Friday, 30 September 2011

Rice fields provide habitat for breeding bitterns

Anthropogenic habitat changes affect a variety of habitat types, but few more so than wetlands. In Europe alone, wetlands have been reduced by 80-90%, often to make room for agricultural fields. Typically, species that once lived in wetland areas find themselves homeless once humans alter the habitat, but every now and then there is an encouraging story of adaptation to and, ultimately, success in, anthropogenic areas.

Such appears to be the case for Eurasian bitterns (Botaurus stellaris), a species of European conservation concern that declined drastically between the 1970's and 1990's. This species traditionally lives in reed beds, which not only offer foraging habitat but also provide cover for individuals and active nests. In Italy, however, bitterns have been observed increasingly often in rice fields, which have many structural similarities to the birds' natural habitats.

 (A stealthy Eurasian bittern, Botaurus stellaris)

Recently, collaborators from the Universita degli Studi di Pavia and the Universita degli Studi di Milano examined bittern rice field use in greater detail, in the hopes of understanding what habitat characteristics the birds are paying attention to when they decide to utilize these anthropogenic areas. Bitterns are notoriously difficult to spot, since they have camouflaging plumage and a knack for positioning their bodies to blend in with the vegetation. However, males produce loud percussive vocalizations--known as booms--during the breeding season; by listening for booms over successive nights, the researchers were able to find fields that were frequented by bitterns and, by process of elimination, also identify fields that the birds decided to avoid.

Over the course of a breeding season, the scientists were able to locate 9 booming males. Habitat characteristics were measured in each of the territories associated with these individuals; traits included those that varied throughout the season--height/density of rice plants, characteristics of vegetation along the borders of the field, amount of visible weed cover, degree of flooding--and those that were invariant--distance from buildings, distance from natural wetlands, and total area of the field. The same characteristics were measured in nearby control fields that were located adjacent to the territories but were never occupied. By comparing traits in the two types of field, the researchers could see whether the males were consistently attracted by particular habitat features--or, conversely, whether there were some things that consistently repelled them. The scientists also tracked these preferences throughout the breeding season in order to assess whether they changed over time.

(Agricultural field in Lomellina, Italy, where the study was conducted.)

Bitterns were more likely to choose larger rice fields with taller plants, more vegetation along the borders, and greater weed cover. The taller plants likely offer more camouflage and shelter; this is probably also a benefit of the border vegetation and weed cover, which also provide extra foraging space.  In larger fields, bitterns would have more space in which to hide during the times when farmers come out to work. Bitterns were also more likely to be found in rice fields that were located near natural wetlands, which may be useful as sheltered resting sites or alternative foraging locations. Interestingly, the birds did not appear to be impacted by distance from buildings, suggesting that the birds could be fairly tolerant of humans.

Over the course of the study, during which time the rice plants grew considerably higher, bitterns seemed to care increasingly more about the overall size of the field, and less about whether the field banks were vegetated. The importance of larger fields later in the season coincides with the sensitive egg-laying period of the birds--a time when they would want to be as furtive as possible in order to ensure the success of their breeding attempts. More space offers more places to hide far away from the nest, thus helping keep its location secret. The decreasing relevance of vegetated borders throughout the season likely reflects the fact that the higher rice plants offer places to hide and forage (mainly on invertebrates and amphibians), so alternative tall vegetation is not needed nearby.

(A Eurasian bittern nest with one hatched chick)

Previous studies have found that rice fields are species-poor and less diverse than their natural wetland counterparts. Despite this, they do seem to act as refugia for some species--for instance, herons and egrets, which, like bitterns, are wading birds in the family Ardeidae. The current study found several habitat characteristics that were preferred by bitterns; this suggests that agricultural areas could be deliberately managed in such a way as to encourage bitterns to settle there--assuming that farmers can be convinced that the birds are not doing their fields any harm.

One worrying trend is the increased use of drought-tolerant rice, which does not require water until later in the summer. In agricultural areas where this strain of rice is used, fields may not be flooded during the time when bitterns are searching for suitable breeding habitat--causing the birds to bypass these areas and find alternatives, where possible, or skip breeding altogether. It is also possible that pesticide use could be harmful to the birds, both directly and by reducing food availability. Thus, the authors suggest that conservationists should work closely with farmers to teach them about bittern preservation and encourage them to use agricultural techniques that are beneficial not only to their crops, but also to local birds. The seasonality of bittern habitat preference should also be taken into consideration--no matter how well-intended, conservation efforts that occur during the wrong time of year may not do the birds any good.

Longoni, V., Rubolini, D., Ambrosini, R., Bogliani, G. 2011. Habitat preferences of Eurasian bitterns Botaurus stellaris booming in ricefields: implications for management. Ibis 153:695-706.

Thanks to the following websites for providing the images used in this post:

Thursday, 29 September 2011

Invasive forest insects damage budgets as well as trees

Most of us encounter invasive species every day--for instance, pigeons and house sparrows eating crumbs underneath sidewalk cafe tables, or the ornamental plants in our own front yards. Because the sight of these species becomes routine, it is easy to forget that they don't belong in many of the places where they are currently found, and that their presence may come with a cost. In the case of invasive insects, that cost can be counted in dollars--millions and billions of them each year. Exact numbers have traditionally been hard to come by because of the difficulty of tracking down informative datasets and taking into account complex interactions and often high amounts of uncertainty associated with estimates.

However, these difficulties have been overcome in a new study published earlier this month, which utilized advanced mathematical modeling techniques to estimate the total direct annual costs of invasive forest insects in the continental United States. The scientists who conducted the study represent a variety of disciplines from several research institutions, including the National Center for Ecological Analysis and Synthesis, McGill University, the University of Minnesota, the US Forest Service, Arizona State University, Michigan State University, and the University of Central Florida.

(Damage to a hemlock tree by the hemlock woolly adelgid, Adelges tsugae, which feeds on sap.)

(The hemlock woolly adelgid.)

They made use of a database cataloging the 455 nonnative forest insect species known to be established in the US. Each species was assigned to one of 4 feeding guilds based on its primary or most damaging mode of feeding--phloem/wood borers, sap feeders, foliage feeders, or "other." Although invasive insects can have a variety of direct and indirect costs, the researchers focused on those that are best documented and, therefore, easiest to measure accurately: federal government expenditures (on surveying, research, regulation, management, and outreach), local government expenditures (tree removal, replacement, treatment), household expenditures (tree removal, replacement, treatment), residential property value losses, and timber value losses to forest landowners.

(An emerald ash borer, Agrilus planipennis, a "poster pest" from the borer guild.)

Wood borers, often invade by hitching a ride on wood packaging materials shipped from overseas, while sap and foliage feeders probably originate on shipments of live plants. When all spending categories were considered cumulatively, the most economically destructive group was the wood boring guild, despite the fact that this group contained the smallest number of species. These insects cost local governments approximately $1.7 billion and reduced property values by about $830 million.  The least harmful group overall was the sap feeding guild, despite the fact that it comprised the largest number of species. These insects caused "only" $14 million in federal spending and $260 million in real estate losses each year.  Foliage feeders were intermediate to these two extremes, costing the federal government approximately $110 million per year, and reducing property values by $410 million. For each guild, only the two most costly categories are listed here; each group of insects was also associated with millions more dollars of expenditures by other entities, such as forest landowners.

(A gypsy moth, Lymantria dispar, which originates in Eurasia but has been increasing its range in North America since introduction in the 1860's.)

These estimates emphasize that invasive species can be not just ecologically, but also economically, destructive. Because these figures do not take into account indirect costs, the tallies presented here are just the lower bound of the total cost of forest insect pests. Overall, the bulk of costs were borne by homeowners and municipal governments, whose efforts were focused on removing damaged or dying trees, replacing them with healthier specimens, and/or applying anti-pest treatments. The authors of the study suggest that some of these costs could be offset by targeted import taxes or fees, such that the same people responsible for introducing the pests would also be the ones supplying at least some of the funds used to eradicate or control them.

The data are also useful for evaluating the utility/efficacy of current spending. For instance, although wood borers cause more damage than foliage feeders, the federal government spends similar amounts on efforts to control species from both guilds.  Likewise, while sap feeders cause more localized host mortality, they receive less funding than defoliators and borers. These spending "mismatches" do not necessarily represent poor decision-making skills among the powers that be. Rather, they may reflect the difficulty of controlling pests that can be spatially and temporally unpredictable, as well as a lack of regulatory/management options, or even the effects of external pressures (e.g., lobbying) on spending decisions.

The modeling framework developed in the study can be updated as new information becomes available, and can be tweaked to include other ecosystem services--and ecological impacts--as they are increasingly better understood. Hopefully, shareholders, policy-makers, and managers alike can use the results of this and future models to make decisions ensuring that they are spending their money wisely.

Aukema, J.E., Leung, B., Kovacs, K., Chivers, C., Britton, K.O., Englin, J., Frankel, S.J., Haight, R.G., Holmes, T.P., Liebhold, A.M., McCullough, D.G., Von Holle, B. 2011. Economic impacts of non-native forest insects in the continental United States. PLoS ONE: 6(9):e24587.

Thanks to the following websites for providing the images used in this post:

Tuesday, 20 September 2011

Impacts of impervious surface cover in watersheds

Two classically understudied topics in the disturbance/conservation literature are bacterial communities and aquatic habitats, both of which have traditionally been neglected because they are simply not as easy to study as larger organisms living on land. However, modern techniques--particularly chemical and genetic analyses--have helped make these subjects more accessible. In fact, some research has addressed both topics at once, as in a recent study that examined the effects of urbanization on stream bacteria and ecosystem function.

The work, which was conducted by collaborators from Duke University and the Colorado State University, sought to quantify differences in stream microbial communities as a function of nearby impervious surface cover. The researchers focused on 8 streams in the Raleigh-Durham area of North Carolina. Their watersheds contained anywhere from 1-39% impervious surface cover (ISC), a metric that was used to classify streams as having low (<3%), intermediate (9-10%), or high (>10%) amounts of human-related disturbance--including heavy water flow, increased stream temperatures, and inputs of contaminants and nutrients.

Composition of bacterial communities was significantly different at the 3 different levels of ISC. This was true when the researchers considered all bacteria sampled, and also when they considered genetically distinct subsets of bacteria. Specifically, they focused on two different types of "denitrifying" bacteria--species that play an important role in stream health by transforming nitrate to nitrogen gas, thus removing the nitrate from the water (if left in the water, nitrate can reduce water quality and lead to human health problems). Denitrification is a particularly important task in more urbanized streams, which often have higher nitrate levels because of the nearby use of fertilizers during landscaping and farming activities. 

 (A denitrifying bacterium)

Previous studies have shown that the presence of heavy metal contamination in stream sediments can affect bacterial community composition--particularly among denitrifiers. Unsurprisingly, the researchers found that highly urbanized streams were significantly more likely to have nickel, and were also strongly associated with aluminum, cadmium, and lead.

Because differences in stream microbial communities could affect stream denitrification, the researchers collected soil samples from their different study sites and took them back to the lab to measure denitrification potential. To do this, they put the samples in flasks, added identical amounts of chemical-filled medium (mimicking stream water), and incubated the samples for 90 minutes. Because each flask was stoppered, nitrous oxide built up as denitrification progressed, and by measuring the amount of gas, the scientists could quantify how much denitrification had taken place. Surprisingly, despite the fact that denitrification potentials ranged hugely (from 41 to 561 ng of nitrogen per gram of sediment per hour), this variation was not related to variations in ISC. In other words, even the most urbanized streams provide adequate conditions for denitrification.

(An urbanized watershed--complete with ISC)

In the final part of the study, the scientists constructed statistical models to identify the factors with the greatest effects on denitrification potential. Possible influences included the amount of nitrate and total organic carbon in the stream water at each site, and composition of the local denitrifying bacteria community. They found that denitrification was best predicted by the composition of the nosZ bacterial community--which comprises species that accomplish denitrification using an enzyme (nitrous oxide reductase) produced by the nosZ gene (as opposed to nirK bacteria, which rely on nitrite reductase).

Cumulatively, the results of the study show that streams can vary greatly across an urban gradient, and that urban stressors are capable of driving changes in the composition of stream bacterial communities. The current results agree with patterns found during previous work in the same study sites: Amount of impervious surface cover in watersheds impacts chemical, physical, and hydrological gradients in streams, which then influence local fauna (and probably also flora, but this has not yet been reported for these focal streams). 

The authors point out that it would be useful for future studies to investigate the effects of urbanization and ISC on other functional groups--for instance, bacteria involved in decomposition and carbon cycling, two other processes that are key to ecosystem health. It might also be helpful to know whether there is a disturbance threshold beyond which the nosZ bacterial communities cannot be maintained, or can no longer achieve adequate levels of denitrification. This information could be vital in predicting the effects of higher levels of ISC (e.g., >39.4%) than those observed in the watersheds examined here.

Wang, S.-Y., Sudduth, E.B., Wallenstein, M.D., Wright, J.P., and Bernhardt, E.S. 2011. Watershed urbanization alters the composition and function of stream bacterial communities. PLoS ONE 6(8):e22972.

Thanks to the following websites for providing the images used in this post:

Sunday, 18 September 2011

Road ecology: an often overlooked field of conservation research

Earlier this year, the open access journal Ecology and Society had a special feature called The Effects of Roads and Traffic on Wildlife Populations and Landscape Function. The issue featured 17 papers plus a guest editorial all devoted to discussing the ecological impacts of roads, the efficacy of road-related mitigation techniques, and the future of road ecology research.

One of the themes highlighted in the introductory editorial is the need for greater public awareness about the effects of roads, as well as a greater emphasis on collaboration between managers, politicians, and road ecology experts. There are an estimated 750 million vehicles utilizing approximately 50 million kilometers of road worldwide, with increases in both road network and traffic volumes continuing to occur globally--especially in developing areas such as eastern Europe, China, India, and Latin America. The known effects of roads include loss and fragmentation of habitat, injury and/or death of wildlife after collision with vehicles, changes in microhabitat (e.g., light, moisture, wind) due to increased exposure, pollution, and facilitation of invasion by non-native species.

Roads can have negative impacts on humans, as well--by reducing the aesthetic qualities and the recreational value of habitats. Perhaps more immediately important is the massive amount of injury and property damage associated with animal-vehicle collisions; in North America alone, 1-2 million collisions are thought to occur each year, causing several hundred human fatalities and over a billion US dollars in damage.

Despite the clear drawbacks of roads, they are often not considered in the same light as other human-induced habitat changes; ask the average person what anthropogenic disturbances are most destructive, and he/she is more likely to focus on factors such as pollution, urban sprawl, and climate change. Although roads are often associated with these things, they are generally not pinpointed as a dangerous habitat feature unto themselves, perhaps because they lie flat to the ground and do not have the same obvious physical presence of, say, skyscrapers or strip malls.

Pretty much since the first road ecology study was conducted  in 1925, it has been clear that animal abundance is negatively impacted by the presence of roads. The extent of the effect varies for different species and at different distances from road. Among frogs studied in Utah, for instance, the "road-effect zone" included land up to 1000 m from the roadside, and reduced abundances of 4 of 7 frog species examined; among small mammals evaluated in a desert environment, however, 11 of 13 species could be captured under vegetation right next to the road. Roads may also affect behaviors. Traffic noise, for instance, causes some bird and frog species to alter their vocalizations, thus potentially impacting their ability to communicate effectively. Roads can also disturb migration patterns, slowing down progress or forcing migrating individuals to take more circuitous routes, as observed in northern leopard frogs (Rana pipiens).

Encouragingly, some relatively simple mitigation techniques are effective at reducing the negative impacts of roads on wildlife. For instance, moving salt pools farther from roadsides can reduce moose (Alces alces)-vehicle collisions by 50%, saving lives of both moose and drivers. The installation of both over- and underpasses can allow safe passage of wildlife, not only preventing accidents on the road, but also preserving or even improving current rates of gene flow. Preliminary cost-benefits analyses have already suggested that, in many areas, the efficacy of some management methods exceed the costs required to employ them, indicating that increased implementation of a few simple techniques could save money and animals.

Because roads keep humans connected, they are an integral part not only of our daily lives, but also of our social structure. In other words, they are not going anywhere soon. Although the major goal among road ecologists is developing mitigation techniques that keep wildlife safe for generations to come, there is still a dearth of the very data that are necessary to make logical management plans. For instance, we still know relatively little about how roads affect long-term viability of populations, what effects they have on the landscape, and whether/how they alter ecosystem function. The authors of the guest editorial encourage road ecologists to center their future work around these questions. Furthermore, they encourage research that is applied in nature and can have direct, obvious value for road agencies, conservationists, wildlife, and motorists alike. One important goal--shared by biologists in a number of fields--is education of the public, including those individuals that make policy decisions. Hopefully, greater awareness will lead to smarter, safer use of roadways, and choices that bode well for the future of wildlife that live in heavily-trafficked areas.

van der Ree, R., Jaeger, J.A.G., van der Grift, E.A., and Clevenger, A.P. 2011. Effects of roads and traffic on wildlife populations and landscape function: road ecology is moving toward larger scales. Ecology and Society 16(1): 48 [online].

Thanks to the following websites for providing the images used in this post:

Saturday, 17 September 2011

Avian populations are not impacted by collision with anthropogenic structures

Experts have estimated that, of all birds migrating through eastern North America, approximately 70% pass through at least one major metropolitan area per migration event. During this time, birds face a number of dangers, one of the least understood of which is collision with anthropogenic structures. Conservationists and bird-lovers alike have frequently called attention to the avian mortality caused when the birds fly into buildings and communication towers. Indeed, there are several long-term datasets that document the many deaths caused by collision--as many as 25 million total mortalities from towers, and 1 billion mortalities from windows, cumulatively representing nearly a quarter of all breeding land birds in North America.

However, absolute numbers are not really informative. The loss of 1,000 individuals of a given species has very different impacts on the species' long-term population numbers depending on whether there are, say, 10,000 or 1,000,000 birds comprising the entire breeding population. In other words, to really understand the implications of collision mortality, it is important to put the body counts into perspective.

This is exactly what collaborators from the University of Minnesota did in a study published this past week. They utilized existing datasets describing birds that were found dead after collisions with towers (180,832 mortalities of 188 species at 39 sites) and buildings (62,271 mortalities of 147 species in 3 cities) in the eastern half of North America. The researchers focused on species that bred, migrated through, and/or wintered in habitats near the towers, and on species that occurred regularly in the area near the buildings. For each focal species, they calculated per capita collision risk by comparing total known collision mortality (determined by body counts at each site) with total population size (determined by Breeding Bird Survey data). 

(Black-throated blue warbler (Dendroica caerulescens), identified as one of the birds most likely to fly into buildings and towers.)

Unsurprisingly, the researchers found a significant correlation between collision mortalities and population size: More common birds are more likely to fly into towers and buildings. However, there are a few species that were identified as "super colliders" and "super avoiders," indicating that the extent of their presence or absence in the body count data was unexpected given their population numbers. Black-throated blue warblers (Dendroica caerulescens) are "super colliders" at both towers and buildings, while cliff swallows (Petrochelidon pyrrhonota) and horned larks (Eremophila alpestris) are consistently "super avoiders." The extent of vulnerability to collision varied by species; at towers, for instance, the top 5 most vulnerable species are 96-236 times more likely to experience collisions than expected by chance, while the 5 least vulnerable species are 222-688 times less likely to to experience collisions than expected by chance.

(Cliff swallows (Petrochelidon pyrrhonota), one of the two "super avoiders" of both towers and buildings.)

Even more interestingly, when the researchers compared the likelihood of collisions relative to total population size with raw body count data, only two "super colliding" species were also found among the top 5 species identified by the body count data (swamp sparrows, Melospiza georgiana, at buildings, and bay-breasted warblers, Dendroica castanea, at towers). Vulnerability appears to be a shared trait within avian families: Wood warblers are much more vulnerable to collision, for instance, while swallows are much less so.

(Swamp sparrow (Melospiza georgiana), one of the two "super colliders" that also made up a high proportion of the total body count at buildings.)

A second component of the study involved determining the impact of species' migratory habits, including total distance traveled, migration destination, and whether the birds traveled at night or during the day. Species that travel farther--either in terms of absolute distance or simply being "Neotropical" versus "local" migrants--are more likely to experience collisions than short-distance birds and non-migratory species. Furthermore, collisions are significantly more likely among nocturnal migrantsthan diurnal migrants.

Despite the high numbers of mortality at these two types of anthropogenic structure, the authors did not detect any impact of collision rates on avian population health. In fact, there was a weak positive relationship between population change and vulnerability to collision with buildings, indicating that more vulnerable species were actual more likely to have increasing population numbers.

(Window decals, one mitigation method that can reduce avian collisions with buildings.)

Overall, the results of the study indicate that, while buildings and towers certainly aren't doing migratory birds any favors (an issue also addressed in this recent post), they also don't appear to be having any long-term negative impacts on avian populations, either. The authors are careful to say that this is no reason for managers to stop employing mitigation techniques, such as turning out lights during peak migration and minimizing vegetation near glass building faces, that can reduce avian collision mortality. Rather, they emphasize that these methods do not address the factors that are actually responsible for avian population declines--including habitat destruction, climate change, house cats, and poisoning, to name a few.


Thanks to the following websites for providing the images used in this post:

Friday, 16 September 2011

How does urban growth affect the availability of green spaces in cities?

Over the course of 2011, there have been several reports on urbanization, densification, ecosystem function, and/or the preservation of nature. The prominence of these topics (or, rather, meta-topic, since they are so closely linked) this year reflects our struggle to find space for an ever-growing human population while maintaining natural habitats that not only perform valuable services--providing crops, keeping flood waters at bay, etc.--but also simply make us happy.

This week, a new study joins the fray, and, if you are a lover of green spaces, its findings are depressing. It reports that, across the 13 largest cities in the UK, the total area devoted to green habitat has declined since 2000, when the national policy shifted to encourage urban development in areas such as gardens, parks, and allotments. These and other "brownfield sites"--abandoned and underused facilities considered available for re-use--were seen as alternatives to expansion into the wider countryside; in other words, the government promoted densification, at the expense of green space, over sprawl.

(Bird's-eye-view of some London allotments)

The research was conducted by collaborators from the University of Sheffield, Peking University, and the University of Kent, who used a combination of different satellite imagery techniques to evaluate temporal changes between 1991 and 2008 in the extent of developed and undeveloped areas in Bournemouth, Portsmouth, Brighton, Bristol, London, Birmingham, Leicester, Nottingham, Sheffield, Liverpool, Manchester, Leeds, and Newcastle. To look for correlations between land use change and population growth, they also used census and survey data to evaluate shifts in population size and the number of dwellings within each of these 13 study cities over the same time period. In addition to looking at broad changes over time, the researchers broke their study period into two parts: before the government-recommended shift in urbanization strategy, and after.

Between 1991 and 2006, all but one of the study cities (Manchester) experienced increases in the amount of green space--despite concurrent growth of both human population numbers and the number of dwellings within city limits. In particular, London, Birmingham, and Newcastle, saw notable rises in proportion of green space. However, this potentially encouraging news was swept aside by results of separate analyses on the pre- and post-policy change periods. The bulk of positive green space changes occurred prior to 2001, since which time urban centers have become markedly less "natural." Nine of the 13 cities have experienced reductions in green space since 2001, since when there have also been increases in the numbers of dwellings per city, along with decreases in average household size (a trend recently bemoaned by the Royal Institute of British Architects).

(Aerial view of Manchester, showing a few secluded bits of green. This was the only city not to experience net increases in green space during the 1991-2006 period.)

One of the most notable patterns emerging from the data is a strong latitudinal gradient: Since 1981, southern cities (e.g., Brighton, Bournemouth, and London), which have the least amount of green habitat, have generally experienced the greatest increases in number of dwellings and total amount of built-up area. On the other hand,  northern cities (e.g., Sheffield, Liverpool, and Newcastle) have generally experienced the least. This pattern mimics one that has also been found across Europe as a whole.

Since announcement of the new urbanization policy at the turn of the century, urban developers have exceeded targets--for instance, increasing densities to 40 housing units/hectare rather than the recommended 30 units per hectare. In 2010, new policy changes were announced that may cause a shift in focus from densification to sprawl; however, it is not yet clear whether this has begun to influence the amount of green space available in and around cities like the 13 examined here.

(Signs of densification can be found all over the world)

The current study shows that land use is both dynamic and highly sensitive to changes in policy. Thus, it can be hard to make long-term predictions about urban habitats; although we know about biological processes and are increasingly well-informed about how they are impacted by human disturbance, it is not easy to make projections about ecosystem services when planning policies could lead to dramatic changes in short periods of time. Perhaps even more disturbing is our lack of knowledge on the long-term repercussions of our current choices. What are the effects of urban growth on ecosystem services? How will the mental health of urban residents be affected by a dearth of accessible green spaces? Additional research is clearly in order, along with innovative solutions to problems associated with population growth and urbanization.

Dallimer, M., Tang, Z., Bibby, P.R., Brindley, P., Gaston, K.J., and Davies, Z.G. 2011. Temporal changes in greenspace in a highly urbanized region. Biology Letters 7:763-766.

Thanks to the following websites for providing the images used in this post:

Thursday, 15 September 2011

Urban soils may be higher quality than you think

One common assumption about urban ecosystems is that they are lower quality than their more rural counterparts. In some cases, this is true; species assemblages are generally less diverse in urban areas, while ecosystem functions are often relatively impaired. However, this is not always the case, as was recently shown in a study on soil compaction in the UK city of Leicester.

Soils are literally and metaphorically the foundation of most terrestrial ecosystem services, playing an integral role in nutrient and carbon storage, purification and drainage water, and provision of anchorage and physical support to both plants and buildings. The ability of soil to perform these functions is reduced by the process of compaction, which decreases the space between individual grains of dirt and makes it harder for plant roots and water to penetrate the earth. Compaction can be caused by heavy machinery and foot traffic--two things that can be found in abundance in urban areas.

Collaborators from the University of Sheffield, the University of Kent, and the NERC Centre for Ecology and Hydrology joined forces to measure soil compaction at 136 sites within Leicester and 28 agricultural sites around the city's border. The sites included gardens, non-domestic land, pasture, and arable fields; at each, soil samples were taken from the top 0-7 cm of earth and also at 7-14 cm below the surface. Bulk density--a measure of soil compaction--was measured in each sample. In addition to investigating whether bulk density differed at each depth over the urban and rural sites, the researchers also looked to see whether it was significantly associated with land cover type (including lawn, woody vegetation, herbaceous growth, shrubs, and trees) at each sample site.

At the 0-7 cm depth, bulk density of urban soils varied much more widely than that of agricultural soils, but overall had a significantly smaller mean value. In other words, agricultural soils, rather than urban soils, were consistently more compacted. At the 7-14 cm depth, there was again more variation at urban sites, but average bulk density did not differ much between the agricultural and urban areas. Across both types of site, soil compaction was significantly influenced by land cover: Bulk density at arable sites was higher than in urban sites with woody garden, tree and shrub, and tall shrub land cover. The least amount of soil compaction was found in green spaces dominated by woody vegetation (e.g., trees and shrubs).

Bulk density values measured within Leicester and its surrounding agricultural fields are comparable to those reported for similar sites elsewhere in Europe, as well as in the United States and Asia. Thus, worldwide, urban soils are in pretty good shape--in comparison with agricultural fields, anyway, where decades of management with heavy machines and inputs have led to much higher levels of soil compaction in the top 7 cm of earth. Another encouraging sign is that the highest bulk density recorded in urban sites was below the "cutoff" after which root growth is halved. 

Thus, it appears that urban sites are more than adequate for providing habitat for plant growth and reducing flooding by allowing storm water drainage. Therefore, it looks like we need to rethink our prejudiced opinions about urban habitats (in the case of soil compaction, anyway). Further, it might be time to think about what can be done to improve soil quality at agricultural sites--not only to increase output but also to ensure that farm habitats are capable of performing other valuable ecosystem services.

Edmondson, J.L., Davies, Z.G., McCormack, S.A., Gaston, K.J., Leake, J.R. 2011. Are soils in urban ecosystems compacted? A citywide analysis. Biology Letters 7:771-774.

Thanks to the following websites for providing the images used in this post:

Wednesday, 14 September 2011

Survival of the brainiest?

With their often unexpected and novel stimuli, urban environments can be chaotic, variable, and harsh. This is why only some species are able to thrive in urban areas, while others vacate disturbed habitats and retreat to more rural locations. But why don't all species respond to cities in the same way? A new study suggests that it may be a matter of brain power.

(London skyline)

Researchers from Sweden's Evolutionary Biology Centre and Spain's Estación Biológica de Doñana analyzed the relationship between brain size and success in urban habitats for 82 songbird species found in and around 12 representative cities in France and Switzerland. "Success" was defined as the ability to breed within the middle of the city, while "avoidance" was defined as breeding around the city, but not at its center. Using these criteria, the authors defined 38 species as "successful" and 44 as "unsuccessful" in urban areas.
They performed both species- and family-level analyses; the first looked at whether individual species with larger brains were more tolerant of urban habitats, while the second investigated whether average brain size within phylogenetic families (e.g., "finches," "corvids") predicted success of entire groups of birds within cities. Both analyses controlled for the relationship between brain size and body size (since larger animals tend to have larger brains), as well as accounting for similarities between animals as a result of shared ancestry.

(Male house sparrow, Passer domesticus)

At both the species and family level, larger brain size was strongly positively associated with the ability to put up with the stresses of city life. Dunnocks (Prunella modularis), starlings (Sturnus vulgaris), wrens (Troglodytes troglodytes), and goldcrests (Regulus regulus) are a few of the brainiacs that drove this pattern, along with all 6 tit species surveyed. One surprise was the corvids, extremely intelligent birds that are, as a group, ubiquitous in cities. However, while two-thirds of corvids were classified as urban dwellers, approximately one third (or about 3 species) made the list of urban avoiders.

(Starling, Sturnus vulgaris)

Large brains have frequently been linked with the ability to adapt to new or unpredictable environments, as well as helping determine whether animals will display innovation. Thus, the results from the current study are not entirely surprising, though they are interesting. It's also not surprising to find measurable differences between city birds and their country cousins; other characteristics that are known to vary between these two groups include song type, diet, size, cholesterol, hormone levels, and tolerance of human approach, to name a few.
The authors of the study point out that it might also be interesting to examine relative size of particular brain features, rather than just the whole brain. In birds, for instance, the ability to innovate is associated with the nidopalium/mesopallium complex in the telencephalon; so, birds with a relatively large telencephalon--regardless of total brain size--might be expected to do well in urban environments. However, since the sizes of at least some individual brain components are highly correlated with total brain size, these specific analyses might not add much to the story.
Perhaps even more intriguing is the possibility that life in the city could select for inhabitants with larger brains--and, therefore, presumably more smarts--than their rural relatives. Given enough time, this could drive evolution of subspecies, as well as causing the evolution of new behaviors. However, it remains to be seen just how widespread these patterns are; additional studies are needed in other avian taxa (and, indeed, other types of animals in general) living in different urban areas around the globe.

Maklakov, A.A., Immler, S., Gonzalez-Voyer, A., Rönn, J., and Kolm, N. 2011. Brains and the city: Big-brained passerine birds succeed in urban environments. Biology Letters 7:730-732.

Thanks to the following for providing the images used in this post:

Long-term biological sites offer protection to hunted animals

Poachers are usually less active in places where they are more likely to be caught, which is why the presence of ecotourists and ranger patrols is known to benefit protected wildlife by reducing trespassing. However, many parks and preserves also have another type of potential guardian--scientific and conservation researchers. Until recently, the potential direct conservation benefits of scientists was unknown, but a new paper reports that the presence of field researchers helps turn their long-term study sites into "refugia" for local wildlife.

Collaborators from the Max Planck Institute for Evolutionary Anthropology, Agro Paris Tech, Office Ivoirien des Parcs et Réserves, the Wild Chimpanzee Foundation, and the Centre Suisse de Recherche Scientifiques investigated the impacts of researchers in the Taï National Park, Côte d’Ivoire, a  5400-km2 park that is home to the Taï Chimpanzee Project (running since 1979) and the Taï Monkey Project (running since 1989). For the current study, the scientists picked out 75 1-km-long transects spread across the chimpanzee and monkey project areas and neighboring habitats. They walked each transect 3 times over the course of a year, looking for both direct (animal sightings) and indirect (chimpanzee nests, duiker feces) signs of primates and duikers--the two groups traditionally most affected by poaching in the area. The scientists also focused on population densities of three individual species from these two broad groups--red colobus monkeys (Procolobus badius), the Diana monkey (Cercopithecus diana), and Maxwell's duiker (Philantomba maxwellii), which have traditionally been the victims of overharvesting.

(Red colobus monkey, Procolobus badius)

(Diana Monkey, Cercopithecus diana)

(Maxwell's duiker, Philantomba maxwellii)

While conducting population surveys, the researchers also searched for signs of poaching, including trails, camps, snares, and empty cartridges. These data had two purposes. First, they were used to generate park-wide maps comparing the amount poaching activity with densities of red colobus monkeys. Second, they were included in statistical analyses investigating whether poaching activity decreased with proximity to the long-term research sites within the national park. The analyses also investigated whether numbers of all duikers, all primates, and the three focal species increased with increasing proximity to the long-term study area.

The results of the analyses were striking. In the maps, moderate to high densities of red colobus monkeys could be seen within the long-term study site, but almost no signs of poaching were found within, or in a band of habitat around, the study area. This same pattern is reflected in the results of the statistical analyses, which showed that numbers of duikers, primates, and all three overharvested species significantly increased with increasing proximity to research area, while visible evidence of poaching significantly decreased. 

(Hikers in the Taï National Park, Côte d’Ivoire)

Such positive impacts will probably only be observed in locations where biological research is sustained over a long period of time--once the scientists leave, there is nothing to stop trespassers from wreaking havoc. This was recently observed at the Marahoué National Park, where 93% of forest cover was lost after the departure of a conservation organization. Given that long-term researchers can gather informative datasets about behavior, life history, ecology, and evolution (among other things) while also performing a conservation role, it seems that their research funding is more than justified--something that might be worth mentioning in future grant proposals.

Campbell, G., Kuehl, H., Diarrassouba, A., N'Goran, P.K., and Boesch, C. 2011. Long-term research sites as refugia for threatened and over-harvested species. Biology Letters 7:723-726.

Thanks to the following websites for providing the images used in this post:

Ecology and culture jointly shaped Polynesian canoe design

Even in our modern world, where transportation and communication technologies can lead to cultural homogenization by making it easy to share ideas and objects, it's still possible to see distinct regional differences. In the US, for instance, adobe houses are primarily found in the Southwest, while brightly painted clapboard houses have a distinctly New England feel. Differences like these are driven partly by culture: Inhabitants of these areas build, or built, houses like those of their ancestors, who built houses like those of their ancestors, and so on. But local ecology also plays a role: Builders are more likely to use materials that are abundant nearby, and will produce homes specifically tailored for comfort in local weather and temperatures.

So which of these factors--culture or ecology--has a bigger impact on design? This question, which has been debated among anthropologists for years, was recently answered by two researchers from the University of California at Davis. Their work focused on variations in canoe designs across Polynesia, a series of archipelagos settled in four distinct clumps (Fiji/Samoa/Tonga, Cook/Society/Tuamotos/Austral, Hawaii, and New Zealand).

 (An aerial photograph of a Polynesian island)

Because the general pattern of settlement throughout Polynesia is known, it was possible to assess whether designs in one region were influenced by designs from ancestral areas. On top of this, previous work has indicated that there were six "interaction spheres" within which the Polynesian peoples traded goods, labor, and ideas. In other words, inhabitants of one group of islands could teach others new and different ways of making canoes or canoe components. By considering these two methods of information transfer--"inheritance" and "learning"--the researchers could investigate the impacts of both old and current cultural influence on canoe design.

Canoes, of course, are used in the water, but very different water conditions can be found throughout the Pacific archipelagos. Islands with protective reefs or enclosed lagoons will have relatively still water, whereas those bordered directly by open ocean will have much choppier waters. Thus, vessels may need to be shallow-keeled in some areas and deeper-keeled elsewhere, or have two hulls instead of one. Some islands may have very different resources available for building, which should also influence canoe design; large islands, for instance, generally have a wider variety of building materials to choose from.

(Modern-built "voyaging canoe" in the traditional Hawaiian style)

In order to evaluate the relative importance of these cultural and ecological factors, the scientists generated a set of 27 different models to predict a total of 65 different canoe traits classified into 6 different categories: hull design, decoration, rigging, paddles, outrigger traits, double-hulled canoe traits. Each different model was capable of explaining different amounts of canoe variation, and models were ranked depending on how much variation they could explain. Therefore, models with higher ranks explain more variation.

Overall, the researchers found that models including both cultural and ecological factors were consistently ranked higher than models including either culture or ecology alone. Specifically, island settlement sequence, island size, and geological type of island seem to be the most important predictors of several Polynesian canoe traits, including hulls, sails, rigging, and outrigging.

Occasionally, models that considered only cultural features did rank more
highly than culture/ecology models, but in no case was this true of an ecology-only model. The culture-only models were particularly important for paddle and double-hull canoe traits. The authors also considered decorative traits, which generally serve no functional purpose. Perhaps unsurprisingly, these were not influenced by ecological factors, but they were also not particularly well explained by culture. Clearly, Polynesian canoes aren't ready to yield all their secrets just yet.

Cumulatively, the results indicate that neither culture nor ecology is the more important variable; rather, the two interacted to shape canoe design. It would be dangerous to generalize too much from these results, since the study only included one type of technology within a single region. All the same, the models clearly show that, in some cases at least, both environment and human-to-human transmission of information are important drivers of technological design and innovation.
Beheim, B.A. and Bell, A.V. 2011. Inheritance, ecology, and the evolution of the canoes of east Oceania. Proceedings of the Royal Society B 278:3089-3095.

Thanks to the following websites for providing the images used in this post:

Monday, 12 September 2011

Not all nest boxes are equal when it comes to Eurasian roller conservation

In dealing with anthropogenic disturbance, one of the most difficult conservation/management issues is that different human activities impact different species in different ways. Removal of trees and hedges, for example, may reduce the amount of feeding or sheltering area available for some animals, while for others it may take away places for potential nest building. For the Eurasian roller (Coracias garrulus), it is this last problem that causes the biggest difficulties, but only in an indirect way. These birds are secondary cavity-nesters, meaning that, because they cannot excavate their own cavities to nest in, they rely on holes dug by others. Thus, when species like woodpeckers have nowhere to create their nests, neither do rollers. This is one of the main reasons why roller population numbers are severely declining in 18 of 29 European countries, with a total estimated reduction of 30% over the past 10 years.

(Eurasian rollers, Coracis garrulus)

To combat this, managers have distributed nest boxes far and wide, hoping to provide otherwise lacking breeding habitat. However, there have been no concerted efforts to deliberately position the boxes in places where they are most likely to promote maximum breeding success. As a result, the boxes are in all types of habitat, sometimes near human structures and disturbances, at different heights and in different orientations. Researchers from the Departamento de Ecología Funcional y Evolutiva thought perhaps it would be a good idea to take a step back and investigate which of these box characteristics are associated not only with likelihood of box occupation, but also preference for, and breeding success in, particular boxes.

They performed their study in the Extremadura region of Spain, where local and regional conservation plans have relied heavily on nest box provisioning, and where agricultural intensification has previously been linked with breeding success in nest box-provisioned habitats. They focused on roller occupation in a 183-box network over a 3-year period. In addition to measuring variables associated with each box (height off ground, orientation of opening, whether it was exposed or concealed), they also measured a variety of large-scale environmental characteristics of the breeding territories, including altitude, distance to various habitat features, and land use patterns. This allowed them to ask four major questions: What factors contributed to the probability of occupation of a nest box? Did boxes with certain features produce more chicks than others? What factors made frequently-chosen boxes seemingly more attractive to the rollers? Did higher-quality birds choose boxes with particular features?

 (Nest boxes have been provided for the roller throughout its range; here, volunteers for Serbian Roller Protection Program brave the heights to install a roller-sized nest box.)

Boxes were more likely to be occupied if they were farther from motorways, were surrounded by smaller portions of almond grove and/or pine plantation, and were more exposed. This last relationship, in particular, seems to suggest that rollers might have difficulty finding more hidden boxes. Unfortunately, the obvious boxes are also those that predators are most likely to discover--at these boxes, overall productivity--in other words, the number of fledglings produced--was significantly lower. There was also some evidence that boxes produced higher-condition chicks when the box opening was facing the northwest, away from cool winds coming off the nearby Sierra Nevada and Sierra de Baza mountain ranges.

None of the variables predicted which boxes were preferred. However, earlier breeders--those likely in better condition--were more likely to choose boxes facing the northwest. Given the relationship between chick condition and northwest orientation of the boxes, these parents were not only high-quality because they started breeding earlier, but also because they were making good decisions for the health of their young.

Overall, breeding rollers appeared to avoid highly human-manipulated areas, initially suggesting they were actively trying to avoid disturbance. However, further analysis indicated that the birds were probably not sensitive to human noise or activities--surprisingly enough--but, rather, that motorway and plantation habitats simply did not offer good places for foraging. In the future, then, it would be better to position boxes near fairly open habitat with a few good perches from which to hunt. Alternatively, conservationists could try to create more of these habitats in which to place the boxes.

(Roller habitat in the Extremadura region of Spain)

It is a bit alarming that rollers most often chose to nest in exactly the type of box--those with high exposure--associated with the lowest level of productivity. This suggests that these highly visible boxes might be acting as ecological traps--poor quality habitats that look good and can therefore "trick" species into living or breeding there when they shouldn't. However, the researchers are quick to point out that productivity is only one of many breeding success indicators, and that their study only lasted three years; over a longer period of time, using different breeding metrics, they might find that these exposed boxes aren't actually so bad.

Still, they suggest that future nest box provisioning efforts should be focused on placing the boxes somewhere not too obvious, but also not too concealed--otherwise the birds might never find them. It might even be possible to use pruning efforts to artificially create better habitats. However, it is important to note that rollers that don't find nest boxes at all might go an entire breeding season--or more--without ever nesting. Thus, even a low quality breeding site is probably better than nothing, since one fledgling is better than none at all.

Rodríguez, J., Avilés, J.M., and Parejo, D. 2011. The value of nestboxesin the conservation of Eurasian rollers Coraciasgarrulus in southern Spain. Ibis 153:735-745.

Thanks to the following websites for providing the images used in this post: