Mark J. McKone

Community- and ecosystem-level changes in a species-rich tallgrass prairie restoration

Changes in the plant community and ecosystem properties that follow the conversion of agriculture to restored tallgrass prairies are poorly understood. Beginning in 1995, we established a species-rich, restored prairie chronosequence where ∼3 ha of agricultural land have been converted to tallgrass prairie each year. Our goals were to examine differences in ecosystem properties between these restored prairies and adjacent agricultural fields and to determine changes in, and potential interactions between, the plant community and ecosystem properties that occur over time in the restored prairies. During the summers of 2000–2002, we examined species cover, soil C and N, potential net C and N mineralization, litter mass, soil texture, and bulk density across the 6- to 8-year-old prairie chronosequence and adjacent agricultural fields in southern Minnesota. We also established experimentally fertilized, watered, and control plots in the prairie chronosequence to examine the degree of nitrogen limitation on aboveground and belowground net primary production (ANPP and BNPP). Large shifts in functional diversity occurred within three growing seasons. First-year prairies were dominated by annuals and biennials. By the second growing season, perennial native composites had become dominant, followed by a significant shift to warm-season C4 grasses in prairies ≥3 yr old. Ecosystem properties that changed with the rise of C4 grasses included increased BNPP, litter mass, and C mineralization rates and decreased N mineralization rates. ANPP increased significantly with N fertilization but did not vary between young and old prairies with dramatically different plant community composition. Total soil C and N were not significantly different between prairie and agricultural soils in the depths examined (0–10, 10–20, 20–35, 35–50, 50–65 cm). We compared the results from our species-rich prairie restoration to published data on ecosystem function in other restored grasslands, such as Conservation Reserve Program (CRP) and old-field successional sites. Results suggest that rapid changes in functional diversity can have large impacts on ecosystem-level properties, causing community- and system-level dynamics in species-rich prairie restorations to converge with those from low-diversity managed grasslands.

Sex allocation and outcrossing rate: a test of theoretical predictions using bromegrasses (Bromus)

Predictions of sex‐allocation theory were tested by comparisons among hermaphroditic bromegrass (Bromus) species that differed in outcrossing rate. Relative maternal and paternal investment were calculated using both the ratio of pollen to seed production, and absolute allocations in units of energy, nitrogen, phosphorus, potassium, magnesium, and calcium. Outcrossing rate had a large effect on sex allocation; species having greater outcrossing rates had relatively more paternal reproductive effort. Bromus inermis was obligately outcrossing, and nearly half of its reproductive effort was devoted to pollen production. Three partially outcrossed species, B. kalmii, B. ciliatus, and B. latiglumis, invested between 5% and 11% of reproductive effort in pollen production. Paternal investment was less than 2% in the selling species B. tectorum. Estimates of sex allocation were relatively unaffected by the resource currency used in calculation. The differences among species in sex allocation were mostly due to differences in anther size and seed set.>

Diversity and abundance of understorey plants on active and abandoned nests of leaf-cutting ants ( Atta cephalotes ) in a Costa Rican rain forest

Nests of leaf-cutting ants (Hymenoptera: Formicidae: Attini) are abundant disturbances in Neotropical rain forests, and could affect the plant community both while the nests are active and after they are abandoned. We measured the diversity and abundance of understorey plants (>1 m in height) in the area around active and abandoned nests of leaf-cutting ants ( Atta cephalotes ) at the La Selva Biological Station in Costa Rica. Sampel quadrats on active nests had reduced diversity (number of morphospecies) and abundance of both small (height >10 cm) and large (10 cm–1 m) understorey plants, when compared to the nearby forest floor (3 and 13 m from the nest edge). Abandoned nests had greater diversity and marginally greater abundance of small understorey plants relative to nearby forest; there was no difference in diversity or abundance of large understorey plants. Leaf-cutting ant nests create gaps in the plant understorey when active, but serve as centres of recruitment for small plants after they are abandoned. Thus, like canopy gaps, ant nests could play an important role in recruitment of new individuals and maintenance of plant species diversity in tropical forests.

Statistical analysis of experiments conducted at multiple sites

Ecological phenomena are often spatially variable, and hence it is desirable to simultaneously conduct experiments at multiple sites. However, the number of sites is often constrained to be few, resulting in many more replicates within sites than there are sites. Such experiments are commonly analyzed by mixed model ANOVAs (where site is the random effect), which are very powerful at detecting differences among sites and siteby-treatment interactions. But, since few sites are sampled, there is little statistical power to make inferences concerning the treatment effects themselves. This lack of power is unfortunate whenever the primary reason for conducting the experiment is to test for differences among treatments, as is true for most ecological field experiments. In this note we suggest that, when there are few sites, the treatment effect should be examined only within sites using a nested ANOVA.

The Evolution of Androgenesis

It is well known that some species produce offspring carrying only female chromosomes by processes such as apomixis and parthenogenesis (generically termed “gynogenesis”). There are also several cases of natural reproduction by androgenesis in which diploid offspring carry nuclear chromosomes from only the male parent. We used population genetics models to investigate the conditions for invasion of rare androgenesis alleles and the consequences of their spread. Our models predict that androgenesis alleles often spread to fixation. If fixation causes the loss of females or female function in the population, population extinction occurs. Therefore, androgenesis alleles represent a new class of selfish genetic elements. Extinction is more likely in dioecious species than in hermaphrodites. Within dioecious species, extinction is more likely when androgenesis occurs via paternal apomixis (vs. fusion or doubling of haploid nuclei) and when females are the heterogametic sex (vs. male heterogamety). The apparent rarity of androgenesis compared to gynogenesis could be because androgenesis is harder to detect and more often leads to population extinction. Also, there could be greater evolutionary constraints on the origin of mutations for androgenesis. We suggest characteristics of groups in which further cases of androgenesis are more likely to be found.

Biology of insects that feed in the inflorescences of Chionochloa (Poaceae) in New Zealand and their relevance to mast seeding

Abstract Inflorescences of New Zealand Chionochloa species are attacked by at least three insects, two flies and a moth. There has been disagreement about the identity of various life stages of these insects. We followed the seasonal pattern of occurrence of the two fly species that fed in Chionochloa pallens inflorescences in a population on Mt Hutt, Canterbury. Eggs and larvae of Diplotoxa similis (Diptera: Chloropidae) appeared in the inflorescences as soon as they emerged. The larvae are principally flower feeders, and most D. similis individuals had pupated by the end of the flowering period. Diplotoxa similis adults emerged from the puparia at the end of the season, and probably overwintered as adults. The second fly was an undescribed cecidomyiid (Diptera: Cecidomyiidae). Eggs of the cecidomyiid are laid into the C. pallens florets at the time of flowering, and hatch into mobile, translucent, early‐instar larvae. Late‐instar larvae were less mobile and opaque orange, and probably dropped from the inflorescences late in the season. The third species, Megacraspedus calamogonus (Lepidoptera: Gelechiidae) has large mobile caterpillars which appear early in the season, but there is doubt about its egg morphology and oviposition sites. On two dates there was a negative correlation between densities of the two fly species among plants. Since D. similis appears first, it may be able to usurp resources and reduce densities of the cecidomyiid. A review of known occurrences of the three insects suggests that, compared to D. similis and M. calamogonus, the cecidomyiid (1) has a greater geographic range and (2) occurs on more Chionochloa species; however, these trends might be due to poor sampling of D. similis and M. calamogonus early in the season. The cecidomyiid appears to be less easily satiated than D. similis by masting in Chionochloa.

An exception to Darwin's syndrome: floral position, protogyny, and insect visitation in Besseya bullii (Scrophulariaceae)

Darwin pointed out that plants with vertical inflorescences are likely to be outcrossed if the inflorescence is acropetalous (flowers from the bottom up), the flowers are protandrous (pollen is dispersed before stigmas are receptive), and pollinators move upward on the inflorescence. This syndrome is common in species pollinated by bees and flies, and very few exceptions are known. We investigated flowering phenology and pollinator behavior in Besseya bullii (Scrophulariaceae) and found that it did not fit Darwins syndrome. The vertical inflorescence was acropetalous but the flowers were distinctly protogynous, so flowers with newly receptive stigmas appeared on the inflorescence above those with dehiscing anthers. A number of small insects visited B. bullii; bees in the family Halictidae (Augochlorella striata and Dialictus spp.) were most common. When insects moved between gender phases within inflorescences, they moved up more often than down (61% versus 39% of observations, respectively) but this difference was only marginally significant. Most visits were to male-phase flowers only, and this preference was more pronounced for pollen-foraging insects than for nectar-foraging insects. B. bullii was self-compatible, so its flowering characteristics potentially could result in considerable self-pollination. However, an average of 38% of the lowermost flowers opened before any pollen was available on the same inflorescence; these “solo females” had a high probability of outcrossing (though fruit set was relatively low in the bottom portion of the inflorescence). Upper flowers may also be outcrossed because downward insect movement was not uncommon. Therefore protogyny in B. bullii may not necessarily lead to more selfing than would protandry.

Random amplified polymorphic DNA markers reveal genetic variation in the symbiotic fungus of leaf-cutting ants

RAPD markers were used to examine the degree of genetic variation within the putatively asexual basidiomycete fungus (Lepiotaceae: provisionally named Leucoagaricus gongylophorus) associated with the leaf-cutting ant species Atta cephalotes. We analyzed fungal isolates from ant nests in two geographically distant sites, two isolates from Panama and five isolates from Trinidad. Ten decamer primers were used to amplify total DNA from these seven fungal isolates, and RAPD banding patterns were compared. Genetic similarity among isolates was determined by pair-wise comparisons of the shared number of DNA bands on an agarose gel. There was considerable genetic variation among isolates of the symbiotic fungus even within sites. Pairs of fungal isolates from the two different sites shared an average of only 36% of the bands in their RAPD profiles, while pairs from the within sites shared an average of 72% of the bands. RAPD markers may be useful for further investigation of the genetic structure of the fungal symbiont within species of leaf-cutting ants.

Choosing an appropriate ANOVA for experiments conducted at few sites

In our paper on experiments conducted at multiple sites, we suggested that there are two options for statistical analysis (McKone and Lively 1993). One option is the standard ANOVA where sites are crossed with treatment, and the F-ratio for testing treatment effects is calculated using the treatment-by-site interaction. This analysis is obviously preferable when the number of sites is large, because it has the advantage of being able to test for treatment effects averaged over all sites. However, when the number of sites is small, there is very little power for testing for a treatment effect in this way. We therefore recommended that treatment be analyzed as nested within site when there are few sites. Nesting allows for the assessment of treatment effects within individual sites, but has the drawback that the effect of treatment cannot be generalized to the population of sites. Greenwood (1994) thinks that our recommendation is mistaken, precisely because one cannot generalize using the nested approach. But he does not address what we see as the real problem. We agree unconditionally that it would always be preferable to generalize. Our main point is that it will often be counterproductive to attempt generalization from a handful of sites. The question here is not which is better in a perfect world with a large random sample of sites. The answer to that is clear. The question is: what should we do when only a few sites are manageable, due to unavoidable constraints? We see three options: (1) recognize the futility of generalizing from these few sites, and simply not conduct the experiment; (2) analyze the data using a factorial ANOVA, and hope that the interaction term is not significant and therefore possibly combinable with the error term (Sokal and Rohlf 1981), or (3) use the nested approach to analyze site effects and treatment effects within sites. Nothing is learned from the first approach; the second approach lacks statistical power, and has a high risk of type II error. The third (nested) approach is not generalizable, but something is learned. In our example of the nested method, herbivores had significant effects at all three sites. This is an important finding for these sites, whether or not the result is later shown to be general. We stick by our recommendation.

Importance of pollen and nectar in flower choice by hummingbird flower mites, Proctolaelaps kirmsei (Mesostigmata: Ascidae)

Abstract Proctolaelaps kirmsei (Mesostigmata: Ascidae) is a specialist flower mite that feeds on pollen and nectar in flowers of Hamelia patens (Rubiaceae). We offered mites a choice between pairs of manipulated H. patens flowers in petri dishes in the laboratory, then counted the number of mites inside the flowers after 1 hour. Relative to flowers with stamens and nectaries removed, there were more mites in flowers with only stamens, with only nectaries, and with both stamens and nectaries. When offered a choice between flowers with stamens but no nectaries versus flowers with nectaries but no stamens, the mites did not show a preference. Mites preferred H. patens flowers with H. patens stamens over those with the stamens of a coexisting non-host plant (Heliconia pogonantha; Heliconiaceae) inserted, so the identity of the pollen is important in flower choice. These results and previous work on the importance of nectar show that the presence of both pollen and nectar can be used by P. kirmsei to discrimin...