What explains persistent species' rarity in communities?

Someone asked me what is the most important or lingering issue in community ecology recently. (There’s probably a whole post to answer that question (to come...)). One answer is the mystery of species coexistence: for more than 50 years (from Hutchinson’s paradox of the plankton through today) we have tried to explain the immense and variable diversity on earth by understanding what allows two or more species to coexist. There are many ways to explain coexistence, and yet the details and the specifics for any given system are also still usually incompletely understood.

A good and fascinating example is that of persistent rarity. Why are so many species in communities rare? What allows species to remain rare for long periods of time, given that small populations should be at greater risk for stochastic extinction? A new preprint from Yenni et al. (1) considers the empirical evidence for one potential explanation for persistent rarity: asymmetric negative frequency dependence (see also Yenni et al. 2012 (2)).

Coexistence theory (Chesson 2000) considers stabilizing mechanisms to be those that allow intraspecific competition to be greater than interspecific competition (often defined as ‘niche’ mechanisms). The strength of such stabilizing mechanisms can be estimated by looking at how a species’ population growth rate is limited by the frequency of conspecifics compared to the frequency of heterospecifics in the community. Negative frequency dependence is expected when stabilizing mechanisms are strong. This allows species to increase when rare, since limitation by conspecifics is low, followed by a decline in growth rates as conspecific frequency increases.

Asymmetric negative frequency dependence may explain persistent rarity, since it suggests especially strong conspecific limitation. As a species’ frequency increases, their growth rate greatly declines and intraspecific interactions, rather than interspecific competition, determine abundances. Species are rare, but also less likely to experience extinctions through competition with other species. The authors suggest that as a result of this, we should expect rare species to have stronger negative frequency dependence, in comparison to more common species. They look for evidence for asymmetric frequency dependence using data from 148 communities collected across multiple taxonomic groups (birds, fish, herpetofauna, invertebrates, mammals, and plants), 5 continents, and 3 trophic levels. The data represented time series of species abundances, which the authors used to estimate negative frequency dependence as the relationship between a species’ frequency in the community and their annual per capita population growth rate.

Several aspects of the results are particularly interesting. First, the authors had to omit rare species that are not persistent, since other processes likely explain the presence of such ephemeral members of communities. The frequency of ephemeral species (not stably coexisting at a local scale), for example, was quite high, particularly in plant communities (average of 82 species per community, of which only 22.6 species were on average identified as ‘persistent’). This may suggest the importance of spatial mechanisms for coexistence or co-occurrence. Their overall prediction of stronger negative frequency dependence in rare species appeared to holds in 46% of the communities they examined, consistently for all of the taxonomic groups but one (herps!). Additionally, the opposite pattern (common species having stronger negative frequency dependence) was never observed.

Rarity in nature is common :-) but not well predicted using most coexistence theory. Many interesting and important questions arise from it, and from results like those shown in Yanni et al. For example, do rare species have rare traits or rare niches? Is the frequency dependent growth rate context dependent (i.e. can a species be strongly limited by conspecifics in one environment but not another)?

*Note I haven’t reproduced any figures here, since this is a preprint. However, it is openly available, so do have a look (link 1 below). I’m not certain if there is a rule of thumb on blogging about preprints, but I imagine it is much like blogging about conference talks. The work may not have been peer reviewed/published yet, but the broad results and ideas remain interesting to discuss.

References:

1. Glenda Yenni, Peter Adler, Morgan Ernest. Do persistent rare species experience stronger negative frequency dependence than common species? doi: http://dx.doi.org/10.1101/040360. Preprint.

2. Yenni, Glenda, Peter B. Adler, and S. K. Ernest. "Strong self‐limitation promotes the persistence of rare species." Ecology 93.3 (2012): 456-461.

A bird in the hand… Worth a bunch in the bush?

Guest post by University of Toronto-Scarborough Masters of Environmental Science Student Amica Ferras

     In less than a week, Christopher Filardi achieved a level of cyber-fame worthy of this digital age— but for all the wrong reasons. If you haven’t heard of him yet, that’s okay. Not all of us peruse biodiversity articles over our morning cereal. Here’s what you’ll need to know to hold your own around the water cooler.

Photo: University of Kansas

Christopher Filardi is the director of Pacific Programs at the American Museum of Natural History’s Center for Biodiversity and Conservation. This past September he and his team were part of an international expedition to the mountains of Guadalcanal, one of the islands in the Solomon Archipelago. Lead by native islanders, the team was on a mission to assess the biodiversity and habitat constraints of this unique region in order to develop a tailored conservation strategy. It was there on those mysterious island mountains that Filardi happened upon a true legend by any biology geek’s standards — the Guadalcanal Moustached Kingfisher. Even if you have zero interest in species biology, the stats on this bird are impressive. Only three sightings of the Kingfisher have been documented in all of history: a single female captured in the 1920’s, and another two in the 1950’s. No male specimen had ever been recorded and no live animal had ever been photographed. This bird can play a mean game of Hide-and-Go-Seek.

Upon discovery of the Kingfisher colony, Filardi and his team set to work. Calls were recorded, habitat was meticulously documented, behavior and motion patterns were scrutinized and population dynamics were assessed. And then, they killed one. (Cue the angry villagers with pitchforks and hippies with signs).

The collection was purely scientific. Filardi and his team stuck to a field biology motto of collect, dissect, but ultimately respect. Filardi hoped that the Kingfisher specimen would open the door to discovering more about the elusive species and their ultra-specific habitat. But the road to media-hell is paved with good intentions, and as the story spread like wildfire Filardi’s actions fell under attack. His ‘collection’ was deemed “perverse, cruel” by a representative from PETA to the Daily News, and the UK online Daily Mail described it as “slaughter”. The story exploded, appearing in the Huffington Post, Washington Post, Nature World News and Audubon, just to name a few. For those links and more I suggest checking the wonderful world of Google, but I will personally recommend that you read Fildari’s self-defense in Audubon https://www.audubon.org/news/why-i-collected-moustached-kingfisher, and the Toronto Star’s coverage of the controversy http://www.thestar.com/news/insight/2015/10/17/why-a-scientist-killed-a-bird-that-hadnt-been-seen-in-50-years.html. The Star does a fabulous job of presenting both sides of the story, and also goes into detail about the rather dubious past of field biology.

In the 1700’s and 1800’s specimen collection was more sport than science. It was a my-stuffed-animal-carcass-is-bigger-than-your-stuffed-carcass race, and rare species paid the ultimate price. Great Auks, for example, upon classification as endangered in 1775, were hunted at an alarming rate by naturalists attracted to its rareness. In 1884 a final pair of Auks was caught by fishermen, and no Auk has ever been sighted since. Specimen collection has come a long way since then though, and field biology has contributed to some groundbreaking scientific discoveries. Consider eggs— comparisons of eggshell thickness from samples collected across decades was used to identify the detrimental effects of DDT and other pesticides to natural ecosystems.

So, those are the facts. And my opinion about it? I’m siding with Filardi. Science has come a long way from naturalist trophy hunting in the 1800’s. Nowadays, before even setting foot outside of the lab scientists must undergo a rigorous evaluation process to determine if collection permits will be granted. Cost-benefit analyses, potential outcomes, and fragility of a species and ecosystem are all heavily weighted in before a decision is reached. Filardi’s expedition was no exception to this rule. (And for anyone questioning the usefulness of collections at all, I suggest you read the following article http://biology.unm.edu/Witt/pub_files/Science-2014-Rocha-814-5.pdf. I’d be happy to argue with you on that front another day).

It wasn’t as if Filardi saw the Kingfisher, pulled a net out of his pack and started swinging. After discovering the Kingfisher colony, the bird was carefully observed over several days. Input from the native islanders, assessments of habitat resilience and population robustness were all carefully analyzed before deciding to humanely collect the single male specimen. The unwilling sacrifice of the Kingfisher was honorably recognized, and the collection will be worthwhile if Filardi has anything to do with it. Scientists now have access to a complete set of genetic information for the Kingfisher. It will now be possible to undertake full molecular, toxicological and evolutionary diagnostics. Scientists may discover disease and pollutant susceptibilities that will guide Kingfisher protection efforts, or identify a direct evolutionary pressure to explain the appearance or behavior of the birds. At a more macro level, the specimen could reveal a shared trait between all high-elevation avian species or allow for an assessment of the particular environmental pressures the island ecosystem exerts over its inhabitants.  

Remember though, the point of the Guadalcanal expedition was not a Kingfisher hunt, but an internationally commissioned excursion to study the biodiversity and ecosystem threats in the Solomon Archipelago. Working with native islanders and Solomon government officials, Filardi’s team was working to establish a conservation strategy to protect the unique island system. The Pacific Island tribes have tended to their mountainous lands for decades, but recent international development has threatened the natural state of the ecosystem. Intensive mining and logging ventures have already begun transforming the lowlands of the islands, and climate change at large is effecting the delicate balance of ocean and forest features that unique species like the Kingfisher rely on. For species limited to a single isolated habitat, even minor changes in soil pH, precipitation or fluid motility can have astronomical effects on species survival. These are not the resilient squirrels and raccoons we in North America watch thrive everywhere from lush forests to derelict urban alleyways. Filardi’s collection will go a long way in identifying what needs to be done to protect these habitat-specific island species.

In fact, it already has. Discovery of the Kingfisher led Filardi to talks with local tribes and the Solomon government which culminated in formal agreements to protect the island mountain region under the recently passed Protected Areas Act. Filardi has already booked a return flight to Guadalcanal to help negotiate the next steps in this exciting conservation effort.

So, what do you think?