competition

A race against pine: Wood-boring wasp in North America threatened by a Eurasian invader

Invasive species have diverse impacts in different locations, including biodiversity loss, as a result of native species being outcompeted for similar resources. A U.S. research team, led by Dr. Ann Hajek, Cornell University, studied the case of an aggressive Eurasian woodwasp that has recently established in North America and poses a threat to a native species. Their study is published in the open-access journal NeoBiota.

Most woodwasps play an essential part in the forest ecosystem, as they decompose wood, preferring dying or felled trees. They do so by laying their eggs in the wood underneath the tree bark. Curiously, the wasps also deposit a symbiotic fungus and venom that shuts down the tree’s defenses. As the tree weakens, the fungal infestation begins and the the tree starts to rot. When the eggs hatch, the larvae feed on the rotten wood before they emerge. This relationship is called obligate since the survival of the wasp is impossible without the fungal infestation.

Originating from Eurasia, the presence of the invasive species is dangerous because it can kill healthier pines. It has long been established in the southern hemisphere causing economic issues due to its attacks on pines. While pines have been introduced to that part of the world, they are native to North America, where the invasive wasp could be far more devastating.

Now that the invasive woodwasp has already been identified in the States, the scientists seek to find a way to protect its frail competitor, reporting a rapid decline in the North American species.

“We would often observe both species emerging from the same infested pine trees, but the ratios changed with time,” explains Dr. Ann Hajek.

“Shortly after the invasive colonizes an area, the native wasps emerging from the trees would equal the invasive. However, a few years later, the natives started to get fewer and fewer.”

It turned out that the Eurasian woodwasp has larger venom glands and produces more eggs, thanks to its greater body size. Furthermore, it emerges earlier than the North American species, so that it can find and colonize the most suitable trees first. By the time the native species lays its eggs, the authors speculate, most of the preferred trees are already occupied by the invasive, leaving a reduced supply of habitat for the newcomer’s larvae.

“Woodwasps are difficult to study and their biologies are generally poorly understood,” note the authors. “While the native species appears to be outcompeted from pines that both species prefer, it is possible that populations of the native can be sustained in trees less desirable to the invasive or unavailable during the time and place that the invasive is present.”

The scientists call for additional research on the native woodwasp in southeastern pine forests in USA, before the invaders spread to that area with extensive pine forests.

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Original source:

Hajek AE, Henry JC, Standley CR, Foelker CJ (2017) Comparing functional traits and abundance of invasive versus native woodwasps. NeoBiota 36: 39-55. https://doi.org/10.3897/neobiota.36.14953

Feeding strategies in competing hummingbird species observed in a small area in Brazil

Being the vertebrates with the highest metabolic rate thanks to their rapid wing flaps, the hummingbirds have evolved various types of feeding behaviour. While the nectar-feeders tend to go for food high in energy, strong competition affects greatly their preferences and behaviour towards either dominance, subordination, a strategy known as trapline and a fourth one named hide-and-wait, conclude the Brazilian scientists Lucas L. Lanna, Cristiano S. de Azevedo, Ricardo M. Claudino, Reisla Oliveira and Yasmine Antonini of Universidade Federal de Ouro Preto. Their conclusions following six months of observations in an Atlantic Forest remnant in southeastern Brazil are published in the open access journal Zoologia.

To test previous assumptions as well as their own hypotheses, the researchers placed artificial bird feeders filled with variable in concentration sugar-water solutions across four separate patches at the core of the forest fragment.

The scientists sought to find out whether the birds would show clear preference for the most sugary food source; whether larger size and heavier weight would guarantee better access to the most nutritious feeders; what strategies would be adopted by each species; and which ones would prove the dominant and most aggressive.

As expected, the scientists concluded that the birds prefer the most sugar-dense solutions. However, when subordinate species, such as the white-throated hummingbird and the versicoloured emerald, confronted dominant species guarding the most nutritious food sources, they would be either frightened or expelled following a short chase. Subsequently, these hummingbirds would resort to the feeders with low-sugar solutions.

In their turn, the Brazilian ruby and the violet-capped woodnymph proved to be the dominant and most aggressive species in the studied area. Upon seeing an ‘intruder’ in their territory, which might be either another species, or belonging to their own, they would vocalise their threats, alert them by perching by the feeder, or expel them following a short pursuit. However, they would only try to limit the access for subordinate hummingbirds if the energy that could be gained from the feeder exceeded the energy loss of the chase.

Contrary to another initial hypothesis, it was not the largest and heaviest species that were the dominant ones. There were two species of hermit hummingbirds which were the largest and the heaviest, however, they expressed no territorial or aggressive behaviour. Instead, they were recorded intruding in the territory of the two dominant hummingbird species. In their turn, the Brazilian ruby and the violet-capped woodnymph would often frighten them. Nevertheless, rather than fleeing, the ‘castaways’ were seen hiding in the shrubs, remaining quiet, and returning to the feeder as soon as the dominant bird was gone. This behaviour strategy, named hide-and-wait, has not been reported in hermit hummingbirds prior to this study, according to the authors.

Having reported all feeding strategies in their study, the scientists conclude that the dominant territorial species and the trapliners feed most frequently and most sufficiently, as they use the most sugary sources.

However, the authors note that the high abundance of food, as well as the presence of aggressive territorial species might have affected the hummingbirds’ behaviour and preferences.

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Original source:

Lanna LL, de Azevedo CS, Claudino RM, Oliveira R, Antonini Y (2017) Feeding behavior by hummingbirds (Aves: Trochilidae) in artificial food patches in an Atlantic Forest remnant in southeastern Brazil. Zoologia 34: 1-9. https://doi.org/10.3897/zoologia.34.e13228

To be fragrant or not: Why do some male hairstreak butterflies lack scent organs?

Female butterflies generally choose among male suitors, but in the tropics with hundreds of close relatives living in close proximity, how can they decide which males are the right ones? After all, if she mates with a male of another species, she is unlikely to have surviving offspring. One solution is that males of some species have scent producing organs on their wings, so if a male has the right smell, the female will presumably be receptive to his advances. Strangely, males of some species lack these scent producing organs, which would seem to be a huge disadvantage.

Biologists have theorized that when a species loses a male scent producing organ during evolution, its closest relatives do not occur in the same places. In other words, the female does not have to choose among males of the most closely related species, and the males do not devote energy to producing scents.

A team of researchers, led by Dr. Robert Robbins from the Smithsonian Institution, digs into this question in a small group of Latin American butterflies in a study published in the open access journal ZooKeys. Two newly discovered representatives in this butterfly group possess scent pads while their closest relatives do not. The researchers report that scent pads were lost evolutionarily twice in this group, and as predicted, in each case, the species without the scent pad does not co-occur with its closest relative. The present study adds more evidence to accumulating support for the explanation why some males lack scent pads.

Evolutionary losses, such as the one observed herein in Thereus oppia and related butterflies, are quite common, as Dr. Robbins and collaborators have observed in a previous research. Such disappearances of male secondary sexual features have been explained by geographic isolation of a species from its closest relatives, and the butterflies in this study are no exception.

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Original source:

Robbins RK, Heredia AD, Busby RC (2015) Male secondary sexual structures and the systematics of the Thereus oppia species group (Lepidoptera, Lycaenidae, Eumaeini). ZooKeys520: 109-130. doi: 10.3897/zookeys.520.10134