Farmland biodiversity and associated ecosystem services across tropical rural- urban landscapes
by Arne Wenzel
Date of Examination:2020-07-14
Date of issue:2022-07-20
Advisor:Prof. Dr. Teja Tscharntke
Referee:Prof. Dr. Teja Tscharntke
Referee:Prof. Dr. Kerstin Wiegand
Referee:Prof. Dr. Holger Kreft
Referee:Prof. Dr. Ingo Grass
Persistent Address:
http://dx.doi.org/10.53846/goediss-9341
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Abstract
English
Driven by the unprecedented growth of human populations, cities all over the globe are expanding. Especially Asia and Africa are hotspots of current and future urbanization. As cities grow, they extend in area, sprawling into surrounding rural landscapes, creating a mosaic of urban developments and agricultural land-uses. At the same time, small-scale urban agriculture within city limits becomes increasingly popular. Given the alleged negative environmental impacts of urbanization, the question arises whether and how sustainable agriculture can be maintained in an increasingly urbanized world. In particular, functional biodiversity, indispensable for ecosystem services such as pollination or biological pest control, could be adversely affected. In my dissertation, I dwell on this question, focusing on the tropical south Indian megacity Bangalore. Megacities represent the most extreme form of urbanization. However, they have received only limited research attention so far, especially in tropical locations. As part of my dissertation, I conducted a systematic literature review on the effects of urbanization on pollination and pollinators and showed that urbanization is not always a threat for wildlife. To the contrary, I found that pollinator responses to urbanization are actually highly mixed with both positive and negative outcomes. Positive responses were often associated with urban sprawl, i.e. moderate levels of urbanization of rural, mostly agricultural land, whereas high levels of densification with high percentages of impervious area largely led to pollinator declines and loss of pollination. Furthermore, urbanization reduced pollinator diversity when compared to semi-natural areas, but also enhanced it when compared to intensified agricultural landscapes. In addition, I show that pollinator responses were commonly highly trait- and scale- specific and identified local and landscape drivers of urbanization for pollinator diversity. I conclude that urbanization can be designed to be biodiversity-friendly and that it could make a valuable contribution to pollinator conservation, in particular in the face of the continued intensification of rural agriculture. However, it also became clear that qualitative research is missing from tropical locations in the global south. In my second chapter, I studied farmland birds within Bangalore’s rural-urban interface. As farmland birds are important biocontrol agents, they occupy a key role for sustainable agricultural production. I sampled birds via point counts on 36 farms across three seasons and along two urbanization gradients. I employed a paired site design to separate local and landscape effects. I found high bird species losses through urbanization. With an increase of Abstract 6 landscape-wide impervious surface area from 0 % to 20 %, farmland bird abundance and diversity were halved. Particularly high declines in invertebrate-feeding birds indicated a great decrease in biocontrol potential, especially amid the monsoon, the high crop season, during which I found a drop of overall bird abundance by 20 %. However, bird richness benefitted from woody vegetation in combination with fallow fields, which could be employed in conservation efforts. In conclusion, my second chapter, somewhat contrary to the encouraging results of the review, revealed that the urbanization of Bangalore causes dramatic, exponential losses of farmland birds both locally as well as on a landscape scale. A worrying finding, not only for bird conservation but also for urban farmers, as the provision of biological pest control by local bird communities may be undermined. Lastly, I conducted a pollinator exclusion experiment and sampled bees on small-scale lablab (Lablab purpureous) farms along an urbanization gradient in Bangalore. I found that wild bees benefited from initial to intermediate urbanization, confirming the results of the review. Both the abundance and richness of bees increased with increasing grey areas in the farms’ surroundings. In particular, large bodied and ground nesting species were more abundant under grey shares of up to 30 %. I argue that these positive effects were associated to a higher availability of forage and nesting resources near built-up areas, as well as a landscape-wide reduction in pesticide applications. I could also show that wild bees did significantly boost lablab yields and hence farmers’ income. However, I could not find any evidence for a better pollination due to urbanization, as pollination was unaffected by grey proportions or differences in local bee communities. Overall, I found that responses of farmland dwelling birds and bees to urbanization were diverged in the Bangalore area. Many of my results are partly contrasting to findings from temperate cities. This highlights that our knowledge of the effects of urbanization in the tropics is limited. In the face of dramatic urbanization prospects, more research efforts are desperately needed.
Keywords: urbanization; biodiversity; pollination; functional ecology; bees; birds; agroecology
