By Colin Beale and Sue Hartley
Over the past few weeks the BES journal blogs have been taking a look at the impacts of climate change on grasslands, celebrating the recent publication of David Gibson and Jonathan Newman’s new edited book on the subject.
For many people, grasslands bring to mind wildlife documentaries showing lines of thirsty wildebeests trekking across drying grasslands and dodging hungry crocodiles in their eternal quest of green grass and water. Or maybe herds of American Bison dotting lush green prairies. Grasslands are immediately linked with the wildlife that occupies them, so in any overview of climate impacts on grasslands it is important to explore how climate change alters the ecology of the animals that both depend upon them and in turn alter their structure and function. Obviously, not all grasslands today are home to these sorts of populations of large herbivores, but herbivores are still central to the likely impacts of climate change on grasslands: even the smaller patches of grasslands in the UK host diverse insect populations, and in Africa’s great grasslands, invertebrates eat as much or even more of the grass than the mammalian ones!
In order to understand how climate change may alter the interactions between plants and animals we identified three key questions that needed to be answered: What sort of nutritional changes are likely to occur in grasslands in response to climate change? How are herbivores likely to respond to these changes? And how might these responses influence other trophic levels? Although the questions themselves are fairly simple, as usual in ecology, the answers are far more complex! For a start, what climate changes are we talking about? Climate change affects how warm it is, how much rain falls on average, when the rain falls and how frequent floods and droughts may be. Trying to separate the impacts of each type of change and how they might interact is already complicated, before we remember that the increased CO2 that is causing climate change probably has direct and interacting impacts on grasses too.
In researching our chapter of the book (Impacts of climate change on trophic interactions in grasslands), we discovered that ecologists have been pretty good at assessing how the different types of climatic change operate in isolation. For example, warming usually increases the growth rate and nutritional quality of individual grasses, but simultaneously favours grass species that are less palatable to most herbivores. By contrast, increased CO2 tends to decrease palatability of grasses by allowing plants to convert more carbon into chemical defenses against herbivores. Since these two factors seem to be working against each other, it is obvious we need to know how they might interact. We found that while there are a few studies combining temperature and CO2 treatments, most of which report interactions that vary greatly in both direction and magnitude from species to species, very few look at interactions with changes in water availability, and studies that compared nutritional changes across multiple interacting drivers are essentially absent. This makes understanding how herbivores are likely to respond to changes in the palatability of grasses a rather speculative task, but clearly there are a host of entomologists out there who are up for the challenge.
Studies involving rainfall were rather more common in the entomological literature than the plant physiological literature, with impacts of drought a particular research focus. For example, we found papers reporting that warming and increased variability in rainfall patterns are associated with outbreaks of locusts, while populations of aphids feeding on grass were generally higher in drought conditions, but again we found a very strong tendency for studies to focus on relatively simple combinations of conditions and the more complex the sets of interactions, the more complex the reported outcomes. Moving from insects to vertebrates, ecologists generally switch from neatly controlled experiments to observations of existing variation in the field, with the key focus again being on the impacts of changed rainfall. Vertebrate herbivores can generate strong feedbacks on the nutritional content of grass through the generation of grazing lawns where regularly cropped grass is constantly resprouting and consequently has less time to accumulate chemical defenses, encouraging further grazing and leading to stable, highly palatable short-grass patches. During periods of high rainfall, grass growth rates can exceed herbivore grazing rates, leading to the lawns becoming overgrown and declines in the overall palatability of the landscape. To what degree this can actually impact animal populations is currently largely left to the reader’s imagination, but studies are starting to link population dynamics to the variation in grass growth rates and nutritional quality of grasses at larger scales. So far, however, the impacts of such changes on predators and parasites are only just beginning to be assessed, so there are lots of research opportunities for ecologists who are interested in understanding climate change impacts on interactions between species.
We end by concluding that because rainfall, temperature and CO2 changes often interact to generate complex changes in palatability, which itself generates variable responses in herbivores, the one thing that we can really be sure of as the climate changes is that there will be more surprises ahead! We hope our chapter offers a useful overview of research progress in this area to date, but will also encourage ecologists to rise to the challenge of larger-scale analyses of complex interacting processes if we really want to generate a predictive ecology for the 21st Century.
Read David Gibson’s introduction to the book on the Journal of Ecology blog.