Treefall gaps deliver diversity

When John Terborgh began research at Cocha Cashu Biological Station in Peru back in 1974, he probably did not expect to still be working there 43 years later, doing research and publishing papers about the astounding species diversity in its tropical floodplain rainforest.

JT_TreefallLisa Davenport

John Terborgh leans against a fallen tree that has created a gap in the forest canopy. Credit: Lisa Davenport.

One contributor to species diversity in tropical forests is treefall gaps, which form when a mature tree falls down, opening up a gap in the overhead canopy. The most obvious change associated with treefall gaps is an increase in light that reaches the canopy floor. In comparison to the closed canopy, treefall gaps may be dryer, warmer, have increased plant transpiration rates, and may host many different species that colonize the new environment.

Treefallgap Irina Skinner

Small treefall gap in a dense rainforest. Credit: Irina Skinner

While it’s clear that gaps influence the physical environment of the forest floor, it is not clear how a changed physical environment translates to biological diversity of the treefall gap community. Comparing treefall gaps to closed canopy communities, Terborgh and his colleagues explored this relationship.

First the researchers asked whether the seed rain into tree gap communities is different from the seed rain into closed canopy communities. Seed rain describes the types and abundance of seeds that are dispersed into communities. Usually seeds are blown into communities by the wind, or enter attached to the bodies or excrement of animals. Alternatively, some seeds are autochorous – self-dispersing, in some cases aided by a change in fruit shape that causes seeds to be ejected explosively.

To do this analysis Terborgh and his colleagues needed a systematic way to measure seed rain. The researchers set up a regularly-spaced grid of small containers (seed traps) that collected a portion of the seeds that entered the community. They also needed a way to describe whether the canopy was closed, somewhat open, or very open as in a treefall gap. For each seed trap they calculated a canopy cover index (CCI), which measured the amount of vegetation found at different levels directly above the traps. A value of 0 indicated no vegetation (a completely open canopy), while a value of 6 indicated dense vegetation at all levels (a completely closed canopy).

As the graphs below indicate, there were some dramatic differences between gaps and canopies. Note that the x-axis has been log-transformed so CCI = 1 transforms to a log(CCI) = 0, and a CCI = 6 transforms to log(CCI) = 0.778. All four major groups of animal seed dispersers dispersed many more seeds into closed canopy forest than into treefall gaps. The relationship between seed abundance and canopy cover was strikingly linear for primates and small arboreal animals. This makes sense, as these animals tend to sit on trees, and spread seeds either through defecation of already eaten fruit, or by eating fruits and inadvertently spilling some seeds in the process. So very few trees in treefall gaps translates to many fewer seeds in treefall gaps, with most (76%) being blown in by the wind.


The log abundance of potentially viable seeds (PV seeds on y-axes) collected in seed traps in relation to the log (canopy cover index) for six different types of seed dispersal agents/mechanisms.

Terborgh and his colleagues realized that differences in seed dispersal could profoundly influence the number and types of plants that were recruited into the population. Despite the scarcity of animals in tree fall gaps, most of the saplings (79%) that recruited into gaps were animal dispersed, whereas wind-dispersed species made up only 1% of the saplings.

Sapling species diversity was greater under a closed canopy.


Sapling species diversity (measured as log(Fisher’s alpha)) in relation to canopy cover (measured as log (canopy cover index)).

Though species diversity was lower in tree fall gaps in comparison to the closed canopy, species composition (the types of species found there) was very different in treefall gaps. There were many species that recruited only under gaps, and were never found under a closed canopy. Interestingly, there is good evidence that the small treefall gaps in this study recruited a different set of tree species than do larger treefall gaps, which tend to recruit species that do best under conditions of very bright sunlight. Thus the researchers conclude that treefall gaps, small and large, offer a wide range of environmental conditions not found in the closed canopy,  that ultimately help to promote astoundingly high tropical forest tree diversity.

note: the paper that describes this research is from the journal Ecology. The reference is Terborgh, J., Huanca Nuñez, N., Alvarez Loayza, P. and Cornejo Valverde, F. (2017), Gaps contribute tree diversity to a tropical floodplain forest. Ecology, 98: 2895–2903. doi:10.1002/ecy.1991. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Metallic starlings: a rain of terror

I am a slow learner. Several times in the past few years I have paddled my canoe under a particular sycamore tree in the New River in Radford, Virginia. Each time I do so, I am greeted by large numbers of cormorant poop bombs dropped by the dozens of cormorants that seem to find that particular tree to their liking, and this particular canoeist not to their liking. Fortunately, cormorants have bad aim, but unfortunately it is not that bad.

Daniel Natusch and three other researchers wanted to know how an analogous form of nutrient enrichment from large colonies of nesting Metallic Starlings (Aplonis metallica) affects the nearby ecosystem in a tropical Australian rainforest. They were interested in this question because it was obvious that the ground below the nesting colony trees was basically devoid of vegetation; they describe it as “an open moonscape”, contrasting sharply with the thick rainforest nearby. Other studies have shown that nutrient enrichment from bird guano leads to increased vegetation density – so why is this ecosystem different?


Lockerbie Scrub rainforest, in Cape York Peninsula, Australia, showing colony tree with dead zone (left) and a continuous rainforest (right)



Dan Natusch conducts herpetological research with his son Huxley. Credit: Jessica Lyons













The researchers compared the biological, chemical and physical environment underneath 27 different colony trees to the environment underneath a randomly chosen tree 100-200 meters from the colony tree. As expected, they found very little vegetation near colony trees, in contrast to relatively dense vegetation near the randomly chosen trees.




Vegetation cover (left) and number of live stems (right) in relation to distance from the colony or randomly chosen tree (Point 0 on X-axis).  Negative numbers are downslope and positive numbers are upslope from the tree.


Soil analyses showed that the soils under the colony trees had much higher concentrations of important nutrients. For example, phosphorus levels were more than 30 times greater, and ammonium nitrogen was about four times greater under colony trees than under the randomly chosen trees. The researchers wondered whether these nutrient levels were so high that they were toxic to vegetation. That would account for the dead zone under the colony trees. An alternative hypothesis is that animals (pigs and turkeys in particular) may be attracted to these high nutrient areas under the colonies, and may either kill germinating plants by eating or trampling them.

Feral pigs (Sus scrofa) rooting and trampling under a colony tree. Credit Daniel Natusch

To test both hypotheses, at the beginning of the breeding season the researchers covered a portion of the colony tree region with metal cages (exclosures) that prevented turkeys and pigs from gaining access. They discovered a much greater number of seedlings under the exclosures in comparison to the areas where turkeys and pigs could access the seedlings.

natuschfig7They concluded that nutrient levels were not toxic to seedlings, but that pigs and turkeys were either eating or trampling the seedlings as they emerge. As you can see, the number of exclosure seedlings dropped sharply in July, in part because rainfall declines sharply in June, which leads to high plant mortality, particularly in the unshaded dead zone. But in addition, feral pigs broke into all of the exclosures that summer to access the seedlings and the nutrient-rich soil.

Do these dead zones actually benefit the starlings in any way? One possible advantage is that dead zones prevent snakes from climbing nearby trees and vines to gain access to the nests that are located high in the canopy of the colony tree. However there is good evidence that colony trees suffer high mortality, as 10 of the 27 colony trees died within three years of the study. Trees that fall during the nesting period could lead to the failure of all of the nests within that colony tree.



A scrub python (Morelia amethistina) puts the squeeze on a juvenile Metallic Starling. Credit Daniel Natusch.


Why do we find dead zones beneath colonies of Metallic Starlings, and increased plant growth rate, larger plant size and greater plant diversity beneath the colonies of several other bird colonies? Most previous studies have looked at sea-bird colonies on small islands that have few terrestrial herbivores, so germinating seedlings are relatively undisturbed. This study occurred in a continuous forest in tropical Australia, which harbored a large population of hungry herbivores. These contrasting findings show the important role of environmental context for understanding how ecological interactions will play out. Given that we humans are continually adding nutrients to our environment (through natural bodily function and when we fertilize our fields), we need to carefully consider the biotic and abiotic players in the ecosystem, so we can predict the effects we are having on the environment.

note: the paper that describes this research is from the journal Ecology. The reference is Natusch, D. J. D., Lyons, J. A., Brown, G. P., & Shine, R. (2017). Biotic interactions mediate the influence of bird colonies on vegetation and soil chemistry at aggregation sites. Ecology 98(2): 382-392. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.