Intricate combination

An ecosystem is the intricate combination of a community of organisms and their environment. My dictionary (the American Heritage Dictionary, Fourth Edition) defines a ‘system’ as “a group of interactive elements forming a complex whole,” and for an ecosystem, sometimes it’s not immediately obvious how the discrete elements interact.

Scientists have long studied the importance of trees and other vegetation growing on riverbanks, known collectively as riparian vegetation. (A quick Google Scholar search for that term returns more than 57,000 articles and books, the oldest of which were published in the late 1700s.)

“Such vegetation [has] a crucial role in stabilizing streambanks, moderating microclimates and shading streams, delivering litter and large wood to the river, providing habitat and food-web support for a wide range of terrestrial and aquatic animals, and supplying other ecosystem services,” write two scientists from Oregon State University who recently published a paper in the journal Ecohydrology.

Riparian vegetation is a fundamental part of many ecosystems; these researchers were interested in investigating the link between riparian cottonwood trees and the reintroduction of wolves to Yellowstone National Park. Wolves, historically part of the Yellowstone ecosystem, were eliminated from the park in the mid-1920s and reintroduced in the mid-1990s. During their absence, elk were released from the pressure of wolf predation and proliferated, and the consequences ricocheted throughout the ecosystem – the sprouts and seedlings of the plants that herbivores like elk eat, including cottonwoods, were unable to grow into mature trees.

Meandering stream and wide floodplain in Yellowstone National Park with almost no riparian trees. 

(Image by Emily Benson)

The scientists found that riparian cottonwood trees were rebounding in the part of a river valley where elk density had declined following wolf reintroduction, but not in a similar, nearby area with a large bison population; whereas cottonwoods more than five feet tall were absent from both locations prior to the reintroduction of wolves, by 2009 the researchers found about 380 cottonwoods over five feet tall in the section of the river valley with fewer elk, and only seven in the area teeming with bison. At the site where bison were plentiful, they write, “[t]he initial pattern of browsing suppression of young cottonwoods . . . by elk, which began several decades earlier when wolves were absent, is being continued by bison even though wolves are again present.”

Wolves, elk, bison, cottonwood trees, and streambanks are all part of the same ecosystem, their fortunes intertwined in an elaborate web; circumstances that affect one reverberate throughout that web to influence them all.

Bison in Yellowstone National Park.

(Image by Emily Benson)

Sandy spawning

Fish live underwater, so it seems like it would be safe to assume that they always spawn underwater, too – but, as it turns out, there are exceptions to that rule.

California grunion are small, silvery fish, typically about five or six inches long when full-grown, that live along the Pacific coast between Punta Abreojos, Baja California Sur, Mexico and Point Conception, Calif. (they’re occasionally found as far north as Monterey, Calif.).

For a few nights during full moons and new moons (when high tides are particularly high), between March and September, sandy beaches along the coast can be overrun with the flipping and flopping of California grunion, starlight reflecting off their iridescent scales, as the females release their eggs under the cover and relative safety of wet sand and the males circle around to fertilize them. Their tasks complete, the adults ride the tide back underwater, and the eggs incubate under the sand for about ten days before they hatch and the larvae are washed out into the ocean to join their parents. (The California Department of Fish and Wildlife has a nice description of California grunion spawning behavior on their website.)

Female California grunion bury themselves in wet sand as they release their eggs; males curl their bodies around the females to fertilize the eggs with their milt. 

(Image by arne heijenga via Flickr)

One spring, a team of researchers from Stony Brook University visited some of the sandy beaches where California grunion reproduce to collect fertilized eggs. The researchers were interested in investigating another unusual feature of the small fish – sex differentiation doesn’t occur until they are about two months old, and it appears to be determined, at least partially, by the environmental conditions the fish experience rather than completely by their genetics.

Those scientists recently reported the results of their study in the Journal of Experimental Marine Biology and Ecology. They raised larval California grunion at three different temperatures and under two different light regimes – one approximating longer day-lengths, such as would be experienced by the larval fish at mid-summer, and one approximating the shorter days late in the spawning season.

The researchers found that the environmental conditions that would indicate to the larval fish that they had been born earlier in the spawning season – colder temperatures and longer day-lengths – led to a higher proportion of female fish, presumably because the females born early in the spawning season gain an advantage over those born later. With more time to grow, the early-born females can maximize their size, and, eventually, their ability to reproduce – bigger females produce more eggs. (There doesn’t appear to be a similar size advantage for male California grunion.)

California grunion rely on at least two environmental cues – day-length as well as temperature – that indicate seasonality. As the scientists write, California grunion “appears to be the first documented case of a vertebrate with [environmental sex determination] that is cued by both temperature and photoperiod. Photoperiod may provide [California grunion] with a more reliable cue than temperature alone, given the small seasonal changes in temperature along the North American Pacific coast.”

On those summer nights when adult California grunion are racing up onto sandy beaches to spawn, the females laying the most eggs – the biggest females – may have been some of the earliest-born fish in previous years.

California grunion spawn on sandy beaches during the three or four nights surrounding full and new moons, when high tides are highest. 

(Image by arne heijenga via Flickr)

Zebrafish cannibals

Cannibalism is not uncommon in the animal kingdom. Many different types of organisms occasionally prey on members of their own species – house mice, monarch butterfly larvae, wandering spiders, crows, and many kinds of fish, just to name a few. As Laurel Fox, the author of a scientific paper entitled “Cannibalism in natural populations,” (the source of the examples of cannibalistic species listed above) wrote in 1975, “[c]annibalism is not an aberrant behavior limited to confined or highly stressed populations, but is a normal response to many environmental factors.”

Fox details a number of reasons why cannibalism can occur among animal populations, including a lack of other sources of food, overcrowding, stress, and simple availability. “In many examples initiation and control of cannibalism has not been ascribed to any obvious factor,” Fox writes, “and in these cases cannibalism may be a response primarily to the presence of vulnerable individuals.”

In fact, the behavior is so common among zebrafish (a small, freshwater species popular in both living room fish tanks and research labs) that online guides dedicated to the care of pet zebrafish warn their readers to keep adults separate from eggs and larvae. It’s also common enough that when a group of researchers was searching for a predator of zebrafish larvae to use in a study, they settled on the adult form of the species.

Those researchers recently published the findings of their study – an investigation of the mechanisms behind the ability of larval zebrafish to evade predators – in The Journal of Experimental Biology. Based on earlier experiments, the scientists already knew that the larvae sense their predators by feeling the flow of water that the larger fish push before them as they move rather than by seeing them coming, but in order to successfully avoid being eaten, the larvae need to avoid their predators as well as sense their presence.

This, the researchers found, is exactly what zebrafish larvae do; based on the cues they got from the water flowing around a predator (a dead adult zebrafish that had been preserved with formalin, and was guided through the experimental tank with a robotic arm), the “larvae direct[ed] their escape away from the side of their body exposed to more rapid flow. This suggests that prey fish use a flow reflex that enables predator evasion by generating a directed maneuver at high speed.”

Zebrafish larvae are equipped with the ability to feel a predator coming, and the reflex to swim away from it, even when that predator is an adult zebrafish.

Zebrafish, like many other animals, sometimes exhibit cannibalism. 

(Image by Soulkeeper via Wikimedia Commons)

Sound pressure

In the autumn of 2001, ships employing compressed air guns were prospecting for gas and oil off the northwest coast of Spain. In the process of mapping the sea floor, they employed pulses of air that created explosively loud, high-intensity, low-frequency waves of sound. During September and October, five giant squid, cephalopods which can grow to be over 40 feet long, were found stranded along the coast, a much higher number than usual; and in 2003, amidst continued oil and gas exploration, four giant squid were found, dead or dying, on the coast or near the shore.

Examination of the animals and further scientific investigation suggested that the squid died as a result of physical damage to their statocysts, the structures they use to detect sound and orient their bodies in the water. (I read about these events here [pdf], here, and here.)

New research conducted on another cephalopod, the common cuttlefish, suggests that, in addition to being damaging at high intensities, lower intensity sound may be an important way that cephalopods sense the environment around them.

A team of scientists monitored the behavior of cuttlefish exposed to a series of sounds of different frequencies and intensities, each lasting for three seconds. The most extreme behaviors the researchers observed were “escape responses” – jetting quickly away, or inking. (Cuttlefish, like most cephalopods, can produce a cloud of ink that obscures them from a predator as they escape.) These behaviors were elicited by low-frequency, high-intensity tones, while lower intensity tones were associated with less extreme behaviors like body movements and changes in body coloring. (Cuttlefish are capable of altering both the color and the pattern of their body, in order to deter predators, attract mates, or blend into their surroundings.)

The researchers also assessed the extent to which cuttlefish become habituated to certain sounds – in other words, whether or not they will stop responding to a repeated tone. They found that “[a]fter several exposures and no imminent threat, the number of escape responses decreased, suggesting the cuttlefish were able to filter out the ‘irrelevant’ acoustic stimuli,” though the cuttlefish never completely stopped responding to the repeated sounds.

Sound may be an important indicator of their surroundings for cuttlefish – the scientists write that the “evasion responses suggest that the cuttlefish initially reacted to the [sound] stimulus as they would react to a predator or other form of danger, and that sound detection could be a mechanism for predator detection” in cuttlefish.

Sound can have disastrous effects on cephalopods – but in other circumstances, or at different intensities or durations, it may serve ecologically important functions like announcing the presence of a predator.

Common cuttlefish (Sepia officinalis) can grow to be up to two feet long. 

(Image by David Sim via Wikimedia Commons)