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.

All About Anoles

The second general meeting of the FIU Ecology club GLADES featured a reptilian guest-the Anole. This week graduate student James Stroud of the Feeley lab was generous enough to host an interactive workshop with GLADES members on anoles. It began with a short presentation on the species found at our university that make up our unique anole community. James explained how to identify these anoles based on their most obvious physical characteristics and on their most common habitat preferences.

GLADES members learning about common South Florida anole species.

GLADES members learning about common South Florida anole species.

After the presentation, the fun began! Our group headed outside armed with lizard “nooses” to look for any anoles around campus. With our newly acquired knowledge of anole ecology and identification we all had a sharp eye ready for any small movement along a tree, post, wall, or on the ground. The day had a slight case of rain, which meant the anoles were hiding from us while we walked around campus getting rained on. As budding ecologists, this didn’t stop us.

On the search for anoles.

On the search for anoles.

With some careful eyes, we spotted one anole in a young slash pine tree in the Nature Preserve and used a lizard noose to catch it. This was a great opportunity for GLADES members to try their hand at lizard catching and identification. Later on in the workshop every member had a chance to catch an anole on their own. Anoles are tough little critters, so they didn’t mind being held in order for members to get a better look at their unique features.

The brown anole (Anolis sagrei).

Anoles really are prevalent here in south Florida. They can be found all over campus, in your backyard, and even in your boots (this happened to me once!) Having grown up in Miami, I recall as a child catching these little lizards left and right and would anger them just enough to get them to bite my fingers (and sometimes my earlobes to wear them as earrings….according to James this seems to be a Miami-only phenomenon).

GLADES members had a unique experience at this workshop, and from it took more nuggets of information which they can utilize to enhance their continuing education in ecology.

An Afternoon with Tropical Plants

The student members of Florida International University’s ecology club GLADES enjoyed an afternoon learning how to identify common woody plant families of the Neotropics earlier this month. We were pleased to have Dr. Scott Zona of the Department of Biological Sciences and Curator of the Wertheim Conservatory here at FIU leading the workshop. GLADES members met at the north entrance of the FIU Nature Preserve where Dr. Zona began with a short introduction on tropical plant families.

GLADES members with Dr. Scott Zona

GLADES members with Dr. Scott Zona

Dr. Zona provided for each student a packet that listed each of the 10 plant families students were going to encounter during the workshop and included information such as the number of genera and species in each family, special identification features, leaf type, flower type, fruit type, and a familiar species example. The plant family list included: Arecaceae, Annonaceae, Bignoniaceae, Euphorbeaceae, Fabaceae,Lauraceae,  Moraceae, Myrtaceae,Sapotaceae, and Rubiaceae.

GLADES members during the identification workshop.

GLADES members during the identification workshop.

This workshop was the first of the semester for GLADES. As the founding president of this truly unique student organization, I was moved by both the presence and engagement of the members during the workshop. What students took home from this experience was a new-found skill in plant identification with a complementary botanical vocabulary, and an appreciation for the diversity of plant life found at our university. At one point during the workshop, Dr. Zona quizzed our group on identifying a woody plant to its family. With our new skills and a trusty hand lens, we easily identified it was in the Rubiaceae (opposite leaves and interpetiolary stipules!). All of the amazing photos were taken by our GLADES Webmaster, Ashley Lambert. I look forward with excitement for what the rest of the semester has in store for GLADES. 

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the start of a new blog

5799409117_2d9273858b_zThe Tropical Ecology and Conservation (TREC) research group at Florida International University (FIU, Miami USA) has decided to try our hands at blogging.  The blog will focus on sharing ideas and information related to our research on the effects of climate change on tropical and subtropical systems.  Posts will primarily be regurgitations of new papers that we find to be particularly interesting or noteworthy, musing about our lives as undergrads, grad students, postdocs and faculty members, and general commentary. More information about who we are and what we do can be found at: http://www2.fiu.edu/~kfeeley/

–Ken Feeley