Dennis J. O'Dowd

Evolutionarily enlightened management

Here we review growing evidence that microevolutionary changes may often be rapid and, in many cases, occur on time frames comparable to human disturbance and anthropogenic change. Contemporary evolutionary change has been documented in relatively pristine habitats, in disturbed populations, under captive management, and in association with both intentional and inadvertent introductions. We argue that evolutionary thinking is thus relevant to conservation biology and resource management but has received insufficient consideration. Ignoring evolution may have a variety of consequences, including unpredicted evolutionary responses to disturbance and naive or inappropriate management decisions. Philosophically, we must also grapple with the issue of whether the evolution of adaptations to disturbance and degraded habitats is sometimes beneficial or something to be rigorously avoided. We advocate promoting evolutionarily enlightened management [Lecture Notes in Biomathematics 99 (1994) 248], in which both the ecological and evolutionary consequences of resource management decisions are considered.

Multalism between Harvester Ants and a Desert Ephemeral: Seed Escape from Rodents

The harvester ants Veromessor pergandei and Pogonomyrmex californicus modify the highly localized seed shadow of the Sonoran Desert ephemeral Datura discolor (Solanaceae) through (1) attraction to food bodies attached to seeds released beneath the parent plant, (2) immediate transport of diaspores to the ant nest, (3) subsequent removal of the food body in underground granaries, and (4) final expulsion of intact seeds onto the colony midden. D. discolor seed without food bodies are not attractive to ants and remain beneath the parent canopy. At Nude Wash, Cali- fornia, USA, diaspore removal distances are generally small, averaging 2.3 m away from the nearest D. discolor canopy in March-May 1977. Two lines of evidence indicate that this limited transport has a large effect on seed detection and utilization by rodents: (1) seeds placed in ant-proof dishes beneath the D. discolor canopy were about 10 times more likely to be removed by nocturnally foraging rodents than seeds placed in locations similar to those where ants locate their nests; (2) using D. discolor as bait, significantly more rodents were trapped beneath the plant canopy than in open areas where seeds were transported by ants. Manipulation of the density of D. discolor seed in these tests suggests that seed utilization by rodents is distance-responsive to the parent canopy rather than dependent on the density of seeds. Diaspore analyses indicate that investment in the transport system is relatively small and repre- sents <5% of the total investment in seeds.

Ecological and socioeconomic impacts of invasive alien species in island ecosystems

Minimizing the impact of invasive alien species (IAS) on islands and elsewhere requires researchers to provide cogent information on the environmental and socioeconomic consequences of IAS to the public and policy makers. Unfortunately, this information has not been readily available owing to a paucity of scientific research and the failure of the scientific community to make their findings readily available to decision makers. This review explores the vulnerability of islands to biological invasion, reports on environmental and socioeconomic impacts of IAS on islands and provides guidance and information on technical resources that can help minimize the effects of IAS in island ecosystems. This assessment is intended to provide a holistic perspective on island-IAS dynamics, enable biologists and social scientists to identify information gaps that warrant further research and serve as a primer for policy makers seeking to minimize the impact of IAS on island systems. Case studies have been selected to reflect the most scientifically-reliable information on the impacts of IAS on islands. Sufficient evidence has emerged to conclude that IAS are the most significant drivers of population declines and species extinctions in island ecosystems worldwide. Clearly, IAS can also have significant socioeconomic impacts directly (for example human health) and indirectly through their effects on ecosystem goods and services. These impacts are manifest at all ecological levels and affect the poorest, as well as richest, island nations. The measures needed to prevent and minimize the impacts of IAS on island ecosystems are generally known. However, many island nations and territories lack the scientific and technical information, infrastructure and human and financial resources necessary to adequately address the problems caused by IAS. Because every nation is an exporter and importer of goods and services, every nation is also a facilitator and victim of the invasion of alien species. Wealthy nations therefore need to help raise the capacity of island nations and territories to minimize the spread and impact of IAS.

Predator Satiation and Site Alteration Following Fire: Mass Reproduction of Alpine Ash (Eucalyptus Delegatensis) in Southeastern Australia

This study examines seedfall, seed predation, and seedling recruitment in a relatively fire—sensitive tree, the alpine ash (Eucalyptus delegatensis), in the Brindabella Range, Australian Capital Territory (A.C.T.), Australia. An experimental high—intensity fire was used at Bushrangers Creek (BC) to determine whether massive, synchronized seedfall and subsequent seedling recruitment are linked to (1) satiation of postdispersal seed predators and/or (2) reduction of the physical and biotic resistance to seedling growth and survivorship. Results were contrasted with findings at Moonlight Hollow (MH), a site which served as a natural, unburned control. Overall results wee consistent with the predator—satiation hypothesis. High—intensity fire killed 93% of the alpine ash trees at BC and caused a massive, synchronized release of seeds, averaging 405 viable seeds/m2 (4021 total ovular structures/m2) in the 3 wk after fire. Fire precipitated an immediate drop in weekly removal of seeds from trays, from an average of 65.3% prefire to 13.6% postfire. Postfire seed removal at BC dropped to significantly lower levels than corresponding measures at unburned MH. Caging experiments demonstrated that ants were the only important removal agents of alpine ash seed at BC and MH. Although abundance and species richness of ants trapped on the seedbed increased following fire, the number of ants trapped per available seed significantly declined due to the massive input of seed. Fire and subsequent seedfall resulted not only in a greater absolute number of emergent seedlings (66.1 seedlings/m2 at BC; 0.8 seedlings/m2 at MH) but also in a significantly greater proportion of emergence from seed input from the canopy (14.7% at BC; 2.7% at MH). Seedling mortality was great, and after 72 wk mean density at BC was 1.9 seedlings/m2. Successful recruitment did not occur at unburned MH. A phytometer experiment demonstrated that fire neither enhanced the physical environment for early growth and survivorship of seedlings nor reduced the effect of natural enemies on E. Delegatensis seedlings. Greater frequency of grazing on seedlings growing in burned soil and a more rigorous physical environment at BC appeared to account for lack of enhanced growth and survivorship at postfire BC. Massive, synchronized release of stored seed from shrub and tree species following fire is a common feature in many fire—prone environments. Parallels in seed storage, postdispersal seed predation, and regeneration following fire in Eucalyptus and Pinus suggest that postdispersal seed predators are important selective agents in the evolution of seed storage and induction of mass seed release by fire.

Plants, mites and mutualism : leaf domatia and the abundance and reproduction of mites on Viburnum tinus (Caprifoliaceae)

Associations between mites and leaf domatia have been widely reported, but little is known about their consequences for either plants or mites. By excising domatia from leaves of the laureltinus, Viburnum tinus L. (Caprifoliaceae), in the garden and laboratory, we showed that domatia alter the abundance, distribution, and reproduction of potential plant mutualists. Over 4 months, leaves with domatia on six garden shrubs had 2–36 times more predatory and microbivorous mites, and more mite eggs than leaves without domatia. However, this effect varied among plants and was weaker on one shrub with few mites on its leaves. Domatia also influenced the distribution of mites on leaves. A significantly higher fraction of mites, representing all life stages, was found in vein axils of leaves with domatia than in vein axils on leaves without domatia. Single-leaf experiments in the laboratory showed that domatia enhanced reproduction by the predatory mite, Metaseiulus occidentalis, especially at low relative humidity (30–38%). When domatia were removed, oviposition was reduced significantly only at low relative humidity, suggesting that domatia provide mites with refuge from environmental extremes on the leaf surface. Moreover, the use of domatia by predatory mites may reduce the impact of some plant enemies. In two experiments where prey consumption was measured, M. occidentalis ate significantly higher percentages of the eggs of the two-spotted spider mite (Tetranychus urticae). Our results are consistent with the viewpoint that mite-domatia associations are mutualistic. By directly aiding and abetting the third trophic level, plants with leaf domatia may increase the efficiency of some predaceous and microbivorous mites in consuming plant enemies.

Seed dispersal by ants: behaviour-releasing compounds in elaiosomes

SummaryIn a study of the biochemical basis of seed dispersal by ants, elaiosomes of Acacia myrtifolia and Tetratheca stenocarpa induced seed collection: intact diaspores and elaiosomes were taken rapidly by ants while most seeds remained on the forest floor. Extracts of elaiosomes (non-polar lipids, polar lipids, and aqueous fractions) were differentially collected by ants. Small pieces of pith impregnated with the polar lipid fraction from elaiosomes of either species elicited a removal rate by ants equivalent to that of intact elaiosomes and significantly higher than that of untreated pith. The non-polar lipid fraction, highest in concentration in elaiosomes of both species, elicited removal that did not differ from that of untreated pith. In T. stenocarpa, however, the aqueous fraction also induced removal equivalent to the polar lipid fraction. 1,2-Diglycerides with unsaturated groups are present in the active polar fractions of both species and unsaturated oleate is the major acid group of the glycerides in elaiosomes. Most oleate-containing compounds tested were taken more rapidly by ants than saturated compounds, and oleic acid, associated with corpse-carrying in ants, induced rapid removal. 1,2-Diolein, but not 1,3-diolein, was taken suggesting that the specific configuration of fatty acid moieties influences collection by ants. We hypothesize that a small suite of oleyl-containing compounds in elaiosomes elicit a stereotyped carrying response by a variety of ants. While the nutrient composition of elaiosomes may provide the underlying selective advantage for ants in seed dispersal, specific compounds may manipulate their behaviour and maximize seed dispersal.

Invasional meltdown: Invader–invader mutualism facilitates a secondary invasion

In multiply invaded ecosystems, introduced species should interact with each other as well as with native species. Invader-invader interactions may affect the success of further invaders by altering attributes of recipient communities and propagule pressure. The invasional meltdown hypothesis (IMH) posits that positive interactions among invaders initiate positive population-level feedback that intensifies impacts and promotes secondary invasions. IMH remains controversial: few studies show feedback between invaders that amplifies their effects, and none yet demonstrate facilitation of entry and spread of secondary invaders. Our results show that supercolonies of an alien ant, promoted by mutualism with introduced honeydew-secreting scale insects, permitted invasion by an exotic land snail on Christmas Island, Indian Ocean. Modeling of land snail spread over 750 sites across 135 km2 over seven years showed that the probability of land snail invasion was facilitated 253-fold in ant supercolonies but impeded in intact forest where predaceous native land crabs remained abundant. Land snail occurrence at neighboring sites, a measure of propagule pressure, also promoted land snail spread. Site comparisons and experiments revealed that ant supercolonies, by killing land crabs but not land snails, disrupted biotic resistance and provided enemy-free space. Predation pressure on land snails was lower (28.6%), survival 115 times longer, and abundance 20-fold greater in supercolonies than in intact forest. Whole-ecosystem suppression of supercolonies reversed the probability of land snail invasion by allowing recolonization of land crabs; land snails were much less likely (0.79%) to invade sites where supercolonies were suppressed than where they remained intact. Our results provide strong empirical evidence for IMH by demonstrating that mutualism between invaders reconfigures key interactions in the recipient community. This facilitates entry of secondary invaders and elevates propagule pressure, propagating their spread at the whole-ecosystem level. We show that identification and management of key facilitative interactions in invaded ecosystems can be used to reverse impacts and restore resistance to further invasions.

Associations between mites and leaf domatia

Associations between mites and leaf domatia are widespread, abundant and probably ancient. Recent research has shown that mites commonly shelter, develop and reproduce within domatia on plants in many geographic regions, from the tropics to the temperate zone, and most of these mites belong to predaceous or fungivorous taxa. Of hypotheses offered to explain these associations, protective mutualism is most consistent with the reported characteristics and patterns of mitedomatia association.

Red crabs in rain forest, Christmas Island: removal and relocation of leaf-fall

Field experiments comparing leaf-fall in the presence and absence of the red land crab, Gecarcoidea natalis, in rain forest on Christmas Island, Indian Ocean showed that, by eating and returning leaves to their burrows, they significantly reduced accumulation of leaf-fall on the forest floor. Red crabs removed 30-50% of the leaf-fall at the experimental sites during the study. Differences in leaf-fall mass between crab-access and exclusion quadrats were positively correlated with the density of foraging red crabs. Red crabs also generated spatial heterogeneity in the distribution of litter on the forest floor by returning litter to their burrows. Leaves lined chambers of 64% of excavated burrows and litter biomass around the entrances was significantly greater than that on off-burrow locations. This was reflected in the significantly higher concentrations of organic matter and nutrients (N, P, K, Ca, Na, and Mg) associated with soils near burrow entrances. These results suggest that a single species, G. natalis, is the major processor of leaf-fall in rain forest on Christmas Island and may affect (1) the temporal and spatial patterns of nutrient availability and (2) the diversity and biomass of the litter fauna.

Resistance of island rainforest to invasion by alien plants: influence of microhabitat and herbivory on seedling performance

Experiments have promise in determining mechanisms by which communities resist invasion. Growth and survivorship of transplanted seedlings of introduced tree species (Leucaena leucocephala, Muntingia calabura, Adenanthera pavonia, and Clausena excavata) were used to assess abiotic (light regime) and biotic resistance (herbivory) to invasion of rainforest on Christmas Island (Indian Ocean). At four sites, seedlings were transplanted into the forest edge along roadside verges and into adjacent intact forest; half were caged to prevent access by the dominant seedling consumer, the red land crab (Gecarcoidea natalis). Red crab densities did not differ between roadside and forest plots. Red crabs initially reduced survivorship of Leucaena in both edge and interior plots but virtually all seedlings in the forest interior were dead after 41 weeks. Survivorship of Muntingia was also initially reduced by crabs in the forest edge, but again, all but one seedling died by the end of the experiment. Seedlings of Adenanthera and Clausena fared much better overall, surviving well in both locations. Red crabs had no overall effect on Adenanthera survival but significantly reduced survival of Clausena in forest plots. For both species, seedling performance was greater in the forest edge than in the forest interior. Red crabs had no effect on height increment for either species; however, for Clausena, red crabs reduced seedling mass in the forest interior. Both Adenanthera and Clausena were able to persist in the intact forest. Clausena is now actively invading intact rainforest, but Adenanthera appears dispersal-limited. Resistance factors in intact forest appear hierarchical: biotic resistance afforded by land crabs can impede establishment of some plant invaders but seedling responses to abiotic factors (e.g., the light regime) largely overwhelm its effect. Together, these two community attributes are likely to restrict the range of plant invaders to a small suite of species that can successfully establish in intact rainforest on the island. However, increased propagule pressure from a variety of shade-tolerant species and further declines in the abundance of the dominant seedling consumer may lead to increasing invasion success in this island rainforest.