David M Watson

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

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Publications (21)43.36 Total impact

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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
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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
  • 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: 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.
    01/2011;
  • 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
  • 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
  • David M. Watson
    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
  • Wendy A March, David M Watson
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    ABSTRACT: The importance of litter in regulating ecosystem processes has long been recognised, with a growing appreciation of the differential contribution of various functional plant groups. Despite the ubiquity of mistletoes in terrestrial ecosystems and their prominence in ecological studies, they are one group that have been overlooked in litter research. This study evaluated the litter contribution from a hemiparasitic mistletoe, Amyema miquelii (Lehm. ex Miq.) Tiegh., in an open eucalypt forest (Eucalyptus blakelyi, E. dwyeri and E. dealbata), at three scales; the forest stand, single trees and individual mistletoes. Litter from mistletoes significantly increased overall litterfall by up to 189%, the amount of mistletoe litter being proportional to the mistletoe biomass in the canopy. The high litter input was due to a much higher rate of mistletoe leaf turnover than that of host trees; the host litterfall and rate of leaf turnover was not significantly affected by mistletoe presence. The additional litter from mistletoes also affected the spatial and temporal distribution of litterfall due to the patchy distribution of mistletoes and their prolonged period of high litterfall. Associated with these changes in litterfall was an increase in ground litter mass and plant productivity, which reflects similar findings with root-parasitic plants. These findings represent novel mechanisms underlying the role of mistletoes as keystone resources and provide further evidence of the importance of parasites in affecting trophic dynamics.
    Oecologia 12/2007; 154(2):339-47. · 3.01 Impact Factor
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    ABSTRACT: Abstract Occurrence patterns of parasitic plants are constrained by the distribution of suitable hosts and movement patterns of seed vectors and, accordingly, represent a simplified system to study many aspects of spatial ecology and determinants of distribution. Previous work has focused on the aerially hemiparasitic mistletoes, and it is unclear whether root parasites are affected by similar factors. Here, we evaluate spatial patterns in the root parasitic Santalum lanceolatum in an arid shrubland in north-western New South Wales, central Australia. In this region, the principal host is a long-lived nitrogen fixing shrub Acacia tetragonophylla closely associated with ephemeral creek-lines. The location of 765 individuals of both species was mapped along a 250-m section of creek-line using a total survey station, and occurrence patterns of the root parasite related to host distribution and landscape context. We used Ripley's K-function and the O-ring statistic to determine whether the distribution of S. lanceolatum was random, aggregated or regular; the spatial scales at which these patterns occurred; and to quantify any spatial associations between the parasite and its host, A. tetragonophylla. While acacias were closely associated with the creek-line, S. lanceolatum plants were more tightly clustered, displaying significant clustering at two spatial scales (1.2 m and 8.8 m). We suggest that host quality may act as an important constraint, with only those acacias growing in or near the creek-line being physiologically capable of supporting a parasite to maturity. Insights gained from spatial analysis are used to guide ongoing research in this system, and highlight the utility of the O-ring statistic for understanding patterns of distribution affected by multiple processes operating at critical scales.
    Austral Ecology 05/2007; 32(4):359 - 369. · 1.74 Impact Factor
  • DAVID M. WATSON
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    ABSTRACT: Abstract Bird surveys are among the most widely used biodiversity inventories and serve as the basis for an increasing proportion of pure and applied ecological research. It is rarely possible to conduct exhaustive censuses of all individuals present at a particular site, so stopping rules are routinely used to determine when sampling should finish. Most bird survey methods use (implicit) effort-based stopping rules, either fixed times, fixed sampling areas (quadrats) or both, to standardize samples of different sites. If between-site variation is high, however, a fixed sampling effort will generate samples of variable completeness with samples from smaller, less complex sites being more representative and complete than samples from larger, more complex sites. More importantly, quadrat-based methods shift the scope of the overall study from bird occurrence in sites to bird occurrence in quadrats within sites, diminishing the impact of the research given that results cannot be extrapolated to relevant biological and management scales. Here I advocate an alternative means of conducting bird surveys, whereby the entire site is sampled and a results-based stopping rule is used to ensure sample completeness is uniform across all sites. For example, a researcher may decide to continue sampling each site until two or fewer previously unencountered species are recorded in a 40-min period. Samples of different sites will vary in both area and duration but will all be equivalently accurate estimates of species richness. This approach allows the avifauna of entire sites (whether territories, woodland remnants or catchments) to be sampled and compared directly, generating results and implications at the appropriate scale. In addition to yielding reliable measures of species richness, data collected this way can be used to calculate estimates of sample completeness and species incidence, two valuable metrics for ecological studies. This paper includes detailed worked examples of how to conduct a ‘standardized search’ and calculate sample completeness and species incidence estimates. I encourage further research on bird survey methods, and suggest that most current methods are insufficient, inconsistent and unreliable.
    Austral Ecology 09/2003; 28(5):515 - 525. · 1.74 Impact Factor
  • David M Watson
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    ABSTRACT: Studies of habitat fragmentation have been restricted primarily to anthropogenically-altered habitats, with most research conducted 60–90 years post-fragmentation. It is unclear whether patterns in older systems concur with results from these dynamic landscapes, and hence the long-term viability of populations inhabiting habitat fragments remains largely unexplored. I focused on resident birds in fragments of humid pine-oak forest in Oaxaca, southern Mexico, isolated over 5000 years ago by climate-change. Seventeen fragments, ranging from 2 ha to over 150,000 ha were sampled in 1997 and 1998 yielding 141 species, of which 60 residents were used for analysis. Avian assemblages exhibited a highly nested structure and, with several notable exceptions, assemblages of birds in low-richness fragments were predictable subsets of those in more diverse fragments. Patch-scale factors—area, shape, elevation, habitat diversity and fractal dimension of edge—all exerted strong univariate influence on avian richness but were so closely inter-related that none had a significant independent effect. Thus, larger fragments were more complex in shape, included higher peaks, supported more diverse forests, and contained higher diversities of resident species. In contrast, the landscape-scale index used—distance from nearest large fragment (>50,000 ha)—had little effect on richness. This was reinforced by species-level analyses—one species was significantly influenced by isolation, compared with 31 species that displayed significant minimum-area distributions, restricted to patches larger than a particular threshold value. In terms of autecology, vagility, relative abundance and elevational breadth were closely related to distribution—those species with greater mobility, higher abundances and broader elevational tolerances were consistently more widespread. I suggest that more abundant species were less prone to extinction initially, more vagile species were better dispersers and species with broader elevational tolerances more likely to be successful colonists. As with previous research from older landscapes, patch-scale factors were consistently found to be influential, with high quality fragments supporting diverse communities regardless of landscape context. This suggests that the influence of landscape-scale factors noted in younger, anthropogenically fragmented systems may be transitory, overwhelmed by patch-scale factors with time. Which patch attributes are most influential could not be resolved, however, indicating that even thousands of years after fragmentation, they affect diversity patterns in concert. Rather than differentiating effects of area from habitat heterogeneity and other patch-level factors, I advocate resource-based approaches to understand and manage diversity in habitat fragments.
    Biological Conservation. 01/2003;
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    David M. Watson
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    ABSTRACT: Aim I propose and develop a new classification system to explain diversity patterns in habitat fragments, equally applicable to islands and other inherently patchy ecosystems. My primary goal is to provide an inclusive model to improve the comparability of studies and enhance future efforts to synthesize their findings, yielding a generalized basis for understanding species composition in patchy ecosystems. Results Differentiating islands from fragments and incorporating patch age and patch: matrix contrast, eight classes of patch are distinguished, spanning a range of geographical features. To compare studies of diversity patterns among and between patch types, patch biota are divided into three categories based on their origin—relict species (present before fragmentation), matrix-derived species and interpatch dispersers. Applying this novel scheme to existing data, the effects of insularization are synthesized. Direct comparisons among fragments revealed broad similarity in the long-term effects of habitat fragmentation compared with highly divergent patterns in younger landscapes (<200 years). Holding patch: matrix contrast and age constant, fragments and islands were compared. Despite initial differences in community assembly, the biota of islands and fragments converge in several properties over time, as diversities stabilize and patch biotas become distinct from the surrounding matrix. Main conclusions Although necessarily broad, this framework provides an explicit context within which to test forty-four specific predictions regarding the distribution of diversity in patchy landscapes and thereby gain a clearer understanding of the long-term biological consequences of insularization. I propose that the fragments-as-islands analogy be revisited, potentially yielding valuable insight into the long-term future awaiting anthropogenically altered ecosystems.
    Journal of Biogeography. 01/2002; 29(5-6):823-834.
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    David M Watson
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    ABSTRACT: Mistletoes are a diverse group of parasitic plants with a worldwide distribution. The hemiparasitic growth form is critical to understanding their biology, buffering variation in resource availability that constrains the distribution and growth of most plants. This is manifested in many aspects of mistletoe life history, including extended phenologies, abundant and high-quality fruits and nectar, and few chemical or structural defenses. Most mistletoe species rely on animals for both pollination and fruit dispersal, and this leads to a broad range of mistletoe-animal interactions. In this review, I summarize research on mistletoe biology and synthesize results from studies of mistletoe-animal interactions. I consolidate records of mistletoe-vertebrate interactions, incorporating species from 97 vertebrate families recorded as consuming mistletoe and from 50 using mistletoe as nesting sites. There is widespread support for regarding mistletoe as a keystone resource, and all quantitative data are consistent with mistletoe functioning as a determinant of alpha diversity. Manipulative experiments are highlighted as a key priority, and six explicit predictions are provided to guide future experimental research. The facts which kept me longest scientifically orthodox are those of adapta-tion—the pollen-masses in Asclepias—the misseltoe, with its pollen carried by insects and seed by Birds—the woodpecker, with its feet and tail, beak and tongue, to climb the tree and secure insects. To talk of climate or Lamarckian habit producing such adaptation to other organic beings is futile. This diffi-culty, I believe I have surmounted. From a letter to Asa Gray by Charles Darwin, 1857.
    Annu. Rev. Ecol. Syst. 01/2001; 32:219-49.
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    David Roshier, David M Watson