Sumanta Bagchi

Introduced grazers can restrict potential soil carbon sequestration through impacts on plant community composition.

Grazing occurs over a third of the earths land surface and may potentially influence the storage of 10(9) Mg year(-1) of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazer-exclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m(-2) year(-1). Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems.

Empirical assessment of state‐and‐transition models with a long‐term vegetation record from the Sonoran Desert

Resilience-based frameworks, including state-and-transition models (STM), are being increasingly called upon to inform policy and guide ecosystem management, particularly in rangelands. Yet, multiple challenges impede their effective implementation: (1) paucity of empirical tests of resilience concepts, such as alternative states and thresholds, and (2) heavy reliance on expert models, which are seldom tested against empirical data. We developed an analytical protocol to identify unique plant communities and their transitions, and applied it to a long-term vegetation record from the Sonoran Desert (1953-2009). We assessed whether empirical trends were consistent with resilience concepts, and evaluated how they may inform the construction and interpretation of expert STMs. Seven statistically distinct plant communities were identified based on the cover of 22 plant species in 68 permanent transects. We recorded 253 instances of community transitions, associated with changes in species composition between successive samplings. Expectedly, transitions were more frequent among proximate communities with similar species pools than among distant communities. But unexpectedly, communities and transitions were not strongly constrained by soil type and topography. Only 18 transitions featured disproportionately large compositional turnover (species dissimilarity ranged between 0.54 and 0.68), and these were closely associated with communities that were dominated by the common shrub (burroweed, Haplopappus tenuisecta); indicating that only some, and not all, communities may be prone to large compositional change. Temporal dynamics in individual transects illustrated four general trajectories: stability, nondirectional drift, reversibility, and directional shifts that were not reversed even after 2-3 decades. The frequency of transitions and the accompanying species dissimilarity were both positively correlated with fluctuation in precipitation, indicating that climatic drivers require more attention in STMs. Many features of the expert models, including the number of communities and participant species, were consistent with empirical trends, but expert models underrepresented recent increases in cacti while overemphasizing the introduced Lehmanns lovegrass (Eragrostis lehmanniana). Quantification of communities and transitions within long-term vegetation records presents several quantitative metrics such as transition frequency, magnitude of accompanying compositional change, presence of unidirectional trajectories, and lack of reversibility within various timescales, which can clarify resilience concepts and inform the construction and interpretation of STMs.

Herbivore effects on above- and belowground plant production and soil nitrogen availability in the Trans-Himalayan shrub-steppes.

Large mammalian herbivores may have positive, neutral, or negative effects on annual net aboveground plant production (NAP) in different ecosystems, depending on their indirect effects on availability of key nutrients such as soil N. In comparison, less is known about the corresponding influence of grazers, and nutrient dynamics, over annual net belowground plant production (NBP). In natural multi-species plant communities, it remains uncertain how grazing influences relative allocation in the above- and belowground compartments in relation to its effects on plant nutrients. We evaluated grazer impacts on NAP, NBP, and relative investment in the above- and belowground compartments, alongside their indirect effects on soil N availability in the multiple-use Trans-Himalayan grazing ecosystem with native grazers and livestock. Data show that a prevailing grazing intensity of 51% increases NAP (+61%), but reduces NBP (−35%). Grazing also reduced C:N ratio in shoots (−16%) and litter (−50%), but not in roots, and these changes coincided with increased plant-available inorganic soil N (+23%). Areas used by livestock and native grazers showed qualitatively similar responses since NAP was promoted, and NBP was reduced, in both cases. The preferential investment in the aboveground fraction, at the expense of the belowground fraction, was correlated positively with grazing intensity and with improvement in litter quality. These results are consistent with hypothesized herbivore-mediated positive feedbacks between soil nutrients and relative investment in above- and belowground compartments. Since potentially overlapping mechanisms, such as N mineralization rate, plant N uptake, compositional turnover, and soil microbial activity, may contribute towards these feedbacks, further studies may be able to discern their respective contributions.

Assessing resilience and state‐transition models with historical records of cheatgrass Bromus tectorum invasion in North American sagebrush‐steppe

Summary 1. Resilience-based approaches are increasingly being called upon to inform ecosystem management, particularly in arid and semi-arid regions. This requires management frameworks that can assess ecosystem dynamics, both within and between alternative states, at relevant time scales. 2. We analysed long-term vegetation records from two representative sites in the North American sagebrush-steppe ecosystem, spanning nine decades, to determine if empirical patterns were consistent with resilience theory, and to determine if cheatgrass Bromus tectorum invasion led to thresholds as currently envisioned by expert-based state-and-transition models (STM). These data span the entire history of cheatgrass invasion at these sites and provide a unique opportunity to assess the impacts of biotic invasion on ecosystem resilience. 3. We used univariate and multivariate statistical tools to identify unique plant communities and document the magnitude, frequency and directionality of community transitions through time. Community transitions were characterized by 37–47% dissimilarity in species composition, they were not evenly distributed through time, their frequency was not correlated with precipitation, and they could not be readily attributed to fire or grazing. Instead, at both sites, the majority of community transitions occurred within an 8–10 year period of increasing cheatgrass density, became infrequent after cheatgrass density peaked, and thereafter transition frequency declined. 4. Greater cheatgrass density, replacement of native species and indication of asymmetry in community transitions suggest that thresholds may have been exceeded in response to cheatgrass invasion at one site (more arid), but not at the other site (less arid). Asymmetry in the direction of community transitions also identified communities that were ‘at-risk’ of cheatgrass invasion, as well as potential restoration pathways for recovery of pre-invasion states. 5. Synthesis and applications. These results illustrate the complexities associated with threshold identification, and indicate that criteria describing the frequency, magnitude, directionality and temporal scale of community transitions may provide greater insight into resilience theory and its application for ecosystem management. These criteria are likely to vary across biogeographic regions that are susceptible to cheatgrass invasion, and necessitate more in-depth assessments of thresholds and alternative states, than currently available.

Distributional correlates of the Tibetan gazelle Procapra picticaudata in Ladakh, northern India: towards a recovery programme

The Tibetan gazelle Procapra picticaudata is on the verge of extinction in India with only c. 100 individ- uals remaining in the Hanle Valley of eastern Ladakh and Sikkim. Conservation planning and initiation of a recovery programme are hindered by lack of ecological information on the species and we therefore assessed the biotic and abiotic correlates of its occurrence in the Hanle Valley. Ecological attributes of areas selected by gazelles were compared with those of adjoining areas without gazelles. Resource selection functions revealed that gazelles use relatively flat areas (6-15°) dispropor- tionately during both summer and winter, and preferred south-facing slopes and avoided north-facing slopes dur- ing winter. Measurements of plant production using exclosures showed that herbivores removed up to 47% of the forage biomass from areas without gazelles, whereas only 29% was removed from areas with gazelles. Although areas selected by gazelles were only marginally more productive than areas not selected, the proportional representation of forbs in plant biomass was signifi- cantly higher in the former. Spatial co-occurrence pat- terns examined using null models revealed a significant negative relationship between distribution of gazelles and goats and sheep, and a significant positive relation- ship between gazelles and wild kiang Equus kiang and domestic yak Bos grunniens. Future in situ recovery programmes for the Tibetan gazelle in Ladakh need to focus on securing livestock-free, forb-dominated areas, with participation from the local pastoral community.

Relationship between size hierarchy and density of trees in a tropical dry deciduous forest of western India

Questions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density. Location: Western India. Methods: Density was measured as the number of trees in 10-m radius circular plots (n = 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n = 1239 trees). Results: Size inequality between neighbours decreased with density but in a non-linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ̒ depletive ̓resource such as below-ground water (rather than a ̒ pre-emptive ̓resource such as light), which is very plausible in a semi-arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ ≈ –1/4), is higher than expected from existing models of allometric plant growth (γ ≈ –1/3) which are based on above-ground interactions alone. Seen in conjunction, these results suggest that above-ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below-ground resources, such as water, may be more important under semi-arid conditions. Conclusions: The non-linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above-ground biomass becomes sluggish in semi-arid, deciduous forests. Since current allometric models do not incorporate below-ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.

Applied ecology in India: scope of science and policy to meet contemporary environmental and socio-ecological challenges

Summary 1. India, a mega-diverse country in terms of both biodiversity and people, is battling environmental problems on many fronts: chronic dependence on natural resources, dwindling ecosystem services, declining environmental quality, effects of climate change and a biodiversity crisis. 2. We review the current focal areas and infrastructure for ecological research and education in India, along with the surrounding legal and policy aspects of related socio-economic issues. 3. Currently, ecological and applied research is predominantly focused on charismatic species within protected areas. This scope could be broadened beyond organismal biology towards functional landscapes and ecosystems; the education system also needs to promote ecology as a career choice for scientists. Expectedly, many environmental challenges are generic in nature, occur in other regions of the world, are primarily biophysical in origin but extend into human dimensions; some challenges are socio-political and have implications for biodiversity conservation. 4. Synthesis and applications. India’s environmental concerns include, but are not restricted to, the biodiversity crisis. The biodiversity crisis, in turn, includes, but is not restricted to, the most charismatic species. Greater integration and alignment among the mandates of government agencies, scientists, policymakers and educators are needed to meet contemporary environmental issues.

Body size and species coexistence in consumer-resource interactions: A comparison of two alternative theoretical frameworks

Species coexistence involving trophic interactions has been investigated under two theoretical frameworks—partitioning shared resources and accessing exclusive resources. The influence of body size on coexistence is well studied under the exclusive resources framework, but has received less attention under the shared-resources framework. We investigate body-size-dependent allometric extensions of a classical MacArthur-type model where two consumers compete for two shared resources. The equilibrium coexistence criteria are compared against the general predictions of the alternative framework over exclusive resources. From the asymmetry in body size allometry of resource encounter versus demand our model shows, counterintuitively, and contrary to the exclusive resource framework, that a smaller consumer should be competitively superior across a wide range of supplies of the two resource types. Experimental studies are reviewed to resolve this difference among the two frameworks that arise from their respective assumptions over resource distribution. Another prediction is that the smaller consumer may have relatively stronger control over equilibrium resource abundance, and the loss of smaller consumers from a community may induce relatively stronger trophic cascades. Finally, from satiating consumers’ functional response, our model predicts that greater difference among resource sizes can allow a broader range of consumer body sizes to coexist, and this is consistent with the predictions of the alternative framework over exclusive resources. Overall, this analysis provides an objective comparison of the two alternative approaches to understand species coexistence that have heretofore developed in relative isolation. It advances classical consumer–resource theory to show how body size can be an important factor in resource competition and coexistence.

Comparing the effects of livestock and native herbivores on plant production and vegetation composition in the Trans-Himalayas

Grazing can have implications for ecosystem management, biodiversity conservation, human livelihoods and global biogeochemical cycles. Grazers can either depress or promote plant production, with weak or strong effects on vegetation composition. Such variability is a major challenge for sustaining production while avoiding undesirable vegetation shifts. It is also uncertain how knowledge obtained from native herbivores can be used to manage domestic livestock and vice versa. In addition, grazer effects on production and vegetation composition tend to vary along prominent environmental gradients and are also negatively related to each other. Here, we evaluate these patterns for both livestock and native grazers under comparable grazing intensity and evaluate competing hypotheses that can account for the negative co-variation between these two types of grazer effects. A dataset from a four-year herbivore exclusion experiment in the Trans-Himalayan ecosystem in northern India shows the following: (a) grazer effects on plant production and on vegetation composition were indeed negatively correlated, but the relationship depends on the choice of data metrics; (b) incidental autocorrelation due to an underlying soil moisture gradient does not fully explain this pattern; instead, (c) their relationship is explained by variation in local plant species richness. Vegetation responses after excluding livestock and native grazers were qualitatively similar. But, despite comparable grazing intensity, livestock had quantitatively stronger effects on plant species composition. Production in species-rich communities was more grazing-tolerant and showed greater compositional stability. So, understanding the determinants of variation in species richness and how it is, in turn, influenced by grazing can offer a framework to interpret and manage highly variable impacts of herbivores on grazing ecosystems.

Quantifying long‐term plant community dynamics with movement models: implications for ecological resilience

Quantification of rates and patterns of community dynamics is central for understanding the organization and function of ecosystems. These insights may support a greater empirical understanding of ecological resilience, and the application of resilience concepts toward ecosystem management. Distinct types of dynamics in natural communities can be used to interpret and apply resilience concepts, but quantitative methods that can systematically distinguish among them are needed. We develop a quantitative method to analyze long-term records of plant community dynamics using principles of movement ecology. We analyzed dissimilarity of species composition through time with linear and nonlinear statistical models to assign community change to four classes of movement trajectories. Compositional change in each sampled plot through time was classified into four classes, stability, abrupt nonlinear change, transient reversible change, and gradual linear drift, each representing a different aspect of ecological resilience. These competing models were evaluated based on estimated coefficients, goodness of fit, and parsimony. We tested our methods accuracy and robustness through simulations, or the ability to distinguish among trajectories and classify them correctly. We simulated 16,000 trajectories of four types, of which 94-100% were correctly classified. Next, we analyzed 13 long-term vegetation records from North American grasslands (annual grasslands with warm-season and cool-season communities, shortgrass, mixedgrass, and tallgrass prairies, and sagebrush steppe), and a record of primary succession at Mt. St. Helens volcano. Collectively, we analyzed 14,647 observations from 775 plots, between 1915 and 2012. Dynamics could be reliably assigned for 705 plots (91%), and overall statistical fit was high (goodness of fit, 0.77xa0±xa00.15 SD). Among the perennial grasslands, stability was most common (44% of all plots), followed by gradual linear (22%), abrupt nonlinear (17%), and reversible (6%) change. Among annual grasslands, abrupt nonlinear shifts (33%) were more common in the warm-season community than in the cool-season (20%). As expected, abrupt nonlinear change was common during primary succession (51%) while reversible change was rare (3%). Generally, reversible dynamics often required 2-3 decades. Analysis of long-term community change, or trajectories, with principles of movement ecology provides a quantitative basis to compare and interpret ecological resilience within and among ecosystems.