David M. Watson

Comparison of dwarf mistletoes (Arceuthobium spp., Viscaceae) in the western United States with mistletoes (Amyema spp., Loranthaceae) in Australia: ecological analogs and reciprocal models for ecosystem management

Whereas the biology, physiology and systematics of mistletoes have been explored in considerable detail, their ecology has received less attention and our understanding is highly fragmentary. A conspicuous exception is the dwarf mistletoes (Arceuthobium spp.)—a genus that exclusively parasitises coniferous trees, including many commercially valuable species in the forests of the western United States. Accordingly, these plants have been the subjects of intensive cross-disciplinary research for the past five decades, initially from a control and management perspective but extending into most aspects of their ecology and life history. This review summarises our understanding of dwarf mistletoes, focusing on recent developments in the areas of mistletoe–wildlife interactions, fire, ecosystem ecology and conservation biology. We also compare dwarf mistletoes with Australian mistletoes in the genus Amyema, a diverse suite of species found throughout the continent. Despite fundamental differences in their evolutionary origin and most aspects of their autecology and life history, the genera exhibit many similarities in terms of their ecological role in forests and woodlands, and their influence on stand- and forest-scale dynamics. In particular, both groups provide nesting resources for a range of birds and mammals, and nutritional resources for a diverse assemblage of species. Both also interact with fire, potentially leading to changes in successional dynamics at the stand scale. At an applied level, both groups are widely considered as pests but, as our understanding of these keystone species improves, they have the potential to serve as sensitive ecological indicators for their respective ecosystems. Key research priorities are identified for further research on both groups of mistletoes and more explicit comparative research, with Arceuthobium serving as a valuable template for future work on Amyema and Australian mistletoes in general.

A productivity-based explanation for woodland bird declines: poorer soils yield less food

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.

Mistletoe as a keystone resource: an experimental test

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.

Parasites boost productivity: effects of mistletoe on litterfall dynamics in a temperate Australian forest.

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.

Long-term consequences of habitat fragmentation—highland birds in Oaxaca, Mexico

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.

Mistletoe nesting in Australian birds: a review

Abstract Interactions between birds and mistletoes have been described in many regions worldwide, with most research focusing exclusively on the role of birds as seed and pollen vectors for these hemiparasitic plants. Mistletoe is also widely used by birds as a nesting site, with a recent family-level compilation identifying species in 43 families worldwide nesting in mistletoes. We reviewed breeding and nesting accounts of Australian birds to explore the extent of mistletoe nesting at the species level within an entire avifauna. In total, 217 species of Australian arboreal nesting bird from 29 families are here reported nesting in mistletoes, representing 66% of Australian species that nest in the foliage of trees. A further 28 species are also known to nest in mistletoes incidentally. This increases the total number of avian families known to exhibit this behaviour worldwide to 60, across 16 orders. Although no species can be considered an obligate mistletoe nester, several families regularly nest in mistletoes with >90% of Australian species known to have nested in mistletoes, including Pomatostomidae, Artamidae, Corvidae and Ptilonorhynchidae. Determining preference for mistletoe nesting is a priority for understanding this behaviour and we present guidelines for evaluating whether a particular species preferentially uses mistletoe as a nest-site. We postulate that the evergreen, dense habit of mistletoes provide a strong structural substrate on which to build a nest, offering a higher level of concealment and a more moderate nest microclimate than otherwise similar arboreal nest-sites. These features may also have a role in reducing nest predation and enhancing survivorship of nestlings. Future studies should focus on the mechanisms underlying this pattern using field experiments to evaluate the influence of mistletoe on nest microclimate, rates of predation and nest success.

Effects of mistletoe on diversity: a case-study from southern New South Wales

Abstract The influence of mistletoe density on avian diversity has been noted previously, with several studies demonstrating a close positive relationship between the two variables. All previous work has been correlative, exploiting naturally occurring variation in mistletoe density, and hence unable to demonstrate a causative link between mistletoe density and avian richness. Here I compare the avifauna of two adjacent woodland remnants, one of which has had most mistletoe plants removed, but otherwise comparable in area, vegetation and grazing history. Ten-hour inventories were conducted in each remnant in both spring and summer, resulting in a total of 40 hours of censuses. Of the 71 species recorded overall, 52 were recorded from the treatment site (with reduced mistletoe density) and 61 species from the control site. Significantly more woodland-dependent species and species known to feed on mistletoe were recorded in the control site, while there was no significant difference for those species known to nest in mistletoe. These results broadly support the idea that mistletoe is a keystone resource, with mistletoe density having a significant positive effect on species richness. These findings reinforce previous correlative studies, and further highlight the importance of mistletoe in Australian woodlands and forests.

Determinants of parasitic plant distribution: the role of host quality.

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 oc...

Comparative evaluation of new approaches to survey birds

Two recently devised approaches for sampling bird communities – time-balanced area-proportionate transects and standardised searches – aim to yield bird surveys of improved resolution specifically for studying distribution of terrestrial birds in patchy landscapes. Here, I compare these two approaches with the two most common methods presently used in Australian ornithological research: the fixed-effort (or 20-min) search and the repeat transect. The woodland-dependent avifauna of four reserves was sampled using all four methods. Total woodland richness of the four reserves was estimated by combining exhaustive surveys and incidental records to provide a benchmark for subsequent comparisons. The standardised search yielded the most complete richness estimates (i.e. closest to total woodland richness), averaging 78.4% under the strict stopping rule after an overall sampling effort of 740 min. Using a more lenient rule, mean completeness was slightly lower (72.5%) but overall sampling effort fell to 320 min. Coincidentally, this was the same total effort as required for the three fixed-effort methods, but resultant surveys were all less consistent and less complete. Fixed-effort searches (four 20-min searches per patch) yielded richness estimates with a mean completeness of 68.3%, compared with 37.1% for area-proportionate transects and 33.8% for repeat transects. Thus, for the same sampling effort, the lenient standardised search yielded data approximately twice as complete as transect-based methods. Moreover, resultant data are of uniform completeness and are expressed at the patch scale, and thus have greater biological value and relevance for management. In addition to being the most efficient and yielding the most complete data, the standardised search also yielded incidence estimates for all species recorded. While subject to the same limitations as abundance estimates, this information can complement richness data and allows more subtle comparisons of habitat preference and site quality. The use of transect counts and other fixed-effort sampling methods is discouraged for studies comparing variable sites, and standardised searches and other approaches relying on results-based stopping rules are advocated.

Land‐use change: incorporating the frequency, sequence, time span, and magnitude of changes into ecological research

The frequency and extent of human-induced land-cover changes is escalating worldwide. Recurrent turnover of land-cover types will affect ecosystems over and above major, one-time changes (eg deforestation). Here, we show how a deeper appreciation of the temporal dynamics of land-cover change is needed to understand its effects on ecosystems. We distinguish between four key components of land-change regimes: (1) frequency of land-cover changes over a period of time, (2) the sequence of land-cover types, (3) the time span over which each land-cover type extends, and (4) the magnitude of difference between land-cover types. We synthesize the impacts of these four components on ecological communities, showing that frequent land-cover changes are likely to favor species that are habitat and dietary generalists. Greater attention to the complex dynamics of land-cover changes is critical for a better understanding of the future impacts that human-generated land-use changes will have on global biodiversity.