Introduction

The understorey ground layer typically supports the majority of the biodiversity of Northern Territory ecosystems. Native plants and most wildlife (perhaps with the exception of birds and some invertebrates) are all more numerous and diverse in the ground layer than in the overstorey.

Spinifex hummock grass

Photo: © Mel Pegg

Understorey ground layers vary enormously in structure and composition across the Northern Territory. Ground layer structure and composition is determined by a combination of factors including rainfall, soil drainage properties, parent geology, soil fertility and management history.

Spinifex hummock grasses are a distinctive and characteristic component of extensive areas of the Northern Territory, either as the understorey ground layer or even as the dominant vegetation stratum.

Tussock grasses make up the understorey ground layer for much of the Northern Territory savanna woodlands. There are both native perennial and annual (notably sorghum) tussock grasses. In some situations a few grass species dominate and in other areas a diverse suite of tussock grasses occur together. The inter-tussock spaces are typically occupied by numerous native herbs and sub-shrubs, many of which appear only during the tropical wet season. Some have perennial root systems or storage organs such as tubers which allow the plants to persist after their wet season shoots have died off or been burnt. Others are very short lived, with only seed stored in the soil surviving the long dry season.

Other understorey ground layers include those completely dominated by herbs, or mat-forming (rhizomatous) grasses or sedges in wet situations.

In the arid zone many annual or ephemeral plants persist in the seed bank. These seeds respond to good rain by rapidly germinating, growing, flowering and setting seed. In this way they are able to thrive in areas with irregular rainfall.

A number of threatened plants grow in the understorey ground layer. Several ferns, orchids and typhoniums grow only where the ground layer is shaded by a rainforest canopy. Swamp Twig-rush, Caldwell's Club-rush and several bladderworts grow on the edges of wetlands or in shallow water. These habitats are highly restricted in the Northern Territory. Other threatened ground layer plants, including Tobermorey Melon and Fitzgerald's Spinifex, are found in drier environments, which appear to be more widespread. These plants may be limited to a specific soil type or landform.

Numerous threatened animals live in the ground layer. Plains mice shelter in cracks in the soil, Arnhem Rock-rats in rock crevices, hopping mice in burrows and Yellow-snouted Geckos in the leaf litter. Spinifex hummock grasslands provide specialised habitat for many species, particularly in the arid zone, such as hopping mice, dragons, skinks and grass-wrens. All of these animals feed amongst grasses or in the bare ground in between.

An intact ground layer provides shelter from predators and may also be important for providing a moist microhabitat for animals such as Howard Springs Toadlet. The ground layer is particularly important for species which nest on the ground or within clumps of grasses, including the White-throated and Carpentarian grass-wrens, Partridge Pigeon, Flock Pigeon and Night Parrot.

Ground layer plants also provide food for a range of threatened species. Many small animals feed on grass seeds. Threatened seed-eaters include Partridge Pigeon and three species of hopping mice. Cockatoo Grass is particularly important for Gouldian Fiches and Brush-tailed Rabbit-rats. Spinifex seed is the main food taken by Central Rock-rats.

Woodland with understory

Photo: © John Westaway

Seed resources may be scarce at particular times of the year or in response to longer climatic cycles. In the Top End, grass seeds are produced and shed in abundance at the start of the dry season, but their availability drops through the dry season as a result of burial, fire, insect attack and consumption by seed-eating birds and mammals. Finally, wet season rains germinate remaining seed supplies, leading to a period of low food availability for seed-eaters. This means the early wet season can be a difficult time for seed-eaters.

In the deserts, long and irregular periods between rainfall events mean that seed supplies are unpredictable. This possibly explains the apparently nomadic movements of Night Parrots and Princess Parrots, and the boom and bust population cycles of the more sedentary Plains Rats.

Ground layer plants also provide habitat for a range of prey species fed on by threatened animals, including insects and spiders eaten by Yellow-snouted Geckos.

Woodland with native grasses understorey

Photo: © John Westaway

Issues

The native tussock grass ground layer of savanna woodlands underpins the Northern Territory pastoral industry. Pastoral management practices have the potential to impact significantly on the biodiversity of these ecosystems. Grazing practices, weed incursion and fire regime all influence the ground layer.

Grazing animals can increase the stress placed on threatened species that depend on seed reserves by removing the grasses that provide seeds or preventing them from setting seed. Cattle selectively graze palatable plants, trample others and can compact the soil. This disturbance affects which animals are able to persist in the presence of grazing.

Twelve granivorous birds have declined across northern Australia since European settlement, including threatened species such as Golden-shouldered Parrot, Star Finch and Gouldian Finch. Medium and small-sized mammals which feed and shelter in the grass layer have also declined across northern Australia, including Bridled Nail-tailed Wallaby and Northern Bettong. Similarly, in the arid zone numerous mammals have become locally extinct at least in part due to the loss of grass layer habitat including Sandhill Dunnart, Central Hare-wallaby and Mala. Areas of eucalypt woodland with intact canopies have not been immune to this fauna decline, indicating the importance of understorey habitat components.

Well managed cattle grazing can co-exist with healthy populations of many ground layer plants and animals. However, there is a suite of plants and animals that are particularly sensitive to grazing. These species are only found in areas at some distance from permanent water where grazing pressure is low. Numbers of Hooded Robins, for example, increase with distance from permanent water.

Invasion of the ground layer by weeds reduces plant diversity and decreases food and habitat available for wildlife. Several grasses introduced as pasture plants have since become significant weeds including Gamba Grass, which leaves little or no space between clumps. This has a negative effect on animals, such as Gouldian Finch and Partridge Pigeon, which rely on such spaces to find fallen seed.

Annual burning has been found to reduce ground cover plant diversity and this effects wildlife causing a reduction in biodiversity. The timing of burning also affects species that rely on the understorey ground layer for shelter and food. Red-backed Fairy Wrens have a lower reproductive output after late dry season fires than after early dry season fires. This is thought to be a reason for their decline after fire in northern savannas.

Flock pidgeons need native grasses

Photo: © Peter Dostine Flock Pigeons rely on ground layer vegetation, especially tussock grasses for cover and for nest sites, and feed on the seeds of grasses and herbs.

Management

While it is important to preserve ground cover, a continuous ground cover is not essential, as many smaller animals shelter in the grass or litter and hunt across the bare patches of ground in the inter-tussock spaces. This complex structure is disrupted when the ground layer is cleared, over-grazed, or gets invaded by aggressive introduced grasses.

Maintaining ground cover and minimising erosion are the basis of good pasture management and benefit many native species. Grasses that are important for cattle grazing, such as Cockatoo Grass, Plume Sorghum and Giant Spear, are also important for wildlife. A variety of herb seeds are also important. Maintaining the ground layer is possibly the best protection against vegetation thickening, as a healthy grass cover competes with suckering woody plants.

Maintaining the ground layer on pastoral properties means grazing moderately and allowing plants to regenerate by spelling pasture every few years, either over the wet season in the Top End or after good rainfall in the arid zone. This will help maintain the seed bank and benefit species that depend on seed reserves. Controlling grazing patterns through fencing and careful placement of water points can also help preserve biodiversity.

Introduced pasture plants may be good for cattle, but are not always good for wildlife. Before using them, ask advice on the likely impacts. Species that smother or out-compete native plants, or lead to overgrazing or an increase in fire intensity are best avoided. Introduced species that have been highlighted for their negative impact on wildlife include Para Grass, Buffel Grass and Gamba Grass.

Careful management of fire is also important. Late dry season fires remove grass cover, causing erosion. Also if early dry season burns are too small in area, the green pick they produce may be overgrazed by cattle. Burning the same areas repeatedly can cause permanent loss of ground cover and soil compaction. Fire management is especially important for maintaining the diversity and abundance of ground layer species and seed production. A fine-scale mosaic of burnt and unburnt ground is recommended for maintaining grass seed supply for granivorous birds.

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