Large herbivore impact on plant biomass along multiple resource gradients in the Serengeti

Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom-up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources. We test four alternative hypotheses for the relationship between plant biomass, herbivore impact on plant biomass, and plant resource gradients, each based on how resources might affect plant abundance and quality to herbivores. We measured plant biomass for four non-consecutive years in a long-term grazing exclosure experiment in the Serengeti National Park that includes seven sites that vary substantially in rainfall and soil and plant nitrogen (N) and phosphorus (P). Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N. Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized.

Naïve plant communities and individuals may initially suffer in the face of reintroduced megafauna: An experimental exploration of rewilding from an African savanna rangeland

Excluding large native mammals is an inverse test of rewilding. A 25-year exclosure experiment in an African savanna rangeland offers insight into the potentials and pitfalls of the rewilding endeavor as they relate to the native plant community. A broad theme that has emerged from this research is that entire plant communities, as well as individual plants, adjust to the absence of herbivores in ways that can ill-prepare them for the return of these herbivores. Three lines of evidence suggest that these “naïve” individuals, populations, and communities are likely to initially suffer from herbivore rewilding. First, plots protected from wild herbivores for the past 25 years have developed rich diversity of woody plants that are absent from unfenced plots, and presumably would disappear upon rewilding. Second, individuals of the dominant tree in this system, Acacia drepanolobium, greatly reduce their defences in the absence of browsers, and the sudden arrival of these herbivores (in this case, through a temporary fence break), resulted in far greater elephant damage than for their conspecifics in adjacent plots that had been continually exposed to herbivory. Third, the removal of herbivores favoured the most palatable grass species, and a large number of rarer species, which presumably would be at risk from herbivore re-introduction. In summary, the native communities that we observe in defaunated landscapes may be very different from their pre-defaunation states, and we are likely to see some large changes to these plant communities upon rewilding with large herbivores, including potential reductions in plant diversity. Lastly, our experimental manipulation of cattle represents an additional test of the role of livestock in rewilding. Cattle are in many ways ecologically dissimilar to wildlife (in particular their greater densities), but in other ways they may serve as ecological surrogates for wildlife, which could buffer ecosystems from some of the ecological costs of rewilding. More fundamentally, African savannah ecosystems represent a challenge to traditional Western definitions of “wilderness” as ecosystems free of human impacts. We support the suggestion that as we “rewild” our biodiversity landscapes, we redefine “wildness” in the 21st Century to be inclusive of (low impact, and sometimes traditional) human practices that are compatible with the sustainability of native (and re-introduced) biodiversity.

Experimental Evidence for Opposing Effects of High Deer Density on Tick-Borne Disease Prevalence and Hazard

Identifying the mechanisms driving disease risk is challenging for multi-host pathogens, such as Borrelia. burgdorferi s.l., the tick-borne bacteria causing Lyme disease. Deer are tick reproduction hosts but do not transmit B. burgdorferi s.l., resulting in potentially opposing effects on transmission. Here, we use a deer exclosure experiment to test three hypotheses for how high deer density shapes B. burgdorferi s.l. prevalence in ticks: (H1) high transmission on rodents due to higher tick densities; alternatively, (H2) low B. burgdorferi s.l. prevalence because more ticks feed on deer rather than transmission-competent rodents (dilution effect); (H3) ecological cascades, whereby lower vegetation decreases rodent abundance thus reducing transmission. Although we found support for all three mechanisms, prevalence was reduced almost 3-fold in high deer density plots compared to exclosures, suggesting that the dilution (H2) and cascade (H3) mechanisms outweighed the increased opportunities for transmission (H1). High deer density led to lower vegetation and fewer rodents, providing evidence for an ecological cascade. However, Lyme disease hazard (density of infected ticks) was increased 5-fold at high deer densities due to an 18-fold rise in tick density. This demonstrates that reproduction hosts like deer can drive up vector-borne disease hazard at high densities, despite simultaneously reducing pathogen prevalence.

Termite mound cover and abundance respond to herbivore-mediated biotic changes in an African savanna

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 remains 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 and the second a tree removal experiment– to demonstrate that patterns in termite mound abundance and cover are unexpectedly dynamic. Termite mound abundance, but not areal cover not significantly, is positively associated with experimentally controlled presence of cattle, but not wild mesoherbivores (15-1000 kg) or megaherbivores (elephants and giraffes). Herbaceous productivity and tree density, termite dietary resources that 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 is intimately tied to herbivore-driven resource availability.

Are Yellow Sticky Cards and Light Traps Effective on Tea Green Leafhoppers and Their Predators in Chinese Tea Plantations?

In Chinese tea plantations, yellow sticky cards and light traps are increasingly used to control insect pests, especially the tea green leafhopper Empoasca onukii. In this study, a 16-week open-field experiment with daily weather monitoring was designed to test the responses of tea green leafhopper, parasitoids and spiders to yellow sticky cards and three light traps with different wavelengths (covered with sticky cards). An exclosure experiment was also designed to further test the influence of the three light systems (without sticky card) on the same species. The results showed that all three light emitting diode (LED) light traps (white, green and yellow) and yellow sticky cards attracted many more E. onukii male adults than females during the course of the open field experiment, with less than 25% of trapped adults being females. Parasitoids and spiders were also attracted by these systems. Weather variables, especially rainfall, influenced the trapping efficiency. In the exclosure experiment, the population of leafhoppers in the yellow sticky card treatment did not decline significantly, but the number of spiders significantly decreased. The green and white light treatments without sticky cards showed a significant control of E. onukii and no obvious harm to spiders. These results suggest that yellow sticky cards and light traps have limited capacity to control tea green leafhoppers. However, light, especially green light, may be a promising population control measure for tea green leafhoppers, not as killing agents in the traps, but rather as a behavioral control system.

Chronic deer browsing leads to biotic homogenization of minerotrophic peatlands

Peatlands could become important foraging habitats, and their plant communities threatened, in areas with an overabundance of large herbivores. Peatland response to herbivore exclusion may vary widely according to abiotic conditions and associated species because of a strong minerotrophic gradient. We assessed the impact of white-tailed deer (Odocoileus virginianus Zimm.) on peatland vegetation using an exclosure experiment. A total of 53 pairs of exclosures and unprotected plots were set up in bogs (13 pairs), sedge fens (20), shrub fens (7), and laggs (13), and surveyed prior to exclosure construction as well as three, five, and eight years after. Vascular plant composition of exclosures diverged from that of unprotected plots through time only in shrub fens and laggs. Bryophytes remained constant in all habitats. On average, shrub cover was 30% higher in exclosures in all habitats after five years, whereas herb cover increased only in laggs, by 43%, after eight years. Reclassification of sites by pH showed deer exclusion promoted alpha diversity in low- and high-moderate rich fens (pH 5.3–6.8) and beta diversity in the latter as well as in rich fens (pH 6.3–7.5). Overall, our results suggest that conservation efforts in areas with overabundant large herbivores should target richer peatland habitats since they showed a higher resilience and fostered alpha and beta diversity.