Early Plant Succession in Abandoned Pastures in Ecuador1

We compared early plant succession in four abandoned pastures of differing age since abandonment and a nearby secondary forest site in northwestern Ecuador. Two “Open” pastures had no tree canopy covering, and two “Guava” pastures had a well-developed canopy cover of Psidium guajava. No site had been seeded with pasture grasses. All pastures were compared in a chronological sequence; two were monitored for 18 months. Species richness was consistently higher in Guava sites than in Open sites and it continued to increase over time, whereas it remained static in Open sites. Species richness was highest in secondary forest. Recruitment of tree saplings in Guava sites was lower than in secondary forest; however, it was nearly absent in Open sites. The seed bank contained predominantly herbaceous species at all sites, and was highly dissimilar to aboveground vegetation. Dominance-diversity curves for Guava sites showed a more equitable distribution of species that increased over time. In contrast, dominance-diversity curves for Open sites were static and indicated dominance by a few aggressive species. Soil characteristics among sites were variable; however, a principal components analysis on soils isolated the older Open site from all others. The older Open site had the lowest species richness and was dominated by Baccharis trinervis, an aggressive shrub species. The site appears to be in a state of arrested succession and some form of restorative intervention may be necessary to initiate succession toward a forested condition. Succession in Guava sites appears headed toward secondary forest, whereas it does not in Open sites.     

Chloroplast DNA restriction site variation in theVernonieae (Asteraceae), an initial appraisal of the relationship of New and Old World taxa and the monophyly ofVernonia

Chloroplast DNA restriction site variation was examined for 35 taxa in theVernonieae and four outgroup tribes, using 17 restriction enzymes mapped for ca. 900 restriction sites per species; 139 mutations were found to be phylogenetically informative. Phylogenetic trees were constructed using Wagner and weighted parsimony, and evaluated by bootstrap and decay analyses. Relationships of Old and New World taxa indicate complex geographical relationships; there was no clear geographic separation by hemisphere. The relationships between Old and New World Vernonias found here support prior morphological analyses. The sister group to all New and most Old World taxa was composed of a small group of Old World species including yellow-flowered, trinervate-leaved species previously postulated to be basal in the tribe. The majority of both New and Old World taxa are derived from a lineage beginning with the monotypic genusStokesia, an endemic of the southeastern United States. The genusVernonia was also found to be paraphyletic within both the New and Old World. Available data do not support either the separation ofVernonia or the tribeVernonieae into geographically distinct lineages. The pattern of relationships within theVernonieae for taxa from North America, Asia, Africa, Central and South America is most similar to that of several other groups of both plants and animals with a boreotropical origin, rather than an origin in Gondwanaland. Such a pattern of distribution suggests more ancient vicariant events than are routinely postulated for theAsteraceae.     

The relationship between species richness of vertebrate mutualists and their food plants in tropical and subtropical communities differs among hemispheres

Non-random statistical patterns have long been of interest to community ecologists. Recent studies of communities of mutualists have revealed non-random patterns in terms of connectance, degree of specialization, and nestedness. Currently unstudied, however, are the detailed statistical relationships between tropical vertebrate mutualists and their food plants in different parts of the world. Here, I review 87 studies that quantify the relationship between species richness of nectar- or fruit-eating birds and bats and species richness of their food plants in New and Old World, mostly tropical, communities. This analysis revealed that in the New World, number of plant-visiting birds and bats per community was significantly correlated with number of food plants and that the slopes of regression equations were the same for nectarivores and frugivores. The New World quantitative assembly rule states that it takes about three species of flowers or fruits to support one species of plant-visiting bird or bat. This relationship does not appear to exist in Old World communities, in which species richness of nectar- or fruit-eating birds or bats was independent of species richness of their food plants. These geographic differences likely reflect a greater degree of feeding specialization in plant-visiting vertebrates in the New World than in the Old World. I hypothesize that hemispheric differences in the spatio-temporal predictability (STP) of food resources ultimately determine levels of dietary specialization and structure in communities of New and Old World plant-visiting vertebrates.     

Secondary forest succession differs through naturalised gorse and native kānuka near Wellington and Nelson

The dominant native woody species forming early-successional vegetation on formerly forested sites in lowland New Zealand were kānuka (Kunzea ericoides) and mānuka (Leptospermum scoparium) (Myrtaceae). These have been replaced extensively by gorse (Ulex europaeus, Fabaceae), a naturalised species in New Zealand. Because gorse typically gives way to native broadleaved (angiosperm) forest in about 30 years, it is often considered desirable for facilitating native forest restoration. We tested three hypotheses, derived from the New Zealand literature, on gorse and kānuka: (1) kānuka stands have a different species composition and greater species richness than gorse stands at comparable successional stages; (2) differences between gorse and kānuka stands do not lessen over time; and (3) several native plant taxa are absent from or less common in gorse than in kānuka stands. We sampled 48 scrub or low-forest sites in two regions, Wellington and Nelson. Sites were classified into one of four predefined categories – young gorse, young kānuka, old gorse, old kānuka – based on canopy height of the succession and the dominant early-successional woody species. Few characteristics of the sites and surrounding landscapes differed significantly among site categories, and none consistently across regions. The vegetation composition of gorse and kānuka and their immediate successors differed in both regions, mainly in native woody species. Species richness was often lower in gorse and there were fewer smallleaved shrubs and orchids in gorse. Persistent differences at the older sites suggest the successional trajectories will not converge in the immediate future; gorse leads to different forest from that developed through kānuka. Gorse-dominated succession is therefore not a direct substitute for native successions. We suggest areas of early native succession should be preserved, and initiated in landscapes where successions are dominated by gorse or other naturalised shrubs.     

Vegetation Succession After Bauxite Mining in Western Australia

Alcoa World Alumina Australia has been rehabilitating bauxite mines in the jarrah forest of Western Australia for more than 35 years. An experiment was established in 1988 using three different seed treatments (legume and small understorey mix, small understorey mix only, and no seed) and two fertilizer treatments (N and P, and P only). The objectives of this study were to (1) document vegetation changes in the first 14 years after bauxite mining; (2) assess whether the vegetation is becoming more similar to the unmined forest; and (3) gain a better understanding of successional processes. Seed treatments significantly affected 13 of the 14 measured vegetation characteristics. Native species richness was higher in seeded than in unseeded sites at 1, 2, and 5 years of age, whereas diversity and evenness were generally higher at all assessment ages. Exotic species density was higher in unseeded than in seeded sites from 5 years onward, whereas richness was higher from 8 years onward. Nitrogen fertilizer significantly increased exotic species richness, density, and cover. Ephemerals dominated plant density in all rehabilitation treatments over time, whereas seeder species dominated cover. In contrast, resprouting species dominated density and cover in the unmined forest. Orchids were the only species that were not present in the first year in rehabilitated sites but increased in abundance over time. Vegetation composition in rehabilitated areas did not become more similar to the unmined forest during the 14 years since seeding, instead strongly reflected the initial species mix. Rehabilitated bauxite mines appear to follow the initial floristic composition model of succession.     

Nectar-feeding bird and bat niches in two worlds: pantropical comparisons of vertebrate pollination systems

Aim We review several aspects of the structure of regional and local assemblages of nectar‐feeding birds and bats and their relationships with food plants to determine the extent to which evolutionary convergence has or has not occurred in the New and Old World tropics. Location Our review is pantropical in extent and also includes the subtropics of South Africa and eastern Australia. Within the tropics, it deals mostly with lowland forest habitats. Methods An extensive literature review was conducted to compile data bases on the regional and local species richness of nectar‐feeding birds and bats, pollinator sizes, morphology, and diets. Coefficients of variation (CVs) were used to quantify the morphospace occupied by the various families of pollinators. The extent to which plants have become evolutionarily specialized for vertebrate pollination was explored using several criteria: number and diversity of growth forms of plant families providing food for all the considered pollinator families; the most common flower morphologies visited by all the considered pollinator families; and the number of plant families that contain genera with both bird‐ and bat‐specialized species. Results Vertebrate pollinator assemblages in the New World tropics differ from those in the Old World in terms of their greater species richness, the greater morphological diversity of their most specialized taxa, and the greater degree of taxonomic and ecological diversity and morphological specialization of their food plants. Within the Old World tropics, Africa contains more specialized nectar‐feeding birds than Asia and Australasia; Old World nectar‐feeding bats are everywhere less specialized than their New World counterparts. Main conclusions We propose that two factors – phylogenetic history and spatio‐temporal predictability (STP) of flower resources – largely account for hemispheric and regional differences in the structure of vertebrate pollinator assemblages. Greater resource diversity and resource STP in the New World have favoured the radiation of small, hovering nectar‐feeding birds and bats into a variety of relatively specialized feeding niches. In contrast, reduced resource diversity and STP in aseasonal parts of Asia as well as in Australasia have favoured the evolution of larger, non‐hovering birds and bats with relatively generalized feeding niches. Tropical Africa more closely resembles the Neotropics than Southeast Asia and Australasia in terms of resource STP and in the niche structure of its nectar‐feeding birds but not its flower‐visiting bats.     

Chronosequential change in a butterfly community after clear-cutting of deciduous forests in a cool temperate region of central Japan

Transect counts of butterflies were conducted in the northern part of Ibaraki, central Japan, from 1997 to 2001 at 11 census sites, composed of successive stages of deciduous forest development: grassland (one site, early abandoned stage); cutover land (one site, 1–5 years after clear‐cutting); secondary forests (very young (two sites, 6–9 years), middle (two sites, 16–22 years) and old (two sites, 47–51 years)) and old‐growth natural forests (three sites, ≥124 years old). A total of 86 species and 8858 individual butterflies were recorded by 29 sets (406 times) of transect counts. The species richness (number of species), abundance (number of individuals) and diversity indices (Shannon–Wiener H′ and Simpson's 1–λ) of butterflies were high in the early stages (grassland, cutover land and very young secondary forests) of secondary succession. Typical natural forest species increased with the progress of succession, and the old secondary forests and old‐growth natural forests had similar species composition. In contrast, most of the typical natural grassland species were recorded only in the grassland site. In the cutover land site, the number of individuals of grassland species considerably decreased from the first to the second year; furthermore, only one typical natural grassland species was recorded. Thus, the suitable stage for grassland butterfly species lasts for only 1–2 years after clear‐cutting. Old secondary forests (approximately>50 years old) would be able to give refuge to the forest butterfly species, including typical natural forest species. Based on the results, a practical, forestry‐based plan to conserve butterfly diversity was proposed.     

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition. 2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River. 3. Patterns of species richness varied with temporal scale of analysis, that is between single‐year and multi‐year time frames. At the annual timescale, quadrat species richness (m^?2) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single‐year pattern, the highest long‐term richness occurred at intermittent sites. 4. Quadrat species richness, total species richness at a site (per 18 1‐m² plots) and cover were more variable year to year at non‐perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter‐annual compositional variance, while the perennial sites had the lowest. 5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types. 6. High long‐term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non‐perennial sites.     

Plant species richness in ephemeral and perennial reaches of a dryland river

Ephemeral reaches are common along desert rivers but are less well studied than those with perennial stream flow. This study contrasted riparian plant species richness and composition (extant vegetation and soil seed bank) between stream reaches with different low-flow conditions (perennial vs. ephemeral flow) but similar flood patterns and similar watershed-derived species pools. Data were collected at Cienega Creek (Arizona, USA) over a 2 year period spanning drought conditions and wetter conditions. Consistent with expectations relating to water limitation effects on diversity, species richness in the riparian zone was lower at ephemeral-flow sites during a season with minimal precipitation and no overbank flooding; under these conditions, the more permanent water sources of the perennial-flow sites sustain the larger number of species. During seasons with greater precipitation and elevated stream flows, in contrast, species richness at ephemeral-flow sites increased to levels at or slightly above those of perennial-flow sites. For values pooled across two wet seasons of a calendar year, year-round richness was greater at the two ephemeral-flow sites (total of 92 vascular plant species) than at the two perennial-flow sites (68 species). This greater year-round richness was a combination of multiple factors: greater light, space, and bare ground, a diverse soil seed bank (with the seed banks equally species-rich among hydrologic types), and moderately abundant precipitation and flooding sufficient to stimulate establishment of opportunistic species (mainly annuals) during the bimodal wet seasons. These results indicate that long-term patterns of site water availability, by influencing woody plant cover, mediate the diversity response to episodic water pulses in dryland rivers. The results also have implications for riparian conservation efforts, which to date have focused primarily on perennial stream reaches: ephemeral reaches of spatially intermittent rivers harbor many riparian plant species, and warrant conservation efforts, as well.     

Ants as Indicators of Restoration Success at a Uranium Mine in Tropical Australia

The composition and structure of ant communities were used to assess the success of the preliminary restoration program at Ranger uranium mine in the seasonal tropics of northern Australia. Ants were surveyed at eight sites, including two relatively undisturbed control sits, within the Ranger lease. The revegetated sites represented a range of variables likely to influence restoration success: revegetation age (two, four, and eight years), proximity to undisturbed sites (which act as potential sources of recolonization), and burning treatment. Revegetation at most sites was dominated by fast-growing species of Acacia. There was a clear succession of ant species across revegetated sites. Initial colonization was by species of Iridomyrmex, but as plant cover and litter development increased these were replaced by broadly adapted, opportunist species, especially the introduced Paratrechina longicornis. Ant recolonization was very slow at isolated sites, with only 12 species present after eight years (the oldest site available). This compares with 21 species after only four years at a site located close to potential sources of recolonization. The ant community at this site, however, was very similar to that at another site located close to colonization sources, but eight years old. Ant succession therefore appeared to have stalled at this point, with species richness and composition bearing little resemblance to that at control sites. The heavy shade and litter produced by acacias were considered to be the major impediment to further change. Results from a site that had undergone a prescribed burn after two years, thereby breaking dominance by acacias and allowing for the establishment of a wide variety of plant taxa, suggest that such management practices may promote further colonization by ant species.     

Influence of late season fire on early successional vegetation of an Oklahoma prairie

Abstract. The study of vegetation dynamics in tallgrass prairie in response to fire has focused on dormant season fire in late successional prairies. Our objective was to determine if late season fire of varying frequency results in divergent successional patterns in an early successional tallgrass prairie disturbed by grazing and cultivation. Specifically, we evaluated the influence of late‐summer fires of varying frequency on community composition and species richness. We collected vegetation and environmental data on two sites burned in the late growing‐season at varying frequencies. These communities differed in composition depending primarily on edaphic factors, time since the last burn, and year‐to‐year variation. We interpret the time effect as related to changes in species composition accompanying plant succession that followed disturbance either from cropping and heavy grazing on the loamy site or heavy grazing on the shallow site. Other unidentified factors also have a role in vegetation dynamics on this prairie. Community composition and species richness were not consistently responsive to frequency of growing‐season fires.     

Global patterns in peatmoss biodiversity

DNA sequence data from the nuclear ribosomal internal transcribed spacers (ITS) and the trnL-trnF chloroplast DNA regions were used to quantify geographical partitioning of global biodiversity in peatmosses (Sphagnum), and to compare patterns of molecular diversity with patterns of species richness. Molecular diversity was estimated for boreal, tropical, Neotropical, nonboreal (tropical plus Southern Hemisphere), Old World and New World partitions, based on a total of 436 accessions. Diversity was partitioned among geographical regions in terms of combined nuclear and chloroplast sequence data and separately for the ITS and trnL-trnF data sets. Levels of variation were estimated using phylogenetic diversity (PD), which incorporates branch lengths from a phylogenetic tree, and the number of polymorphic nucleotide sites. Estimates of species richness suggest that peatmoss diversity is higher in New World than Old World regions, and that the Neotropics constitute a "hotspot" of diversity. Molecular estimates, in contrast, indicate that peatmoss biodiversity is almost evenly divided between New and Old World regions, and that the Neotropics account for only 20-35% of global peatmoss diversity. In general, levels of tropical and boreal peatmoss molecular diversity were comparable. Two species, S. sericeum from the Old World tropics and S. lapazense from Bolivia, are remarkably divergent in nucleotide sequences from all other Sphagna and together account for almost 20% of all peatmoss diversity, although they are represented by only three of the 436 accessions (0.7%). These species clearly demonstrate the nonequivalence of species biodiversity value.     

Secondary succession is influenced by belowground insect herbivory on a productive site

We investigated the effects of insect herbivory on a plant community of a productive old-field community by applying foliar and soil insecticides in a full factorial design. During the first 3 years of succession, insecticide treatments had only minor effects on total cover abundance and species richness. However, species ranking within the plant community was strongly affected by soil insecticide but not by foliar insecticide. Creeping thistle, Cirsium arvense , dominated the experimental plots with reduced root herbivory, while square-stemmed willow-herb, Epilobium adnatum , dominated the control and the plots with foliar insecticide. When soil insecticide was applied, cover abundance of monocarpic forbs increased and cover abundance of polycarpic herbs decreased compared to the control. However, this effect was due to a few abundant plant species and is not based on a consistent difference between life history groups. Instead, application of soil insecticide promoted persistence of species that established at the start of succession, and suppressed species that established in the following years. We conclude that below-ground herbivory reduces competitive ability of resident species and, thus, facilitates colonization by late-successional species. Hence, soil insects can exert strong top-down effects on the vegetation of productive sites by affecting dominant plant species and altering competitive balances.     

Long‐term vegetation changes in experimentally grazed and ungrazed back‐barrier marshes in the Wadden Sea

Abstract. Vegetation succession in three back‐barrier salt marshes in the Wadden Sea was studied using a data set comprising 25 years of vegetation development recorded at permanent quadrats. The effect of livestock grazing on succession was assessed by comparing quadrats where grazing was experimentally prevented or imposed. We studied changes at the species level as well as at the level of the plant community. Special attention is given to effects on plant species richness and community characteristics that are relevant for lagomorphs (hares and rabbits) and geese. Inundation frequency and grazing were most important in explaining the variation in species abundance data. The three marshes studied overlap in the occurrence of different plant communities and the observed patterns were consistent between them. Clear differences in frequency and abundance of plant species were observed related to grazing. Most plant species had a greater incidence in grazed treatments. Species richness increased with elevation, and was 1.5 to 2 × higher in the grazed salt marsh. Grazing negatively influenced Atriplex portulacoides and Elymus athericus, whereas Puccinellia maritima and Festuca rubra showed a positive response. The communities dominated by Elymus athericus, Artemisia maritima and Atriplex portulacoides were restricted to the ungrazed marsh. Communities dominated by Puccinellia maritima, Juncus gerardi and Festuca rubra predominantly occurred at grazed sites. As small vertebrate herbivores prefer these plants and communities for foraging, livestock grazing thus facilitates for them.     

Climatic niche evolution in Smilacaceae (Liliales) drives patterns of species diversification and richness between the Old and New World

Geographical variation in species richness in plant groups is determined by the interplay between historical, evolutionary, and ecological processes. However, the processes underlying the striking disparity in species richness between Asia and the Americas remain poorly understood. Here, we synthesize global phylogenetic and macroecological data on the diversification of Smilacaceae, deciphering potential drivers underlying the species diversity pattern biased toward Asia. We compiled global distributions of all Smilacaceae species, and reconstructed the biogeographic history and niche evolution using a new time-calibrated phylogeny (eight genes, 135 species). Integrating these data sets, we estimated evolutionary histories and diversification rates for each region, and tested correlations among species diversification, niche evolution, and niche divergence. Smilacaceae probably originated during the Late Cretaceous/Early Palaeocene and began to diversify in middle to low latitudes in Central America and Eurasia during the Late Eocene. Both the Old and New World clades exhibited a steady, albeit slight, increase of species diversification from the Late Eocene to Early Miocene. However, the Old World clade experienced an abrupt increase in net diversification during the Late Miocene. Our findings also revealed that species diversification rates were positively correlated with ecological niche evolution and niche divergence. Niche shifts and climatic niche evolution since the Middle Miocene played crucial roles in species diversification dynamics within Smilacaceae. The high plant richness in Asia may be explained by greater diversification in this region, potentially promoted by heterogeneous environments.     

Rate of succession in restored wetlands and the role of site context

Question: Are changes in plant species composition, functional group composition and rates of species turnover consistent among early successional wetlands, and what is the role of landscape context in determining the rate of succession? Location: Twenty‐four restored wetlands in Illinois, USA. Methods: We use 4 years of vegetation sampling data from each site to describe successional trends and rates of species turnover in wetlands. We quantify: (1) the rate at which composition changes from early‐successional to late‐successional species and functional groups, as indicated by site movement in ordination space over time, and (2) the rate of change in the colonization and local extinction of individual species. We correlate the pace of succession to site area, isolation and surrounding land cover. Results: Some commonalities in successional trends were evident among sites. Annual species were replaced by clonal perennials, and colonization rates declined over time. However, differences among sites outweighed site age in determining species composition, and the pace of succession was influenced by a site's landscape setting. Rates of species turnover were higher in smaller wetlands. In addition, wetlands in agricultural landscapes underwent succession more rapidly, as indicated by a rapid increase in dominance by late‐successional plants. Conclusions: Although the outcome of plant community succession in restored wetlands was somewhat predictable, species composition and the pace of succession varied among sites. The ability of restoration practitioners to accelerate succession through active manipulation may be contingent upon landscape context.     

Coexistence of arbuscular mycorrhizae and dark septate endophytic fungi in an undisturbed and a disturbed site of an arid ecosystem

Effect of disturbance on root colonization and vertical distribution of arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) was investigated at two adjacent sites of Lal Suhanra Biosphere Reserve, Pakistan. Disturbance clearly affected AMF and DSE colonization, vertical distribution of AMF and plant community structure. Mean colonization of AMF and DSE was slightly less at the disturbed site. Average spore densities, diversity and richness of AMF and DSE were higher at the undisturbed site. A study of the vertical distribution of AMF associated with the five plant species most common to each study site indicated that beside AMF and DSE colonization disturbance may affect AMF species composition. Correlation of AMF with DSE is also discussed.     

Plant succession in areas of scorched and blown-down forest after the 1980 eruption of Mount St. Helens,Washington

Abstract. Patterns of plant succession were studied in areas of scorched and blown-down forest resulting from the 1980 eruption of Mount St. Helens, Washington. Changes in species abundance were observed for 7 years in permanent sample plots representing four post-disturbance habitats, or site types. Total plant cover and species richness increased with time on all site types. In blown-down forests supporting snowpack at the time of eruption, understory recovery was dominated by the vegetative regeneration of species persisting through disturbance. In forests without snowpacks, plant survival was poorer. Increases in cover and diversity were dominated first by introduced grasses, then by colonizing forbs characteristic of early successional sites. Epilo-bium angustifolium and Anaphalis margaritacea showed widespread recruitment and clonal expansion throughout the devastated area. As a result, species composition on previously forested sites converged toward that on formerly clearcut sites, where early serai forbs resprouted vigorously from beneath the tephra. Total plant cover and species diversity were poorly correlated with post-disturbance habitat and general site characteristics (e.g. distance from the crater, elevation, slope, and aspect). However, distributions of several life-forms (e.g. low sub-shrubs and tall shrubs) were strongly correlated with depth of burial by tephra and with cover of tree rootwads. Thus, early community recovery may reflect microsite variation or chance survival and recruitment rather than broad-scale gradients in environment or disturbance. Recovery of pre-disturbance composition and structure will undoubtedly be much slower than after other types of catastrophic disturbance. The rate and direction of community recovery will largely depend on the degree to which original understory species survived the eruption.     

Variation in Species and Trophic Composition of Insect Communities in Puerto Rico1

Insects are important participants in many ecosystem processes, but the effects of anthropogenic and natural disturbances on insect communities have been poorly studied. To describe how disturbances affect insect communities, we addressed two questions: Do insect communities return to a pre‐hurricane composition? And how do insect communities change during succession? To answer these questions, we studied insect communities in a chronosequence of two abandoned pastures (5 yr and 32 yr) and a mature forest (>80 yr) that were recently disturbed by two hurricanes (Hurricane Hugo, 1989) Hurricane Georges, 1998). Although insect abundance and richness fluctuated during the study, all sites returned to pre‐hurricane (Hurricane Georges) abundance and richness in less than one year. All trophic categories present before Hurricane Georges were present after the hurricane, but richness within categories fluctuated greatly. Insect richness did not increase during succession; the 5 yr site had the highest richness, the >80 yr site had an intermediate richness, and die 32 yr site the lowest. Nevertheless, the species composition of the two forested sites was different in comparison to the 5 yr site. These results suggest that trophic structure varies little in time and space, but the species composition within each trophic category is highly variable.     

Experimental analysis of patch dynamics and community heterogeneity in tallgrass prairie

Effects of fire and small-scale soil disturbances on species richness, community heterogeneity, and microsuccession were investigated in a central Oklahoma tallgrass prairie. In the fall of 1985, 0.2 m² soil disturbances were created on burned and unburned tallgrass prairie. Vegetation on and off disturbances was sampled at monthly intervals over two growing seasons. During the first growing season, the cover of forbs and annuals, and species richness were significantly greater on versus off disturbances, but these differences did not persist through the second year. The variation in species composition among disturbed plots (heterogeneity) was significantly greater compared to undisturbed areas throughout the study. Fire had no consistent effect on richness and heterogeneity of vegetation on soil disturbances but fire reduced heterogeneity on undisturbed vegetation. Rate of succession, based on an increase in cumulative cover of perennial grasses over time, did not differ among treatments during the first growing season. During the second year, rate of succession was significantly greater on burned soil disturbances compared to unburned soil disturbances. These results suggest that while small-scale soil disturbances have primarily short-lived effects on grassland community structure, disturbances do help to maintain spatial and temporal variation in tallgrass prairie communities. Unlike in undisturbed vegetation, however, species richness and heterogeneity on soil disturbances were little effected by fire, but the rate of colonization onto disturbances appeared to be enhanced by fire.