Changes In Herbivore Densities Research Articles

Comparison of Sporormiella dung fungal spores and oribatid mites as indicators of large herbivore presence: evidence from the Cuzco region of Peru

The ability of sedimentary proxies (especially dung fungal spores) to reflect the past presence and density of large herbivores on a landscape has been receiving increasing scrutiny. Here we examine the Sporormiella spore record from a well-dated, multi-proxy, highly organic sedimentary record from the small lake basin of Marcacocha in the Cuzco region of Peru. The basin, a wetland since ca. AD 1840, existed as a small lake for at least 4000 years prior. Previous work at Marcacocha has shown that changes in herbivore densities reflect the abundances of oribatid mites preserved in the lake sediments. This study tests the assumption that both the Sporormiella and the oribatid mite records responded in broadly the same way to changes in herbivore densities over the past 1200 years.Analysis reveals a very low covariance between concentrations of Sporormiella spores and the two major components of the oribatid mite assemblage: a single aquatic species of Hydrozetes and a less abundant, wider grouping comprising members of the mostly terrestrial superfamily Ceratozetoidea (referred to as the ‘ceratozetoids’). Variations in the Hydrozetes assemblage clearly dovetail with known historical events, including the massive decline of indigenous and camelid populations following the collapse of the Inca Empire in the mid-sixteenth century. Comparison with other environmental proxies from the Marcacocha sequence, including diatoms and charophyte oospores, suggests that the ceratozetoids appear to reflect increasing terrestrialization. In contrast, although the Sporormiella record indicates the presence of livestock, it fails to register the major historical changes in herbivore presence, though reflects well episodes of lake-level lowering. In small lake settings, therefore, the use of Sporormiella to estimate the density of herbivores in the catchment might be considered a relatively blunt instrument when compared with some other indicators.

Nutrient and herbivore alterations cause uncoupled changes in producer diversity, biomass and ecosystem function, but not in overall multifunctionality

Altered nutrient cycles and consumer populations are among the top anthropogenic influences on ecosystems. However, studies on the simultaneous impacts of human-driven environmental alterations on ecosystem functions, and the overall change in system multifunctionality are scarce. We used estuarine tidal flats to study the effects of changes in herbivore density and nutrient availability on benthic microalgae (diversity, abundance and biomass) and ecosystem functions (N2-fixation, denitrification, extracellular polymeric substances -EPS- as a proxy for sediment cohesiveness, sediment water content as a proxy of water retention capacity and sediment organic matter). We found consistent strong impacts of modified herbivory and weak effects of increased nutrient availability on the abundance, biomass and diversity of benthic microalgae. However, the effects on specific ecosystem functions were disparate. Some functions were independently affected by nutrient addition (N2-fixation), modified herbivory (sediment organic matter and water content), or their interaction (denitrification), while others were not affected (EPS). Overall system multifunction remained invariant despite changes in specific functions. This study reveals that anthropogenic pressures can induce decoupled effects between community structure and specific ecosystem functions. Our results highlight the need to address several ecosystem functions simultaneously for better ecosystem characterization and management.

The importance of herbivore density and management as determinants of the distribution of rare plant species

Herbivores are often drivers of ecosystem states and dynamics and in many situations are managed either as livestock or through controlled or exploitative hunting of wild populations. Changes in herbivore density can affect the composition of plant communities. Management of herbivore densities could therefore be regulated to benefit plant species of conservation concern. In this study we use a unique spatial dataset of large herbivores in Norway to test whether herbivore density affects the distribution of rare red-listed plant species in tundra ecosystems, and to identify regions where herbivore density is the most important factor in determining the habitat suitability for the plant species. For all selected species a climatic variable was the most important determinant of the distribution, but herbivore density was an important determinant of some species notably Primula scandinavica. Herbivore density was the most important factor determining habitat suitability for this species in 13% of mainland Norway. Regions of Norway where the management of herbivore densities is most strongly linked to the habitat suitability of red-listed plant species are mapped. However, there was very low concordance in the localities of these areas; at any individual locality, habitat suitability was limited by herbivore density for only a small subset of the species. This suggests that management of herbivores for the benefit of rare plant species needs to be tailored for individual locations or species.

Herbivore density mediates the indirect effect of herbivores on plants via induced resistance and apparent competition

AbstractHerbivore‐induced plant resistance and apparent competition are two indirect ways herbivores interact. If a less damaging herbivore indirectly suppresses the abundance of a more damaging herbivore via these mechanisms, then plants may ultimately benefit. Changes in herbivore density, however, can dictate the intensity of species interactions and may play a critical role in determining the outcome of plant‐ and predator‐mediated herbivore interactions. We tested the effects of herbivore density on the strength of indirect interactions among phloem‐feeding aphids and herbivorous caterpillars and the outcome of these interactions for their shared host plant, cotton (Gossypium hirsutum). We quantified the survival of caterpillars on host plants that were infested with varying densities of aphids in the presence and absence of predators (ladybeetles). We found that aphids induced defensive proteins in cotton plants and that caterpillar survival was negatively affected by induced resistance. Likewise, we found that the presence of aphids increased predation of caterpillars by ladybeetles, but that apparent competition between aphids and caterpillars was density dependent. Ladybeetles consumed relatively high numbers of small caterpillars at low to intermediate aphid densities, but essentially became aphid specialists at high aphid densities. Aphid induced defenses and apparent competition combined such that plant damage by caterpillars was lowest when predators were present at low aphid density (induced resistance + highest level of apparent competition). This suggests that herbivores can benefit plants, but the effect on host plants is mediated by herbivore density. Indirect herbivore‐plant mutualisms may increase plant quality, plant fitness, and yield of crop plants and these interactions need to be considered in ecologically based pest management plans. In addition, these interactions likely alter arthropod community structure and natural selection on anti‐herbivore defense traits in plants in natural systems.

Volatile-mediated foraging behaviour of three parasitoid species under conditions of dual insect herbivore attack

Infochemicals play an important role in structuring intra- and interspecific interactions. Many parasitoid wasp species rely on herbivory or oviposition-induced plant volatiles (HIPVs/OIPVs) to locate their herbivorous hosts, and must cope with variation in the volatile blends due to factors such as plant/host species, herbivore density or attack by several herbivores. However, little is known about how dual herbivory or changes in herbivore density affect multiple parasitoid species, each attacking a different herbivore, in the same system. In a natural system, we investigated the effect of dual attack on the ability of three parasitoid species to differentiate between volatiles induced by hosts and those induced by a combination of hosts and nonhosts. Black mustard, Brassica nigra, plants were infested with eggs or caterpillars of Pieris brassicae, alone or in combination with different densities of Brevicoryne brassicae aphids. We determined the ability of three different parasitoid species that parasitize either P. brassicae eggs (Trichogramma brassicae), caterpillars (Cotesia glomerata) or B. brassicae aphids (Diaeretiella rapae) to discriminate between the induced volatiles, and analysed the plant volatile blends. Dual infestation did not affect the parasitoid species equally and aphid infestation altered, in a density-dependent manner, the volatile-mediated foraging of all three parasitoid species. Chemical analyses of the volatile blends revealed nonlinear emission patterns in relation to aphid density in both plants attacked by aphids alone and in plants attacked by a combination of aphids and caterpillars. Simple correlations between behaviour and volatile emissions in pairwise comparisons suggest the importance of certain volatiles explaining attraction, whereas dose-response type analyses reveal that these simple correlation analyses provide an incomplete picture.

Herbivore diversity improves benthic community resilience to ocean acidification

Ocean acidification is expected to alter a wide range of marine systems, but there is great uncertainty about the outcome because indirect effects are often crucial in ecology. Work at volcanic seeps has shown that major ecological shifts occur due to chronic exposure to acidified seawater. Changes in herbivore densities are often seen and this may interact with direct CO2 effects to determine benthic community structure. Here, an exclusion experiment was used to test effects of herbivory in benthic communities along a pCO2 gradient off Methana (Greece). A manipulative experiment was used to examine how large herbivores affected sublittoral algal communities as seawater carbon dioxide levels increased. Sea urchins and herbivorous fish dramatically reduced macroalgal biomass at background carbon dioxide levels; this effect was not hampered by increased pCO2 despite lower sea urchin densities near the seeps, since herbivorous fish abundances increased concurrently. We found that carbon dioxide levels up to about 2000μatm are unlikely to reduce the role of herbivory in structuring benthic communities if tolerant species are able to replace those that are vulnerable. A shift from sea urchins to fish as main grazers highlights that ocean acidification may cause unexpected responses at the community level, and that maintaining high functional redundancy in marine ecosystems is key to improving their resilience.

Linking sheep density and grazing frequency to persistence of herb species in an alpine environment

AbstractLarge herbivores are important drivers in ecosystems worldwide. Changes in herbivore densities are predicted to especially affect herbs that are strongly preferred by herbivores. The persistence of herbs could be challenged by enhanced grazing, but also grazing cessation may affect persistence, especially for prostrate herbs, which might be out‐competed. To test how different herbivore densities (high, low, and no sheep) affect grazing frequency and plant responses (plant height, flowering frequency, and plant density) at the herb community and species level, we conducted a fully replicated, landscape‐scale experiment in an alpine environment. We found that none of the herb species changed their densities after 5 years with experimental changes in grazing pressure. Sheep density affected grazing and flowering frequency at the herb community level. Eight herb species were more grazed at high sheep density as compared to enclosures with no sheep. Herb height decreased at high sheep density as compared to no sheep for the two species with the highest grazing frequency. Increased height and flowering frequency were found for small herbs at high sheep density. Our experiment clearly shows that herbs do not constitute a homogeneous functional group and that, in particular, tall and small herbs are affected in contrasting ways but all species (n = 15) tolerated changes in grazing regimes as densities were maintained at both enhanced grazing and grazing cessation.