How often do you need to see a species, before you view it as being ‘characteristic’ of a particular ecosystem?
Some species are widespread and abundant. Others occur sparsely, but are found only in one ecosystem. Some of these sparse species become common after rare weather events or disturbances. All of these species might be viewed as being ‘characteristic’ of a particular ecosystem, even though they have different distribution patterns.
But if you hardly ever saw a species in a particular ecosystem, and you usually found it in a different ecosystem, you’d be hard pressed to call it ‘characteristic’ of the ecosystem you never saw it in. Seems obvious, yes? So, one more question – what if the species was always common in the soil seed bank, but never found in the vegetation? Would you say this species was ‘characteristic’ of the ecosystem? After all, it’s always there.
I pondered this question after reading a recent paper on grassland seed banks. The study found that many of the most common species in the seed bank were never found in the vegetation. Consequently, the authors described the seed bank species as being ‘not characteristic’ of grasslands. I’ve probably expressed similar sentiments in my papers too.
Which is curious. If we count up all the plants and all the seeds, then the most abundant species in many ecosystems are those in the seed bank that we rarely see. So we probably should view these species as being ‘characteristic’ of the ecosystem. After all, soil mycorrhizae are characteristic of grasslands (and other habitats), even though we don’t see them. Similarly, the bacteria E. coli is ubiquitous in the gut biota of humans.
No matter how much I try, I just can’t deny that the bacteria E. coli is characteristic of all my best friends.
Persistent soil seed banks
In ecosystems across the world, soil seed banks contain species that are rarely seen in the vegetation. Seeds of many of these ‘invisible’ species are extremely abundant. In a recent blog, I talked about grassland plants that don’t persist in the seed bank, and the high risk of local extinction that these species face. But Australian grasslands and woodlands also contain species with persistent seed banks, many of which are rare in the vegetation.
One group of plants that is really common in grassland and woodland seed banks includes the rushes and sedges, such as Juncus, Isolepis, Cyperus, Schoenus and other genera. These include annual species (e.g. Juncus bufonius, J. capitatus and many Isolepis species) and many large, long-lived, perennial rushes and sedges.
In a recent study from northern Victoria, two thirds of all of the seeds that were found belonged to just four species of Juncus or Isolepis. In another study, two thirds of all of the seeds in black box woodlands were from a single native sedge, Cyperus difformis. This sedge wasn’t found in the vegetation, despite its abundance in the seed bank. One soil sample contained 206,000 Cyperus seeds per square meter of soil, which is a lot of seeds.
Most plant species have very ‘patchy’ soil seed banks. Seeds may be abundant in one area (under an old plant for example) but completely absent just a few centimeters away. By contrast, seeds of some of these rushes and sedges are so abundant that they tend to be found almost everywhere. I once studied seed banks in remnant grasslands and woodlands in Gippsland, Victoria. There were seeds of the Broadleaf Rush, Juncus planifolius, in virtually every soil sample in the remnant forests, even though I never saw the species in a forest (it did occur in some roadside ditches and wetlands). I still find this amazing, as I’d walked and worked in these forests for many years.
There’s a wetland in my grassland
Many of the species that form persistent seed banks in grasslands are really common on damp mudflats and in wetlands. Indeed many of the ‘non characteristic’ species in grassland seed banks are ‘characteristic’ wetland species. Wetland plants are renowned for forming large, persistent seed banks. This is a great strategy for surviving in temporary pools that intermittently flood and dry out. Plants flower and set seed during wet seasons, and then die back to persist in the soil seed bank during dry periods.
But how do so many of these wetland seeds get into dry grasslands?
Many ‘mudflat species’ grow in small damp depressions in grasslands and woodlands, and seeds from these plants may gradually accumulate in the seed bank. Seeds of other species might disperse into dry grasslands. Water birds are renowned for spreading seeds from wetland to wetland on their muddy feet. However, it would take a lot of ducks, with really dirty boots, to disperse millions of Juncus or Cyperus seeds across the countryside!
Perhaps the Juncus planifolius seeds were deposited in the Gippsland woodlands long, long ago. Big floods may have triggered Juncus to grow and set seed in forests that are normally far too dry. Large areas of eastern Australia flooded this year and last year (see John Morgan’s blog). Do these rare events refill the seed bank with sedges and rushes?
If the world was heading for ‘global wetting’ rather than global warming, I wonder… would we view our grasslands as being inherently ‘resilient’ to global change, since they contain abundant seeds of native plants ready to thrive when sites get wetter? Persistent seed banks impart resilience to ecosystems, not stability. Regardless of whether we like the invisible ‘uncharacteristic’ species that they contain, seed banks enable ecosystems to respond rapidly to variable environmental circumstances.
A characteristic feature of all of our grasslands and woodlands is the presence of an abundant, invisible seed flora, full of ‘uncharacteristic’ grassland species, quietly waiting for their moment in the sun.
I am grateful to photographer Bob Trlin for permission to copy the photo of Juncus planifolius from his wonderful collection of photographs of the Bush Flora of Ku-ring-gai. Bob’s web site is well worth visiting. The seed photo in the header image is from wikipedia.
Brock MA (2011) Persistence of seed banks in Australian temporary wetlands. Freshwater Biology 56, 1312-1327.
Eldridge DJ, Lunt ID (2010) Resilience of soil seed banks to site degradation in intermittently flooded riverine woodlands. Journal of Vegetation Science 21, 157-166.
Leck MA, Schutz W (2005) Regeneration of Cyperaceae, with particular reference to seed ecology and seed banks. Perspectives in Plant Ecology Evolution and Systematics 7, 95-133.
Lunt ID (1997) Germinable soil seed banks of anthropogenic native grasslands and grassy forest remnants in temperate south-eastern Australia. Plant Ecology 130, 21-34.
Scott AJ, Morgan JW (2012) Resilience, persistence and relationship to standing vegetation in soil seed banks of semi-arid Australian old fields. Applied Vegetation Science 15, 48-61.