EARTHWORM INVASIONS ON THE GALAPAGOS: PATTERNS, PROCESSES AND THREATS
Ortiz, Jaime David
The Galapagos forest ecosystems are facing profound alterations as a result of plant and animal introductions. Historically the forests of Santa Cruz have been dominated by Scalesia pedunculata Hook. F. (Asteraceae), a fast-growing, shade-intolerant tree species endemic to the Galapagos. However, introduced vegetation has transformed these forests and Himalayan blackberry (Rubus niveus) is the most devastating introduced plant in the Galapagos. Non-native earthworms as invasive species have been overlooked on the Galapagos with potentially important consequences for native biota. We sampled in four areas of interest defined according to their management: 1) Agricultural: those private properties where agriculture and cattle ranching were allowed; 2) Park(int): buffer zone of protected areas adjacent to agricultural areas where control activities for R. niveus and other invasive plant species are often implemented, 3) PA(cont): protected area in Los Gemelos where mechanical and chemical control of R. niveus is implemented as a continuous activity and located within a 100m buffer area from the airport highway; and 4) PA(int): protected area in Los Gemelos where mechanical and chemical control of R. niveus is implemented as an intermittent activity, based on the needs of certain scientific and conservation projects and located within 100m - 200m of the buffer area from the airport highway. We established seven 200 m long transects that crossed perpendicular from highly disturbed agricultural areas through the buffer zone into the protected areas of the Galapagos National Park (i.e., from agricultural to Park(int)). In addition, to evaluate effects of land management for conservation purposes (i.e., mechanical and chemical control of R. niveus), we included five additional 200 m transects that crossed from a buffer zone into the remnant Scalesia forests in Los Gemelos. Within each transect, we established 20 1 m2 quadrats at different distances. We investigated the effects of non-native earthworms in the Galapagos in several ways. First, we used a combination of parametric and spatial statistics to understand the invasion patterns of non-native earthworms and their relationships with invasive plants, specially R. niveus. Second, to understand the possible impacts of non-native earthworms on seed germination of invasive plants, we conducted a microcosm experiment. Finally, using a field experimental approach, we examined the growth of S. pedunculata seedlings in response to sites invaded by non-native earthworms and R. niveus. Surprisingly, we found a highly significant positive correlation between abundance of non-native earthworms and R. niveus density, but only in the presence of invasive ants. We found that Park areas, where intermittent control of R. niveus is implemented, had a significantly higher non-native earthworm density compared with agricultural areas. Non-native earthworm density showed spatial autocorrelation, however this was highly dependent on land management. In areas where land management driven disturbances were occasional (i.e., Park(int) and PA(int)), we found that the probability of occurrence of R. niveus increased rapidly as a function of non-native earthworm biomass. Whereas, in areas continually managed for R. niveus invasion in Los Gemelos, we found a strong negative spatial correlation between non-native earthworm density and R. niveus at short ranges and a positive spatial correlation at long ranges. These results suggest that the spatial pattern of R. niveus invasion in the highlands of Santa Cruz might depend only to some degree on the spatial distribution of non-native earthworms, which in turn depend on land disturbances at the local scale (i.e., <100 m). A critical factor in this possible invasional meltdown scenario is the presence of invasive ants that may alter non-native earthworm behavior to benefit R. niveus. In the case of seed germination, we found a clear negative effect of non-native earthworm presence on R. niveus germination on soils collected in areas invaded by R. niveus outside of Scalesia forest and free from earthworm colonization. Our results suggested that there were strong effects of non-native earthworms on seed germination rates and species composition, however, the direction and magnitude of the effects depended on plant species, seed bank composition, and the invasion history of the soil. Finally, we found that the main cause of mortality for the Scalesia seedlings was mechanical control of R. niveus. In addition, our results suggested that overall mortality of Scalesia seedlings decreased as non-native earthworm biomass increased. At the end of our study period, average height of Scalesia seedlings was highest in the Park(int) areas compared with agricultural areas, PA(int), and PA(cont). However, we did not detect any significant difference in growth between the four areas of interest. We found that, at the end of our experimental period, mean height of Scalesia seedlings decreased as a function of earthworm biomass in Park(int) and PA(int) areas. These results suggest that earthworm invasion can play an important role on Scalesia growth, however these effects are mediated by land management activities (i.e. mechanical control of invasive plants). Overall, our findings shed new light on the unintended effects of mechanical control of invasive plants and provide indicative patterns of the possible effects of non-native earthworms on invasive and native plant establishment on tropical forests and remote islands. However, the magnitude and direction of the effects of non-native earthworm on these ecosystems are mediated by (1) the local spatial distribution of non-native earthworm communities, (2) presence of other invasive soil dwelling invertebrates (i.e. invasive ants), (3) seedbank composition, and (4) land management activities (i.e. control of invasive plants).
Conservation biology; Ecology; Amynthas; Galapagos; invasion meltdown; Land management; non-native earthworms; invasive species
Fahey, Timothy James
M.S., Natural Resources
Master of Science
dissertation or thesis