Grassland structural heterogeneity in a savanna is driven more by productivity differences than by consumption differences between lawn and bunch grasses

Savanna grasslands are characterized by high spatial heterogeneity in vegetation structure, aboveground biomass and nutritional quality, with high quality short- grass grazing lawns forming mosaics with patches of tall bunch grasses of lower quality. This heterogeneity can arise because of local differences in consumption, because of differences in productivity, or because both processes enforce each other (more production and consumption). However, the relative importance of both processes in maintaining mosaics of lawn and bunch grassland types has not been measured. Also their interplay been not been assessed across landscape gradients. In a South African savanna, we, therefore, measured the seasonal changes in primary production, nutritional quality and herbivore consumption (amount and percentage) of grazing lawns and adjacent bunch grass patches across a rainfall gradient. We found both higher amounts of primary production and, to a smaller extent, consumption for bunch grass patches. In addition, for bunch grasses primary production increased towards higher rainfall while foliar nitrogen concentrations decreased. Foliar nitrogen concentrations of lawn grasses decreased much less with increasing rainfall. Consequently, large herbivores targeted the biomass produced on grazing lawns with on average 75 % of the produced biomass consumed. We conclude that heterogeneity in vegetation structure in this savanna ecosystem is better explained by small- scale differences in productivity between lawn and bunch grass vegetation types than by local differences in consumption rates. Nevertheless, the high nutritional quality of grazing lawns is highly attractive and, therefore, important for the maintenance of the heterogeneity in species composition (i.e. grazing lawn maintenance)

Spatiotemporal habitat use of large African herbivores across a conservation border

The rapid expansion of human populations in East Africa increases human-wildlife interactions, particularly along borders of protected areas (PAs). This development calls for a better understanding of how human-modified landscapes facilitate or exclude wildlife in savannas and whether these effects change through time. Here, we used camera traps to compare the distribution of 13 large herbivore species in Serengeti National Park with adjacent village lands used by livestock and people at both seasonal and diel cycle scales. The results show that body weight and feeding guild predict habitat use. Smaller sized grazers and mixed-feeders occurred more in village lands than larger herbivores. Across seasons, mixed-feeders and large browsers used edge areas year-round, while grazers were largely excluded during the dry season. At the diel cycle scale, wild herbivores' activity shifts towards the night in village lands compared to the protected area. A closer look revealed that wildebeest (Connochaetes taurinus) and zebra (Equus quagga) mainly used village lands from pre-dusk to midnight when pastoralists and their livestock were absent. Wildebeest and zebra activity in village lands peaked around dusk, which overlapped with peaks in predator activity. These results suggest that edge areas of PAs can provide valuable habitat to native herbivores. Intensive use of village lands by grazing herbivores during the wet season – particularly at night – suggests grazers benefit from high-quality grazing lawns and increased safety from predators during part of the year. Herbivores that (also) forage on browse can benefit year-round from both food availability and decreased predation risk. We further note that excluding nighttime observations in similar studies may underestimate the use of edge areas by wildlife. Understanding how edge areas fit into the risk-resource landscape will help identify the unique benefits of edge areas to wildlife and promote coexistence of people and wildlife around protected areas

Integrating defense and leaf economic spectrum traits in a tropical savanna plant

IntroductionAllocation to plant defense traits likely depends on resource supply, herbivory, and other plant functional traits such as the leaf economic spectrum (LES) traits. Yet, attempts to integrate defense and resource acquisitive traits remain elusive.MethodsWe assessed intraspecific covariation between different defense and LES traits in a widely distributed tropical savanna herb, Solanum incanum, a unique model species for studying allocations to physical, chemical, and structural defenses to mammalian herbivory.ResultsWe found that in a multivariate trait space, the structural defenses - lignin and cellulose - were positively related to the resource conservative traits - low SLA and low leaf N. Phenolic content, a chemical defense, was positively associated with resource acquisitive traits - high SLA and high leaf N - while also being associated with an independent third component axis. Both principal components 1 and 3 were not associated with resource supply and herbivory intensity. In contrast, spine density - a physical defense - was orthogonal to the LES axis and positively associated with soil P and herbivory intensity.DiscussionThese results suggest a hypothesized “pyramid” of trade-offs in allocation to defense along the LES and herbivory intensity axes. Therefore, future attempts to integrate defense traits with the broader plant functional trait framework, such as the LES, needs a multifaceted approach that accounts for unique influences of resource acquisitive traits and herbivory risk

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

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Cross-boundary human impacts compromise the Serengeti-Mara ecosystem

Protected areas provide major benefits for humans in the form of ecosystem services, but landscape degradation by human activity at their edges may compromise their ecological functioning. Using multiple lines of evidence from 40 years of research in the Serengeti-Mara ecosystem, we find that such edge degradation has effectively “squeezed” wildlife into the core protected area and has altered the ecosystem’s dynamics even within this 40,000-square-kilometer ecosystem. This spatial cascade reduced resilience in the core and was mediated by the movement of grazers, which reduced grass fuel and fires, weakened the capacity of soils to sequester nutrients and carbon, and decreased the responsiveness of primary production to rainfall. Similar effects in other protected ecosystems worldwide may require rethinking of natural resource management outside protected areas

Spatial redistribution of nutrients by large herbivores and dung beetles in a savanna ecosystem

1. Territorial or resting behaviour of large herbivores can cause strong local deposits of dung, in different places than where they graze. Additionally, dung beetles and other macrodetritivores can subsequently affect local nutrient budgets through post-depositional re-dispersion of dung and accompanying nutrients. Such horizontal displacement of nutrients by animals has strong implications for savanna ecosystem functioning, but remains poorly studied as it is notoriously difficult to accurately map these flows and incredibly time-consuming. 2. In an African savanna, with alternating patches of lawn, bunch grasses and trees/shrubs, we undertook such effort and studied nutrient aggregation and redistribution by different large herbivore functional groups and dung beetles for a full growing season. We used movable cages to quantify herbivore consumption rates and measured nutrient return through biweekly dung counts. Furthermore, we estimated the offtake of nitrogen (N) and phosphorus (P) by the dominant megagrazer (white rhinoceros) to middens (dung deposition hotspots). Last, we experimentally measured the removal amount and movement paths of telocoprid dung beetles to quantify their nutrient redistribution effects. 3. Our estimates suggest white rhinoceros to cause a large export of nutrients from grazing areas towards middens resulting in negative nutrient budgets for both lawn and bunch grassland types. Mesograzers (50-600kg) realized a net nitrogen input towards high forage quality lawn vegetation at the expense of lower quality bunch grasslands. Browsers caused a net flow from trees/shrubs towards grassland patches. 4. Interestingly, while the magnitude of our estimated flows of N consumption and return by large herbivores were rather similar, the P returns were about half of what has been consumed. This is in agreement with ecological stoichiometry theory that predicts that large herbivores should recycle more N than P, due to their relatively high P demand. Furthermore, dung-rolling beetles had a small, but significant, directed movement from lawn to bunch grassland vegetation. 5. Synthesis. We conclude that within-ecosystem nutrient redistributions by animals are important and approximately of the same order of magnitude as regional atmospheric nutrient in and outputs (e.g. fire emissions, atmospheric N deposition, biological N fixation), and hence are important for understanding savanna ecosystem functioning

Termites promote resistance of decomposition to spatiotemporal variability in rainfall

The ecological impact of rapid environmental change will depend on the resistance of key ecosystems processes, which may be promoted by species that exert strong control over local environmental conditions. Recent theoretical work suggests that macrodetritivores increase the resistance of African savanna ecosystems to changing climatic conditions, but experimental evidence is lacking. We examined the effect of large fungus-growing termites and other non-fungus-growing macrodetritivores on decomposition rates empirically with strong spatiotemporal variability in rainfall and temperature. Non-fungus-growing larger macrodetritivores (earthworms, woodlice, millipedes) promoted decomposition rates relative to microbes and small soil fauna (+34%) but both groups reduced their activities with decreasing rainfall. However, fungus-growing termites increased decomposition rates strongest (+123%) under the most water-limited conditions, making overall decomposition rates mostly independent from rainfall. We conclude that fungus-growing termites are of special importance in decoupling decomposition rates from spatiotemporal variability in rainfall due to the buffered environment they create within their extended phenotype (mounds), that allows decomposition to continue when abiotic conditions outside are less favorable. This points at a wider class of possibly important ecological processes, where soil-plant-animal interactions decouple ecosystem processes from large-scale climatic gradients. This may strongly alter predictions from current climate change models.</p

A novel mechanism for grazing lawn formation: large herbivore-induced modification of the plant-soil water balance

Veldhuis MP, Howison RA, Fokkema R, Tielens E, Olff H. A novel mechanism for grazing lawn formation: large herbivore-induced modification of the plant-soil water balance. Journal of Ecology. 2014;102(6):1506-1517

Data from: A novel mechanism for grazing lawn formation: large herbivore-induced modification of the plant-soil water balance

1. Large herbivores play a key-role in creating spatial heterogeneity through the formation of grazing lawns. Recent research suggests that the currently accepted nutrient-based theory on the formation of these grazing lawns cannot universally explain their formation in all ecosystems where they are found. 2. We developed and investigated an alternative hypothesis on grazing lawn formation and maintenance based on herbivore effects on the plant-soil water balance. We propose that large herbivores change the soil water balance in grazing lawns through defoliation and soil compaction, causing a shift in vegetation composition towards a drought tolerant plant community. 3. Investigating this idea in a tropical savanna, we indeed found profound differences in grazing lawn soil properties and water balance. In particular, defoliation increased soil temperatures and potential evaporation rates while soil compaction increased bulk density and decreased water infiltration rates, especially on fine-textured soils. Soil moisture was therefore generally much lower in grazing lawns than in adjacent bunch grass areas. 4. Furthermore, we found that grazing lawn species show drought-tolerant traits, with higher leaf sodium levels, suggesting evolutionary adaptation to these herbivore-induced dry conditions. However, leaf water potentials did not differ between grazing lawn and bunch grass species. 5. Synthesis: This study shows that large herbivores might form grazing lawns through previously underestimated effects on water balance. Thus, future studies on large herbivore effects on vegetation should increasingly focus on additional pathways of soil compaction and defoliation. While nutrient-based processes driving grazing lawn formation may operate during the wet season in savannas, we suggest that water balance-based processes are additionally important during the dry season

The metamicrobiome: Key determinant of the homeostasis of nutrient recycling

The metamicrobiome is an integrated concept to study carbon and nutrient recycling in ecosystems. Decomposition of plant-derived matter by free-living microbes and fire - two key recycling pathways - are highly sensitive to global change. Mutualistic associations of microbes with plants and animals strongly reduce this sensitivity. By solving a fundamental allometric trade-off between metabolic and homeostatic capacity, these mutualisms enable continued recycling of plant matter where and when conditions are unfavourable for the free-living microbiome. A diverse metamicrobiome - where multiple plant- and animal-associated microbiomes complement the free-living microbiome - thus enhances homeostasis of ecosystem recycling rates in variable environments. Research into metamicrobiome structure and functioning in ecosystems is therefore important for progress towards understanding environmental change