Lions constrain herbivore’s options to deal with high temperatures

One of the fun parts of field work are the nightly gatherings where scientist and park staff come together for some social activities. Working in tropical savannas this comes with additional excitement as you drive back home seeing all kinds animals that you don’t or rarely see during daytime. It’s a different world out there. An observation that stuck with me since my PhD field work is that the rate at which you bump into the largest animals, e.g. elephants, rhino’s, buffalo, is way higher at night than during daytime.

Hippo feeding at night in Hluhluwe-iMfolozi Park captured by a camera trap. Photo credit: Joris Cromsigt

Around the same time a fellow PhD student and her supervisor Joris Cromsigt, also working in Hluhluwe-iMfolozi Park (South-Africa), observed that dense vegetation plots were more frequently used by white rhino’s than open plots. The hypothesis that these megaherbivores prefer shady habitats and nocturnal activity for thermoregulatory reasons seemed plausible. It didn’t take long to expand these observations to a more general concept of a tradeoff between predation risk and thermoregulation mediated by body size, where small herbivores were expected to be active during the day and large herbivores active during the night.

Zebra’s walking in the heat at noon in Lake Manyara National Park, Tanzania. 

Joris and I went off to test this hypothesis using the camera trap surveys done in the park by Panthera over the years. It didn’t take long before we had a story ready describing this tradeoff beautifully with real data, including some iconic species (e.g. bushpig) as the exceptions proving the rule. Most small species were diurnal, most large species nocturnal and intermediate-size species showed crepuscular activity patterns. This made perfect sense as we generally scheduled our own fieldwork between dawn and mid-day to avoid being scared to death on the one hand and a heatstroke on the other. Nevertheless, our data was correlational, so not necessarily evidence of a tradeoff. Were predation and thermoregulation indeed the driving processes of herbivore activity patterns?

Male lion resting in the shade during daytime in Serengeti National Park, Tanzania.

We needed to show that these intermediate-sized herbivores would become nocturnal in the absence of predation and/or that megaherbivores would be more active during the day when in cooler areas. The latter one is tricky because these cameras are triggered by temperature differences, but the former was possible if we could compare protected areas with and without large carnivores.

So we contacted Guy Balme, our collaborator from Panthera, and asked whether they maybe had done surveys in other areas without lions which we could use as a comparison. Turned out that they had done surveys in 32 different protected areas across South-Africa of which about half had lions. Lions have been extirpated across almost all of South-Africa mid-way the past century but have been reintroduced in many areas. Panthera’s camera traps surveys, initiated to study big cats, thus allowed us to “experimentally” investigated the trade-off between predation risk and thermoregulation. True experiments with large carnivores are extremely rare or non-existent and this design is probably the closest we will get to replicated manipulations of carnivore effects.

Waterbuck, gemsbok and zebra changed their daily activity patterns most when lions were present. Photo credit: Tim Hofmeester (gemsbok and zebra)

The reversed timescapes of risk and heat have important consequences for the adaptive capacity of large herbivore to global warming. Maybe more important is that not all species respond in the same way. Their responses are most likely related to thermoregulatory traits and anti-predator strategies. Would it be possible that the predation-thermoregulation trade-off serves as an additional dimension of niche partitioning?

Our study demonstrates that species interactions change the set of possible responses to deal with a changing climate. We have focused on predation, but other interactions like parasitism, competition or positive interactions might all change the predicted responses of species to a changing environment. We are just beginning to unravel these complex interactions leading to context-dependent adaptation. I strongly believe we can learn a lot from these large-scale monitoring efforts, like the camera trap survey by Panthera, especially when undertaken across environmental gradients and/or combined with real-worlds experimental manipulations. ​

Animals move around substantial amounts of essential plant nutrients

Animals move around through the landscape and can therefore disperse materials from one place to another. Among these materials are essential nutrients for plants like nitrogen and phosphorus. For example, large herbivore ingest plant nutrients in one place by eating plant material, but generally excrete them hours or days later at a different place. Although these principles are well known, the actual balances for different areas are hard to quantify as it is practically impossible to follow all these nutrients continuously. Furthermore, herbivore species differ in what they eat and how they move around.
A way to overcome these practical problems is to quantify local nutrient budgets and upscale these finding to the ecosystem. We therefore measured the consumption of grasses and the nutrients inside these grasses for a full year to estimate the amount of nutrients extracted from the vegetation by herbivores. At the same time we counted the amount of dung that was produced by different herbivores species and converted this to nutrients using data from the literature as an estimate of nutrient return. The balance between consumption (nutrient extraction) and excretion (nutrient return) provides an estimate of whether the amount of plant available nutrients increase or decrease over time. We did this for three distinct vegetation types in an South-African savanna ecosystem: short lawn grasses (highly nutritious, preferred by grazers), long bunch grasses (much less nutritious) and woody species (preferred by browsers). From these local balances we could then estimate the nutrient redistributions between the different vegetation types.

We measured biomass consumption using movable cages, where the difference in grass biomass inside and outside the cage is attributed to grazing (left). Dung beetles were collected and identified to species by Matty Berg (right). Photos: Moniek Gommers

Intermediate-sized grazers increase nutrient availability of grazing lawns.
We found that intermediate-sized herbivores (warthog, impala, zebra) moved nutrients from bunch grasslands to grazing lawns, thereby fertilizing their own preferred grazing areas. This confirms the hypothesized positive feedback loop between grazers and grazing lawns. Browsers redistributed similar amounts of nutrient from woody patches to grasslands as grazers moved from grasslands to woody patches, balancing each other’s effect.

Intermediate-sized grazers in Hluhluwe-iMfolozi park. Warthog piglets (left), wildebeest (middle) and zebra (right). Photos: Michiel Veldhuis

White rhinoceros exports nutrient to middens
The megagrazer white rhinoceros defecates in middens (large dung heaps) as part of their territorial behavior, resulting in a continuous export of nutrients from grasslands. This effect overrides the effects of intermediate-sized grazers. White rhino densities are high in the study area and their large body size and specific behavior make their effect on the spatial distribution of nutrients extremely high, with important consequences for ecosystem functioning. A still open question is what happens to all the nutrients in the middens at longer time-scales (decades).

Megagrazer white rhinoceros (left) and a midden (right) in Hluhluwe-iMfolozi park. Photos: Michiel Veldhuis (left) and Ruth Howison (right)

Dung beetles also contribute significantly to nutrient dispersal
Once herbivores have dropped their dung pellets, dung beetles initiate a second phase of nutrient dispersal through rolling dung balls. These can be moved over significant distance (>50m). We therefore executed a small side-experiment to investigate their effect and found that dung beetles prefer to bury their dung balls in taller bunch grass vegetation, probably because the soil is more loose and wet here. This caused a net movement of nutrients from grazing lawns to bunch grasses, the opposite effect of intermediate-size herbivores. However, from our experiment it was not yet possible to obtain reliable flow estimates, so the magnitude of dung beetle nutrient dispersal on the overall nutrient budgets of African savanna ecosystems remains to be seen.

Dung beetles moving balls in Hluhluwe-iMfolozi park. Photos: Michiel Veldhuis (left), Matty Berg (right)

Animals move significant amounts of nutrients and contribute to spatial heterogeneity
This study provides estimates of the spatial redistributions of nutrient by animals in savanna ecosystems and has shown that these flows are significant. Compared to other nutrient flows, like nitrogen deposition, nitrogen fixation or nitrogen emissions through fire, the movement of nutrients by animals is about the same order of magnitude. More importantly, through there nutrient redistribution act they increase the heterogeneity of these incredible ecosystems, contributing to their well-known biodiversity.