Common good

about  Bon Bon Station Reserve  
on 23 Oct 2014 

When we talk about conservation work the focus is often on threatened species but biodiversity encompasses much more. Here's a quick reminder that, beyond merely existing, species are of value for the important functional roles they often play.

Scientists have shown how natural feedback loops can impact on individual species, or groups of species that share foods, and influence their roles within food webs. These roles include helping to regulate populations of prey, competitors and their own species.

This was dramatically illustrated to me at a monitoring exercise on Bon Bon Station Reserve in South Australia’s arid rangelands.

Every day, while travelling between pitfall trap lines to inspect them for mammals and reptiles, I noticed a large influx of masked woodswallows – a nomadic species that isn’t predictably present on Bon Bon. We also had a lesser number of white-browed woodswallows alongside the resident black-faced woodswallows breeding too.

I also noticed that at every stop grasshoppers of several species could be seen crawling around. A little investigation suggested it was mostly grasshoppers that these woodswallows were eating and feeding to their young.

Virtually everywhere I stopped I saw one or more woodswallow pairs nesting in dead mulga trees. I made a quick estimate that there was about one breeding pair per hectare and after looking at the vegetation map, estimated about 125,000 hectares on this massive reserve that were likely to have at least a few dead mulga trees present and a relatively good growth of grasshopper food - grass.

I quickly calculated that over the three species of woodswallows, if there were two adults and two or more young per hectare, we were potentially looking at 500,000 individual woodswallows!

Clearly the food bonanza drove these nomadic birds to temporarily move in and supplement the resident black-faced woodswallows. All these species are common but that doesn’t mean we shouldn’t be concerned about their numbers - to maintain a functional ecosystem it’s very important they remain common.

Stabilising an ecosystem by regulating insects and dispersing seeds are roles often associated with birds. Other essential functions are maintained by common plant species. Things such as carbon accumulation, water infiltration, climate amelioration, and atmospheric composition.

Ecosystem ecologists combine field information like this with information on the densities and life spans of grasshoppers. The rate at which grasshoppers consume food, multiplied over grasshopper lifetimes expressed in units of mass or energy provides an estimate of how much plant material these creatures harvest per hectare.

This can then be compared with the kilograms of foliage produced by grasses on which they feed. Ultimately they can work out how much grass foliage is needed to support a breeding pair of woodwallows. Furthermore if they had the time to collect information on the mortality rates and reproductive rates of the grasshoppers they could determine if and when the woodswallows, by themselves or in combination with other consumers such as dunnarts and skinks, (small marsupial and reptilian insectivores respectively) began to exert control of their prey.

Movements by migratory and nomadic species help to maintain the predator/prey equilibrium when the resident species can’t do it by population growth alone. Together, communities of co-existing species help maintain stability – insectivores control insects most of the time, nectarivores pollinate plants, granivores manipulate seeds on the soil surface, frugivores disperse seeds of fruit, carnivores regulate other consumers and carrion feeders clean up.

While this was a stark but simple example, we find in our rigorous monitoring programs that there’s typically a strong connection between the species richness of birds that feed on a shared resource (insects, seeds, vertebrates, nectar, fruit, herbage) and patterns in resource availability.

Birds and other higher order consumers show population cycles over time that follow seasonal patterns. Often populations are reduced by dry spells that reduce plant growth and food to herbivorous insects such as grasshoppers.

We also find consumers often lag behind cycles in their foods until they can reproduce or otherwise increase in numbers. That is, until growth of a seasonal food resource declines and dispersal or death are the ultimate outcomes - necessary parts of population regulation cycles.

Ultimately collective consumption can’t exceed production of each resource.

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