Australian eruptions

Red foxes were introduced to Australia in the mid-19th century to provide hunting opportunities for the English colonists.  Since then, the fox population has exploded and spread throughout the continent, creating happy hunters, but probably leading to the decline of many native mammal species.  As a result, conservation ecologists have attempted to reduce the fox population, primarily using a program of poison baits, but also by reintroducing other predators, such as dingos, that might be able to outcompete the foxes. 

A red fox on the hunt

There are many challenges associated with eliminating foxes and restoring endangered native mammal populations.  As it turns out, red foxes are primarily nocturnal so they are not so easy to hunt, and some of their prey have either gone extinct or are reduced to very small remnant populations.  In cases where fox prey have been reduced but not eliminated, conservation ecologists want to understand what the numerical response of a recovering population will look like.  Will the recovering population increase immediately until another factor, such as food availability, becomes limiting, and then level out at a stable equilibrium?  Or might the recovering population show boom-bust dynamics, increase rapidly until it overeats its resources, then decline sharply from food deprivation, which allows food availability to recover, at which point the recovering population may go through another boom phase? If these boom-bust dynamics (also known as eruptive dynamics) are predictable, that knowledge would allow conservation ecologists to know how long they need to monitor a recovering population to establish its long-term trajectory.

Richard Duncan and his colleagues used data from two types of sources.  Most of the data were population abundance time series of at least seven years (mean = 17 years) from 169 populations of 20 native Australian mammalian species that were recovering following fox control programs.  In addition, the researchers compared their Australian data with time series from seven populations of ungulates that were translocated to northern or mountain regions of Canada, Alaska and Europe.

More than half of the populations were either unchanged or continued to decline following fox removal.  Duncan and his colleagues suggest that fox control might have been inadequate in these cases, or that some other factor continued to limit the native mammalian populations. But 46% of the populations did increase following fox control.  Some of these species, for example the brushtail possum, increased to much higher abundance, and then leveled off. 

The brushtail possum in Austin’s Ferry, Tasmania. Credit: J. J. Harrison.
Abundance of the brushtail possum following intensive red fox control that began in 2003 in Boodoree National Park in eastern Australia. The black-dotted, blue-dashed and solid red curves (in this graph and the graph below) were generated by three different models that were fitted to the data. In this case, the blue dashed curve was the best fit.

But most (56%) of the populations that increased following red fox control showed evidence of eruptive dynamics. For example, the long-nosed bandicoot increased sharply following intensive fox control for about two years, but then crashed sharply, approaching pre-removal levels after another three years.

The long-nosed bandicoot at Craters Lake National Park in Queensland, Australia. Credit: Joseph C. Boone.
Abundance of the long-nosed bandicoot following intensive red fox control that began in 2004 in Boodoree National Park in eastern Australia. In this case the solid red curve was the best fit.

Species with higher maximum rates of increase tended to reach their eruptive peak more rapidly.  In addition, larger species, such as the translocated ungulates, took longer to reach their eruptive peaks.

Time until a mammal population reaches its eruptive peak in relation to the maximum annual rate of population growth (top graph), and the mean body mass of each species (bottom graph). The solid line is the best fit generated by the model, while the dashed lines represent 95% confidence intervals. Both axes use logarithmic scales.

These findings demonstrate that it is not sufficient to show that a threatened population is recovering following removal of an invasive species such as the red fox.  It is possible that even with continued fox control, the recovery will be short-lived, only to be followed by a population crash. Density dependent factors – factors that become more important at high population densities – are probably responsible for many of the observed population crashes.  We have already discussed food availability dynamics; other density dependent factors could include predators being attracted to large prey populations, or disease being more easily transmitted when populations reach a certain level. Because they take longer to reach their eruptive peak and then crash, larger species need to be monitored by conservation ecologists for a longer period of time than do smaller species. Conservation managers need to anticipate these eruptive dynamics as they create their species recovery plans following predator removal.

note: the paper that describes this research is from the journal Ecology. The reference is Duncan, R. P.,  Dexter, N.,  Wayne, A., and  Hone, J.  2020.  Eruptive dynamics are common in managed mammal populations. Ecology  101( 12):e03175. 10.1002/ecy.3175  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Seagrass stimulated by the return of the green turtle

Christopher Columbus’s journal describes how his ships had to plow through masses of sea turtles to reach the shore of Caribbean Islands. Since the 15th century, populations of the green turtle (Chelonia mydas) were nearly extirpated, primarily to feed the expanding human population. Recent conservation programs have led to a partial recovery in the Caribbean, but current green turtle populations are still a small fraction of what they were historically.

The green turtle, Chelonia mydas.

While the green turtle recovery is good news for turtles, it’s not clear how their favorite food in the Caribbean, the seagrass Thalassia testudinum feels about green turtle resurgence.  When green turtle populations were at their lowest, many lush seagrass meadows performed ecosystem services such as sequestering carbon via photosynthesis, stabilizing marine sediment  and providing important nursery habitats for commercial fisheries.  

Alexandra Gulick and her colleagues were inspired by a similar situation that has been occurring in terrestrial ecosystems.  Since around 1960, wildebeest and buffalo populations in the Serengeti have increased sharply – a result of the sharp decline (or possible elimination) of the rinderpest virus, which previously had controlled the abundance of those two large mammals.  The increase in buffalo and wildebeest populations has profoundly affected the distribution, abundance and productivity of grasses and trees, which of course impacts the entire ecosystem.  Gulick wondered whether the return of green turtles was an analogous situation, in which increases in green turtles would dramatically reduce seagrass meadows and alter ecosystem functioning.

Alexandra Gulick assisted the National Park Service and the U.S. Geological Survey with a mark recapture study of juvenile green turtles. Credit: Kristen Hart.

Gulick and her colleagues were looking for evidence of compensatory growth – increased seagrass growth in response to grazing. Green turtles use a cultivation grazing strategy, in which they select and repeatedly crop the same meadows.  Such behavior would make sense if grazed meadows compensated for grazing by producing biomass at a higher rate, or by producing leaves that were more digestible or nutritious.

A sharp boundary between a grazed and ungrazed seagrass meadow. Credit: Alexandra Gulick.

Working at the Buck Island Reef National Monument, off St. Croix, Virgin Islands, the researchers studied both grazed and ungrazed seagrass meadows, in both shallow water (3-4 meters) and deeper water (9-10 meters). They placed 129 turtle-proof exclosures over grazed and ungrazed meadows during August-October 2017 and January-February 2018. After 7-10 days they measured how much growth had occurred in both types of meadows.  

Divers set up an exclosure in a grazed meadow at Buck Island Reef National Monument. Credit: Alexandra Gulick.

The data table below shows some good evidence for compensatory growth in grazed meadows, particularly in shallow water, but also in some of the deep water meadows. Grass blades grew longer and achieved greater surface area in grazed meadows in shallow water, and also in deep water during the winter (their growth rate was slightly greater during the summer as well, but this increase was not statistically significant).  However, the seagrass in grazed meadows added much less biomass (dried weight) per day per m2.

Seagrass growth in grazed and unglazed meadows at different water depths and seasons. Mass is the increase in biomass (dry weight) per m2 per day. Statistically significant differences between grazed and unglazed meadows are boldfaced.

How can a seagrass blade have more surface area but less biomass?  There are at least two answers to this question.  First, seagrass biomass was measured on a per m2 basis, and ungrazed meadows had more blades per m2. Second, while achieving greater surface area, the seagrass blades from grazed meadows were much thinner, so when dried they weighed much less.  This is important, because putting their resources into surface area allows the seagrass blades to achieve a high photosynthetic rate, which should allow them to recover relatively quickly from sea turtle grazing.  The bottom row in the data table above is a measure of production (measured as mass growth) in relation to initial biomass (P:B).  You can see that P:B in deep water is similar in grazed vs, ungrazed meadows, while P:B in shallow meadows is substantially greater in grazed meadows. This indicates that despite continuous cropping by sea turtles, the grazed seagrass can recover quite nicely.

Gulick and her colleagues wanted to know whether the intensity of grazing might affect productivity.  They counted the number of grazed vs. ungrazed shoots, and the length of grazed vs. ungrazed blades for each sample site, and used those data to calculate grazing intensity.  The researchers then generated a model that calculated P:B in relation to grazing intensity.  The model shows that high grazing intensity increased P:B, indicating that grazing is stimulating increased leaf tissue production.

Increase in production (P:B) in relation to grazing intensity. Dashed lines indicate 95% confidence interval of the linear model.

These findings indicate that increased grazing intensity by recovering sea turtle populations is sustainable in Caribbean seagrass meadows, as seagrass growth was still stimulated at relatively high grazing intensities. Many of the meadows had been grazed continuously for at least two years, and still showed no evidence of being overly stressed by the attention that turtles had given them.  Presumably, compensatory growth by seagrass is an adaptation resulting from the co-evolution of seagrass with green turtles and other hungry herbivores. In support of this coevolution scenario, seagrass in grazed areas reduces the height of its flowers and fruits, reducing consumption of these structures by green turtles, and allowing it to achieve reproductive success.  As green turtle populations continue to recover, it is likely that seagrass meadows will be grazed more heavily, but, at least in most cases, will be able to successfully compensate for even greater grazing levels.

note: the paper that describes this research is from the journal Ecology. The reference is Gulick, A. G.,  Johnson, R. A.,  Pollock, C. G.,  Hillis‐Starr, Z.,  Bolten, A. B., and  Bjorndal, K. A..  2020. Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem. Ecology 101( 12):e03180. 10.1002/ecy.3180.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Forest Physiognomy

I am old enough that I attended school at a time when educators still taught physiognomy to their students. I recall being attracted to the idea that you could predict someone’s character, criminal or violent inclinations, passions and general temperament by the location of bumps or indentations on the head, the shape of the nose, or the forward projection of the jaw. Dampening my enthusiasm, we were taught physiognomy as an example of pseudoscience, and that we should make sure to not embrace ideas simply because they were intuitively attractive. And this letdown came after I had spent several precious moments learning how to pronounce the word.

Two tranquil foreheads. Credit: Giambattista della Porta: De humana physiognomonia libri IIII. From website of the National Library of Medicine:

Later, I was delighted to learn in my plant communities class in graduate school that forests had physiognomy, and that reputable scientists actually studied it. Forest physiognomy is the general appearance of a forest, including the height, spacing and structural growth forms of its dominant species.  Michelle Spicer described to me that she went to Central America as an engineering undergraduate student, and became enraptured with tropical forests, including their physiognomy.

Tropical forest showing vast collection of lianas and a few epiphytes. Credit: Michelle Spicer.

Spicer switched from engineering to ecology, and as a graduate student realized that nobody had actually rigorously compared tropical and temperate forest physiognomy. Textbooks might talk about the importance of lianas (vines) and epiphytes (plants that grow on other plants and get nutrients from the air, water or debris lodged in their host plants) in tropical forests.  These same texts might also highlight the importance of the herbaceous layer in temperate forests. 

A temperate forest in the Smokey Mountains, USA. Credit: Michelle Spicer.

But there were few organized data to compare forest physiognomy in the two biomes. Spicer, an undergraduate student in her lab (Hannah Mellor), and her advisor (Walter Carson) chose to compare nine temperate forests and nine tropical forests, spreading across the Americas from Brazil to Canada. Each of these forests (studied by other researchers) had detailed downloadable plant species lists, which also included data about their height and reproductive status.  In total, the researchers went through over 100,000 records to create their dataset.

The figure below highlights the plant physiognomy concept. You can see that most of the species in temperate forests are herbs residing primarily in the forest floor layer.  In contrast, tropical forests have a much more even distribution of types of species, and location of growth.

The physiognomy of temperate and tropical forests. Credit: Jackie Spicer.

Quantitatively, 80% of temperate forest plant species are herbs, while only 7% are trees, and there are relatively few lianas and epiphytes.  In contrast, tropical forests boast a much more even distribution of each plant growth form.

Relative species richness of trees, shrubs, lianas, herbs and epiphytes in temperate and tropical forests.

Going along with the growth form distribution finding, most temperate plant species grow on the forest floor, while more tropical species are actually higher up (upshifted) in the understory – in part due to the prevalence of lianas and epiphytes in the understory layer.

Relative species richness of plants at different layers of temperate and tropical forests.

Spicer and her colleagues caution us that the up-shift in the tropical forest profile may be understated by the data, because even the best inventories are likely to miss epiphytes growing high in the canopy.

The tropical forest epiphyte Guzmania musaica. Credit: Michelle Spicer.

These findings have important implications for conservation and forest management.  Logging of tropical forests removes trees, but also removes lianas and epiphytes associated with trees. Lianas recover well from disturbance, but epiphytes take a long time to return following disturbance. Thus even relatively small-scale logging will significantly reduce biological diversity, not only in the plant communities, but in the many species of animals, fungi and microorganisms that interact with these plants. In contrast, temperate forests may be more resilient to logging, because the diverse herbaceous community can recover quickly, particularly if some canopy cover remains after logging.  Spicer and her colleagues argue that over-browsing by large ungulates, and changes in herbaceous species composition resulting from years of fire suppression are the two primary threats to the extensive biological diversity in the temperate forest herbaceous layer. With many species missing from the herbaceous plant community from these two sources, invasive species can take over, changing forest ecosystem functioning.  The researchers suggest that forest managers should prioritize managing the vast diversity of plant species that inhabit the temperate forest floor and understory.

note: the paper that describes this research is from the journal Ecology. The reference is Spicer, M. E., H. Mellor, and W. P. Carson. 2020. Seeing beyond the trees: a comparison of tropical and temperate plant growth-forms and their vertical distribution. Ecology 101(4):e02974. 10.1002/ecy. 2974.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Invading hippos

Jonathan Shurin was studying declining water quality in Lago de Tota, Colombia’s largest lake, when he learned about a local invasion of the common hippopotamus, Hippopotamus amphibius.  Four hippos were imported to Colombia by the notorious drug lord Pablo Escobar to populate his private zoo.  Following Escobar’s shooting death in 1993, the zoo fell into disrepair and the hippos wandered off free.  The population now numbers between 65-80, and breeding individuals have been seen 150 km from the zoo.


Hippos wallow in a lake framed by cattle egrits. Credit J. Shurin

Common hippos are native to central and southern Africa; as their scientific name implies they divide their existence between land (mostly at night) and water (keeping cool during the day).  These are huge animals, weighing up to 1500 kg and capable of running a surprising 30 kg/hr.  Apparently it is very easy to annoy a hippo.  From an ecosystem standpoint, hippos in their native Africa have been shown to have a strong impact on ecosystems by grazing on land at night and then releasing processed nutrients into lakes during the day.  Their influence is greatest during the dry season when they’re concentrated at high densities.  Jonathan Shurin and his colleagues wanted to know whether hippos were having a discernable effect on lakes and rivers in Colombia.  Given an expectation that the hippo population will continue to grow, this question has important management implications.


A grazing hippo. Credit: J. Shurin

The researchers sampled 14 small lakes at Hacienda Napoles in Antioquia, Columbia during the wet season and the dry season.  All lakes were sampled from shore because entering a lake containing hippos can be hazardous to a researcher’s health.peligrohippo

Two lakes were found to contain hippos, while the other 12 did not (though some had been observed with hippos on other occasions).  The analysis compared the two lakes with hippos to the 12 lakes without hippos for nutrients, conductivity, pH, temperature and chlorohyll-a concentration (a measure of photosynthetic activity).  The researchers sampled for phytoplankton, zooplankton and used dip nets to sample macroinvertebrates.  They found few differences in most categories except for the composition of the phytoplankton community. As you can see below, lakes with hippos had considerably more cyanophytes (photosynthetic bacteria often associated with harmful algal blooms), and fewer chlorophytes and charophytes (types of green algae) than did lakes without hippos.


Mean relative density of different divisions of phytoplankton in the two lakes with hippos (left bar) and the 12 lakes without hippos (right bar).

Shurin and his colleagues also estimated net production of each lake by systematically measuring dissolved oxygen concentration throughout the day. Photosynthetic organisms in highly productive lakes should take up lots of carbon dioxide during the day, and release considerable oxygen into the water.  Thus the difference in oxygen levels during the day (when photosynthesis occurs) vs. night (when there is no photosynthetic activity) would be greatest in highly productive lakes. The researchers discovered from multiple samples that the two lakes with hippos had an average range of 3.6 mg/L in dissolved oxygen levels which was significantly greater than the average range of 2.1 mg/L measured in three of the lakes without hippos (it was not feasible to measure all of the no hippo lakes). Presumably, this difference occurs from high photosynthetic rates during the day in the lakes with hippos.


Time series of dissolved oxygen in the sampled lakes.  Notice how dissolved oxygen levels peak in the late afternoon (hour 12 = noon), but decline overnight without input from photosynthesis.

In addition to comparing the quantity of nutrients, Shurin and his colleagues wanted to know the source of the nutrients.  Stable isotopes are forms of elements (in this case carbon and nitrogen) that differ in number of neutrons.  They are called stable, because they don’t undergo radioactive decay.  Stable isotope analysis measures the ratio of rare isotopes of a particular element in comparison to the more common isotope (for example 13C compared to 12C). Relevant to the hippo study, plants growing on land tend to have a higher (less negative for carbon, more positive for nitrogen) stable isotope ratio of carbon (delta13C) and nitrogen (delta15N) than do plants growing in water.  So if hippos were bringing nutrients into the lakes, the researchers expected the two hippo lakes to have higher stable isotope ratios of carbon and nitrogen.

As you can see from the graph below, on average, the two hippo lakes had higher stable isotope ratios of carbon, but not of nitrogen.  This indicates that hippos are importing carbon into the lake – presumably eating 13C rich plants during the evening, and then pooping out the remains when they return to the water.  However there is no evidence that hippos are importing nitrogen into the lakes.


Stable C and N isotopic ratios for samples collected from lakes with (green) and without (orange) hippo populations. Solid circles are the mean values of multiple samples collected at different times from the same lake, and open circles are the individual observations from each sample.

Shurin and his colleagues acknowledge the difficulty of drawing conclusions on ecosystem impact based on only two lakes with hippos.  On the other hand, finding significant differences with such a small sample is noteworthy, particularly when considering that the hippo invasion may be in its early stages.  If we extrapolate, from four hippos in 1993 to the lower estimate of 65 hippos at the time of the study, and assume exponential growth, we should find 785 hippos by 2040 and over 7000 hippos by 2060.  There are several assumptions with this extrapolation, but if unchecked the hippo population could expand dramatically, impacting ecosystem functioning in many different ways.


Observed (solid circles) and projected (open circles) growth of the hippo population in Antioquia, Columbia, assuming exponential growth.

But should we worry about this?  After all, hippos are amazingly cool, and tourists have begun visiting Hacienda Napoles specifically to see the hippos.  This is an example of a social-ecological mismatch, where the societal value placed on a species may oppose potential negative environmental impact. Conservation ecologists will need to work with the local community to devise a plan that serves the best interests of the ecosystem, and the humans who live there.

note: the paper that describes this research is from the journal Ecology. The reference is Shurin, J. B., Aranguren-Riaño, N., Duque Negro, D., Echeverri Lopez, D., Jones, N. T., Laverde‐R, O., Neu, A., and Pedroza Ramos, A. 2020. Ecosystem effects of the world’s largest invasive animal. Ecology 101(5):e02991. 10.1002/ecy.2991. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Fast living vs. slow and steady

Fast living makes headlines, as evidenced by such notables as Freddie Mercury, Paul Walker and Lamar Odom.  Unfortunately the first two are dead while Odom was narrowly brought back from a near-death experience – all were victims of their fast life styles.  Like humans, some birds live fast and die young, while others live slow, but may survive to relatively ripe old ages.

Pic 2

The tiny rifleman (Acanthisitta chloris), reintroduced in Tiritiri Matangi, New-Zealand.  This endangered bird feeds on insects that it gleans from tree trunks.  It often has two clutches of 2-5 young per year. Credit: Simon Ducatez.

Simon Ducatez studied invasive cane toads with Rick Shine in Australia, and became interested in why some species were more likely than others to successfully invade new habitat.  The problem for answering that question is that most invasions are not studied until after the invasive species becomes established; by that time it may be too late to identify exactly what factors were responsible for the successful invasion. On his first visit to New-Zealand in 2016, Ducatez discovered ecosanctuaries – enclosed wildlife reserves where invasive predators are eliminated, and native animals (mostly birds) are introduced. He realised that these introductions could provide invaluable information on why species thrive or fail to become established in a new environment. At about the same time, a colleague drew his attention to a database developed by the Lincoln Park Zoo (LPZ) in Chicago, Illinois, which contains data on hundreds of intentional release events (translocation attempts), including information on the survival and reproduction of the released individuals. Analyzing how a species life history could affect the survival and reproduction of these voluntarily introduced populations would provide answers useful for restoration biologists who wish to return native species to habits where they were now extinct, and to ecologists who want to identify the factors promoting biological invasions.

Pic 7

The relatively massive and flightless south island takahe (Porphyrio hochstetteri), reintroduced in New-Zealand.  This bird was thought to be extinct but was rediscovered in 1948 and has benefited from active restoration programs. Credit: Simon Ducatez.

Life history traits are adaptations that influence growth, survivorship and reproduction of individuals of a particular species.  For each species in the LPZ dataset, Ducatez and Shine used the bird literature to gather data on body mass and four life history traits: maximum lifespan, clutch size, number of clutches per year, and age at first reproduction. They then used a statistical procedure – principle components analysis – which described each species based on their life history strategy.  Fast life styles were associated with small bodies, short lifespans, large clutch size and number, and early reproduction.  Slow life styles were associated with large bodies, long lifespans, small clutch size and number, and delayed reproduction. Ducatez and Shine then asked a simple question based on 1249 translocation events in the LPZ dataset – how do fast life style birds perform in comparison to slow life style birds following translocation?

It turns out that slow life style birds are much better at surviving translocation than are fast life style birds, at least when measured in the short term (one week) and the medium term (one month).


Association between life style as measured by principle component analysis (PC1 on X-axis) and survival (proportion of translocated individuals still alive on Y-axis). The left graph is survival to one week, while the right graph is survival to one month. 

In contrast, following translocation fast life style birds are more likely to attempt breeding and successfully breed than are slow life style birds.


Association between life style and probability of attempting breeding (left graph) and successfully breeding (right graph).

Ducatez and Shine suggest that both restoration biologists and invasion ecologists could use these findings to address major questions in their respective fields.  Restoration biologists wishing to return native species to previously occupied habitat might adopt different approaches based on a species life style. Species with fast life styles suffer from low survival, so restoration biologists should focus on promoting survival by controlling predators or provisioning extra food. Species with slow life styles suffer from low reproductive success, so conservation managers might consider providing extra nest boxes or other resources that promote successful breeding.


A successful foraging event for an Atlantic puffin (Fratercula arctica), reintroduced in Maine, USA. Credit: Simon Ducatez.

This research informs invasion ecologists that the same trait can have opposite effects on the likelihood that a biological invasion will actually happen.  Thus a slow life style species is more likely to survive moving to a novel habitat, but is unlikely to breed successfully once it gets there.  In contrast a fast life style species is less likely to survive the move, but if it does survive, it may be more likely to successfully reproduce. How this plays out in actual biological invasions is yet to be determined, but at least we now have a better grasp on what factors we should be examining.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Ducatez, S. and Shine, R. (2019), Life‐history traits and the fate of translocated populations. Conservation Biology, 33: 853-860. doi:10.1111/cobi.13281. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Fewer infections found in forest fragments

As human populations expand, we are converting ecosystems from one state to another.  In the case of tropical forests, conversion of forest to cropland may leave behind fragments of relatively undisturbed forest surrounded by a matrix of cropland or other forms of development.  Conservation ecologists are exploring whether ecological processes and ecosystem structure in these fragments work pretty much like normal forested regions, or whether fragments behave differently.  To do this, in a few locations around the world such as the Wog Wog Fragmentation Experiment in New South Wales, Australia, researchers have systematically created forest fragments of various sizes.  They can then ask a variety of questions comparing fragments vs. intact forest. For example,  how does species diversity, or how do processes such as competition, predation and mutualism differ in the two landscapes?

aerial photo2

Aerial photo of Wog Wog Fragmentation Experiment at the time the experiment began in 1987. Credit: Chris Margules.

Julian Resasco was working as a postdoctoral associate in Kendi Davies’ lab at the University of Colorado on a study that looked at changes in invertebrate communities in response to fragmentation at Wog Wog. Beginning in 1985, researchers had set up a network of pitfall traps, which are cups that are buried with their tops level to the ground, so that any careless organism that wanders in will be trapped in the cup.  Some pale-flecked garden skinks, Lampropholis guichenoti, also had the misfortune to become entrapped and became subjects for the study. The invertebrates, and the 186 unfortunate skinks were preserved in alcohol and stored as part of the Australian National Wildlife Collection.


Skink museum specimens at the Australian National Wildlife Collection. Credit: Julian Resasco.

Much later, Resasco arrived and began dissecting skink guts to analyze the prey items for a study that looked at how the skinks shifted prey consumption (their feeding niche) in response to fragmentation. While dissecting the skink guts, he noticed that some of the skinks had worms (nematodes) inside their guts.  These nematodes were relatively common among skinks from continuous eucalypt forests, rare among skinks from eucalypt fragments, and absent from skinks in the cleared, pine plantation matrix.


Top. The study area in southeast Australia, showing location of continuous forest, forest fragments and surrounding matrix.  Dots indicate locations of pitfall traps. The matrix was planted in pine seedlings soon after fragmentation.  Bottom. The pale-flecked garden sunskink Lampropholis guichenotti. Credit: Jules Farquar

As it turned out, the nematode was a new species, which Resasco and a colleague (Hugh Jones) named Hedruris wogwogensis. Nematodes in the genus Hedruris use crustaceans as intermediate hosts, which alerted Resasco and his colleagues that the terrestrial amphipod Arcitalitrus sylvaticus, which was very common in the pitfall traps, was probably an important intermediate host.  When amphipods from pitfall traps were examined microscopically, a small portion of them were infected with Hedruris wogwogensis. The researchers concluded that amphipods became infected when they ate plants that harbored nematode eggs or young nematodes, which then developed in amphipod guts, and were passed on to skinks that ate the amphipods.  Thus somewhat inadvertently, one aspect of the study transitioned into the question of how fragmentation can influence the transmission of parasites.

After concluding their skink dissections, Resasco and his colleagues discovered that skinks in continuous forest had five times the infection rate as did skinks in fragmented forest.  In addition, no skinks collected in the matrix were infected. Infected skinks harbored a similar number of nematodes, whether they lived in continuous forest or fragments (see the Table below). Lastly, amphipods were considerably more common in skink guts and pitfall traps from continuous forest, less so in fragments, and least in the matrix.


Summary of data collected by Resasco and his colleagues. Nematode intensity is the mean number of nematodes per infected skink. Nematode abundance is the mean number of nematodes per skink (infected and uninfected). 

The researchers put these findings together in a structural equation model.  The boxes in the model below represent the variables, while the numbers in smaller boxes over the arrows are the regression coefficients, with larger positive numbers (in black) indicating stronger positive effects, and larger negative numbers (in red) indicating stronger negative effects.  The model revealed three important findings.  First, habitat fragmentation strongly reduced amphipod abundance.  High amphipod abundance was associated with high nematode abundance (that is the +0.20 in the model), so lower amphipod abundance from fragmentation reduced nematode abundance. Second, habitat fragmentation positively affected skink abundance – more skinks were captured in fragments than in intact forest, but this increase had no effect on nematode abundance in skinks.  Finally, note the direct arrows connecting “Fragmentation” to “Log nematode abundance in skinks”.  This indicates that other variables (beside amphipod abundance) are reducing infection rates in skinks that live in fragments and the matrix.


Structural equation model showing effects of fragmentation on nematode infection in skinks. Amphipods are the intermediate host.  Black arrows indicate significant positive effects of one variable on the other, while red arrows indicate significant negative effects. Solid lines represent fragments compared to controls and dashed lines represent the matrix compared to controls. Thicker lines are stronger effects.

At this point, we still have an incomplete understanding of the system.  We know that fragmentation reduces amphipods, which require a moist and shaded environment to thrive.  Reduced amphipod abundance leads to lower nematode infection rates in skinks.  But we know that other variables are important as well; perhaps nematodes survive more poorly in fragment and matrix soils. Interestingly, pine trees were planted in the matrix and are beginning to mature and shade out the matrix environment. Amphipod abundances are on the rise, so the researchers predict that nematode infection rates will begin to increase accordingly.  Those studies have begun.


Eucalypt forest canopy at Wog Wog. Credit: Julian Resasco.

Looking at the bigger picture, it is clear that fragmentation may decrease (as in this study) or increase the abundance of an intermediate host. As an example of fragmentation increasing intermediate host abundance, the researchers describe a study in which fragmentation increased the abundance of the white footed mouse, an intermediate host for black-legged ticks (that host the bacteria that causes Lyme disease). We need to unravel the connections between landscape factors and the various species they influence, so we can begin to understand how human changes to the landscape can influence the transmission of diseases.

note: the paper that describes this research is from the journal Ecology. The reference is Resasco, J.,  Bitters, M. E.,  Cunningham, S. A.,  Jones, H. I.,  McKenzie, V. J., and  Davies, K. F..  2019. Experimental habitat fragmentation disrupts nematode infections in Australian skinks. Ecology  100( 1):e02547. 10.1002/ecy.2547. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Invasive crayfish depress dragonflies and boost mosquitoes

Paradoxically, obliviousness and intense focus can be two sides of the same coin, as the following story highlights.  As a new graduate student at the University of Minnesota, I took a field ecology course at the University’s field station at Lake Itasca (famously known as headwaters of the Mississippi River).  One afternoon we watched dragonflies at a small pond; the male dragonflies were obviously patrolling territories and behaving thuggishly whenever intruders came by, and amorously whenever females of their species approached.  Surprisingly, territorial males chased off male intruders of any species, even though they posed no reproductive threat to them.  Why, I wondered…  So I sat there for many hours and kept very careful track of who chased whom, and for how long.  Big focus time. Ultimately, these observations blossomed into my doctoral dissertation.  Unfortunately, these observations also blossomed into the most virulent case of poison ivy known to humanity, as my intense focus on dragonflies obliviousized me to the luxurious patch of poison ivy, which served as my observation perch.

Anax junius Henry Hartley

Anax junius dragonflies in copula.  The male has the bright blue abdomen.  Credit: Henry Hartley.

Despite this ignoble incident, dragonflies remain one of my favorite animal groups.  They are strikingly beautiful, brilliant flyers, and fun to try to catch. In addition, they have so many wonderful adaptations, including males with penises that are shaped to scoop out sperm (previously introduced by another male) from their mate’s spermatheca, and females who go to extremes to avoid repeated copulation attempts, for example, by playing dead when approached by a male. Thus I was delighted to come across research by Gary Bucciarelli and his colleagues that highlighted the important role dragonflies play in stream ecosystems just west of Los Angeles, California.

Back Camera

Captured dragonfly nymph.  Dragonflies require from one to four years to develop in aquatic systems, before they metamorphose into terrestrial winged adults. As nymphs, they are fearsome predators on aquatic invertebrates. As adults, they specialize on winged insects, though there are stories of them killing small birds. Credit: Gary Bucciarelli

Bucciarelli and his colleagues came up with their research question as a result of working in local streams with students on a different project.  They wanted to know if invasive non-native crayfish (Procambarus clarkii) affect the composition of stream invertebrates and whether removal of crayfish could lead to rapid recovery of these invertebrate communities.

crayfish, egg masses, clutches

Invasive crayfish, P. clarkii, sits on the stream bottom. Credit: Gary Bucciarelli

The researchers collected stream invertebrate samples and noticed a dramatic pattern – in all the streams with crayfish there were numerous mosquito larvae, but in all of the streams without crayfish there were no mosquito larvae and much greater numbers of dragonfly nymphs. This led them to formulate and test two related hypotheses. First, dragonfly nymphs (Aeshnaspecies) are more efficient predators on mosquitoes (Anopheles species) than are the invasive crayfish. Second, crayfish interfere with dragonfly predation on mosquitoes in streams where crayfish and dragonflies are both present.

Field Sampling

Student researchers collect stream samples. Credit: Gary Bucciarelli

Bucciarelli and his colleagues systematically sampled 13 streams monthly from March to October 2016 in the Santa Monica Mountains. Eight streams have had crayfish populations since the 1960s, while four streams never had crayfish, and one stream had crayfish removed as part of a restoration effort in 2015. Overall, streams with crayfish had a much lower number of dragonfly nymphs than did streams without crayfish.  In addition, streams with crayfish had substantial populations of Anopheles mosquitoes, while streams without crayfish (but much higher dragonfly populations) had no Anopheles mosquitoes in the samples.


Number of mosquito larvae (MSQ) and dragonfly nymphs (DF)  by month in streams with crayfish (CF – top row of data) or without crayfish (CF Absent – bottom row)

This field finding supports both of the hypotheses, but the evidence is purely correlational.  So the researchers brought the animals into the laboratory to test predation under more controlled conditions.  They introduced 15 mosquito larvae into tanks, and exposed them to one of four treatments: (1) a single crayfish, (2) a single dragonfly nymph, (3) one crayfish and one dragonfly nymph, or (4) no predators. The researchers counted the numbers of survivors periodically over the three day trials. As the graph below indicates, dragonflies are vastly superior consumers of mosquito larvae compared to crayfish.  However, when forced to share a tank with crayfish, dragonflies stop hunting, either huddling in corners or actually perching on the crayfish.  By 36 hours into the experiment, all of the dragonflies had been eaten by the crayfish.  After three days, mosquito survival was similar when comparing tanks with crayfish alone with tanks that had both a crayfish and a dragonfly.


Mean number of surviving mosquito larvae in tanks with a lone dragonfly (DF), a lone crayfish (CF), one crayfish and one dragonfly (CF+DF) in comparison to controls with no predators.

Bucciarelli and his colleagues conclude that dragonfly nymphs are much more efficient predators of mosquito larvae than are crayfish. But when placed together with crayfish, dragonfly foraging efficiency plummeted. Field surveys showed a negative correlation between crayfish abundance and dragonfly larvae, and much greater mosquito larva populations in streams with crayfish.  This supports the conclusion that invasive crayfish cause mosquito populations to increase sharply by depressing dragonfly populations and foraging efficiency.  This is a complex trophic cascade because crayfish increase mosquito populations despite eating a substantial number of mosquito individuals.

The researchers argue that crayfish probably relegate dragonfly larvae to inferior foraging habitats, thereby limiting their efficiency as mosquito predators. As such, ecosystem services provided by dragonflies to humans are greatly diminished.  Recently, several new mosquito species that are disease vectors have moved into California.  Thus the loss of dragonfly predation services could pose a public health threat to the human population.  Bucciarelli and his colleagues recommend removing the invasive crayfish to restore the natural community of predators, including dragonflies, which will then naturally regulate the increased number of potential disease vectors.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Bucciarelli, G. M., Suh, D. , Lamb, A. D., Roberts, D. , Sharpton, D. , Shaffer, H. B., Fisher, R. N. and Kats, L. B. (2019), Assessing effects of non‐native crayfish on mosquito survival. Conservation Biology, 33: 122-131. doi:10.1111/cobi.13198. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Rewilding tropical forests: dung is the key

Rewilding means different things to different people. Basically, it involves restoring a species, or several species to an area from which they have been extirpated by humans. Conservation biologists might study the population size and distribution of the returned species, ecologists might focus on interactions between the returned species and other species, while anthropologists might investigate how humans in the area are adjusting to having a new species in their lives.  One of the most famous examples of rewilding is the return of gray wolves to the Greater Yellowstone Ecosystem in western U.S.A., which can be looked at from the perspective of how the wolf populations are doing numerically, how they affect their prey (elk) or their prey’s prey (willow and aspen in the case of elk), and how they affect ranchers in the surrounding areas.

Conservation ecologists have begun a major rewilding program in Tijuca National Park in Brazil, introducing agoutis in 2010 and brown howler monkeys (Alouatta guariba clamitans) in 2015. Howler monkeys were extirpated from this park over a century ago, so ecologists worried that the monkeys might interact with the remaining species in unexpected ways.  For example, this forest hosts several species of invasive fruit trees, such as the jackfruit (Artocarpus heterophyllus). Luisa Genes and her colleagues were concerned that howler monkeys might eat fruits from these trees, and poop out the seeds in new forest locations, causing the invasive species to spread more rapidly.


Introduced howler monkey holding the second baby born to her in the forest. Credit: L. Genes.

Even a disturbed rainforest such as Tijuca National Park hosts a large number of plant species, so the interactions can be complex and difficult to study.  As is so often the case in ecology, one very important complex of interactions involves poop.  Specifically, howler monkeys eat fruit off of trees, and poop the seeds out, usually at a new location, effectively dispersing the seeds.  But there is a second link in this seed dispersal interaction.  Twenty-one species of dung beetles use howler monkey poop for food for themselves and their offspring, breaking off small sections into balls and rolling the balls to a new location.  This process of secondary dispersal is nice for the beetles, but also for the seeds within the balls, which can now germinate in a new location without competing with the large number of seeds in the original howler monkey pile.


Two dung beetles battle over a dung ball. Credit: Rafael Brix.

Genes and her colleagues were interested in two basic questions.  First, were the howler monkeys eating fruit from a few select tree species, or were they eating from many different types of trees, thereby dispersing seeds from many species?  Before releasing the monkeys (two females and two males), they attached radio transmitters to the monkeys so they could easily track them, and note what they ate.  Based on 337 hours of observation, the howler monkeys ate fruit from 60 different tree species out of 330 possible species in the forest (18.2%).  This is an underestimation of actual howler monkey contribution to seed dispersal, because the researchers observed the monkeys for a relatively brief time, and fruit consumption by the monkeys should increase over time as the population of monkeys (and possibly tree diversity), continues to increase.

bugio reintroduzido no PN Tijuca 2015

Male howler monkey released in 2016.  Note the radio transmitter on its right rear leg. Credit: L. Genes.

The second question is whether secondary dispersal by dung beetles was reestablished following reintroduction of howler monkeys.  To answer this question quantitatively, Genes and her colleagues set up an experiment that used plastic beads of various sizes instead of seeds. The researchers set up circular plots of 1m diameter with 70 grams of howler monkey poop in the middle.  Each pile was mixed with seeds (actually beads) of four different sizes (3, 6, 10 and 14 mm diameters) to mimic the range of seed sizes. The researchers measured secondary seed dispersal by returning 24 hours later and counting the remaining beads, reasoning that the rest had been moved by dung beetles (along with the poop) to a new location.

Genes and her colleagues discovered that the median rate of seed dispersal (bead removal) was 69% with larger seeds being moved at a significantly lower rate than smaller seeds.  Thus secondary seed dispersal by dung beetles was still operating in this ecosystem even after howler monkeys had been absent for over 100 years.


Removal rate of beads (seed mimics) from dung piles by dung beetles in relation to bead size.  Different letters above treatments indicate statistically significant differences between treatments. 

Overall, ecological interactions among howler monkeys, plants, and dung beetles were rapidly reestablished once howler monkeys were reintroduced to the community.  There are plans to introduce five more howler monkeys this year, which should further increase beneficial seed dispersal, and hopefully allow plant diversity to increase as well.  One problematic observation was that howler monkeys also ate invasive jackfruit, which could promote its dispersal within the community.


Luisa Genes monitors howler monkeys in the forest. Despite its apparent lushness, the forest still lacks many species and interactions that you would expect to find in an intact forest. Credit: L. Candisani.

The researchers discovered only 21 species of dung beetles, which was somewhat lower than other studies have found.  It is probable that conversion of this land into farmland in the 19thcentury led to the decline and/or demise of some dung beetle species.  With reintroduction of howler monkeys, and the passage of time, Genes and her colleagues expect that this rewilding effort should lead to a more robust ecosystem, with increased howler monkey populations supporting high dung beetle abundance and diversity, and more effective dispersal of many plant species. To understand the overall impact on forests, the researchers recommend that future studies should compare seedling survival and forest regeneration in areas where howler monkeys were reintroduced to areas where howler monkeys are still missing.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Genes, L. , Fernandez, F. A., Vaz‐de‐Mello, F. Z., da Rosa, P. , Fernandez, E. and Pires, A. S. (2019), Effects of howler monkey reintroduction on ecological interactions and processes. Conservation Biology, 33: 88-98. doi:10.1111/cobi.13188. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Quoll vs. toad: a toxic brew

A native of Central and South America, the cane toad, Rhinella marina, was introduced to Australia in 1935 with great fanfare. The plan was for the voracious cane toad to eat all of the grey-backed cane beetles that were plaguing sugar cane plantations in northern Australia (a similar introduction had been successful in Puerto Rico).  But the plan failed, in part because there was no cover from predators, so the toads were not enthusiastic about hanging out in sugar cane plantations, and in part because adult beetles live primarily near the tops of sugar cane, and cane toads are poor climbers.


A cane toad. Credit: Ben Philips

So now, northern Australia has a cane toad plague, which is wreaking havoc on ecosystems, and threatening many native species, including the northern quoll, Dasyurus hallucatus. These omnivorous marsupials eat fruit, invertebrates and small vertebrates.  Unfortunately, their long list of food items includes cane toads, which are highly toxic to most consumers, having poison glands that contain bufotoxin, a composite of several very nasty chemicals.  If a northern quoll eats a cane toad, it’s bye bye quoll.

Male captive born northern quoll_EllaKelly

A northern quoll. Credit: Ella Kelly.

Unfortunately most quolls have not gotten the message; huge numbers are dying, and populations are going extinct.  As toads continue their invasion from north to south, more quoll populations, particularly those in northwestern Australia, will be at risk.


Map of Australia showing past (light shading) and recent (dark shading) northern quoll distribution, and present (solid line) and future (dashed line) cane toad distribution.

Some quolls show “toad-smart” behavior and don’t eat toads. Ella Kelly and Ben Phillips are trying to understand how this happens. This is particularly important because a few quoll populations have managed to survive the cane toad plague by virtue of being toad-smart (though 95% of quoll populations have gone extinct in the wake of the cane toad wave). The researchers reason that if there is a genetic basis to toad-smart behavior, it might be possible to introduce toad-smart individuals into populations that have not yet been overrun by cane toads.  These individuals with toad-smart genes would breed and spread their genes through their adopted population.  This strategy of targeted gene flow would give the recipient population the genetic variation needed, so that some individuals (those with toad-smart genes) would be more likely to survive the cane toad invasion.  Over time toad-smart behavior would spread throughout the population via natural selection.

Targeted gene flow requires the trait to be influenced by genes.  To test for a genetic basis to the toad-smart trait, Kelly and Phillips designed a common-garden experiment, capturing some quolls that had survived the cane toad invasion (toad-exposed), and others from regions that had not yet been exposed (toad-naïve).  At Territory Wildlife Park, Northern Territory, Australia, the researchers bred these quolls to create three lines of offspring: Toad-exposed x toad-exposed, toad-exposed x toad-naïve (hybrids), and toad-naïve x toad-naïve.  They raised these three lines under identical conditions at the park. Kelly and Phillips then asked, are there behavioral differences in how these three lines respond to cane toads?


Northern quoll captured in Northern Territory, Australia. Credit: Ella Kelly.

The researchers set up two experiments.  First they asked, which would a quoll (that had never before experienced a cane toad) prefer to investigate if given the choice: a dead cane toad or a dead mouse? It turned out that the quoll offspring with two toad-exposed parents were somewhat more interested in mice than in cane toads.  The same was true for the hybrids.  However, the toads with two toad-naïve parents showed little preference.

Second, and more important, the researchers gave quolls from the three lines the opportunity to eat a toad leg (which does not have enough poison to harm the quoll). The results of this experiment were striking; offspring of toad-naïve parents were twice as likely to eat the toad leg than were offspring of toad-exposed parents, or hybrids with one parent of each type.


Proportion of toad-naive (both parents toad-naive), hybrid and toad-exposed (both parents toad-exposed) quoll offspring that ate a cane toad leg. Error bar = +/- 1 SE.

Kelly and Phillips conclude that toad-smart behavior is a genetically-based trait that has been under strong natural selection in populations of quolls that survived the cane toad invasion.  Hybrid offspring behave similarly to the offspring of two toad-exposed parents, suggesting that toad-smart behavior has a dominance inheritance pattern. The researchers propose using targeted gene flow, in this case introducing toad-adapted individuals into populations prior to the arrival of cane toads. Recently, Kelly and Phillips released 54 offspring with toad-smart genetic backgrounds onto Indian Island, which is about 40 km from Darwin.  The island has a large cane toad population, so the researchers will follow the introduced quoll population to see whether it is genetically equipped to survive in the presence of the cane toad scourge.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Kelly, E. and Phillips, B. L. (2019), Targeted gene flow and rapid adaptation in an endangered marsupial. Conservation Biology, 33: 112-121. doi:10.1111/cobi.13149. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Coral can recover (occasionally)

Coral reefs have amazing species diversity, which depends, in part, on a mutualism between the coral animal and a group of symbiotic algae that live inside the coral. The algae provide the coral host with approximately 90% of the energy it needs (from photosynthetic product).  In return, the algae are rewarded with a place to live and a generous allotment of nitrogen (mostly fecal matter) from the coral.  Unfortunately, coral are under attack from a variety of sources. Most problematic, humans are releasing massive amounts of carbon dioxide into the atmosphere, which is increasing ocean temperatures and also making the ocean more acidic.  Both processes can kill coral by causing coral to eject their symbiotic algae, making it impossible for the coral to get enough nutrients.

Moorea Coral Reef LTER site

The coral reef at Mo’orea. Credit: Moorea Coral Reef LTER site.

But other factors threaten coral ecosystems as well. For example, the reefs of Mo’orea , French Polynesia (pictured above), were attacked by the voracious seastar, Acanthaster planci, between 2006-2010, which reduced the coral cover (the % of the ocean floor that is covered with coral when viewed from above) from 45.8% in 2006 to 6.4% in 2009.  Then, in Feb 2010, Cyclone Oli hit, and by April, mean coral cover had plummeted down to 0.4%.

Moorea swimmers

Researchers survey the reef at Mo’orea. Credit: Peter Edmunds.

Peter Edmunds has been studying the coral reef ecosystem at Mo’orea for 14 years, and has observed firsthand the sequence of reef death, and the subsequent recovery.  Working with Hannah Nelson and Lorenzo Bramanti, he wanted to document the recovery process, and to identify the underlying mechanisms.  Fortunately Mo’orea is a Long Term Ecological Research (LTER) site, one of 28 such sites funded by the United States National Science Foundation.  Consequently the researchers had long term data available to them, so they could document how coral abundance had changed since 2005.  Their analysis showed the decline in coral cover from 2007 to 2010, but a remarkable rapid recovery beginning in 2012 and continuing through 2017.


% cover (+ SE) of all coral , Pocillopora coral (the species group that the research team focused on). and macroalgae at Mo’orea over a 13 year period based on LTER data. The horizontal bar with COTs above it represents the time period of maximum seastar predation.

What factors caused this sharp recovery? One general process that could be part of the answer is density dependence, whereby populations have high growth rates when densities are low and there is very little competition, and low growth rates when densities are high and there is a great deal of competition between individuals, or in this case, between colonies. The problem is that though density dependence makes intuitive sense, it is difficult to demonstrate, as other factors could underlie the coral recovery.  Perhaps after 2011 there was more food available, or fewer predators, or maybe the weather was better for coral growth.


High density (top) and low density (bottom) quadrats of Pocillopora coral established by the research group.

To more convincingly test for density dependence, Edmunds and his colleagues set up an experiment, establishing 18 1m2 quadrats in April, 2016. The researchers reduced coral cover in nine quadrats to 19.1% by removing seven or eight colonies from each experimental quadrat (low density quadrats), and left the other nine quadrats as unmanipulated controls, with coral cover averaging 32.5% (high density quadrats).  They then asked if, over the course of the next year, more recruits (new colonies < 4cm diameter) became established in the low density quadrats.

Returning in 2017, the researchers discovered substantially greater recruitment in the low density quadrats than in the high density quadrats. This experiment provides strong evidence that the rapid recovery after devastation by seastars and Cyclone Oli was helped by a density dependent response of the coral population – high recruitment at low population density.


Density of recruits just after (left), and one year after (right) the quadrats were established. Solid bars are means (+ SE) for high density quadrats, while clear bars are means (+ SE) for low density quadrats.

In recent years, many coral reef systems around the world have experienced declining coral cover, a loss of fish and invertebrate diversity and abundance, and an increase in abundance of macroalgae.  While many of these reefs continue to decline, others, such as the reefs at the Mo’orea LTER site, are more resilient, and are able to recover from disturbance.  The researchers argue that we need to fully understand the mechanisms underlying recovery – in other words what is causing the density dependent response? Is it simply competition between coral that cause high recruitment under low density, or may interactions between coral and algae be important?  And what types of interactions influence recruitment rates under different densities?  One possibility is that at high densities, coral are eating most of the tiny coral larvae as they descend from the surface after a mass spawning event.  This raises the important question of why many reefs around the world do not show this density dependent response.  Clearly there is much work remaining to be done if we want to preserve this critically endangered marine biome.

note: the paper that describes this research is from the journal Ecology. The reference is Edmunds, P. J., Nelson, H. R. and Bramanti, L. (2018), Density‐dependence mediates coral assemblage structure. Ecology, 99: 2605-2613. doi:10.1002/ecy.2511. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.