A conceptual model of land conversion processes: predictions and evidence from a microlandscape experiment with grassland insects

Land conversion due to human activities produces distinctive spatial patterns across the landscape. It remains unclear, however, how particular spatial arrangements of remnant habitat patches influence species persistence. We present a conceptual model of landscape change that focuses explicitly on habitat spatial arrangement. Four sequences, characterized as shrinkage, bisection, fragmentation, and perforation, differ qualitatively in the spatial arrangement of intact habitat and differ quantitatively in boundary length, amount of interior area, and connectivity. We tested the predictions of this conversion sequence model in a "microlandscape" field experiment with insects in a native grassland habitat. The four conversion sequences significantly influenced the species composition of the above-ground insect community, and insect species groups varied in their responses. Four patterns were evident from the field experiment: (1) as habitat decreased, insect density rose sharply on small dispersed patches of the bisection and fragmentation sequences, an effect less evident in the large remnant patch of the shrinkage sequence; (2) species richness declined only in the case of shrinkage, whereas richness increased in the fragmentation sequence; (3) most individual species examined responded similarly to the community as a whole; and (4) large rare species were concentrated in remaining habitat of the shrinkage and fragmentation sequences. Overall, the shrinkage and perforation sequences, which had high connectivity, were similar to one another in species composition, while the bisection and fragmentation sequences, which had low connectivity, were similar to one another. Based on these results, we conclude that a spatial pattern of land conversion that maintains one or more large, closely spaced remnant patches of native vegetation is essential for species persistence.