Leon A. Barmuta

Habitat structural complexity mediates the foraging success of multiple predator species

We investigated the role of freshwater macrophytes as refuge by testing the hypothesis that predators capture fewer prey in more dense and structurally complex habitats. We also tested the hypothesis that habitat structure not only affects the prey-capture success of a single predator in isolation, but also the effectiveness of two predators combined, particularly if it mediates interactions between the predators. We conducted a fully crossed four-factorial laboratory experiment using artificial plants to determine the separate quantitative (density) and qualitative (shape) components of macrophyte structure on the prey-capture success of a predatory damselfly, Ischnura heterosticta tasmanica, and the southern pygmy perch, Nannoperca australis. Contrary to our expectations, macrophyte density had no effect on the prey-capture success of either predator, but both predators were significantly less effective in the structurally complex Myriophyllum analogue than in the structurally simpler Triglochin and Eleocharis analogues. Furthermore, the greater structural complexity of Myriophyllum amplified the impact of the negative interaction between the predators on prey numbers; the habitat use by damselfly larvae in response to the presence of southern pygmy perch meant they captured less prey in Myriophyllum. These results demonstrate habitat structure can influence multiple predator effects, and support the mechanism of increased prey refuge in more structurally complex macrophytes.

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.

Habitat structural complexity mediates food web dynamics in a freshwater macrophyte community.

A considerable amount of research has investigated the influence of habitat structure on predator success, yet few studies have explored the implications for community structure and food-web dynamics. The relative importance of macrophyte structure and fish predation on the composition of the macroinvertebrate and periphyton communities in a lowland river was investigated using a multifactorial caging experiment. We hypothesised that: (1) fish predators are less effective in a more structurally complex macrophyte analogue; (2) strong direct and indirect effects of fish predators (e.g. trophic cascades) are less likely to occur in a structurally complex habitat; and (3) the strength of these patterns is influenced by the composition of the prevailing community assemblage. We measured the abundance and composition of the macroinvertebrate and periphyton communities associated with three different-shaped macrophyte analogues, under different fish predator treatments and at different times. Macrophyte analogue architecture had strong, consistent effects on both the macroinvertebrate and periphyton communities; both were most abundant and diverse on the most structurally complex plant analogue. In contrast, the fish predators affected only a subset of the macroinvertebrate community and there was a suggestion of minor indirect effects on periphyton community composition. Contrary to expectations, the fish predators had their strongest effects in the most structurally complex macrophyte analogue. We conclude that in this system, macrophyte shape strongly regulates the associated freshwater assemblage, resulting in a diverse community structure less likely to exhibit strong effects of fish predation.

NICHE DIFFERENTIATION AMONG SYMPATRIC AUSTRALIAN DASYURID CARNIVORES

Abstract Niche differentiation occurred on 3 dimensions in a size-structured guild of marsupial carnivores (Marsupialia: Dasyuridae) that included the Tasmanian devil (Sarcophilus laniarius), spotted-tailed quoll (Dasyurus maculatus), and eastern quoll (D. viverrinus). Diet was partitioned on a body size–prey size axis, but substantial overlap occurred between adjacent species. Complementary niche differentiation also occurred on horizontal (vegetation type and structure) and vertical (arboreal) habitat dimensions. Eastern quolls separated completely by using grasslands more than the other 2 species. Spotted-tailed quolls were distinguished by a greater degree of arboreal activity that reflected a large proportion of arboreal prey species in their diet. A strong relationship was found between body size, arboreal activity, and morphologic adaptations for climbing between predators and prey. Coevolution in phenotype among predators and prey and consequent constraint on performance in different habitat types were the most easily measured explanations for habitat differentiation among these species, particularly between spotted-tailed quolls and devils. The “ghost of competition past” is an alternative and nonexclusive explanation that can equally explain arboreal habitat use by spotted-tailed quolls and separation between the eastern quoll and the spotted-tailed quoll. Risk of predation and prey availability also may be important factors.

Methodological and conceptual issues in the search for a relationship between animal body-size distributions and benthic habitat architecture

Benthic ecologists have studied the distribution of animal body sizes because it is a form of ‘taxon-free’ classification that may be a useful metric for describing variation within and between ecological communities. In particular, the idea that the allometry of physiological and life-history traits may control species composition and relative abundances implies a functional link between body-size distributions and communities. The physical structure of aquatic habitats has often been cited as the mechanism by which habitat may determine body-size distributions in communities. However, further progress is hindered by a lack of theoretical clarity regarding the mechanisms that connect body size to the characteristics of ecological communities, leading to methods that may obscure interesting trends in body-size data. This review examines the methodological and conceptual issues hindering progress in the search for a relationship between animal body size and habitat architecture and suggests ways to resolve these issues. Problems are identified with current methods for the measurement of animal body size, the data and measures used to quantify body-size distributions and the methods used to identify patterns therein. Fundamentally, renewed emphasis on the mechanisms by which animal body sizes are influenced by habitat architecture is required to refine methodology and synthesise results from pattern-seeking and mechanistic studies.

Upstream heterogeneous zones: small stream systems structured by a lack of competence?

Abstract Small headwater streams have a suite of physical eccentricities that distinguishes them from the rest of the river system. These differences are now recognized in the study of sediment transport and channel morphology, but their implications for other limnological disciplines are less apparent. We suggest a zonation scheme for the upper stream system that acknowledges obvious geomorphic boundaries while highlighting differences in fine-scale habitat features that are likely to be biologically relevant. The upstream heterogeneous zone (UHZ) is distinguished by a high ratio of structural component size to stream width. Structural components include large rocks exposed from the regolith or from colluvial sources, tree roots, and woody debris, all of which are stochastically distributed and can constrain the morphology, hydraulics, and habitat distribution of small headwater streams. The high ratio of structural component size to stream width is a geomorphic phenomenon linked to the streams lack of competence to move the material that forms its bed and banks. It follows that the incompetence of streams within the UHZ is ultimately responsible for their greater internal physical heterogeneity than downstream reaches and is a fundamental driver of their physical structure, dynamics, and possibly ecology. Benign hydraulic conditions in small headwater streams have the potential to uncouple the link between physical and biological heterogeneity, resulting in a faunal community composed of highly mobile generalists.

Amplified predation after fire suppresses rodent populations in Australia’s tropical savannas

Abstract Context. Changes in abundance following fire are commonly reported for vertebrate species, but the mechanisms causing these changes are rarely tested. Currently, many species of small mammals are declining in the savannas of northern Australia. These declines have been linked to intense and frequent fires in the late dry season; however, why such fires cause declines of small mammals is unknown. Aims. We aimed to discover the mechanisms causing decline in abundance of two species of small mammals, the pale field rat, Rattus tunneyi, and the western chestnut mouse, Pseudomys nanus, in response to fire. Candidate mechanisms were (1) direct mortality because of fire itself, (2) mortality after fire because of removal of food by fire, (3) reduced reproductive success, (4) emigration, and (5) increased mortality because of predation following fire. Methods. We used live trapping to monitor populations of these two species under the following three experimental fire treatments: high-intensity fire that removed all ground vegetation, low-intensity fire that produced a patchy burn, and an unburnt control. We also radio-tracked 38 R. tunneyi individuals to discover the fates of individual animals. Key results. Abundance of both species declined after fire, and especially following the high-intensity burn. There was no support for any of the first four mechanisms of population decline, but mortality owing to predation increased after fire. This was related to loss of ground cover (which was greater in the high-intensity fire treatment), which evidently left animals exposed to predators. Also, local activity of two predators, feral cats and dingoes, increased after the burns, and we found direct evidence of predation by feral cats and snakes. Conclusions. Fire in the northern savannas has little direct effect on populations of these small mammals, but it causes declines by amplifying the impacts of predators. These effects are most severe for high-intensity burns that remove a high proportion of vegetation cover. Implications. To prevent further declines in northern Australia, fire should be managed in ways that limit the effects of increased predation. This could be achieved by setting cool fires that produce patchy burns, avoiding hot fires, and minimising the total area burnt.

River conservation in a changing world: invertebrate diversity and spatial prioritisation in south-eastern coastal Australia

Concentration of human populations with likely impacts of climate change present major challenges for river conservation in the south-eastern coastal region of Australia. Quantitative methods for spatial prioritisation of conservation actions can play a major role in meeting these challenges. We examined how these methods may be applied to help plan for potential impacts of climate change in the region, using macroinvertebrate assemblages as surrogates of river biodiversity. Environmental gradients explaining broad-scale patterns in the composition of macroinvertebrate assemblages are well represented in protected areas; however, their effectiveness for conserving river biodiversity with climate change depends on linking management inside and outside protected areas. Projected increases in temperature and sea level may be used to prioritise conservation to counter likely major impacts in high-altitude zones and the coastal fringes, whereas elsewhere, considerable uncertainty remains in the absence of better downscaled projections of rainfall. Applying such spatial prioritisations using biodiversity surrogates could help river-focussed conservation around the world.

Effects of exotic riparian vegetation on leaf breakdown by shredders: a tropical-temperate comparison

Abstract.  Displacement of riparian vegetation by exotic species is a global phenomenon with the potential to affect leaf breakdown rates by shredders. We predicted that exotic riparian vegetation would have a greater effect on leaf breakdown by temperate than by tropical shredders because temperate shredders usually feed on a limited range of generally palatable leaves, whereas tropical shredders are naturally exposed to a higher variety of leaves, many of them unpalatable. We tested 3 hypotheses with common shredder assemblages from tropical Queensland and temperate Tasmania (Australia): 1) tropical shredders are equally efficient at breaking down native and exotic vegetation, whereas temperate shredders are less efficient at breaking down exotic vegetation; 2) tropical shredders are more generalist in their leaf choices than temperate shredders; and 3) shredders are more generalist in their leaf choices among exotic than among native vegetation. Hypothesis 1 was not supported. Caddisflies (tropical or temperate) were able to consume both native and exotic leaves, whereas non-caddisfly shredders fed only on native leaves, a result suggesting that shredding capacity depends on the identity of the shredder species or their phylogenetic position rather than on their origin. Hypothesis 2 was supported. Tropical shredders fed on various leaf types, whereas most temperate shredders chose one leaf type and fed on it for the duration of the experiment. Hypothesis 3 was not supported. Specificity of shredder choices did not differ between native and exotic leaves, a result suggesting that shredding behavior is not flexible, regardless of the leaf litter available. Thus, invasive riparian plants may affect leaf breakdown by shredders, particularly in temperate streams, but effects may vary depending on assemblage composition, the nature and timing of litterfall, and interactions with climate.

Forest clearance increases metabolism and organic matter processes in small headwater streams

Abstract Small headwater streams are abundant components of the riverine landscape where critical biochemical processes occur that provide clean water, energy, and nutrients to downstream reaches. Disturbance to these systems as a result of human land use has the potential to affect downstream health. Rates of metabolism and organic matter processing were measured in 22 small forested headwater streams in 2 regions of Tasmania, Australia, to evaluate the effects of forestry disturbance. Twelve of these streams had been subjected to recent clearfell-burn-and-sow (CBS) harvest. Benthic metabolism was measured in small in situ chambers (production ranged from <0.001 to 21.845 mg C m−2 h−1 and respiration from <0.001 to 4.976 mg C m−2 h−1), whole-system metabolism was estimated based on relative habitat abundance (gross primary production ranged from <0.001 to 0.297 g C m−2 d−1 and daily respiration from 0.003 to 0.072 g C m−2 d−1). Algal growth potential was measured on nutrient diffusing pots (chlorophyll a ranged from <1.0 to 40.1 mg/m2), and cellulose decomposition potential was assessed with a cotton-strip assay (cotton tensile strength loss ranged from 17.8% to 38.3% in 28 d). Sometimes an increase in the variability of response is a consequence of disturbance, but in our study, the difference between forested streams and clearcut streams was a significant increase in the mean values of all functional variables. The degree of response depended on the underlying geology (broad-scale spatial variability) of the streams. Current management practices for small headwater streams in Tasmania do not protect instream processes from forestry disturbance in the short-term (i.e., 2–5 y), and we suggest that an investigation of long-term response is warranted.