Herpetofauna responses to agricultural matrix management

Abstract

Preventing biodiversity loss in fragmented agricultural landscapes is a global problem that requires knowledge of how species move through landscapes. Farming practices can increase the contrast between remnant patches of native vegetation and adjoining habitats, with negative consequences for species movement and the preservation of biodiversity. Yet, the relationships between habitat attributes and the presence of species are poorly understood, and the mechanisms influencing cross-habitat movement has rarely been empirically tested, particularly for less mobile organisms such as frogs, snakes and lizards (herpetofauna). To address this important knowledge gap, I used a large-scale, empirical experiment to examine herpetofauna abundance, species richness, body condition, risk of predation, and inter-habitat movement between remnant woodland patches, edges and four contrasting farm land use types: 1) crop fields, 2) pasture paddocks, 3) linear plantings, and 4) woody mulch applied to a crop paddock after harvest. I also examined the effect of crop harvesting and seasonal effects on the distribution and abundance of herpetofauna. In the second and third chapters of the thesis, I compared the habitat preferences and abundance and, richness and body condition (frogs only) of reptiles and frogs to predictions developed from a conceptual matrix model and literature review. I found the structure and quality of the matrix was a stronger influence on reptile abundance, species richness and movement than the temporal effect of crop harvesting (chapter 2). My results demonstrate remnant patches and farmland, particularly those with woody vegetation plantings, can provide suitable habitat for common reptile species, but this environment is not readily interchangeable as habitat for uncommon reptile species. The negative response of uncommon reptile abundance and species richness to farmland, irrespective of restoration, indicate that farmland not only reduced their abundance but also the likelihood of presence. I also observed a negative trend in uncommon reptile abundance in remnant patches adjacent to crop paddocks. These findings suggest that crop paddocks and associated farming activities may be negatively influencing animals within both patches and matrix, further isolating populations, and reinforcing the negative consequences of conversion to agriculture for reptile populations. In Chapter three, my study revealed while frog abundance was positively associated with woody vegetation plantings within farmland, many frog species were found ubiquitously throughout the landscape, reflecting the dominance of a few disturbance tolerant species in the amphibian assemblage. My movement data demonstrated that frogs used multiple farmland types, moved between remnant patches and farmland, and into and out of cropped paddocks across harvesting periods. Unexpectedly, I found crop harvesting did not reduce frog abundance in crop paddocks, with some individuals persisting in farmland after harvesting. Body condition analyses indicated that farmland areas may provide good quality habitat for frogs and allow movement, dispersal, and foraging opportunities. My findings provide important insights into the utilisation of highly modified farmland as terrestrial habitat for frogs. This new knowledge is valuable in the context of land use intensification, and global amphibian declines. While much effort has focused on the protection of aquatic habitats for amphibians, management strategies must also consider the range of contrasting farmland types present in cropping areas, and the habitat requirements of the target species. In the fourth chapter, I explored how contrasting habitats and farm management influences predation risk in lizards using wildlife cameras and plasticine replica lizard models. I found predation pressure from multiple predators, operating across the landscape, could be compounding the effects of habitat degradation and fragmentation. Predation attempts on models were highest at habitat edges, with a reduced risk within farmland and remnant patches. My findings suggest edge habitats are ‘risker’ for lizards than farmland, and may act as a population sink, preventing reptiles from moving into the farmland if individuals suffer increased mortality at the edge. Such trends have not been previously reported for cropped landscapes. Finally, in the fifth chapter, I examined detailed movement patterns and behavior of a patch-dependent gecko species, Eastern Tree Dtella Gehyra versicolor, using radio-telemetry, fluorescent powder and an experiment displacement to examine movement and avoidance behaviour in response to different woodland to farmland edges. Unexpectedly, I found edge effects with a strong partitioning of individuals away from farmland, irrespective of the presence of complex habitat (e.g. linear plantings), in preference for remnant patches. We found pasture environments promoted direct movements of displaced geckos at a set distance from remnant patches into pastures. However, none of the radio-tracked geckos crossed the woodland-pasture edge, suggesting pastures may reduce homing ability of geckos. These findings contrast with the known ecology of the species and suggest, despite efforts to improve farmland quality by planting, farmland is not a preferred habitat for this species and does not promote longer distance dispersal movements. By examining the influence of anthropogenic habitat change on herpetofauna it is possible to improve our understanding of the distribution of species outside of remnant native vegetation. My research findings provide new insights into the complex responses of herpetofauna to contrasting farmland uses in cropping landscapes. I provide evidence for potential drivers to explain herpetofauna distribution within highly disturbed areas. Importantly, I have demonstrated the relative value of different farmland types for facilitating and reducing movement and providing habitat across diverse agricultural landscapes. Through my series of inter-connected studies, I highlight important opportunities to promote herpetofauna conservation in agricultural landscapes by protecting remnant native vegetation and by increasing restoration efforts in cropping landscapes by establishing linear plantings. I also identify management practices to reduce mortality risk in areas where dispersal may be important by focusing on implementing strategies that increase shelter opportunities for lizards and to reduce the size of edges particularly where dispersal may be important (such as between remnants and linear plantings). This new knowledge is valuable in the context of promoting landscape connectivity, targeted restoration programs, and arresting global herpetofauna decline.