EEB Faculty Publications

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Effects of above-ground herbivory on soil microbial biomass in conventional and no-tillage agroecosystems
(Elsevier, 1995) Holland, J. Nat
The effects of above-ground herbivory on soil microbial biomass associated with maize (Zea muys) roots were measured in field experiments. Different grazing intensities were established by placing grasshoppers (0, 5, 10 and 20) in cages around individual maize plants, and allowing them to graze for 5 days. Soil samples were taken 12 days later at a depth of between 8 and 12 cm. Soil microbial biomass was measured using the chloroform fumigation direct extraction method. Results indicated that intermediate levels of herbivory increased soil microbial biomass in the no-tillage system. However, in a conventional tillage system no significant differences were found among the grazing treatments. It is hypothesized that increases in soil microbial biomass in the no-tillage system were attributable to increases in root exudates resulting from above-ground herbivory.
Post-hibernation movement and foraging habitat of a male Indiana bat, Myotis sodalis (Chiroptera: Vespertilionidae), in western Virginia
(1995-12) Hobson, Christopher S.; Holland, J. Nathaniel
We investigated departure patterns of bats from a higernculum and use of tree roosts and foraging habitat by a male Indiana bat (Myotis sodalis) in western Virginia with radio-telemetric techniques, cave populaiton surveys,and habitat assessment.
Herbivore-induced changes in plant carbon allocation: Assessment of below-ground C fluxes using carbon-14
(Springer-Verlag, 1996-03) Crossley, D.A. Jr; Cheng, Weixin; Holland, J. Nathaniel
Effects of above-ground herbivory on shortterm plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25-50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with 14CO2. During the next 8 h, ~4C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and 14C recovered from soil. While herbivory increased root and soil respiration, the peak time for ~4CO2 evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.
Habitat use and resource tracking by African Ceratogymna hornbills: implications for seed dispersal and forest conservation
(Blackwell Publishing, 1998) Smith, Thomas B.; Whitney, Kenneth D.
Conservation of tropical forests requires an understanding of the seasonal patterns of abundance and distribution of important seed dispersers. In the forests of Central Africa, three species of hornbills in the genus Ceratogymna (Aves: Bucerotidae) provide dispersal for over 50 species of trees and lianas, yet their ecology is poorly known. We present the first empirical evidence for resource tracking and large-scale movements by two of these species, which were previously considered resident. Hornbill numbers and fruit production were estimated over a one-year period in the Dja Reserve, Cameroon. Monthly abundances of C. atrata and C. cylindricus varied four- and 12-fold, respectively, and were predicted by community fruit availability as measured by fruitfall on a 4·3 km trail. In contrast, C. fistulator abundances varied little and were not related to fruit availability. Synchronous population fluctuations in sites 22 km distant were observed for atrata and cylindricus, but not for fistulator. Together, these results indicate that atrata and cylindricus populations track fruit resources via seasonal emigrations, while the fistulator population appears to be relatively sedentary. Comparisons of hornbill densities in primary and secondary forests suggest that they are important dispersers in both habitats, and will likely play larger roles in central African forest regeneration as more vulnerable seed dispersers are removed by hunting. If large seasonal movements of hornbills turn out to be a general phenomenon, effective management of central African forest reserves will require greater emphasis on reserve connectivity and on seasonal and spatial patterns of fruit production.
The evolution of obligate pollination mutualisms: senita cactus and senita moth
(Springer-Verlag, 1998) Holland, J. Nathaniel; Fleming, Theodore H.
We report a new obligate pollination mu- tualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran De- sert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incom- patible columnar cactus. Beginning at sunset, its ¯owers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on ¯ower stigmas, and oviposit a single egg on a ¯ower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-polli- nation and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita ¯owers abort by day 6 after ¯ower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, ¯owers usually closed be- fore sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The in- dependent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal ¯ower opening, self-incompati- ble breeding systems, and resource-limited fruit pro- duction appear to be important during this evolution
Seed dispersal by Ceratogymna hornbills in the Dja Reserve, Cameroon
(Cambridge University Press, 1998-05) Smith, Thomas B.; Parker, V. Thomas; Hardesty, Britta Denise; Stauffer, Donald J.; Holbrook, Kimberly M.; Lamperti, Aaron M.; Fogiel, Mark K.; Whitney, Kenneth D.
Seed dispersal is a process critical to the maintenance of tropical forests, yet little is known about the interactions of most dispersers with their communities. In the Dja Reserve, Cameroon, seed dispersal by the hornbills Ceratogymna atrata, C. cylindricus and C. fistulator (Aves: Bucerotidae) was evaluated with respect to the taxonomic breadth of plants dispersed, location of seed deposition and effects on seed germination. Collectively, the three hornbill species consumed fruits from 59 tree and liana species, and likely provided dispersal for 56 of them. Hornbill-dispersed tree species composed 22% of the known tree flora of the site. Hornbill visit lengths, visit frequencies, and seed passage times indicated that few seeds were deposited beneath parent trees; in five hornbill/tree species pairings studied, 69–100% of the seeds ingested were deposited away from the parent trees. Germination trials showed that hornbill gut passage is gentle on seeds. Of 24 tree species tested, 23 germinated after passage by hornbills; of 17 planted with controls taken directly from trees, only four species showed evidence of inhibition of germination rate, while seven experienced unchanged germination rates and six experienced enhanced germination rates. Results suggested that Ceratogymna hornbills rank among the most important seed dispersers found in Afrotropical forests, and they deserve increased conservation attention. Ceratogymna hornbills are likely to become increasingly important in forest regeneration as populations of larger mammalian seed dispersers (such as forest elephants and primates) diminish.
Mutualistic interactions between Upiga virescens (Pyralidae), a pollinating seed-consumer, and Lophocereus schottii (Cactaceae)
(Ecological Society of America, 1999) Fleming, Theodore H.; Holland, J. Nathaniel
Pollinating seed-consuming interactions are rare, but include fig–fig wasp and yucca–yucca moth interactions, both of which are thought to be coevolved. Conditions favoring such mutualisms are poorly known but likely include plants and pollinators whose life cycles are synchronized. In this paper, we describe a new pollinating seed-consumer mutualism between a Sonoran Desert cactus, Lophocereus schottii (senita cactus), and a pyralid moth, Upiga virescens (senita moth). We compare this mutualism with the yucca mutualism in terms of life history traits, active pollination, and selective abortion. Senita cactus flowers were pollinated nearly exclusively by nocturnal senita moths, but a few halictid bees also pollinated flowers. Only 40% of flowers set fruit during the years of study, apparently due to resource limitation. All phases of the senita moth’s life history were associated with the senita cactus. During flower visitation, female senita moths collected pollen, actively pollinated flowers, and oviposited one egg. After flowers closed, emerging larvae bored into the tops of developing fruit, where they consumed seeds and fruit tissue. However, not all seeds/fruit were consumed by larvae because only 20% of eggs produced larvae that survived to be seed/fruit consumers. Senita cactus and senita moth interactions were mutualistic. Moths received food resources (seeds, fruit) for their progeny, and cacti had a 4.8 benefit-to-cost ratio; only 21% of developing fruit were destroyed by larvae. Life history traits important to this mutualism included low survival of senita moth eggs/larvae, several moth generations per flowering season, host specificity of senita moths, active pollination, oviposition into flowers, and limited seed/fruit consumption. Active pollination by senita moths in the presence of co-pollinators supports the prediction that active pollination can evolve during a period of coexistence with co-pollinators. The specialization of both senita and senita moths in the presence of co-pollinators makes the senita mutualism quite remarkable in comparison with fig–fig wasp and yucca– yucca moth mutualisms.
Geographic and population variation in pollinating seed-consuming interactions between senita cacti (Lophocereus schottii) and senita moths (Upiga virescens)
(springer-Verlag, 1999) Fleming, Theodore H.; Holland, J. Nathaniel
Interspecific interactions can vary within and among populations and geographic locations. This variation can subsequently influence the evolution and coevolution of species interactions. We investigated population and geographic variation in traits important to pollinating seed-consuming interactions between the senita cactus (Lophocereus schottii) and its obligate pollinating moth (Upiga virescens), both of which are geographically restricted to the Sonoran Desert. Female moths actively pollinate senita flowers and oviposit onto flowers. Their larvae consume developing seeds and fruit of flowers pollinated by females. Traits important to this interaction include fruit set from moth pollination, fruit survivorship, and costs of fruit consumption by larvae. We studied these traits for five populations at two widely separated geographic locations. On average, 37% of flowers set fruit, 22% of flowers produced mature fruit, and larvae consumed 25% of immature fruit pollinated by female senita moths. Senita cactus and senita moth interactions were strongly mutualistic in all populations that we studied. Although one population had statistically lower fruit set and fruit production than the other four, all five populations were qualitatively similar in fruit production, costs, and patterns of fruit survivorship. Hand-pollination experiments suggested that fruit set was resource-limited in all but this one population. Apparent pollen limitation in the one population explains the quantitative differences in fruit set and fruit survivorship among the populations. As predicted by theory and exemplified by the senita mutualism, specialized and/or obligate interactions vary little among populations and geographic locations.
Sonoran Desert columnar cacti and the evolution of generalized pollination systems
(Ecological Society of America, 2001) Hamrick, J.L.; Nason, John D.; Holland, J. Nathaniel; Sahley, Catherine T.; Fleming, Theodore H.
We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant–pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied threefold over six years. In senita, peak flowering varied by as much as 5–8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64–87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The ‘‘missing’’ pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycteris bats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita–senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats and birds for pollination
Population dynamics and the stability of obligate pollination mutualisms
(Springer-Verlag, 2001) Holland, J. Nathaniel; DeAngelis, Donald L.
Mutualistic interactions almost always produce both costs and benefits for each of the interacting species. It is the difference between gross benefits and costs that determines the net benefit and the per-capita effect on each of the interacting populations. For example, the net benefit of obligate pollinators, such as yucca and senita moths, to plants is determined by the difference between the number of ovules fertilized from moth pollination and the number of ovules eaten by the pollinator’s larvae. It is clear that if pollinator populations are large, then, because many eggs are laid, costs to plants are large, whereas, if pollinator populations are small, gross benefits are low due to lack of pollination. Even though the size and dynamics of the pollinator population are likely to be crucial, their importance has been neglected in the investigation of mechanisms, such as selective fruit abortion, that can limit costs and increase net benefits. Here, we suggest that both the population size and dynamics of pollinators are important in determining the net benefits to plants, and that fruit abortion can significantly affect these. We develop a model of mutualism between populations of plants and their pollinating seed-predators to explore the ecological consequences of fruit abortion on pollinator population dynamics and the net effect on plants. We demonstrate that the benefit to a plant population is unimodal as a function of pollinator abundance, relative to the abundance of flowers. Both selective abortion of fruit with eggs and random abortion of fruit, without reference to whether they have eggs or not, can limit pollinator population size. This can increase the net benefits to the plant population by limiting the number of eggs laid, if the pollination rate remains high. However, fruit abortion can possibly destabilize the pollinator population, with negative consequences for the plant population.
Confronting climate change in the Gulf region: prospects for sustaining our ecological heritage
(Union of Concerned Scientists, 2001-10) Zimmerman, R.J.; Wetzel, R.G.; Siemann, E.H.; Reed, J.B.; Miller, R.L.; Harwell, M.A.; Gholz, M.A.; Barron, E.J.; Twilley, R.B.
From Texas to Florida, the Gulf coast region is rich with ecological resources that support the region’s economic wealth. Over time, human activities from dam construction to shoreline development have dramatically altered natural landscapes, waterways, and ecological processes. Pressures from human activities remain the most important agents of ecological change in the region today. Over the century ahead, land-use changes are likely to increase as rapid population growth continues. Global climate change, driven by rising levels of carbon dioxide and other heat-trapping greenhouse gases in the atmosphere, will interact with, and magnify, other human stresses on Gulf Coast ecosystems and the goods and services they provide. Confronting Climate Change in the Gulf Coast Region explores the potential risks of climate change to Gulf Coast ecosystems in the context of pressures from land use. Its purpose is to help the public and policymakers understand the most likely ecological consequences of climate change in the region over the next 50 to 100 years and prepare to safeguard the economy, culture, and natural heritage of the Gulf Coast. This summary highlights key findings.
Co-pollinators and specialization in the pollinating seed-consumer mutualism between senita cacti and senita moths
(Springer-Verlag, 2002) Fleming, Theodore H.; Holland, J. Nathaniel
Specialization of a plant on a particular pollinator may not evolve if co-pollinators are effective and abundant. This is particularly evident if fruit set is resource limited and cannot be increased above the levels produced by the actions of co-pollinators. The pollinating seed-consuming interaction between senita cacti and senita moths in the Sonoran Desert presents a paradox because it exhibits many traits resembling those of the highly specialized yucca/yucca moth system, but also involves co-pollinators. For 6 years, we studied how contributions of nocturnal senita moths and diurnal co-pollinating bees to fruit set depended on resource and pollen limitation, time of flower closing, and the onset and phenology of flowering. Fruit set was typically resource limited. Fruit set of flowers exposed only to senita moths was not different from resource-limited fruit set of control flowers. When only co-pollinating bees were allowed to visit flowers, however, fruit set became pollen limited. Only in one year when fruit set was pollen limited were bees able to increase fruit set beyond the level resulting from senita moth pollination. High temperatures commonly induced flowers to close before sunrise so that diurnal bees were unable to visit flowers. This was particularly important from 1998 to 2000, when flowering did not begin until late in spring when temperatures were already high enough to induce flowers to close before sunrise. Bees were typically functionally redundant with senita moths; excluding bees from visiting flowers did not alter fruit set. Nevertheless, extreme specialization of floral traits to exclude co-pollinators has not evolved in senita, possibly because there are times when bees do increase fruit set. This can occur when senita moths are rare, fruit set is pollen limited, cool temperatures prevent flowers from closing before sunrise, and flowering begins early in spring.
Experimental evidence that both parties benefit in a facultative plant-spider mutualism
(Ecological Society of America, 2004-06) Whitney, Kenneth D
Spiders are known to influence plant fitness, and vice versa. Yet, it has not been appreciated that these facultative relationships can be mutualistic. I examined the interaction between Phryganoporus candidus, a subsocial Australian spider, and the extrafloral nectary-bearing shrub Acacia ligulata to explore variability in mutualistic interactions over a three-year period. Spiders enhanced seed production by reducing seed predation by heteropterans, wasps, and weevils. Because spider colonies occupy only a fraction of a plant’s volume, average benefits ranged from 0.4 to 6% increases in whole-plant seed production. These benefits were strongest in years of low seed production, suggesting that spiders may buffer plants against female reproductive failure. To evaluate benefits for spiders, I established experimental spider colonies on three common hosts. Spider performance (persistence and prey capture rates) on live A. ligulata and live hopbush Dodonaea viscosa exceeded that on dead acacia, suggesting that live hosts are more beneficial than dead hosts. Stable-isotope analyses demonstrated that colonies living on the three hosts differed substantially in diet, providing a possible mechanism for the observed differential suitability of hosts. However, the analyses were unable to establish conclusively that A. ligulata extrafloral nectar was an important reward for spiders. Variability in the A. ligulata– P. candidus system suggests that plant–spider associations, like other facultative protection relationships, likely vary along a continuum from antagonism to mutualism.
Insect seed predators as novel agents of selection on fruit color
(Ecological Society of America, 2004-08) Stanton, Maureen L; Whitney, Kenneth D
The ecological and evolutionary dynamics of fruit color polymorphisms remain poorly known because patterns and agents of selection have rarely been identified. Here, we examine Acacia ligulata, a shrub of the Australian arid zone characterized by a red/yellow/orange aril color polymorphism. Seed production patterns over four populations and three years suggested that spatially variable selection may be acting to maintain the polymorphism: red and yellow aril color morphs each had the highest seed production in alternate sites. Seed production differences between morphs were a function of both intrinsic plant characters (fruit production) and predispersal seed predation, which affects the number of viable seeds matured per ovule. Fruit production differences are hypothesized to result from a genotype-by-environment interaction, perhaps related to plant vigor. In contrast, morph differences in the numbers of viable seeds per ovule are produced via differential seed predation by heteropteran insects, as demonstrated by exclusion experiments. Because these predators feed when aril color is not visible, differential predation is evidently a response to pleiotropic effects of fruit color alleles. We suggest that such pleiotropic effects may be a common feature of fruit color polymorphisms, and that the most obvious selective agents (that is, seed dispersers) may not always be the most important.
Using scalar models for precautionary assessments of threatened species
(Blackwell Publishing, 2006-10) Bridges, Todd S; Akcakaya, H. Resit; Dunham, Amy E.
Scalar population models, commonly referred to as count-based models, are based on time-series data of population sizes and may be useful for screening-level ecological risk assessments when data for more complex models are not available. Appropriate use of such models for management purposes, however, requires understanding inherent biases that may exist in these models. Through a series of simulations, which compared predictions of risk of decline of scalar and matrix-based models, we examined whether discrepancies may arise from different dynamics displayed due to age structure and generation time. We also examined scalar and matrix-based population models of 18 real populations for potential patterns of bias in population viability estimates. In the simulation study, precautionary bias (i.e., overestimating risks of decline) of scalar models increased as a function of generation time. Models of real populations showed poor fit between scalar and matrix-based models, with scalar models predicting significantly higher risks of decline on average. The strength of this bias was not correlated with generation time, suggesting that additional sources of bias may be masking this relationship. Scalar models can be useful for screening-level assessments, which should in general be precautionary, but the potential shortfalls of these models should be considered before using them as a basis for management decisions.
Biodiversity maintenance in food webs with regulatory environmental feedbacks
(Elsevier, 2007) Rauscher, Daniel; Brown, Christopher G.; Dunham, Amy E.; Bagdassarian, Carey K.
Although the food web is one of the most fundamental and oldest concepts in ecology, elucidating the strategies and structures by which natural communities of species persist remains a challenge to empirical and theoretical ecologists. We show that simple regulatory feedbacks between autotrophs and their environment when embedded within complex and realistic food-web models enhance biodiversity. The food webs are generated through the niche-model algorithm and coupled with predator–prey dynamics, with and without environmental feedbacks at the autotroph level. With high probability and especially at lower, more realistic connectance levels, regulatory environmental feedbacks result in fewer species extinctions, that is, in increased species persistence. These same feedback couplings, however, also sensitize food webs to environmental stresses leading to abrupt collapses in biodiversity with increased forcing. Feedback interactions between species and their material environments anchor food-web persistence, adding another dimension to biodiversity conservation. We suggest that the regulatory features of two natural systems, deep-sea tubeworms with their microbial consortia and a soil ecosystem manifesting adaptive homeostatic changes, can be embedded within niche-model food-web dynamics. r 2007 Elsevier Ltd. All rights reserved.
Battle of the sexes: Cost asymmetry explains female dominance in lemurs
(Elsevier, 2008) Dunham, Amy E.
Above and below ground impacts of terrestrial mammals and birds in a tropical forest
(Blackwell Publishing, 2008) Dunham, Amy E.
Understanding the impact of losing trophic diversity has global significance for managing ecosystems as well as important theoretical implications for community and ecosystem ecology. In several tropical forest ecosystems, habitat fragmentation has resulted in declines and local extinctions of mammalian and avian terrestrial insectivores. To assess the ability of a tropical rainforest community in Ivory Coast to resist perturbation from such loss of trophic diversity, I traced feedbacks in above and below ground communities and measured changes in nutrient levels and herbivory rates in response to an experimental exclosure of avian and mammalian terrestrial insectivores. I present evidence that loss of this functional group may result in increased tree seedling herbivory and altered nutrient regimes through changes in the abundance and guild structure of invertebrates. Exclusion of top predators of the forest floor resulted in increased seedling herbivory rates and macro-invertebrate ( 5 mm) densities with strongest effects on herbivorous taxa, spiders and earthworms. Densities of microbivores including Collembola, Acarina and Sciaridae showed the opposite trend as did levels of inorganic phosphorus in the soil. Results were evaluated using path analysis which supported the presence of a top down trophic cascade in the detrital web which ultimately affected turnover of phosphorus, a limiting nutrient in tropical soils. Results illustrate the potential importance of vertebrate predators in both above and belowground food webs despite the biotic diversity and structural heterogeneity of the rainforest floor.
Evaluating effects of habitat loss, hunting, and El Niño on a threatened lemur
(Elsevier Science, 2008) Erhart, Elizabeth M.; Dunham, Amy E.; Wright, Patricia C.; Overdorff, Deborah J.
Madagascar ranks as one of the world’s top extinction hotspots because of its high endemism and high rate of habitat degradation. Global climate phenomena such as El Nin˜ o Southern Oscillations may have confounding impacts on the island’s threatened biota but these effects are less well known. We performed a demographic study of Propithecus edwardsi, a lemur inhabiting the eastern rainforest of Madagascar, to evaluate the impact of deforestation, hunting, and El Nin˜ o on its population and to re-evaluate present endangerment categorization under the IUCN. Over 18 years of demographic data, including survival and fecundity rates were used to parameterize a stochastic population model structured with three stage classes (yearlings, juveniles, and adults). Results demonstrate that hunting and deforestation are the most significant threats to the population. Analysis of several plausible scenarios and combinations of threat revealed that a 50% population decline within three generations was very likely, supporting current IUCN classification. However, the analysis also suggested that changing global cycles may pose further threat. The average fecundity of lemurs was over 65% lower during El Nin˜ o years. While not as severe as deforestation or hunting, if El Nin˜ o events remain at the current high frequency there may be negative consequences for the population. We suggest that it is most critical for this species continued survival to create more protected areas, not only to thwart hunting and deforestation, but also to give this endangered lemur a better chance to recover from and adapt to altered climate cycles in the future.
Global climate cycles and cyclones: consequences for rainfall patterns and lemur reproduction in southeastern Madagascar
(Global Change Biology, 2010) Dunham, Amy; Erhart, E.; Wright, P. C.

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