Islands and introduced herbivores: conservation action as ecosystem experimentation

Published source details Donlan C.J., Tershy B.R. & Croll D.A. (2002) Islands and introduced herbivores: conservation action as ecosystem experimentation. Journal of Applied Ecology, 39, 235-246.
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Summary

In this study, non-native vertebrate herbivores (mostly rabbits) were removed from the San Benito Islands, Mexico, to investigate the impact on the plant community. This was done by contrasting one island, from where herbivores were removed, with another adjacent similar island, where they remained.

Study sites: The San Benito Islands, located 65 km west of Point Eugenia, Baja California, north-west Mexico, are one of the driest areas in North America (average annual rainfall < 100 mm). These desert islands are low in diversity but high in endemism, including at least eight endemic plants. They have no native terrestrial mammals. The dominant plant community is maritime desert scrub.

Experimental design: The study comprised an experimental island (San Benito West, SBW - 3.5 km²) and a control island (San Benito East, SBE - 1.1 km²) which are separated by 2 km of sea and are similar both in vegetation and fauna.

European rabbits Oryctolagus cuniculus, introduced during the early 1990s, have caused significant damage to the vegetation. On SBW, rabbits were introduced in 1991; goats Capra hircus and donkeys Equus asinus have had a discontinuous presence since 1948. At the beginning of the study, rabbits were abundant and a few goats and donkeys (11 in total) were present on SBW. On SBE, rabbits were introduced between 1995 and 1996 and were abundant in March 1996. There is no history of goats or donkeys on SBE. Rabbits were considered the primary vertebrate herbivore on both islands.

Removal of exotic herbivores Removal of exotic herbivores was part of a regional island conservation programme. Herbivores were removed from both SBW and SBE, but removal on SBE was postponed to facilitate this experiment. Removal efforts began in January 1998. Most rabbits (> 95%) and all seven goats were removed during the first 2 months. Complete removal was accomplished in spring 1998.

Plant cover and diversity: Changes in the plant communities (percentage cover and alpha diversity) on the islands were measured using repeated-sampling permanent (100 m) transects (21 on SBW, 11 on SBE) over 20 months. Both islands were sampled once prior to (December 1997) and three times (June–July 1998, December 1998 and June–July 1999) after herbivore removal on SBW.

Herbivore preference: Food preference tests were conducted on three rabbits during July 1998 and five during July 1999. on SBE. Each was placed in a 3 × 3 m enclosure with no standing vegetation, and habituated for at least 24 h. The rabbits, originally from domestic stock, were relatively tame and habituated quickly. Five native, perennials were chosen: Malva pacifica (an island endemic), and four species native throughout Baja California i.e. Atriplex barclayana, Euphorbia misera, Lycium californicum and Suaeda moquinii. Plants were chosen based on field observations of rabbit browsing and general plant abundance. The first plant species consumed by the rabbit was scored as preferred for each trial conducted.

Results from the trials were used to predict the direction and magnitude of perennial plant cover change.

Herbivore exclosures: Three 27 × 27 m exclosures (metal fence posts and chicken wire, trenched to 0.5 m deep, 1.25 m high) on SBE were constructed in July 1998. Similar control plots (27 × 27 m) were located 25 m away from the exclosures. No evidence of rabbits breaching the exclosures was observed. Vegetation was sampled using five 25 m transects in each plot. Transects were at least 2 m away from the edge of the exclosure, to minimize edge effects. Percentage cover for each transect was estimated for three sampling periods: July 1998, December 1998 and July 1999.

El Niño-related precipitation dominated vegetation dynamics early in the study. Differences in plant community structure due to selective rabbit herbivory between the experimental and control islands were detectable in the second year.

Results from the rabbit food-preference trials accurately predicted changes in the perennial plant community. When herbivores were removed from the experimental island, the abundance of their preferred plants increased while unpalatable species decreased. On the control island (herbivores still present), the opposite trend was observed. However, no recovery of vegetation occurred inside the exclosures on the control island, constructed after the El Niño rains. This was probably due to the lack of rainfall.

Conclusions Upon herbivore removal (mostly rabbits), preferred forage plants increased while unpalatable species decreased. The influence of El Niño precipitation highlights the importance of factors, such as water availability, in the recovery of these arid plant communities.

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