The global conservation status of many widespread and rare tropical tree species remains uncertain

Below is a copy of recent commentary that I published in Frontiers of Ecology and Evolution based on the article by ter Steege et al on “Estimating the global conservation status of more than 15,000 Amazonian tree species“.

Amazonian forests provide ecosystem services that are critical at the planetary scale. Unfortunately, human land use threatens to drive many rainforest species to extinction. In a recent study, ter Steege et al. (2015) provide valuable insight into the threats that current and future deforestation potentially pose for Amazonian tree species. In any such large-scale analysis dealing with thousands of poorly-known species, there are clearly going to be many assumptions and possible sources of uncertainty. Here, I highlight two major assumptions used by ter Steege et al. (2015) to simplify their analyses—namely in the handling of widespread species and rare species. These assumptions have the potential to strongly influence predictions of how many and which species are at risk of being lost to deforestation over the coming decades.

Some tree species are likely to be endemic to the lowland Amazon; however, there are also certain to be many species that have ranges extending to higher elevations, different ecoregions, or even different continents. While ter Steege et al. perfunctorily acknowledge (in their online Supplemental Material) the potential problems caused by widespread tree species with geographic ranges extending beyond the defined Amazonian study area, they make no attempt to quantify how pervasive of a problem this may be or to account for it in any of their analyses. Rather, ter Steege et al. assume that rates and patterns of deforestation outside the Amazon mirror those occurring inside the Amazon. This goes against the core proposition of the study that spatial patterns of species’ distributions, population densities, and the rates of deforestation, all combine in determining the degree to which species are threatened by habitat loss.

To get a sense of how many species may have ranges extending beyond the Amazon, I mapped the locations where Amazonian tree species are known to occur based on their herbarium collections records. More specifically, I downloaded all georeferenced occurrence records available through the Global Biodiversity Information Facility (GBIF; http://www.gbif.org/) for the nearly 5000 Amazonian tree species occurring in the Amazon Tree Diversity Network’s (ATDN;http://atdn.myspecies.info/) forest plots and queried how many of these “common” species have recorded occurrences outside of the Amazon. I found that the vast majority (81%) of species have ≥1 occurrence outside the defined study region, one-fourth of the species have ≥50% of their occurrences outside the study region, and one-tenth of species have >90% of their occurrences outside the study region. Even if these extra-Amazonian populations are in some cases cryptic species, it is clear that many, if not most, Amazonian tree species are not actually endemic to the Amazon. For at least these widespread species, the data and methods employed by ter Steege et al. (2015) are insufficient to accurately estimate their true “global conservation status.”

In the case of rare species, there are believed to be ~11,000 Amazonian tree species (i.e., ~2/3 of total Amazon tree diversity) that are too rare to occur in any of the ATDN’s networked inventory plots (ter Steege et al., 2013). ter Steege et al. (2013, 2015) estimated the population sizes of these rare species based on an extrapolation of a rank-abundance curve created for the common species that do occur in their plots. ter Steege et al. (2015) then estimated the range sizes for rare species by assuming a fixed relationship between population size and range size. This methods explicitly disregards the different ways that species can be rare (i.e., the classic “7 forms of rarity”;Rabinowitz, 1981) by assuming that all rare species have small geographic ranges and that no rare species have large, low-density ranges. It is difficult to test this assumption due to the inherent relationship between a species’ density and its detection probability. However, it is easy to imagine that there may exist widespread species that occur at such low densities that they are effectively “invisible” to current census techniques—especially considering that the ATDN’s plots include < 0.8 million of the nearly 400 billion trees that they estimate to be growing in the Amazon (i.e., a sampling intensity of 0.0002%; ter Steege et al., 2013, 2015). In some cases, the ATDN may get “lucky” and a widespread low-density species will occur as a singlet or small number of individuals within one of their plots. According to the methods of ter Steege et al. (2015), however, the ranges of all species occurring in only a single plot, regardless of the number of individuals, are truncated to an arbitrarily set area (e.g., < 444 km from the plot where it occurs). A clear priority for future research in tropical forests is to understand the true nature of rarity.

The handling of rare and widespread species by ter Steege et al. likely adds large uncertainties to the predicted global extinction risks of many individual species. However, it is still possible that the cumulative result, that between about 30 and 60% of Amazonian tree species are threatened with extinction due to deforestation, is valid. The same two concerns about widespread and rare species were raised in a response to a previous study by Hubbell et al. (2008) that estimated the extinction risks posed by Amazonian deforestation (Feeley and Silman, 2008). A subsequent analyses byFeeley and Silman (2009) was then attempted with the explicit goal of at least partially bypassing these assumptions through the use of occurrence records, habitat maps and estimates of deforestation rates outside the Amazon (at the same time introducing other assumptions and possible sources of errors). Feeley and Silman (2009) predicted that Amazonian plant species will lose an average of 17 or 30% percent of their ranges by 2050 under Increased-Governance or Business-As-Usual models of deforestation—estimates that are strikingly similar to the new loss rates predicted by ter Steege et al. (ter Steege et al. predict that the population sizes of common Amazonian tree species will decrease by an average of 11 or 35%). In other words, while the data, methods, assumptions, and limitations differed greatly between studies, the final predictions were accordant. If nothing else, these studies all indicate that very high numbers of Amazonian species are already, or soon will be, threatened by deforestation. Add in the largely-unexplored effects of other human disturbances such as climate change, fire, forest degradation and defaunation (Peres et al., 2010), and it is clear that no matter what the underlying assumptions, the Amazon’s future is very dire indeed.

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Evan Rehm, a former graduate student with FIU Department of Biology and ICTB (currently a postdoc at Colorado State University), and Dr. Kenneth Feeley have published a new article in the open-access journal Frontiers of Biogeography.  The article is entitled “Many species risk mountaintop extinctions long before they reach the top“.  In their article, Rehm and Feeley discuss the importance of ecotones, such as the alpline treeline, in setting current and future species’ distributions.  They highlight the fact that many species’ range boudaries are set by ecotones and that these ecotones may not shift concurrently with climate change – potentially resulting in rapid range compressions and elevated extinction risks.

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Fall 2016 in Colombia

Ken Feeley has been awarded a Fulbright Research Fellowship.  With the support of the Fulbright Fellowship, Ken will spend the Fall 2016 semester on a sabbatical leave at the National University of Colombia in Medellin working on a collaborative study with Dr. Alvaro Duque. Ken’s research will build off of previous work that he and Alvaro conducted looking at climate-driven changes in the composition of Andean forests (published in the Proceedings of the National Academy of Sciences: http://www.pnas.org/content/112/34/10744.abstract).  Ken and Alvaro will now look at how the observed changes in composition relate to species’ functional traits and climatic tolerances.

transplants help to reveal the complex factors setting high Andean treeline

Evan Rehm, a former biology grad student at FIU, and Ken Feeley have published a new article in the journal Oecologia entitled “Seedling transplants reveal species-specific responses of high-elevation tropical treeline trees to climate change” (http://link.springer.com/article/10.1007/s00442-016-3619-0).  The article is based on a study transplanting seedling of high-elevation Andean tree species across an elevational gradiant and under experimental heating and shading treatments. The study revealed that tree species responded differently to the environmental manipulations and that warming decreased survivorship in the most common species (Weinmannia fagaroides).  These results highlight the need for species specific studies and models to predict the effects of climate change in this biodiversity hotspot.

 

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Thermal trouble in the tropics

Tim Perez, James Stroud and Ken Feeley published a new Perspectives article in Science Magazine entitled “Thermal  Trouble in the Tropics”.  The article explains why tropical species are at extra risk of extinction under climate change compared to their temperate counterparts.  They also discuss the need for additional studies of tropical plant species and their climatic tolerances. A copy of the article is available HERE.

Ken Feeley discussed the article during a radio interview with John Batchelor.  A podcast of the interview is available to download or listen to HERE.

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Priority effects in a changing climate

A new publication from the Feeley Lab in Frontiers in Ecology and Evolution addresses the importance of priority effects on species range shift. The publication is a comment to a paper published in Nature by Alexander et al (2015).

Commentary: Novel competitors shape species´ responses to climate change

Belen Fadrique and  Kenneth Feeley

There is a growing appreciation of the need to understand the effects of climate change on species interactions and how changes in interactions can influence the ability of species to persist in the face of climate change (Araújo and Luoto, 2007; Thuiller et al., 2008; Svenning et al., 2014). However, empirical or experimental studies investigating species interactions under climate change remain extremely scarce. Alexander et al. (2015) use experimental transplants of European alpine plant species and communities to provide valuable insight into some of the novel competitive interactions that may emerge as species migrate upslope to keep pace with rising temperatures. More specifically, they look at performance of plant species under simulated upslope migrations into preexisting higher-elevation plant communities as well as the performance of plant species that fail to migrate and find themselves competing with new suites of species migrating into their community from below. This is a useful approximation of some of the scenarios that are already being created by the unequal responses of species to climate and the creation of novel communities.

One limitation of the study by Alexander et al. (2015) is the omission of the earliest phases of establishment when processes such as dispersal and germination are crucial in the encroachment of initial populations of migrant species into the new locations (Hampe, 2011). In particular, the experimental set up fails to account for one of the potentially most important drivers of community assembly—priority effects. Priority effects refer to the observation that early colonists will often inhibit, or alternatively facilitate, the establishment of subsequent colonizers (Connell and Slatyer, 1977).

Keep on reading HERE.

OTS leftovers: Does leaf pH influence herbivory?

My research focuses on how species have adapted to environmental variation and how these adaptations influence species’ niche breadths and geographic distributions. Although IUntitled focus on tropical plants, and insect herbivores are a substantial biotic selection pressure, herbivory is not a topic I actively pursued until I participated in an OTS course last summer. On my OTS course I was able to investigate an array of topics from plant defense to plant-mediated tri-trophic interactions, all of which are documented in our 2015 OTS coursebookOf all the projects I participated in, the last project of the course remains my favorite because it has provoked new questions, some of which have informed my thesis research.

In this last project, my classmate and I were following-up a previous study in which we investigated the ability of leaf pH to predict herbivory in plants. Literature suggested that low pH values deter herbivores in the sub-arctic where ungulates, not insects, are the dominant herbivores. Consequently, the influence of leaf pH on herbivory in tropical to sub-tropical regions where insects are the dominant herbivores is unclear. In our first project, my classmate and I found no effect of leaf pH on standing herbivore damage in the 5 Piper species we measured. For our follow-up project we were interested in determining the sources of variation that may have influenced leaf pH, and thus, the results of our previous study. Ultimately, we decided to investigate diurnal and ontogenic changes in leaf pH for two Piper species. 

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Figure 1: Ontogenic changes in leaf pH of P. multiplinervum & P. hispidum

Interestingly, our immature leaves exhibited the lowest pH values (Fig 1), and immature leaves have previously been shown to be the most palatable to insect herbivores. Compared to mature leaves, immature leaves tend to receive more insect herbivory because they are softer, are higher in nutrients, but also produce more chemical defense compounds. Therefore, leaf acidity may be correlated with leaf age and linked to chemical defenses that deter herbivores. If so, leaf pH could be an easily-measured trait used to understand the susceptibility of plants to insect herbivory.

In addition to ontogeny, leaves also exhibited diurnal changes in their chemistry (Fig 2). We found that both immature and mature leaves that were measured in the morning (~9am) had lower pHs than leaves measured in the the afternoon (~4pm). These diurnal fluctuations may have resulted from changes in carbonic acid and cytoplasmic proton gradients throughout the day. However, given the potential relationship of pH to chemical defenses, it is easy to speculate that some chemical defenses could fluctuate throughout the day in addition to ontogenic phase. 

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Figure 2: Diurnal changes in leaf pH of P. multiplinervum & P. hispidum

Since our first study did not properly measure herbivore damage, the effect of pH on insect herbivory remains an unanswered question. The observations from our follow-up study indicate pH is probably correlated with ontogeny, which has been shown to influence the production of chemical defenses that deter herbivory. pH was also observed to fluctuate diurnally, and may indicate diurnal changes in herbivore deterrence.

What I think is exciting about these results is that if pH does influence herbivory, insects may feed upon leaves according to temporal differences in leaf chemistry. In turn, these short-term temporal feeding preferences could promote co-existence of insect species, which greatly outnumber plant species, and are likely to overlap in diet-breadth. There are many unanswered questions that this project has induced that warrant further investigation before any concrete conclusions can be made about the influence of pH on insect herbivory. 

My uncharacteristic foray into plant-insect interactions has led me to a new understanding of niche-breadth and coexistence theories through concepts such as the storage effect that incorporate temporal niche partitioning – a topic of great relevance to my thesis. Moreover, my newfound interest in herbivory highlights the benefit of stepping outside the territory of your own research in order to gain a better understanding of it. 

Now that I am in Florida, where Pipers are introduced and rare,  I’ve been
gathering information on the native species Psychotria nervosa to address some of the questions I developed while on my OTS course. I’m currently collecting and identifying insects that feed on P. nervosa, and have already quantified ontogenic changes in leaf nutrients. As my course-load dwindles and the resources at the International Center for Tropical Botany 
become available, I hope to dedicate more time to understanding how leaf chemistry influences insect herbivory in the sub-tropics as a side-project…so stay tuned.

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Me (Tim Perez) happily collecting Piper cenocladum at La Selva Biological Station, Costa Rica.