General Interests
I am interested in a wide range of research topics including population ecology, population dynamics, community ecology, landscape ecology, and breeding ecology. I prefer strong applied and conservation aspects in my research and prefer to work with birds; I have worked with shorebirds at important stop-over sites in Delaware, the Netherlands, and the United Kingdom, and at breeding grounds in Alaska. I further worked with breeding songbirds in Kansas and the Netherlands.
Comparing network connectivity of the historical and current wetlands of the Rainwater Basin, Nebraska (2017 – 2020)
Agricultural practices have led to large-scale losses of wetlands in the Great Plains, but what effects do those losses have for wildlife populations in the remaining wetlands and the connectivity among them?
For my current postdoc project, I am comparing network characteristics between the historical network and currently remaining wetlands in the Rainwater Basin region in south-central Nebraska, USA. The Rainwater Basin supports a complex network of spatially-isolated shallow wetlands that harbors diverse floral and faunal communities. Since European settlement, many wetlands have been lost to the network due to drainage, deliberate filling, land-use change, and increased sedimentation rates. Large-scale losses of wetlands have not only reduced the total available number and area of extant wetlands, but also increased the distance among remaining wetlands. Many species of plants, insects, and amphibians rely on these wetlands for reproduction and survival, but have limited dispersal capabilities. Populations may therefore become isolated and face higher extinction rates if distances among wetlands become too large.
What did we find?
So far, we have found that large-scale losses of wetlands in the Rainwater Basin have substantially reduced network connectivity. Species now need to be able to disperse at least 10 km to travel throughout the whole network, while this was only 3.5 km before European settlement. Furthermore, the relative importance of individual wetlands was dependent on the dispersal capabilities of a species of interest as well as the location of remaining wetlands. Conservation efforts should therefore focus on maintaining or increasing the connectivity of the network. However, which wetlands should be prioritized is likely dependent on the dispersal capabilities of the species or taxa of interest.
PhD Research: Effects of Patch-burn Grazing on the Demography of Grassland Songbirds (2012 – 2017)
Why are populations of grassland songbirds doing so bad? And what can we do to stop those declines?
Grasslands are among the most rapidly declining ecosystems in the world. The Flint Hills ecoregion contains one of the largest remaining tracts of tallgrass prairie, but most of the area is managed with high densities of grazing animals and frequent prescribed burns, which reduces variation in vegetative structure. A homogeneous landscape leads to lower diversity and abundance of wildlife species, including grassland songbirds.
Patch-burn grazing management could aid birds by more closely matching the historical interaction between fire and selective grazing by large ungulates. By burning only part of the pasture, and allowing cattle to roam free to selectively graze on recently burned patches, pastures have greater variety in vegetative structure and higher plant diversity. As a result, the diversity, abundance, and reproductive success of birds is often higher in pastures that are managed with patch-burn grazing compared to annually burned and grazed pastures.
However, grasslands show considerable regional variation in a whole array of factors, including temperature, precipitation, predator abundance and community composition, and the abundance of Brown-headed Cowbirds (Molothrus ater), a common brood-parasite. Effects of patch-burn grazing management on bird populations are therefore likely not the same at every site or every year.
Over a six year period and at two tallgrass prairie sites, we tested regional and temporal variation in the effects of patch-burn grazing on the reproductive success of three native grassland songbird species, Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna), and Grasshopper Sparrows (Ammodramus savannarum), as well as subsequent effects on the space use, movements, and survival of fledgling Dickcissels.
What did we find?
We found that patch-burn grazing had only minor effects on the reproductive success of any of our three species, potentially due to the high rates of nest depredation and brood-parasitism at our grassland sites. However, patch-burn grazing management might benefit grassland songbirds in other ways, even if depredation and parasitism rates are high. We found that densities of male Dickcissels were higher and territories tended to be smaller on unburned patches within patch-burned and grazed pastures compared to annually grazed and burned pastures, potentially indicating higher quality habitats.
We also found evidence for a potential tradeoff between habitat selection for nests vs. fledglings of Dickcissels. Parents that realized high reproductive success by nesting in annually burned and ungrazed pastures due to lower cowbird densities, produced fledglings that faced higher rates of depredation and tried to move away from those pastures. Since survival rates and movements of fledglings are low, especially during the first week after leaving the nest, local habitat conditions are crucial for breeding Dickcissels and patch-burn grazing management could provide a wide array of habitats within close proximity.
Compared to annual burning and grazing of tallgrass prairie, patch-burn grazing management is not worse for breeding grassland songbirds at our sites. Many other studies have found that patch-burn grazing could benefit plants, invertebrates, mammals, and larger birds such as Greater Prairie-chickens and Upland Sandpipers. Patch-burn grazing might therefore still be an improvement over the more traditional use of annual burning and grazing. However, regional variation in predator community composition and abundance of Brown-headed Cowbirds could play a large role in how rangeland management effects the population dynamics of grassland songbirds.
Master’s Research Projects (2007 – 2010)
1) How do vegetation characteristics influence the choice of breeding site of the Black-tailed Godwit (Limosa limosa)? The number of breeding Black-tailed Godwits in the Netherlands is rapidly declining. While some godwits realize high reproductive success in extensively managed wet meadows, others establish territories in intensively managed meadows of much lower quality. As part of a much larger research effort, I was interested in how vegetation characteristics varied between intensively and extensively managed meadows when godwits arrive in the Netherlands and establish breeding territories. Want to know more about Black-tailed Godwits? Click here, or read our publication!
2) The hydrologic effects of restoration by rewetting of the Peenetal Moor, Germany. Fens, such as the Peenetal Moor, often have high biodiversity and harbor many endangered species. However, many fens have been drained, and remaining wetlands are often damaged or degraded. Because the Peenetal Moor was damaged by direct and indirect drainage, land managers attempted to restore its hydrology by blocking up local ditches in 1994. In 2008, we found that water levels had indeed increased. However, the source of available water had largely shifted from groundwater to rainwater, which could have large implications for local plant communities.
3) Evaluation of vegetation changes in Midden-Groningen using plant and environmental traits. Habitat fragmentation has large consequences for the plant diversity of small remaining grasslands in a “sea” of row crop agriculture and intensively managed meadows. Although agricultural lands can be purchased and restored to connect remaining grasslands, it might take a long time for native plant communities to reestablish. At several grassland sites in Groningen, we assessed how abiotic conditions and plant community composition had shifted during an eight-year period after the sites had been taken out of agricultural use. We found that restoration effort increased water levels and decreased pH and nitrogen availability. Despite these changes, most plant communities showed only minor shifts, with many native species with low dispersal abilities unable to reach our sites after eight years.
Supervisors: Dr. R. van Diggelen and Dr. T. Piersma.