Spatial, bottom-up, and top-down effects on the abundance of a leaf miner

Many plant and animal species have higher densities at the centre of their distribution, with a gradual decline in abundance towards the edge of the range, though reasons for this pattern is not well known. We examined the abundance of the leaf miner Cameraria sp. nova over the range of its host plant Quercus myrtifolia in Florida and addressed how bottom-up and top-down factors varied over its whole distribution. Leaf miner densities, plant quality and natural enemy effects on mine survivorship were evaluated in 40 sites and spatially structured models were used to determine the effects of spatial location on the abundance of Cameraria and effects of both bottom-up (tannin concentration, foliar nitrogen, soil nitrogen, and leaf area) and top-down factors (larval parasitism and predation) on abundance and survivorship. Cameraria mines were, on average, three times more abundant on edge/coastal sites compared to centre/inland sites and did not support the hypothesis of higher abundance on the centre of the distribution. Differences in plant quality, larval parasitism and successful emergence of mines on edge versus central sites might be partially responsible for this finding. A trend surface equation with latitude and longitude combined explained almost 52% of the variation in Cameraria density and a trend surface map also revealed peaks of Cameraria abundance on the edges of the plant distribution. Correlograms also indicated a significant spatial structure of Cameraria as mines were positively spatially autocorrelated at small distances (≈122 km). Partial regression analyses indicated that 69% of the variation in Cameraria abundance was explained by the effects of latitude, longitude, elevation and percentage of foliar nitrogen. Our results indicated that variation in Cameraria abundance was mostly explained by spatial position and significant effects of bottom-up and top-down factors were not detected in our large-scale study.