Nothing in Biology Makes Sense: Making sense of inbreeding depression

Bighorn sheep. Photo by Noah Reid, via Nothing in Biology Makes Sense.

This week at the collaborative blog Nothing in Biology Makes Sense!, Noah Reid returns to discuss the bane of small, isolated populations: inbreeding depression:

Iconic North American species such as grizzly bears, red-cockaded woodpeckers, and the American burying beetle today inhabit only small fractions of the ranges they occupied only 100 years ago. A result of this fragmentation is that many individuals exist in small, isolated populations. In these populations, a curious phenomenon often emerges, one that can only be understood in light of some basic evolutionary theory.

To find out more about that phenomenon, and when it can become hazardous to a population’s health, go read the whole thing. ◼

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Pollinating birds leave plants in the lurch

ResearchBlogging.orgPlants’ ancient relationship with animal pollinators is pretty crazy, when you think about it. On the one hand, it gives plants access to mates they can’t go find on their own, and it’s more efficient than making scads of pollen and hoping the wind blows some onto another member of your species. On the other hand, it can leave a plant totally dependent upon another species for its reproduction.

This catch is probably why lots of plants still use wind pollination strategies, or reserve the option to pollinate themselves if animals don’t do the job for them. Avoiding complete dependence on animal pollinators is likely to become more important in the modern era, as human disruption of the environment amplifies the inherent risk of entrusting your reproduction to another species [$a], a study in the latest issue of Science shows.

A flower of Rhabdothamnus solandri, waiting for pollinators who may never show up. Photo by Tonyfoster.

Sandra Anderson and her coauthors examined the health of populations of Rhabdothamnus solandri, a forest shrub native to the North Island of New Zealand. The flowers of R. solandri are classic examples of the pollination syndrome associated with birds—bright red-orange, with long nectar tubes. Rhabdothamnus solandri is incapable of self-pollinating, because its The flowers attract three native bird species, the tui, the bellbird, and the stitchbird. Thanks to human activity, all three of these birds “functionally extinct” in most of the range where R. solandri grows.

The bellbird and the stitchbird were eliminated from much of the North Island in the Nineteenth Century as European colonists cleared forests for farmland and introduced cats, rats, and dogs that preyed on the native fauna. Tuis have persisted, but tend to stay in the upper forest canopy—possibly to avoid rat predation—and don’t visit lower-growing shrubs. However, all three birds are still living as they did before Europeans arrived on two island nature preserves just a few kilometers off the North Island’s shores. This creates an inadvertent experiment in pollinator loss, allowing Anderson et al. to compare R. solandri populations on the mainland with those on the preserve islands to see how the plant gets on without its pollinators.

The short answer is: not well.

The three principle pollinators of R. solandri, the tui, the bellbird, and the stitchbird. Only the Tui is still common in most R. solandri habitat. Photos by kookr, angrysunbird, and digitaltrails.

To test whether R. solandri‘s reproduction is limited by pollen supply (as opposed to water or nutrients), the authors compared flowers that were either enclosed to prevent pollinator access, left open to natural pollination, or pollinated artificially. On the islands, plants left open set about as much fruit as plants pollinated by hand—but on the mainland, plants pollinated by hand set much more fruit than those left open. Mainland plants also produced smaller fruits, with fewer seeds per fruit, than island plants. The enclosed flowers set very little fruit, so it seems clear that pollen is the limiting factor for island and mainland R. solandri populations, and mainland populations aren’t getting enough.

The age structure of island and mainland R. solandri populations bears this out. Anderson et al. surveyed the island and mainland sites and counted the number of “adult” shrubs in a given area relative to recently sprouted seedlings. Island and mainland sites had similar densities of adult shrubs, but mainland sites had much lower densities of seedlings. It looks very likely that R. solandri populations on the North Island mainland are in decline as a direct result of losing pollinator services.

As Cagan Sekercioglu points out in an invited commentary [$a], this study demonstrates that species’ ecological roles can be strongly compromised even if they don’t go extinct. Tuis and bellbirds are not considered particularly endangered, and the stitchbird is classified as “vulnerable,” the lowest level of “threatened” under the system used by the International Union for the Conservation of Nature. Yet these birds’ local losses and adaptation to human activity have left R. solandri without adequate pollination services. Conserving biodiversity requires more than preventing extinction—but it can be quite a bit harder to preserve important relationships between species such as this one.

References

Anderson, S., Kelly, D., Ladley, J., Molloy, S., & Terry, J. (2011). Cascading effects of bird functional extinction reduce pollination and plant density. Science, 331 (6020), 1068-1071 DOI: 10.1126/science.1199092

Sekercioglu, C. (2011). Functional extinctions of bird pollinators cause plant declines. Science, 331 (6020), 1019-20 DOI: 10.1126/science.1202389

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Invasive species not so bad?

Over on Slate, Rebecca Tuhus-Dubrow says some conservation biologists are starting to question the importance of preventing species invasions:

Certainly, they say, non-native plants and critters can be terribly destructive—the tree-killing gypsy moth comes to mind. Yet natives such as the Southern Pine Beetle can cause similar harm. The effects of exotics on biodiversity are mixed. Their entry into a region may reduce indigenous populations, but they’re not likely to cause any extinctions (at least on continents and in oceans—lakes and islands are more vulnerable). Since the arrival of Europeans in the New World, hundreds of imports have flourished in their new environments.

Tuhus-Dubrow cites the case of Tamarisk in the U.S. Southwest — an aggressive introduced shrub that has also ended up providing important nesting sites for the endangered southwestern willow flycatcher.

The fact of the matter is that human-introduced species can eventually integrate into an ecological community; once they do it’s hard to get them out, and problematic as to whether it’s a good idea. In Australia, dingoes helped extirpate many other large predators when they were introduced by the first humans to arrive on that continent — and now they’re critical to controlling other, later-introduced mammal species.

(Thanks to Ephraim Zimmerman for point this one out to me!)


Invasive pest, or critical flycatcher habitat? Maybe both. Photo by Anita363.
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How fast do ecosystems recover from disturbance? It’s complicated.

ResearchBlogging.orgIn the 21st century, human activity promises to impact the natural world on an unprecedented scale. In order to decide where to focus conservation effort, one thing we need to know is how permanent the damage from a forest clear-cut or a collapsed fishery actually is. A paper in this week’s PLoS ONE looks at natural systems’ ability to recover after human and natural disturbances, and the authors say the results are hopeful. I’m not so sure.


A clearcut.
Photo by : Damien.

The authors, Jones and Schmitz, assemble a meta-dataset of ecological studies published from 1910 to 2008, all examining the recovery of either ecosytem functions (like total nutrient cycling rates) or plant or animal diversity following disturbances as diverse as hurricanes and oil spills. They then calculated the proportion of measured variables that recovered, or failed to, within the period studied by each paper in the dataset, how much the measured variables had been altered by the disturbance, and how long it took before they returned to their pre-disturbance state.

The results are complicated, to say the least. For example, here’s Figure 2, which charts the times to recovery for variables measuring animal community recovery (black bars), ecosystem function (white bars) or plant community (gray bars), broken down by ecosystem type in the top panel, and by disturbance type in the bottom panel:

The authors’ conclusion? There is “no discernable pattern.” Which I can’t really dispute — recovery times look highly idiosyncratic. An ANOVA performed on the data finds significant effects of ecosystem type and disturbance type, but what does that tell us? Different ecosystems recover differently. Forests take the longest to recover, which makes sense given that trees grow slowly, and succession from clearcut to mature forest can take centuries. Similarly, ecosystems experiencing multiple types of disturbance took the longest to recover.

Of the ecosystems that do recover, the authors point out, recovery occurs comparatively rapidly:

Among studies reporting recovery for any variable, the average recovery time was at most 42 years (for forest ecosystems) and typically much less (on the order of 10 years) when recovery was examined by ecosystem. When examined by perturbation type, the average recovery time was no more than 56 years (for systems undergoing multiple interacting perturbations) and typically was 20 years or less …

The authors then perform a regression of the strength of disturbance (i.e., how much the measured variables changed due to disturbance) against the time needed for recovery. The data set is necessarily small, because not many studies follow an ecosystem all the way from disturbance to complete recovery, and they find a significant effect of disturbance strength on recovery time mostly because of a single data point.

Jones and Schmitz conclude from this dataset that ecosystem recovery from human disturbance is frequently possible within human lifetimes, especially if we put in the effort for restoration. I’ll buy that; but I think the more important lesson to draw from this paper is that, after a century of watching the natural world respond to human activity, we still can’t predict what the results of our actions will be. It shouldn’t need saying, but when we fiddle with our life-support systems, we must proceed cautiously.

Reference

Jones, H., & Schmitz, O. (2009). Rapid recovery of damaged ecosystems PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005653

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A second chance for Last Chance to See

Just discovered: Stephen Fry joins Mark Carwardine in returning to the places and creatures visited by Carawardine and chronicled by Douglas Adams in the excellent little book Last Chance to See, a travelogue of desperately endangered animals. The second Last Chance, like the first, is principally a BBC documentary project – we shall have to see if a book grows out of Fry’s new journey. Regrettably, none of the video seems to be viewable this side of the Atlantic.

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Human populations grow faster near protected areas

New in Science: Protected areas like national parks and forests seem to stimulate economic development [abstract only]. The study’s authors examine more than 300 protected areas in 45 African and Latin American countries, and find that human populations near the preserves are growing nearly twice as rapidly as those in rural areas farther away from preserves.

On the one hand, this is good news – it might be evidence that, rather than depressing economic development, natural reserves actually provide benefits to locals in the form of jobs as park staff or ecotourism guides, or development projects coupled with conservation efforts. On the other hand, though, it could be that development associated with tourism actually puts more pressure on the land just outside protected areas, making the preserves more isolated and less ecologically functional. The paper also shows that deforestation of the areas around preserves increases as the human population growth rate increases.

Reference

Wittemeyer G, P Elsen, WT Bean, A Coleman, O Burton, and JS Brashares. Accelerated human population growth at protected area edges. Science 321:123-6.

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