Kyle W. Tomlinson

Predicting the effect of climate change on African trypanosomiasis: integrating epidemiology with parasite and vector biology

Climate warming over the next century is expected to have a large impact on the interactions between pathogens and their animal and human hosts. Vector-borne diseases are particularly sensitive to warming because temperature changes can alter vector development rates, shift their geographical distribution and alter transmission dynamics. For this reason, African trypanosomiasis (sleeping sickness), a vector-borne disease of humans and animals, was recently identified as one of the 12 infectious diseases likely to spread owing to climate change. We combine a variety of direct effects of temperature on vector ecology, vector biology and vector–parasite interactions via a disease transmission model and extrapolate the potential compounding effects of projected warming on the epidemiology of African trypanosomiasis. The model predicts that epidemics can occur when mean temperatures are between 20.7°C and 26.1°C. Our model does not predict a large-range expansion, but rather a large shift of up to 60 per cent in the geographical extent of the range. The model also predicts that 46–77 million additional people may be at risk of exposure by 2090. Future research could expand our analysis to include other environmental factors that influence tsetse populations and disease transmission such as humidity, as well as changes to human, livestock and wildlife distributions. The modelling approach presented here provides a framework for using the climate-sensitive aspects of vector and pathogen biology to predict changes in disease prevalence and risk owing to climate change.

Leaf adaptations of evergreen and deciduous trees of semi-arid and humid savannas on three continents

Summary 1. Drought stress selects for a suite of plant traits at root, stem and leaf level. Two strategies are proposed for trees growing in seasonally water-stressed environments: drought tolerance and drought avoidance. These are respectively associated with evergreen phenology, where plants retain their leaves throughout the year, and deciduous phenology, where plants drop their leaves during dry seasons. Evergreen species are thought to have leaf traits supporting lower photosynthesis and transpiration rates, in order to conserve water during dry periods. 2. We evaluated 18 morphological, chemical and physiological leaf traits of 51 abundant savanna tree species that differed in leaf habit (deciduous and evergreen), selected from two climate types (semi-arid and humid) in three continents (Australia, Africa and South America) (annual rainfall range: 500–1550 mm), and grown in a common garden experiment. We hypothesised that evergreen species have more conservative water use and differ more across climate types than deciduous species because evergreen species are forced to endure extended water deficits during dry seasons. 3. Trait shifts between semi-arid and humid savannas did not differ between evergreen and deciduous species. 4. Evergreen species had similar assimilation rates but lower photosynthetic water-use efficiency (PWUE) than deciduous species, possibly to extend their leaf lifespans by protecting their photosynthetic machinery from overheating through evaporative cooling. 5. Species of humid and semi-arid environments did not differ with respect to assimilation rate or PWUE, but semi-arid species did have smaller leaf sizes and greater leaf potassium and phosphorus concentrations. These traits may enable semi-arid species to maximize growth during episodes of favourable moisture availability. 6. Species from the three continents differed in their leaf traits. These probably reflect the greater proportion of evergreen species in Australia as compared to the other continents and generally infertile soils in the South American sampling sites compared to the wider fertility range in the African sites. 7. Synthesis: Water stress in savannas does not select for more conservative water use, but may select for rapid adjustment to prevailing water conditions and for heat avoidance mechanisms.

The utility of DNA metabarcoding for studying the response of arthropod diversity and composition to land-use change in the tropics

Metabarcoding potentially offers a rapid and cheap method of monitoring biodiversity, but real-world applications are few. We investigated its utility in studying patterns of litter arthropod diversity and composition in the tropics. We collected litter arthropods from 35 matched forest-plantation sites across Xishuangbanna, southwestern China. A new primer combination and the MiSeq platform were used to amplify and sequence a wide variety of litter arthropods using simulated and real-world communities. Quality filtered reads were clustered into 3,624 MOTUs at ≥97% similarity and the taxonomy of each MOTU was predicted. We compared diversity and compositional differences between forests and plantations (rubber and tea) for all MOTUs and for eight arthropod groups. We obtained ~100% detection rate after in silico sequencing six mock communities with known arthropod composition. Ordination showed that rubber, tea and forest communities formed distinct clusters. α-diversity declined significantly between forests and adjacent plantations for more arthropod groups in rubber than tea, and diversity of order Orthoptera increased significantly in tea. Turnover was higher in forests than plantations, but patterns differed among groups. Metabarcoding is useful for quantifying diversity patterns of arthropods under different land-uses and the MiSeq platform is effective for arthropod metabarcoding in the tropics.

Tree species from different functional groups respond differently to environmental changes during establishment.

Savanna plant communities change considerably across time and space. The processes driving savanna plant species diversity, coexistence and turnover along environmental gradients are still unclear. Understanding how species respond differently to varying environmental conditions during the seedling stage, a critical stage for plant population dynamics, is needed to explain the current composition of plant communities and to enable us to predict their responses to future environmental changes. Here we investigate whether seedling response to changes in resource availability, and to competition with grass, varied between two functional groups of African savanna trees: species with small leaves, spines and N-fixing associations (fine-leaved species), and species with broad leaves, no spines, and lacking N-fixing associations (broad-leaved species). We show that while tree species were strongly suppressed by grass, the effect of resource availability on seedling performance varied considerably between the two functional groups. Nutrient inputs increased stem length only of broad-leaved species and only under an even watering treatment. Low light conditions benefited mostly broad-leaved species’ growth. Savannas are susceptible to ongoing global environment changes. Our results suggest that an increase in woody cover is only likely to occur in savannas if grass cover is strongly suppressed (e.g. by fire or overgrazing). However, if woody cover does increase, broad-leaved species will benefit most from the resulting shaded environments, potentially leading to an expansion of the distribution of these species. Eutrophication and changes in rainfall patterns may also affect the balance between fine- and broad-leaved species.

Relative growth rate variation of evergreen and deciduous savanna tree species is driven by different traits.

BACKGROUND AND AIMS Plant relative growth rate (RGR) depends on biomass allocation to leaves (leaf mass fraction, LMF), efficient construction of leaf surface area (specific leaf area, SLA) and biomass growth per unit leaf area (net assimilation rate, NAR). Functional groups of species may differ in any of these traits, potentially resulting in (1) differences in mean RGR of groups, and (2) differences in the traits driving RGR variation within each group. We tested these predictions by comparing deciduous and evergreen savanna trees. METHODS RGR, changes to biomass allocation and leaf morphology, and root non-structural carbohydrate reserves were evaluated for juveniles of 51 savanna species (34 deciduous, 17 evergreen) grown in a common garden experiment. It was anticipated that drivers of RGR would differ between leaf habit groups because deciduous species have to allocate carbohydrates to storage in roots to be able to flush leaves again, which directly compromises their LMF, whereas evergreen species are not subject to this constraint. KEY RESULTS Evergreen species had greater LMF and RGR than deciduous species. Among deciduous species LMF explained 27 % of RGR variation (SLA 34 % and NAR 29 %), whereas among evergreen species LMF explained between 2 and 17 % of RGR variation (SLA 32-35 % and NAR 38-62 %). RGR and LMF were (negatively) related to carbohydrate storage only among deciduous species. CONCLUSIONS Trade-offs between investment in carbohydrate reserves and growth occurred only among deciduous species, leading to differences in relative contribution made by the underlying components of RGR between the leaf habit groups. The results suggest that differences in drivers of RGR occur among savanna species because these have different selected strategies for coping with fire disturbance in savannas. It is expected that variation in the drivers of RGR will be found in other functional types that respond differently to particular disturbances.

Savannahs of Asia: antiquity, biogeography, and an uncertain future

The savannahs of Asia remain locally unrecognized as distinctive ecosystems, and continue to be viewed as degraded forests or seasonally dry tropical forests. These colonial-era legacies are problematic, because they fail to recognize the unique diversity of Asian savannahs and the critical roles of fire and herbivory in maintaining ecosystem health and diversity. In this review, we show that: the palaeo-historical evidence suggests that the savannahs of Asia have existed for at least 1 million years, long before widespread landscape modification by humans; savannah regions across Asia have levels of C4 grass endemism and diversity that are consistent with area-based expectations for non-Asian savannahs; there are at least three distinct Asian savannah communities, namely deciduous broadleaf savannahs, deciduous fine-leafed and spiny savannahs and evergreen pine savannahs, with distinct functional ecologies consistent with fire- and herbivory-driven community assembly. Via an analysis of savannah climate domains on other continents, we map the potential extent of savannahs across Asia. We find that the climates of African savannahs provide the closest analogues for those of Asian deciduous savannahs, but that Asian pine savannahs occur in climates different to any of the savannahs in the southern continents. Finally, we review major threats to the persistence of savannahs in Asia, including the mismanagement of fire and herbivory, alien woody encroachment, afforestation policies and future climate uncertainty associated with the changing Asian monsoon. Research agendas that target these issues are urgently needed to manage and conserve these ecosystems. This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.

An approach to evaluate the effect of property size on land-use options in semi-arid rangelands

It is claimed that high returns can be achieved from hunting and ecotourism operations. As a result wildlife production is a rapidly growing form of land-use in South Africa. Lately, rural African communities have approached regional conservation agencies for aid to establish small game reserves so that they too may benefit from wildlife production. However, wildlife operations have high input costs relative to domestic stock operations and no attempt has been made to determine the effect of property size on the costs and revenue generated by wildlife. This paper attempts to develop a method for identifying the relevant economic variables of wildlife production, subsistence production and commercial beef production and the revenues that these separate land-uses generate. Thence to observe their relationship with property size by means of an illustrative example. In this way the size ranges for which each of the three land-uses is most appropriate can be determined. Finally, the method is evaluated against the results of the example to identify future refinement. The results of this example indicate that the profit curve of wildlife rises far more steeply than those of either subsistence production or commercial beef production. However, due to the effect of high input costs associated with wildlife, both commercial and subsistence beef production are more profitable at small land sizes. This indicates that investing large sums of money into small game reserves of the order of 3000 ha or less may not be justified on the basis of profits alone.

Deciduous and evergreen trees differ in juvenile biomass allometries because of differences in allocation to root storage

BACKGROUND AND AIMS Biomass partitioning for resource conservation might affect plant allometry, accounting for a substantial amount of unexplained variation in existing plant allometry models. One means of resource conservation is through direct allocation to storage in particular organs. In this study, storage allocation and biomass allometry of deciduous and evergreen tree species from seasonal environments were considered. It was expected that deciduous species would have greater allocation to storage in roots to support leaf regrowth in subsequent growing seasons, and consequently have lower scaling exponents for leaf to root and stem to root partitioning, than evergreen species. It was further expected that changes to root carbohydrate storage and biomass allometry under different soil nutrient supply conditions would be greater for deciduous species than for evergreen species. METHODS Root carbohydrate storage and organ biomass allometries were compared for juveniles of 20 savanna tree species of different leaf habit (nine evergreen, 11 deciduous) grown in two nutrient treatments for periods of 5 and 20 weeks (total dry mass of individual plants ranged from 0·003 to 258·724 g). KEY RESULTS Deciduous species had greater root non-structural carbohydrate than evergreen species, and lower scaling exponents for leaf to root and stem to root partitioning than evergreen species. Across species, leaf to stem scaling was positively related, and stem to root scaling was negatively related to root carbohydrate concentration. Under lower nutrient supply, trees displayed increased partitioning to non-structural carbohydrate, and to roots and leaves over stems with increasing plant size, but this change did not differ between leaf habits. CONCLUSIONS Substantial unexplained variation in biomass allometry of woody species may be related to selection for resource conservation against environmental stresses, such as resource seasonality. Further differences in plant allometry could arise due to selection for different types of biomass allocation in response to different environmental stressors (e.g. fire vs. herbivory).

Effects of fragmentation on the seed predation and dispersal by rodents differ among species with different seed size

Abstract Fragmentation influences the population dynamics and community composition of vertebrate animals. Fragmentation effects on rodent species in forests may, in turn, affect seed predation and dispersal of many plant species. Previous studies have usually addressed this question by monitoring a single species, and their results are contradictory. Very few studies have discussed the fragmentation effect on rodent–seed interaction among tree species with different seed sizes, which can significantly influence rodent foraging preference and seed fate. Given that fruiting periods for many coexisting plant species overlap, the changing foraging preference of rodents may substantially alter plant communities. In this study, we monitored the dispersal and predation by rodents of 9600 seeds, belonging to 4 Fagaceae species with great variation in seed size, in both the edge and interior areas of 12 tropical forest fragments ranging in area from 6.3 to 13872.9 ha in Southwest China. The results showed that forest fragmentation altered the seed fates of all the species, but the intensity and even the direction of fragmentation effect differed between species with large versus small seeds. For the seeds harvested, fragment size showed negative effects in forest interiors but positive effects at edges for the 2 large‐seeded species, but showed little effect for the 2 small‐seeded species. For the seeds removed, negative effects of fragment size only existed among the small‐seeded species. The different fragmentation effect on seed dispersal and predation among plant species may, in turn, translate into the composition differences of the regeneration of the whole fragmented forest.

Comment on “The extent of forest in dryland biomes”

Bastin et al. (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, leading to inappropriate management policies and practices in tropical grassy ecosystems.