We use landscape data to understand spatial patterns of diversity of one of the smallest groups of insects: thrips. The study was performed in Reunion island, a small volcanic island of the Indian Ocean. The dramatic changes in elevation, abiotic conditions and anthropisation allowed us to explore multiple variables affecting diversity revealing why, despite more than a century of research, the elevational diversity gradient is far from being fully understood.
How diversity is distributed on our planet is one of the patterns that has been most intensely studied by ecologists. One such pattern is the change in diversity along elevational gradients, which was already noticed by Alexander von Humboldt in the 18th century. Since then many studies have tried to find general rules behind this gradient, but the more the pattern is studied the more complex it seems to be. The reasons are many and include that the elevational diversity gradient depends on both abiotic conditions and biotic interactions, it may change with latitude, taxa or climatic regions, and the relationship is not always linear with diversity being often larger at mid-elevations. In addition, human impacts on ecosystems also co-vary with elevation with concomitant effects on diversity. Early researchers were able to observe this pattern in pristine ecosystems, which are nowadays rare. Human activities are currently the strongest drivers of diversity erosion, and including these activities on the study of diversity gradients can help to better understand spatial patterns of diversity.
(Above) Lentil crops embedded within the National Park in the Cirque de Cilaos, Reunion island.
Our study aimed to explore whether diversity changes along elevation could be better understood if human impacts on landscape structure were taken into account. We selected three important variables that can be obtained from highly-resolved vector layers: the amount of habitat available, habitat diversity, and fragmentation. The study was performed in Reunion, a small island in the Indian Ocean recognized as a diversity hotspot. This volcanic island offered a unique opportunity to test our hypotheses given its dramatic changes in elevation as well as at the landscape level. Low elevation areas are dominated by human settlements, mid elevation ones are mostly devoted to agriculture (and sugarcane in particular), and higher elevation areas are protected by a National Park. The study was done using as model system a group of minute insects known as thrips (Thysanoptera), and was part of Niry Dianzinga’s PhD, a project that required intense sampling and identification of thrips, with the new species Thrips reunionensis being discovered.
FROM THE COVER:
Dianzinga, N.T., M.-L. Moutoussamy, J. Sadeyen, L.H.R. Ravaomanarivo, & E. Frago (2020) The interacting effect of habitat amount, habitat diversity and fragmentation on insect diversity along elevational gradients. J. Biogeogr. 47 (11): 2377-2391. https://doi.org/10.1111/jbi.13959
The model system used in our study imposed an important challenge because, as far as we are aware, we were among the first to use this group of insects in a diversity study. It was rewarding to find out that a large part of the diversity of this group (a total of 40 species) could be obtained by sampling insects from flowers using the beating sheet technique, a simple, cheap, and quick method to sample insects. Despite many previous studies suggesting that these insects mostly disperse passively with wind currents, we found that they establish tight interactions with flowering plants, which may allow us to use this insect group to study insect-plant interaction networks in the future. This model is also interesting given its large functional diversity: most species are herbivorous, but several species feed on fungi, and some species are predators, like the species Franklinothrips vespiformis depicted on the cover of issue 47(11) of the Journal of Biogeography.
Savannah ecosystem in La Savane, Saint-Paul, Reunion island.
Our results are a good example of the challenges to understand the elevational diversity gradient because variables affecting thrips diversity were multiple, and often interacted in an intricate manner. During the rainy season, rainfall was one of the most important variables affecting insect diversity, but during the dry season elevation dominated. Among landscape variables, the amount of habitat available and its diversity were also important but one variable modulated the effect of the other: insect diversity increased with habitat diversity, but this effect was offset in areas of low habitat amount. Overall our results suggest that landscape effects can help us to better understand the elevational diversity gradient, but also that diversity may change a lot between seasons. Most research on diversity patterns has focused on spatial patterns, and we believe that temporal trends in diversity will provide exciting new knowledge on how diversity is maintained or lost. Temporal diversity trends may be particularly important to explore given the unprecedented loss of insect diversity occurring during the last decades.
Another important challenge for future studies will be to dig deeper into the landscape data that can be obtained from satellite imagery. In our study we obtained data from vector layers that were very accurate in delineating different land uses, but that did not distinguish, for example, vegetation types among the natural forest category, or did not provide any information on plant or soil diversity. It thus remains to be answered whether insect communities are more unique in areas where plant communities are unique too, or whether areas dominated by invasive or agricultural plans, are also dominated by exotic or pest insects. These questions will for sure provide a more detailed understanding of insect diversity patterns, but answering them will require intense field work that is not always possible. As it commonly happens in ecological studies, the balance between the quality of the information obtained, and the human endeavor needed to obtain such data will need to be evaluated depending on access to study sites, the questions asked, the model system and the funding available, among others.
Enric Frago and Niry T Dianzinga
CIRAD, CBGP, Montpellier, France, and CIRAD-UMR PVBMT, Saint-Pierre, La Réunion, France.
Bois de corail, Chassalia corallioides, a Reunion endemic in the Cirque de Mafate that owes its name to the coral-like structure of the flowers