Wild geese of the Yangtze River: their ecology and conservation

Habitat selection is a process in which organisms decide to choose a suitable site for nesting, roosting or foraging. The question where the organisms are, and when they will leave are two of the fundamental questions frequently asked by ecologists. Habitat selection is affected by various abiotic and biotic determinants, varying over different spatial and temporal scales. In addition, an animal’s body size, determining its daily demands and its digestion capacity, plays an important role in foraging and habitat selection. This is because forage quality often decreases with increasing forage quantity. Therefore, herbivores often face a trade-off between forage quality and quantity. Although studies on habitat selection have offered substantial insights into the effect of various ecological factors, myriad effects of habitat and its’ surrounding are still not clearly understood, as former studies concerning this topic normally focus on a single species or a single spatial scale.

Migrating goose species are herbivorous with more or less similar habitat requirements and hence often mix in the field. Studying habitat selection of different goose species is attractive as they are from the same guild but differ in body size. In this thesis, I study the effects of various variables on habitat selection of different Anatidae species over different spatial scales, answering the question how ecological and anthropogenic variables affect Anatidae species habitat selection and population sizes and if these effects vary over different spatial scales.

First, I studied the habitat selection of Anatidae species under the condition with and without interference competition using an experimental approach in Chapter 2. To do this, I offered geese and ducks foraging patches with various swards heights. My results showed that all three species acquired the highest nitrogen intake at relatively tall swards (on 6 or 9 cm, but not on 3 cm) when foraging in single species flocks in the functional response experiment. When they were offered foraging patches differing in sward height with and without competitors, their mean percentage of feeding time did not change, whereas all species increased their percentage of time being vigilant except for the dominant swan goose. All species utilized strategies that increased their peck rate on patches across different sward heights when foraging together with other species, resulting in the same instantaneous and nitrogen intake rate than when foraging in a single species flocks. My results suggest that variation in peck rate over different swards height permits Anatidae herbivores to increase nitrogen intake under competition to compensate for the loss of intake, illustrating the importance of behavioural plasticity in heterogeneous environments when competing with other species for resources.

In Chapter 3, using a correlative field study, I analysed the habitat selection of two differently sized grazing goose species at site level. I found that both species selected lower lying area where the swards became recently exposed, due to receding water levels. However, the smaller species was more sensitive to this elevation gradient. Moreover, sward height negatively affected both species habitat selection with a stronger effect on the smaller species. This result highlighted the importance of body size on facilitating species coexistence and habitat segregation. Not in agreement with the results from most experimental studies, I found that nitrogen content did not influence habitat selection of both species. This conflicting result suggests that additional factors should be carefully considered when applying outcomes from experimental studies to field situations.

In Chapter 4, I studied habitat selection of the two goose species at a lake level by analysing the effect of ecological and anthropogenic variables. My results supported the individual-area relationship as only patch area had a significant effect on both species habitat selection, and other variables that were related to food availability and disturbance, were not significant. In addition, a facilitation effect of grazing livestock on geese habitat selection was detected, indicating that larger grazing herbivores can facilitate geese foraging by removing the taller and lower quality food from the top. As patch area size in wetlands is directly linked to water levels fluctuations, this result demonstrated that modifying hydrological regimes can enlarge the capacity of wetlands for migratory birds.

In Chapter 5, I further expanded my study area to the flood plain level of the Yangtze, testing for the effect of various abiotic and biotic variables on several Anatidae species habitat selection and population trends. I showed that slope and climate factors were the most important ones affecting habitat selection and distribution of Anatidae species. Furthermore, I demonstrated that the current protection policies may not stop the declining population trends but might buffer to some extent against a rapid decline in numbers in wetlands with a higher level protection status. This result points out that the conservation effectiveness is still low and larger conservation efforts are urgently needed to maintain the Anatidae populations, especially in wetlands with a lower level protection status. I recommend several protection measures to stop the decline of Anatidae species in wetlands of the Yangtze River flood plain and I called for more research efforts in this area in particularly, but also at a larger scale, the entire East Asian-Australasian Flyway.

In Chapter 6, I synthesized these results and draw conclusions from the preceding chapters, and highlighted the importance of spatial scales when studying the effect of abiotic and biotic variables on animals’ habitat selection. I also propose to modify hydrological regimes, aimed at creating enhanced habitat and improved forage accessibility conditions over the entire wintering period for herbivorous birds species in the Yangtze River flood plain. In summary, this thesis offers a framework for the effects of various variables on habitat selection and population sizes of herbivorous Anatidae species over different spatial scales, and a scientific basis for policy-makers and managers to enhance the efficiency of conservation actions in wetlands along the Yangtze River flood plain and also for similar ecological systems.