Stock grazing has reduced the conservation value of many native grasslands and woodlands in southern Australia. Not surprisingly, we often remove stock to help restore degraded areas. But how well does this work? Can damage caused by past grazing be reversed, or will the removal of stock create new, unexpected communities?
In a fantastic new report, Josh Dorrough assesses how stock grazing affects native and exotic plants in lowland (non-alpine) grasslands and woodlands in southern Australia. Josh collated data from all available grazing studies (79 in all) and used a meta-analysis to analyze the pooled dataset. He asked two questions: (1) how does grazing affect plants, and (2) how does the removal of grazing affect plants?
To answer the first question, Josh analyzed studies that compared ungrazed remnants (such as cemeteries and rail-lines) against unfertilized, grazed pastures. For the second, he analyzed studies that compared unfertilized grazed pastures against pastures that had been fenced to exclude stock.
If grazing effects are reversible, species that decrease when grazed will increase when fenced, while those that increase when grazed will decrease when fenced. However, if grazing effects are irreversible, we might get surprising outcomes. For example, a species might decrease when grazed, and then decrease even faster when fenced.
[Note: I use the term ‘fencing’ to refer to the total exclusion of livestock, as this is how fences are often used for conservation. The terms ‘grazing’ and ‘pastures’ refer to livestock grazing in unfertilized pastures only; fertilizer affects native plants much more than grazing alone].
How many ways can plants respond to grazing and grazing removal? The range of outcomes can be shown in simple charts. Figure 1a below shows how five imaginary species (each represented by a dot, A-E) respond to initial grazing. Species on the left (e.g. species A) are abundant in ungrazed remnants and rare in pastures; species in the middle are equally common in remnants and pastures (species C); and species on the right are abundant in pastures and rare in ungrazed remnants (species E).
Similarly, Figure 1b shows how plants might respond when pastures are fenced. Species A is abundant in fenced areas and absent from pastures; species C is equally common in both areas; and Species E is abundant in pastures and absent from fenced areas. These patterns aren’t very exciting, but things get more interesting when we combine the two charts. Figure 2 shows how five species might respond to initial grazing (from left to right) and fencing (from top to bottom).
Species A and B, in the top-left corner, decrease when initially grazed (i.e. are more common in remnants than pastures) and then increase when stock are removed following fencing. We might expect many ‘grazing-sensitive’ native plants to behave like this. By contrast, species D and E in the bottom-right corner behave in the opposite way. They increase when grazed (i.e. are more common in pastures than remnants) and decrease when fenced. We might expect (or hope) that many ‘grazing-increaser’ exotic plants would behave this way. Species C, in the middle, is equally abundant in all situations, and isn’t consistently affected by grazing or grazing removal.
The patterns in Figure 2 illustrates reversible changes: species A and B decrease when grazed and increase when fenced, and species D and E do the opposite. But what happens if grazing has irreversible effects on some species? To represent irreversible changes, our chart needs to be expanded to show two extra patterns.
Each corner of Figure 3 represents a different combination of the effects of initial grazing and fencing. Reversible effects are shown in green and unexpected irreversible changes are shown in blue. Species that plot in the center of the chart are either unaffected by grazing or fencing, or respond differently in different areas.
Josh’s report has a complex chart that shows how every analyzed species responded to initial grazing and fencing (Fig. 6 in his report). In the charts below, I’ve simplified Josh’s figure by using shading to indicate the proportion of species that lie in each corner of the chart. I’ve ignored the many species in the center, and only show species that consistently responded in a particular way. In the charts below, dark blue indicates that many species lie in that corner (i.e. many species showed that response), and light blue indicates that few species are in that corner.
For example, if nearly all exotic species increase when grazed and decrease when fenced, the bottom-right corner would be dark blue and the other corners would be light blue, as in Figure 4. By contrast, if nearly all native species decrease when grazed and then increase when fenced, the top-left corner would be dark blue.
Before reading on, take a break and think – If you had 100 native species, how many would sit in the middle and how many would sit in each corner of the chart? How many would consistently increase or decrease when initially grazed? How many would increase and decrease when pastures were fenced? How many would show surprising patterns? What about exotics? Would they show similar or different patterns to native plants?
If we know the answers to these questions, then we know how well fences can work. If we don’t, we may get some surprising outcomes. So read on… and perhaps be surprised.
How do native plants respond?
I imagine that many people would think that most native plants will decrease when grazed and then increase (perhaps slowly) when fenced. If this was the case, the top-left corner of the chart would be dark blue and the other corners would be light blue.
Josh’s analysis shows that things aren’t that simple. Many native species showed no consistent response, and responded differently in different studies. (These species sit in the middle of the charts, and I haven’t shown them in my charts for clarity). Consequently it is difficult to predict how lots of plant species will respond to grazing or fencing in any particular situation.
These inconsistent species aside, different natives respond to grazing and fencing in different ways, as shown in Figure 5 and as listed at the end of the blog. Most species respond in predictable, reversible ways (top-left and bottom-right) and relatively few show surprising, irreversible outcomes.
As expected, many natives consistently decrease when grazed and increase when fenced (top-left); Bulbine Lily (Bulbine bulbosa) and Yam Daisy (Microseris lanceolata) do this. However, many natives do the opposite (bottom-right), and consistently increase when grazed and decrease when fenced, such as Red-leg Grass (Bothriochloa macra) and the ground-hugging daisy, Smooth Solenogyne (Solenogyne dominii).
A smaller group of natives act ‘surprisingly’ and increase when grazed and then increase even more when fenced (top-right), for example Purple Wire-grass (Aristida ramosa). Luckily, only a few natives decrease when grazed and decrease even faster when fenced (bottom-left). The daisy, Scaly Buttons (Leptorhynchos squamatus) is a notable example. Fencing is hell for Leptorhynchos.
How do exotic plants respond?
Like natives, many exotics respond inconsistently to grazing and fencing. Of the consistent species, most increase when grazed and decrease when fenced (bottom-right in Figure 6). Common examples are the annuals, Cape Weed (Arctotheca calendula), Barley Grass (Hordeum leporinum) and Brome grasses (Bromus hordeaceus and B. rubens). However, the next most common response by exotics is a ‘surprising’ outcome. A group of exotics increase when grazed and increase even more when fenced (top-right). Wild Oats (Avena species), Rye Grasses (Lolium species) and Sweet Vernal-grass (Anthoxanthum odoratum) are some of the worst offenders. All three are vigorous, transformative weeds, capable of out-competing many natives.
How did you expect native and exotic plants would respond? Did any of these patterns surprise you? Did you expect to see such a diversity of responses by natives?
What do these patterns tell us?
Many of the messages in Josh Dorrough’s report aren’t new, but they are worth repeating. The biggest message is that grazing and fencing have unpredictable impacts on many species. Peter Vesk also found this in an earlier, excellent study.
The next obvious message is that grazing and fencing affect different species in different ways. This isn’t surprising. Notably though, many exotic and native species respond in the same way, especially by increasing when grazed and decreasing when fenced. Much as we’d like to promote natives and reduce exotics, we have to accept that native-exotic mixes are here to stay.
How good is fencing? Fencing has inconsistent impacts on many species, but consistently benefits some natives and consistently reduces others; it isn’t an obvious win-win outcome for all natives. On the other hand, continued grazing isn’t win-win either, as it also consistently benefits some natives and consistently reduces others.
Many of the natives that decline after fencing are abundant in unfertilized grazed pastures, so their decline may not be a problem at regional scales. Fortunately, very few natives consistently decline when grazed and decline even faster when fenced. Ungrazed remnants appear to be the best refuges for these species.
The biggest problem with fencing is that it consistently leads to increases in some very problematic exotics, especially Wild-oats (Avena species) and Rye-grass (Lolium species), which compete strongly against natives. We need to develop better strategies to control these problem species.
Where do we go from here?
It’s worth emphasizing that Josh’s review (and this blog) only examines ground plants, and grazing and fencing both affect many other natural features, including soils, water courses, regeneration of trees and shrubs, and habitat for birds and animals.
Notably, the review compares two very simple options: ‘grazing’ versus ‘no grazing’. Few people would debate the effects of ‘burning’ versus ‘no burning’, as most people readily accept that fire regimes influence outcomes, not fire per se. However, many people view grazing as being intrinsically ‘good’ or intrinsically ‘bad’, and grazing exclusion is a common management response in many areas. Clearly, we need to refine the dialogue about grazing, and we need to do much more work to examine how different grazing regimes, such as short-duration grazing or seasonal grazing, affect vegetation composition.
A key conclusion from the review is that the effects of grazing and fencing are highly variable, across and within regions. Local experiences can’t be extrapolated to other sites and regions, as variability and uncertainty are the norm, not the exception.
Given this unpredictability, the safest management strategy is to avoid ‘putting all your eggs in one basket’. At regional scales, we have created – and will continue to create – mosaics of patches, each managed in different ways, and each promoting different species. A range of management styles may promote diversity across regional scales, even if diversity isn’t extremely high within each patch.
Josh’s fantastic review suggests that it may be useful to review our expectations about fencing. The major benefit from a fence may not be that it increases the diversity of native plants within it. Instead, a major benefit may be that fences increase the diversity of management regimes across regions, and thereby increase the diversity of vegetation and habitat types across different patches.
If fences bring flexibility, then flexibility may bring diversity.
Dorrough J (2012) How do different levels of grazing and fertilisation affect vegetation composition in temperate Australian grassy ecosystems? Systematic review and meta-analysis. Final Report by Natural Regeneration Australia to the Victorian Dept of Sustainability and Environment. [You can download a copy of the report from this link].
Vesk PA & Westoby M (2001) Predicting plant species’ responses to grazing. Journal of Applied Ecology 38, 897-909. [You can download a copy of the article for free from this link].
Footnote: Native species lists
This section lists many of the native plants that consistently displayed each of the four responses. These species have responded in a similar way in past studies, so we can be reasonably confident that they will respond this way in the future.
The native plants that consistently decline when grazed and recover when fenced include Bulbine bulbosa (Bulbine Lily), Microseris lanceolata (Yam Daisy), Senecio quadridentatus (Cotton Fireweed), Enteropogon acicularis (Curly Windmill Grass) and Gonocarpus tetragynus (Common Raspwort). Most geophytes also do this.
The surprising ‘fence-winners’, that increase when grazed and increase even more when fenced are an odd group. They include Aristida ramosa (Purple Wire-grass), Lomandra filiformis (Wattle Mat Rush), Small Poranthera (Poranthera microphylla ) and Wurmbea dioica (Early Nancy). Based on past performance, these species are ‘sure-fire winners’ from fencing schemes, if they are present.
Many natives consistently increase when grazed and decrease when fenced. These include some Wallaby-grasses (Austrodanthonia racemosa and A. setacea), Red-leg Grass (Bothriochloa macra) and Slender Rat’s Tail Grass (Sporobolus creber) and the small herbs, Austral Stonecrop (Crassula sieberiana), Smooth Solenogyne (Solenogyne dominii) and Common Sunray (Triptilodiscus pygmaeus).
Finally, the ‘surprising’ casualties of fencing schemes, that decline when grazed and decline even more when fenced, include the daisy, Leptorhynchos squamatus (Scaly Buttons) and Schoenus apogon (Common Bog-rush).