Geochemical landscapes as drivers of wildlife reproductive success: Insights from a high-Arctic ecosystem

The bioavailability of essential and non-essential elements in vegetation is expected to influence the performance of free-ranging terrestrial herbivores. However, attempts to relate the use of geochemical landscapes by animal populations directly to reproductive output are currently lacking. Here we measured concentrations of 14 essential and non-essential elements in soil and vegetation samples collected in the Zackenberg valley, northeast Greenland, and linked these to environmental conditions to spatially predict and map geochemical landscapes. We then used long-term (1996-2021) survey data of muskoxen (Ovibos moschatus) to quantify annual variation in the relative use of essential and non-essential elements in vegetated sites and their relationship to calf recruitment the following year. Results showed that the relative use of the geochemical landscape by muskoxen varied substantially between years and differed among elements. Selection for vegetated sites with higher levels of the essential elements N, Cu, Se, and Mo was positively linked to annual calf recruitment. In contrast, selection for vegetated sites with higher concentrations of the non-essential elements As and Pb was negatively correlated to annual calf recruitment. Based on the concentrations measured in our study, we found no apparent associations between annual calf recruitment and levels of C, Mn, Co, Zn, Cd, Ba, Hg, and C:N ratio in the vegetation. We conclude that the spatial distribution and access to essential and non-essential elements are important drivers of reproductive output in muskoxen, which may also apply to other wildlife populations. The value of geochemical landscapes to assess habitat-performance relationships is likely to increase under future environmental change.

Effect of fire on the palatability of plants in an African woodland savanna: varying impacts depending on plant functional groups

Fire and herbivores are two important drivers of changes in vegetation composition, quality and dynamics and both are highly related to each other. Herbivores are known to respond to fire both in terms of foraging decisions and distribution. However, little is known about the actual changes in plant chemistry following a fire event and how long these changes will last. We investigated the effect of fire on two different plant functional groups (grasses and woody species) in a woodland savanna of southern Africa. We studied chemical compounds known to be important for palatability of five perennial grass and seven woody species (trees and shrubs) common in the woodland savanna and known to be utilized by herbivores. We wanted to know if plant chemistry differs between a recently burned site (burned 2 years ago) and a control site, burned 16 years ago, and if grasses and woody species show similar relative differences between sites (i.e., the plants' response to fire). We found a clear difference in chemical composition patterns between the plant functional groups, with an almost homogenous response to fire among woody species, but higher variability in response among grass species. Furthermore, we found that woody species maintained a higher nutritional value even 2 years after burning, whereas grasses did not show clear differences among the two investigated sites. Hence, few years after burning, woody plants might still serve as an attraction for herbivores, especially browsers, in contrast to grasses. The knowledge about these differences between the two functional groups in response to fire is beneficial for the development of management strategies for large herbivores whether domestic or wild.

Integrating plant stoichiometry and feeding experiments: state-dependent forage choice and its implications on body mass

Intraspecific feeding choices comprise a large portion of herbivore foraging decisions. Plant resource quality is heterogeneously distributed, affected by nutrient availability and growing conditions. Herbivores navigate landscapes, foraging not only according to food qualities, but also energetic and nutritional demands. We test three non-exclusive foraging hypotheses using the snowshoe hare (Lepus americanus): (1) herbivore feeding choices and body conditions respond to intraspecific plant quality variation; (2) high energetic demands mitigate feeding responses; and (3) feeding responses are inflated when nutritional demands are high. We measured black spruce (Picea mariana) nitrogen, phosphorus and terpene compositions, as indicators of quality, within a snowshoe hare trapping grid and found plant growing conditions to explain spruce quality variation (R² < 0.36). We then offered two qualities of spruce (H1) from the trapping grid to hares in cafeteria-style experiments and measured their feeding and body condition responses (n = 75). We proxied energetic demands (H2) with ambient temperature and coat insulation (% white coat) and nutritional demands (H3) with the spruce quality (nitrogen and phosphorus content) in home ranges. Hares with the strongest preference for high-quality spruce lost on average 2.2% less weight than hares who ate the least high-quality spruce relative to low-quality spruce. The results supported our energetic predictions as follows: hares in colder temperatures and with less-insulative coats (lower % white) consumed more spruce and were less selective towards high-quality spruce. Collectively, we found variation in plant growing conditions within herbivore home ranges substantial enough to affect herbivore body conditions, but energetic stats mediate plant-herbivore interactions.

Bottom-up regulation of a tritrophic system by Beet yellows virus infection: consequences for aphid-parasitoid foraging behaviour and development

Effects of plants on herbivores can cascade up the food web and modulate the abundance of higher trophic levels. In agro-ecosystems, plant viruses can affect the interactions between crops, crop pests, and natural enemies. Little is known, however, about the effects of viruses on higher trophic levels, including parasitoids and their ability for pest regulation. We tested the hypothesis that a plant virus affects parasitoid foraging behaviour through cascading effects on higher trophic levels. We predicted that the semi-persistent Beet yellows virus (BYV) would influence plant (Beta vulgaris) quality, as well as aphid host (Aphis fabae) quality for a parasitoid Lysiphlebus fabarum. We determined amino acid and sugar content in healthy and infected plants (first trophic level), lipid content and body size of aphids (second trophic level) fed on both plants, as well as foraging behaviour and body size of parasitoids (third trophic level) that developed on aphids fed on both plants. Our results showed that virus infection increased sugars and decreased total amino acid content in B. vulgaris. We further observed an increase in aphid size without modification in host aphid quality (i.e., lipid content), and a slight effect on parasitoid behaviour through an increased number of antennal contacts with host aphids. Although the BYV virus clearly affected the first two trophic levels, it did not affect development or emergence of parasitoids. As the parasitoid L. fabarum does not seem to be affected by the virus, we discuss the possibility of using it for the development of targeted biological control against aphids.

Effectiveness of fencing and hunting to control Lama guanicoe browsing damage: Implications for Nothofagus pumilio regeneration in harvested forests

Browsing damage by native ungulates is often to be considered one of the reasons of regeneration failure in Nothofagus pumilio silvicultural systems. Fencing and hunting in forests at regeneration phase have been proposed to mitigate browsing effects. This study aims to determine effectiveness of these control methods in harvested forests, evaluating browsing damage over regeneration, as well as climate-related constraints (freezing or desiccation). Forest structure and regeneration plots were established in two exclosures against native ungulates (Lama guanicoe) by wire fences in the Chilean portion of Tierra del Fuego island, where tree regeneration density, growth, abiotic damage and quality (multi-stems and base/stem deformation) were assessed. Exclosures did not influence regeneration density (at the initial stage with < 1.3 m high, and at the advanced stage with >1.3 m high). However, sapling height at 10-years old was significantly lower outside (40-50 cm high) than inside exclosures (80-100 cm), and also increased their annual height growth, probably as a hunting effect. Likewise, quality was better inside exclosures. Alongside browsing, abiotic conditions negatively influenced sapling quality in the regeneration phase (20%-28% of all seedlings), but greatly to taller plants (as those from inside exclosure). This highlights the importance of considering climatic factors when analysing browsing effects. For best results, control of guanaco in recently harvested areas by fencing should be applied in combination with a reduction of guanaco density through continuous hunting. The benefits of mitigation actions (fencing and hunting) on regeneration growth may shorten the regeneration phase period in shelterwood cutting forests (30-50% less time), but incremental costs must be analysed in the framework of management planning by means of long-term studies.

The effect of wilting on palatability of plants to Schistocerca gregaria, the desert locust

Previous work has shown that host choice by acridids (grasshoppers and locusts) is sensitive to alterations in host quality. In particular, reduced plant water content has been found to increase palatability of certain plant species. To determine if this phenomenon is general, and to gain preliminary information on causes, turgid and wilted plant material of forty-one species was tested using nymphs of the desert locust, Schistocerca gregaria. Twelve plant species (29%) had increased and five (12%) had decreased palatability (as measured by meal size) when wilted. Among fifteen families tested, the increases occurred in six, the decreases in three. The greatest change occurred in Taraxacum officinale; further tests on this plant revealed the increase to be continuous, rising with decreasing water content. The behavioral observations combined with the pattern of the results across plant species suggest that changes are due to alterations in specific deterrents or stimulants, rather than to decreased water content or increased concentrations of amino acids and/or sugars. The implications of these results for understanding drought-associated population outbreaks are discussed.