It Pays to Break from the Crowd

Image Credit: Pete, CC BY-NC 2.0

Increased reproductive success through parasitoid release at a range margin: Implications for range shifts induced by climate change (2020) MacKay, Gross, & Ryder, Journal of Biogeography, https://doi.org/10.1111/jbi.13795

The Crux

Predicting the response of organisms to climate change is a challenge for ecologists and wildlife managers alike. Fortunately, some responses are common enough that it is still possible to make fairly accurate predictions about them without too much information. One common response is that of the range shift, whereby a population of organisms facing some alteration (eg. climate change) in their current habitat, making it unfavorable, begin to move to another location. This allows them to track favorable environmental conditions and possibly mitigate any negative effects of climate change.

Sounds easy, right? Just pack it all up and move when things get hard? Well, for some organisms it may be that simple (looking at you, birds), but for others (like trees) it is significantly harder to do so. Trees (and other plants) are limited in that they depend on other organisms or things like wind to help disperse their seeds. Making things even more difficult are plant species that depend on specific pollinators, and in order for a successful range shift to happen trees AND their pollinators have to make the move. Today’s authors wanted to study how relationships between trees and their pollinators changed at the leading edge of a range shift, allowing them to understand how and why trees succeed during a range shift.

What They Did

The authors used fig trees (Ficus rubiginosa) and their mutualist fig wasp (Pleistodontes imperialis) pollinators as their model system. They collected an impressive amount of data to understand one basic idea: how does being at the forefront of a range shift (and far from other trees) affect relationships with pollinators.

To answer this question, the authors made comparisons between single, isolated trees, trees in small groups, and trees in large groups. To measure how well-pollinated each tree was they counted the number of fruits produced by each tree, the number of developed seeds per fruit, how many wasps were found within each fruit, and the amount of wasp parasitism recorded.

Did You Know: Fig Wasps

Fig wasps are a classic system in ecology for studying mutualisms. They lay eggs inside the fruit of the fig trees, where the larvae eventually hatch, feed on the fruit, gather pollen, and fly off to another tree to pollinate it and start the cycle all over again. This mutualism developed tens of millions of years ago, and since then has grown to include ~900 species. The fig/fig wasp partnership has spawned an entire unique ecosystem, with predators, parasites, and parasitoids flocking to these fig trees to take advantage of the fig wasps. Predators eat the wasps, wasp parasites lay eggs in the fruits but don’t spread pollen, and wasp parasitoids lay their eggs inside of the pollinator wasps.

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Fig wasps are incredibly small insects (they’re the black spots there in the photo) Despite this size these little champions can fly up to 14.2km/8.8 miles in order to pollinate new trees. Without these impressive flight abilities fig tree at the species range likely wouldn’t succeed. (Image Credit: Jnzl’s Photos, CC BY 2.0)

What They Found

The isolated trees at the range edge were more fit, meaning that both the number of fruits (female fitness) and pollen distributed (male fitness) were higher than when trees were growing together in large groups. Although fewer wasps were found in the isolated trees, pollinator fitness was still higher in the isolated trees. This was because there were both fewer parasitoids killing the wasps and fewer wasps per fruit (reducing competition for food).

Problems

The methods involved in collecting all of the data for this paper were very thorough, yet my issue with this study isn’t in the methods, but something that the authors didn’t touch on. Despite isolated trees (and the wasps in isolated trees) having higher fitness, it is unclear as to why there aren’t more trees at the range front. The environment may not be ideal for these trees, and it may be difficult for more trees to establish themselves, but I would’ve appreciated some discussion as to why these fitter trees are all alone on the landscape (minimum of 1km from their nearest neighbor).

So What?

Isolated trees in a fragmented landscape appear to act as refuges for pollinators where they can escape parasitism and competition, increasing their fitness. This increased pollinator fitness results in an increase in fig fitness. These advantages allow for the persistence of figs in an increasingly fragmented and arid landscape, and they also provide crucial data needed to better predict the outcomes of species range shifts due to climate change.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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