Climate change and caterpillars
In this, the International Year of Forests (IYF), designated by the United Nations, Earthwatch scientists are finding evidence that climate change is having a significant impact on our forests by altering interactions between species and shifting ecological processes.
We still know little about the effect that increased carbon dioxide (CO2) levels and associated warmer temperatures are having on the planet's biodiversity and ecosystem functioning. The flora and fauna of tropical and temperate rainforests construct a particularly delicate ecosystem of complex food webs that are likely to be some of the first to feel the effects of climate change. Current impact predictions are based largely on studies of individual species, but Earthwatch scientists and volunteers are making their mark as some of the first people to document how climate affects the interactions that exist between different animal and plant species.
A forest canopy walkway, La Selva, Costa Rica.
New species and climate change discoveries
In 2010 six teams of Earthwatch volunteers joined Dr. Lee Dyer and his colleagues in Costa Rica, Ecuador, and the US states of Louisiana, Arizona, Nevada, and California. The teams collected valuable field and laboratory data to contribute towards a long term study of the relationships between climate and biodiversity - in particular certain plants, caterpillars, and the parasitic wasps (parasitoids) that attack them. In doing so they have not only discovered species previously unknown to science, but identified that future changes in climate will disrupt interactions between these species with potentially far reaching ecosystem impacts.
Important ecological interactions affected by climate
Data collected in laboratory experiments was combined with 2010 field data and used to study the interaction between caterpillars and parasitoid wasps under increased levels of CO2 and temperature. Results indicated that changes in these climatic variables, and the extreme weather events that would result, could have a significant disruptive effect by upsetting the balance between caterpillars and parasitic wasps - the naturally occurring biological controls that keep caterpillar populations in check.
The investigation showed that changes in climate caused parasitoid wasps to fail to parasitise caterpillars and die, leading to a sharp rise in caterpillar numbers and overall decrease in species diversity. Experiments revealed that the rate of caterpillar development (growth) increased under higher temperatures. This caused caterpillars to develop before their natural parasitoid predators were mature, effectively de-linking their life cycles. This change led to decreases in the level of parasitism as without hosts, the parasites could not survive, eventually leading to total parasite mortality. In essence this sounds like it could be a good thing, but in practice it caused an outbreak of caterpillars which had negative knock-on effects, causing a decrease in overall species diversity as a factor of not only parasite decline, but because in the absence of parasites, caterpillars out-competed other species and decimated plants.
Elevated levels of CO2 and temperature were additionally found to decrease plant quality. Results showed that under higher temperatures the levels of defensive chemicals used by plants to deter herbivores were increased. This subsequently took energy away from growth and decreased plant biomass (organism mass at a given time) by up to 50%.
Climate change events such as those simulated by this experiment are predicted to occur more regularly in the future. If warming causes a breakdown of host-parasite relationships and decline in plant biomass as indicated by these results, in the future this could have large scale implications for ecosystem function and service provision, by reducing the beneficial resources they provide such as carbon and nutrient cycling.
Impacts on species diversity
Experiments from 2010 highlighted the importance of interactions between herbivores and plants as a mechanism which maintains high levels of species diversity in the flora of the understory of tropical forests. Experimenting with removal of caterpillars in forest patches over 15 months resulted in more than 40% reduction in the number of individual seedlings, more than 40 per cent fewer species and 40 per cent greater seedling evenness, on average in those patches, compared to control plots where the herbivore (i.e. caterpillar) populations were left intact.
Investigations using damage keys created by Earthwatch volunteers to assess the level of damage to plants showed that certain plant species are the source of caterpillars that cause the widespread seedling mortality of understory plants. This subsequently allows plant species that would otherwise be out-competed to colonise the area, as dominant species are prevented from monopolising the understory. These findings highlight the important role that caterpillars play in maintaining species diversity within rainforest ecosystems when populations remain naturally controlled by parasitic wasps.
A saturniid caterpillar.
Feeding habits and diversity
Data indicating a relationship between elevation (height above sea level) and herbivore diversity collected by 2010 Earthwatch teams revealed that currently reported levels of herbivore diversity may be a significant underestimate. Experts have previously based diversity estimates on data from lowland areas, yet Earthwatch results indicate that species diversity increases at higher elevations. Data collected by volunteers has shown that the feeding specialisations of caterpillars living at high elevations have given rise to greater herbivore diversity. This is because co-existence of a higher number of species is possible when there are a high number of specialist feeders, each utilising their own ecological niche and not competing with each other for food. As a result total diversity estimates based on lowland diversity are likely to be too low, missing out many species.
The work carried out by Earthwatch scientists and volunteers in 2010 led not only to the advancement of scientific knowledge, but to the discovery of new species and previously unknown host-parasite interactions. Research teams have to date documented a number of new caterpillar and parasitoid species at their field sites. Last year's research in California, however, unveiled a new three-way interaction taking place between a plant, new species of caterpillar and previously unknown parasitic wasp.
A newly discovered caterpillar species in California (undescribed species of Noctuidae) eats the plant Amelanchier alnifolia and is attacked by a new species of parasitic wasp (undescribed species of Eulophidae). These were all previously unknown associations. The caterpillar pictured here is being consumed by larvae of the parasitoid wasp.
Impact of the project, and the future
The results of this project, in line with the aim of IYF to promote sustainable forest management and conservation through improved knowledge, have contributed to important theoretical advances in ecology, and provided insight into the effect of climate change on forest ecosystem functioning. The 2010 field season additionally saw the publication of a number of scientific papers based on the results collected by Earthwatch scientists with the help of volunteers.
Dr. Lee Dyer and his team with their on-going work, aim to encourage other ecologists to adopt the approach of studying multiple species to improve understanding of ecological interactions at different trophic levels (an organism's position in the food web), and how these change under different climate scenarios. Exploring these relationships is vital to accurately guide future management decisions and develop a successful approach to conservation strategy in the light of global change.
Find out more about our Climate Change and Caterpillars expeditions, and other Earthwatch climate change expeditions you can join.
Report by Danielle Parks.