ICC's Work in the Democratic Republic of the Congo (DRC)

Between Rhetoric and Action: The Politics, Processes and Practice of the ICC's Work in the Democratic Republic of the Congo (DRC)

If terrorism and insurgency are the “weapons of the weak,” why do they sometimes win?

It has been argued that terrorism and insurgency are weapons of the weak. It is thus surprising that there are clear instances where the weaker actor, adopting such tactics, has emerged victorious. This paper identifies three arguments put forth in the current literature to explain this phenomenon. First Mack’s interest asymmetry thesis, second Arreguin-Toft’s strategic interaction approach, and finally the role of external support as identified by Record. This paper however proposes that while all of these are true in certain circumstances, single factor explanations are rarely useful when considering conflict outcomes. Rather, the literature has a predisposed bias toward the stronger actor. The literature presupposes that wars should be won by the stronger actor, and that consequently the fight is lost by the power with the most material capability, not won by the weaker side. This paper argues however that the focus on material capabilities is misguided, and does not offer any true predictive accuracy....

The Political Geographies of Urban Polarization: A Critical Review of Research on Divided Cities

AbstractThis article aims at providing a review of various streams of literature dealing with the spatial fragmentation of cities. In the last two decades many different contributions emphasized the growing fragmentation of the urban environment; the idea of “divided city” covers a multiplicity of approaches, methodologies and field of research. Still, the “divided city” literature tends to elevate a small number of cities to paradigmatic examples and to focus on abstract categories based on single factor explanations of urban fragmentation. The article, while accepting the hypothesis that the urban context is increasingly fragmented, argues that systems of coordinates – more than a taxonomy or a hierarchy – is needed to make sense of this phenomenon and draw significan comparison between cities. The article also maintains that a significant angle to look at urban polarization relates to the configuration of urban governance and political power. The article therefore considers three main areas connected to the distinction between polity, politics, and policy – and the three corresponding ideal‐types of partitioned, contested and discrete city – where the conflict focuses respectively on the jurisdictional shape of governance, the degrees and venues of access to decision‐making, and frame and contents of public policies.

Degradation of copepod faecal pellets in the upper layer: role of microbial community and Calanus finmarchicus

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 462:39-49 (2012) - DOI: https://doi.org/10.3354/meps09808 Degradation of copepod faecal pellets in the upper layer: role of microbial community and Calanus finmarchicus Camilla Svensen*, Christian Wexels Riser, Marit Reigstad, Lena Seuthe Faculty of Biosciences, Fisheries and Economics, University of Tromsø, 9037 Tromsø, Norway *Email: camilla.svensen@uit.no ABSTRACT: Copepod faecal pellets (FP) are considered important contributors to vertical carbon flux, but investigations comparing FP production with FP export using sediment traps conclude that vertical export is not their only fate. FP are degraded to a large extent in the upper 60 m, and even among large, fast-sinking FP, only a fraction reaches sediment traps deeper than 200 m. Retention mechanisms for copepod FP are still not well understood. In order to investigate the relative importance of the small (<180 µm) compartment of the plankton community versus larger filter-feeding copepods for degradation of large, fast-sinking FP, we incubated FP produced by Calanus finmarchicus (Gunnerus) in 180 µm-filtered water from the chlorophyll a maximum. From a series of experiments, we found that the degradation of large FP is time-dependent, as no degradation was apparent after 20 or 48 h of incubation, but after 72 h FP volume was reduced by 32%. We also found that large filter-feeding copepods may facilitate the degradation process, since FP degradation increased from 0 to 75% after 48 h of incubation in the presence of 5 C. finmarchicus. We conclude that ciliates and dinoflagellates are able to degrade large copepod FP, but that this process is too slow to explain observed retention of large FP in the upper 200 m of the water column due to fast sinking of large particles. Rather than looking for single-factor explanations for flux-regulating processes, we stress the importance of investigating combined effects in relevant time frames to understand the complexity of carbon flux regulation in natural systems. KEY WORDS: Faecal pellet · Faecal pellet retention · Microbial degradation · Faecal pellet fragmentation · Calanus finmarchicus · Carbon cycling Full text in pdf format PreviousNextCite this article as: Svensen C, Wexels Riser C, Reigstad M, Seuthe L (2012) Degradation of copepod faecal pellets in the upper layer: role of microbial community and Calanus finmarchicus. Mar Ecol Prog Ser 462:39-49. https://doi.org/10.3354/meps09808 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 462. Online publication date: August 21, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Constraints on host choice: why do parasitic birds rarely exploit some common potential hosts?

1. Why are some common and apparently suitable resources avoided by potential users? This interesting ecological and evolutionary conundrum is vividly illustrated by obligate brood parasites. Parasitic birds lay their eggs into nests of a wide range of host species, including many rare ones, but do not parasitize some commonly co-occurring potential hosts. 2. Attempts to explain the absence of parasitism in common potential hosts are limited and typically focused on single-factor explanations while ignoring other potential factors. We tested why thrushes Turdus spp. are extremely rarely parasitized by common cuckoos Cuculus canorus despite breeding commonly in sympatry and building the most conspicuous nests among forest-breeding passerines. 3. No single examined factor explained cuckoo avoidance of thrushes. Life-history traits of all six European thrush species and the 10 most frequently used cuckoo hosts in Europe were similar except body/egg size, nest design and nestling diet. 4. Experiments (n = 1211) in several populations across Europe showed that host defences at egg-laying and incubation stages did not account for the lack of cuckoo parasitism in thrushes. However, cross-fostering experiments disclosed that various factors during the nestling period prevent cuckoos from successfully parasitizing thrushes. Specifically, in some thrush species, the nest cup design forced cuckoo chicks to compete with host chicks with fatal consequences for the parasite. Other species were reluctant to care even for lone cuckoo chicks. 5. Importantly, in an apparently phylogenetically homogenous group of hosts, there were interspecific differences in factors responsible for the absence of cuckoo parasitism. 6. This study highlights the importance of considering multiple potential factors and their interactions for understanding absence of parasitism in potential hosts of parasitic birds. In the present study, comparative and experimental procedures are integrated, which represent a novel approach that should prove useful for the understanding of interspecific ecological relationships in general.

Understanding African Conflicts: Theoretical, Historical, and Contemporary Perspectives

On the eve of the first decade of the twenty-first century?and some fifty years after independence?many states continue to be plagued by ethnoregional conflict, while number of others are barely emerging from decades of devastating civil wars. Peace and security?essential prerequisites for democracy and development?continue to elude far too many countries. Building upon the pioneering work of Christopher Clapham in Guerillas (James Currey, 1998), Morten B0?s, Kevin Dunn, and their nine contributors?including Clapham, author of the concluding chap ter?revisit the analysis of guerilla movements. Rejecting single-fac tor explanations (such as greed, competition over resources, and cultural conflict), the editors argue for a more nuanced, holistic approach that is historically grounded and integrates multiple levels of analysis, from the local and national to the regional and global (3). Further inspired by the groundbreaking research of the anthropologist Paul Richards (No Peace, No War, James Currey, 2005), B0?s and Dunn believe that African insurgen cies are best understood as rational responses to the composition of states and their polities (4); and they approach war as a social drama over

The influence of science on Canada's foreign policy on persistent organic pollutants (1985–2001)

The role of science in environmental foreign policymaking has received little attention in the international environmental politics literature, and systematic, in-depth case studies of the influence of science on environmental foreign policy with respect to persistent organic pollutants (POPs) are entirely missing. We present a case study of the influence of science on Canada's foreign policy on POPs from the discovery of the transboundary nature of the POPs problem around 1985 to the signing of the United Nations Stockholm Convention on Persistent Organic Pollutants in 2001. Influence was analyzed in terms of a knowledge–action methodology that focused on the types of scientific knowledge and the actions taken by scientists that were influential. Data were collected through interviews and extensive document analysis. We conclude that, while individual knowledge types and actions were influential during various periods (single-factor explanations), it was an increasingly layered and integrated “package” of knowledge types and actions, with human health impacts as its touchstone and partnering between scientists and non-scientists as its watchword, that propelled and sustained Canada's foreign policy on the POPs issue.

Consociationalism and Power Sharing in Africa: Rwanda, Burundi, and the Democratic Republic of the Congo

Although there are obvious merits to the consociational argument, including the need to recognize the claims of minorities through power-sharing arrangements, translating theory into practice has generally failed in much of Africa. The reasons for this are many and are by no means reducible to single-factor explanations. Looking at the recent experiments in power sharing in former Belgian Africa, this article offers a comparative assessment of the radically different trajectories followed by Rwanda, Burundi, and the Democratic Republic of the Congo (DRC) in their efforts to regulate conflict through consociational formulas. Although Rwanda stands as a textbook example of failed power sharing, and the DRC as a less than successful experiment, Burundi, which comes nearest to institutionalizing the Lijphart model, offers grounds for cautious optimism about the merits of a consociational polity. On the strength of the evidence from Burundi, one might conceivably argue that the key to success lies in the extent to which the technicalities of power sharing tend to approximate the conditions spelled out by Lijphart, notably group autonomy, proportionality, and the minority veto. Closer scrutiny of the cases at hand suggests a somewhat different conclusion. Perhaps even more importantly than the mechanics of power sharing, the socio-political context is what spells the difference between success and failure.

Paul Richards, ed. No Peace, No War: An Anthropology of Contemporary Armed Conflicts. Athens: Ohio University Press/Oxford: James Currey, 2005. x + 214 pp. Appendix. References. Bibliography. Index. $49.95. Cloth. $24.95. Paper.

Paul Richards, ed. No Peace, No War: An Anthropology of Contemporary Armed Conflicts. Athens: Ohio University Press/Oxford: James Currey, 2005. x + 214 pp. Appendix. References. Bibliography. Index. $49.95. Cloth. $24.95. Paper. In one of rare photo-ops from Ituri zone, couple of militias crouch before camera. In front of them ground are heads of their enemies. One man has placed his left hand one of heads to make it face camera, while right one is holding severed arm by hand, as moved to offer last handshake to his victim. Next to him is another militia man holding gun in one hand and in other forearm hacked off at elbow. He holds it close to his nose, as tangle of blood-stained ligaments, tendons, and pieces of flesh are bunch of flowers. In another scene four human heads are neatly aligned ground. Huddled over these trophies are other militias, all holding AK47s, except one, whose hands are holding to his mouth large chunk of freshly cut human flesh, ready to be eaten. Are such chilling scenes to be treated as evidence of Robert D. Kaplan's new barbarism? Or sinister incarnation of Malthus with gun? Or do they reflect perverse effects of greed versus creed dialectic? None of above, suggests Paul Richards in opening chapter of this rich collection of case studies. War, he tells us, must be seen as a social process, and if we are to understand and in processual terms we must first comprehend practices of and peace: how people mobilize and organize for war, and role played by ideational factors in such mobilization and organization. The emphasis, therefore, must not be triggered but on exploring how people make and peace (13). The Ituri photos, in word, need to be contextualized. The book is testament to quality of research done at Department of Cultural Anthropology of University of Uppsala, until recently under guidance of late Bernhard Helander, to whose memory this volume is dedicated. Seven of ten contributors are either Uppsala Ph.D.s or faculty of same department. The case studies cover wide gamut: six are from Africa (Burkina Faso, Somalia, Uganda, Sierra Leone, Liberia, Zimbabwe), two from Asia (Cambodia and Tibet), one from Latin America (Guatemala), and another from Europe (Bosnia). For bringing such diverse cases into coherent theoretical frame editor deserves full credit. Best known for his classic work Sierra Leone, Fighting for Rainforest: War, Youth and Resources in Sierra Leone (1996), Paul Richards offers brilliant critique of some of most influential explanations of African conflicts in introductory chapter to this book. He successfully demolishes Malthusian theories, delivers coup de grâce to Kaplan's long moribund new barbarism thesis, and goes to tackle Paul Collier's greed not grievance theory. The nub of his critique is as straightforward as it is convincing: [Collier's] analysis shows that internal wars are more likely where mineral wealth combines with poverty, and where there is high unemployment among young men with limited education, but (perversely) he considers neither circumstance grounds for valid grievance. As he goes to note: This seems very odd to anyone with on-the-ground knowledge of youth activism against oil companies in Niger Delta or rebels facing mercenary-backed kimberlite concession holders in Sierra Leone. Why it is 'greedy' to want basic education or job Collier does not explain (10). Richards makes no effort to offer an alternative etiology of warfare. His aim is to sketch outlines of what he calls the ethnographic perspective. Drawing from B. M. Knauft's discussion of Melanesian warfare, he rejects single-factor explanations, while emphasizing fact that war belongs within society and must therefore be seen as an aspect of social process where boundary between and is blurred and periodically renegotiated. …

Do we understand the causes of bush encroachment in African savannas?

Bush encroachment affects the agricultural productivity and biodiversity of 10–20 million ha of South Africa. Many people believe that we understand the causes of bush encroachment. We do not. Many people believe that either fire or heavy grazing by domestic livestock is the sole cause of bush encroachment. This is wrong. Bush encroachment occurs in many arid regions where fuel loads are insufficient for fires to be an important causal factor. Belief in grazing as the sole cause of bush encroachment stems from Walter's two-layer model. This model states that grasses typically outcompete trees in open savannas by growing fast and intercepting moisture from the upper soil layers, thereby preventing trees from gaining access to precipitation in the lower soil layers where their roots are mostly found. When heavy grazing occurs, grasses are removed and soil moisture then becomes available to the trees, allowing them to recruit en masse. The fact that many bush-encroached areas are heavily grazed means neither that grazing causes encroachment nor that Walter's model is correct. Bush encroachment is widespread in areas where there is a single soil layer and where grazing is infrequent and light. We need to move away from observational studies and these single-factor explanations. If we are to understand the causes of bush encroachment, we need mechanistic models to guide us and multi-factorial experiments to tease out the interactions among causal factors. Current disturbance-based models have many of the right elements necessary to make mechanistic predictions but need to be appropriately parameterised. Some patch dynamic models also appear to hold great promise in this regard. Field experiments carried out to date show that support for factors conventionally claimed to cause bush encroachment is underwhelming, and that rainfall amount and frequency, coupled with specific soil nutrient levels, may drive this phenomenon.

RESPONSES OF AN INSECT FOLIVORE AND ITS PARASITOIDS TO MULTIYEAR EXPERIMENTAL DEFOLIATION OF ASPEN

Foliage quality may decline in deciduous trees following defoliation, thus affecting the insect generation responsible for the herbivory (rapid induced resistance, RIR), or future generations (delayed induced resistance, DIR). During outbreaks, trees often suffer partial or complete defoliation for two or more successive years, yet most studies have examined induced resistance following only one season of defoliation, which may not reveal its full impact on herbivores. In a field experiment, 40 trees from each of two clones of trembling aspen, Populus tremuloides, were severely defoliated for one, three, and four years in succession by experimentally manipulating densities of an outbreak folivore, the forest tent caterpillar, Malacosoma disstria. Treatments were applied such that the individual and combined effects of RIR and DIR on the fitness of the forest tent caterpillar could be assessed independently. In field assays, defoliation treatments did not affect larval development time and only marginally affected survival. However, fecundity in both clones was significantly reduced by a single season of defoliation concurrent with the bioassay (effects of RIR), by three consecutive years of defoliation prior to the year of the bioassay (effects of DIR), and by four consecutive years of defoliation (combined effects of RIR and DIR). There were no differences among the three defoliation treatments, indicating that the effects of DIR and RIR combined were not greater than each acting alone. Reductions in fecundity were less than half those observed during natural outbreaks, suggesting that other factors also must contribute to declining fecundity during the collapse of outbreaks. Short-term laboratory bioassays indicated that defoliation effects observed in long-term field assays were not due to changes in relative growth rate (RGR) of second instars or final-instar males, which were unaffected, possibly because of increased relative consumption rates (RCR). Defoliation treatments decreased RGR of final-instar females in laboratory bioassays, despite elevated RCR. Both defoliation treatment and aspen clone influenced parasitism by tachinid flies that detect hosts through volatiles released from leaves damaged by caterpillars. Parasitism was highest on trees defoliated concurrently with the larval bioassay. However, there were no differences between trees defoliated prior to the bioassay and control trees, indicating that effects on parasitism were not due to defoliation-induced changes in host quality per se. Thus, there were no additive interactions between DIR and parasitism that would amplify delayed density-dependent effects on population dynamics. Spatial responses of these tachinids to host density or to current defoliation rather than the defoliation history of the trees may enhance the stabilizing effect of RIR on population dynamics. Conversely, differences in parasitism among clones could contribute to spatial variation in tent caterpillar population density. Neither defoliation effect on host quality nor parasitism was sufficient to slow reproductive rates to levels observed in declining outbreaks in nature, suggesting that single-factor explanations for tent caterpillar population dynamics are unlikely.