Thursday, 1 March 2012

The role of termites in the savanna biome

The ground is crawling with termites! Nr. Tarangire, Now 2011.
Termites are hugely important to the of the savanna biome. We've covered some of their roles here before when we talked about termite mounds and when we covered nutrients and nitrogen in the savanna biome. The numbers of termites in savanna habitats can be quite extraordinary: with over 400/m2 of soil, their biomass can exceed that of mammals in the ecosystem. Such a huge abundance of animals mean that termites, by weight of numbers alone, must have a massive impact on the ecosystem. We've seen how they are crucial for keeping nutrients cycling rapidly in the savanna, how their excavations can change the texture of the soil and how these impacts change the plants and, ultimately, the behaviour of animals within the savanna. Despite this obvious importance, however, there's surprisingly little research on what they actually get up to and where they really are - I guess researchers are generally too busy tracking lions sleeping under a bush than worrying about termites under their feet... It's important though, as processes that cause spatial variation in patterns of nutrients and such-like are increasingly being perceived as vital to the ecosystem as a whole, and if we don't understand the processes that cause variation, it will be much harder to understand what's going on at larger levels. Still, some work is coming out now, and a paper last year caught my eye.

Dead wood too is covered with the things!
This paper (hidden here, I'm afraid) is by Buitenwerf and colleagues and reports on an experiment they did to measure termite activity across patches of Hluhluwe–iMfolozi Park, South Africa. In some ways it's a direct follow up to a similar study carried out here in Serengeti and in fact Han Olff is an author on both studies. The idea is simple. It's possible to count termite mounds and get some ideas of the distribution of termite that way, but (a) most termite species don't actually make termite mounds and (b) even if you know where the mounds are, you don't necessarily know exactly where the termites are. So as well as doing that the authors of both studies set out a network of termite baits. In both places these consisted of cut, dried grass tucked up in a mesh bag that was big enough to let termite in and not much more. In Serengeti they also filled some bags with wildebeest dung. Each bag they then took to the field and nailed to the ground, around which they recorded a few things that might affect termite activity (like the amount of grass and bushy cover present). They left the bags out in the field for a month and then went back and brought them all in again. Since they knew how much grass or dung they put in the mesh bags at the start, all they had to do to get an idea of termite activity at each spot was to sift the soil out (termite often forage in soil tunnels they create) and weigh the remaining matter. Pretty simple set of experiments.
Females emerge with the rains and are food for everything!

Termite study area in Serengeti and shaded map of termite activity (B)
- dark is more active. From Freyman et al 2010

In the Serengeti they did the experiment twice - once in the dry and once in the wet season, in Kruger the experiment only happened once (and we're not told when). Whilst the Serengeti experiment is clearly a much better job than the South African one, we actually know that termite activity is far from simple to predict and changes dramatically month to month so I think there should be a serious health warning on both these studies. Never-the-less, there are some interesting results - in Serengeti (the first study) they discovered that during the dry season activity was much higher on the top of the ridges (where the soils are sandier, and you'll remember there are often more termite mounds) than on the bottom (where the soils are deeper, but more full of clay). In fact, there was almost no activity at the bottom of the ridges at all during the dry season. That makes a certain amount of sense - especially as the study are was the long grass plains south and south-east of Seronera where the Thomson's Gazelle's hang out in the dry season, and they prefer to be on the ridges with a good view of cheetahs: if you like nicely packaged dung to eat, with a bit more moisture than simply dry grass, you're better off concentrating on the ridges during the dry season.  During the wet season the gazelles aren't in the long grass plains and the story is rather different, with activity both top and bottom of the ridges, but - it seems - a lot more variation within these areas.

Major differences in activity in relation to rainfall and large mammal presence.
Note each dot only represents two plots(!). From Buitenwerf et al 2011
With this as background the second experiment in South Africa set out to study that fine-scale variation in much greater detail, whilst also aiming to study directly the impacts of grazing animals and rainfall gradients on termite activity. Unfortunately, the experiment seems much less fully thought through than the Serengeti one - using only four pairs of plots (in each pair one was in an area fenced to exclude large mammals, and one beside the area where mammals are present) across the landscape to investigate two main variables: rainfall and mammal activity. Within each plot they had 100 bags of grass, but that still only leaves eight plots to explore the larger-scale differences that they were most interested in. Not a good survey design at all (not to mention the unknown season, when they already knew season makes a huge difference)... Still, the results are worth noting as a start, if nothing else, especially as there might be something interesting here. Unexpectedly, the total activity is higher in the lower rainfall areas than in the higher rainfall areas (though the difference is only from 600 to 850mm, so nothing like the gradients available in TZ parks). We'd probably have expected the opposite, as high rainfall areas have more grass that can provide more food to termites. So there's the first surprise. Secondly, the effect of mammals seemed to differ depending on whether the plot was in the wetter or drier area. In the wetter areas (where activity was generally low anyway), activity was lower in the grazed areas than in the grazing exclosure. In the drier area it was the other way around. Now, bearing in mind the (huge!) caveat that these samples sizes are so small as to be nearly useless, and the fact that activity can vary seasonally, but that the degree of seasonal variation might be related to overall aridity (less seasonal variation in drier areas), let's think about what this might mean if, indeed, it is real. The authors suggest that in wetter areas it might be a direct competition effect: grazers eat the grass, so there's not much for the termites. That might be true, but if so, why isn't it also true in the drier areas? They suggest that in the drier areas the activities of herbivores actually provide extra food for the termites - through dung and by dropping bits of grass, but have no ideas why it might be other way around in the wet areas.

Matabele ants on a raid - everything likes a termite... Mara River, Sep 2011.
My suspicion is that this is probably an artifact of a too-small experiment, perhaps being carried out at an inappropriate (or at least not complete) time of the year. But if not, it's something to puzzle about next time you come across a bunch of termites busy foraging... And if you don't fancy that, at least remember how hugely important termites are in the savanna - their activity certainly is patchy, which must be generating a lot of variation in grazing quality at both fine and large scales. Certainly worth some more study! [And that's before we remember just how many things eat them! There are specialist birds, mammals, invertebrates and all the rest - it's tough being a termite!]

Main references:
ResearchBlogging.orgBuitenwerf, R., Stevens, N., Gosling, C., Anderson, T., & Olff, H. (2011). Interactions between large herbivores and litter removal by termites across a rainfall gradient in a South African savanna Journal of Tropical Ecology, 27 (04), 375-382 DOI: 10.1017/S0266467411000125

Freymann, B., de Visser, S., & Olff, H. (2010). Spatial and temporal hotspots of termite-driven decomposition in the Serengeti Ecography DOI: 10.1111/j.1600-0587.2009.05960.x

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