Long-term livestock exclusion increases plant richness and reproductive capacity in arid woodlands

Aim Herbivore exclusion is implemented globally to recover ecosystems from grazing by introduced and native herbivores, but evidence for large-scale biodiversity benefits is inconsistent in arid ecosystems. We examined the effects of livestock exclusion on dryland plant richness and reproductive capacity. Location Central Australia. Methods We collected data on plant species richness and seeding (reproductive capacity), rainfall, vegetation productivity and cover, soil health, and herbivore grazing intensity from 68 sites across 6500 km2 of arid Georgina gidgee (Acacia georginae) woodlands between 2017 and 2020. Sites were on an actively grazed cattle station and two destocked conservation reserves. We used structural equation modelling to examine indirect (via soil or vegetation modification) versus direct (herbivory) effects of grazing intensity by two introduced herbivores (cattle, camels) and a native herbivore (red kangaroo), on seasonal plant species richness and seeding. Results Soil health and rainfall were the strongest drivers of variation in richness and seeding. Cattle and camel grazing indirectly led to lower seasonal richness and seeding by reducing soil health. Kangaroos had a small but negative direct impact on richness, but no impact on soil health. Both introduced and native herbivores reduced annual chenopod shrub richness and seeding, whereas only cattle directly reduced perennial shrub richness and seeding. Camels indirectly reduced perennial shrub richness by impacting shrub abundance. Introduced herbivores reduced native grass richness and seeding indirectly via impacts on soil health, whereas forbs responded positively to cattle and camel activity. Main conclusion Considering indirect impacts improves evaluations of the effects of disturbances on biodiversity, as focusing only on direct effects can mask critical mechanisms of change. Our results indicate substantial biodiversity benefits from excluding livestock and controlling camels in drylands. Reducing introduced herbivore impacts will improve soil and vegetation condition, ensure reproduction and seasonal persistence of species, and protect native plant diversity.Funding provided by: Australian Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923Award Number: DE170100599Site background and selection Our study occurred across 6600 km2 of the north-eastern Simpson Desert, central Australia, on three properties, Ethabuka Reserve (23°51'S, 138°28'E, 2330 km2, cattle removed 2004), Pilungah Reserve (23°8'S, 138°26'E, 2130 km2, cattle removed 2006, previously known as Cravens Peak) and Carlo Station (23°29'S, 138°32'E, 2110 km2, still grazed by cattle). The average annual precipitation of nearby long-term weather stations is 198 mm (Bedourie; 1933–2020) to 211 mm (Boulia; 1887-2020) (Bureau of Meteorology, 2021), but rainfall is highly variable and years with below-average rainfall are more frequent than years with above average rainfall (Dickman, Mahon, Masters, & Gibson, 1999). The area is on the eastern edge of the Simpson Desert, and is dominated by linear sand dunes 5–20 m high vegetated with spinifex (Triodia basedowii E.Pritz.) grassland and scattered shrubs. Georgina gidgee (Acacia georginae F.M.Bailey) woodlands occur patchily on clay swales between dunes as well as on calcareous and alluvial soils fringing the dunefields, with typical patch sizes ranging from 0.5 ha to 10 ha. We selected 17 gidgee woodland patches across the three properties. Patches were selected to represent a range of grazing intensities but located specifically to avoid areas immediately adjacent to watering points, which are often extremely heavily grazed. Each patch had four sites set up at 300-m intervals along a 1 km line, resulting in 16 sites on Carlo Station (in 4 patches; site selection was limited due to the density of artificial watering points in relation to accessible gidgee patches), 24 sites on Pilungah (in 6 patches) and 28 sites on Ethabuka (in 7 patches). Each site comprised a 100-m-long transect. Sites were monitored during 4 trips: September 2018, May and September 2019, and September 2020. Appropriate permissions and licences to conduct the fieldwork were obtained from the Queensland Government (Permits WITK15192514 and WISP15192514). Grazing intensity To assess the recent relative abundance of large herbivorous mammals we conducted dung surveys every monitoring session. All dung was removed from transects after counting, ensuring that transects would be clear for the next visit. Soil health At each site, we measured soil surface attributes and ground cover characteristics along a 100-m line transect, with the direction of each transect determined randomly. We took a measurement every 5 m along the transect, resulting in 20 measurements per transect. Plant foliage projected ground cover (vegetation