Familiarity breeds contempt: Mockingbirds recognize and react to repeat intruders

ResearchBlogging.orgHumans are a fact of life for many, many parts of the natural world. This doesn’t always have to be a bad thing — some critters adapt to human-dominated landscapes pretty well. A paper in this week’s PNAS, for instance, shows that Northern Mockingbirds nesting on a busy university campus learn to differentiate between uninterested passers-by and people who repeatedly disturb the nest site [$-a].

Northern Mockingbird with fledglings
Photo by mjmyap.

When potential predators come too close to a nest, wild birds try to distract the threat with harassing alarm calls, diving attack flights, or “flushing” to draw attention away. This doesn’t work so well if you’re nesting near a busy sidewalk; you’d spend all you time trying to drive off passers-by who pose no real threat. And, as Levey et al. show in the new paper, mockingbirds seem to have solved this problem by reacting more strongly to people who approach the nest repeatedly.

The experimental evidence is elegant: Individual researchers approached occupied mockingbird nests on four consecutive days, and recorded the birds’ reactions. The birds flushed farther, gave more alarm calls, and attacked more often with each repeat visit. When a new person approached the nest on a fifth day, though, the birds’ reactions were equivalent to their behavior on the first day. Furthermore, nesting mockingbirds flushed farther if their nests were near less-busy sidewalks. This isn’t evidence for an evolved response, but for learning; and it suggests that mockingbirds are able to recognize individual humans, and apply that familiarity in assessing the danger posed when someone approaches the nest.

This is just one example of the evolved and learned adaptations the living world has made in response to human activity. Last year, for instance, a study showed that a French wildflower has evolved wingless seeds in response to urban growing conditions — although winged, wind-dispersed seeds do better in the wild because they’re less likely to compete with their siblings, in a heavily paved environment the best spot to germinate is more likely to be close to the parent plant. And perhaps one of the best-known examples of natural selection in action is the increased frequency of dark-colored peppered moths in response to industrial pollution. Nature is nothing if not flexible.


Cheptou, P., Carrue, O., Rouifed, S., & Cantarel, A. (2008). Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta Proc. Nat. Acad. Sci. USA, 105 (10), 3796-9 DOI: 10.1073/pnas.0708446105

Grant, B.S., Owen, D.F., & Clarke, C.A. (1996). Parallel rise and fall of melanic peppered moths in America and Britain Journal of Heredity, 87, 351-7 DOI: http://jhered.oxfordjournals.org/cgi/content/abstract/87/5/351

Levey, D., Londono, G., Ungvari-Martin, J., Hiersoux, M., Jankowski, J., Poulsen, J., Stracey, C., & Robinson, S. (2009). Urban mockingbirds quickly learn to identify individual humans Proc. Nat. Acad. Sci. USA, 106 (22), 8959-62 DOI: 10.1073/pnas.0811422106

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.


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

Getting away from it all: Why are invasive species invasive?

ResearchBlogging.orgWhen humans move from place to place, we almost always bring other organisms with us. Sometimes it’s intentional — domestic animals carried along with Polynesian colonists, for instance. Just as often, it’s accidental, as with mice stowing away on Viking longships. A lot of these introduced species have done so well in their new habitats that they become invasive, outcompeting natives and disrupting local ecosystem processes. But the species that go crazy-invasive — the cane toads and the purple loosestrife — are probably only the very successful subset of the species that hitch rides in cargo holds and ballast tanks. What sets the dangerously successful invasive species apart from others?

Purple loosestrife, introduced
from Europe, now common in East
Coast wetlands.
Photo by Muffet.

A new dataset published in last week’s PNAS suggests that it may be an interaction between available resources in a new habitat and a lack of compatible pathogens [$-a]. This is an amalgam of two hypothesized causes for successful invasion: access to new resources, and escape from antagonistic species. Focusing on European plant species that have successfully invaded North America, the authors, Blumenthal et al., assembled records of viral and fungal infections on each plant species in its native range, and in North America. They classified the plant species based on the habitats each occupies — wet vs. dry, nitrogen-rich vs. -poor — and on whether the plants tended to grow slowly or rapidly. This is because plant species adapted to rich, wet environments are generally thought to evolve fewer defenses against infection and herbivores; they can “afford” to grow new tissue instead of fight to keep it.

Garlic mustard, another invasive
from Europe.
Photo by clspeace.

If resource availability interacts with freedom from infectious agents to spur a successful invasion, then invasive plants adapted to rich conditions should tend to host more pathogens in their home ranges than they do in their introduced range; and this difference should be less pronounced in invasive plants adapted to dry, resource-poor conditions. This is exactly what the analysis found — plants adapted to richer habitats saw a larger reduction in the number of pathogen species attacking them in their new ranges than plants adapted to less-productive conditions.

This is a valuable result for its basic application — helping to predict which introduced species are likely to become invasive, and target them for eradication efforts before they become well-established. But it also provides us with an insight into how evolution works. Many authors, particularly G.G. Simpson and Dolph Schluter, have described ecological conditions that set the stage for adaptive radiation — the rapid diversification of a lineage into many species — which sound a lot like the “ecological release” that invasive species seem to experience.

Rapid evolutionary diversification may be triggered by the evolution of a key innovation; by colonization of a new, empty habitat; or the removal of antagonistic species (usually by their extinction). These three classes of conditions are closely related, and they can be mimicked, or even replicated, when humans move species to new habitats [$-a]. Blumenthal et al. suggest, for instance, that species invasions entail both colonization of a new habitat and escape from pathogens. This is a broad observation; a good next step would be to directly compare natural selection acting on invasive plants in their native and introduced ranges. Through day-to-day processes like this, the specific ecology of a species can ultimately shape its evolutionary fate.


Blumenthal, D., Mitchell, C., Pysek, P., & Jarosik, V. (2009). Synergy between pathogen release and resource availability in plant invasion. Proc.Nat. Acad. Sci. USA, 106 (19), 7899-904 DOI: 10.1073/pnas.0812607106

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification. Trends Ecol. & Evol., 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017

Speciation changes ecosystem

ResearchBlogging.orgWe know that ecosystem processes can act on organisms to help create reproductive isolation and speciation – now, a new paper released online in advance of publication in Nature shows that speciation can change the ecosystem [$-a].

The study’s authors are a group of University of British Columbia scientists, including Luke Harmon (who occasionally blogs at Dechronization) and Simone Des Roches, who have since come to my department at UI as a faculty member and doctoral student, respectively. They focus on the case of ecological speciation in sticklebacks (Gasterosteus aculeatus), which have repeatedly into evolved two reproductively isolated, ecologically different forms [$-a] after colonizing North American freshwater lakes from the ocean about 10,000 years ago. One of the two forms is “limnetic,” living in open water near the surface and feeding on plankton; the other is “benthic,” living on lake bottoms and feeding on invertebrates.

A stickleback
Photo by frequency.

Harmon et al. reasoned that the presence of these two different fish must have a substantial effect on lake food webs. To test this hypothesis, they set up mesocosms – big cattle tanks seeded with a standard mix of sediment, plankton, and invertebrates – and introduced either (1) sticklebacks of the “generalized” type ancestral to the benthic and limnetic types, (2) either the benthic or limnetic type alone, or (3) both the benthic and limnetic types together. They found that the fish species present in the mesocosm strongly affected the plankton species diversity – limnetic-type nearly eliminated one of their preferred prey species – and on measures of total ecosystem productivity and metabolic activity.

Perhaps the most important effect was on dissolved organic content (DOC) and light transmission in the water column. Mesocosms containing both fish types had about the same amount of (non-living) organic material as those containing the generalist ancestor, but the two-species treatment changed the DOC composition to make the water column more transparent to light. In a real lake, this effect could significantly change the productivity and composition of the aquatic plant community, which would in turn reshape the rest of the food web.

The buzzword for this phenomenon is “ecosystem engineering,” which the ESA blog puts front-and-center in its discussion of this paper. I think Harmon et al.‘s result is most interesting as the closing of a feedback loop between the ecosystem and a population undergoing speciation. It’s evidence that a speciation event can actually alter the conditions that created it in the first place – which might prevent future speciation events, or create opportunities for new ones.


Harmon, L., B. Matthews, S. Des Roches, J. Chase, J. Shurin, & D. Schluter (2009). Evolutionary diversification in stickleback affects ecosystem functioning Nature DOI: 10.1038/nature07974

Vines, T., & D. Schluter (2006). Strong assortative mating between allopatric sticklebacks as a by-product of adaptation to different environments Proc. R. Soc. B, 273 (1589), 911-6 DOI: 10.1098/rspb.2005.3387

The environmental impacts of war

ResearchBlogging.orgLast year Bioscience published a review article proposing a new discipline in conservation ecology: warfare ecology [PDF]. It’s now making the rounds in the science blogosphere, with good coverage at Conservation Blog and Deep Sea News, where I first happened upon it – and it deserves all the attention it can get.

In the U.S., at any rate, war and preparation for war tend to get priority over everything – especially tree-hugging environmental concerns. Exhibit A is last year’s Supreme Court decision that the Navy’s need to practice with sonar trumps the damage sonar can do to whale populations, to the extent that the Navy could not be required to do an environmental impact assessment before beginning the exercise. War is treated as an emergency, and who worries about environmental impacts during emergencies?

Yet environmental damage caused in the course of war has direct impact on the human aftermath of conflict. Refugees provided with nowhere else to go will often set up camp in protected lands. Materials used in warfare – Agent Orange defoliant used in Southeast Asia, depleted uranium in Iraq – can continue to kill people long after the fighting ends. On the other hand, the review’s authors, Machlis and Hanson, point out that demilitarized zones and military training grounds often serve as (perhaps overly-well protected) accidental preserves.

This is a subject I’ve thought about quite a bit before – way back in my undergraduate days, I won a Mennonite Central Committee oratorical contest with a speech that connected peace theology to environmental concerns. That speech now looks to me like slightly embarrassing juvenalia, but the central idea still holds, and it’s great to see that working ecologists are thinking along similar lines. By laying out a framework for thinking about the environmental impacts of war, Machlis and Hanson’s paper can hopefully help push governments to consider the longer-term environmental, economic, and social consequences of ecological decisions made in the course of preparing for and prosecuting war.


G. Machlis, & T. Hanson (2008). Warfare ecology BioScience, 58 (8), 729-36 DOI: 10.1641/B580809

Parasites like their hosts clustered

ResearchBlogging.orgIn epidemiology the importance of ecological and evolutionary processes comes into sharp relief: questions about the networks of interactions between species in a community, or about the evolution of parasite specificity, virulence, and contagiousness have immediate implications for human health, as well as in animal husbandry and conservation. One of the most basic of these questions is, what determines the community of parasites that infect a species? One answer is in this month’s issue of The American Naturalist, where a neat meta-analysis shows that the size of mammals’ home ranges shapes the number of parasite species they attract [$-a].

A tapeworm parasitic worm
Photo by pinkcigarette.

For mammals, we already know that parasite communities are shaped by the host’s body size, geographical range, and population density. In this new study, Bordes et al. propose another factor: the host’s home range, the area that a single individual occupies. There are two major ways that home range might shape the diversity of parasites infecting host. Greater home range could mean that the host encounters a broader array of habitats, and opportunities for infection, so that home range and parasite diversity are positively correlated. Alternatively, hosts with smaller home ranges effectively live at higher density, which should create more opportunities for parasite transmission between hosts, generating a negative correlation between home range and parasite diversity.

Bordes et al. test these hypotheses by collecting published studies of the number of parasitic worm (helminth) species infecting mammals, and then performing regressions (corrected for phylogenetic relationships between host species) of parasite species richness on a variety of possible causal factors, including home range. They find that host home range is a stronger predictor of parasite species diversity than host body size, and that home range is negatively correlated with parasite diversity.

In a way, then, this result confirms the importance of host density in host-parasite interactions. But it’s not an obvious outcome – it is intuitive that more densely populated hosts should be more susceptible to parasitism in general, but not that they should also be attacked by a wider array of parasites. Maybe dense host populations are more productive habitat to parasites, so that there’s ecological “space” to support a greater diversity of parasites. Or maybe these dynamics are a result of the specific biology of helminth parasites, many of which have different hosts for different parts of their life cycle.


F. Bordes, S. Morand, D.A. Kelt, D.H. Van Vuren (2009). Home range and parasite diversity in mammals The American Naturalist, 173 (4), 467-74 DOI: 10.1086/597227

Invasive plants turn out to be native

ResearchBlogging.orgBotanists digging through bog sediments on the Galapagos island Santa Cruz have discovered that six plant species thought to be invasive were actually already there when humans first arrived [$-a]. The key data are fossilized pollen grains buried in the sediments – pollen from all six plants are found in sediments formed up to 7,000 years before humans settled the Galapagos.


J.F.N. van Leeuwen, C.A. Froyd, W.O. van der Knaap, E.E. Coffey, A. Tye, K.J. Willis (2008). Fossil Pollen as a Guide to Conservation in the Galapagos Science, 322 (5905) DOI: 10.1126/science.1163454

Nowhere to go but uphill

ResearchBlogging.orgHistorical data sets are invaluable in assessing the impact of climate change on natural systems. Case in point: in today’s issue of Science, a new paper uses a century-old survey of small mammals in Yosemite National Park to see how the park’s community has shifted as climate warmed [$-a].

Belding’s ground squirrels contracted their
high-altitude range as climate warmed.

Photo by infinite wilderness.

Moritz et al. repeated a survey of small mammals – chipmunks, shrews, ground squirrels, and the like – originally conducted by the biologist Joseph Grinnell between 1914 and 1920. Since that time, average minimum monthly temperatures in the Yosemite area have increased approximately three degrees Centigrade (five and a half degrees Fahrenheit), and Moritz et al. found significant changes in the distributions of small mammals associated with that warming.

In the face of warming temperatures, the easiest thing for animals to do is move up to the cooler climes at higher elevations, and this is what many species did. Those at lower elevations expanded their ranges uphill. But small mammals already living at high elevations, like the aptly named Alpine Chipmunk (Tamias alpinus) have nowhere cooler to go – so their ranges contracted over the last century. As the globe warms up, this pattern is likely being repeated in ecosystems everywhere – not a happy prospect for critters that live at high elevations.


C Moritz, JL Patton, CJ Conroy, JL Parra, GC White, SR Beissinger (2008). Impact of a Century of Climate Change on Small-Mammal Communities in Yosemite National Park, USA. Science, 322, 261-4 DOI: 10.1126/science.1163428