Australian eruptions

March 25, 2021 by fredsingerecology

Red foxes were introduced to Australia in the mid-19^th 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.

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.

Invading hippos

August 9, 2020 by fredsingerecology

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.

Gone gorilla

January 14, 2020 by fredsingerecology

Humans and lowland gorillas (Gorilla gorilla gorilla) share many features, including strong social bonds among members of their group. Lowland gorillas differ from humans in that one male (the silverback) dominates the group, which is composed of several females and their offspring. Some mature males are unable to attract females and may be consigned to a solitary existence. The silverback male mates with females in his group, and may allow other females to join. However, if a female joins a new group with an unweaned child, there is a high probability that the silverback will kill the child, as a way of getting the female into estrous more quickly, so that he can be the father of more future children.

A group of gorillas ranges over the landscape. Credit: Céline Genton CNRS/University of Rennes

The Odzala-Kokoua National Park in the Republic of Congo is home to several thousand lowland gorillas. Nelly Ménard and Pascaline Le Gouar (in affiliation with the ECOBIO laboratory CNRS/University of Rennes) have been studying two populations of these gorillas for over 20 years, and have identified and collected long-term data on 593 individuals from the two populations in their study. Working with their student, Alice Baudouin, and several other researchers, they documented that about 22% of the individuals were suffering from a yaws-like disease – an infectious skin disease caused by the bacterium Treponema pallidum pertenue.

A mother carries her infected infant. Credit: Ludovic Bouquier CNRS/University of Rennes

Females may disperse from their social group several times over the course of their lifetime. Factors influencing the decision to disperse include availability of a higher quality silverback, reduction of predation, and avoiding inbreeding, resource competition and disease. Given the prevalence and conspicuousness of yaws, the researchers suspected that these highly intelligent animals would use a variety of cues to inform them of whether they should disperse and which group they should attempt to join. They expected that females should leave diseased silverbacks for healthy ones, that they should leave groups with numerous diseased individuals and immigrate into groups with healthy individuals, and that diseased females would be less likely to leave their group. Other factors influencing a gorilla’s decision might include group size, group age and whether she had an unweaned infant in her care.

Silverback gorilla viewed from the mirador (observation post). Credit: Céline Genton CNRS/University of Rennes.

Because they considered so many variables, the researchers used their dataset to construct models of the probability of emigration (leaving the group) and immigration (entering a new group). The research team categorized each breeding group based on the age of the oldest offspring: young (oldest offspring less than 4 years), juvenile (<7.5 years), mature (<11 years) and senescent (< 14 years). Female gorillas were more likely to emigrate if their group had numerous infected individuals (graph a below) and if the silverback was severely infected (graph b). They were also more likely to leave an older breeding group, perhaps understanding that the silverback would be losing effectiveness in the near future (graph c). Lastly, females with unweaned infants were very unlikely to leave a group (graph d), presumably unwilling to accept the risk that their infant might starve or be killed if they attempted to join a new group.

Probability an adult female emigrates from her group in relation to (a) number of severely diseased individuals within her group, (b) presence of severe lesions on the silverback, (c) age of the breeding group, and (d) presence of an unweaned infant. Dotted lines (in graph a) and bars (in graphs b, c and d) indicate 95% confidence intervals.

The research team did a similar analysis of factors associated with female gorillas immigrating into a new breeding group.

Probability an adult female immigrates into a group in relation to (a) age of group, (b) presence of severely diseased individuals, and (c) group size. Bars (in graph a and b) and dotted lines (graph c) indicate 95% confidence intervals.

They discovered that females were much more likely to join younger groups which had younger silverbacks (graph a). In addition, females tended to join groups without any severely diseased individuals (graph b). They were also attracted to smaller groups (graph c).

Based on these data, it is clear that disease strongly influences female dispersal decisions. Females were much more likely to disperse from breeding groups with numerous infected individuals, and strongly avoided groups with more than two diseased individuals. This is not surprising, given how conspicuous these skin lesions are, particularly in the facial regions. Contrary to expectation, female disease status (infected or not) did not influence female dispersal tendency. The researchers suggest that dispersal might not be particularly costly to the female (assuming she does not have an unweaned infant) because the home range of social groups overlap broadly so it is easy to move from one group to another, and food is also plentiful throughout the range.

Many features of a gorilla’s social environment influence its dispersal decisions. Because diseased females are as likely to disperse as healthy females, the disease pathogen may be more easily spread into previously uninfected gorilla populations. On the other hand, dispersing female avoidance of diseased populations has the effect of quarantining the diseased populations. The researchers hope to get a better understanding of the mechanisms of female appraisal of their social environment, so they can predict changes in the prevalence of this pathogen.

note: the paper that describes this research is from the journal Ecology. The reference is Baudouin, A., S. Gatti, F. Levrero, C. Genton, R. H. Cristescu, V. Billy, P. Motsch, J.-S. Pierre, P. Le Gouar, and N. Ménard. 2019. Disease avoidance, and breeding group age and size condition the dispersal patterns of western lowland gorilla females. Ecology 100(9): e02786. 10.1002/ecy.2786. 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.

Fast living vs. slow and steady

September 10, 2019 by fredsingerecology

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.

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.

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.

Rewilding tropical forests: dung is the key

May 25, 2019 by fredsingerecology

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.

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 19^thcentury 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.

Coral can recover (occasionally)

March 7, 2019 by fredsingerecology

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.

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%.

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.

Meandering meerkats

November 14, 2018 by fredsingerecology

Dispersal – the movement of individuals to a new location – is a complex process that ecologists divide into three stages: emigration (leaving the group), transience through an unfamiliar landscape, and settlement in a suitable habitat. Dispersal is fraught with danger, as dispersers usually have a higher chance of starving, of getting eaten by predators, and may suffer a low reproductive rate. So why move?

The problem is that there are major issues with not moving. First, if nobody disperses, population densities could increase alarmingly, putting strains on resources and increasing the incidence of disease transmission. Second, if nobody disperses, close relatives would tend to live near each other. If these relatives mate, there would be a high probability of bad combinations of genes being expressed, leading to developmental abnormalities or high offspring mortality (geneticists call this inbreeding depression). In social species, such as meerkats, Suricata suricatta, the issues are even more complex, as dispersal could break up social groups that work well together to detect predators or find resources. Nino Maag and his colleagues explored what factors influence meerkat dispersal decisions, their survival and reproduction, and how those factors affected overall population dynamics in the Kuruman River Reserve in South Africa.

A group of vigilant meerkats. Credit: Arpat Azgul

Meerkats live in groups of 2-50 individuals, with a dominant pair that monopolizes reproduction. While pregnant, the dominant female usually evicts some subordinate females from the group; this coalition of evictees will either remain apart from the group (but within the confines of the territory) and eventually be allowed back in, or else emigrate to a new territory. By attaching radio collars to subordinate females, the researchers were able to follow emigrants to determine their fates.

Nino Maag collects data in the Kalahari Desert while a meerkat, wearing a radio collar, strolls by. Credit: Gabriele Cozzi.

How does population density affect emigration rates of evicted females? You might think that meerkats would be most likely to emigrate at high population density, as a way of avoiding resource competition. As it turns out the story is more complicated. First, individual females (solid lines in graph below) are more likely to remain with the group (not emigrate) than are groups of two or more females (dashed lines). Second, emigration rates were highest at low population density, intermediate at high population density and lowest at intermediate population density. This nonlinear effect can be explained by low benefits of remaining in a very small group, so evictees are more likely to emigrate. But as population density (and group size) increase, then the meerkats enjoy higher success as a result of cooperation between individuals (in particular, detecting and avoiding predators). But when population densities get too high, there are not enough resources to go around, and evictees are more likely to emigrate.

Proportion of evicted female meerkats that had not yet emigrated in relation to time since eviction at low (red), medium (light blue) and high (dark blue) population density. Solid lines represent individual females, while dashed lines are coalitions of two or more females.

In addition to the density effects we just discussed, association with unrelated males from other groups early after eviction increased the probability that females would emigrate – presumably this increased the probability females would quickly create offspring in their new territory. Females also dispersed longer distances if unrelated males did not meet up with them, possibly to avoid inbreeding with closely-related males from neighboring groups.

Coalitions were more likely to return to the group if females were not pregnant – in fact 62% of pregnant evictees aborted their litters before being allowed back into the group. Of the ones that did not abort before returning, only 42% of their litters survived to the first month.

The period of transience, when emigrators are seeking new territories can be prolonged and dangerous. The mean dispersal distance was 2.24 km, and averaged about 46 days. Larger coalitions with males present tended to disperse the shortest distances (left graph below). Dispersers took longest to settle at high population density – perhaps there were fewer available territories under those conditions (right graph below).

A. Effect of coalition size and presence of unrelated males on dispersal distance. B. Effect of population density on transience time (interval between emigration and settling).

Large coalitions settled more quickly than did small coalitions, particularly if accompanied by unrelated males. Once settled, females successfully carried through 89% of their pregnancies (compare that to the 62% abortion rate of females that returned to their original group). These females had a litter survival rate (to the first month) of 65%.

Social and non-social species are influenced by population density in different ways. The situation is relatively simple for non-social species; as population size increases, competition between individuals increases, so dispersal is more likely. However, even for non-social species, we might expect dispersal at very low population levels, if there are no mates available. For social species such as meerkats, the situation is more complex. Cooperation enhances survival and reproduction, so it is better to be in a larger group (with more cooperators). At the same time, if the group is too large, then resource competition starts being an increasingly disruptive factor. As ecologists collect more dispersal data from other social species, they will be able to test the hypothesis that population density in many species influences dispersal in a non-linear way.

note: the paper that describes this research is from the journal Ecology. The reference is Maag, N. , Cozzi, G. , Clutton‐Brock, T. and Ozgul, A. (2018), Density‐dependent dispersal strategies in a cooperative breeder. Ecology, 99: 1932-1941. doi:10.1002/ecy.2433. 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.

Climate changes a bird’s life in shrinking grasslands

September 21, 2018 by fredsingerecology

Back in graduate school, a couple of my grad student buddies and I would get together to fish for brown trout in the Kinnickinnic River in western Wisconsin. We were students at the University of Minnesota (Twin Cities), but the Kinni was the closest trout stream. Tired of catching small brown trout, we consulted a trout fishing map and discovered that the headwaters of the Kinni were rich in brook trout. So early one morning, map in hand, we followed strange paths and found our sacred brook trout haven. Alas, the only thing it was rich in was corn, now about two feet high – though there was a modest depression where trout waters once had flowed. Our personal depression was perhaps more than modest – having been robbed of brook trout, and the opportunity to experience some pristine waters flowing through a beautiful grassland.

Grasslands, one of the biomes native to parts of Wisconsin and Minnesota, are globally one of the most endangered biomes, because they usually are relatively easy to convert into farmland and suburban developments. Native grasslands harbor a wide biological diversity; consequently conservation biologists are concerned about their continued loss.

Cool-season grassland in southwest Wisconsin. Credit: John Dadisman.

Ben Zuckerberg, Christine Ribic and Lisa McCauley wanted to know how environmental factors influenced the nesting success of grassland birds, in particular, because as obligate ground nesters, they might be susceptible to changing weather conditions that will be affecting the climate in coming decades. A nest built on the ground is much less insulated from the environment than one built in or on a tree or even a ledge.

Seven day old bobolink chicks in a ground nest. Credit: Carolyn Byers.

Zuckerberg and his colleagues used Google Scholar and the ISI Web of Science to comb the literature (1982-2015) for studies that explored the nest success of obligate grassland birds in the United States. They identified 12 bird species from 81 individual studies of 21,000 nests. Based on their experience and the literature, both precipitation and temperature were likely to influence nest success, which is the proportion of nests that fledge at least one young. They considered three precipitation time periods: (1) Bioyear – previous July through April of the breeding season, (2) May of the breeding season, (3) June – August of the breeding season. They considered breeding season temperatures during May, and during the period from June-August. The researchers were also interested in the size of the grassland (grassland patch size), reasoning that a larger grassland might provide more diverse microclimates, so, for example, a bird might be able to find a dry microhabitat for nesting in a large grassland, even in a wet breeding season.

Map of the identity and location of species considered for this study.

The researchers discovered that both temperature and precipitation were important. Nest success increased steadily with bioyear precipitation (Figure (a) below). Presumably, more rain led to more plant growth and more insect survival, which would help feed the young. Taller plants could also help shade or hide the nests. In contrast, nest success declined sharply with precipitation during spring and summer of the breeding season (Figure (b) and (c)). Heavy rains during the breeding season can flood nests, and also decrease the foraging efficiency of parents who might need to spend more time incubating nests during rainstorms. Lastly, extreme (low or high) May temperatures depressed nest success, which was highest at intermediate temperatures (Figure (d)). Egg viability depends on maintaining a constant temperature, and the parents may be more challenged to thermoregulate at extreme temperatures. Temperatures later in the breeding season did not affect nest success.

Effects of (a) bioyear precipitation (previous July – April of the breeding season), (b) May precipitation during the breeding season, (c) June – August precipitation during the breeding season, and (d) May temperature on nest success. Shaded area represents 95% confidence interval.

But all is not straightforward in the grassland nest success world. These main findings about precipitation and temperature interacted with grassland size in interesting ways. For example high bioyear precipitation, which overall increased nest success, only did so for smaller grassland patches (dashed line in top graph below), but not for larger patches (solid line). Extreme May temperatures had different effects on nest success in relation to grassland patch size. Low May temperatures were associated with high nest success in small patches (dashed line in bottom graph) and with low nest success in large patches (solid line). High May temperatures were associated with high nest success in large patches, and with low nest success in small patches.

Predicted nest success of grassland birds in relation to bioyear precipitation (top graph) and May temperature (bottom graph) in relation to grassland patch size. Solid lines represent large grasslands, while dashed lines represent small grasslands. Shaded area is 95% confidence interval.

The researchers were surprised to discover that patch size affected how weather influenced grassland bird nesting success. Some of the patterns seem intuitively logical; for example, in unusually hot breeding seasons birds had higher nest success in larger grasslands than in smaller grasslands. Presumably, birds were more likely to find a cooler microclimate for their nests in a large grassland. However it is puzzling why in unusually cold breeding seasons birds had higher nest success in smaller grasslands. The researchers are planning a follow-up study to better document and measure the existence of microclimates in grasslands of different sizes, and explore how different microclimates influence the nesting success of vulnerable grassland birds. Finding that warmer temperatures and drought generally reduce nest success to the greatest extent in small grassland patches is strong incentive for conservation mangers to establish large core grasslands as a tool to maintain bird populations in the wake of present and future changes to the climate.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Zuckerberg, B. , Ribic, C. A. and McCauley, L. A. (2018), Effects of temperature and precipitation on grassland bird nesting success as mediated by patch size. Conservation Biology, 32: 872-882. doi:10.1111/cobi.13089. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2018 by the Society for Conservation Biology. All rights reserved.

Recruiting rhinoceros

August 30, 2018 by fredsingerecology

Despite their immense size and unfriendly disposition, we humans have done an excellent job decimating the black rhinoceros (Diceros bicornis) population from several hundred thousand individuals around 1900 to fewer than 3000 individuals by 1990. Three subspecies are extinct, one is on the brink, while even the most successful remaining subspecies, the south-central black rhinoceros, is extinct over much of its former range, or only found in nature reserves. Humans prize its horns, which historically were used for making wine cups, ceremonial daggers, and a variety of medicines that purportedly revive comatose patients, detoxify infections, and aid male sexual stamina. Trade of rhinoceros horn has been illegal since 1977, but poaching abounds.

Rhinoceros horn products seized by the Hong Kong Government. Credit: U.S. Government Accountability Office from Washington, DC, United States [Public domain], via Wikimedia Commons

This population crisis has motivated conservation ecologists to evaluate the best approaches to conserving the black rhinoceros, and to restoring it to parts of its former range. Translocation, moving rhinoceroses from one location where they are relatively well-established, to another where they are extinct or at very low numbers, is a viable approach to restoring populations. However, translocation did not always work well, as some rhinoceroses died following translocation, or failed to reproduce even if they survived translocation. While a post-doctoral researcher at the Zoological Society in San Diego, California, Wayne Linklater recognized that there were large datasets collected by government and non-government agencies that might answer the question about what drives better rhinoceros survival and breeding. Unfortunately some of these datasets were difficult to access or interpret, but Jay Gedir, Linklater and several other colleagues persevered and combed through the data to identify the factors associated with successful translocation.

Black rhino is airlifted to temporary captivity before being translocated to a release site. Credit: Andrew Stringer

Many translocation studies use a short-term measure of success, such as survival or fecundity (fertility) of the translocated individuals. The researchers reasoned that the most important measure, when it is available, is the number of offspring produced by translocated females that survive to an age that they too can reproduce, which in the case of rhinoceros is four years. Based on existing long-term studies, Gedir, Linklater and their colleagues compiled the offspring recruitment rate (ORR), which combines the variables of survival, fecundity, and offspring survival to sexual maturity. They found that ORR was greatest when mature females were translocated into a population that had a female-biased sex ratio.

Offspring recruitment rate (per year) in relation to age at release and sex ratio bias of the recipient population. Female bias is greater than 60% female, male bias is greater than 60% male. Numbers above graph are sample size for each category. Error bars are 95% confidence intervals.

So why does sex-ratio matter? The researchers are not certain, but females are subjected to considerable sexual harassment by very aggressive males, so a female-biased sex ratio may lead to less harassment and improved survival and reproductive success for translocated females.

Translocated juveniles took longer to produce their first calf after reaching sexual maturity than did adults after being released. Again this effect was stronger with a male-biased sex ratio.

Mean time (years) to produce their first calf after reaching sexual maturity for juveniles, or after release for adults.

Many females (47%) produced no surviving offspring. This pattern of recruitment failure was most common in juveniles, and least common in adults translocated into populations with a female-biased sex ratio.

Recruitment failure of translocated females in relation to age at release and sex ratio bias of the recipient population.

Several factors can cause recruitment failure: 23% of females died following translocation and 24% of surviving females never produced calves. However, calf survival to sexual maturity was a robust 89%, and this survival rate was independent of age at translocation or population sex ratio. Among surviving translocated females, juveniles were about twice as likely as young or old adults to fail to produce a calf, but were equally successful at raising the calves they were able to produce.

Linklater was surprised at how important sex ratios (and presumably social relationships) were, particularly given that black rhinoceroses spend much of their time alone or with one offspring.

Neto (the ranger) and Gwala (the rhino) size each other up. Credit: Wayne Linklater.

The study did not support a major role for many other factors that had previously been considered important in translocation success, such as habitat quality, population density, number of rhinoceroses released and reserve size. Based on their analysis, the researchers recommend that conservation biologists should translocate mature females into populations with female-biased sex ratios to reduce rates of recruitment failure. If juveniles must be translocated, they too should be moved into populations that already have a female-biased sex ratio to reduce levels of sexual harassment by males after they mature.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Gedir, J. V., Law, P. R., du Preez, P., & Linklater, W. L. (2018). Effects of age and sex ratios on offspring recruitment rates in translocated black rhinoceros. Conservation Biology, 32(3), 628-637. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2018 by the Society for Conservation Biology. All rights reserved.

Homing in on the micro range

February 8, 2018 by fredsingerecology

I’ve always been fascinated by geography. As a child, I memorized the heights of mountains, the populations of cities, and the areas encompassed by various states and countries. I can still recite from memory many of these numbers – at least based on the 1960 Rand McNally World Atlas. Part of my fondness for geography is no doubt based on my brain’s ability to recall numbers but very little else.

Most geographic ecologists are fond of numbers, exploring numerical questions such as how many organisms or species are there in a given area, or how large an area does a particular species occupy? They then look for factors that influence the distribution and abundance of species or groups of species. Given that biologists estimate there may be up to 100 million species, geographic ecologists have their work cut out for them.

As it turns out, most geographic ecologists have worked on plants, animals or fungi, while relatively few have worked on bacteria and archaeans (a very diverse group of microorganisms that is ancestral to eukaryotes).

Two petri plates with pigmented Actinobacteria. Credit: Mallory Choudoir.

Until recently, bacteria and archaeans were challenging subjects because they were so small and difficult to tell apart. But now, molecular/microbial biology techniques allow us to distinguish between closely related bacteria based on the sequence of bases (adenine, cytosine, guanine, and uracil) in their ribosomal RNA. Bacteria which are identical in more than 97% of their base sequence are described as being in the same phylotype, which is roughly analogous to being in the same species.

As a postdoctoral researcher working in Noah Fierer’s laboratory with several other researchers, Mallory Choudoir wanted to understand the geographic ecology of microorganisms. To do so, they and their collaborators collected dust samples from the trim above an exterior door at 1065 locations across the United States (USA).

Dr. Val McKenzie collects a dust sample from the top of a door sill. Credit: Dr. Noah Fierer.

The researchers sequenced the ribosomal RNA from each sample to determine the bacterial and archaeal diversity at each location. Overall they identified 74,134 gene sequence phyloypes in these samples – that took some work.

On average, each phylotype was found at 70 sites across the USA, but there was enormous variation. By mapping the phylotypes at each of the 1065 locations, the researchers were able to estimate the range size of each phylotyope. They discovered a highly skewed distribution of range sizes, with most phylotypes having relatively small ranges, while only a very few had large ranges (see the graph below). As it turns out, we observe this pattern when analyzing range sizes of plant and animal species as well.

Mean geographic range (Area of occupancy) for each phylotype in the study. The y-axis (Density) indicates the probability that a given phylotype will occupy a range of a particular size (if you draw a straight line down from the peak to the x-axis, you will note that most phylotypes had an AOO of less than 3000 km²

Taxonomists use the term phylum (plural phyla) to indicate a broad grouping of similar organisms. Just to give you a feel for how broad a phylum is, humans and fish belong to the same phylum. Some microbial phyla had much larger geographic ranges than others. Interestingly, it was not always the case that the phylum with the greatest phylotype diversity had the largest range. For example, phylum Chrenarchaeota had the greatest median geographic range (see the graph below), but ranked only 19 (out of 50 phyla) in number of phylotypes (remember that a phylotype is kind of like a species in this study).

Box plots showing range size distribution for individual phyla. Middle black line within each box is the median value; box edges are the 25th and 75th percentile values (1st and 3rd quartiles). Points are outlier phylotypes. Notice that the y-axis is logarithmic.

With this background, Choudoir and her colleagues were prepared to investigate whether there were any characteristics that might influence how large a range would be occupied by a particular phylotype. We could imagine, for example, that a phylotype able to withstand different types of environments would have a greater geographic range than a phylotype that was limited to living in thermal pools. Similarly, a phylotype that dispersed very effectively might have a greater geographic range than a poor disperser.

The researchers expected that aerobic microorganisms (that use oxygen for their metabolism) would have larger geographic ranges than nonaerobic microorganisms, which are actually poisoned by oxygen. The data below support this prediction quite nicely.

Geographic range size in relation to oxygen tolerance. In this graph, and the graphs below, the points have been jittered to the right and left of their bar for ease of viewing (otherwise even more of the points would be on top of each other).

Some bacterial species form spores that protect them against unfavorable environmental conditions. The researchers expected that spore-forming bacteria would have larger geographic ranges than non-spore-forming bacteria.

Geographic range in relation to spore formation (left graph) and pigmentation (right graph).

Choudoir and her colleagues were surprised to discover exactly the opposite; the spore forming bacteria had, on average, slightly smaller geographic ranges. Choudoir and her colleagues also expected that phylotypes that are protected from harsh UV radiation by pigmentation would have larger geographic ranges than unpigmented phylotypes – this time the data confirmed their expectations.

The researchers identified several other factors associated with range size. For example, bacteria with more guanine and cytosine in their DNA or RNA tend to have larger geographic ranges. Some previous studies have shown that a higher proportion of guanine and cytosine is associated with greater thermal tolerance, which should translate to a greater geographic range. Choudoir and her colleagues also discovered that microorganisms with larger genomes (longer DNA or RNA sequences) also had larger ranges. They reason that larger genomes (thus more genes) should correspond to greater physiological versatility and the ability to survive variable environments.

This study opens up the door to further studies of microbial geographic ecology. Some patterns were expected, while others were surprising and beg for more research. Many of these microorganisms are important medically, ecologically or agriculturally, so there are very good reasons to figure out why they live where they do, and how they get from one place to another.

note: the paper that describes this research is from the journal Ecology. The reference is Choudoir, M. J., Barberán, A., Menninger, H. L., Dunn, R. R. and Fierer, N. (2018), Variation in range size and dispersal capabilities of microbial taxa. Ecology, 99: 322–334. doi:10.1002/ecy.2094. 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.

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Category Archives: Populations