Kelp consumption curtailed by señorita

Miranda Haggerty was diving through a kelp forest, and noticed that many kelp bore a large number of tiny limpets that were housed in small scars that they (or a fellow-limpet) had excavated on the kelp’s surface. This got her thinking about how these scars might affect the kelp, and equally relevant, whether there were any limpet predators that might lend the kelp a hand (or a mouth) by removing limpets.

Jerry Kirkhart

A limpet grazes on a kelp frond. Credit: Jerry Kirkhart

Feather boa kelp (Egregia menziesii) is a foundation species within the subtidal marine system off the California coast, providing food and habitat for many species that live on or among its fronds. The tiny seaweed limpet, Lottia insessa, specializes on feather boa kelp, grazing on its fronds and living within the scars. Many invertebrates and fish live within the kelp forest, but the most abundant fish is the señorita, Oxyjulis californica. Haggerty wondered whether the señorita might benefit the kelp (directly) by removing limpets, or (indirectly) by scaring limpets away – what ecologists call a trait-mediated indirect interaction.

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The señorita – a fearsome predator of limpets.  Credit: Miranda Haggerty

The first order of business was to determine whether the limpets were actually harming the kelp.  Haggerty and her colleagues approached this in two ways.  First they chose 94 kelp plants from kelp forests off the California coast.  From each individual they chose one grazed and one ungrazed frond (each 3 m long). Grazed fronds averaged 5-10 scars and at least 2 limpets per meter of length.  Every three weeks they visited their kelp to score for broken fronds. In 29 of 30 cases, the grazed frond broke before the ungrazed frond (in the remaining cases the entire plant was missing, or both fronds broke and the researchers could not tell which had broken first).

HaggertyFigS1

Photo of feather boa kelp showing grazing scars, including one housing a limpet (left).  Diagram of feather boa kelp showing multiple fronds (right).

But the researchers were concerned that perhaps limpets chose to graze on weaker fronds, so the breakage was not caused by grazing scars, but by limpet choice.  To account for this concern, Haggerty and her colleagues chose 43 ungrazed kelp plants, placed three  limpets on one frond, and chose a second, equal-sized frond as an unmanipulated control. Once again, they visited their plants every three weeks, and discovered that grazed fronds broke first in all 20 pairs that the sequence of frond breakage could be determined.  Clearly, limpet grazing is bad news for feather boa kelp.

How does the señorita fit into this system? The researchers designed a laboratory experiment to address this question.  They used 10 large tanks (1700 L), and set up five different experimental treatments to compare direct effects of predation, and indirect effects of predator presence, on limpet grazing, and ultimately on kelp survival. To isolate the direct effects of predation from the indirect effects of predator cues on limpets, Haggerty and her colleagues placed four kelp fronds into fish exclosure cages, which were housed in the large tanks, and placed three limpets onto some of these fronds.  To mimic actual predation (CE treatment in Table below), they removed limpets by hand at a constant rate typical of señorita predation. For the NCE treatment (testing indirect effects of predator presence) they introduced señorita into the large tank so the limpets experienced the predator cues, but were not eaten. The different treatments are summarized in the table below. These experiments ran for one week and each treatment was replicated 10 times.

HaggertyTableFinalEach day the researchers monitored the number of limpets and grazing scars.  After one week, Haggerty and her colleagues counted the number of grazing scars, and measured the breaking strength of each frond by clamping the frond’s end to a table and pulling on the opposite end with a spring scale until it broke. They then recorded the amount of force needed to break the frond.

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Clamped kelp frond whose breaking strength has been tested.  Notice that the frond broke at a grazing scar (right). Credit Miranda Haggerty.

Not surprisingly, the predator control (PC) kelp (limpets present without señorita) had the most scars and lost the greatest amount of tissue.  Kelp receiving the consumptive predator effect treatment (CE) had fewer scars and lost less tissue than PC.  But interestingly, kelp receiving NCE and TPE treatments had significantly fewer scars than the CE kelp, and were statistically indistinguishable from each other.  Thus, in the laboratory, the presence of señorita cues (NCE treatment) was more important than actual predation (CE treatment) in reducing kelp scarring and tissue consumption (top and middle graph below).  As a result, the NCE treated kelp were stronger (had greater breaking strength) than were the CE treated kelp (bottom graph below).

HaggertyFig2

Mean (+ standard error) number of grazing scars (top), mass of tissue consumed (middle) and breaking strength (bottom) of kelp in response to five experimental treatments. CE = consumptive effect, NCE = non-consumptive effect, TPE = total predator effect, PC = predator control, LC = limpet control. Different letters above bars indicate significant differences between the means when comparing treatments.

Haggerty and her colleagues replicated this experiment, with a few experimental design modifications, in a field setting.  As with the laboratory experiment we’ve just discussed, the researchers found a very strong non-consumptive effect. The researchers suspect that these limpets respond to chemical cues emitted by their señorita predators. They could not respond to many types of sensory cues because they lack auditory organs, and the experimental design prevented fish from transmitting any shadows (visual cues) or vibrational cues. In addition previous studies have shown that some limpet species use chemoreception for predator avoidance, foraging and homing. However, the nature of this chemical cue is yet to be discovered for this predator-prey system.

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Schooling señorita. Credit: Miranda Haggerty

Trophic cascades occur when the effects of one species on another species cascade down through the ecosystem. In this case, señorita predators directly and indirectly reduce limpet density, which increases the survival of kelp – a foundation species for this ecosystem. The researchers point out that this trophic cascade only occurs in the southern feather boa kelp range, because señorita are absent further north.  We don’t know if limpets have other predators in the northern range, but we do know that the kelp are structurally more robust further north, so they (and the ecosystem) may be relatively immune to limpet-induced destruction.

note: the paper that describes this research is from the journal Ecology. The reference is Haggerty, M. B., Anderson, T. W. and Long, J. D. (2018), Fish predators reduce kelp frond loss via a trait‐mediated trophic cascade. Ecology, 99: 1574-1583. doi:10.1002/ecy.2380. 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.

Mangroves partner with rats in China

Many of us have seen firsthand the havoc that invasive plants can wreak on ecosystems.  We are accustomed to think of native plants as unable to defend themselves, much like a skinny little kid surrounded by a group of playground bullies. ‘Not so fast’ says Yihui Zhang.  As it turns out, many native plants can defend themselves against invasions, and they do so with the help of unlikely allies.

In southern China, mangrove marshes are being invaded by the salt marsh cordgrass, Spartina alterniflora, which is native to the eastern USA coastline. Cordgrass seeds can float into light gaps among the mangroves, and then germinate and choke out mangrove seedlings.  However, intact mangrove forests can resist cordgrass invasion.  Zhang and his colleagues wanted to know how they resist.

mangrove-Spartina ecotone

Cordgrass (pale green) meets mangrove (bright green) as viewed from space. Credit: Yihui Zhang.

Cordgrass was introduced into China in 1979 to reduce coastal erosion.  It proved up to the task, quickly transforming mudflats into dense cordgrass stands, and choking out much of the native plant community.  Dense mangrove forests grow near river channels that enter the ocean, and are considerably taller than their cordgrass competitors.  The last player in this interaction is a native rat, Rattus losea, which often nests on mangrove canopies above the high tide level. At the research site (Yunxiao), many rat nests were built on mangroves, using cordgrass leaves and stems as the building material.

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Rat nest constructed from cordgrass shoots rests upon a mangrove tree.  Credit Yihui Zhang.

Zhang and his colleagues suspected that cordgrass invasion into the mangrove forest was prevented by both competition from mangroves and herbivory by rats on cordgrass.

Baby rat in the nest

Baby rats in their nest. Credit Yihui Zhang.

 

To test this hypothesis, they built cages to exclude rats from three different habitats: open mudflats (primarily pure stands of cordgrass), the forest edge, and the mangrove forest understory, (with almost no cordgrass). They set up control plots that also had cages, but that still allowed rats to enter.

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Arrow points to resprouting cordgrass. Credit Yihui Zhang.

The researchers planted 6 cordgrass ramets (genetically identical pieces of live plant) in each plot and then monitored rodent grazing, resprouting of original shoots following grazing, and shoot survival over the next 70 days.

They discovered that the cages worked; no rats grazed inside the cages.  But in the control plots, grazing was highest in the forest understory and lowest in the mudflats (Top figure below).  Most important, both habitat type and exposure to grazing influenced cordgrass survival.  In the understory, rodent grazing was very important; only one ramet survived in the control plots, while 46.7% of ramets survived if rats were excluded.  In the other two habitats, grazing did not affect ramet survival, which was very high with or without grazing (Middle figure). Rodent grazing effectively eliminated resprouting of ramets in the understory, but not in the other two habitats (Bottom figure).

Zhangfig2

Impact of rat grazing on cordgrass in the field study in three different habitats.  Top figure is % of stems grazed, middle figure is transplant survival, and bottom figure is resprouting after grazing (there was no grazing in the rodent exclusion plots). Error bars are 1 standard error. Different letters above bars indicate significant differences between treatments.

The researchers suspected that low light levels in the understory were preventing cordgrass from resprouting after rat grazing. This was most easily tested in the greenhouse, where light conditions could be effectively controlled.  High light was 80% the intensity of outdoor sunlight, medium light was 33% (about what strikes the forest edge) and low light was 10% the intensity of outdoor sunlight (similar to mangrove understory light).  Rat grazing was simulated by cutting semi-circles on the stembase, pealing back the leaf sheath, and digging out the leaf tissue. Cordgrass ramets were planted in large pots, exposed to different light and grazing treatments, and monitored for survival, growth and resprouting following grazing.

Greenhouse setup

Cordgrass growing in greenhouse under different light treatments. Credit: Yihui Zhang.

Zhang and his colleagues found that simulated grazing sharply reduced cordgrass survival from 85% to 7% at low light intensity, but had no impact on survival at medium or high light intensities.  Cordgrass did not resprout after simulated grazing at low light intensity, in contrast to approximately 50% resprouting at medium and high light intensity.

ZhangFig4

Survival (top) and resprouting (bottom) of cordgrass following simulated grazing in the greenhouse experiment.

The researchers conclude that grazing by rats and shading by mangroves are two critical factors that make mangroves resistant to cordgrass invasion. Rats tend to build their nests near the mangrove forest edge, so it is not clear how far into the forest the rat effect extends. Rats do prefer to forage in the understory (rather than right along the edge), presumably because the understory helps to protect them from predators.  In essence, mangroves compete directly with cordgrass by shading them out, and also indirectly by attracting cordgrass-eating rats. Conservation biologists need to be aware of both direct and indirect effects when designing management programs for protecting endangered ecosystems such as mangrove forests.

note: the paper that describes this research is from the journal Ecology. The reference is Zhang, Y. , Meng, H. , Wang, Y. and He, Q. (2018), Herbivory enhances the resistance of mangrove forest to cordgrass invasion. Ecology. Accepted Author Manuscript. doi:10.1002/ecy.2233. 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.

Plant communities bank against drought

Many plants shed their young embryos (seeds) into the soil where they may accumulate in a dormant (non-growth) state over years before germinating (resuming growth and development). Ecologists describe this collection of seeds as a seed bank.  Marina LaForgia describes how scientists were able to germinate and grow to maturity some 32,000 year old Silene stenophylla seeds that was stashed, probably by an ancient squirrel, in the permafrost! With increased climatic variation predicted by most climate models, she wanted to know how environmental variability might affect germination of particular groups of species within a community.  In addition, she and her colleagues recognized that most ecological studies investigate community responses to disturbances by looking at the aboveground species.  It stands to reason that we should consider the below-surface seed bank as a window to how a community might respond in the future.

LaForgiaSeedlings

Some seedlings coming up from the seed bank. Credit:Marina LaForgia.

Seed banks can be viewed as a bet-hedging strategy that spreads out germination over several (or many) years to reduce the probability of catastrophic population decline in response to one severe disturbance, such as drought, flood or fire. In some California annual grassland communities, species diversity is dominated by annual forbs – nonwoody flowering plants that are not grasses. Many forbs produce seeds that can lie dormant in the seed banks for several years. Though these forbs are the most diverse group, there are also about 15 species of exotic annual grasses that dominate the landscape in abundance and cover. These grasses dominate because they produce up to 60,000 seeds per m2, they grow very quickly, and they build up a layer of thatch that suppresses native forbs. However, seeds from these grasses cannot lie dormant in the seed bank for very long.

 

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Area of field site dominated by Delphinium (purple flower) and Lasthenia (yellow flower).  Looking closely you can also see some tall grasses rising. Credit Marina LaForgia.

How is drought affecting these two major components of the plant community? LaForgia and her colleagues answered this question by collecting seeds from a northern California grassland at the University of California McLaughlin Natural Reserve in fall 2012 (beginning of the drought) and fall 2014 (near the end of the drought). They used a 5-cm diameter 10-cm deep cylindrical sampler  to collect soil and associated seeds from 80 different plots.  The researchers also used these same plots to estimate aboveground-cover, and to identify the aboveground species that were present. The research team germinated and identified more than 11,000 seeds.

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Plants germinating in the greenhouse. Credit Marina LaForgia.

The researchers knew from previous work on aboveground vegetation that exotic annual grasses declined very sharply in response to drought.  In contrast, the native forbs did relatively well, in part depending on their specific leaf area (SLA) – a measure of relative leaf size, with low SLA plants conserving water more efficiently. It seemed reasonable that these same patterns would be reflected belowground. Recall that most grass seeds are incapable of extended dormancy, while many forbs can remain dormant for several years. Consequently, LaForgia and her colleagues expected that grass abundance in the seed bank would decline more sharply than would forb abundance. In addition, they expected that high SLA forbs would not do as well as low SLA forbs during drought.

The researchers discovered very sharp differences between the two groups over the course of the drought. Exotic annual grasses declined sharply in the seed bank, while native annual forb abundance tripled.  Aboveground cover of grasses declined considerably, while aboveground cover of forbs increased modestly.  Clearly the exotic grasses were suffering from the drought, while the forbs were doing quite well.

LaForgiaFig1

(a) Seed bank abundance of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). (b) Percent cover of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). Data are based on samples from 80 plots. Error bars indicate one standard error.

We can see these differences on an individual species basis, with most of the grasses declining modestly or sharply in abundance, while most of the forbs increased.

LaForgiaFig2

Mean change in seed bank abundance per species based on 15 exotic grass species and 81 native forb species.

It is not surprising that the grasses do so poorly during the drought.  Presumably, less water causes poorer germination, growth, survival and seed production.  In addition, because grass seeds have a low capacity for dormancy, grass abundance will tend to decrease in the seed bank very quickly with such a low infusion of new seeds.

But why are the forbs actually doing better with less water available to them?  One explanation is that grass abundance and cover declined sharply, causing the forbs to experience reduced competition with grasses that might otherwise inhibit their growth, development and reproductive success. The tripling of native forbs in the seed bank was much greater than the 14% increase in aboveground forb cover.  The researchers reason that the drought caused many of the forb seeds to remain dormant, leading to them building up in the seed bank. This was particularly the case for low SLA forbs, which increased much more than did high SLA forbs in the seed bank.

We can understand exotic grass behavior in the context of their place of origin – the Mediterranean basin, which tends to have wet winters.  In that environment, natural selection favored individuals that germinated quickly, grew fast and made lots of babies. Since their introduction to California in the mid 1800s, 2014 was the driest year on record.  It will be fascinating to see if these exotic grasses can recover when, and if, wetter conditions return.  Can we bank on it?

note: the paper that describes this research is from the journal Ecology. The reference is LaForgia, M.L., Spasojevic, M.J., Case, E.J., Latimer, A.M. and Harrison, S.P., 2018. Seed banks of native forbs, but not exotic grasses, increase during extreme drought. Ecology99 (4): 896-903. 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.

Successful scavengers

Scavengers have a bad reputation. They reputedly eat foul smelly stuff, and are too lazy or incompetent to track down prey on their own, depending on “noble” beasts such as lions to kill prey, and then sneaking a few bites when the successful hunters are not looking (or after they’ve stuffed themselves). Of course the reality is that scavenging is simply one way that animals make a living. Many different species, including lions, will scavenge if given the opportunity, and from a human perspective, scavengers provide several important ecosystem services. As one example described by Kelsey Turner and her colleagues, ranchers in parts of Asia gave diclofenac, a non-steroidal anti-inflammatory drug, to their cattle, which had the unintended consequence of killing much of the vulture community. Losing vultures from the scavenging community increased the prevalence of rotting carcasses, which caused feral dog and rat populations to skyrocket, resulting in a sharp increase of human rabies cases in India. The take-home message is that we need to understand what factors influence scavenging behavior and scavenging success.

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Golden eagle overwintering in South Carolina scavenges a pig carcass in a clearcut. Credit: Kelsey Turner.

Turner and her colleagues were particularly interested in whether the size of a carcass, the habitat in which an animal dies, and the time of year, influence scavenging dynamics.   The researchers varied carcass size by using three different species: rats (small), rabbits (medium) and pigs (large). Habitats were clearcuts, mature hardwood, immature pine, and mature pine forest. Time of year was divided into two seasons: warm (May – September) and cool (December – March). I should point out that the cool season was mild by many standards, as the research was conducted at the Savannah River Site in South Carolina, with a mean winter temperature of about 10 ° C.

TurnerFieldSite

Map of Savannah River Site showing the study sites and diverse habitats.

The researchers collected data by laying down carcasses of varying size in each of the habitats in both summer and winter. Each carcass was observed by a remote sensing camera that captured the scavenging events, allowing the researchers to identify the species of each scavenger and how long it took for the carcass to be detected and consumed.

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Two coyotes captured by a remote sensing camera scavenging a pig carcass on a rainy day. Credit: Kelsey Turner.

Scavengers discovered 88.5% of the carcasses placed during the cool season, but only 65.4% of carcasses placed during the warm season. Carcass size was also important, with only 53.9% of rats detected, in contrast to 78.5% of rabbits and 97.8% of pigs detected. But habitat interacted with these general findings: for example scavengers consumed all (23) rabbits in clearcuts, but only about 70% of rabbits placed in the other three habitats.

Detection time also varied with carcass size; in general scavengers found pigs more readily than rats or rabbits. As the graphs below show, this relationship was quite complex. Pigs were detected much more quickly than the smaller carcasses in clearcuts, and somewhat more quickly in mature pine. Additionally, this difference between pigs and the other species is stronger in the warm season (left graph) than in the cool season (right graph). In fact, there is no difference in detection time of pigs, rabbits and rats placed in mature pine during the cool season.

EcologyFig1Turner

Natural log of mean detection time (in hours) of rat, rabbit and pig carcasses in warm season (left) and cool season (right) in different habitats.  CC = clearcut, HW = mature hardwood, IP = immature pine, MP = mature pine.

Not surprisingly pigs tended to persist longer (before being totally consumed) than the other two species. More strikingly, persistence time for all three species was much greater in the cool season than in the warm season.

EcologyFig3Turner

Natural log of mean carcass persistence time (in hours) of rat, rabbit and pig carcasses during the cool and warm seasons.

Turner and her colleagues identified 19 different scavenger species; turkey vultures, coyotes, black vultures, Virginia opossums, raccoons and wild pigs were the most frequent. The first scavengers to detect pig carcasses were usually turkey vultures (76.0%) or coyotes (17.3%). An average of 2.8 different species scavenged at pig carcasses, in contrast to 1.5 at rabbit carcasses and 1.04 at rat carcasses. As you might imagine, most scavengers made short work of rat carcasses, so there was not much opportunity for other individuals or species to move in. Carcasses that persisted longer generally had a greater diversity of scavengers; for example, carcasses scavenged by 1, 2 or 3 species persisted, on average, for 90.5 hours, while those scavenged by 4, 5 or 6 species persisted, on average for 216.5 hours.

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A flock of turkey vultures in a clearcut surround and scavenge a pig carcass. Credit: Kelsey Turner.

Early ecologists viewed feeding relationships within an ecological community as a linear process in which plants extract nutrients from soils and calories from the air, which they pass onto herbivores and then to carnivores, with considerable energy being lost in each transfer. Now, we use a food web perspective, which considers the essential contributions of scavengers and decomposers (among others) to these feeding relationships. Carcasses decompose much more quickly during the warm season, returning calories and nutrients to lower levels of the food web. Microbial decomposers are, in essence, competing with vertebrates for carcasses, and being metabolically more active in warm months, are able to extract a greater portion of the resources from the carcass than they can during the winter. Slow decomposition in winter allows longer carcass persistence, leading to a greater number and greater diversity of scavengers. As a bonus for those who believe in human primacy, these same scavengers help to create a cleaner and healthier world.

note: the paper that describes this research is from the journal Ecology. The reference is Turner, K. L., Abernethy, E. F., Conner, L. M., Rhodes, O. E. and Beasley, J. C. (2017), Abiotic and biotic factors modulate carrion fate and vertebrate scavenging communities. Ecology, 98: 2413–2424. doi:10.1002/ecy.1930. 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.

Treefall gaps deliver diversity

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

JT_TreefallLisa Davenport

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

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

Treefallgap Irina Skinner

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

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

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

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

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

TerbprghFig2

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

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

Sapling species diversity was greater under a closed canopy.

TerborghFig5scanpub

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

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

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

River restoration responses

The Lippe River in Germany has been subjected to many decades of channelization, deepening, floodplain drainage, straightening and consequent shortening, with one result being that the modern Lippe is 20% shorter than it was two centuries ago. Beginning in 1996, conservation managers began reversing this trend by widening the river, raising the level of the river bed, constructing small islands within the river and terminating floodplain drainage operations over a stretch of 3.3 km. As a result of these activities, a small portion of the river looks much like it did 200 years ago.

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A section of the Lippe River before (left) and after (right) restoration.

Over a 21-year period, researchers from Arbeitsgemeinschaft Biologischer Umweltschutz have conducted systematic surveys of fish communities at the restored and unrestored sections of the river. Researchers sampled the fish community with electrofishing – inputting a direct electrical current into the river – which causes the fish to swim towards the boat where they are easily collected with nets, identified by species, and returned unharmed into the river. A data set over this length of time in association with a restoration project is very unusual; oftentimes (in part due to funding issues) only one survey is conducted to assess the fish community response to river restoration.

About eight years ago, while a postdoctoral researcher at Senckenberg Research Institute in Frankfurt, Germany, Stephan Stoll was asked to analyze some river restoration outcomes, and, as he describes, “became hooked to the topic.” To evaluate the response of the Lippe River fish community to restoration, a group of researchers headed by Stephanie Höckendorff, a Master’s student with Stoll, first asked a very simple question – how did fish abundance and species richness (the number of fish species) compare in the restored and unrestored regions of the river.

The graph below shows several striking trends. Abundance peaked about 2-3 years after restoration, declined sharply the next year, and recovered in subsequent years to about three times the abundance found in unrestored sections. Importantly, abundance varied extensively year-to-year. For example, if you had done only one survey in 2000, you would have erroneously concluded that restoration had no effect, which is why the researchers emphasize the importance of collecting data over a long stretch of time.

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Abundance of fish in restored (Rest-gray curve) and unrestored (Cont-black curve) sections of the Lippe River.  The gray vertical bar indicates the start of the restoration project in 1997.

Species richness increased sharply, but did not reach its peak until nine years after restoration. Again, there was extensive year-to-year variation in species richness.

rivrest2b

Fish species richness in restored (Rest-gray curve) and unrestored (Cont-black curve) sections of the Lippe River.  The gray vertical bar indicates the start of the restoration project in 1997.

Höckendorff and her colleagues were intrigued by this delay in species richness, and turned their attention to understanding what types of species benefited most from the restoration. Their analyses indicated that colonizing species, such as common minnows and three-spined sticklebacks, tended to have short life spans, early female maturity, several spawning events per year and a fusiform body shape – a body that is roughly cylindrical and tapers at both ends. Interestingly, some of the most successful colonizers took quite a long time to get well-established within the community.

Minnow

Common minnows, Phoxinus phoxinus. Credit: Carlo Morelli (Etrusko25)

Stickleback

The three-spined stickleback, Gasterosteus aculeatus. Credit: Ron Offermans

The restored habitat was highly dynamic, experiencing periodic flooding and the formation of temporary shallow bays and shifting sandbanks. These types of habitats tend to select for minnows, sticklebacks and other opportunistic species that are attracted to periodic disturbances. These opportunistic species were quick to move in, and continued to increase in abundance over time. Importantly, several rare and endangered species also colonized the restored habitat. However, large, deep-bodied, slow maturing and long-lived species did not benefit (at least over the 17 years of the survey), as these types of species are generally favored in less dynamic habitats, which are more stable and uniform.

Overall, these findings demonstrate the benefits of river restoration to the fish communities they harbor. But some species are more likely to benefit than others, and the time-scale over which recolonization occurs is highly variable. Surveys must be repeated over a long time-scale to tell conservation managers whether their restoration efforts are successful, and how they might change their future river restoration efforts.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Höckendorff, S., Tonkin, J. D., Haase, P., Bunzel-Drüke, M., Zimball, O., Scharf, M. and Stoll, S. (2017), Characterizing fish responses to a river restoration over 21 years based on species’ traits. Conservation Biology, 31: 1098–1108. doi:10.1111/cobi.12908. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Seagrass scourge: when nutrient enrichment reaches the tipping point

Sean Connell has watched as south Australia has lost vast expanses of kelp forest and seagrasses over the past years. One of the primary culprits associated with loss of seagrass meadows is excessive nutrients, particularly nitrogen, which enters the ecosystem with runoff, and causes an increase in algal epiphytes (epiphytes are small plants that grow on other plants). Epiphytes can negatively affect seagrass by blocking sunlight needed for photosynthesis, and indirectly, by increasing the rate of cellular respiration within the ecosystem, thus using up oxygen needed by seagrass for metabolic processes.

DolphinConnell

Two dolphins swim above a bed of seagrass off the south Australian coast.

Connell and his colleagues noticed that seagrass loss was often sudden; a large seagrass meadow would appear to be in good shape, and then it would abruptly disappear. They suggested that there might be a threshold effect in nutrient levels that seagrasses can tolerate; that these systems function well until a certain threshold in nutrient levels is crossed, above which there is an abrupt loss of seagrasses. They tested this hypothesis by subjecting plots of the seagrass Amphibolis antarctica to seven different concentrations of dissolved inorganic nitrogen (DIN) over a 10 month period, and monitored the abundance of epiphytes and seagrass over that timespan.

The meadows were about two km offshore from Lady Bay, Fleurieu Penninsula, Australia, in about 5 meters of water. Different amounts of nitrogen fertilizer were wrapped in nylon bags (for slow continuous release of DIN) and staked to the ocean floor. Amphibolis antarctica grows by producing new leaves at the top of each leaf cluster, but at the same time it drops old leaves. Leaf turnover, the researchers’ measure of growth, is simply new leaf production minus old leaf drop. The researchers tied on a small nylon cable at known locations on selected plants, noted how many leaves were above and below each tie at the beginning of the experiment, and recounted leaf number 10 months later. Finally, the researchers measured epiphyte growth by microscopically viewing a sample of seagrass leaves, and counting the number seagrass leaf cells that were covered by epiphytes.

Seagrass growth was relatively unaffected by all tested DIN levels.

ConnellFigA

Leaf production per day in relation to concentration of DIN.

However, leaf drop showed a strong threshold effect; leaf drop rates increased sharply between 0.13 – 0.15 mg/L of DIN.

ConnellFigB

Leaf drop per day in relation to concentration of DIN.

Putting these two graphs together, you can see (below) that leaf turnover switched from positive to negative at 0.13 – 0.15 mg/L of DIN. Negative leaf turnover translates to a sudden loss of seagrass at that threshold. At least in this system, at this location, 0.13 – 0.15 mg/L of DIN is the tipping point, beyond which the seagrass system suddenly goes into decline.

ConnellFig1

Leaf turnover per day (left y-axis and red data), and Epiphyte cover (% – right y-axis and green data), in relation to concentration of dissolved inorganic nitrogen.

The graph also shows that the tipping point coincides with an epiphyte cover of approximately 60%. It is possible that increased epiphyte cover may reduce seagrass photosynthetic rates (particularly in lower leaves), so that leaf turnover suddenly shifts into the negative zone, but the study was not designed to identify the underlying mechanism.

Seagrass meadows perform important ecosystem services, such as absorbing excess nutrients from the sediment, and providing habitat and food for a diverse group of grazers and indirectly, for their consumers. Thus seagrass conservation is vital. The danger here is that moderate levels of nutrients do not appear to have much effect on seagrass populations, but there is an abrupt shift to seagrass loss once the nutrient threshold is crossed. This makes the system very difficult to manage, because the loss occurs without warning. Australian ecologists have repeatedly failed to restore lost seagrass meadows, as simply reducing nutrient levels does not reverse the process. Thus anticipating seagrass loss before it happens is the most viable management solution for this critical ecosystem.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Connell, S. D., Fernandes, M., Burnell, O. W., Doubleday, Z. A., Griffin, K. J., Irving, A. D., Leung, J. Y.S., Owen, S., Russell, B. D. and Falkenberg, L. J. (2017), Testing for thresholds of ecosystem collapse in seagrass meadows. Conservation Biology, 31: 1196–1201. doi:10.1111/cobi.12951. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.