F. Leland Russell

Effects of White-tailed Deer (Odocoileus virginianus) on Plants, Plant Populations and Communities: A Review

Abstract Large effects of white-tailed deer (Odocoileus virginianus) upon individual plants, plant populations and communities have been documented in a number of studies. However, well-supported experimental measures of the magnitude and geographical extent of these effects are still surprisingly scarce. Deer-caused changes in stem morphology and reductions in plant growth rates are well-documented in some parts of the North America. Furthermore, deer have been shown to affect the composition of several plant communities in the north-central and northeastern United States. There are some documented cases of deer-caused reductions in plant survival; most of these are tree seedlings and saplings. However, many studies have detected no effects on plant survival or fecundity, or have found that negative effects occur only in a fraction of years, seasons, sites or deer densities. Little is known about population-level or ecosystem-level impacts. Many regions and plant communities with large deer populations have not been studied. Whereas deer density is clearly important in determining spatial and temporal variation in the presence and magnitude of deer effects, other factors that may modify the effects of deer density are poorly understood.

Phenological synchrony affects interaction strength of an exotic weevil with Platte thistle, a native host plant

Phenological synchrony of a consumer population with its resource populations is expected to affect interaction intensity. We quantified phenological variation and synchrony of populations of an invasive Eurasian flower head weevil, Rhinocyllus conicus, that consumes florets, ovules, and seeds of developing flower heads of a native North American thistle, Cirsium canescens, in Sand Hills prairie in Nebraska, USA. Variation in timing of adult activity among weevil populations was larger than variation in timing of flower head development among C. canescens populations, and it drove the observed variation in the phenological synchrony between weevil and host plant populations. Furthermore, the degree of phenological synchrony between populations was significant in explaining variation in weevil egg load on the newly acquired host plant. Because population growth of C. canescens is limited by predispersal seed losses to floral herbivores, variation in the synchrony of herbivore and plant flowering will affect the density of the plant population. These results provide strong quantitative support for the hypothesis that the synchrony of insect activity with plant resources can determine the magnitude of impact of floral herbivores on their host plant populations.

VARIATION IN HERBIVORE-MEDIATED INDIRECT EFFECTS OF AN INVASIVE PLANT ON A NATIVE PLANT

Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001-2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0-220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (<5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistles co-occurrence with the coevolved preferred exotic host plant and not from the weevils response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants.

Failure of Adult Recruitment in Quercus buckleyi Populations on the Eastern Edwards Plateau, Texas

Abstract In many semiarid regions woodlands, savannas and grasslands form an unstable landscape mosaic; the physiognomy of a particular patch in the mosaic changes over time. To explain such temporal and spatial variation in tree abundance in semiarid landscapes, greater understanding of woodland tree population dynamics and factors that affect population dynamics of trees in woodlands is required. We examined the apparent failure of adult recruitment in Quercus buckleyi (Spanish oak) populations in woodlands on the Edwards Plateau, Texas. To verify adult recruitment failure, we quantified age structures of adult stems (stems taller than 150 cm) in five Q. buckleyi stands. We found that adult recruitment has been low to absent for 35 to 60 y in four of our five sites. At the fifth site, some stems recruited above 150 cm (the browseline) in the past 30 y. Most recruitment of adult stems occurred between 1900 and 1935, a period of low deer abundance. At the one site with recent adult stem recruitment, relatively low deer densities have been maintained since 1970 by hunting and by a perimeter fence that prevents immigration. Based on coincidence of adult stem recruitment with low deer populations, we hypothesize that intense browsing pressure or the interaction between fire suppression and intense browsing pressure is limiting adult recruitment of Q. buckleyi. If the documented patterns persist, many Q. buckleyi stands on the eastern Edwards Plateau are unlikely to replace themselves.

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.

Seed availability and insect herbivory limit recruitment and adult density of native tall thistle

Understanding spatial and temporal variation in factors influencing plant regeneration is critical to predicting plant population growth. We experimentally evaluated seed limitation, insect herbivory, and their interaction in the regeneration and density of tall thistle (Cirsium altissimum) across a topographic ecosystem productivity gradient in tallgrass prairie over two years. On ridges and in valleys, we used a factorial experiment manipulating seed availability and insect herbivory to quantify effects of: seed input on seedling density, insect herbivory on juvenile density, and cumulative impacts of both seed input and herbivory on reproductive adult density. Seed addition increased seedling densities at three of five sites in 2006 and all five sites in 2007. Insect herbivory reduced seedling survival across all sites in both years, as well as rosette survival from the previous years seedlings. In both years, insecticide treatment of seed addition plots led to greater adult tall thistle densities in the following year, reflecting the increase in juvenile thistle densities in the experimental year. Seedling survival was not density dependent. Our analytical projection model predicts a significant long-term increase in adult densities from seed input, with a greater increase under experimentally reduced insect herbivory. While plant community biomass and water stress varied significantly between ridges and valleys, the effects of seed addition and insect herbivory did not vary with gradient position. These results support conceptual models that predict seedling and adult densities of short-lived monocarpic perennial plants should be seed limited. Further, the experiment demonstrates that even at high juvenile plant densities, at which density dependence potentially could have overridden herbivore effects on plant survival, insect herbivory strongly affected juvenile thistle performance and adult densities of this native prairie species.

Integral projection models show exotic thistle is more limited than native thistle by ambient competition and herbivory.

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Effects of apical meristem mining on plant fitness, architecture, and flowering phenology in Cirsium altissimum (Asteraceae)

UNLABELLED • PREMISE OF THE STUDY Interactions that limit lifetime seed production have the potential to limit plant population sizes and drive adaptation through natural selection. Effects of insect herbivory to apical meristems (apical meristem mining) on lifetime seed production rarely have been quantified experimentally. We studied Cirsium altissimum (tall thistle), whose meristems are mined by Platyptilia carduidactyla (artichoke plume moth), to determine how apical damage affects plant maternal fitness and evaluate both direct and indirect mechanisms underlying these effects.• METHODS In restored prairie, apical mining was manipulated on tall thistles by applying insecticide, water, or no spray to apical meristems. We quantified effects on lifetime seed production, plant architecture, and flowering phenology. Seed germinability and seedling mass were evaluated in a greenhouse.• KEY RESULTS Apical meristem miners decreased lifetime seed production of C. altissimum, but not seed quality. Higher mortality rates of damaged plants contributed to reduced seed production. Apical damage reduced plant height and increased the proportion of blooming flower heads in axial positions on branches. Apical damage delayed flowering and shortened flowering duration.• CONCLUSIONS Apical meristem mining reduced plant maternal fitness. The shift in the identity of blooming flower heads from terminal to axial positions contributed to this reduction because axial heads are less fecund. Shorter, meristem-mined plants may have been more susceptible to competition, and this susceptibility may explain their higher mortality rates. The kinds of changes in architecture and phenology that resulted from apical damage to C. altissimum have been shown to affect floral visitation in other plant species.

Patterns in beaver herbivory in south-central Kansas riparian woodlands

Beaver (Castor canadensis) herbivory can strongly affect the physiognomy and successional dynamics of forests and woodlands near bodies of water. In the central and southern Great Plains, the riparian zones that occur in a grassland / cropland matrix are foci of biodiversity and beaver activity, but little is known about patterns of beaver damage in these areas. We conducted a regional survey of beaver girdling of trees in riparian woodlands in south-central Kansas to determine 1) how the frequency of girdling was related to distance from streams, 2) whether there were differences among tree species in the frequency of girdling and, if so, which species were damaged most and 3) how the probability of beaver girdling varied with tree diameter. The frequency of girdling of trees declined more rapidly with distance from water (< 10 m) than in most northern ecosystems where beaver foraging patterns often have been quantified. Damage occurred significantly more frequently on Celtis occidentalis, Morus rubra and Salix sp. than on Maclura pomifera, Catalpa speciosa and Ulmus sp. Beavers avoided very small- and large-diameter stems, but within the range of stem diameters that beavers gnawed, the probability of girdling decreased with increasing stem diameter. Distance to the stream did not alter the distribution of girdling among tree species nor did it change the relationship between girdling probability and stem diameter. In south-central Kansas riparian woodlands, beavers have the potential to alter woodland physiognomy by reducing the density of the sapling-small adult tree layer. Further, their strong preferences among tree species suggest the potential to alter tree species composition by affecting regeneration patterns. However, at most sites these effects will be weak >10 m from streams.