Scott A. Parsons

Plant litter decomposition and nutrient cycling in north Queensland tropical rain-forest communities of differing successional status

Soil processes are essential in enabling forest regeneration in disturbed landscapes. Little is known about whether litterfall from dominating pioneer species in secondary rain forest is functionally equivalent to that of mixed rain-forest litter in terms of contribution to soil processes. This study used the litterbag technique to quantify the decomposition and nutrient dynamics of leaf litter characteristic of three wet tropical forest communities in the Paluma Range National Park, Queensland, Australia over 511 d. These were: undisturbed primary rain forest (mixed rain-forest species), selectively logged secondary rain forest (pioneer Alphitonia petriei) and tall open eucalypt forest (Eucalyptus grandis). Mass loss, total N, total P, K, Ca and Mg dynamics of the decaying leaves were determined, and different mathematical models were used to explain the mass loss data. Rainfall and temperature data were also collected from each site. The leaves of A. petriei and E. grandis both decomposed significantly slower in situ than the mixed rain-forest species (39%, 38% and 29% ash-free dry mass remaining respectively). Nitrogen and phosphorus were immobilized, with 182% N and 134% P remaining in E. grandis, 127% N and 132% P remaining in A. petriei and 168% N and 121% P remaining in the mixed rain-forest species. The initial lignin:P ratio and initial lignin:N ratio exerted significant controls on decomposition rates. The exceptionally slow decomposition of the pioneer species is likely to limit soil processes at disturbed tropical rain-forest sites in Australia.

Volume measurements for quicker determination of forest litter standing crop

Litter standing crop (LSC) is the quantity of plant detritus on the floor in forested environments. Knowledge of LSC is important in understanding many ecological phenomena. These include studies of litterfall, decomposition/litter turnover rates and nutrient cycling (Anderson et al. 1983, Dent et al. 2006), general plant performance (Benitez-Malvido & Kossmann-Ferraz 1999), other ecosystem processes such as the effects of fire (Odiwe & Muoghalu 2003) and fauna (Frith & Frith 1990, Giaretta et al. 1999, Levings & Windsor 1985). The determination of accurate annual average LSC data, may require monitoring over long periods due to seasonality and sometimes sporadic nature of litterfall and decomposition rates (Clark et al. 2001). Furthermore, the effects of topography and water movement create the need for both representative site selection and sufficient spatial coverage.

Rapid differentiation of sexual signals in invasive toads: call variation among populations

Advertisement calls tend to differ among populations, based on morphological and environmental factors, or simply geographic distance, in many taxa. Invasive cane toads (Rhinella marina) were introduced to Australia in 1935 and their distribution has expanded at increasing rates over time. Rapid evolution occurred in morphological and behavioural characters that accelerate dispersal, but the effects of rapid expansion on sexual signals have not been examined. We collected advertisement calls from four populations of different ages since invasion, and analysed the geographic differentiation of seven call parameters. Our comparisons indicate that the calls of R. marina differ among Australian populations. The signal variation was not simply clinal with respect to population age, climate, or morphological differentiation. We suggest that selection on signalling among populations has been idiosyncratic and may reflect local female preferences or adaptation to environmental factors that are not clinal such as energy availability.

Assessing the temporal dynamics of aquatic and terrestrial litter decomposition in an alpine forest

1. Litter decomposition supplies nutrients and energy within and among aquatic and terrestrial ecosystems. It is driven by several biotic and abiotic factors, the relative importance of which may change during litter decay. However, to date, very few studies have addressed the temporal dynamics of such factors across aquatic and terrestrial ecosystems, which limits our understanding of litter decomposition process. 2. To assess the temporal dynamics of major abiotic and biotic litter decomposition drivers, we conducted a 2-year field experiment to evaluate the losses of foliar litter carbon (C) and nitrogen (N) in alpine streams, riparian zones and forest floors. Environmental (soil, water and climatic) factors were continuously monitored, and incubated plant litter was sampled over time to assess temporal changes in litter chemistry and microbial diversity. 3. We analysed sequential litter decomposition stages based on mass-loss intervals and used structural equation modelling to disentangle the relative importance of each biotic and abiotic driver. 4. Our results suggested that across the aquatic and terrestrial ecosystems, litter C and N loss was generally controlled by a common hierarchy of drivers: (a) Environment and initial litter quality regulated C and N loss via both direct and indirect effects, and their total effects were mainly significant in the early and late decomposition stages, respectively; (b) changes in litter chemistry significantly influenced litter decomposition throughout the decomposition process, mainly via direct effects; and (c) microbial diversity per se showed minimal effects on litter C and N loss. 5. The identified common hierarchy of biotic and abiotic drivers and their direct and indirect effects on C and N loss at different decomposition stages across aquatic and terrestrial ecosystems indicates the possibility of integrating aquatic and terrestrial decomposition into a single framework for future construction of models accounting for temporal dynamics of litter decomposition.

Exploring relationships between native vertebrate biodiversity and grazing land condition

Although commercial grazing can degrade natural habitats, sustainably grazed land may be effective for wildlife conservation. Thus, land condition frameworks that assess the landscape quality of grazed land may also be useful for assessment of habitat quality for wildlife. However, the relationship between the condition of grazed land and native biodiversity is mostly unknown, and this knowledge gap must be addressed to adequately balance commercial production and conservation. In the present case study we determined the relevance of a widely used grazing land condition scale to understanding native vertebrate species richness and abundance (birds, reptiles, amphibians, mammals and all these vertebrate classes grouped) in grazed rangelands in northern Australia (~24–13°S; annual rainfall ranging from >1200 to <400 mm), sampled over approximately 10 years from 17 unique sites, containing 381 1-ha study plots. We defined the land condition scale relative to climate and comprehensive assessment of habitat attributes, and then described the relationships between land condition, habitat and biodiversity. The land condition scale partially explained richness and abundance patterns only for mammals (especially rodents), which tended to be higher in better-condition pasture. For other vertebrate groups, the scale was a very poor descriptor of richness and abundance. The land condition scale was not useful to assess wildlife diversity primarily because ‘woody thickening’ (increases in woody vegetation on grazed land, including shrubs and trees) lowers the ‘grazing value’ of land while also generally promoting vertebrate diversity. In line with this, biodiversity decreased with increasing bare ground and erosion, together with, and in the absence of, vegetation cover (i.e. desertification), consistent with grazing land degradation. The present study supports observations that land clearing and reductions in woody vegetation on grazed rangelands are particularly detrimental to native vertebrates.