Showing papers by "Guy F. Midgley published in 2012"

Carbon dioxide and the uneasy interactions of trees and savannah grasses

Abstract: Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help mainta...

How to understand species' niches and range dynamics: a demographic research agenda for biogeography

Abstract: Range dynamics causes mismatches between a species’ geographical distribution and the set of suitable environments in which population growth is positive (the Hutchinsonian niche). This is because source–sink population dynamics cause species to occupy unsuitable environments, and because environmental change creates non-equilibrium situations in which species may be absent from suitable environments (due to migration limitation) or present in unsuitable environments that were previously suitable (due to time-delayed extinction). Because correlative species distribution models do not account for these processes, they are likely to produce biased niche estimates and biased forecasts of future range dynamics. Recently developed dynamic range models (DRMs) overcome this problem: they statistically estimate both range dynamics and the underlying environmental response of demographic rates from species distribution data. This process-based statistical approach qualitatively advances biogeographical analyses. Yet, the application of DRMs to a broad range of species and study systems requires substantial research efforts in statistical modelling, empirical data collection and ecological theory. Here we review current and potential contributions of these fields to a demographic understanding of niches and range dynamics. Our review serves to formulate a demographic research agenda that entails: (1) advances in incorporating process-based models of demographic responses and range dynamics into a statistical framework, (2) systematic collection of data on temporal changes in distribution and abundance and on the response of demographic rates to environmental variation, and (3) improved theoretical understanding of the scaling of demographic rates and the dynamics of spatially coupled populations. This demographic research agenda is challenging but necessary for improved comprehension and quantification of niches and range dynamics. It also forms the basis for understanding how niches and range dynamics are shaped by evolutionary dynamics and biotic interactions. Ultimately, the demographic research agenda should lead to deeper integration of biogeography with empirical and theoretical ecology.

Diverse functional responses to drought in a Mediterranean-type shrubland in South Africa

Abstract: Summary •Mediterranean-type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. •We imposed a 6-month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep-rooted proteoid shrubs, shallow-rooted ericoid shrubs and graminoid restioids). •There was considerable variation in drought response both between and within the growth forms. The shallow-rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow-rooted, isohydric restioids and deep-rooted, isohydric proteoid shrubs were largely unaffected by the drought. •Rooting depth and degree of iso/anisohydry allow a first-order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.

Biodiversity and Ecosystem Function

Abstract: How is the biodiversity within an ecosystem related to the ecosystem's function? Quantifying and understanding this relationship—the biodiversity-ecosystem function (BEF) ( 1 )—is important because socio-economic development is almost always accompanied by the loss of natural habitat and species ( 2 ). Short-term economic gains may thus trump longer-term benefits for human society, creating vulnerabilities that could be avoided or corrected with enough knowledge about the role of biodiversity. Erosion of biodiversity at local and regional scales may also reduce resilience at larger spatial scales as a result of degradation of ecosystem function ( 3 ). On page 214 of this issue, Maestre et al. ( 4 ) report an important step toward extending our understanding of BEF to globally important ecosystems.

A physiological analogy of the niche for projecting the potential distribution of plants

Abstract: Aim To develop a physiologically based model of the plant niche for use in species distribution modelling. Location Europe. Methods We link the Thornley transport resistance (TTR) model with functions which describe how the TTR’s model parameters are influenced by abiotic environmental factors. The TTR model considers how carbon and nutrient uptake, and the allocation of these assimilates, influence growth. We use indirect statistical methods to estimate the model parameters from a high resolution data set on tree distribution for 22 European tree species. Results We infer, from distribution data and abiotic forcing data, the physiological niche dimensions of 22 European tree species. We found that the model fits were reasonable (AUC: 0.79–0.964). The projected distributions were characterized by a false positive rate of 0.19 and a false negative rate 0.12. The fitted models are used to generate projections of the environmental factors that limit the range boundaries of the study species. Main conclusions We show that physiological models can be used to derive physiological niche dimensions from species distribution data. Future work should focus on including prior information on physiological rates into the parameter estimation process. Application of the TTR model to species distribution modelling suggests new avenues for establishing explicit links between distribution and physiology, and for generating hypotheses about how ecophysiological processes influence the distribution of plants.

Novel methods reveal shifts in migration phenology of barn swallows in South Africa

Abstract: Many migratory bird species, including the barn swallow (Hirundo rustica), have advanced their arrival date at Northern Hemisphere breeding grounds, showing a clear biotic response to recent climate change. Earlier arrival helps maintain their synchrony with earlier springs, but little is known about the associated changes in phenology at their non-breeding grounds. Here, we examine the phenology of barn swallows in South Africa, where a large proportion of the northern European breeding population spends its non-breeding season. Using novel analytical methods based on bird atlas data, we show that swallows first arrive in the northern parts of the country and gradually appear further south. On their north-bound journey, they leave South Africa rapidly, resulting in mean stopover durations of 140 days in the south and 180 days in the north. We found that swallows are now leaving northern parts of South Africa 8 days earlier than they did 20 years ago, and so shortened their stay in areas where they previously stayed the longest. By contrast, they did not shorten their stopover in other parts of South Africa, leading to a more synchronized departure across the country. Departure was related to environmental variability, measured through the Southern Oscillation Index. Our results suggest that these birds gain their extended breeding season in Europe partly by leaving South Africa earlier, and thus add to scarce evidence for phenology shifts in the Southern Hemisphere.

Do niche-structured plant communities exhibit phylogenetic conservatism? A test case in an endemic clade

Abstract: 1. The growing literature on the phylogenetic structure of plant communities places great emphasis on the role of phylogenetic niche conservatism (PNC) in community assembly. However, the patterns revealed by such analyses are difficult to interpret in the absence of independent data on niche structure. While there is increasing evidence that plant coexistence does depend upon niche differences, it is still not clear in most cases what the relevant niche axes are. 2. We address this problem by testing for PNC within the African Restionaceae (‘restios’), a clade endemic to the Western Cape where we have shown niche segregation along soil moisture gradients to be common. 3. Significant niche segregation on soil moisture gradients occurred among restios in 7 of 10 communities sampled, but PNC was detectable in only one of these and then only by one of three methods used. 4. Phylogenetic analysis of the evolution of hydrological niche traits for the species pool of 37 Restionaceae in the study showed tolerance of drought to be convergent rather than conserved. 5. Synthesis. The demonstration that clear niche segregation may occur among related species without PNC being detectable supports the hypothesis that hydrological niche responses are evolutionarily labile. More generally, the results demonstrate that phylogenetic analysis can be a poor guide to the process of community assembly. We argue that it may in future be better to apply ecological data to the interpretation of phylogenies, rather than to follow the current preoccupation with the application of phylogenies to ecology.

Costs of Expanding the Network of Protected Areas as a Response to Climate Change in the Cape Floristic Region

Abstract: The expansion of protected areas is a critical component of strategies to promote the continued existence of biodiversity (i.e., life at all levels of biological organization) as climate changes, but scientific, social, and economic uncertainties associated with climate change are some of the major obstacles preventing such expansion. New models of climate change and species distribution and new methods of conservation planning now make it possible to explore the uncertainties associated with climate changes and species responses. Yet few reliable estimates of the costs of expanding protected areas and methods for determining these costs exist, largely because of the many (and uncertain) determinants of these costs. We developed a cost-accounting model to estimate the range in costs of various options for expanding protected areas and to explore the variables that drive these costs. Model development was informed by an existing plan to expand protected areas in the Cape Floristic Region of South Africa to address species conservation under a scenario of climate change. The 50-year present value of total costs varied from US$260 million ($1077/ha) for an off-reserve option that involves agreements with landowners and no compensation of forgone production and associated revenue to $1020 million ($4228/ha) for an on-reserve option that involves land acquisition and protection. The costs of acquiring land or compensating landowners for forgone production and development opportunities were the major drivers of the total costs across all options because most of the area identified in the protected-area expansion plan consisted of urban and high-quality agricultural lands. Total costs were also affected by changes in protected area extent and discount rate. Model-generated outputs such as these may be useful for informing implementation strategies and the allocation of future efforts in monitoring, data collection, and model development. Resumen: La expansion de areas protegidas es un componente critico de las estrategias para promover la existencia continua de la biodiversidad (i.e., vida en todos los niveles de organizacion biologica) a medida que cambia el clima, pero las incertidumbres cientificas, sociales y economicas asociadas con el cambio climatico son algunos de los principales obstaculos para dicha expansion. Los modelos nuevos de cambio climatico y distribucion de especies y los metodos nuevas para planificar la conservacion hacen posible explorar las incertidumbres asociadas con cambios en el clima y las respuestas de las especies. Sin embargo, existen pocas estimaciones confiables de los costos de la expansion de las areas protegidas y de los metodos para determinar esos costos, debido principalmente a los muchos (e inciertos) determinantes de estos costos. Desarrollamos un metodo que considera costos para estimar el rango de los costos de varias opciones para la expansion de areas protegidas y explorar las variables que influyen en estos costos. El desarrollo del modelos fue informado por un plan existente para expandir las areas protegidas en la Region Floristica del Cabo en Sudafrica para atender la conservacion de especies en un escenario de cambio climatico. El valor actual de los costos totales a 50 anos vario de US$260 millones ($1077/ha) para una opcion que implica acuerdos con propietarios de tierras y sin compensacion por la produccion de oportunidad y los ingresos asociados hasta $1020 millones ($4288/ha) para una opcion que implica la adquisicion y proteccion de tierras. Los costos de adquisicion de tierras o de compensacion a propietarios por produccion de oportunidad y desarrollo de oportunidades fueron los factores principales de los costos totales en todas las opciones debido a que casi toda el area identificada en la expansion de areas protegidas consistio de terrenos urbanos y agricolas de alta calidad. Los costos totales tambien fueron afectados por cambios en la extension de areas protegidas y la tasa de descuento. Los resultados generados por modelos como estos pueden ser utiles para informar la implementacion de estrategias y la asignacion de esfuerzos futuros de monitoreo, recoleccion de datos y desarrollo de modelos.

Exploring the significance of land-cover change in South Africa

Abstract: . Changes in land cover include the conversion of natural vegetation to agricultural crops and forest plantations, changes to natural vegetation through bush encroachment and overgrazing, soil erosion, invasion by alien plant species, and accelerating urbanisation. Land-cover changes increasingly relate to climate and atmospheric changes in ways that are currently poorly understood but potentially significant, especially in terms of compromising or enhancing the delivery of vital ecosystem services from rangelands, agricultural croplands, water catchments and conservation areas. Land-cover change is being studied in different ways, and at different scales, by ecologists, plant physiologists, applied biologists and social scientists. A core group of scientists has recently formed the Land Cover Change Consortium (LCCC), which aims to begin integrating the results of the varied approaches to studying land-cover change, and to guide future research directions, with a view to building a better science base for informing policy and management decision-making in conservation, agriculture and environmental management. The group has developed a simple conceptual outline that links field experiments, observation and monitoring, modelling and prediction of land-cover change (Figure 1), and is currently developing a funding base to support collaboration in addressing fundamental questions about how ecosystems might change in the coming decades, in training new graduates, and in communicating effectively with policymakers. The LCCC hopes to provide a theoretical and practical multidisciplinary platform for scientific collaboration on global change issues that also includes different stakeholder groups and contributes to policy and decision-making. Multidisciplinary collaboration is notoriously challenging, but holds great promise for novel insights.

The reforestation of Africa

Abstract: 1poses a challenging question for African ecologists and environmentalists: do we, in the subcontinent, face not a contraction, but a vast and inevitable expansion of subtropical tree cover, driven by levels of CO 2 that have not been seen in the past several million years? Higgins and Scheiter’s paper warrants our attention because it projects, for the first time, the continent-wide implications of the decade-old hypothesis, originally formulated by South African ecologists, of CO 2 -driven woody expansion in fire-prone savannas. 2 The paper supports concerns that the expansion of woodlands and forest may be an imminent threat to ecosystem structure, function and biodiversity across extensive landscapes in the sub-continent. 3 If its projections are correct, then we stand on the brink of massive ecosystem change in the ‘savanna–complex’ vegetation (i.e. tropical grasslands, savanna and forests) of Africa. But how credible are these projections? Unlike more intensively researched temperate ecosystems, the vegetation structure and land cover of huge tracts of sub-Saharan Africa may be highly sensitive to increasing levels of atmospheric CO 2 . Vast areas of the subcontinent are currently dominated by C 4 grasses – a photosynthetic mode that owes much of its competitive advantage to the low CO 2 levels of pre-industrial and, even more so, glacial times. 4 Grasses do not require the large amounts of carbon that woody plants do to support their photosynthetic tissue. This low carbon demand for growth allows grasses to outcompete woody plants under low CO 2 conditions by building up a flammable layer of grass fuel – the savanna fire trap – that immolates slower growing woody plants and maintains the system in its grassy state. Under high levels of CO 2 , trees are thought to regain the advantage, escaping the fire trap and converting the system into forest. 2 This mutable balance of trees versus grasses, mediated by atmospheric CO 2 levels and fire, results in ‘bi-stable’ systems 5 in which

Exploring the significance of land-cover change in South Africa : news and views

Abstract: Changing land cover is a phenomenon that is growing in magnitude and significance, both globally and in South Africa. Changes in land cover include the conversion of natural vegetation to agricultural crops and forest plantations, changes to natural vegetation through bush encroachment and overgrazing, soil erosion, invasion by alien plant species, and accelerating urbanisation. Land-cover changes increasingly relate to climate and atmospheric changes in ways that are currently poorly understood but potentially significant, especially in terms of compromising or enhancing the delivery of vital ecosystem services from rangelands, agricultural croplands, water catchments and conservation areas.

Climate Change, Extinction Risk, and Public Policy

Abstract: When initial estimates of the extinction risk from climate change appeared, there was immediate public and media interest. Authors of Thomas et al. appeared on CNN, BBC, and other major national and international television networks. Newspaper headlines, often front page, appeared on the day of the report’s release. Magazine, radio, and other media treatments of the subject followed for weeks after. But to what extent was this media interest driven by policy relevance, and to what extent were the implications of the extinction risk estimates taken up in policy dialogue?

Cape Floristic Region, South Africa

Abstract: The highly diverse and unique Cape Floristic Region near the southern tip of South Africa has been a productive testing ground for evolving systematic conservation planning techniques. As protea plant species ranges contract, expand, and shift in response to climate change, new reserve selection methods are needed to account for this dynamism. Conservation planners are applying cutting-edge methods for integrating climate change species-distribution models into the design of connectivity and protected areas in the Cape Floristic Region. These modeling advances, when linked to specific land-use policies and opportunities for acquisition, provide a promising basis for implementing a set of actions to make conservation in the Cape robust to climate change.