Steven N. Handel

Floral Evolution: Attractiveness to Pollinators Increases Male Fitness

Because availability of resources often limits seed or fruit set, increased visits by pollinators may not always lead to increases in maternal reproduction. This observation has led evolutionary biologists to hypothesize that a plants ability to attract pollinators may have its primary impact on male fitness achieved through the fertlization of ovules. This interpretation of angiosperm reproductive ecology is supported by field experiments. Pollinating insects strongly discriminated between two Mendelian petal-color morphs in Raphanus raphanistrum, a widespread, self-incompatible crucifer. In experimental populations composed of petal-color homozygotes. color discrimination by naturally occurring pollinators had no statistically significant effect on relative maternal function (fruit and seed production) in the two morphs. In contrast, yellow-flowered individuals were far more successful as fathers (pollen donors) than were the less visited whites. These results suggest that the evolution of floral signals such as petal color may be driven primarily by selection on male function.

The impact of a flower-color polymorphism on mating patterns in experimental populations of wild radish (Raphanus raphanistrum L.)

We conducted field experiments to determine how a naturally occurring petal‐color polymorphism influences mating patterns in wild radish (Raphanus raphanistrum). The polymorphism is controlled at a single genetic locus, with white petal color being completely dominant to yellow. In experimental populations with equal numbers of yellow‐ and white‐flowered homozygous individuals, insect visitors strongly discriminated against white flowers. Pieris rapae, the most frequent pollinator, was almost 50% more likely to visit yellow than white flowers. Maternal fecundity did not differ between the morphs and was not significantly influenced by a plants compatibility with potential donors, suggesting that seed production was not limited by receipt of compatible pollen. In contrast, the yellow‐flowered morph sired approximately 75% of all seeds produced during the study. This paternity proportion was consistently greater than that expected on the basis of postpollination compatibility measures and was indistinguishable from that expected on the basis of pollinator‐visitation frequency. We conclude that the male‐fitness advantage of the yellow morph resulted from enhanced pollen export due to the greater attractiveness of its flowers to insect pollinators. With color morphs evenly distributed in experimental arrays, insects did not move assortatively on the basis of petal color, and we found no evidence for assortative pollen flow due to the floral polymorphism. Once postpollination compatibility relationships within populations were taken into account, paternal success of yellow donors did not differ between yellow‐ and white‐flowered maternal plants.

Advantages of Clonal Growth in Heterogeneous Habitats: An Experiment with Potentilla Simplex

1 The ability to share resources between modules (ramets) is considered to be a benefit of the clonal growth habit. This type of physiological integration may buffer the entire clone against locally adverse conditions resulting from the patchy distribution of resources. The hypothesis that physiological integration is most advantageous in a heterogeneous habitat was tested using the clonal, perennial species Potentilla simplex (Rosaceae). 2 Five artificial habitat types differing in nutrient status were constructed in the glasshouse and the performance of intact and disconnected clones (clones in which all ramets were severed from each other following the establishment of roots) growing in them was compared. The habitat types ranged from homogeneously poor via three levels of spatial heterogeneity in nutrient supply to homogeneously rich. 3 Intact clones growing in the heterogeneous habitats weighed significantly more than their disconnected counterparts, supporting the hypothesis. The benefits of resource sharing were enhanced with increasing patchiness of the habitat. No differences in biomass between intact and disconnected clones appeared in the homogeneous settings. 4 Parent ramets supported their offspring at a large cost in biomass to themselves, but the offspring ramets benefited from clonal integration irrespective of the quality of the patches they occupied. 5 Across all habitats, intact stolons elongated more than the severed ones. This increased mobility of intact clones implies that clonal integration may allow this species to escape from unfavourable patches. The enhanced performance of connected ramets rooted in rich patches shows that clonal integration also enables clones to exploit resource-rich microhabitats, thereby maximising foraging ability and resource acquisition, particularly when resources are distributed in patches. 6 The cumulative effects of parent and offspring biomass and offspring ramet number appear to explain the trend in performance of intact and disconnected clones observed in the homogeneous and heterogeneous habitats.

POLLINATORS DISCRIMINATE AMONG FLORAL HEIGHTS OF A SEXUALLY DECEPTIVE ORCHID: IMPLICATIONS FOR SELECTION

Pollinators have influenced the evolution of many morphological floral traits, although few studies have shown that pollinators have influenced plant height. Chiloglottis trilabra is one of many Australian orchids that deceive and attract male pollinators by mimicking the sex pheromones and morphology of females insects. Orchids in this genus have unusually short flowers whose peduncle elongates dramatically after pollination to approximately twice the original height. In a series of choice experiments in the field, we show that pollinators of C. trilabra strongly discriminate among floral heights, preferring flowers presented at 15 cm‐20 cm over flowers presented at lower and higher positions (ranging from 2 cm‐100 cm). Our results suggested pollinators have the potential to mediate stabilizing selection for floral height when pollination is limiting. However, the natural height range of the orchid (mean = 10 cm, range 5 cm–15 cm) was lower than the experimentally determined optimum for visitation frequency. This difference may indicate that pollinator‐mediated selection does not occur in this species, perhaps because seed set is not sufficiently limited. Alternatively, other life‐history factors may counteract pollinator‐mediated selection, yielding an evolutionary compromise in height.

A century of change in the Staten Island flora: Ecological correlates of species losses and invasions'

ROBINSON, GEORGE R., MARY E. YURLINA AND STEVEN N. HANDEL (Department of Biological Sciences, Rutgers University, Piscataway, New Jersey 08855-1059). A century of change in the Staten Island Flora: Ecological correlates of species losses and gains. Bull. Torrey Botanical Club 121: 119-129. 1994.-We analyzed three consecutive floristic censuses of Staten Island, New York, to determine (1) the general pattern of vascular plant species gains and losses over 112 years (1879-1991), (2) the extent of increases in non-native species, and (3) ecological features that characterize those species that were lost. Over forty per cent of the original native species (53% of regionally rare and endangered species) are presently missing. Most of the losses have occurred during an accelerating period of suburbanization over the last 60 yr. At the same time, the proportion of non-native species has increased from 19% to > 33% of the flora, while average abundances of most native species apparently declined. Species from different habitats were lost in similar proportions, indicating that broad-scale habitat alteration has been the main factor in the floras decline. Only two ecological features examined were statistically correlated with the missing species: (1) herbaceous species were more vulnerable than woody plants; (2) species reported to be uncommon in a previous census were more likely to be absent in the next. Unless conservation measures are improved, further degradation of this historically rich and varied flora can be anticipated.

Jumping spiders (Salticidae) enhance the seed production of a plant with extrafloral nectaries

Abstract Many plants secrete nectar from extrafloral nectaries (EFNs), specialized structures that usually attract ants which can act as plant defenders. We examined the nectar-mediated interactions between Chamaecrista nictitans (Caesalpineaceae) and jumping spiders (Araneae, Salticidae) for 2 years in old fields in New Jersey, USA. Previous research suggests that spiders are entirely carnivorous, yet jumping spiders (Eris sp. and Metaphidippus sp.) on C. nictitans collected nectar in addition to feeding on herbivores, ants, bees, and other spiders. In a controlled-environment experiment, when given a choice between C. nictitans with or without active EFNs, foraging spiders spent 86% of their time on plants with nectar. C. nictitans with resident jumping spiders did set significantly more seed than plants with no spiders, indicating a beneficial effect from these predators. However, the presence of jumping spiders did not decrease numbers of Sennius cruentatus (Bruchidae), a specialist seed predator of C. nictitans. Jumping spiders may provide additional, unexpected defense to plants possessing EFNs. Plants with EFNs may therefore have beneficial interactions with other arthropod predators in addition to nectar-collecting ants.

Additive and nonadditive effects of herbivory and competition on tree seedling mortality, growth, and allocation.

The interaction between simulated cotyledon herbivory and interspecific competition was studied in a greenhouse experiment using two species of trees, Acer rubrum and Quercus palustris, which commonly invade abandoned agricultural fields. Herbivory treatments were applied as a gradient of cotyledon removal for A. rubrum with 0, 25, 50, 75, and 100% of cotyledon tissue removed. Cotyledons from Q. palustris were clipped and removed (control, early, and late removal) to create a gradient of seed reserve availability. The competition treatment consisted of plugs of old-field vegetation that filled the pots with perennial cover. Mortality of seedlings was higher with competition. There was a significant interaction between herbivory and competition with the highest mortality occurring with competition at the highest intensity of herbivory in both species. Herbivory reduced biomass for Q. palustris only, while competition reduced biomass in both species. Neither species showed an interaction between herbivory and competition for growth. There was a significant interaction between herbivory and competition on allocation patterns for both species, with greater allocation to roots with competition at the highest intensity of herbivory. This study demonstrates the potential for cotyledon herbivory and competition to interact, altering the invasion of tree seedlings into abandoned agricultural land.

Fruit type, life form and origin determine the success of woody plant invaders in an urban landscape

The spread of alien plant species is a critical ecological event worldwide, but the forces that control this spread are not well documented. Alien plant species are well known to disrupt ecological services of native ecosystems, change the composition of native habitats, and often lead to the extirpation of native flora and fauna. Here, we report on life history patterns of plant species with rapidly spreading and declining ranges in North America’s major urban region. We tested for differences in life history traits between the 466 native and alien woody flora of the New York metropolitan area. We also examined the relationship between life history traits and change in distribution in the New York metropolitan area between 1900 and 2000. Native and alien species of the New York metropolitan area differ with respect to pollination vector and breeding system. However, pollination vector and breeding system are not associated with success, defined here as increasing range spread in the urban environment; instead, fruit type (dispersal), life form and origin are important determinants of success. Alien species that are deciduous trees, shrubs or vines with fleshy fruit are the most successful in increasing their distribution in this urban landscape. Newly introduced species with these characteristics are expected to have a better chance at establishing in similar urban landscapes and should be targets for intensive management. The ability to predict which alien species will become invasive is also a valuable tool for the prevention of invasions by newly introduced plant species.

PROBABILITY OF TREE SEEDLING ESTABLISHMENT CHANGES ACROSS A FOREST–OLD FIELD EDGE GRADIENT

Forest edges affect many aspects of plant communities, causing changes in microclimate, species composition, and community structure. However, the direct role of edges in regulating forest regeneration is relatively unknown. The pattern of tree establishment across a forest-old field edge was experimentally examined to determine the response of three tree species to the edge gradient. We placed 100 1-m(2) plots in a 90 × 90 m grid that began 30 m inside the forest, extended across the edge, and ended at 60 m into the old field. Into each plot, we planted seeds of Acer rubrum, Acer saccharum, and Quercus palustris. Emergence increased with distance into the field for both A. saccharum and Q. palustris. Emergence for A. rubrum increased from forest to field, reaching a maximum near 20 m into the field, and then declined with further distance. Nearly all A. rubrum seedlings died shortly after emergence. Survival of A. saccharum increased with distance into the old field, while survivorship of Q. palustris did not respond to the edge gradient. Establishment probabilities increased with distance into the old field for both A. saccharum and Q. palustris. Growth of Q. palustris and allocation patterns of A. saccharum also varied across the edge gradient. These results suggest that edges have complex, species-specific effects on tree establishment and growth that can influence the spatial pattern and species composition of regenerating forests.

Herbivory constrains survival, reproduction and mutualisms when restoring nine temperate forest herbs'

RUHREN, SCOTT AND STEVEN N. HANDEL (Department of Ecology, Evolution, and Natural Resources, and Center for Urban Restoration Ecology, Rutgers, The State University of New Jersey, 1 College Farm Road, New Brunswick, NJ 08901-1582). Herbivory constrains survival, reproduction and mutualisms when restoring nine deciduous forest herbs. J. Torrey Bot. Soc. 130:34-42. 2003.-Restoration of perennial herbs may be influenced by plant-animal interactions. For example, projects may be aided by native pollinators or seed dispersers or impeded by herbivores. Our previous work at Jockey Hollow, a U.S. National Park, indicated that abundance of native woodland herbs was low and herbivory by Odocoileus virginianus Zimmerman (white-tailed deer) was intense. The goal of our experiment was to test the feasibility of herbaceous plant restoration in spite of a large 0. virginianus herd. By examining small-scale reintroductions of nine species of native herbaceous perennials (Asarum canadense L., Geranium maculatum L., Mitella diphylla L., Polemonium reptans L., Sanguinaria canadensis L., Smilacina racemosa (L.) Desf., Tiarella cordifolia L., Uvularia grandiflora J.E. Smith and Viola spp.) we determined which plants survived and reproduced. After 3 years, 46% of plants survived and 5% flowered inside exclosures, while 23% of plants planted outside survived; no plants surviving outside exclosures flowered. Within deer exclosures, native bees visited five species and ants dispersed Sanguinaria seeds in experimental depots. However pollination and seed dispersal occurs only with protection from 0. virginianus. Overabundant 0. virginianus and the relatively long generation time of perennial herbs may prevent restoration of similar plant communities at this time.