Cover image: sweeping views over the Laikipia Plateau west to the Longonot volcanic arc. © Jonathon Roberts, 2017.
When you imagine a scientist at work, you might imagine a begoggled middle-aged man in a long white lab coat, squinting at the contents of a test tube (and usually not wearing gloves, which is a big mistake in most labs). With ecologists, the truth is usually less glamorous – we spend most of our time sat behind a computer, trying to make sense of data or figuring out where we went wrong in five hundred lines of code. But somebody has to collect all that data, so just occasionally we’re let off the leash, and left to roam free in the field. Again, not always as glamorous as advertised. Depending on where your field site is, you might have to contend with blizzards or dust storms, thin air or high humidity, total darkness, parasites, diseases, large predators, drug cartels, enraged hippos, or a host of other potential dangers. But don’t let that put you off – for those who love it, fieldwork can be the most rewarding aspect of the job, and it’s hard to argue with glorious landscapes like the one above.
In Kenya, by far the most renowned field site is Mpala Research Centre, snugly located in Laikipia County near the very centre of the country. Mpala is a cattle ranch managed for the benefit of nature, boasting important populations of elephants, Grevy’s zebra, and African wild dogs. Many savanna researchers base themselves at Mpala for the duration of their fieldwork, as Mpala sits on a strong climatic gradient, giving easy access to both semi-arid and mesic (moderately moist) environments in the north and south respectively. Every year, scientists at Mpala churn out several dozen research papers, from graduate theses and dissertations to decades-long experiments. Here I’ll be taking a look at just a few of the most recent.
Ants in Your Plants
East Africa’s savannas are ruled by acacia trees – generally medium-sized, extremely thorny trees in the pea family (Fabaceae), with characteristic flat-spreading canopies. In Mpala, one of the dominant species is Vachellia drepanolobium, the whistling-thorn acacia – so named because of the whistling of the wind through the swollen, hollow bases of its thorns. These swollen thorns, termed ‘domatia’, are often inhabited by ants of four species. Crematogaster mimosae, C. nigriceps, and Tetraponera penzigi are all phytoecious (they have to inhabit the acacia, and cannot survive elsewhere); the fourth species, C. sjostedti, nests under the bark of any rotting wood, but acacia is the most common tree on the savanna. The ants use the domatia for shelter from the elements and the harsh sun, and to store food (the acacia secretes sugary nectar from glands at the base of its leaves, which the ants harvest). In return, the tree receives protection – when any herbivore dares to browse from a colonised tree, the ants attack with bites and stings, repelling all but the most determined browsers.
Each tree is usually occupied by just one nest of a single species, so competition between the ants is fierce. Some ants are good at defending their home tree, but struggle to colonise other, occupied trees. Others are the opposite, able to rapidly colonise occupied trees and oust the resident ants, but unable to hold their conquests for long if another ant species attacks. In other ants, this trade-off between competition and colonisation can be explained by having multiple queens in a single nest, allowing the colony to grow rapidly.
Boyle et al. (2017) studied this in the acacia ants, sampling DNA to determine whether a colony was descended from a single queen. As it turns out, the most dominant species (C. sjostedti) usually only has a single queen, so colonies grow relatively slowly – but they still occupy the largest and most desirable trees, and tend to include the most trees in the range of a single colony. C. mimosae and T. penzigi usually have multiple queens in a colony: in C. mimosae these are usually sisters working together for mutual benefit, but in T. penzigi they are often unrelated. It is likely that several queens colonise a tree at the same time, but kill each other until only one remains, incorporating her rivals’ workers into her own nest – hence mature colonies only have one queen, but workers from multiple mothers.
KLEE – Keep Out!
The Kenya Long-term Exclosure Experiment (KLEE) at Mpala has been running for twenty years. Tall electrified fences enclose parcels of savanna, and are designed to keep out megaherbivores (elephants and giraffes) or all large herbivores; cattle are permitted in some of the exclosures, but all large herbivores are kept out of others. This allows scientists to study the impacts of different types of herbivores on aspects of the savanna ecosystem, and whether they add to each other or not. This provides data to make the case for why wildlife conservation is important, why megaherbivores should be allowed into grazing areas (their actions maintain nutritious grass, by knocking down trees and keeping them from covering the savanna), and how many cattle can be grazed in an area without damaging the wildlife.
It is widely accepted in the ecology community that moderate levels of herbivory can make the aboveground parts of plants more productive, improving the quality of grazing – this is often used to maintain species-rich wildflower meadows in the UK, and prairies and steppes in Asia and North America. Using data from KLEE, Charles et al. (2016) found a positive relationship between mean productivity and total ungulate density: the more hoofed mammals there are, the better grasses and other plants grow. Mid-sized wild herbivores, like impala and gazelles, make the environment more homogeneous, with grass dominating and other plants much rarer. With cattle, the opposite is true – herbaceous plants are often less palatable due to toxins in their leaves, so cattle leave them alone, and they become more common.
Parasites are a big part of zoology, and incredibly common – most are specific to a single host, so logically at least half of the species on earth are parasites. Some parasites even have parasites of their own! They are particularly important when considering the spread of zoonotic diseases, those which usually exist in animals but can be spread to humans, often through livestock coming into contact with wildlife. High-profile examples include Ebola, which can be spread through bodily fluids and tick bites; Zika fever transmitted through infected mosquitoes; and of course, the bubonic plague, spread by fleas. Serious zoonotic diseases can cause human epidemics, and even milder ones can devastate livestock herds.
Titcomb et al. (2017) used the KLEE exclosures to examine the effects of wildlife loss on the abundance of ticks and the diseases they carry, including Q fever Coxiella burnetii, and Rickettsia spp., a genus of bacteria which includes typhus, African tick bite fever, and Boutonneuse fever. Where all wildlife was excluded, ticks were 130-225% more abundant, depending on how wet the environment was. Where only megaherbivores (giraffes and elephants) were absent, ticks were 170% as common; and when all large wildlife was removed, tick abundance shot up by 360%. This seems to show that in environments with fewer native herbivores, cattle, goats and other livestock, as well as humans, are more likely to be bitten by disease-carrying ticks. This could be made even worse by increasing aridity across much of East Africa, caused partly by global climate change and partly by the water demands from expanding agricultural industries and urban settlements.
The Family That Feeds Together, Breeds Together
As mentioned in a previous blog, cooperative social breeding is more common in arid environments, like Kenya’s savannas, than it is in most other habitats. Working as a family group helps animals raise more offspring, indirectly passing on more of their genes to the next generation. A lot of cooperative breeding behaviour is determined by hormones within the animals, particularly testosterone – while important in the production of gametes, this hormone also influences social behaviour, particularly aggression. It peaks when competition for mates is fierce, and declines through the course of a parental care period.
The superb starling, Lamprotornis superbus, is a common social breeding in Mpala, often coming to dining areas to scavenge morsels of food. Each group tends to be composed of a dominant male and female, which breed; and their offspring from the most recent brood, which only help. Pikus et al. (2018) found that breeding males have significantly more testosterone in their bloodstreams than male helpers, and females, during the incubation period. However, testosterone levels decline through the incubation and chick rearing stages, at which point all individuals reach similar levels. Occasionally, larger groups form, with helpers coming from several past broods, or perhaps one brood that survived particularly well. In these situations, helpers have higher testosterone levels than in small groups, as competition to inherit the breeding slot when one of the dominant pair dies is much more intense. However, more (or less) testosterone does not make for a better helper, so has no impact on how much the nestlings are fed.
Trojan Baboons and Sneaky Seeds
Baboons are the dominant primates on semi-open savanna, adapted for life out in the open rather than clinging to trees. They rove around in large troops with strong social hierarchies, meaning that at least one individual is always on the lookout for approaching predators (particularly leopards) while the others feed. Baboons have varied diets, based on vegetable matter but also including meat, and virtually anything else edible they can get their hands on. Fleshy fruit is often eaten when it is available, including the fruits of the invasive prickly pear, Opuntia stricta. Originating in the American subtropics, this large spiny cactus has spread rapidly across much of eastern Africa, often through birds or baboons – after eating the fruit, the animals disperse the plants’ seeds far and wide in their dung, helping the cactus spread quickly. Dyck (2017) studied the effect of prickly pear removal at Mpala, between 2014 and 2016, over the course of which the cactus and its fruits became much less available. However, baboons actively sought out these fruits, in favour of much more abundant alternative foods. Consequently, they spent more time foraging and less time feeding, and also consumed fewer native fruits, heavily reducing seed dispersal in these species.
The Human Factor
The savannas of eastern Africa have been home to nomadic cattle farmers for several thousand years, and remain crucial to this economy today. Globally, 10-20% of arid and semi-arid rangelands are severely degraded, including 70% of those south of the Sahara. It’s crucial that these areas remain productive, as they make up 43% of Africa’s land surface, and support 45% of the continent’s human population.
Kimiti et al. (2017) provided an overview of recent changes to the savanna rangelands in the Laikipia and Samburu counties of central Kenya. Here, degradation through overgrazing leads to more bare ground (which, baked hard in the sun, is difficult for plants to recolonise). Perennial grasses are replaced by red-bark acacia Vachellia reficiens and prickly pear, which are difficult for cattle to eat due to their thorns and spines. Lots of time, money and energy has been dedicated to mechanically clearing V. reficiens from the areas it dominates, and reseeding them with grasses, to mixed success. Apparently more useful are bomas, mobile cattle enclosures that keep the livestock safe from predators at night, and can be moved around every few weeks or months, depending on how rapidly they fill up with dung. While reducing conflict with lions and other large mammal predators, both traditional and modern bomas concentrate nutrients from manure, creating vegetation patches in areas where bare ground is becoming a problem.
Why is ecological research important? Simply, we don’t know what we don’t know, and the more information we have at our disposal, the more confident we can be about making the right decisions to manage species and ecosystems. In a part of the world coming under increasing pressure from a fast-growing human population, knowing the impacts we have is crucial. And having seen first-hand the splendour of East Africa’s dry grasslands, the elegance and power of its large animals (and the dazzling beauty of its small ones), and the breathtaking majesty of those wide-open skies – that’s not a treasure we should let go lightly.
Well, that wraps up my thoughts from my visit to Kenya, almost a year ago at time of writing. With this series drawing to a close, I’ll be coming back soon with something brand-new – although it might take me a while to decide which idea to run with. Until then,
Boyle, J. H., Martin, D. J., Pelaez, J., Musili, P. M., Kibet, S., Ndung’u, S. K., Kenfack, D. & Pierce, N. E. (2017) Polygyny does not explain the superior competitive ability of dominant ant associates in the African ant-plant, Acacia (Vachellia) drepanolobium. Ecology and Evolution 8(3), pp. 1441-1450. https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.3752.
Charles, G. K., Porensky, L. M., Riginos, C., Veblen, K. E. & Young, T. P. (2016) Herbivore effects on productivity vary by guild: cattle increase mean productivity while wildlife reduce variability. Ecological Applications 27(1), pp. 143-155. https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/eap.1422.
Dyck. M. A. (2017) Restoration of native baboon-plant mutualisms following biocontrol of the invasive prickly pear cactus (Opuntia stricta) in Kenya. Honors Theses AY 16/17, 44. http://repository.uwyo.edu/honors_theses_16-17/44/.
Kimiti, D. W., Hodge, A. C., Herrick, J. E., Beh, A. W. & Abbott, L. E. (2017) Rehabilitation of community-owned, mixed-use rangelands: lessons from the Ewaso ecosystem in Kenya. Plant Ecology 218(1), pp.23-37. https://link.springer.com/article/10.1007/s11258-016-0691-9.
Pikus, A. E., Guindre-Parker, S. & Rubenstein, D. R. (2018) Testosterone, social status and parental care in a cooperatively breeding bird. Hormones and Behavior 97, pp. 85-93. https://www.sciencedirect.com/science/article/pii/S0018506X17302428.
Titcomb, G., Allan, B. F., Ainsworth, T., Henson, L., Hedlund, T., Pringle, R. M., Palmer, T. M., Njoroge, L., Campana, M. G., Fleischer, R. C., Mantas, J. N. & Young, H. S. (2017) Interacting effects of wildlife loss and climate on ticks and tick-borne disease. Proceedings of the Royal Society B 284(1862), 20170475. http://rspb.royalsocietypublishing.org/content/284/1862/20170475.