Intensification of cultivation methods and land use types, particularly in the face of climate change, has steadily weakened the habitat function of rice fields for waterbirds.

Intermediate egrets (Ardea intermedia) are highly dependent on rice field habitats. Here, we examine the effect of a shift from rice monoculture to rice–barley double cultivation on the use of rice fields by intermediate egrets in South Korea. Rice-barley double cultivation has been increasing steadily in the southern region of South Korea due to changes in the rice growth period under climate change and maximization of economic benefits.

We studied rice fields to determine the effect of the rice-barely double cultivation area on the abundance of intermediate egrets on the regional scale and their potential distribution pattern in response to double cultivation in relation to climate change using national-scale climate, altitude, and land cover data from May 1 to June 23, 2018.

Compared to the irrigation practices used for rice monoculture, irrigation was stopped for some time in rice-barley double cultivation prior to planting, compromising the role of rice fields as a food ground for intermediate egrets. The potential abundance of intermediate egrets rapidly decreased by half when the double cultivation area increased to >40% of the total rice-field area, and was less than one individual if the area increased to >80%. Furthermore, according to climate change projections, by 2100, the double cultivation area would account for 75.74% of the predicted range of intermediate egret foraging ground.

Synthesis and applications. Double cultivation areas should be <40% of existing rice fields to minimize the impact of the change in cultivation practices. This approach can also convert rice fields to other types of land use. Consequently, appropriate conservation and management measures may be developed to maintain suitable habitats despite rapidly changing cultivation practices resulting from climate change.

The study was carried out from 1 May to 23 June 2018 in rice fields at a regional scale (105,271 ha, including part of the provinces of Chungcheongnam-do (CN) and Jeollabuk-do (JN)) and national scale (10,021,000 ha, including eight provinces: Gyeonggi-do (GG), Gangwon-do (GW), Chungcheongbuk-do (CB), CN, Gyeongsangbuk-do (GB), Gyeongsangnam-do (GN), Jeollabuk-do (JB) and JN) to determine the abundance of intermediate egrets based on the double cultivation area and their potential distribution range according to climate change. A survey area surrounded by rice fields was randomly selected to minimize the influence of the landscape (approximately 80% of the area in each plot).

At the regional scale, 40 plots (1 × 1 km) were randomly selected in CN and JB, located in the southwestern region of South Korea, where rice fields used for rice mono- and double cultivation were distributed. The rice cultivation areas in CN and JB were 133,986 ha and 113,775 ha, respectively, and the double cultivation area was 65 and 10,362 ha, respectively (KOSIS, 2022a). Surveys were carried out in rice fields in Seocheon-gun and Nonsan-si of CN and Iksan-si, Wanju-gun, Gimje-si, Jeongeup-si and Buan-gun of JB, with a mixed distribution used for rice monoculture and double cultivation. The distance between the plots was set at 5 km to minimize the effect of spatial autocorrelation (Amano et al., 2008). The average number of rice fields in a plot was 190.6 ± 11.4 units (range: 168-209) and the average size of the standardized unit paddies (separated by levee and farm road) was 0.46 ± 0.06 ha (range: 0.32–0.68 ha).

Fifteen sites (1 × 1 km) were randomly selected in seven of the eight provinces, except GW, to understand intermediate egret occupancy with respect to the rice-barley double cultivation area according to climate change on a national scale. GW mainly comprises mountainous areas, with a few areas with rice fields of at least 1 × 1 km; therefore, six sites were randomly selected.

During the survey period, irrigation in rice monoculture fields in the survey plots began on 9 May. In contrast, barley was grown in a dry state in rice fields used for double harvesting. On 17 June, rice was planted in flooding rice fields or barley was harvested from rice fields used for double cultivation, and the fields were irrigated. In the fields used for rice monoculture, irrigation and rice planting began at the beginning of May, and in the rice fields used for double cultivation, barley was cultivated immediately after rice was harvested in October. The barley cultivated during this period was harvested in June of the following year. During barley cultivation, rice fields were not irrigated.

Both regional and national surveys were conducted on days without rain or strong wind. The distribution and population of intermediate egrets were recorded using binoculars (Leica 10 × 40 BN, Solms, Germany) or spotting scopes (Leica 25–50 × 82) while traveling on farm roads adjacent to the main rice fields. The surveys were conducted between 06:00 and 12:00.

On the regional scale, two teams (two observers per team) conducted intermediate egret surveys once a week from 1 May to 23 June 2018 for eight weeks, and each plot was investigated eight times for 40 plots. For each survey, the 40 fixed plots were targeted and the plots were divided into two groups of 20 each and randomly assigned to two teams for conducting the survey. The number of intermediate egrets that occurred within each plot during a 15-min observation period were recorded. While intermediate egrets were surveyed, the percentage of barley cultivation area under double cultivation was confirmed in the selected plots by simultaneous aerial photography and direct observation.

During the same survey period on the regional scale, the survey points were randomly selected for each province and the distribution status was surveyed by visiting the plots centered on the selected points twice to investigate the distribution of intermediate egrets and rice-barley double crop areas on the national scale. The two teams alternated visits, visiting once each for two visits. The start and end times of the investigation were nearly identical for both teams. When the distribution of intermediate egrets and rice-barley double cropping area was confirmed, the coordinate values of the corresponding points were also confirmed. The field surveys did not require any licences or permits.

To investigate whether the abundance of intermediate egrets is also influenced by distance to breeding colonies (Fasola et al., 2022), the distance to the nearest colony of intermediate egrets from the rice field where intermediate egrets were observed was determined. The coordinates of intermediate egret colonies, based on a nationwide survey of herons conducted by the Ministry of Environment in Korea, were extracted and used for analyses (NIBR, 2020).