Species composition, herbage mass and grass productivity influence pasture responses to kangaroo grazing in a temperate environment

Abstract

SummaryUnderstanding the complexities of pasture responses (the overall seasonal gains or losses in herbage mass) under different environmental conditions is critical to the effective management of total grazing pressure, whether to achieve agricultural, cultural or biodiversity conservation outcomes. Whilst large‐scale management programmes for kangaroos (Macropus spp. and Osphranter spp.) occur across much of south‐eastern Australia, there is limited understanding of how such programmes influence the overall pasture response across different environmental contexts in the temperate environment. This study investigates the effects of potential grass productivity (measured as the growth of pasture protected from large herbivore grazing), initial herbage mass (measured as the attached, above‐ground component of grass tussocks), kangaroo density (measured at the site scale) and season on the overall pasture response in plots dominated by six common grass types from south‐eastern Australia. Using generalised additive mixed modelling, we demonstrate potential grass productivity (related to but not synonymous with grass palatability) to be the major predictor variable for pasture responses across all six grass types. Kangaroo density, either directly or via an interaction with initial herbage mass, was related to overall pasture responses for most grasses in spring and remained an important variable in favoured areas dominated by preferred Kangaroo Grass (Themeda triandra) and Redleg Grass (Bothriochloa macra) throughout the autumn survey period. Plots dominated by grazing‐tolerant species, including wallaby grasses (Rytidosperma spp.) and Phalaris (Phalaris aquatica; an introduced pasture grass), showed marginally positive pasture responses under higher kangaroo densities, whilst plots with Wild Oats (Avena fatua; an introduced annual grass) and less preferred native spear grasses (Austrostipa spp.) showed no significant effect of kangaroo density on pasture response. Results support a context‐specific mix of ‘bottom‐up’ (resource‐driven) and ‘top‐down’ (consumer‐driven) processes in temperate grassy ecosystems, highlighting the importance of considering landscape productivity, existing herbage mass and grass species composition when informing kangaroo management decisions.