Is plant community richness regulated over time? Contrasting results from experiments and long-term observations

There is considerable debate among ecologists as to whether or not communities are saturated. In saturated communities, species richness should remain relatively constant over time, despite compositional turnover, because richness is negatively correlated with colonization and positively correlated with local extinction. Few studies have tested for saturation using temporal observational data as well as diversity-perturbation experiments. We analyzed 10 years of data for plant species richness at 71 sites on contrasting serpentine and non-serpentine soils within Californian (USA) grasslands. We also manipulated local richness and measured its effects on immigration and extinction. Consistent with saturation, we observed that richness was positively correlated with extinction rates and negatively correlated with colonization rates, and randomization tests confirmed that diversity fluctuated less than expected by chance. However, experimental species additions and removals did not affect extinction or colonization, suggesting that richness is not regulated by local species interactions. Instead, we propose three reasons why richness may fluctuate within narrow limits causing the appearance of saturation in temporal observational data sets: negatively autocorrelated patterns of biotic response to yearly conditions, differential affinities of particular species for local conditions, or stochastic abundance-dependent colonization and extinction rates. We illustrate the latter using a metacommunity model.     

Herbivores, tidal elevation, and species richness simultaneously mediate nitrate uptake by seaweed assemblages

In order for research into the consequences of biodiversity changes to be more applicable to real-world ecosystems, experiments must be conducted in the field, where a variety of factors other than diversity can affect the rates of key biogeochemical and physiological processes. Here, we experimentally evaluate the effects of two factors known to affect the diversity and composition of intertidal seaweed assemblages--tidal elevation and herbivory--on nitrate uptake by those assemblages. Based on surveys of community composition at the end of a 1.5-year press experiment, we found that both tide height and herbivores affected seaweed community structure. Not surprisingly, seaweed species richness was greater at lower tidal elevations. Herbivores did not affect richness, but they altered the types of species that were present; seaweed species characterized by higher rates of nitrate uptake were more abundant in herbivore-removal plots. Both tide height and herbivores affected nitrate uptake by seaweed assemblages. Individual seaweed species, as well as entire seaweed assemblages, living higher on the shore had greater rates of biomass-specific nitrate uptake, particularly at high ambient nitrate concentrations. Grazed seaweed assemblages exhibited reduced nitrate uptake, but only at low nitrate concentrations. We evaluated the effect of seaweed richness on nitrate uptake, both alone and after accounting for effects of tidal elevation and herbivores. When only richness was considered, we found no effect on uptake. However, when simultaneous effects of richness, tide height, and herbivores on uptake were evaluated, we found that all three had relatively large and comparable effects on nitrate uptake coefficients and that there was a negative relationship between seaweed richness and nitrate uptake. Particularly because effects of richness on uptake were not apparent unless the effects of tide height and herbivory were also considered, these results highlight the importance of considering the effects of environmental context when evaluating the consequences of biodiversity change in more realistic systems.     

A direct comparison of the consequences of plant genotypic and species diversity on communities and ecosystem function

Biodiversity loss is proceeding at an unprecedented rate, yet we lack a thorough understanding of the consequences of losing diversity at different scales. While species diversity is known to impact community and ecosystem processes, genotypic diversity is assumed to have relatively smaller effects. Nonetheless, a few recent studies suggest that genotypic diversity may have quantitatively similar ecological consequences compared to species diversity. Here we show that increasing either genotypic diversity of common evening primrose (Oenothera biennis) or species diversity of old-field plant species resulted in nearly equivalent increases (approximately 17%) in aboveground primary production. The predominant mechanism explaining this effect, niche complementarity, was similar for each type of diversity. Arthropod species richness also increased with both types of plant diversity, but the mechanisms leading to this effect differed: abundance-driven accumulation of arthropod species was important in plant genotypic polycultures, whereas resource specialization was important in plant species polycultures. Thus, similar increases in primary productivity differentially impacted higher trophic levels in response to each type of plant diversity. These results highlight important ecological similarities and differences between genotypic and species diversity and suggest that genotypic diversity may play a larger role in community and ecosystem processes than previously realized.     

Ecology of hemiepiphytism in fig species is based on evolutionary correlation of hydraulics and carbon economy

Woody hemiepiphytic species (Hs) are important components of tropical rain forests, and they have been hypothesized to differ from non-hemiepiphytic tree species (NHs) in adaptations relating to water relations and carbon economy; but few studies have been conducted comparing ecophysiological traits between the two growth forms especially in an evolutionary context. Using common-garden plants of the genus Ficus, functional traits related to plant hydraulics and carbon economy were compared for seven NHs and seven Hs in their adult terrestrial "tree-like" growth phase. We used phylogenetically independent contrasts to test the hypothesis that differences in water availability selected for contrasting suites of traits in Hs and NHs, driving evolutionary correlations among functional traits including hydraulic conductivity and photosynthetic traits. Species of the two growth forms differed in functional traits; Hs had substantially lower xylem hydraulic conductivity and stomatal conductance, and higher instantaneous photosynthetic water use efficiency. Leaf morphological and structural traits also differed strikingly between the two growth forms. The Hs had significantly smaller leaves, higher leaf mass per area (LMA), and smaller xylem vessel lumen diameters. Across all the species, hydraulic conductivity was positively correlated with leaf gas exchange indicating high degrees of hydraulic-photosynthetic coordination. More importantly, these correlations were supported by correlations implemented on phylogenetic independent contrasts, suggesting that most trait correlations arose through repeated convergent evolution rather than as a result of chance events in the deep nodes of the lineage. Vatiation in xylem hydraulic conductivity was also centrally associated with a suite of other functional traits related to carbon economy and growth, such as LMA, water use efficiency, leaf nutrient concentration, and photosynthetic nutrient use efficiency, indicating important physiological constraints or trade-offs among functional traits. Shifts in this trait cluster apparently related to the adaptation to drought-prone canopy growth during the early life cycle of Hs and clearly affected ecophysiology of the later terrestrial stage of these species. Evolutionary flexibility in hydraulics and associated traits might be one basis for the hyper-diversification of Ficus species in tropical rain forests.     

Evaluating post-Katrina recovery in Mississippi using repeat photography

Hurricane Katrina of August 2005 had extensive consequences for the state of Mississippi in the United States. Widespread infrastructure and property damage, massive social dislocation, and ecological loss remain among the many challenges faced by communities as they work towards 'normalcy'. This study employs repeat photography to understand differential recovery from Hurricane Katrina in Mississippi. Revealing change with conventional landscape photography, a process known as repeat photography, is common in the natural sciences. Simply stated, repeat photography is the practice of re-photographing the same scene as it appears in an earlier photograph. Photographs were taken at 131 sites every six months over a three-year period. Each photograph was assigned a recovery score and a spatially interpolated recovery surface was generated for each time period. The mapped and graphed results show disparities in the progression of recovery: some communities quickly entered the rebuilding process whereas others have lagged far behind.     

An independent review and prioritization of past radionuclide and chemical releases from the Los Alamos National Laboratory--implications for future dose reconstruction studies

From 1999 through 2010, a team of scientists and engineers systematically reviewed approximately eight million classified and unclassified documents at Los Alamos National Laboratory (LANL) that describe historical off-site releases of radionuclides and chemicals in order to determine the extent to which a full-scale dose reconstruction for releases is warranted and/or feasible. As a part of this effort, a relative ranking of historical airborne and waterborne radionuclide releases from LANL was established using priority index (PI) values that were calculated from estimated annual quantities released and the maximum allowable effluent concentrations according to The U.S. Nuclear Regulatory Commission (USNRC). Chemical releases were ranked based on annual usage estimates and U.S. Environmental Protection Agency (USEPA) toxicity values. PI results for airborne radionuclides indicate that early plutonium operations were of most concern between 1948 and 1961, in 1967, and again from 1970 through 1973. Airborne releases of uranium were found to be of most interest for 1968, from 1974 through 1978, and again in 1996. Mixed fission products yielded the highest PI value for 1969. Mixed activation product releases yielded the highest PI values from 1979 to 1995. For waterborne releases, results indicate that plutonium is of most concern for all years evaluated with the exception of 1956 when (90)Sr yielded the highest PI value. The prioritization of chemical releases indicate that four of the top five ranked chemicals were organic solvents that were commonly used in chemical processing and for cleaning. Trichloroethylene ranked highest, indicating highest relative potential for health effects, for both cancer and non-cancer effects. Documents also indicate that beryllium was used in significant quantities, which could have lead to residential exposures exceeding established environmental and occupational exposure limits, and warrants further consideration. In part because of the close proximity of residents to LANL, further study of historical LANL releases and the potential impact to public health is recommended for those materials with the largest priority index values; namely, plutonium, uranium, and selected chemicals.     

Can spatial isolation help predict dispersal-limited sites for native species restoration?

When the distribution of species is limited by propagule supply, new populations may be initiated by seed addition, but identifying suitable sites for efficiently targeted seed addition remains a major challenge for restoration. In addition to the biotic or abiotic variables typically used in species distribution models, spatial isolation from conspecifics could help predict the suitability of unoccupied sites. Site suitability might be expected to increase with spatial isolation after other factors are accounted for, since isolation increases the chance that a site is unoccupied only because of propagule limitation. For two native annual forbs in Californian grasslands, we combined experimental seeding and niche modeling to ask whether suitability of unoccupied sites could be predicted by spatial variables (either distances from, or densities of, conspecific populations), either by themselves or in combination with niche models. We also asked whether experimental tests of these predictions held up not only in the short term (one year), but also in the longer term (three years). For Lasthenia californica, seed additions were only successful relatively near existing populations. For Lupinus nanus, seeding success was low and was positively related to the number of conspecifics within 1 km. For both species, a few previously unoccupied sites remained occupied three years after seeding, but this subset was not predictable based on either spatial or niche variables. Seed addition alone may be a limited means of native forb restoration if suitable unoccupied sites are either rare or unpredictable, or if they tend to be close to where the species already occurs.     

Remote analysis of biological invasion and the impact of enemy release

Escape from natural enemies is a widely held generalization for the success of exotic plants. We conducted a large-scale experiment in Hawaii (USA) to quantify impacts of ungulate removal on plant growth and performance, and to test whether elimination of an exotic generalist herbivore facilitated exotic success. Assessment of impacted and control sites before and after ungulate exclusion using airborne imaging spectroscopy and LiDAR, time series satellite observations, and ground-based field studies over nine years indicated that removal of generalist herbivores facilitated exotic success, but the abundance of native species was unchanged. Vegetation cover <1 m in height increased in ungulate-free areas from 48.7% +/- 1.5% to 74.3% +/- 1.8% over 8.4 years, corresponding to an annualized growth rate of lambda = 1.05 +/- 0.01 yr(-1) (median +/- SD). Most of the change was attributable to exotic plant species, which increased from 24.4% +/- 1.4% to 49.1% +/- 2.0%, (lambda = 1.08 +/- 0.01 yr(-1)). Native plants experienced no significant change in cover (23.0% +/- 1.3% to 24.2% +/- 1.8%, lambda = 1.01 +/- 0.01 yr(-1)). Time series of satellite phenology were indistinguishable between the treatment and a 3.0-km2 control site for four years prior to ungulate removal, but they diverged immediately following exclusion of ungulates. Comparison of monthly EVI means before and after ungulate exclusion and between the managed and control areas indicates that EVI strongly increased in the managed area after ungulate exclusion. Field studies and airborne analyses show that the dominant invader was Senecio madagascariensis, an invasive annual forb that increased from < 0.01% to 14.7% fractional cover in ungulate-free areas (lambda = 1.89 +/- 0.34 yr(-1)), but which was nearly absent from the control site. A combination of canopy LAI, water, and fractional cover were expressed in satellite EVI time series and indicate that the invaded region maintained greenness during drought conditions. These findings demonstrate that enemy release from generalist herbivores can facilitate exotic success and suggest a plausible mechanism by which invasion occurred. They also show how novel remote-sensing technology can be integrated with conservation and management to help address exotic plant invasions.     

Within-group behavioral variation promotes biased task performance and the emergence of a defensive caste in a social spider

The social spider Anelosimus studiosus exhibits a behavioral polymorphism where colony members express either a passive, tolerant behavioral tendency (social) or an aggressive, intolerant behavioral tendency (asocial). Here we test whether asocial individuals act as colony defenders by deflecting the suite of foreign (i.e., heterospecific) spider species that commonly exploit multi-female colonies. We (1) determined whether the phenotypic composition of colonies is associated with foreign spider abundance, (2) tested whether heterospecific spider abundance and diversity affect colony survival in the field, and (3) performed staged encounters between groups of A. studiosus and their colony-level predator Agelenopsis emertoni (A. emertoni)to determine whether asocial females exhibit more defensive behavior. We found that larger colonies harbor more foreign spiders, and the number of asocial colony members was negatively associated with foreign spider abundance. Additionally, colony persistence was negatively associated with the abundance and diversity of foreign spiders within colonies. In encounters with a colony-level predator, asocial females were more likely to exhibit escalatory behavior, and this might explain the negative association between the frequency of asocial females and the presence of foreign spider associates. Together, our results indicate that foreign spiders are detrimental to colony survival, and that asocial females play a defensive role in multi-female colonies.     

Simultaneous nitrogen and silicate deficiency of a phytoplankton community in a coastal jet-front

The nutritional status of a phytoplankton community was investigated in a coastal jet-front located in the Gulf of St. Lawrence, Canada, in 1987. During the sampling period, the frontal community was mainly composed of the diatomsChaetoceros debilis, Skeletonema costatum, Thalassiosira gravida andC. pelagicus. As previously reported for the St. Lawrence, some frontal stations were depleted both in nitrate and silicate. At stations impoverished in nitrate, internal nitrate pool concentrations were low or undetectable, suggesting that cells had not, recently, been exposed to a nitrate flux which exceeded the nitrate assimilation rate. At these impoverished stations, however, ambient and intracellular concentrations of ammonium and urea were high, suggesting that the community was not nitrogen-deficient. The comparison between the ambient silicate concentrations and the silicate requirement (K _s ) of the dominant diatoms suggests thatC. debilis andS. costatum were Si-deficient. This is further supported by the low silicate uptake rates and intracellular concentrations measured at the silicate impoverished stations. The silicate deficiency also resulted in a decrease in the seston and phytoplankton N:C ratios.Please address all correspondence and requests for reprints to Dr Levasseur at his new address: Maurice Lamontagne Institute, 850 Route de la Mer, Mont-Joli, Québec G5H 3Z4, Canada