The unified neutral theory of biodiversity: do the numbers add up?

Hubbell's unified neutral theory is a zero-sum ecological drift model in which population sizes change at random in a process resembling genetic drift, eventually leading to extinction. Diversity is maintained within the community by speciation. Hubbell's model makes predictions about the distribution of species abundances within communities and the turnover of species from place to place (beta diversity). However, ecological drift cannot be tested adequately against these predictions without independent estimates of speciation rates, population sizes, and dispersal distances. A more practical prediction from ecological drift is that time to extinction of a population of size N is approximately 2N generations. I test this prediction here using data for passerine birds (Passeriformes). Waiting times to speciation and extinction were estimated from genetic divergence between sister populations and a lineage-through-time plot for endemic South American suboscine passerines. Population sizes were estimated from local counts of birds in two large forest plots extrapolated to the area of wet tropical forest in South America and from atlas data on European passerines. Waiting times to extinction (ca. 2 Ma) are much less than twice the product of average population size (4.0 and 14.4 x 10(6) individuals in South America and Europe) and generation length (five and three years) for songbirds, that is, 40 and 86 Ma, respectively. Thus, drift is too slow to account for turnover in regional avifaunas. Presumably, other processes, involving external drivers, such as climate and physiographic change, and internal drivers, such as evolutionary change in antagonistic interactions, predominate. Hubbell's model is historical and geographic, and his perspective importantly links local and regional process and pattern. Ecological reality can be added to the mix while retaining Hubbell's concept of continuity of communities in space and time.     

The pursuit of extra-pair copulations by monogamous female razorbills: how do females benefit?

Summary Monogamous female razorbills Alca torda actively sought extra-pair copulations (EPCs) in mating arenas outside their nesting colony. Females showed marked variation in promiscuity, defined as the number of EPCs they accepted (0–7), and in receptivity, defined as the percentage of EPC opportunities accepted (0%–100%). The opportunity of females to encounter males for EPCs was measured by (a) the time spent in the mating arena, and (b) the guarding effectiveness of their mates. Females appeared to encounter males and accept EPCs independently of opportunity, which suggests that variation in promiscuity and receptivity is caused by differences in the degree to which individual females might benefit from EPCs. I tested predictions of four hypotheses which propose benefits which females could obtain from EPCs: good genes, genetic diversity of offspring, insurance against infertility of mate, and acquisition of a new mate. No evidence was found to support two predictions made exclusively by the good genes hypothesis: (a) paired males did not achieve greater EPC success than unpaired males, and (b) males who were cuckolded did not obtain fewer EPCs than males whose mates avoided EPCs, suggesting that a male's attractiveness to other females does not ensure his own mate's fidelity. A prediction of both the insurance and good genes hypotheses was supported: females (who in this species retain their mates between years) showed similar receptivity to EPCs between years. This finding contradicts the genetic diversity hypothesis which predicts that in species that lay one egg per year, females who refuse EPCs in one year will accept them in the next. The good genes and genetic diversity hypotheses both predict that females who accept EPCs will behave as if to maximize their chances of being fertilized by extra-pair males. Results contradicted this prediction: females who accepted EPCs were more, rather than less receptive to their mates' copulation attempts than were females who did not accept EPCs. This finding is compatible with the insurance hypothesis which only requires females to store the sperm of extra-pair males, without necessarily being fertilized by it. Insurance against male infertility was the only hypothesis whose predictions were not contradicted by any of the results.     

Assessing the antipredatory defensive strategies of Caribbean non-scleractinian zoantharians (Cnidaria): is the sting the only thing?

The relative importance of chemical, nematocyst, and nutritional defenses was examined for 18 species of Caribbean sea anemones (actinarians), zoanthids, and mushroom polyps (corallimorpharians) from the Florida Keys and the Bahamas Islands, 2008–2010. Feeding assays were performed using the fish Thalassoma bifasciatum with artificial foods containing crude organic extracts of cnidarian tissues. A novel behavioral assay using brine shrimp nauplii was used to assess nematocyst defenses. The nutritional quality of cnidarian tissues was examined using bomb calorimetry and soluble protein assays. In general, actinarians invested in nematocyst defenses, zoanthids in either nematocyst or chemical defenses, and corallimorpharians lacked both, except for 1 of 3 species that was chemically defended. Relative to other coral reef invertebrates, cnidarian tissues had similar caloric values but lower soluble protein concentrations. Trade-offs between chemical and nematocyst defenses were observed for 65% of species, while habitat and behavior provided a likely explanation for undefended species.     

Does cannibalism in Spodoptera frugiperda (Lepidoptera: Noctuidae) reduce the risk of predation?

The incidence of cannibalism of larval Spodoptera frugiperda (Lepidoptera: Noctuidae) on maize under field conditions was investigated using field cages. Cannibalism was found to account for approximately 40% mortality when maize plants were infested with two or four fourth-instar larvae over a 3-day period. Field trials examined the effect of larval density on the prevalence of natural enemies of S. frugiperda. The abundance of predators (earwigs, staphylinids, other predatory beetles, and Chrysoperla spp.) was significantly greater on maize plants with higher levels of larval feeding damage, while the relationship between predator abundance and number of S. frugiperda larvae per plant was less clear. As larval damage is probably a more reliable indicator of previous larval density than numbers collected at an evaluation, this indicates that predation risk will be greater for larvae living in large groups. Parasitism accounted for 7.1% mortality of larvae in sorghum, and involved six species of Hymenoptera and Tachinidae. There was no effect of larval density or within-plant distribution on the probability of larval attack by parasitoids. The selective benefits of cannibalism, in relation to the risk of predation and parasitism, are discussed. Received: 23 March 2000 / Accepted: 24 June 2000     

The relative influence of habitat loss and fragmentation: do tropical mammals meet the temperate paradigm?

The relative influence of habitat loss vs. habitat fragmentation per se (the breaking apart of habitat) on species distribution and abundance is a topic of debate. Although some theoretical studies predict a strong negative effect of fragmentation, consensus from empirical studies is that habitat fragmentation has weak effects compared with habitat loss and that these effects are as likely to be positive as negative. However, few empirical investigations of this issue have been conducted on tropical or wide-ranging species that may be strongly influenced by changes in patch size and edge that occur with increasing fragmentation. We tested the relative influence of habitat loss and fragmentation by examining occupancy of forest patches by 20 mid- and large-sized Neotropical mammal species in a fragmented landscape of northern Guatemala. We related patch occupancy of mammals to measures of habitat loss and fragmentation and compared the influence of these two factors while controlling for patch-level variables. Species responded strongly to both fragmentation and loss, and response to fragmentation generally was negative. Our findings support previous assumptions that conservation of large mammals in the tropics will require conservation strategies that go beyond prevention of habitat loss to also consider forest cohesion or other aspects of landscape configuration.     

Nestmate recognition in the ant Cataglyphis niger : do queens matter?

This study compares two basic models for the origin and maintenance of colony gestalt odor in the polygynous ant species Cataglyphis niger. In the first model, queens are centers of de novo biosynthesis and distribution of recognition odors (“queen-centered” model); in the second, colony odors are primarily synthesized and distributed by workers (“worker-centered” model). We tested the behavioral patterns that are predicted from each model, and verified by biochemical means the distributional directionality of these signals. Encounters between nestmates originating from split colonies were as amicable as between nestmates from non-split colonies; queenless ants were as aggressive as their queenright nestmates, and both were equally aggressed by alien ants. These results indicate that queens have little impact on the recognition system of this species, and lend credence to the worker-centered model. The queen-centered model predicts that unique queen substances should be produced in appreciable quantities and that, in this respect, queens should be more metabolically active than workers. Analysis of the chemical composition of postpharyngeal glands (PPGs) or cuticular extracts of queens and workers revealed high similarity. Quantitatively, queens possessed significantly greater amounts of hydrocarbons in the PPG than workers, but the amount on the thoracic epicuticle was the same. Queens, however, possess a lower hydrocarbon biosynthesis capability than workers. The biochemical evidence thus refutes the queen-centered model and supports a worker-centered model. To elucidate the directionality of cue distribution, we investigated exchange of hydrocarbons between the castes in dyadic or group encounters in which selective participants were prelabeled. Queens tended to receive more and give less PPG content, whereas transfer to the epicuticle was low and similar in all encounters, as predicted from the worker-centered hypothesis. In the group encounters, workers transferred, in most cases, more hydrocarbons to the queen than to a worker. This slight preference for the queen is presumably amplified in a whole colony and can explain their copious PPG content. We hypothesize that preferential transfer to the queen may reflect selection to maintain her individual odor as close to the average colony odor as possible. Received: 4 November 1997 / Accepted after revision: 5 February 1998     

The effects of adaptation measures on hurricane induced property losses: Which FEMA investments have the highest returns?

This paper evaluates the relative effectiveness of FEMA expenditures on hurricane induced property losses. We find that spending on FEMA ex-ante mitigation and planning projects leads to greater reductions in property losses than spending on ex-post adaptation programs – specifically, a one percent increase in annual spending on ex-ante risk reduction and warning projects reduces damages by 0.21 percent while a one percent increase in ex-post recovery and clean-up spending reduces damages by 0.12. Although both types of program spending are effective, we find the marginal return from spending on programs that target long-term mitigation and risk management to be almost twice that of spending on ex-post recovery programs. With the predicted increases in the frequency and severity of North Atlantic hurricanes in the future, our findings suggest there are important potential gains that could be realized from the further diversification of FEMA spending across project categories.     

Does the conservation of land reduce development? An econometric-based landscape simulation with land market feedbacks

We use an econometrically-based landscape simulation to investigate the effect of conservation on the net change in local development – the amount of land directly protected from development minus the amount of development that may occur on neighboring unprotected private land in response to conservation. First, we use spatial-panel data from Wisconsin to estimate parcel-level subdivision probabilities and density expectations, controlling for the endogenous location of open space. Second, we use these subdivision probabilities and density expectations in a landscape simulation model. Our simulation results indicate that 57% of conserved open space created between 1978 and 2009 generated close to zero net change in local development. This suggests that conserved open space mostly reallocated development in a small neighborhood (in a half-mile radius) rather than altering the total amount of development. We explore the landscape conditions that may lead to conservation having either a positive or negative effect on local development.     

Timber Certification: Where Is the Voice of the Biologist?

Editor's note: This Conservation Forum begins with a reprinted editorial by Elizabeth Bennett from the August 2000 issue of Conservation Biology . Several papers were submitted in repsonse to that piece, and it is reprinted here to start this forum.     

How do simple energy activities comprise complex behavior of life systems?: A conceptual synthesis and decomposition of the energy structure of life systems

Reduction and synthesis has always been Science’s way to learn about complex systems. The problem is, of course, that after studying the behavior of the system components, the reconstruction of system behavior cannot be any simple summation of the behavior of the individual components. How should we go about with the reconstruction is an umbrella question addressed in the paper. In particular, one should ask what is the contribution of each component to the system, and what kind and quantitative relations do the connections among the components signify in the total system? Answering these questions is necessary to understand the internal mechanism of a life system, the regularity of its dynamics, and the nature of the control mechanisms.The general approach of the paper is based on the assumption that the life energy transmission structure of the system is the fundamental principle of reduction and synthesis.The paper generalizes the energy transmission in terms of two basic types, based upon the mode of linkage in energy activities, such as sequential or parallel. The basic mathematical concept is an eigenparameter analysis in a model called Life Energy System Model (LESM). Simple examples illustrate the models application. It is true that the energy transmission structure of a natural life system is usually very complex, in which case the analysis has to incorporate more details than shown in the paper. Yet, the analysis still follows the same fundamental principle as outlined.     

Evaluating animal personalities: do observer assessments and experimental tests measure the same thing?

The animal personality literature uses three approaches to assess personality. However, two of these methods, personality ratings and experimentation, have been little compared in captivity and never compared in the wild. We assessed the boldness of wild chacma baboons Papio ursinus using both ratings and experimental methods. Boldness was experimentally assessed when individuals were presented with a novel food item during natural foraging. The boldness of the same individuals was rated on a five-point scale by experienced observers. The ratings and experimental assessments of boldness were found to correlate positively and in a linear fashion. When considered categorically the two approaches showed variable agreement depending on the number of categories assigned and the cut-off criteria adopted. We suggest that the variation between approaches arises because each method captures different aspects of personality; ratings consider personality in absolute terms (using predefined criteria) and multiple contexts, while experimental assessments consider personality in relative terms (using experimental scores relative to the population average) and in limited contexts. We encourage animal personality researchers to consider adopting both methodologies in future studies. We also propose that future studies restrict their analyses to continuous data, since the greatest comparability between methods was found with these data. However, if individuals must be categorised, we suggest that researchers either (a) analyse only those individuals categorised as bold or shy by both ratings and experimental approaches or, if these methods cannot be employed simultaneously, (b) do not use approach-specific criteria but choose a cut-off that can be compared by both approaches.     

Duration of paternal care in pine engraver beetles: why do larger males care less?

Male pine engravers, Ips pini (Coleoptera: Scolytidae), assist their mates during reproduction by removing the debris that accumulates while females excavate oviposition tunnels in the phloem tissue of host tree bark. Although duration of paternal care and male reproductive success are positively correlated, large males leave their mates and brood sooner than small males. We address two hypotheses to explain the earlier departure of larger males from their breeding galleries: (1) females oviposit most rapidly when paired with large males, thereby reducing the length of time that paternal care increases male reproductive success, (2) larger males have better prospects for future reproduction, and thus leave their galleries in search of new breeding opportunities sooner than smaller males. Contrary to the first hypothesis, when females were paired either with large or small males, there was no effect of male size on their rate of oviposition. Consistent with the second hypothesis, males that initiated breeding galleries were larger than males from the general population. In addition, large males flew farther and faster on flight mills than small males, which may indicate that large males have an advantage in locating suitable breeding sites. Thus, we suggest that large male pine engravers leave their galleries earlier than small males because large individuals have better prospects for future reproduction. Received: 30 November 1997 / Accepted after revision: 23 May 1998     

Antipredator strategies of house finches: are urban habitats safe spots from predators even when humans are around?

Urbanization decreases species diversity, but it increases the abundance of certain species with high tolerance to human activities. The safe-habitat hypothesis explains this pattern through a decrease in the abundance of native predators, which reduces predation risk in urban habitats. However, this hypothesis does not consider the potential negative effects of human-associated disturbance (e.g., pedestrians, dogs, cats). Our goal was to assess the degree of perceived predation risk in house finches (Carpodacus mexicanus) through field studies and semi-natural experiments in areas with different levels of urbanization using multiple indicators of risk (flock size, flight initiation distance, vigilance, and foraging behavior). Field studies showed that house finches in more urbanized habitats had a greater tendency to flock with an increase in population density and flushed at larger distances than in less urbanized habitats. In the semi-natural experiment, we found that individuals spent a greater proportion of time in the refuge patch and increased the instantaneous pecking rate in the more urbanized habitat with pedestrians probably to compensate for the lower amount of foraging time. Vigilance parameters were influenced in different ways depending on habitat type and distance to flock mates. Our results suggest that house finches may perceive highly urbanized habitats as more dangerous, despite the lower number of native predators. This could be due to the presence of human activities, which could increase risk or modify the ability to detect predators. House finches seem to adapt to the urban environment through different behavioral strategies that minimize risk.     

Can ocean acidification affect population dynamics of the barnacle Semibalanus balanoides at its southern range edge?

The global ocean and atmosphere are warming. There is increasing evidence suggesting that, in addition to other environmental factors, climate change is affecting species distributions and local population dynamics. Additionally, as a consequence of the growing levels of atmospheric carbon dioxide (CO2), the oceans are taking up increasing amounts of this CO2, causing ocean pH to decrease (ocean acidification). The relative impacts of ocean acidification on population dynamics have yet to be investigated, despite many studies indicating that there will be at least a sublethal impact on many marine organisms, particularly key calcifying organisms. Using empirical data, we forced a barnacle (Semibalanus balanoides) population model to investigate the relative influence of sea surface temperature (SST) and ocean acidification on a population nearing the southern limit of its geographic distribution. Hindcast models were compared to observational data from Cellar Beach (southwestern United Kingdom). Results indicate that a declining pH trend (-0.0017 unit/yr), indicative of ocean acidification over the past 50 years, does not cause an observable impact on the population abundance relative to changes caused by fluctuations in temperature. Below the critical temperature (here T(crit) = 13.1 degrees C), pH has a more significant affect on population dynamics at this southern range edge. However, above this value, SST has the overriding influence. At lower SST, a decrease in pH (according to the National Bureau of Standards, pHNBs) from 8.2 to 7.8 can significantly decrease the population abundance. The lethal impacts of ocean acidification observed in experiments on early life stages reduce cumulative survival by approximately 25%, which again will significantly alter the population level at this southern limit. Furthermore, forecast predictions from this model suggest that combined acidification and warming cause this local population to die out 10 years earlier than would occur if there was only global warming and no concomitant decrease in pH.     

Making scents of defense: do fecal shields and herbivore-caused volatiles match host plant chemical profiles?

Many plant families have aromatic species that produce volatile compounds which they release when damaged, particularly after suffering herbivory. Monarda fistulosa (Lamiaceae) makes and stores volatile essential oils in peltate glandular trichomes on leaf and floral surfaces. This study examined the larvae of a specialist tortoise beetle, Physonota unipunctata, which feed on two M. fistulosa chemotypes and incorporate host compounds into fecal shields, structures related to defense. Comparisons of shield and host leaf chemistry showed differences between chemotypes and structures (leaves vs. shields). Thymol chemotype leaves and shields contained more of all compounds that differed than did carvacrol chemotypes, except for carvacrol. Shields had lower levels of most of the more volatile chemicals than leaves, but more than twice the amounts of the phenolic monoterpenes thymol and carvacrol and greater totals. Additional experiments measured the volatiles emitted from M. fistulosa in the absence and presence of P. unipunctata larvae and compared the flower and foliage chemistry of plants from these experiments. Flowers contained lower or equal amounts of most compounds and half the total amount, compared to leaves. Plants subjected to herbivory emitted higher levels of most volatiles and 12 times the total amount, versus controls with no larvae, including proportionally more of the low boiling point chemicals. Thus, chemical profiles of shields and volatile emissions are influenced by the amounts and volatilities of compounds present in the host plant. The implications of these results are explored for the chemical ecology of both the plant and the insect.     

Nest-site selection in honey bees: how well do swarms implement the "best-of-N" decision rule?

This study views a honey bee swarm as a supraorganismal entity which has been shaped by natural selection to be skilled at choosing a future home site. Prior studies of this decision-making process indicate that swarms attempt to use the best-of-N decision rule: sample some number (N) of alternatives and then select the best one. We tested how well swarms implement this decision rule by presenting them with an array of five nest boxes, only one of which was a high-quality (desirable) nest site; the other four were medium-quality (acceptable) sites. We found that swarms are reasonably good at carrying out the best-of-N decision rule: in four out of five trials, swarms selected the best site. In addition, we gained insights into how a swarm implements this decision rule. We found that when a scout bee returns to the swarm cluster and advertises a potential nest site with a waggle dance, she tunes the strength of her dance in relation to the quality of her site: the better the site, the stronger the dance. A dancing bee tunes her dance strength by adjusting the number of waggle-runs/dance, and she adjusts the number of waggle-runs/dance by changing both the duration and the rate of her waggle-run production. Moreover, we found that a dancing bee changes the rate of her waggle-run production by changing the mean duration of the return-phase portion of her dance circuits. Differences in return-phase duration underlie the impression that dances differ in liveliness. Although a honey bee swarm has bounded rationality (e.g., it lacks complete knowledge of the possible nesting sites), through its capacity for parallel processing it can choose a nest site without greatly reducing either the breadth or depth of its consideration of the alternative sites. Such thoroughness of information gathering and processing no doubt helps a swarm implement the best-of-N decision rule.     

Diet of the northern fulmar Fulmarus glacialis: reliance on commercial fisheries?

The massive expansion in breeding numbers and range of the northern fulmar Fulmarus glacialis over the last two centuries is generally attributed to an increased availability of fish offal and discarded fishes from commercial fisheries. This implies that discards should be a major component in fulmar diets in the more recently colonised areas in the south of their range. This paper examines the contemporary diet of the northern fulmar at three major breeding sites, Fair Isle (Shetland, UK), Iceland and Disko Fjord (western Greenland). At Fair Isle, 89% of regurgitates contained fishes, with sandeels (Ammodytidae) recorded in 37%; 32% contained crustaceans (mainly decapods); 8% contained squid. There was also a temporal trend; sandeels declining and crustaceans increasing in the diet from July to August. In Iceland, fishes were also the main prey (47 to 93% of wet mass), with sandeels common in the south and west, and capelin (Mallotus villosus) in the north and east. Other fish species were mainly discards, and together with discarded decapods and fish offal formed 5 to 72% of the diet, depending on the sector. Euphausiids, amphipods and copepods constituted 3 to 13% of the diet. At Disko, 39% of samples contained fishes, 64% contained crustaceans (mainly amphipods), 22% contained squid, and 16% contained pteropods. There was a clear temporal trend, with the bulk of the samples made up of crustaceans in mid-June, capelin from late June to late July, and crustaceans and pteropods from late July to late August. An extensive review of published studies was also carried out. The general pattern was for birds in more southerly populations to consume more discarded fishes, fish offal and benthic invertebrates. However, a considerable proportion of their diet also consisted of juvenile gadids, sandeels, capelin and pelagic zooplankton, which fulmars catch for themselves, and we suggest that breeding adults in the south are less dependent on fishing waste than is generally assumed. Received: 25 September 1998 / Accepted: 7 May 1999