Stephen D. Hopper

OCBIL theory: towards an integrated understanding of the evolution, ecology and conservation of biodiversity on old, climatically buffered, infertile landscapes

OCBIL theory aims to develop an integrated series of hypotheses explaining the evolution and ecology of, and best conservation practices for, biota on very old, climatically buffered, infertile landscapes (OCBILs). Conventional theory for ecology and evolutionary and conservation biology has developed primarily from data on species and communities from young, often disturbed, fertile landscapes (YODFELs), mainly in the Northern Hemisphere. OCBILs are rare, but are prominent in the Southwest Australian Floristic Region, South Africa’s Greater Cape, and Venezuela’s Pantepui Highlands. They may have been more common globally before Pleistocene glaciations. Based on the premise that natural selection has favoured limited dispersability of sedentary organisms, OCBILs should have elevated persistence of lineages (Gondwanan Heritage Hypothesis) and long-lived individuals (Ultimate Self Hypothesis), high numbers of localised rare endemics and strongly differentiated population systems. To counter such natural fragmentation and inbreeding due to small population size, ecological, cytogenetic and genetic mechanisms selecting for the retention of heterozygosity should feature (the James Effect). The climatic stability of OCBILs should be paralleled by persistence of adjacent semi-arid areas, conducive to speciation (Semiarid Cradle Hypothesis). Special nutritional and other biological traits associated with coping with infertile lands should be evident, accentuated in plants, for example, through water-foraging strategies, symbioses, carnivory, pollination and parasitism. The uniquely flat landscapes of southwestern Australia have had prolonged presence of saline lakes along palaeoriver systems favouring evolution of accentuated tolerance to salinity. Lastly, unusual resiliences and vulnerabilities might be evident among OCBIL organisms, such as enhanced abilities to persist in small fragmented populations but great susceptibility to major soil disturbances. In those places where it is most pertinent, OCBIL theory hopefully lays a foundation for future research and for better informed conservation management.

Plant mineral nutrition in ancient landscapes: high plant species diversity on infertile soils is linked to functional diversity for nutritional strategies

Ancient landscapes, which have not been glaciated in recent times or disturbed by other major catastrophic events such as volcanic eruptions, are dominated by nutrient-impoverished soils. If these parts of the world have had a relatively stable climate, due to buffering by oceans, their floras tend to be very biodiverse. This review compares the functional ecophysiological plant traits that dominate in old, climatically buffered, infertile landscapes (OCBILS) with those commonly found in young, frequently disturbed, fertile landscapes (YODFELs). We show that, within the OCBILs of Western Australia, non-mycorrhizal species with specialised root clusters predominantly occur on the most phosphate-impoverished soils, where they co-occur with mycorrhizal species without such specialised root clusters. In global comparisons, we show that plants in OCBILs, especially in Western Australia, are characterised by very low leaf phosphorus (P) concentrations, very high N:P ratios, and very high LMA values (LMA = leaf mass per unit leaf area). In addition, we show that species in OCBILs are far more likely to show P-toxicity symptoms when exposed to slightly elevated soil P levels when compared with plants in YODFELs. In addition, some species in OCBILs exhibit a remarkable P-resorption proficiency, with some plants in Western Australia achieving leaf P concentrations in recently shed leaves that are lower than ever reported before. We discuss how this knowledge on functional traits can guide us towards sustainable management of ancient landscapes.

Little evidence for fire-adapted plant traits in Mediterranean climate regions

As climate change increases vegetation combustibility, humans are impacted by wildfires through loss of lives and property, leading to an increased emphasis on prescribed burning practices to reduce hazards. A key and pervading concept accepted by most environmental managers is that combustible ecosystems have traditionally burnt because plants are fire adapted. In this opinion article, we explore the concept of plant traits adapted to fire in Mediterranean climates. In the light of major threats to biodiversity conservation, we recommend caution in deliberately increasing fire frequencies if ecosystem degradation and plant extinctions are to be averted as a result of the practice.

Phosphorus-mobilization ecosystem engineering: the roles of cluster roots and carboxylate exudation in young P-limited ecosystems

BACKGROUND Carboxylate-releasing cluster roots of Proteaceae play a key role in acquiring phosphorus (P) from ancient nutrient-impoverished soils in Australia. However, cluster roots are also found in Proteaceae on young, P-rich soils in Chile where they allow P acquisition from soils that strongly sorb P. SCOPE Unlike Proteaceae in Australia that tend to proficiently remobilize P from senescent leaves, Chilean Proteaceae produce leaf litter rich in P. Consequently, they may act as ecosystem engineers, providing P for plants without specialized roots to access sorbed P. We propose a similar ecosystem-engineering role for species that release large amounts of carboxylates in other relatively young, strongly P-sorbing substrates, e.g. young acidic volcanic deposits and calcareous dunes. Many of these species also fix atmospheric nitrogen and release nutrient-rich litter, but their role as ecosystem engineers is commonly ascribed only to their diazotrophic nature. CONCLUSIONS We propose that the P-mobilizing capacity of Proteaceae on young soils, which contain an abundance of P, but where P is poorly available, in combination with inefficient nutrient remobilization from senescing leaves allows these species to function as ecosystem engineers. We suggest that diazotrophic species that colonize young soils with strong P-sorption potential should be considered for their positive effect on P availability, as well as their widely accepted role in nitrogen fixation. Their P-mobilizing activity possibly also enhances their nitrogen-fixing capacity. These diazotrophic species may therefore facilitate the establishment and growth of species with less-efficient P-uptake strategies on more-developed soils with low P availability through similar mechanisms. We argue that the significance of cluster roots and high carboxylate exudation in the development of young ecosystems is probably far more important than has been envisaged thus far.

Conservation genetics and clonality in two critically endangered eucalypts from the highly endemic south-western Australian flora

Extensive agricultural development has endangered species diversity in the highly endemic south-western Australian flora. Many relict species with restricted distribution are now on the brink of extinction. Eucalyptus phylacis and E. dolorosa are both known from single locations within pockets of native vegetation surrounded by agricultural land. The development of appropriate conservation strategies required the appraisal of the extent of clonality within both species as well as the definition of species identity for E. phylacis. The use of RAPD analysis revealed contrasting stories for these two threatened species. E. phylacis, for which species identity was successfully established, was shown to be a single clone and as a result, possibly the rarest, largest, potentially oldest mallee eucalypt known. Whereas, despite its isolation, E. dolorosa still retained sufficient genetic variability to enable the production of highly outcrossed seed. Different conservation strategies are suggested for the two species and the importance of understanding basic genetic variability in rare clonal species is discussed.

The mating system and population genetic structure in a bird-pollinated mallee, Eucalyptus rhodantha

The mating system and spatial genetic structure of the rare and endangered bird-pollinated mallee Eucalyptus rhodantha were investigated in a remnant stand, using progeny arrays and pollen assayed at four polymorphic allozyme loci. Comparisons of the genetic diversity within and between the pollen pools and maternal parents of two arbitrary subpopulations indicated the presence of spatial genetic heterogeneity which was not broken down by pollen flow. It was suggested that this is the result of a high level if inbreeding and limited pollen dispersal by birds. Estimates of outcrossing rate ranged between [tcirc ]=0·59 and [tcirc ]=0·67 and were at the low end of the range reported for other eucalypts. It was concluded that E. rhodantha has a mixed mating system with a significant proportion of self-pollination. Biparental inbreeding within small neighbourhoods probably also contributed to the high level of inbreeding. The low level of outcrossing observed in E. rhodantha was not consistent with the hypothesis that bird pollination leads to high levels of outcrossing in the Australian flora. However, the level of outcrossing achieved through bird pollination together with high levels of gene flow between populations contribute to the maintenance of the relatively high levels of diversity which characterise the dissected populations of this species.

Pollination ecology and the possible impacts of environmental change in the Southwest Australian Biodiversity Hotspot

The Southwest Australian Biodiversity Hotspot contains an exceptionally diverse flora on an ancient, low-relief but edaphically diverse landscape. Since European colonization, the primary threat to the flora has been habitat clearance, though climate change is an impending threat. Here, we review (i) the ecology of nectarivores and biotic pollination systems in the region, (ii) the evidence that trends in pollination strategies are a consequence of characteristics of the landscape, and (iii) based on these discussions, provide predictions to be tested on the impacts of environmental change on pollination systems. The flora of southwestern Australia has an exceptionally high level of vertebrate pollination, providing the advantage of highly mobile, generalist pollinators. Nectarivorous invertebrates are primarily generalist foragers, though an increasing number of colletid bees are being recognized as being specialized at the level of plant family or more rarely genus. While generalist pollination strategies dominate among insect-pollinated plants, there are some cases of extreme specialization, most notably the multiple evolutions of sexual deception in the Orchidaceae. Preliminary data suggest that bird pollination confers an advantage of greater pollen movement and may represent a mechanism for minimizing inbreeding in naturally fragmented populations. The effects of future environmental change are predicted to result from a combination of the resilience of pollination guilds and changes in their foraging and dispersal behaviour.

Exploring rock fissures: does a specialized root morphology explain endemism on granite outcrops?

BACKGROUND AND AIMS Worldwide, many plant species are confined to open, shallow-soil, rocky habitats. Although several hypotheses have been proposed to explain this habitat specificity, none has been convincing. We suggest that the high level of endemism on shallow soils is related to the edaphic specialization needed to survive in these often extremely drought-prone habitats. Previous research has shown that species endemic to ironstone communities in SW Australia have a specialized root morphology that enhances their chance to access fissures in the underlying rock. Here we test the generality of these findings for species that are confined to a shallow-soil habitat that is of much greater global significance: granite outcrops. METHODS We compared temporal and spatial root growth and allocation of three endemic woody perennials of SW Australian granite outcrop communities with those of congeners occurring on nearby deeper soils. Seedlings of all species were grown in 1·2 m long custom-made containers with a transparent bottom that allowed monitoring of root growth over time. KEY RESULTS The granite outcrop endemics mostly differed in a predictable way from their congeners from deeper soils. They generally invested a larger portion of their biomass in roots, distributed their roots faster and more evenly over the container and had a lower specific root length. In different species pairs the outcrop endemics achieved their apparent advantage by a different combination of the aforementioned traits. CONCLUSIONS Our results are consistent with earlier work, indicating that species restricted to different types of drought-prone shallow-soil communities have undergone similar selection pressures. Although adaptive in their own habitat in terms of obtaining access to fissures in the underlying rock, these root system traits are likely to be maladaptive in deeper soil habitats. Therefore, our results may provide an explanation for the narrow endemism of many shallow-soil endemics.

Impact of two wildfires on endemic granite outcrop vegetation in Western Australia

Abstract In seasonally dry regions of the world fire is a recurring disturbance but little is known of how fire interacts with granite outcrop vegetation. We hypothesize that the floristic composition in granite vegetation, usually attributed to the edaphic environment, may also reflect the impact of disturbances such as fire. Dramatic differences in floristic composition and cover over 13 years and two fires were observed in vegetation on a Western Australian granite outcrop. This was very marked in the first year following the two fires, with annuals and geophytes showing the greatest turnover of species. Even among the perennial shrubs there was considerable turnover in a number of obligate seeders. After the first fire the number of species declined for woody perennials, herbaceous perennials and annuals, remained unchanged for perennial grasses and sedges, and varied with highest richness 4 yr after fire for geophytes. Demographic studies of two endemic woody obligate seeders and three endemic mallee eucalypt resprouters similarly showed dramatic differences within and between species in seedling recruitment following the two fires. Fire does have a significant impact on the floristic composition of semi-arid granite outcrop vegetation communities. Studies on other granite outcrop systems are needed to test the generality of this conclusion. Nomenclature: follows the database of Western Australian plant names maintained by the Western Australian Herbarium (Paczkowska & Chapman 2000).

Robert Brown's Caladenia revisited, including a revision of its sister genera Cyanicula, Ericksonella and Pheladenia (Caladeniinae: Orchidaceae)

Nomenclatural confusion has been generated regarding the large Australasian terrestrial orchid genus Caladenia following publication from 2001 onwards of three major treatments of Caladeniinae. Here, we review concepts for Caladenia and allied genera in the subtribe, we revise three sister genera of Caladenia (Cyanicula, Ericksonella and Pheladenia), and we present an annotated nomenclatural checklist with many new synonymies and some new combinations. A revised circumscription of ten genera in the Caladeniinae is presented, including both Adenochilus and Eriochilus, which have recently been segregated as monogeneric subtribes by others. We argue for retaining Caladenia in the broad sense, largely reflecting Robert Browns original concept, differing only in the recognition as genera of Cyanicula, Pheladenia, and Leptoceras, as well as two monotypic genera not known to Brown but later described as species of Caladenia (Praecoxanthus and Ericksonella). Thus Caladenia remains a large Australasian genus of terrestrial orchids with 243 species and six subgenera. This approach maximises nomenclatural stability while ensuring that hypothesised monophyly is upheld in the light of molecular phylogenetics analyses. The valid type for Caladenia is C. carnea, while that for Caladenia sect. Calonema is C. longicauda. The genus Jonesiopsis and generic combination Phlebochilus (Benth.) Szlach. were validly published. These conclusions call into question many recently erected taxa and combinations of other authors. Synonyms of Caladenia include Arachnorchis, Calonemorchis, Drakonorchis, Jonesiopsis, Petalochilus, Phlebochilus and Stegostyla. Pentisia is a synonym of Cyanicula. Calonema (Lindl.) Szlach. and Calonema (Lindl.) D.L. Jones and M.A. Clem. are invalid generic combinations as the name Calonema had already been used for a fungal genus. New taxa described herein include Ericksonella, Cyanicula subgenus Trilobatae, C. aperta, C. ixioides subsp. candida, × Cyanthera and × C. glossodioides. New combinations include Caladenia subgenus Stegostyla (D.L. Jones and M.A. Clem.) Hopper and A.P. Br., C. graniticola (Hopper and A.P. Br.) Hopper and A.P. Br., C. saccata (R.S. Rogers) Hopper & A.P. Br., C. orientalis (G.W. Carr) Hopper & A.P. Br., and C. villosissima (G.W. Carr) Hopper & A.P. Br., and Ericksonella saccharata (Reichb.f.) Hopper and A.P.Br.