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Frankincense trees (Boswellia spp.) worldwide are affected by a number of threats, including global warming and changing land management practices. On the Socotra Archipelago (Yemen), which harbours eleven endemic Boswellia species, grazing is generally assumed to be one of the main threats preventing natural regeneration. To test the impact of overgrazing on natural regeneration, we established an in situ experiment on four different Boswellia taxa in different areas of Socotra Island. Mortality and the height increment of seedlings were measured for a period of two/three years in five plots excluded from grazing (fenced) and in five paired control (unfenced) plots. Each plot was 50 m × 50 m in size and contained several adult trees as a source of viable seeds. Our results show that seedling mortality was significantly higher, and seedling height increment generally (4 out of 5 sites) lower in open compared to fenced plots. In the fenced plots, the number of seedlings for all species reached up to 772, with 560 surviving seedlings. In comparison, the control plots reached up to 296 seedlings, with 176 seedlings surviving after 2-3 years. The results of our experimental study indicate that grazing directly threatens the natural regeneration of the endemic Boswellia on Socotra Island. However, seedling mortality remained relatively high inside the exclosures as well, which indicates that even without the pressure of livestock grazing, other impacts remain a challenge for the future conservation of the archipelago's unique frankincense trees.

Frankincense trees (Boswellia spp.) worldwide are affected by a number of threats, including global warming and changing land management practices. On the Socotra Archipelago (Yemen), which harbours eleven endemic Boswellia species, grazing is generally assumed to be one of the main threats preventing natural regeneration. To test the impact of overgrazing on natural regeneration, we established an in situ experiment on four different Boswellia taxa in different areas of Socotra Island. Mortality and the height increment of seedlings were measured for a period of two/three years in five plots excluded from grazing (fenced) and in five paired control (unfenced) plots. Each plot was 50 m x 50 m in size and contained several adult trees as a source of viable seeds. Our results show that seedling mortality was significantly higher, and seedling height increment generally (4 out of 5 sites) lower in open compared to fenced plots. In the fenced plots, the number of seedlings for all species reached up to 772, with 560 surviving seedlings. In comparison, the control plots reached up to 296 seedlings, with 176 seedlings surviving after 2-3 years. The results of our experimental study indicate that grazing directly threatens the natural regeneration of the endemic Boswellia on Socotra Island. However, seedling mortality remained relatively high inside the exclosures as well, which indicates that even without the pressure of livestock grazing, other impacts remain a challenge for the future conservation of the archipelago's unique frankincense trees. [ABSTRACT FROM AUTHOR]

Large mammalian herbivores are well known to shape the structure and function of ecosystems world-wide, and these effects can in turn cascade through systems to indirectly influence other animal species. A wealth of studies has explored the effects of large mammals on arthropods, but to date they have reported such widely varying results that generalizations have been elusive. Three factors are likely drivers of this variability: the widely varying life-history characteristics of different arthropod groups, the highly variable landscapes that mammalian herbivores commonly inhabit and temporal variation in environmental conditions. Here, we use an 18-year-old exclosure experiment stratified across three distinct coastal prairie habitats in northern California to address the effects of a reintroduced mammalian herbivore, tule elk (Cervus canadensis nannodes) on the composition, richness and abundance of ground-dwelling arthropods over two years with very different precipitation regimes. We found that elk shifted the composition of arthropod communities, increasing the abundance of ants, beetles, spiders and mites, decreasing the abundance of woodlice and bristletails in some but not all habitats types, and having no effect on the abundance of bugs, crickets and springtails. Elk also increased richness and changed the composition of ant genera and beetle morpho-species. Interestingly, the effects of elk on arthropod composition, richness and abundance varied little between years, despite very different precipitation levels, biomass accumulation and thatch height. Elk reduced shrub cover, above-ground herbaceous biomass and thatch height and increased soil compaction, and these changes predicted the abundance and richness of arthropods, although taxonomic groups varied in their responses, presumably due to differences in environmental requirements. Synthesis. Our research highlights the importance of using long-term experiments to assess the cascading effects of large herbivores on the composition of grounddwelling arthropod communities and to identify the mechanisms that indirectly shape arthropod responses to herbivores among variable habitats and years in order to develop a greater understanding of the variable responses of arthropods to large mammalian herbivores.

Abstract Both termites and large mammalian herbivores (LMH) are savanna ecosystem engineers that have profound impacts on ecosystem structure and function. Both of these savanna engineers modulate many common and shared dietary resources such as woody and herbaceous plant biomass, yet few studies have addressed how they impact one another. In particular, it is unclear how herbivores may influence the abundance of long‐lived termite mounds via changes in termite dietary resources such as woody and herbaceous biomass. While it has long been assumed that abundance and areal cover of termite mounds in the landscape remain relatively stable, most data are observational, and few experiments have tested how termite mound patterns may respond to biotic factors such as changes in large herbivore communities. Here, we use a broad tree density gradient and two landscape‐scale experimental manipulations—the first a multi‐guild large herbivore exclosure experiment (20 years after establishment) and the second a tree removal experiment (8 years after establishment)—to demonstrate that patterns in Odontotermes termite mound abundance and cover are unexpectedly dynamic. Termite mound abundance, but areal cover not significantly, is positively associated with experimentally controlled presence of cattle, but not wild mesoherbivores (15–1,000 kg) or megaherbivores (elephants and giraffes). Herbaceous productivity and tree density, termite dietary resources that are significantly affected by different LMH treatments, are both positive predictors of termite mound abundance. Experimental reductions of tree densities are associated with lower abundances of termite mounds. These results reveal a richly interacting web of relationships among multiple savanna ecosystem engineers and suggest that termite mound abundance and areal cover are intimately tied to herbivore‐driven resource availability.

Rice is widely consumed as a staple food, being cultivated worldwide. However, in West Africa, production is not enough to satisfy demand. Rice often suffers intensive damage by herbivorous arthropods that affect quality and quantity of the grain. Birds and bats have been shown to suppress arthropod pests, potentially enhancing rice productivity and food security. However, the degree to which these taxa provide nature-based solutions for mitigating pest-induced rice losses is poorly known, especially in West Africa. Here, we used experimental exclosures to investigate whether birds and bats reduce plant damage and boost rice yield by suppressing arthropod abundance. In a rural area in northern Guinea-Bissau, we established 14 sets of paired control and experimental exclosures parcels, precluding access of birds and bats to rice plants. We then quantified how the absence of birds and bats influenced arthropod communities, plant damage, and rice yield over a full rice production cycle (six months). Arthropod numbers in exclosures (10.1 ± 9.1 ind./plot) were nearly double those in control plots (5.8 ± 3.0 ind./plot), a result mostly due to a lower spider abundance in the controls. The percentage of leaf and grain damage showed no difference between exclosure and control. Using Structural Equation Models, we uncovered that the exclusion of birds and bats boosted arthropod abundance but had only marginal effects on rice damage and no detectable effect on yield. The exclusion of flying vertebrates led to a marked increase in spider abundance, suggesting an effect of mesopredator release, which in turn likely helped maintaining pest abundance low and potentially contributing to the small overall effect on rice damage and yield. Enhancing the abundance of birds and bats is an interesting option to suppress agricultural pests, but our results highlight the need for a better understanding of ecological interactions in agricultural landscapes in West Africa. We stress the need for more research to inform evidence-based policies using nature-based solutions that foster the natural consumption of pests by vertebrates, as a means to improve food security. • Pioneer experiment of arthropod consumption by birds and bats in West Africa. • Bird and bat exclusion doubled arthropod abundance mostly due to spiders. • Excluding flying vertebrates did not influence rice yield. • Exclosures may inadvertently promote mesopredator proliferation. • Further studies needed to unravel the complex trophic networks in rice paddies. [ABSTRACT FROM AUTHOR]

Understanding distribution patterns and multitrophic interactions is critical for managing bat- and bird-mediated ecosystem services such as the suppression of pest and non-pest arthropods. Despite the ecological and economic importance of bats and birds in tropical forests, agroforestry systems, and agricultural systems mixed with natural forest, a systematic review of their impact is still missing. A growing number of bird and bat exclosure experiments has improved our knowledge allowing new conclusions regarding their roles in food webs and associated ecosystem services. Here, we review the distribution patterns of insectivorous birds and bats, their local and landscape drivers, and their effects on trophic cascades in tropical ecosystems. We report that for birds but not bats community composition and relative importance of functional groups changes conspicuously from forests to habitats including both agricultural areas and forests, here termed 'forest-agri' habitats, with reduced representation of insectivores in the latter. In contrast to previous theory regarding trophic cascade strength, we find that birds and bats reduce the density and biomass of arthropods in the tropics with effect sizes similar to those in temperate and boreal communities. The relative importance of birds versus bats in regulating pest abundances varies with season, geography and management. Birds and bats may even suppress tropical arthropod outbreaks, although positive effects on plant growth are not always reported. As both bats and birds are major agents of pest suppression, a better understanding of the local and landscape factors driving the variability of their impact is needed.

Both termites and large mammalian herbivores (LMH) are savanna ecosystem engineers that have profound impacts on ecosystem structure and function. Both of these savanna engineers modulate many common and shared dietary resources such as woody and herbaceous plant biomass, yet few studies have addressed how they impact one another. In particular, it is unclear how herbivores may influence the abundance of long‐lived termite mounds via changes in termite dietary resources such as woody and herbaceous biomass. While it has long been assumed that abundance and areal cover of termite mounds in the landscape remain relatively stable, most data are observational, and few experiments have tested how termite mound patterns may respond to biotic factors such as changes in large herbivore communities. Here, we use a broad tree density gradient and two landscape‐scale experimental manipulations—the first a multi‐guild large herbivore exclosure experiment (20 years after establishment) and the second a tree removal experiment (8 years after establishment)—to demonstrate that patterns in Odontotermes termite mound abundance and cover are unexpectedly dynamic. Termite mound abundance, but areal cover not significantly, is positively associated with experimentally controlled presence of cattle, but not wild mesoherbivores (15–1,000 kg) or megaherbivores (elephants and giraffes). Herbaceous productivity and tree density, termite dietary resources that are significantly affected by different LMH treatments, are both positive predictors of termite mound abundance. Experimental reductions of tree densities are associated with lower abundances of termite mounds. These results reveal a richly interacting web of relationships among multiple savanna ecosystem engineers and suggest that termite mound abundance and areal cover are intimately tied to herbivore‐driven resource availability. [ABSTRACT FROM AUTHOR]

ABSTRACT Herbivory is a fundamental process that controls primary producer abundance and regulates energy and nutrient flows to higher trophic levels. Despite the recent proliferation of small-scale studies on herbivore effects on aquatic plants, there remains limited understanding of the factors that control consumer regulation of vascular plants in aquatic ecosystems. Our current knowledge of the regulation of primary producers has hindered efforts to understand the structure and functioning of aquatic ecosystems, and to manage such ecosystems effectively. We conducted a global meta-analysis of the outcomes of plant-herbivore interactions using a data set comprised of 326 values from 163 studies, in order to test two mechanistic hypotheses: first, that greater negative changes in plant abundance would be associated with higher herbivore biomass densities; second, that the magnitude of changes in plant abundance would vary with herbivore taxonomic identity. We found evidence that plant abundance declined with increased herbivore density, with plants eliminated at high densities. Significant between-taxa differences in impact were detected, with insects associated with smaller reductions in plant abundance than all other taxa. Similarly, birds caused smaller reductions in plant abundance than echinoderms, fish, or molluscs. Furthermore, larger reductions in plant abundance were detected for fish relative to crustaceans. We found a positive relationship between herbivore species richness and change in plant abundance, with the strongest reductions in plant abundance reported for low herbivore species richness, suggesting that greater herbivore diversity may protect against large reductions in plant abundance. Finally, we found that herbivore-plant nativeness was a key factor affecting the magnitude of herbivore impacts on plant abundance across a wide range of species assemblages. Assemblages comprised of invasive herbivores and native plant assemblages were associated with greater reductions in plant abundance compared with invasive herbivores and invasive plants, native herbivores and invasive plants, native herbivores and mixed-nativeness plants, and native herbivores and native plants. By contrast, assemblages comprised of native herbivores and invasive plants were associated with lower reductions in plant abundance compared with both mixed-nativeness herbivores and native plants, and native herbivores and native plants. However, the effects of herbivore-plant nativeness on changes in plant abundance were reduced at high herbivore densities. Our mean reductions in aquatic plant abundance are greater than those reported in the literature for terrestrial plants, but lower than aquatic algae. Our findings highlight the need for a substantial shift in how biologists incorporate plant-herbivore interactions into theories of aquatic ecosystem structure and functioning. Currently, the failure to incorporate top-down effects continues to hinder our capacity to understand and manage the ecological dynamics of habitats that contain aquatic plants. [ABSTRACT FROM AUTHOR]

African savannas are highly seasonal with a diverse array of both mammalian and invertebrate herbivores, yet herbivory studies have focused almost exclusively on mammals. We conducted a 2-yr exclosure experiment in South Africa's Kruger National Park to measure the relative impact of these two groups of herbivores on grass removal at both highly productive patches (termite mounds) and in the less productive savanna matrix. Invertebrate and mammalian herbivory was greater on termite mounds, but the relative importance of each group changed over time. Mammalian offtake was higher than invertebrates in the dry season, but can be eclipsed by invertebrates during the wet season when this group is more active. Our results demonstrate that invertebrates play a substantial role in savanna herbivory and should not be disregarded in attempts to understand the impacts of herbivory on ecosystems. [ABSTRACT FROM AUTHOR]

Sunbird-pollinated plants display a suite of floral adaptations, for example, red tubular flowers, significant volumes of dilute nectar, or a suitable perch from which to forage flowers. In this study we show that two sunbird species were important for seed-set in low-growing, perchless plants of Hyobanche sanguinea and Lachenalia luteola on Kommetjie Slangkop, southern Cape Peninsula, South Africa. Remote video camera footage and field observations showed that malachite sunbirds ( Nectarinia famosa ) and orange-breasted sunbirds ( Anthobaphes violacea ) perched on the ground while visiting flowers of both plant species. Open inflorescences of both species yielded significantly higher seed-set than excluded inflorescences, which showed zero to near zero seed-set. Inflorescences shown on video to have been visited by sunbirds showed significantly higher seed-set than those for which video footage was not obtained, suggesting sunbirds are effective pollinators. Besides being the first documented pollinator information on these two species, our results expand upon the known traits associated with sunbird-pollination systems and suggest that low-growing sunbird-pollinated plants need not evolve perching structures in order to achieve effective visitation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Caatinga soil seed banks have low species richness, density, and persistence. • The proportion of viable seeds is similar in goat-exclosure and free-access plots. • Taxonomic and functional seed composition remains unchanged after goat exclosure. • Goat exclosure increases seedling survival in seedling transplantation experiments. • Free-ranging goats have the potential to impair forest regeneration and resilience. Livestock browsing and trampling can deplete plant regeneration and thus drive natural ecosystems towards degradation. This phenomenon may be particularly true in tropical dry forests, which support extensive livestock herds feeding on native vegetation, but has rarely been experimentally addressed. Here we examine how browsing and trampling by exotic goats impacts seed bank and seedling survivorship of woody plant species in a Caatinga dry forest by performing a 4-year exclosure experiment. Soil seed bank and transplanted seedlings from four woody plants species were monitored through a year across 14 and 10 paired plots (free goat-access vs exclosure plots), respectively. Regardless of treatment, the soil seed bank was low in density 14 seeds/m2 (±13, standard deviation) and taxonomically impoverished (24 species). Small to intermediate-sized seeds from dry fruits (75%) and abiotically dispersed species (65%) prevailed. The most common attractive structure was an elaiosome (33%), but pulp (14%) and aril (9.5%) were also recorded. 60% of the species presented some type of dormancy and 32% presented a pleurogram. 57% of the seeds were classified as forming a transient seed bank, while seasonally transient seeds accounted for 43% of the total seed bank. Seed bank attributes were not affected by goats, including proportion of viable seeds (25.85 ± 41.3 in exclosure plots vs 31.45 ± 42.49 in free-access plots) and diversity. However, seedling survival one year after transplantation was higher for the plant species in the exclosure plots (68.4%) as compared to the free-access plots (39%). Although there were no significant differences in seed bank, our results suggest that free-ranging goats in densities typical of the Caatinga region can impose elevated seedling mortality to the dominant woody dry forest species even considering a short period of time. [ABSTRACT FROM AUTHOR]

Abstract: Changes in ground flora were monitored over a 32-year period in deer exclosures in a yew-wood and a neighbouring oakwood in Killarney National Park, southwest Ireland; both woods are Annex I habitats under the European Habitats Directive. Comparison was made with unfenced plots adjacent to each of the exclosures. During the period of the study, both woods were heavily grazed by introduced sika deer (Cervus nippon). In the yew-wood exclosures, total ground flora cover increased markedly during the period of deer exclusion, the main species to increase in abundance being Rubus fruticosus agg. and Hedera helix. Herbaceous species increased initially in frequency but subsequently declined; herbaceous species had higher total cover in unfenced plots than in fenced plots at the end of the study. In the oakwood, Luzula sylvatica and Vaccinium myrtillus expanded their cover in one exclosure with cover in ferns declining, but in other exclosures where dense holly thickets developed, it was instead R. fruticosus and H. helix that became more abundant within the fences than outside. Overall, long-term fencing has caused a shift from vegetation characterised by woodland specialist to woodland generalist species and there are indications of a long-term decline in diversity. We conclude that chronic heavy grazing in these woodlands has strongly influenced the overall abundance and composition of the ground flora, but that complete exclusion of grazing is also undesirable due to potential declines in diversity of woodland specialists. [Copyright &y& Elsevier]

Although birds and mammals play important roles in several mechanisms hypothesized to maintain plant diversity in species-rich habitats, there have been few long-term, community-level tests of their importance. We excluded terrestrial birds and mammals from fourteen 6 x 7.5 m plots in Australian primary tropical rain forest and compared recruitment and survival of tree seedlings annually over the subsequent seven years to that on nearby open plots. We re-censused a subset of the plots after 13 years of vertebrate exclusion to test for longer-term effects. After two years of exclusion, seedling abundance was significantly higher (74%) on exclosure plots and remained so at each subsequent census. Richness was significantly higher on exclosure plots from 1998 to 2003, but in 2009 richness no longer differed, and rarefied species richness was higher in the presence of vertebrates. Shannon's diversity and Pielou's evenness did not differ in any year. Vertebrates marginally increased density-dependent mortality and recruitment limitation, but neither effect was great enough to increase richness or diversity on open plots relative to exclosure plots. Terrestrial vertebrates significantly altered seedling community composition, having particularly strong impacts on members of the Lauraceae. Overall, our results highlight that interactions between terrestrial vertebrates and tropical tree recruitment may not translate into strong community-level effects on diversity, especially over the short term, despite significant impacts on individual species that result in altered species composition. [ABSTRACT FROM AUTHOR]

The objective of this study was to examine how the processes of seed dispersal and seed predation were altered in forest fragments of the dry forest of Madagascar, where the usual seed dispersers and vertebrate seed predators were absent, using a lemur-dispersed tree species ( Strychnos madagascariensis; Loganiaceae) as an example. We then assessed how the changes in vertebrate community composition alter the regeneration pattern and establishment of this tree species and thus, ultimately, the species composition of the forest fragments. By using size-selective exclosures, data from forest fragments were compared with results from continuous forest where vertebrate dispersers and predators were abundant. Visits to the exclosures by mammalian seed predators were monitored with hair traps. In the continuous forest up to 100% of the seeds were removed within the 7 days of the experiments. A substantial proportion of them was lost to seed predation by native rodents. In contrast, practically no predation took place in the forest fragments and almost all seeds removed were dispersed into the safety of ant nests by Aphaenogaster swammerdami, which improves chances of seedling establishment. In congruence with these findings, the abundance of S. madagascariensis in the forest fragments exceeded that of the continuous forest. Thus, the lack of vertebrate seed dispersers in these forest fragments did not lead to a decline in regeneration of this animal-dispersed tree species as would have been expected, but rather was counterbalanced by the concomitant demise of vertebrate seed predators and an increased activity of ants taking over the role of seed dispersers, and possibly even out-doing the original candidates. This study provides an example of a native vertebrate-dispersed species apparently profiting from fragmentation due to flexible animal-plant interactions in different facets, possibly resulting in an impoverished tree species community. [ABSTRACT FROM AUTHOR]

Abstract: Woodland dominated by Taxus baccata is one of the rarest of European woodland types and has priority habitat status under Annex I of the EU Habitats Directive, yet little is known about its stand dynamics or the effects of long-term grazing. The abundance of naturally regenerating seedlings and saplings was monitored over a 32-year period in exclosures in a yew-wood and a neighbouring oakwood in the Killarney National Park, southwest Ireland. Both woods are heavily grazed by introduced sika deer (Cervus nippon). Mortality, recruitment and growth of adult trees were monitored over a 20-year period. Comparison was made with unfenced plots adjacent to each of the exclosures. Regeneration in the yew-wood exclosures was chiefly by Ilex aquifolium, Sorbus aucuparia and Fraxinus excelsior. No Taxus baccata saplings were recorded and seedlings of this species were very rare throughout the duration of the experiment. Taxus baccata trees which died during the 20-year monitoring period were significantly smaller than those which survived, indicating that self-thinning is occurring and the wood in its present form may be of relatively recent origins. Changes in adult tree species composition suggest that yew woodland in Ireland may develop from a yew-hazel scrub woodland sere. A variety of species regenerated in the oakwood exclosures, including Taxus baccata, and dense holly thickets formed in several areas. Quercus petraea failed to regenerate beneath the oak canopy. We conclude that chronic heavy grazing in the Killarney woodlands strongly influences the natural regeneration of several tree species. Research into the scientific manipulation of grazing levels in temperate woodlands is required. However, grazing intensity is not the only factor affecting regeneration, canopy conditions in particular are also likely to be of high significance, and this should be reflected in management plans. [Copyright &y& Elsevier]

The growth and survival of Quercus petraea saplings were monitored over a 25-year period, in an exclosure in a heavily-grazed oakwood on podzol soil in Killarney National Park. Seedlings were subjected to combinations of two levels of shading (under canopy and in an artificial clearing), two levels of competition (weeded and unweeded) and five fertiliser treatments (N, P, NP, Ca and control).Damage by Apodemus sylvaticus (long-tailed fieldmice) was confined to acorns and first-year seedlings, and was not a limiting factor. Invertebrate damage was unimportant. Seedling survival was greatly enhanced within the ungrazed exclosure, even under canopy. In the clearing, following a short-term breach of the fence in the second winter, Cervus nippon (sika deer) browsed 49% of oak seedlings in weeded plots but only 11% in unweeded plots, confirming that surrounding vegetation cover may exercise a protective effect. In later years, seedling survival was significantly higher in the weeded plots, presumably owing to reduced competition. Seedlings under canopy showed no significant response to fertilisation or weeding; the median proportion surviving per plot was zero within 8 years and all had died within 20 years. In the clearing, the median proportion surviving per plot was 0.33 after 8 years and 0.2 after 25 years. In a portion of the clearing with peaty soil and impeded drainage, seedlings showed reduced performance and significantly increased mortality. Microsphaera alphitoides (oak mildew) attacked seedlings mainly in the clearing; its impact appeared negligible. Fertilisation with P significantly enhanced performance in the clearing. No fertiliser treatment enhanced survival. Fertilisation with N alone led to sharply increased mortality in the clearing.I conclude that, in woodlands in western Ireland, successful oak regeneration is to be expected only in unshaded or lightly-shaded sites where grazing levels are low. [Copyright &y& Elsevier]