Amphibians, the oldest group of land vertebrates, are regarded as the most threatened across the globe. From Brazil’s tropical rain forests, to Canada’s temperate boreal forests, from fresh streams in the Alps to Australia’s temporary ponds, most populations are declining. Contributing factors include habitat destruction, invasive species, disease spread and rapid climate change. Most are human induced.
Frogs are ectotherms: they generate their heat from the environment around them. When the temperature changes in their native areas, frogs need to change their behaviour or habitat to stay at their preferred temperature.
Frogs are also generally regarded as poor dispersers among land vertebrates. This means that, unlike birds or large mammals, they find it difficult to move – or they move slowly – across landscapes. As a result, it has been suggested that they will not be able to keep track of rapid climate change because they cannot cover the distances required to match the particular climate they require.
South Africa’s Cape region, comprising both succulent semi-desert and fynbos vegetation, has a unique amphibian fauna that has been recognised globally. It consists of more than 50 species of frogs, 37 of which are endemic (they occur nowhere else in the world). Cape frogs are important because of their global uniqueness.
But their future may be under threat, and the climate of the future looks to be changing faster than the steady climate they evolved into in the past. How can we target conservation interventions to pinpoint which species or sub-populations need particular help?
Confined to smaller and smaller areas
Biologists have already observed that species use different methods to keep up with changes in climate. These methods broadly involve species shifting their distribution to track suitable climate, and changes in behaviour and genetic make-up to enable them to survive new climate regimes.
In a recently published study, we looked at how changes in global climate have affected the spread of the endemic Cape frog community during two key periods in recent history: the Last Glacial Maximum about 21,000 years ago; and the Holocene Glacial Minimum about 6,000 years ago.
We then projected the distributions forward into two possible future scenarios using two emission scenarios. This allowed us to ask whether the forecasted climates would significantly change the distribution of Cape frogs. By predicting these future distributions, we were able to assess whether these species are likely to move in specific directions, like north or south, or whether their distributions are likely to become fragmented. It also established which particular group of frogs is likely to be more negatively affected.
The results were startling. Our models suggested that the area occupied by Cape frogs today is just a fraction of the area of suitable climate space they would have had available at the Last Glacial Maximum. But comparing the current distribution with that at the Holocene Glacial Minimum provided very little evidence for change. The biggest surprise was the massive loss in suitable climate space between current distributions and future forecast scenarios in 2080.
Not only are Cape frogs likely to be confined to a much smaller area, but the rate at which they will be forced to move is faster than anything that they have experienced in a very long time. This would mean that many of the species would experience a fragmentation effect: their sub-populations would be separated beyond what they’re likely to be able to bridge through their own hopping abilities. In other words, sub-populations would become isolated from each other – and too far apart to reach each other to breed. This effectively makes each small sub-population more vulnerable to extinction.
Our models predict that the suitable climate space is likely to shift Cape frog distributions to the north. Based on the estimates, this has been a trend since the Last Glacial Maximum. But in the past this would have seen a movement rate equivalent to 1km per 1,000 years. The movement between the present and the worst emission scenario in 2080 is more than 500 times faster.
Is there any hope?
Our interest is in the trends that the data show, and asking how these potential scenarios could be mitigated by conservation actions today. For example, in the future lowland species are expected to be more fragmented than highland species. This shows the need to establish corridors of suitable habitat between existing sites for many species.
These areas need to be made up of both terrestrial and aquatic habitats, allowing animals to track suitable climate space as they move. Where possible, corridors should not only move between lowland sites, but link lowland to upland sites.
By making predictions of what may happen to the Cape community of frogs, we are providing information that may be typical for any animal with limited dispersal ability, including lizards and flightless insects.
The comparative datasets for many smaller invertebrates are absent. But our results could be generalised to a much larger group of ectothermic animals that are limited in their ability to move.
Our study has shown that despite major changes in climate, as well as available habitat in the Cape, the frogs continue to survive. Their resilience is likely to require our help in the future through conservation actions that not only preserve their current breeding and foraging sites but also enable them to move into areas with suitable climate conditions.
There is hope. If the global community can stick to the recent Paris Agreement, our scenario suggests that the biotic velocity – the distance and/or time to move between the point where a population is and the nearest climatically suitable space – of the Cape frog community will be roughly double that of historical rates.
Many conservation agencies have started taking specific actions that would see many coastal and upland areas linked through areas of continuous reserves.
The study has also helped identify areas where the frogs can be monitored to assess whether climate change is likely to affect them. These include lowland areas that are already very heavily affected by humans for agriculture and living space. These areas won’t be changed back to their original state, but they could be made more tolerant of the flora and fauna that could be used as corridors. Today, frogs are reliant on the spaces that we humans make for them.