Planning Habitat Restoration: quantity, quality and context

A seemingly isolated pond in the White Mountains. Photo Credit: Alena Warren

A seemingly isolated pond in the White Mountains. Photo Credit: Alena Warren

Habitat loss is often to blame when wild animal populations are declining. When we think about habitat loss, we often think of quantity- how many acres of the Amazon rainforest have been burned for agriculture today, or what percent of a watershed is developed land. Quantity of habitat is certainly important when we’re talking about increasing the population of an animal species.

However, protecting species is not a simple math problem- more area doesn’t always equal more animals.

Context matters! I’d like to share a few examples to illustrate.

Sinks

Often, habitats are broken up into pieces and spread across the landscape- like ponds, islands, or forest that is divided by roads and towns. In these cases, we can talk about source and sink populations.

Imagine a large pond with many frogs. There is a lot to eat and lots of places to hide. The frogs have lots of baby frogs. There are so many frogs that many of them hop away in hopes of finding another pond. I.e., the population growth rate is high and at least some individuals emigrate from the pond to populate other ponds. This is a source population.

A pond full of happy frogs: A source population from which individuals emigrate.  

A pond full of happy frogs: A source population from which individuals emigrate.  

Some of the migrating frogs will find a new pond to live in but it might not be as nice as the one they left. It might be more like a puddle, and there’s not too much to eat, and not many places to hide. They get eaten by other animals pretty quickly. I.e., the growth rate is low and the pond frog population relies heavily on immigration. This is a sink population.

A small pond where frogs are more vulnerable: A sink population.  

A small pond where frogs are more vulnerable: A sink population.  

So when people plan habitat restoration, it’s important to think about whether the new habitat will be a source or a sink. A sink could lower the overall growth rate of the larger population, which is detrimental for a species already in decline. A sink will not help improve the genetic diversity and population stability of other nearby populations.

References and examples:

Pulliam, H. R. 1988. Sources, sinks, and population regulation. American Naturalist 132:652-661.

Large Blue Butterfly 

Disconnect

There is more to consider when planning the spatial context of new habitat. The connectivity- or lack thereof- is important too. If animals cannot cross the areas between the new habitat patch and other patches, the long term effects of genetic isolation must be considered. Also, isolation means that if the local population becomes extinct due to disease, weather, etc., the other populations will not be able to re-colonize it. Landscape connectivity has a social importance as well. Will these animals be crossing roads, and if so, will it be a safety hazard that needs to be addressed?

Crossing the road. Photo Credit: Ian Sewell.  

Crossing the road. Photo Credit: Ian Sewell.  

Examples:

Florida panther road mortalities 

Urbanization causes genetic isolation

Social and temporal context

Setting aside land for conservation is beneficial to wildlife, but if we are targeting a certain species for conservation, there are concerns other than the protection of the land from development. Does the animal have specific requirements that will require management over time? For example, plant communities will change over time and a once suitable habitat may become unsuitable without intervention. Will the area be affected by adjacent land uses in the future, like logging, development, industrial uses? Are hunting and poaching going to be problematic? Will the area require surveillance and access control?

So, context matters! In habitat restoration, more habitat isn’t the only goal. More high quality habitat in a suitable spatial context is a better goal.