Arid vegetation in disequilibrium with livestock grazing: Evidence from long‐term exclosures

In recent decades, the conventional equilibrium paradigm for explaining rangeland vegetation dynamics has been challenged. Proponents of an alternative non‐equilibrium paradigm argue that in variable rangeland environments, external climatic events are critical to vegetation dynamics and there is little opportunity for plant–herbivore interactions to reach equilibrium. Understanding which paradigm more effectively describes an ecosystem has important consequences for management. In particular, some authors have argued that a focus on reducing stocking rates in non‐equilibrium systems may be futile, and management should be opportunistic in response to unpredictable rainfall events. We measured herbaceous biomass and plant species richness and abundance at five 14‐year exclosures on Innamincka Regional Reserve. Four were situated in the dunefields land system, and one on the Cooper Creek floodplain. We did not detect any significant differences between grazed and ungrazed treatments in total species richness or abundance, life form richness or abundance, or herbaceous biomass. Only one species, Portulaca oleracea, showed differences in abundance between treatments at more than one site, but the direction of these differences was not consistent. These results suggest that the non‐equilibrium paradigm more accurately describes vegetation dynamics in the dunefields and floodplains of north‐eastern South Australia. It is possible that some species had been lost from the study area prior to the establishment of the exclosures, thereby precluding recovery with protection from grazing; however, a regional analysis of the flora reveals little evidence of this. We argue that the dominance of ephemeral species confers resilience by limiting the development of strong feedbacks between grazing intensity and vegetation dynamics. Current grazing practices seem consistent with the conservation of plant species diversity across the dunefields and floodplains. Future studies should focus on the impacts of cattle grazing on areas of the landscape dominated by palatable perennials, as well as the small number of rare and potentially grazing‐sensitive species identified.     

Effects of long‐term fire exclusion and frequent fire on plant community composition: A case study from semi‐arid shrublands

Time since last fire and fire frequency are strong determinants of plant community composition in fire‐prone landscapes. Our study aimed to establish the influence of time since last fire and fire frequency on plant community composition and diversity of a south‐west Australian semi‐arid shrubland. We employed a space‐for‐time approach using four fire age classes: ‘young’, 8–15 years since last fire; ‘medium’, 16–34; ‘old’, 35–50; and ‘very old’, 51–100; and three fire frequency classes: burnt once, twice and three times within the last 50 years. Species diversity was compared using one‐way ANOVA and species composition using PERMANOVA. Soil and climatic variables were included as covariables to partition underlying environmental drivers. We found that time since last fire influenced species richness, diversity and composition. Specifically, we recorded a late successional transition from woody seeders to long‐lived, arid‐zone, resprouting shrub species. Fire frequency did not influence species richness and diversity but did influence species composition via a reduction in cover of longer‐lived resprouter species – presumably because of a reduced ability to replenish epicormic buds and/or sufficient starch stores. The distinct floristic composition of old and very old habitat, and the vulnerability of these areas to wildfires, indicate that these areas are ecologically important and management should seek to preserve them.     

Rethinking long‐term vegetation dynamics: multiple glacial refugia and local expansion of a species complex

Evidence is accumulating that some arcto‐boreal plant taxa persisted through the last glacial maximum (LGM) in Alaska and adjacent Canada. However, the spatial patterns of glacial persistence and associated postglacial colonization remain largely unknown. In this study, we investigated the LGM refugia of an alder (Alnus) species complex (n = 3 taxa) and assess the spatiotemporal dynamics of Alnus in this vast region. Specifically, we conducted high‐throughput DNA sequencing (ddRADseq) on Alnus foliar samples collected from a dense population network to investigate patterns of genetic structure and infer the presence of glacial lineages. Species distribution modeling (SDM) was used to investigate the probability and possible locations of glacial persistence. These analyses were integrated and then compared with fossil pollen data to identify the locations of refugial populations and spatial patterns of postglacial colonization. Our genetic analyses revealed two glacial lineages with separate geographic origins for each Alnus taxon, suggesting that the genus persisted in multiple LGM refugia. Non‐overlapping hindcast distributions based on SDMs further support the presence of multiple, spatially distinct refugia. These ddRADseq and SDM results, in conjunction with reassessment of fossil pollen records, suggest that Alnus expanded from several population nuclei that existed during the LGM and coalesced during the Holocene to form its present range. These results challenge the unidirectional model for postglacial vegetation expansion, implying that climate buffering associated with landscape heterogeneity and adaptation to millennial‐scale environmental variability played important roles in driving late‐Quaternary population dynamics.     

Population Structure of Woody Plants in Relation to Land Use in a Semi‐arid Savanna,West Africa

Indigenous woody species are important natural resources in West African savannas. Information about their population structures and response to human impact, particularly land use, however, is scarce. In this study, we explored (1) the effect of land use on the population structure of woody savanna species; and (2) searched for species with similar population structures related to comparable ecological preferences. Using generalized linear models, we separately analyzed the size‐class distribution (SCD) of 30 species to reveal the influence of three land cover types (non‐arable land, fallows, and protected areas) on population structures. Generalized linear models were applied to identify comparable population structures of species with similar ecological preferences. We were able to identify five groups for shrub species and four groups for tree species with different population structures and comparable ecological preferences. In terms of human impact, we detected four groups of species responding similarly to land use. Especially for trees, we found a strong influence of local land use on SCD and hence, population structures. The SCD of shrub species tends to be more related to species' ecological preferences. Some of the shrub species may be characterized as ubiquitous species as their SCD is neither related to land use nor ecological preferences, indicating a high tolerance to disturbance. The observed results have implications on local woody species composition in relation to land use. According to this, we propose focusing on trees when developing appropriate local land use management strategies.     

Power analysis for trend in ordinal cover classes: implications for long‐term vegetation monitoring

Question: We provide a method to calculate the power of ordinal regression models for detecting temporal trends in plant abundance measured as ordinal cover classes. Does power depend on the shape of the unobserved (latent) distribution of percentage cover? How do cover class schemes that differ in the number of categories affect power? Methods: We simulated cover class data by “cutting‐up” a continuous logit‐beta distributed variable using 7‐point and 15‐point cover classification schemes. We used Monte Carlo simulation to estimate power for detecting trends with two ordinal models, proportional odds logistic regression (POM) and logistic regression with cover classes re‐binned into two categories, a model we term an assessment point model (APM). We include a model fit to the logit‐transformed percentage cover data for comparison, which is a latent model. Results: The POM had equal or higher power compared to the APM and latent model, but power varied in complex ways as a function of the assumed latent beta distribution. We discovered that if the latent distribution is skewed, a cover class scheme with more categories might yield higher power to detect trend. Conclusions: Our power analysis method maintains the connection between the observed ordinal cover classes and the unmeasured (latent) percentage cover variable, allowing for a biologically meaningful trend to be defined on the percentage cover scale. Both the shape of the latent beta distribution and the alternative hypothesis should be considered carefully when determining sample size requirements for long‐term vegetation monitoring using cover class measurements.     

Gradual changes in range size accompany long‐term trends in species richness

Species richness has long been used as an indicator of ecosystem functioning and health. Global richness is declining, but it is unclear whether sub‐global trends differ. Regional trends are especially understudied, with most focused on island regions where richness is strongly impacted by novel colonisations. We addressed this knowledge gap by testing for multi‐decade trends in species richness in nine open marine regions around North America (197 region‐years) while accounting for imperfect observations and grounding our findings in species‐level range dynamics. We found positive richness trends in eight of nine regions, four of which were statistically significant. Species' range sizes generally contracted pre‐extinction and expanded post‐colonisation, but the ranges of transient species expanded over the long‐term, slowly increasing their regional retention and driving increasing richness. These results provide more evidence that sub‐global richness trends are stable or increasing, and highlight the utility of range size for understanding richness dynamics.     

Damped long‐term host–parasite Red Queen coevolutionary dynamics: a reflection of dilution effects?

An increase in biological diversity leads to a greater stability of ecosystem properties. For host–parasite interactions, this is illustrated by the ‘dilution effect’: a negative correlation between host biodiversity and disease risk. We show that a similar mechanism might stabilise host–parasite dynamics at a lower level of diversity, i.e. at the level of genetic diversity within host species. A long‐term time shift experiment, based on a historical reconstruction of a Daphnia–parasite coevolution, reveals infectivity cycles with more stable amplitude in experienced than in naive hosts. Coevolutionary models incorporating an increase in host allelic diversity over time explain the detected asymmetry. The accumulation of resistance alleles creates an opportunity for the host to stabilise Red Queen dynamics. It leads to a larger arsenal enhancing the host performance in its coevolution with the parasite in which ‘it takes all the running both antagonists can do to keep in the same place’.     

Responses of woody vegetation to exclusion of large herbivores in semi‐arid savannas

The Nkuhlu large‐scale long‐term exclusion experiment in Kruger National Park was designed to study the long‐term effects of large herbivores on vegetation. One treatment excludes elephants, another excludes all herbivores larger than hares and another one comprises an open, control area. Vegetation monitoring was implemented in 2002 when a baseline survey was conducted prior to exclusion. Monitoring was repeated 5 years after exclusion. Data from the surveys were analysed to establish how structure and composition of woody vegetation had changed 5 years after herbivore exclusion. The analysis showed that neither plant assemblage nor mean vegetation height had changed significantly since exclusion. However, both species richness and density of woody plants increased 5 years after exclusion of all large herbivores, but not after the exclusion of elephants alone. One already common species, Dichrostachys cinerea, became more common after excluding all large herbivores compared with either no exclusion or elephant exclusion, possibly leading to competitive suppression of other species. Species other than D. cinerea tended to either increase or decrease in density, but the changes were insufficient to induce significant shifts in the overall assemblage of woody plants. The results indicate that after 5 years of exclusion, the combined assemblage of large herbivores, and not elephants alone, could induce changes in species richness and abundances of woody plants, but the effect was so far insufficient to induce measureable shifts in the assemblages of woody plants. It is possible that assemblages will change with time and increasing elephant numbers may amplify future changes.     

Spatiotemporal variation in mechanisms driving regional‐scale population dynamics of a Threatened grassland bird

To achieve national population targets for migratory birds, landscape‐level conservation approaches are increasingly encouraged. However, knowledge of the mechanisms that drive spatiotemporal patterns in population dynamics are needed to inform scale‐variant policy development. Using hierarchical Bayesian models and variable selection, we determined by which mechanism(s), and to what extent, changes in quantity and quality of surrogate grassland habitats contributed to regional variation in population trends of an obligatory grassland bird, Bobolink (Dolichonyx oryzivorous). We used North American Breeding Bird Survey data to develop spatially explicit models of regional population trends over 25 years across 35 agricultural census divisions in Ontario, Canada. We measured the strength of evidence for effects of land‐use change on population trends over the entire study period and over five subperiods. Over the entire study period, one region (Perth) displayed strong evidence of population decline (95% CI is entirely below 0); four regions displayed strong evidence of population increase (Bruce, Simcoe, Peterborough, and Northumberland). Population trends shifted spatially among subperiods, with more extreme declines later in time (1986–1990: 28% of 35 census divisions, 1991–1995: 46%, 1996–2000: 40%, 2001–2005: 66%, 2006–2010: 82%). Important predictors of spatial patterns in Bobolink population trends over the entire study period were human development and fragmentation. However, factors inferred to drive patterns in population trends were not consistent over space and time. This result underscores that effective threat identification (both spatially and temporally) and implementation of flexible, regionally tailored policies will be critical to realize efficient conservation of Bobolink and similar at‐risk species.     

Dynamic size responses to climate change: prevailing effects of rising temperature drive long‐term body size increases in a semi‐arid passerine

Changes in animal body size have been widely reported as a correlate of contemporary climate change. Body size affects metabolism and fitness, so changing size has implications for resilience, yet the climatic factors that drive size variation remain poorly understood. We test the role of mean and extreme temperature, rainfall, and remotely sensed primary productivity (NDVI) as drivers of body size in a sedentary, semi‐arid Australian passerine, Ptilotula (Lichenostomus) penicillatus, over 23 years. To distinguish effects due to differential growth from changes in population composition, we analysed first‐year birds and adults separately and considered climatic variation at three temporal scales (current, previous, and preceding 5 years). The strongest effects related to temperature: in both age classes, larger size was associated with warmer mean temperatures in the previous year, contrary to Bergmann's Rule. Moreover, adults were larger in warmer breeding seasons, while first years was larger after heatwaves; these effects are more likely to be mediated through size‐dependent mortality, highlighting the role of body size in determining vulnerability to extinction. In addition to temperature, larger adult size was associated with lower primary productivity, which may reflect a trade‐off between vegetative growth and nectar production, on which adults rely. Finally, lower rainfall was associated with decreasing size in first year and adults, most likely related to decreased food availability. Overall, body size increased over 23 years, strongly in first‐year birds (2.7%) compared with adults (1%), with size outcomes a balance between competing drivers. As rainfall declined over time and productivity remained fairly stable, the temporal increase in body size appears largely driven by rising mean temperature and temperature extremes. Body size responses to environmental change are thus complex and dynamic, driven by effects on growth as well as mortality.     

Using a species distribution model to guide NSW surveys of the long‐footed potoroo (Potorous longipes)

Knowledge of threatened species’ distributions is essential for effective conservation decision‐making. Species distribution models (SDMs) are widely used to map species’ geographic ranges, identify new areas of suitable habitat and guide field surveys. In New South Wales (NSW), Australia, there are grave doubts about whether populations of the critically endangered long‐footed potoroo (Potorous longipes) remain extant, and identification of occupied sites is a high priority for its conservation. We used an SDM (Maxent) to identify regions in NSW that may have suitable habitat for the potoroo. The SDM was built with seven climate layers and had strong predictive performance (cross‐validated AUC = 0.94). We then combined this information on habitat suitability with vegetation and topography, to identify 58 survey sites across NSW. From April 2016 to May 2017, we undertook six field trips deploying six to eight cameras at each site for 52–63 days, resulting in 25 120 camera trap nights. A total of 215 759 images captured 43 native and feral animal species, but no long‐footed potoroos. Following the survey, newly available, independent presence and absence data were used to validate our model. A Kruskal–Wallis H test indicated that habitat suitability values were significantly higher at presence locations than absence locations (H = 58.66, d.f. = 1, P < 0.001). Finally, we refitted the Maxent model with the new data and identified additional regions that future surveys could explore. We conclude, however, that if the long‐footed potoroo remains extant in NSW, it is extremely rare.