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

Abstract

1. 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.2. 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).3. 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. 4. Synthesis. 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. Given the uncertainties in rainfall due to climate change and increasing anthropogenic manipulations of global N and P cycles, our findings emphasize the need to consider multiple resources for understanding how trophic interactions might be influenced by environmental variables.

Leaf chemistry of architecturally defended plants responds more strongly to soil phosphorus variation than non‐architecturally defended ones

“…Another observation is the importance of soil P in relation to almost all the leaf chemicals measured, except leaf C. A recent study from the Serengeti also found that herbivore grazing intensity was related to soil phosphorus (P) but not nitrogen (N) (Mohanbabu & Ritchie, 2022), suggesting soil P may be critical to understanding the distribution of defence types. In our analysis, only leaf P was related (positively) to soil P, its resource supply, and we did not see this direct link for K or N. Phenolics and condensed tannins were also negatively related to soil P for the architectural defenders, but only condensed tannins were negatively related to soil P for non‐architectural defenders.…”

“…Thus, N and P are involved in carbohydrate supply and in enzymatic reactions that control carbohydrate use (see PCM above), such as its allocation to phenolics. A role for P in defence is supported by a recent study showing that herbivory pressure on herbaceous plants in the Serengeti ecosystem in East Africa was positively related to plant P (Mohanbabu & Ritchie, 2022). Potassium (K), the most abundant cation within plant cells, controls stomatal function through its role as an osmoticum regulator (Hasanuzzaman et al, 2018), and thus may restrict photosynthetic reactions, and hence carbohydrate supply for phenolics production.…”

Landscape variation in defense traits along gradients of multiple resources in a tropical savanna plant

“…These herbivores often exert intense herbivory pressure with plant losses as high as 60–90% of the annual aboveground biomass (McNaughton 1985) with likely minimal loss to insect herbivores as in other savanna ecosystems (Davies et al 2016). More recent research has also shown that variation in herbivory intensity is associated with resource gradients such that herbivory intensity is negatively associated with rainfall, positively associated with P availability to plants and unassociated with N availability to plants (Mohanbabu and Ritchie 2022) in the park. These associations are a result of resource supplies influencing herbivory intensity as the patterns hold even with nutrients from fenced plots (i.e.…”

“…These associations are a result of resource supplies influencing herbivory intensity as the patterns hold even with nutrients from fenced plots (i.e. no herbivory) indicating that nutrient recycling from herbivory, if any, were weak (Mohanbabu and Ritchie 2022). The inference is further supported by the fact that the underlying variation in soil N and P in the Serengeti has been attributed to volcanic ash deposits in the southeast to highly leached granite‐derived sands in the north (Anderson and Talbot 1965, Sinclair et al 2008).…”

“…‘+' indicate all 61 sites sampled and dots indicate sites with Solanum incanum . The associations of herbivory intensity with the different resource availabilities to plants are based on previous research from the national park (Mohanbabu and Ritchie 2022).…”