Category Archives: Behavior

Timely trophic cascades

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While many of us appreciate oysters as delectable delights, we may underestimate the environmental benefits they also bring to the table. As filter feeders, they remove vast quantities of organic debris from the water, and as reef builders they protect our shorelines from violent wave action.

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Oyster reef. Credit: WFSU, Public Media

Of course, humans are not the only animals to enjoy eating oysters. For example, along portions of the Florida coast dominated by the reef-building oyster Crassostrea virganica, the mud crab, Panopeus herbstii, is a major consumer of juvenile oysters. In some locations, the average abundance of these voracious crabs can exceed 10 adults/m2 of reef. But all is not food and gravy for these crabs, as lurking in nearby burrows are equally voracious crab-eating toadfish, Opsanus tau. When toadfish are detected, the mud crabs will hide within the protective matrix of oyster shells and sediment that form the reef.

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A mud crab hiding among a cluster of oysters. Credit: WFSU, Public Media

By consuming mud crabs, toadfish are indirectly protecting oysters from being eaten. Ecologists call this a consumptive effect (CE). But David Kimbro and his colleagues have also shown than toadfish, by their mere presence, can also protect oysters by scaring the crabs into hiding. Since, in this case, they are not consuming the crabs, ecologists call this a non-consumptive effect (NCE). Together, CEs and NCEs should both increase oyster survival. More surviving oysters lead to higher overall feeding by oysters, which lead to more oyster poop, and more organic matter deposited into the sediment below. Ecologists call this type of relationship a trophic cascade, because the effects on one species cascades down through the ecosystem. In this case, increasing toadfish will decrease crabs, thereby increasing oysters and sediment organic matter. Conversely, decreasing toadfish should increase crabs, thereby decreasing oysters and sediment organic matter.

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Toadfish/mud crab/oyster/sediment organic matter (SOM) cascade. Dotted arrows are indirect effects

Kimbro and his colleagues wanted to explore this trophic cascade in more detail. They set up an experiment with 24 artificial reefs (made out of natural materials, except for the surrounding fence), which included 35 L of live oysters. They supplied each reef with 0, 2, 4, 6, 8 or 10 live crabs, and provided half of the reefs with a caged toadfish. They then measured oyster survivorship in relation to crab density in the presence or absence of predators.

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Setting up an artificial reef. Credit: WFSU, Public Media

The graphs below summarize their findings. The first thing to notice is that mud crabs were bad news for oysters, as survivorship plummeted when mud crabs were abundant. However, early in the experiment (graphs A and B) having a toadfish around helped out considerably. Oysters survived much better in the presence of toadfish (triangles and dotted curve) than they did without toadfish (circles and solid curve). But by the middle of the experiment (Graphs C and D), the toadfish no longer helped. Interestingly, by the end of the experiment (Graph E) the toadfish was once again helping the oyster’s cause, as survivorship was again greater in the presence of toadfish than in its absence. Realize that the difference between the dotted and solid curve is a measure of the NCE, as the toadfish are not eating the crabs (because they are caged). So we can conclude that there was a strong NCE early on, which waned in the middle of the experiment and then returned by the end of the experiment.

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A second finding is that the reef grew (expanded) when there were no crabs present, but that even two crabs were enough to reduce reef growth to zero. In addition sediment organic matter was greatest when there were either none or only two crabs present in the reef. Four or more crabs in the reef reduced the deposition of sediment organic matter. These findings were not influenced by the presence or absence of toadfish.

This is a complicated system, but we (and toadfish, crabs and oysters) live in a complicated world. And there are several other complications that I have not even mentioned! We might argue that the crabs may habituate (get accustomed) to these toadfish, so that by the middle of the experiment, the toadfish NCE had worn off. That begs the question of why the NCE returned towards the end of the experiment. Kimbro suggests that at the beginning of the experiment, the novelty of the predator cue probably caused strong NCEs. But by the middle of the experiment, the crabs became hungry and chose to forage regardless of predator cue. Finally, towards the end of the experiment, the crabs, having filled up on juvenile oysters, opted to hide rather than forage when toadfish were present. Whatever the reason, these findings caution us that if we want to understand trophic cascades, we need to consider the dimensions of both space and time.

note: the paper that describes this research is from the journal Ecology. The reference is Kimbro, D. L., Grabowski, J. H., Hughes, A. R., Piehler, M. F., & White, J. W. (2017). Nonconsumptive effects of a predator weaken then rebound over time. Ecology 98(3): 656-667. 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.

E-lek-trical death to migrants

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Leks have been described as singles bars for birds, though with all the singing and dancing that can go on there, a Karaoke bar might be the closest human analog. Male birds, such as the Great Bustard, Otis tarda, get together at traditional display grounds (leks) and strut their stuff, providing no material resources for females, and being visited by females solely for the purpose of mating.

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Three male Great Bustards on a lek in Central Spain. Credit: Carlos Palacin.

After the mating season concludes, some male great bustards in central Spain fly further north while others remain near the lek area. Migrants benefit from cooler and moister environmental conditions, and, in some cases, greater food availability. But migrants flying to a new area consume calories, and more recently, run the risk of flying into power lines, thereby injuring or killing themselves.

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Newly erected power lines in central Spain. Credit: Carlos Palacin.

 

Carlos Palacín and three other researchers used radio-tracking technology to follow the behavior of 180 male bustards over the course of 16 years. They knew that some bustards died from collision with power lines, but they didn’t know whether these collisions were affecting migrants and non-migrants (sedentary birds) differently, nor if these collisions were changing the migratory behavior of bustards in the 29 breeding groups they studied. So they tracked their birds by ground and by air and determined whether each bird was migrant or sedentary, how long each bird survived, and when possible, the cause of death. For migrant bustards, the researchers measured when and where they migrated, and whether they remained migrants their entire lives.

Palacín and his colleagues discovered that birds migrated away from the lek primarily in May and June, and returned to the breeding grounds over a much more prolonged time period during the autumn and winter.

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About 35% of the birds were sedentary, while 65% migrated an average of 89.9 km, with the longest migration of 261 km. Migrants had much higher mortality rates; for example among 73 birds captured and marked as juveniles, migrants survived an average of 90.6 months (post marking), while sedentary males survived an average of 134.7 months, almost 50% longer! The same pattern follows for 107 birds that were captured and marked as adults. The lesson here is that migration kills.

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So why migrate? Well it appears that before humans (and in particular, before power lines), migration was a much more beneficial strategy. The researchers identified three causes of bustard mortality: collision with power lines in 37.6% of the cases, poaching (9.1%) and collision with fences (2.6%). The bustard forensic team was unable to determine mortality in the remaining cases, so these percentages may underestimate human impact. Importantly, the researchers discovered that death from power lines was more than three times greater in migrants than in sedentary birds.

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This study clearly demonstrates that human infrastructure can shape the migratory behavior of a population. Over the time period of the study, the percentage of sedentary birds has increased sharply even though food availability actually decreased near the breeding grounds as a result of urbanization.

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The decrease in migration may be compounded by a finding that juveniles learn to migrate (or not) from adults during their first three years of life. So if there are more sedentary adults to serve as role models for juvenile behavior, more juveniles will develop into sedentary adults. But sedentary behavior can have several drawbacks. A greater number of sedentary males will increase competition for food and other resources. Also, birds may overheat during particularly hot summers near the breeding grounds. In addition, sedentary birds may have higher inbreeding rates and lower genetic diversity, which in turn can make a local population more susceptible to disease and other environmental changes, ultimately making it more prone to extinction.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Palacín, C., Alonso, J. C., Martín, C. A., & Alonso, J. A. (2017). Changes in bird‐migration patterns associated with human‐induced mortality. Conservation Biology 31: 106-115. 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.

Metallic starlings: a rain of terror

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

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

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

 

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

 

 

 

 

 

 

 

 

 

 

 

 

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

 

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

 

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

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

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

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

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

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

 

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

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