Abstract:

In agro-ecosystems, habitat loss and fragmentation may alter insect assemblages such as ladybug beetles and aphids, potentially affecting important ecological interactions. We used an experimental model system (EMS) with multiple micro-landscapes in which the habitat loss and habitat fragmentation impacts were distinguishable to test the following hypotheses: (1) Habitat removal results in short-term increases in population density in remaining habitat patches (crowding effect); (2) For the same total habitat area on a landscape, insect density will be higher in landscapes with more but smaller patches and more habitat edge than in less fragmented landscapes; (3) This positive effect of fragmentation on density is larger in landscapes with small inter-patch distances, and these last two effects on colonization should be reduced or disappear over time following habitat removal. This EMS included 18 fragmented and 18 clumped experimental plots. Alfalfa was cut to 1 m×1 m patches in fragmented plots (H=1) and retained the whole patches in continual plots (H=0). Habitat loss was designed to 90%, 80%, 70%, 50%, 20% and 0% respectively. Every plot was 3 times replicated and 0% of habitat loss was CK. Net sweeping, barber traps and clapper boards were used to investigate insect species in experimental alfalfa landscapes. 38 species were divided into 4 groups and diversity and abundance of every group were caculated and the relationship between diversity and habitat loss was analyzed. In our EMS, there was little support for the threshold phenomenon or for general effects of habitat loss and fragmentation, although this conclusion needs to be tempered by the limited duration of the experiment. We observed no threshold in species loss after habitat fragmentation and habitat loss in experimental alfalfa landscapes. The species number in fragmented habitat was higher than that in continual habitat under the same degree of habitat loss. Insect abundance was lower in micro-landscapes with 80-90% habitat loss than in CK. As for species, individuals in fragmented habitat were higher than that in continual habitat under the same degree of habitat loss. Insect diversity did not differ among variously treated micro-landscapes. It appears that insect colonization is low in micro-landscapes with 80-90% habitat loss; species richness, abundance and diversity were all significantly lower than that in CK, especially in fragmented habitats. Herbivorous species appear to colonize more rapidly than predatory species after habitat fragmentation and loss in experimental alfalfa landscapes. Our results did not support our first, but provided some support for the other two hypotheses. We suggest that fragmentation decreased the rate of immigration to patches, resulting in lower population densities in more fragmented landscapes. These results could be used to guide spatial and temporal aspects of the design of agricultural systems in order to enhance natural predator populations in agricultural landscapes and suppress pest population to the greatest extent.

Key words: habitat loss, habitat fragmentation,grasslands, insect diversity, landscape structure, threshold