Meandering meerkats

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.

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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.

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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.

MaagFig2A

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).

MaagFig4

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.

Savanna plant survival: hanging out in the right crowd

Tyler Coverdale first visited the Mpala Research Centre in Laikipia, Kenya in 2013, and immediately became painfully aware of the abundant spiny and thorny plants that cover the savanna.  Spines help defend the plants from voracious elephants, giraffes and numerous other herbivores that depend on vegetation for their sustenance.

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Camels browsing on  Barleria trispinosa at Mpala Research Centre, Kenya. Credit Tyler Coverdale.

Acacia trees such as Acacia etbaica (left foreground below) dominate the landscape, and may be associated with smaller shrubs, such as Barleria trispinosa. In the photo below, there is one B. trispinosa plant immediately below (on the right side) the acacia tree, and a second B. trispinosa plant to its right, more out in the open.  Coverdale realized that being situated immediately below a spiny acacia tree might be advantageous to B. trispinosa, which could be protected from the ravages of elephants and giraffes by the acacia thorns .

MRC landscape

Credit: Tyler Coverdale.

As you might guess by its name, B. trispinosa is itself a very spiny plant, which should help protect it from browsers.  Nonetheless, it still gets eaten, so Coverdale and his colleagues explored whether being under acacias would reduce how much it, and two other related species, got browsed.

Barleria trispinosa

Barleria trispinosa out in the open. Credit: Tyler Coverdale.

The first study was observational – a survey of the damage three species of Barleria suffered when they were under (associated with) acacia trees vs. unassociated with acacia trees. For each Barleria species, the researchers haphazardly chose 10 stems from eight associated and eight unassociated plants, and measured the proportion of these stems that showed physical evidence of being browsed.  As the figure below shows, browsing was sharply lower for each species when it was associated with an acacia plant.

CoverdaleFig1A

Percentage of stems damaged by browsers for three Barleria species in relation to whether they were associated or unassociated with an acacia tree.* indicates significant differences between means in all figures.

The understory plant community associated with acacias is much denser than the plant community out in the open, so the researchers wondered whether it was the acacia itself, or the other plants associated with it, that were providing protection. They set up an experiment using focal B. trispinosa plants with four treatments (A) unmanipulated control, (B) overstory removal, (C) overstory + understory removal, (D) a procedural control with overstory + understory removal, with the focal plant enclosed in a metal cage to protect it from predators (see Figure below).

CoverdaleS1

Coverdale and his colleagues ran the experiment for one month.  They discovered that removing overhanging acacia branches sharply increased herbivory, but the additional removal of understory neighbors had little additional effect.  Both the unmanipulated controls and procedural controls were unaffected.

CoverdaleFig1B

Change in % of stems browsed for (A) unmanipulated control (left bar), (B) overstory removal (second from left bar), (C) overstory + understory removal (second from right bar), (D) a procedural control (right bar).  Different letters above bars indicate significant differences between the mean values.

The researchers then investigated how useful these spines are to unassociated B. trispinosa plants. They set up another experiment with four types of spine treatments: (A) unmanipulated controls, (B) 50% spine removal, (C) 100% spine removal, (D) procedural control with 100% spine removal + enclosure within a predator-proof cage. These cages were vandalized shortly after the experiment was set up, so the researchers chose eight plants from a nearby plot (that had all predators excluded for a different experiment) as their procedural control. They discovered that spines are very useful to protect against predators in unassociated B. trispinosa.

CoverdaleFig1C

Change in % of stems browsed for (A) unmanipulated control (left bar), (B) 50% spine removal (second from left bar), (C) all spines removed (second from right bar), (D) procedural control (right bar).

If you were a plant living under the protection of an acacia tree, it would make sense for you to reduce your investment in thorns, so you could allocate more resources to growth and reproduction.  Does Barleria do this?

CoverdaleFig2

Several lines of evidence indicate that all three Barleria species reduce their investment in spines when associated with an acacia. First, a survey of spine density shows a reduced number of spines for all three species when they were associated with acacia trees (top graph).  Second, the spines that are present are significantly shorter in Barleria species associated with acacia trees (middle graph).  In a final survey, Coverdale and his colleagues cut all of the spines off of associated and unassociated Barleria.  For each plant, the researchers calculated the dry weight of spines and of all the other plant tissue.  For each Barleria species, the defensive investment – the ratio of spines to total mass, was substantially reduced in acacia-associated plants in comparison to unassociated plants (bottom graph).

Lastly, can plants react adaptively to browsing?  In other words, will understory plants produce more thorns if they are browsed?  To explore this question, the researchers used scissors to simulate moderate (25%) or heavy (50%) browsing.  They discovered a significant increase in spines produced by unassociated plants one month after clipping. Ecologists call this an induced defense. This induced defense is strongly suppressed in plants that have lived under the protection of acacia trees – in fact there was no significant response to experimental browsing in acacia-associated B. trispinosa plants. The researchers don’t know how long this suppression of induced responses persists. Would browsing induce increased spine growth in B. trispinosa six months, a year or two years after its protective acacia tree died?

Coverdale and his colleagues conclude that the overall benefit of association is positive to the plant populations.  Their studies show better survival and higher reproductive rates of acacia-associated understory plants. There is probably a cost associated with too many offspring competing for resources within a small area, as seedlings tend to grow within 1 meter of their parents.  However the reduction in defense costs probably overrides this cost of competition, leading to increased population size.  The researchers suggest a long-term study of population growth rates for acacia-associated and unassociated plants for several different species to see how general these effects are, and to explore whether other factors, such as soil moisture and nutrient levels influence the allocation and induction of defensive structures such as spines and thorns.

note: the paper that describes this research is from the journal Ecology. The reference is Coverdale, T. C., Goheen, J. R., Palmer, T. M. and Pringle, R. M. (2018), Good neighbors make good defenses: associational refuges reduce defense investment in African savanna plants. Ecology, 99: 1724-1736. doi:10.1002/ecy.2397. 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.

Intertidal tussles: a shifting balance

As an omnivore with a research-oriented palate, I delight in consuming many different food types.  High on my list are crustaceans – in particular the American lobster, Homarus americanus.

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A juvenile American lobster, Homarus americanus. Credit: C. Baillie.

However, another crustacean, the invasive Asian shore crab, Hemigrapsus sanguineus, threatens to disrupt my epicurean delight, by interfering with the growth and development of juvenile lobsters in the low intertidal zone in the north Atlantic. Christopher Baillie and Jonathan Grabowski have explored interactions between these lobsters and crabs to unravel how they might be influencing each other.

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The invasive Asian shore crab, Hemigrapsus sanguineus. Credit: Rhode Island Marine and Estuarine Invasive Species Site.

The Asian shore crab was first detected off the New Jersey coast in 1988 and quickly spread from North Carolina to Maine. Their increase has coincided with a sharp decrease in the abundance of their rival green crabs over the same range. Baillie and Grabowski were concerned that the Asian shore crab could also be adversely affecting lobster populations. They did monthly surveys (May-October) of both lobster and crab densities in Dorothy Cove in Masachusetts, USA, between 2013 and 2017, and discovered that crab populations were increasing sharply at the same time that lobster populations were decreasing steadily.

BaillieFig1

Annual average densities of Asian shore crabs (dark gray) and American lobsters (light gray) from surveys at Dorothy Cove, Nahant, Massachusetts, USA, between 2013 and 2017. Error bars are 1 standard error.

The researchers wanted to know whether the increased number of Asian shore crabs was responsible for the lobster decline. Perhaps the two species competed with each other for shelter. Baillie and Grabowski set up experimental tanks, each containing a wire mesh bottom with a rectangular opening cut in the center, so that a burrow could be excavated.  They then introduced a single lobster or crab to the tank, and allowed it to dig a burrow in the cutout center (we’ll call this individual the resident).

Slide1

In one shelter experiment, the researchers compared the behavior of larger (mean carapace length = 24.7 cm) and smaller (mean carapace length = 9.3 cm) juvenile lobsters in the presence and absence of a variable number of crabs. They discovered that both larger and smaller lobsters spent most of the time in their burrow when no crabs were in the tank. However, introducing crabs was a major disruptor to their mellow existence, with both lobster size classes being more likely to abandon their residences when crabs were present.

BaillieFig3final

Mean (+ standard error) percentage of time spent in shelter by large juvenile lobsters (top graph) and small juvenile lobsters (bottom graph) in relation to absence (Control) or presence of different numbers of crabs.  Different letters above the bars indicate that the means are statistically different from each other.

The reasons for the decline in residence time were very different for large vs. small lobsters.  In an experiment with one large lobster pitted against one crab, resident lobsters initiated an average of 18.00 attacks against crabs, while resident crabs initiated an average of only 0.20 attacks against lobsters. Even if crabs were allowed to establish residency, when a lobster was introduced, it usually picked a fight with the resident crab. So large resident lobsters left their burrows to challenge intruding crabs. Lobsters managed to kill and eat two intruding crabs.

In contrast, smaller lobsters had a much different experience. Crabs attacked resident small lobsters and were able to displace them from their burrow. This was particularly the case when a greater number of crabs were added to the tank.  When eight crabs were added, the poor lobster was kicked out of its burrow, on average, almost 20 times within a six-hour trial.  Under these conditions, crabs attacked the resident lobster almost 40 times per trial.

BaillieFig4

Crab behavior towards a resident lobster in relation to the number of crabs (heterospecific competitors) introduced into the tank. (A) Mean number of times the lobster is displaced. (B) Mean number of fights initiated by an intruder crab. Error bars are 1 standard error. Different letters above the bars indicate that the means are statistically different from each other.

Baillie and Grabowski also conducted feeding trials – but only with a larger lobster pitted against an individual crab (a blue mussel – a preferred food item for both species – was the prey).  Lobsters were much more successful feeders than crabs, and actually increased their feeding rates in the presence of crabs, presumably having no interest in sharing the mussel with its competitor. Taken together, the shelter and feeding experiments suggest a reversal in dominance structure occurs over the course of lobster development.  The abundant Asian shore crab outcompetes small juvenile lobsters for shelter, but once lobsters attain a certain size, they can outcompete crabs for both shelter and food. We still don’t know, for sure, whether the decline in lobsters in the low intertidal zone at the study site was caused by the increase in crabs; the Asian shore crab may still be expanding its range, so it may be possible to more directly study changes in distribution at other sites both north and south of its current range. Fortunately for lobsters (and for lobster consumers), juveniles can also grow and flourish in deeper ocean waters, where Asian shore crabs are much less of a threat.

note: the paper that describes this research is from the journal Ecology. The reference is Baillie, C. J. and Grabowski, J. H. (2018), Competitive and agonistic interactions between the invasive Asian shore crab and juvenile American lobster. Ecology, 99: 2067-2079. doi:10.1002/ecy.2432. 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

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.

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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.

Bobolink 7 days (Carolyn Byers)

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.

ZuckFig1

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.

ZuckFig2

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.

ZuckFig3

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.

 

Carbon dioxide’s complex personality

Carbon dioxide (CO2) deservedly gets a lot of bad press because it is responsible for much of the global warming Earth is currently experiencing.  Less publicized, but perhaps equally important, CO2 is acidifying oceans, thereby threatening the continued existence of some critical biomes such as coral reefs and kelp forests (acid interferes with the ability of many marine organisms to build their shells).  But carbon dioxide also has a kinder, gentler side, as it is an essential resource for plants, and in some cases higher CO2 levels can increase a plant’s ability to carry on photosynthesis.  Sean Connell and his colleagues explored this complex personality by studying a marine ecosystem that experiences naturally varying levels of CO2. High CO2 levels and acidity exist near CO2-emitting vents at the study site – a volcanic island (Te Puia o Whakaari) off the coast of New Zealand.

White_Island_James Shook [CC BY 2.5 (https-::creativecommons.org:licenses:by:2.5)], from Wikimedia Commons

The volcanic Te Puia o Whakaari off the coast of New Zealand’s north island. Credit: James Shook [CC BY 2.5 (https-//creativecommons.org/licenses/by/2.5)], from Wikimedia Commons.

The major players in this ecosystem are the kelp, Ecklonia radiata, several species of turf-forming algae, and two grazers, the snail, Eatoniella mortoni, and the urchin, Evechinus chloroticus.  The typical vegetation in the region is a mosaic of kelp forest, some scattered small patches of algal turf, and sea urchin barrens – hard rock without significant vegetation, a result of overgrazing by sea urchins.  In contrast, extensive algal mats carpeted the rocks near these vents, and the researchers hypothesized that high CO2 levels caused this shift in dominant vegetation.

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Sean Connell collects data in a habitat dominated by algal turf (and numerous fish). Credit: anonymous backpacker.

Connell and his colleagues chose two vents and two nearby control sites at a depth of 6-8 meters. The CO2 levels and acidification near the vents were approximately equal to the amount projected for the end of the 21stcentury, but there were no differences between vents and controls in temperature, salinity or nutrient concentrations. The researchers estimated photosynthetic rates for kelp and turf algae by measuring the rate of oxygen production. They also estimated snail consumption rates by caging them for 3 days and measuring how much algal turf they removed.  They used an analogous approach to measure sea urchin consumption rates.

Conditions at vents had a major impact on both producers and consumers.  Kelp production decreased slightly, while turf production increased sharply at vents (Figures A and B below).  Urchin density declined (almost to nonexistence) while gastropod density increased markedly at vents (Figures C and D).  Lastly, consumption rates (on a per individual basis) by urchins plummeted, while consumption rates by snails increased sharply at vents (Figures E and F).

ConnellFig3

Comparison of production and consumption at control sites vs. carbon dioxide emitting vents.

These patterns converted the normal mosaic of kelp forest, small algal turf patches and urchin barren into turf-dominated habitats.  Algal turf increased in size and frequency near the vents, while kelp forest shrank into near oblivion.

ConnellFig2

Frequency of patches of turf (light gray bars), urchin barren (medium gray) and kelp (black) in relation to patch size (diameter in meters) at control sites (top graph) and sites near vents (bottom graph).

These results can be pictured visually by the graph below.  Under conditions of present-day pH and CO2 levels, gross algal production is relatively low and urchin consumption is relatively high, which results in negligible net algal turf production (net production = gross production – urchin and gastropod consumption).  High CO2 levels sharply increase gross algal turf production while dramatically decreasing consumption by urchins.  Even though gastropod consumption increases slightly at vents, the overall effect on vents is a dramatic increase of net algal turf production. Consequently, the ecosystem experiences regime shift from kelp to algal turf domination.

ConnellFig1

Summary of effects of CO2 release by vents (bottom) vs Controls (top). Net algal production (red circle) = Gross algal production – urchin and gastropod consumption.  Net algal production in dark green zone is predicted to be turf-dominated (as is found near vents), light green is a mosaic, while white zone represents urchin barrens (low production and high consumption). Error bars are 1 standard error. 

Under current conditions, kelp is the dominant producer over turf algae in the near offshore ecosystem. High consumption by urchins keep the turf algae in check.  But near CO2 emitting vents, high levels of carbon dioxide have a dual effect on this ecosystem, disproportionately increasing turf algae production rate and decreasing urchin abundance and consumption rate.  This allows the competitively subordinate turf algae to replace the competitively dominant kelp, resulting in a dramatically changed ecosystem.  This occurs in the absence of an increase in ocean temperature.  Given that ocean temperature will increase sharply by 2100 (along with CO2 levels), many species interactions are expected to change in the next century, and ecosystem structure and functioning will be very different from what we observe today.

note: the paper that describes this research is from the journal Ecology. The reference is Connell, S. D., Doubleday, Z. A., Foster, N. R., Hamlyn, S. B., Harley, C. D., Helmuth, B. , Kelaher, B. P., Nagelkerken, I. , Rodgers, K. L., Sarà, G. and Russell, B. D. (2018), The duality of ocean acidification as a resource and a stressor. Ecology, 99: 1005-1010. doi:10.1002/ecy.2209 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.

Recruiting rhinoceros

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.

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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.

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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.

conbiofig1

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.

conbiofig2a

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.

conbiofig2b

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 and Gwala size each other up

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 Biology32(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.

Eavesdropping on antshrikes

Growing up in the Spy vs. Spy era, and a bit later in the Watergate age, I developed a keen appreciation for clandestine operations, which I assumed at that time were unique to human culture.  As it turns out, eavesdropping is practiced by many different species for a variety of reasons. One important example occurs in bird flocks composed of several species of birds. Antshrikes (Thamnomanes ardesiacus) are sentinel species in multi-species flocks because they produce alarm calls when they spot a predacious raptor flying overhead, alerting other nearby birds of the threat. Ari Martinez and his colleagues wondered whether hanging out with antshrikes allowed these other bird species to expand their niches to forage in areas that might otherwise be too dangerous.

Alarm calling species Thamnomanes ardesiacus Photo cred E. Parra 600dpi (1)

An antshrike perched in the Amazonian rainforest. Credit: E. Parra.

This fear-based niche shift hypothesis makes two related predictions.  First, in the absence of antshrikes, the remainder of the flock should shift its range to areas with lower predation risk.  Second, without antshrikes some birds might leave the flock entirely, because without sentinel services they no longer benefit from hanging with other birds. To test these predictions, Martinez and his colleagues identified eight flocks of 5-8 species (including antshrikes) in a tropical lowland forest in southeastern Peru.  They established four removal flocks from which they removed all antshrikes after capturing them in mist nets. They left four control flocks, in which they captured all antshrikes, but then returned them to the flock (to control for the effects of handling).

Group banding and mist netting birds photo ced Micah Reigner

Research team mist-netting and measuring antshrikes.  Credit Micah Reigner

To determine where the flock was spending its time, researchers used a GPS device every 10 minutes to record the center of the flock. They also censused each flock for species composition from dawn to dusk for three days before removal and three days after removal. In control flocks, home range overlapped extensively (average of 69%) when comparing the first (pre-removal) and second (post-removal) three-day period. In removal flocks, there was only 8% overlap in home range, indicating that the remaining flock was shifting its range when antshrikes were gone.

MartinezFig1

Home ranges of a control flock (top) and a flock which had antshrikes removed (bottom). Red color indicates home range during the three day pre-removal period, while blue color indicates home range during the three day post-removal period.  Deeper colors indicate greater occupancy. 

But are the remaining species shifting their niches to safer locations when antshrikes are no longer available as sentinels? To answer this question the researchers measured the presence or absence of vegetation cover at different height intervals every 10 minutes at the center of the flock. Comparing the second (post-removal) to the first (pre-removal) period, the removal flocks (those without antshrikes) moved into understory vegetation (0-8 meters high) that was substantially denser than was the vegetation inhabited by the control flocks (those with antshrikes). Presumably, dense understory protects birds without sentinels from being spotted or captured by raptors flying overhead. These dense understory areas are usually associated with less tree cover at higher height intervals (above 16 meters), which allows more sunlight to reach the forest floor, resulting in lush vegetation growth.

MartinexFig3

Proportion change in vegetation cover occupied by flocks from pre-trial to post-trial period at different height intervals.  Positive numbers indicate an increase in vegetation density. Error bars are 95% confidence intervals. Data are based on the behavior of four control and four removal flocks.

Flocking occurrence is the proportion of time individuals of a particular species spend in flocks.  The fear-based niche shift hypothesis predicts that flocking occurrence should decrease when sentinel species are removed because the benefits of flocking are reduced for the remaining species. When the researchers compared post-removal to pre-removal time-periods, five species showed strong reductions in flocking occurrence for removal flocks in comparison to control flocks, two were unchanged, and one species showed an increase in flocking occurence.

MartinezFig2

Change in proportion flocking occurrence for eight different flocking species in control and removal flocks.  Error bars are 95% confidence intervals.  Chlorothraupis carmioli (CHCA), Epinecrophylla erythrura (EPER), Epinecrophylla leucophthalma (EPLE), Glyphorynchus sprirus (GLSP), Hylophilus ochraceiceps (HYOC), Myrmotherula longipennis (MYLO), Myrmotherula menetriesii (MYME), Xiphorhynchus elegans (XIEL).

The authors emphasize that though flocking occurrence decreased for most species, the flocks did remain intact, which indicates that there are probably other benefits from flocking besides the opportunity to eavesdrop. There might be safety in numbers – a decrease in individual mortality as group size increases, or the possibility that the remaining flock members do provide some information about imminent predator attacks.

Martinez and his colleagues conclude that sentinels help other bird species succeed in tropical rainforests, thriving in dangerous habitats where they might otherwise fear to tread.  These species may provide important ecosystem services, such as dispersing seeds and eating herbivorous insects that threaten plants that are the foundation of these tropical ecosystems.

note: the paper that describes this research is from the journal Ecology. The reference is Martínez, A. E., Parra, E. , Muellerklein, O. and Vredenburg, V. T. (2018), Fear‐based niche shifts in neotropical birds. Ecology, 99: 1338-1346. doi:10.1002/ecy.2217. 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.