New ways to address an old idea: rethinking the regional species pool

Like many concepts in ecology (metacommunity, community), the idea of a regional species pool is useful, makes conceptual sense, and is incredibly difficult to apply to real data. Originally, the idea of a species pool came from the theory of island biogeography (MacArthur and Wilson, 1967), where it referred to all the species that could disperse to an island. Today, the regional species pool appears frequently, across null models, studies of community assembly both empirical and theoretical, and metacommunity theory. 

Understanding how particular processes shape community membership—whether the environmental, competition, or dispersal limitation—depends on knowing the identity of all the species that could have potentially assembled there. The species pool as defined by the research provides the frame of reference against which to consider a community's composition. Most null models of community assembly rely on correctly identifying this set of species, and worse, tend to be very sensitive to bias in how the regional pool is defined. If you include all species physically present in a region, in your species pool, environmental filtering may appear to be particularly important simply because many of those species can’t actually survive in your community (the narcissus effect). Given the importance of null models to community ecology, defining the species pool appropriately is an ongoing concern.

There are many decisions that can be made when asking 'which species could potentially be members of a community'? You could include all species that can physically arrive at a site (so only dispersal or geographic distance limits membership), or only include those species that can both arrive and establish (both dispersal and environmental conditions limit membership). Further, the availability of data is key: if you use observational data used to determine the environmental limitations, you may also incorporate the outcome of biotic interactions indirectly. If some species are rare and have low observation likelihoods, they will be under-represented. Abundances may be useful but inaccurate depending on how they are measured. Finally, it is common to define species as either present or not present for a species pool; this binary approach may conceal ecologically important information.

The 'filtering' heuristic for understanding community membership. Species groups 1-3 could each be defined as a regional species pool, depending on the definition applied.

A number of recent papers provide alternative approaches to constructing species pools, meant to avoid these pitfalls. Researchers can define multiple contrasting species pools, each pool representing an ecological process (or perhaps multiple processes) of interest. Each species pool can be modified further to reflect the strength of a particular process in constraining membership. The regional pool is not seen as a single entity but as a number of possible configurations whose utility is in comparison.

Lessard et al. (2016) illustrates how to produce this kind of process-based species pool with various constraints (figure below). Their three-step approach is to:

1. Define absolutely all possible members of regional pool. This is determined by identifying all assemblages in the region containing at least one species also found in the focal community (creating a 'dispersion field') (figure below, section A). This delineates a large region and identifies all species within it.
2. Calculate the probability of resampling a species from the focal community elsewhere in the dispersion field. This is done in the context of the process of interest. For example, the probability of observing a species in the focal community and another community might be determined based on the geographical or environmental distance between those sites. Every site in the dispersion field would now have a probability (or distance really) associated with it, representing some similarity with the focal site.
3. Finally, apply constraints to the calculated probabilities. You might choose to consider only the species within communities that are at least 50% similar to the focal community, for example. Such constraints would reflect the strength or importance of filtering by the process of interest.

Another recent paper (Karger et al., 2016) takes an approach with a number of commonalities to the Lessard et al. method. However, rather than resampling to produce potential pools of species (with species being defined as present or absent), they advocate a probabilistic approach to species pools. They suggest that species pools should be thought of as a set of probabilities of membership, which may be more reflective of ecological reality. In some ways, this is a simply a formalization of probabilistic sampling from Lessard, but instead of applying constraints, the researcher acknowledges that probabilities vary for different species. “Hence, a species pool can simply be defined as a function of probabilities of a species’ occurrence in the focal unit given the unit’s environmental and biotic conditions, geographical location and the time frame of interest”.

Both comparative and probabilistic approaches to defining species are logical advances, and one way of dealing with the untidy concept of the species pool. If this topic is of interest, a few other papers, albeit slightly less recent, are definitely worth reading: Pigot and Etienne 2015; Lessard et al. 2012, Carsten et al., 2013.

From Lessard et al., 2016. The three steps to build a species pool.

Reining in traffic –looking to China for solutions?

Human impacts on landscapes are immense. Urban areas represent complete transformations of the geological, hydrological and ecological norms in landscapes. But while urban effects are concentrated to relatively small areas, the roads and rail lines feeding cities create a pervasive and diffuse network of negative impacts. Roads funnel rain runoff and can cause local flooding and this runoff also concentrates pollutants. Further, roads alter wildlife movement. For example, the fragmentation of formerly continuous forest in Florida is worsened by large busy roads, and black bears there are unable to move long distances to find mates. The result of this is that the Florida Black bear populations are getting smaller and more genetically inbreed.

Roads are created to meet traffic demands. The more people drive and the further they drive, the more roads we build. Cities around the world are growing, meaning that more cars are concentrated in small areas. The increase in automobile use also has direct environmental consequences. Cars, thanks to their internal combustion engines, add pollution to our local environments –carbon monoxide, particulate matter, and other toxins create smog, exacerbate respiratory ailments, and contribute to global warming.

More cars also means more traffic congestion and greater difficulty in getting from A to B, meaning that we spend more time travelling to, instead of being, somewhere. Heavy reliance on automobiles directly affects our quality life in both positive and negative ways.

1950s traffic jam in Los Angeles (from Wikipedia)

Given the undesirable consequences of cars, many cities try to reduce car use. In North America, cities employ a number of strategies, including: minor improvements to public transit (while often passing on the costs to riders), creating car free zones (which have been very modest in North America, whereas European cities have been much more successful –Montpellier, France is a great example), introducing tolls, and limiting parking in the city core. It is safe to say that the North American approach to dealing with traffic has been less than spectacular –just drive through Toronto or Los Angeles during rush hour.

Living in China for the past several months, I have been intrigued by how Chinese jurisdictions have dealt with traffic. And traffic was something that needed dealing with here. In the late 1990s and early 2000s, thousands of new cars were added to roads every single day.  The air quality in China is abysmal and having hundreds of millions of cars driving at the same time only make things worse. So governments in China decided to experiment with ways to reduce automobile use.

In China, much of the power to control automobiles resides with municipalities –they are the ones who set local traffic laws and issue license plates. From conversations with scientists from different regions of China, I have compiled ways different municipalities deal with traffic and reduce automobile use. Here are some of the ways that municipalities try to reduce automobile traffic:

1) Massive investments in public transit

There can be no real traffic solutions without building fast, efficient and affordable public transit. China has been a world leader in public infrastructure development over the past ten years. For example, Shanghai has one of the largest metro systems in the world, and has opened a new line every other year since 1999! They are currently building two new lines, which will give Shanghai 18 metro lines and about 400 stations. In Guangzhou, where I currently live, they also have a very modern and rapidly expanding metro system. Guangzhou currently has 8 lines with 4 more under construction! In all the Chinese cities I’ve been in, the metro systems are modern, efficient, heavily used, and very affordable. In Guangzhou, a bus ride works out to be about 35 cents US and a metro trip to the airport (the longest trip you can take in Guangzhou I believe) is about $1.15 US.

In Toronto, where I normally live, and like most other large North American cities, subway construction has not been sufficient to keep up with population growth. Local politicians seem to be unable to make the tough decisions to get public transit infrastructure built. But this infrastructure is the linchpin for any successful reduction in automobile usage.

2) Driving days

During the 2008 summer Olympics, Beijing created a system where cars were allowed on the roads only on certain days. Which days people could drive their cars depended on the last number of their license plates. This scheme was successful in reducing traffic congestion and air pollution. Since then, they have periodically reinstated this policy, especially during extremely bad air pollution days. I was there in early December, and road sharing was in effect then.

3) Making license plates really, really expensive (or difficult to get).

In Guangzhou, Beijing, and Shanghai, getting a car is easy, but getting a license plate, now there is the real hurdle. Since 2012, Guangzhou and other cities have severely limited the number of license plates issued, and now people can get a plate in one of two ways in these cities: by joining a lottery or going to an auction. In the lottery, a person submits an application and waits for the results. One person told me it took them three years to get their plate in the lottery. In the auction, the plates go to the highest bidder and the price for a license plate at auction has sky rocketed. A person told me that plates at auction now go for more than 60,000 RMB (about  $10,000 USD), which costs more than an economy car here! This person also quipped that the plates have become more of a status symbol than the actual car.

4) Your license plate will die

In Guangzhou and other cities, license plates expire. No, not like they expire in North America where you pay an annual license fee. They expire after 10 years and are no longer valid, and the driver must re-enter the lottery. 

5) Pay the toll

Many of the intercity highways have tolls here. While this is not a policy that affects travel behaviour within cities, it does influence driver choices traveling outside the city. Tolls only work when there are decent alternatives, and the rail system in China is excellent. There are frequent trains and many high speed lines in operation (where the trains go faster than 250 km/h). We don't have many toll roads in Ontario, but the one we have near Toronto, hwy 407, doesn't go into the city (so doesn't influence commuter decisions), and does not have viable options for alternative travel. This highway is an example of poor government policy and it was one of the worst policy decisions by a government who thought private companies should run public infrastructure. Its nothing more than a cash grab that doesn't serve the broader good. But I digress.

I have been struck by the variety of approaches and the experimental nature of policy making. What I mean by experimental, is that some policies seem to be ‘test run’ to see how people respond and if the policies result in the desired effects. China is able to institute creative and extreme measures because of the government’s unique ability to change policy without public debate. Often these policies are instituted overnight with little warning. In China, people seem to take government edicts with a “well, what can you do?” attitude. But if there is a country that can change the automobile culture, China is a good candidate. They did change what a family was with the one-child policy.

While most North Americans would certainly have a problem with the lack of transparency and seemingly impulsive nature of government decisions, China is providing the world with working examples of how to reduce the number of automobiles. It is clear to most, that without strong governmental leadership, a robust set of policies, and massive infrastructure investment, heavy automobile traffic will be unavoidable.