Treefrogs: Understanding species diversity
It’s been a long-standing mystery in biology. Why do some habitats have more species diversity than others? New research on the worldwide distribution of treefrogs could help answer that question. EarthSky spoke to Dr. John Wiens at the Department of Ecology and Evolution at Stony Brook University in New York. He and a team published their findings in the May, 2011 issue of Ecology Letters. Their research indicates that the degree of treefrog species diversity in a region is related to the evolutionary history of the treefrog group at that location.
For instance, Dr. Wiens said treefrogs have existed in the Amazon Basin for as long as 60 million years. That region also has a rich diversity of treefrog species. Other tropical rainforest locations, like in Central America and and northern Australia, have a relatively lower diversity of treefrog species, and at those locations, DNA data on those treefrogs show that their evolutionary history is not as deep-rooted as that of the Amazon Basin treefrogs.
EarthSky asked Dr. Wiens about the conventional thinking among biologists about why some places have higher species diversity than others.
There are literally dozens of hypotheses that have been proposed to explain why some places have higher diversity than others, as this is one of the major research questions in the field of ecology. These hypotheses involve things like area, disturbance, favorable climate, and interactions between species. The problem is that many of these hypotheses do not have very much to do with the processes that actually create new species, because those processes (like speciation) tend to be studied by evolutionary biologists instead of ecologists.
For example, many groups of organisms tend to have more species in places that are relatively warm and wet, like tropical rainforests. Flowering plants are a good example. However, a favorable climate alone cannot create new species out of thin air (or even warm humid air!). There has to be some interaction between climate and the processes that are involved in the origin of new species.
What were the inconsistencies in those conventional theories that led Wiens and his colleagues to pursue their new line of research?
What we have learned from the treefrogs is that just being in a warm, tropical climate is not enough to explain the high species richness in some tropical rainforest sites, like those in the Amazon Basin.
We find that many sites that have nice, wet, warm tropical rainforest, including rainforest sites in Australia, Central America, and northwestern South America, have the same number of species as some sites in the temperate zone.
One factor can explain the high [species] richness in the Amazon Basin, but also the low [species] richness in these other tropical sites: the amount of time that hylids have been present in each region. There are lots of species in the Amazon Basin because many different groups of treefrogs have been there for so long. [However, treefrogs] have colonized these other [tropical] regions more recently. These recently colonized regions have had much less time to build up species richness, even though they may have wet tropical rainforest like in the Amazon Basin.
EarthSky asked Wiens why the richness of species diversity at a particular geographic location was dependent on how long that species group had been established there.
The idea that a group will have more species in a place that it has inhabited longer is really the simplest explanation for species richness patterns. Unless some regions tend to have a higher rate of speciation or extinction than other regions, then you should always expect more species in the region that has been colonized longer.
Species seem to arise most frequently when their geographic ranges are split into two, and this seems to happen every few million years or so. A region that has had treefrogs present and speciating for 50 million years should have many more treefrog species than one that has only had treefrogs for 10 million years. In some ways, this importance of time seems pretty obvious. But surprisingly, time has really fallen out of favor as a potential explanation for species diversity patterns among ecologists in recent decades, so much so that most studies do not even bother to test it.
In fact, this idea that time is important is not really a new idea at all. But what we have now shown with our study of treefrogs is that the importance of time applies not only to large-scale geographic regions (like continents) but also to much smaller spatial scales, like sites that are only a few kilometers in size.
To test their theory that treefrog species diversity in a region was related to the treefrogs’ evolutionary history at that region, Wiens and his colleagues compiled data on the population sizes of treefrog species at 123 sites around the world. Treefrogs, Wiens explained, are a particularly important group to study because they make up almost half of all amphibian species in some rainforest sites.
The team analyzed data on species richness by constructing a new treefrog “evolutionary tree,” – a branching diagram showing evolutionary relationships of different species based on DNA sequence data for 360 treefrog species. This analysis revealed, for different sites around the world, how long a group of species had been established at a particular location.
How is DNA sequence data used to create an evolutionary tree and establish ages for species groups? Said Wiens,
To make a long and complex story short and more-or-less simple, the mutations in the DNA sequence of a given gene can tell us how species are related to each other and how long ago they diverged. For example, if two species share a mutation in a given gene that no other species have, then this indicates that they are likely to be each others’ closest relatives.
By combining information from many different mutations in many different genes, we can reconstruct an evolutionary tree for hundreds of species, such as the one that we reconstructed for treefrogs. Also, we know that there is a general relationship between how long ago species have split from each other and how many mutational differences there are between them – there are few differences between species that split recently, and many between those that split tens or hundreds of millions of years ago. Thus, there is a general molecular clock, and we can now use sophisticated statistical methods to account for the fact that this clock does not keep perfect time.
There has been much news recently about how amphibians are declining at a record rate around the world. Did this have an impact on the data used by the team? And how does this decline affect diversity at different locations around the world?
Many of our estimates are based on literature records that predate these recent declines.
I have seen first hand that habitat destruction and diseases (like chytrid fungus) can wipe out the majority of species at a given site. So, to give you one striking example, the site that we discuss with the highest treefrog diversity in the Amazon Basin (Santa Cecilia, Ecuador) was already deforested when I visited it over 20 years ago! When I visited there in the early 1990s, we could find only a fraction of the species that should have been present. It was heartbreaking. Fortunately, there are other sites that still have high diversity, but these will not last forever if deforestation in the western Amazon does not stop.
Earthsky asked Wiens, assuming the treefrog habitat range remains undisturbed, are populations in high species diversity locations more likely to overcome the stressors that are implicated in the amphibians’ demise?
.As long as the habitat remains intact, then I think that the biggest challenges to survival that frog species now face are chytrid fungus infection and global warming. Chytrid fungus has been most destructive to frog diversity at higher elevations in tropical Central America and South America. So, many of the high-diversity lowland sites in the Amazon Basin may be fine.
What will happen in the face of global warming is still uncertain, and frog species all around the world might be in danger from this. It is a very scary prospect.
I do not know of any evidence that populations of species in more diverse sites are more tolerant to stress, but it is an interesting idea.
Can the methods used to analyze treefrog data be generally applied to similar data on other organisms?
Absolutely. The methods can be applied to almost any group of organisms. The critical pieces of information needed are (for a given group of organisms) the local diversity at several sites across several regions, and having a detailed evolutionary tree with divergence times. These should already be available for dozens of groups of organisms.
EarthSky asked Dr. Wiens about future plans in pursuing this line of research.
A lot of the research that I do, either by myself or with my students, postdocs, and other collaborators, centers around the questions of why some locations have more species than others.
Some of the projects that I am most excited about currently involve trying to understand the species diversity of deserts, and why there are so many more species on land than in the ocean.
I am also looking forward to doing more detailed studies on the patterns of diversity in treefrogs, and the mechanisms and processes that have created these patterns. Finally, I am continuing to try and understand why there are more species in the tropics. It is a tough nut to crack!
It’s never been entirely clear why some regions have richer species diversity than others. John Wiens, a scientist at Stony Brook University in New York and his colleagues took a fresh look at the problem, by analyzing data on 360 treefrogs in 123 locations around the world. Their findings, published in the May, 2011 issue of Ecology Letters, show that for treefrogs, richness of species diversity in a region is related to how long a species group has been established in that area. The techniques that Wiens and his team developed for the treefrog analysis can also be applied to study other types of animals as well as plants.
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