Habitat loss.....the signature conservation problem of the 21st century

Extinction from habitat loss has been suggested to be the signature conservation problem of the twenty-first century. Indeed, the Millennium Ecosystem Assessment predicts that near-term extinction rates could be 1,000 to 10,000 times higher than background rates. This alarming statistic seems to have got lost in the current focus on climate change, important as that will likely prove to be.

Images from Google Earth highlight just how much
 woodland clearing has occurred 
in western Victoria for cropping and grazing

One key reason species will go locally extinct due to habitat loss is because fragmentation leads to small populations. A central tenet of conservation biology is that small population size makes them much more vulnerable to inbreeding, recruitment failure, and climate extremes.

In our Lab, we've been testing the idea that small populations are more vulnerable to local extinction than large populations. Despite the simplicity of the hypothesis, there is not a lot of good evidence around in the literature to support or reject this idea (at least for plants in Australian ecosystems).

To address this question, we've employed re-visitation studies. This involves using data on species abundance collected at some previous point in time from well-defined places (lets call this the historical dataset, time1), and revisiting the very same places some time later to determine whether species have persisted (lets call this the contemporary dataset, time2). If you have data on abundance at t1, you can start to understand whether the smallest populations at this time are less likely to be present at t2 (as theory would suggest).

We used data collected in 1975 by a superb amateur botanist called Cliff Beauglehole as our t1 for a number of remnant grasslands and grassy woodlands in western Victoria.

Scaly Buttons is one of the common species
in grasslands and woodlands that has declined
over the last three decades
(Photo: John Morgan)

 Beuglehole recorded species lists for these sites (many of which were very diverse at the time) and allocated an 'abundance' measure for each species at each site that approximates a log scale - a few dozen, up to 100, in the 100s, in the 1000s.

We re-visited these same sites (t2) and recorded all species we could find; importantly, we estimated their population size in the same way as Beuglehole. When comparing the data on abundance at t1 versus t2, remembering that small populations should be more vulnerable to local extinction, we found some very interesting results which we have reported in the Journal of Ecology.

As predicted, the liklihood of local extinction over a  31 yr period was highest (34%) for those species whose population was initially very small. But initially small populations did not always decline - indeed, a small number (9%) actually increased and became abundant (in the 100s) or even very abundant (in the 1000s).

Changes in population abundances of native plants species
from 1975 to 2006 in grassy woodlands in western Victoria
(From: Sutton & Morgan Journal of Ecology)

But the story doesn't end there. We uncovered something in our re-visitation study that we had not predicted, nor even expected.

Some of the plant populations that were initially considered abundant or very abundant in woodland remnants had become locally extinct. Hence, having a large population did not guarantee persistence over the three decade period between observations. There could be a number of reasons for this, but we think the main one relates to habitat quality.

Over the 30 yrs between surveys, it is very likely that the habitat has deteriorated for some species - edge effects, weed invasions (by exotic Iridaceae, a curse in grassy woodlands in western Victoria) and inappropriate disturbance regimes have probably all caused a decline in habitat. In particular, the lack of fire is probably a key driver of change. Fire-sensitive shrubs, as well as dominant tussock grasses, have increased at the expense of poor competitors in the groundlayer.

Our study hints that while there is a strong need to find simple, general rules in conservation ecology, these general rules need to be put to the test. Not all small populations are vulnerable to short-term extinction. Indeed, 'common' species deserve our attention as well, as others have suggested.

And it is the common species that drive ecosystem function and provide much of the habitat for fauna. Simple re-vistation studies like ours are an excellent way to monitor trends in remnant vegetation, provided that old datasets are archived in such a way that they can be re-used in the future.