David M. Watson

Charles Sturt University, Уогга Уогга, New South Wales, Australia

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Publications (37)74.35 Total impact

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    ABSTRACT: This study is the first direct comparison of the diversity of phytophagous insects associated with a parasitic plant and its host plants. Specifically, we compared the species composition, density and host specificity of psylloids or jumping plant lice (Hemiptera: Psylloidea), inhabiting hemiparasitic box mistletoe Amyema miquelii, and three of its host Eucalyptus species: Eucalyptus blakelyi, Eucalyptus melliodora and Eucalyptus polyanthemos. Insects were sampled by restricted canopy fogging in remnant Eucalyptus woodlands in an agricultural region of temperate south-eastern Australia. Although most psylloids are understood to be mono- or oligophagous, most species in our survey were found on the foliage of both mistletoes and eucalypts. Nevertheless, analysis of density patterns and reference to previous work on psylloids supports the high degree of host specificity for psylloids, leading to distinct assemblages on these two intimately associated plants. We show that (1) there were two mistletoe-associated species of psylloid and 18 eucalypt-associated species; (2) there were a large number of tourist species, as indicated by known psylloid/plant host associations; and (3) psylloid density was higher on eucalypt than mistletoe leaves. The different psylloid assemblages found on box mistletoes compared with their host plants are likely to be due to differences in the foliar properties implicated in host specificity and host selection by phytophagous insects. Further research is required to understand the ecological dynamics and evolutionary origins of these arboreal assemblages.
    Austral Entomology. 11/2014;
  • Emma Razeng, David M. Watson
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    ABSTRACT: Food availability is emerging as a key determinant of avian occurrence and habitat use in a variety of systems, but insectivores have received less attention than other groups and the potential influence of nutritional quality has rarely been considered. Rather than a uniform food source, arthropods vary greatly in terms of nutritional composition, but does this variation translate into differential consumption? Building on previous work that demonstrated clear preference for some arthropod groups by 13 species of ground-foraging insectivores, we compare the nutritional composition of these arthropod groups with other groups commonly encountered but seldom consumed in the same habitat types. Using samples of arthropods collected from a eucalypt woodland in southern Australia, we found the high frequency prey groups (Coleoptera, Lepidoptera, Orthoptera and Araneae) consistently contained higher fractions of crude protein and total fat than the low frequency groups (Diptera, Hymenoptera and Odonata). Even more clear-cut differences were noted in terms of micronutrients; high frequency prey containing significantly greater concentrations of seven elements than low frequency prey and significantly greater amounts per individual arthropod for all eleven elements measured. These results indicate that the nutritional quality plays an important role in prey selection in insectivores and suggests that micronutrients may be more important determinants of prey choice than previously recognized. Integrating these findings with previous work suggesting food limitation may constrain distribution patterns of birds in fragmented landscapes, we contend that variation in nutritional quality helps explain observed patterns in insectivore diets and occurrence. In addition to explaining why smaller and more disturbed habitats are unable to support resident insectivore populations, this bottom-up mechanism may underlie the disproportionate sensitivity of insectivores to land-use intensification.
    Journal of Avian Biology 09/2014; · 2.02 Impact Factor
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    ABSTRACT: Five years of research on interrelationships between fauna use of almond plantations and native vegetation in north-western Victoria shows that almond plantations have a strong influence on fauna dynamics and in some cases may provide important habitat for threatened species.
    Blackwell Science Asia Pty Ltd 01/2014; 15(1).
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    Conservation Biology 12/2013; 27(6):1133-5. · 4.36 Impact Factor
  • David M Watson, John Rawsthorne
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    ABSTRACT: Many plants use birds to disperse their propagules, but mistletoes are especially reliant on their services. As aerial parasites, mistletoe seeds need to be deposited upon branches of suitable hosts, and mistletoe specialist frugivores (from eight different avian families) have long been regarded as their coevolved dispersers. Like the pioneer Johnny 'Appleseed' Chapman who established nurseries that helped open up land for settlement, these birds are considered benevolent dispersers of this keystone resource and often invoked as illustrative examples of mutualistic interactions. We have compared recent research on these specialists with studies of other birds with broader diets (generalists) which also disperse mistletoe seed. Rather than mutualists, we suggest that mistletoe specialist frugivores are better considered exploitative, with multiple lineages evolving independently to capitalize on this reliable, nutritious resource. Although mistletoe specialist frugivores are quantitatively important seed dispersers in some regions, their specialized diet restricts them to areas with high mistletoe densities, resulting in contagious dispersal patterns. By intensifying existing infections, mistletoe specialist frugivores increase their own medium-term food security-akin to market gardeners profiting from intensive cultivation. Exploring the ecological and evolutionary implications of this proposition, we evaluate the consequences of different dispersal patterns on mistletoe fitness and highlight the neglected role of dietary generalists in the stabilization of plant-animal interactions.
    Oecologia 06/2013; · 3.01 Impact Factor
  • Laurence P. Barea, David M. Watson
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    ABSTRACT: 1. Nest-site location is a critical component of habitat preference in birds, reflecting a balance between minimizing the likelihood of nest predation while maximizing access to nutritional resources. While many studies have demonstrated the influence of predators in nest-site selection, few studies have explicitly quantified nutritional resources or considered the interacting effects of predation and food availability in determining nest survival. 2. The painted honeyeater Grantiella picta is a mistletoe-specialist frugivore, with fruit from grey mistletoe Amyema quandang representing the main food source for breeding adults and nestlings. Previous work demonstrated that painted honeyeaters prefer to place their nests within mistletoe substrates. Here, we measured the outcome of 63 nests over two years, relating survival to various structural and resource-based variables to discern whether nests placed in mistletoes were more likely to succeed. 3. Twenty-one nests survived the 33 day nest period, with 35 of the 42 failed nests predated. While few significant differences were discerned between successful and unsuccessful nests in terms of nest tree or surrounding habitat, nest substrate emerged as the most important predictor of nest fate. Survival of nests in mistletoe was 16·6% across a 33 day active nest period compared with a mean of 43·1% for nests in other substrates, a difference consistent across both years. 4. Rather than having a positive effect on nest outcome (via access to nutritional resources), proximity to mistletoe had a marked negative effect, with nests in mistletoe suffering a predation rate 2·6 times higher than nests elsewhere. Rather than predators targeting mistletoe clumps, we suggest that this pattern arises from other species visiting fruiting mistletoe clumps, opportunistically predating the nest contents and disturbing attending parents. We interpret this finding as evidence that the painted honeyeater may be caught in an ecological trap; the cues used to select nesting locations are a poor predictor of success.
    Functional Ecology 06/2013; 27(3):766-774. · 4.86 Impact Factor
  • David M. Watson
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    ABSTRACT: Mistletoes rely on birds for seed dispersal, but the presumed importance of mistletoe-specialist frugivores has not been critically examined nor compared with generalist frugivores and opportunistic foragers. The contribution of these three groups was compared directly by quantifying bird visitation to fruiting mistletoe plants ( Oryctanthus occidentalis: Loranthaceae) at Barro Colorado Island, Panama, and by comparing these results with proportions calculated from other empirical studies of mistletoe visitation conducted elsewhere. After more than 100 h of timed watches, 23 bird species were recorded visiting eight heavily infected host trees ( Luehea seemannii: Tiliaceae). Eight of these species visited mistletoe, of which five (all tyrannids) consumed mistletoe fruit. Although two mistletoe specialist frugivores ( Tyrannulus elatus and Zimmerius vilissimus) removed most fruit (73%), more than a quarter was consumed by one generalist frugivore ( Mionectes oleagineus) and two opportunists ( Myiozetetes cayanensis and Myiozetetes similis). Post consumption behaviour varied: the specialists flew from mistletoe to mistletoe, the generalist rested in the subcanopy and understory, and the opportunists spent most time hawking insects and resting high in the canopy. Integrating these data with previous work, the dietary specialization, short gut passage rate and strict habitat preferences of mistletoe specialists suggests that their services relate primarily to intensification and contagious dispersal, while species with broader diets are more likely to visit uninfected trees and establish new infections. The presumed importance of mistletoe-specialist frugivores was not supported and mistletoes are considered to be comparable to many other bird-dispersed plants, relying on both specialist and generalist frugivores, while opportunists may be disproportionately important in long-distance dispersal.
    Biotropica 03/2013; 45(2). · 2.35 Impact Factor
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    David M Watson, Matthew Herring
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    ABSTRACT: Various entities have been designated keystone resources, but few tests have been attempted and we are unaware of any experimental manipulations of purported keystone resources. Mistletoes (Loranthaceae) provide structural and nutritional resources within canopies, and their pervasive influence on diversity led to their designation as keystone resources. We quantified the effect of mistletoe on diversity with a woodland-scale experiment, comparing bird diversities before and after all mistletoe plants were removed from 17 treatment sites, with those of 11 control sites and 12 sites in which mistletoe was naturally absent. Three years after mistletoe removal, treatment woodlands lost, on average, 20.9 per cent of their total species richness, 26.5 per cent of woodland-dependent bird species and 34.8 per cent of their woodland-dependent residents, compared with moderate increases in control sites and no significant changes in mistletoe-free sites. Treatment sites lost greater proportions of birds recorded nesting in mistletoe, but changes in species recorded feeding on mistletoe did not differ from control sites. Having confirmed the status of mistletoe as a keystone resource, we suggest that nutrient enrichment via litter-fall is the main mechanism promoting species richness, driving small-scale heterogeneity in productivity and food availability for woodland animals. This explanation applies to other parasitic plants with high turnover of enriched leaves, and the community-scale influence of these plants is most apparent in low productivity systems.
    Proceedings of the Royal Society B: Biological Sciences 07/2012; 279(1743):3853-60. · 5.68 Impact Factor
  • Emma Razeng, David M. Watson
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    ABSTRACT: Ground-foraging insectivores are prominent among the 26 species considered ‘declining woodland birds’ in southern Australia but the mechanisms driving their declines remain elusive. Nutritional factors may be critical, with larger and more structurally complex woodlands supporting greater arthropod biomass, but these differences need not translate into more arthropods actually consumed by these insectivores. We synthesised existing dietary records of a subset of the 26 declining woodland birds – 13 ground-foraging insectivorous passerines – to determine the range of arthropods consumed and to estimate the relative importance of each prey group for these birds. Declining insectivores consumed a wide array of arthropods, but diets were characteristically dominated by one or two prey groups: Coleoptera, Formicidae and Lepidoptera accounted for 58% of prey records. Coleoptera contributed the greatest proportion of records (27%) and was the dominant prey group in the diets of nine of the 13 birds. These popular prey groups likely represent core resources supporting populations of declining insectivores and measurement of their abundance may provide meaningful estimates of the availability of prey. We highlight the need to quantify the size-range and identity of those prey eaten by declining woodland birds, and propose that reliance on a small number of prey groups may underlie the sensitivity of ground-foraging insectivores to modification of habitat.
    Emu. 06/2012; 112(2):149-156.
  • John Rawsthorne, David M. Watson, David A. Roshier
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    ABSTRACT: Specialist frugivores are the dominant consumers of mistletoe fruit in many regions and have been shown to intensify infections of host plants as a result of their rapid gut passage rates and dependence on existing infections. The role of specialist frugivores in long distance dispersal of mistletoe and establishment of new infections is unclear, and has not been explicitly evaluated previously. Here we critically examine the premise that specialists are the dominant dispersers by examining the role of an Australian mistletoe specialist (mistletoebird Dicaeum hirundinaceum Dicaeidae) in dispersing mistletoe (Amyema preissii Santalales: Loranthaceae) seeds beyond infected host stands. We use two primary lines of evidence – presence of birds using remote call recorders, and presence of dispersed seeds via surveys for defecated seeds on host branches. The observed and inferred movements of the mistletoebird were wholly restricted to habitat patches containing mistletoe, and this bird was not observed to transport seeds to nearby uninfected host stands within the study system. While mistletoe specialists may provide much of the within-stand dispersal service for mistletoes, this serves only to aggregate and intensify existing infections. We suggest that long distance dispersal of mistletoe seeds beyond existing hosts and infection centres is not performed by these dietary specialists, these services more likely to be provided by generalist frugivores and other occasional mistletoe fruit consumers.
    Journal of Avian Biology 01/2012; 43(1). · 2.02 Impact Factor
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    ABSTRACT: 1. Expert knowledge is used routinely to inform listing decisions under the IUCN Red List criteria. Differences in opinion arise between experts in the presence of epistemic uncertainty, as a result of different interpretations of incomplete information and differences in individual beliefs, values and experiences. Structured expert elicitation aims to anticipate and account for such differences to increase the accuracy of final estimates. 2. A diverse panel of 16 experts independently evaluated up to 125 parameters per taxon to assess the IUCN Red List category of extinction risk for nine Australian bird taxa. Each panellist was provided with the same baseline data. Additional judgments and advice were sought from taxon specialists outside the panel. One question set elicited lowest and highest plausible estimates, best estimates and probabilities that the true values were contained within the upper and lower bounds. Asecond question set elicited yes ⁄no answers and a degree of credibility in the answer provided. 3. Once initial estimateswere obtained, all panellists were shown each others’ values. They discussed differences and reassessed their original values.Most communication was carried out by email. 4. The process took nearly 6 months overall to complete, and required an average of 1 h and up to 13 h per taxon for a panellist to complete the initial assessment. 5. Panellists were mostly in agreement with one another about IUCN categorisations for each taxon. Where they differed, there was some evidence of convergence in the second round of assessments, although there was persistent non-overlap for about 2% of estimates. The method exposed evidence of common subjective biases including overconfidence, anchoring to available data, definitional ambiguity and the conceptual difficulty of estimating percentages rather than natural numbers. 6. This study demonstrates the value of structured elicitation techniques to identify and to reduce potential sources of bias and error among experts. The formal nature of the process meant that the consensus position reached carried greater weight in subsequent deliberations on status. The structured process is worthwhile for high profile or contentious taxa, but may be too time intensive for less divisive cases.
    Methods in Ecology and Evolution 01/2012; 3:906-920. · 5.92 Impact Factor
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    David M Watson
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    ABSTRACT: The decline of woodland birds in southern Australia has motivated considerable research, identifying which species, habitats and regions are most affected, but the mechanisms driving these declines remain unclear. Applying findings from plant ecology, hydrology and soil science, I evaluate how availability of water and nutrients has been altered by agricultural development and how those changes have affected woodland food webs. Selective clearing of woodlands on fertile soils and overgrazing of remaining native vegetation have lowered productivity, whereas the storage of water has shifted from within the soil to surface reservoirs. I suggest that these changes have had a profound impact on below-ground decomposer communities, leading to fewer ground-dwelling invertebrate prey and reduced insectivore numbers. This productivity-based hypothesis is congruent with many previous findings, explaining the susceptibility of ground-foraging insectivores to changing land-use (via nutritional limitation), the sensitivity of southern woodlands (via summer drought stress), and the decreased resilience of eucalypt woodlands (via lower litter-fall and greater sensitivity to eutrophication). I detail six testable predictions extending beyond birds to microbial communities, plants, and other woodland-dependent animals. Finally, I explore the implications of this hypothesis, highlighting the value of remnant habitat on productive land to the long-term persistence of woodland bird populations.
    Emu. 03/2011; 111(1).
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    ABSTRACT: Summary1. Parasitic plants are components of many habitats and have pronounced effects on animal diversity; shaping distributions, influencing movement patterns and boosting species richness. Many of these plants provide fleshy fruit, nectar, foliar arthropods and secure nest sites, but the relative influence of these nutritional and structural resources on faunal species richness and community structure remains unclear.2. To disentangle these factors and quantify the resources provided by parasitic plants, we focused on the hemiparasitic shrub Exocarpos strictus (Santalaceae). Twenty-eight Eucalyptus camaldulensis forest plots were studied in the Gunbower-Koondrook forest in southeastern Australia, comparing riparian forests with an Exocarpos-dominated understorey with otherwise similar habitats with or without equivalent cover of the non-parasitic Acacia dealbata. Analyses of avian richness and incidence (overall and in six feeding guilds) were complemented by explicit measures of resources in both shrub types; foliage density, standing crop of fleshy fruit and foliar arthropod abundance and biomass.3. Avian species richness was c. 50% greater and total incidences for five guilds were significantly greater in forests with the parasitic shrub, with no appreciable differences between the other two habitat types. In addition to plentiful fleshy fruits, Exocarpos supported abundant arthropods in their foliage – significantly higher in biomass than for equivalent volumes of Acacia foliage. Exocarpos had a shorter and denser structure, providing a greater range of microhabitats than the more open growing Acacia.4. Our results demonstrate that structural and nutritional resources (both direct and indirect) provided by Exocarpos affect diversity and community composition, with each set of resources affecting different organismal groups. Rather than an exceptional system or an aberrant result, we suggest the influence of Exocarpos on species richness relates to their parasitic habit, supporting the hypothesis that parasitic plants mobilize resources from their hosts and make them available to a range of trophic levels.
    Functional Ecology 02/2011; 25(4):889 - 899. · 4.86 Impact Factor
  • Anna E Burns, Saul A Cunningham, David M Watson
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    ABSTRACT: Parasitic plants, such as mistletoes, are important components of tree canopies, providing food and shelter for a range of vertebrates and invertebrates. Arthropods from several orders are known to inhabit mistletoes but no direct comparisons between these plants and their host plants have been conducted until present. In this study, the composition and abundance of arthropods occurring on hemi-parasitic box mistletoe, Amyema miquelii ((Lehm. ex Miq.) Tiegh., Loranthaceae), on Eucalyptus (L., Myrtaceae) trees from the south-west slopes region of eastern Australia were investigated. Here a comparison of the arthropod assemblages at the ordinal level is presented. Specimens of Insecta and Arachnida were sampled from box mistletoe and three of its most common host species, using restricted canopy fogging, in two consecutive years, in nine remnants of grassy-box woodlands. The same 10 arthropod orders were sampled from the mistletoes and their eucalypt hosts but the total density of arthropods was greater on the eucalypt foliage. The latter result might be attributed to the significantly greater nitrogen content of the eucalypt foliage than the mistletoe foliage. One year after de-faunation, all but one of the arthropod orders had re-colonised the mistletoe plants. The total abundance of arthropods (particularly Hemiptera and Hymenoptera) on the mistletoes was greater in the second year of sampling, in which drought conditions occurred. Future research of arthropod assemblages in tree canopies should be more inclusive of the full range of substrates or habitats within canopies. Furthermore, investigation of the nutritional quality of mistletoe foliage compared with their host trees is required for a better understanding of the factors driving variation in community composition of arthropod assemblages.
    Australian Journal of Entomology 02/2011; 50(3):221 - 230. · 0.88 Impact Factor
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    Andrew F Bennett, David M Watson
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    ABSTRACT: Recent data from the Red List of the International Union for the Conservation of Nature show that 1240 of the world's estimated 10 027 species of birds (12.4%) are listed as threatened (Hoffmann et al. 2010). Globally, many more are 'declining' in conservation status. In Europe, much attention has been given to the marked decline in the abundance and distributional extent of farmland birds associated with the intensification of agricul-tural production (Fuller et al. 1995; Donald et al. 2001). Recent analyses suggest woodland species also may now be experiencing significant declines (e.g. Hewson et al. 2007). In the Americas, the declining status of neotropical migrants has motivated con-siderable research over the last 30 years (e.g. Terborgh 1989; Robinson and Wilcove 1994). In the tropics, narrowly endemic land birds have been identified as those species most at risk of decline globally in coming decades owing to projected changes in land-use (Jetz et al. 2007). Particular taxonomic groups also are experiencing marked declines. Migratory shorebirds, for exam-ple, which depend on key stop-over sites for refuelling during intercontinental migration, are particularly vulnerable to the degradation and destruction of these sites (Barter 2002; Rogers et al. 2010). Such widespread change among the world's avifauna has profound implications for global biodiversity, ecosystem function and the provision of ecosystem services (Sekercioglu 2006). In Australia, the declining conservation status of 'woodland' birds has raised much concern over the last two decades (e.g. Robinson and Traill 1996; Ford et al. 2001). Typically, these are species associated with forests (~30–70% canopy cover) and woodlands (~10–30% canopy cover) of temperate southern Australia: from south-eastern Queensland through parts of New South Wales (NSW), Victoria, Tasmania, South Australia (SA) and Western Australia (WA). Vast tracts of forest and woodland have been transformed into the 'wheat–sheep' belt of Australia, the backbone of Australia's agricultural production and agricultural export industries. Although the nation's economy has benefited from production agriculture, it has come at a substantial ecological cost, including the clearing of native vegetation, typically 80–90% or more in many districts (Hobbs and Yates 2000) and profound changes to the biota of these systems (Lindenmayer et al. 2010). The 'cost' to woodland birds is manifested in terms of population declines (Barrett et al. 2003; Olsen et al. 2005), reductions in geographical ranges (e.g. Franklin et al. 1989) and cumulative losses of species from districts and regions (e.g. Saunders 1989; Ford et al. 2009), ultimately leading to the listing of woodland bird species among Australia's threatened fauna. Woodland birds are the theme of this special issue of Emu – Austral Ornithology, which arose from a symposium of the Fifth Australasian Ornithological Conference held in December 2009 at Armidale, NSW, itself a noted centre for research on woodland birds. In addition to Watson's essay that expands on his keynote address, five of the papers in this issue (Doerr et al. 2011; Ford 2011; Maron et al. 2011; Sunnucks 2011; Szabo et al. 2011) arise directly from that symposium. The remaining three (Bilney et al. 2011; Jenner et al. 2011; Weaving et al. 2011) are additional contributions that complement this theme. Collective-ly, these papers represent the current state of knowledge about woodland birds, including overviews of recent work, reviews of methodological approaches, and stimulating new insights and perspectives. But what does research contribute to the conserva-tion dilemma of population decline in woodland birds? How effective is our science in finding solutions to this problem? Are we making a difference? We suggest that scientific research contributes in at least five ways to knowledge-based solutions to arresting the decline of woodland birds in Australia. These are: (1) documenting declines in woodland birds, including the taxonomic scope, geographical extent, magnitude and rate of declines; (2) identi-fying ecological patterns and correlates of decline; (3) proposing and testing mechanisms potentially associated with population decline; (4) evaluating solutions to arrest declines and promote recovery; and (5) developing conceptual models and analytical tools to enhance conservation management. We use this frame-work to place the papers in this volume in the context of wider work, to highlight the contributions that research is making to the issue of declining woodland birds, and to identify areas for further attention.
    Emu. 01/2011; 111(1).
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    JOHN RAWSTHORNE, DAVID M. WATSON, DAVID A. ROSHIER
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    ABSTRACT: Mistletoes are dispersed primarily by frugivorous birds and have highly aggregated distributions at multiple scales. Mistletoe specialist frugivores have been found to intensify infections within infected hosts and stands, and this is considered the most likely mechanism underlying clumped mistletoe distributions at these scales. How these patchy infections first develop and whether seed dispersers also contribute to aggregated mistletoe distributions at landscape and regional scales have not been evaluated. Here we predict the mistletoe seed shadow of a dietary generalist (spiny-cheeked honeyeater Acanthagenys rufogularis Aves: Meliphagidae), by combining our observations of movements via radio telemetry with previous data on gut passage times to estimate seed dispersal curves for individual birds. There was considerable variation in movements and inferred seed dispersal between individuals, with non-breeding birds predicted to regularly transport Amyema quandang (Santalales: Loranthaceae) seeds up to 700 m; well beyond the boundaries of an existing mistletoe infection. As the first work to consider explicitly the distance component of mistletoe seed dispersal by dietary generalists, this study poses further questions about the relative seed dispersal roles of dietary generalists and mistletoe specialists. Moreover, our findings highlight considerable intraspecific variation in movement and foraging behaviour, suggesting gender and reproductive status of birds should be considered explicitly when quantifying seed dispersal services.
    Austral Ecology 10/2010; 36(6):650 - 655. · 1.74 Impact Factor
  • WENDY A. MARCH, DAVID M. WATSON
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    ABSTRACT: Both nutrient cycling and nutrient relationships between mistletoe and host have been widely studied; yet it is unclear whether high nutrient concentrations commonly found in mistletoes affect rates of nutrient cycling. To address this question, we assessed 13 elements in the leaf litter of a temperate eucalypt forest in southern New South Wales, comparing concentrations from trees (Eucalyptus blakelyi, E. dwyeri, and E. dealbata) with and without the hemiparasitic mistletoe Amyema miquelii. Results were in accord with previous research on fresh leaves showing that concentrations of many elements were higher in the mistletoe than the host. This was not the case for all elements; most notably for N, where concentrations were significantly lower in the mistletoe. However, the return of all elements increased with mistletoe infection because of the combined effect of enrichment in mistletoe tissues and high rates of mistletoe litterfall. Annual returns of N and P in leaf litter increased by a factor of 1.65 and 3 respectively, with the greatest increase being for K by a factor of 43 in spring. These increased element returns were not significantly influenced by any changes in host leaf litter quality, as mistletoe infection was not found to affect host element concentrations. Mistletoe infection also altered the spatial and temporal distribution of element returns because of the patchy occurrence of mistletoes and extended period of mistletoe litterfall compared with the host. These findings provide a mechanistic explanation for the role of mistletoes as a keystone resource and, together with comparable results from root-parasitic plants in boreal tundra and cool-temperate grasslands, suggest that enhancing nutrient return rates may be a generalized property of parasitic plants.
    Austral Ecology 10/2010; 35(7):713 - 721. · 1.74 Impact Factor
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    David M. Watson
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    ABSTRACT: Summary1. Diverse sites have long-attracted ecologists, yet the overwhelming variety of species can confound attempts to enumerate species richness. Various predictive methods estimate species richness by comparing the rate at which species are first detected with the rate at which they are detected again, yielding richness estimates of known precision without exhaustive sampling.2. While frequently used for arthropods, predictive methods are rarely applied to vertebrate surveys where species identity is often a priority. Expressing observed richness as a function of estimated richness, an estimate of survey completeness can be derived, offering the potential for inventories of standardized precision for comparison and further analysis.3. To realize this potential, I conducted 402 h of bird surveys on Barro Colorado Island (Panama) and performed a series of retrospective analyses to address three questions: (i) How much effort is required to achieve complete inventories (maximum completeness)? (ii) What is the least amount of effort required to yield robust richness estimates (maximum efficiency)? and (iii) How much effort is required to optimize sampling, balancing completeness and efficiency?4. Whereas the richness estimate for all species required thirty 6-h samples to attain maximum completeness, once migrants, waterbirds and non-forest-dependent species were excluded, the richness of forest-dependent residents could be estimated to the same precision with fifteen samples and to 80% completeness with four samples.5. Of the 186 bird species detected, 70 represented unique or duplicate records, seen in only one or two sampling periods. These low detectability species were dominated by migrants (28) and raptors (14) and also included seven waterbirds, five nocturnal species and four aerial foragers, justifying the widespread practice of excluding these groups from surveys of forest assemblages.6. In addition to demonstrating the reliability of predictive approaches, this study demonstrates the practicality of results-based stopping rules for sampling diverse sites, especially for targeted groups of species. Combining predictive methods with targeted sampling represents an efficient and rigorous design, increasing the number of sites that can be sampled and enhancing the overall power and value of the study.
    Methods in Ecology and Evolution 05/2010; 1(3):280 - 291. · 5.92 Impact Factor
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    David M. Watson
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    ABSTRACT: Summary • Despite being components of most vegetation types, the community-level effects of parasitic plants are often ignored. The few studies adopting a broader view have revealed that these plants mediate a series of direct and indirect competitive and facilitative effects on community structure and ecosystem processes. • I summarize findings from the two best-studied systems: a set of experimental and manipulative studies from northern Sweden and an integrated research programme in southern Australia, both focusing on the most abundant hemiparasite in the region –Bartsia alpina (Orobanchaceae; Lamiales) and Amyema miquelii (Loranthaceae; Santalales), respectively. • Despite broad-based differences between these regions, their vegetation types and biotic constituents, rates of litter-fall, litter decomposition, nutrient return and plant growth all increased near the hemiparasites in both cases. This leads to changes in the abundance of other plants and the increased species richness and total biomass reflects an indirect form of facilitation. • In addition to reallocation of nutrients from host tissues, some of the additional nutrients may be excreted by other organisms, such as visiting pollinators, seed dispersers, herbivores and members of below-ground decomposer communities. Small-scale heterogeneity in nutrient availability could provide a mechanistic process underlying the role of parasitic plants as keystone resources. • Parasitic plants can be regarded as either malevolent predators (Dracula) or charitable benefactors(Robin Hood), but may be better described as Dryads (Greek deities associated with specific trees, which, in addition to being reliant on their host for their wellbeing, affect nearby trees and visiting animals, rendering the surrounding stand a sacred grove). • Synthesis. The Dryad role may be applicable to a wide range of facilitators, whereby indirect interactions with other organisms affect both hosts and adjacent plants, augmenting direct plant–plant interactions. However, while consistent with both case studies and information from studies of other parasitic plants, this role may be most apparent in low productivity systems. Addition, removal and animal exclosure experiments are highlighted as useful approaches to quantify the community-level influence of parasitic plants, in addition to dedicated work on the below-ground influences of parasitic plants.
    Journal of Ecology 10/2009; 97(6):1151 - 1159. · 5.43 Impact Factor
  • David M. Watson
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    ABSTRACT: Parasitic plants are less affected by resource constraints than other plants and most exhibit broad host tolerances, occupy large distributional ranges, and produce high numbers of propagules. Yet, parasitic plants are characteristically rare in undisturbed habitats, and patterns of distribution within host populations are often highly nonuniform. Previous work on root and shoot parasites has identified strict germination requirements for many species but, while explaining host ranges and site-microsite preferences for particular species, this cannot account for the highly clumped spatial structure of many parasitic plant populations. Other research has examined the role of seed vectors, but in most systems studied, dispersers are not limiting and their dietary breadth, substrate use, habitat preferences, and distributional ranges exceed the extent of the parasitic plant. Here, I propose the “host-quality hypothesis,” suggesting that variation in the quality of potential hosts can account for nonrandom occurrence patterns of parasitic plants in many systems. “Quality” can relate to access to water, nutrients, or other resources that are generally limiting to hosts, whereby parasites are more likely to establish and survive on hosts with greater access (i.e., higher quality from the parasite's perspective). Rather than supplanting germination requirements operating at the individual host plant scale or disperser behaviour operating at the landscape scale, this resource-based hypothesis applies at stand and population scales, explaining why some individuals within a stand or population are infected, while other apparently similar hosts are not susceptible. This hypothesis is explored using case studies on root and shoot hemiparasites, and is consistent with a diverse array of findings from a range of temperate and arid systems.Les plantes parasites sont moins touchées par les contraintes de ressources que d'autres plantes et la plupart font preuve d'une grande tolérance en ce qui concerne leur hôte, occupent une vaste répartition de types de distribution et produisent un grand nombre de ramets. Néanmoins, les plantes parasites ont la caractéristique d'être rares dans les habitats non détériorés et les types de distribution à l'intérieur des populations hôtes sont souvent fortement non uniformes. Les recherches antérieures concernant les parasites de racine et de pousse ont identifié les besoins spécifiques de germination pour un grand nombre d'espèces mais, bien que cela explique les types des hôtes et les préférences de sites/microsites pour des espèces spécifiques, cela ne peut pas expliquer la forte structure spatiale de répartition contagieuse de nombreuses populations de plantes parasites. D'autres recherches ont examiné le rôle de vecteurs de semences mais, dans la plupart des systèmes ayant fait l'objet de recherches, les disperseurs ne sont pas limités et leur largeur alimentaire, le substrat utilisé, les préférences d'habitat et les types de distributions excèdent l'étendue de la plante parasite. Ici, je propose « l'hypothèse de qualité de l'hôte », suggérant que la variation dans la qualité de hôtes potentiels peut expliquer l'occurrence déterminée de modèles de plantes parasites dans de nombreux systèmes. La « qualité » peut se rapporter à un accès d'eau, de nourriture ou autres ressources qui sont en général ce qui limite l'hôte, et donc les parasites ont plus de chance de s'établir et de survivre sur les hôtes avec le plus d'accès (c'est-à-dire, une meilleure qualité du point de vue du parasite). Plutôt que de supplanter les besoins de germination qui opèrent au niveau du hôte individuel ou le comportement de dispersion au niveau du paysage, cette hypothèse basée sur les ressources s'applique au peuplement et à l'échelle de la population, ce qui explique pourquoi certains individus à l'intérieur d'un peuplement ou d'une population sont infectés tandis que d'autres hôtes apparemment similaire ne sont pas réceptifs. Cette hypothèse est explorée en ayant recours à des études de cas d'hémiparasites sur racine ou sur pousse et est compatible avec une variété de groupements dans une répartition de systèmes tempérés et arides.
    Botany 12/2008; 87(1):16-21. · 1.23 Impact Factor