Effects of farmland landscape heterogeneity on biodiversity: is there a spatio-temporal complementation between crop fields?
Since the recognition of the importance of biodiversity for agro-ecosystem functioning and ecosystem services, maintaining biodiversity in agricultural areas has been a major issue. Landscape heterogeneity is a key factor for sustaining and restoring biodiversity. Most studies dealing with landscape effects on biodiversity in agricultural landscapes focused on semi-natural elements. But, many arthropods species are not limited to these habitats and often require cultivated elements (complementation), which results in ‘spill-overs’ from semi-natural to cultivated habitats. Recent studies also suggested that spatio-temporal complementation processes can occur between crops sown at different periods (winter vs. spring), that offer asynchronous shelter and resources for species.
We hypothesised that winter crops are appropriate habitats in spring while spring crops are in autumn, which make them complementary over time. Because these aspects involve population dynamics at landscape scale, we tested the effects of farmland composition and spatial organisation on carabid beetles communities of crop habitats.
Carabids were sampled in wheat (spring season) and maize (spring and autumn season) fields for twenty 1 km² landscapes. Farmland composition and spatial organisation were described by the coverage and interfaces length between spring and winter crops, and we considered the amount and spatial organisation of woody and grassy elements.
Wheat fields had higher carabid species richness and activity-density than maize fields in spring, indicating, as expected, that they are more attractive habitats at this period. Most species were present in the two crops, but some species were more abundant in maize (theoretically less suitable). We found no landscape effect on species richness of either crop in spring. However, ‘woody/crops’ interfaces increased activity-density in both crops, underlining the importance of adjacency between these elements and crops through enhancing spill-overs. Contrariwise, ‘wheat/maize’ interfaces lowered the activity-density in wheat fields, maybe because maize fields act as sinks for wheat carabid populations in spring.
Species composition differed slightly between autumn and spring in maize fields. Also, species richness in autumn benefited from ‘woody/crop’ interfaces. Some late-active species were found in maize in autumn (phenology), probably coming from adjacent woody elements. Otherwise, the farmland heterogeneity did not affect maize species richness, while activity-density showed no influence of landscape at all.
We found no evidence of spatio-temporal complementation between crops, but different species can benefit from wheat and maize in spring and autumn, increasing the overall gamma diversity. Our results confirm the role of permanent habitats for the conservation of abundant carabid communities in crops.