José Sarukhán

Science for managing ecosystem services: Beyond the Millennium Ecosystem Assessment

The Millennium Ecosystem Assessment (MA) introduced a new framework for analyzing social–ecological systems that has had wide influence in the policy and scientific communities. Studies after the MA are taking up new challenges in the basic science needed to assess, project, and manage flows of ecosystem services and effects on human well-being. Yet, our ability to draw general conclusions remains limited by focus on discipline-bound sectors of the full social–ecological system. At the same time, some polices and practices intended to improve ecosystem services and human well-being are based on untested assumptions and sparse information. The people who are affected and those who provide resources are increasingly asking for evidence that interventions improve ecosystem services and human well-being. New research is needed that considers the full ensemble of processes and feedbacks, for a range of biophysical and social systems, to better understand and manage the dynamics of the relationship between humans and the ecosystems on which they rely. Such research will expand the capacity to address fundamental questions about complex social–ecological systems while evaluating assumptions of policies and practices intended to advance human well-being through improved ecosystem services.

Treefall age determination and gap dynamics in a tropical forest

(1) Most individuals of Astrocaryum mexicanum, a monopodial neotropical understorey palm, endure treefalls that form gaps in the forest, by bending under falling trees and limbs. After one year, a bent palm recovers vertical growth at its terminal meristem and forms a permanent kink in its stem. Previous detailed demographic studies have allowed us to determine with accuracy the passage of time, based on the age-constant rate of stem elongation of the palm. (2) Using this morphological feature of the palm, together with its high density (3001230 mature individuals ha-1) in a 5-ha tract of rainforest at Los Tuxtlas, Veracruz, Mx more than 50% of the quadrats suffered disturbance in the last thirty years and 28% suffered more than one disturbance in the last seventy years. These results provide evidence that canopy disturbances capable of promoting the release of suppressed seedlings and saplings of forest trees may occur at the small scale of some tens of square metres. (4) The long-term treefall patterns analysed (up to seventy years) show that canopy disturbances are a permanent ecological factor in the rainforest environment. Annual rainfall explains more than 50% of the annual variation of the proportion of forest opened to gaps per year. (5) The yearly disturbance does not indicate the gap availability for species regeneration: for example, a year of high disturbance (e.g. 6 1 % of the forest opened to gaps) may have a similar number of sizeable gaps suitable for pioneer regeneration as a year of low disturbance (e.g. 1 5% of the forest opened to gaps). (6) The results obtained in this study show that the gap-formation process operating at Los Tuxtlas forest promotes a strong temporally and spatially random variation in the physical environment of plants. This heterogeneity may be one of the factors involved in determining the high biological diversity found in most tropical rainforests.

Effects of experimental defoliation on growth, reproduction and survival of Astrocaryum mexicanum

(1) The effects of three levels of defoliation at four stages of development on leaf production, reproduction probability, fruit production and survival of an understorey palm, Astrocaryum mexicanum, were studied from 1978 to 1981. (2) Three portions of the crown were factorized with the partial defoliation treatments. (3) Completely defoliated juvenile and immature palms produced significantly fewer leaves than control plants. In mature palms, the removal of the oldest portions of the crown increased leaf production by 30%. (4) Complete defoliation and the removal of two-thirds of the oldest leaves had the effect of reducing leaf abscission in all stages, except for seedlings. (5) Defoliation had a significant effect on reproduction during all years, except 1978. Completely defoliated palms had a lower probability of reproduction and produced no fruits for three years. The removal of a small proportion of the oldest tissue significantly increased fruit production. Defoliations greater than 33% of the total number of leaves reduced the reproductive capacity of palms, except when the oldest tissue was removed. (6) The survival of totally defoliated juvenile and immature individuals was 50% less than that of undefoliated plants. The survival of mature individuals was not significanly affected by the defoliation.

Within- and among-year variation in the levels of herbivory on the foliage of trees from a Mexican tropical deciduous forest.

Discrete (i.e., one-time) measurements of the levels of herbivore damage on the foliage of trees (16 species) were made for three consecutive years in the Chamela dry forest. There was a marked similarity in the overall levels of damage despite the fact that there were considerable climatic differences among years and that some of the species showed at least one significant among-year difference. In one of the three years of study, long-term measurements of damage (i.e., on individually marked leaves) were also made to assess within-year variation (by calculating the rates of herbivory early and late in the wet season) and to assess the reliability of the discrete measurements. With this type of measurement the mean leaf area loss of the studied species was 17 percent. This value is higher than the values commonly reported for tropical forests. An outstanding example of defoliation was that of Jatropha standleyi, with 73 percent leaf area eaten. There was a very marked within-season variation in the rates of herbivory: during the early part of the rainy season the mean rate (0.352% leaf area eaten per day) was considerably higher than late in the season (0.097). Analyses of leaf nitrogen and water content early and late in the season indicated that these components of the nutritional quality of the foliage decrease as the season progresses. The discrete measurements of herbivory underestimated the true (long-term) values of leaf damage. A discrepancy ratio long-term/discrete measurement varied considerably among species with an overall mean of 1.99. Thus, in general, discrete measurements underestimated herbivory by half. It appears that the major cause of the underestimates of the discrete measurements is that the leaves of some species are eaten completely and thus missed in a one-time measurement. Another disadvantage of the discrete measurements is that information on the timing of damage is lost. This study showed that folivory is not only concentrated to the season of leaf availability, but that it occurs within an even more restricted period.

Phosphorus cycling in a Mexican tropical dry forest ecosystem

The study was conducted in five contiguous small watersheds (12–28 ha) gauged for long-term ecosystem research. Five 80 × 30 m plots were used for the study. We quantified inputs from the atmosphere, dissolved and particulate-bound losses, throughfall and litterfall fluxes, standing crop litter and soil available P pools. Mean P input and output for a six-year period was 0.16 and 0.06 kg⋅ha−1⋅yr−1, respectively. Phosphorus concentration increased as rainfall moved through the canopy. Annual P returns in litterfall (3.88 kg/ha) represented more than 90% of the total aboveground nutrient return to the forest floor. Phosphorus concentration in standing litter (0.08%) was lower than that in litterfall (0.11%). Phosphorus content in the litterfall was higher at Chamela than at other tropical dry forests. Mean residence time on the forest floor was 1.2 yr for P and 1.3 yr for organic matter. Together these results suggest that the forest at Chamela may not be limited by P availability and suggest a balance between P immobilization and uptake. Comparison of P losses in stream water with input rates from the atmosphere for the six-year period showed that inputs were higher than outputs. Balances calculated for a wet and a dry year indicated a small P accumulation in both years.

Net primary productivity of a tropical deciduous forest ecosystem in western Mexico

Despite their geographical and cultural importance, tropical deciduous forests are among the least known and most endangered tropical ecosystems in the world (Janzen 1988; Martinez-Yrizar, in press; Vogt et al. 1986). Net primary productivity (NPP) of tropical terrestrial ecosystems has been estimated mostly as a gross approximation from information on some component of the total, such as leaf litter (Murphy 1975). Bray & Gorham (1964) found that annual leaf fall represents about one-third of total annual NPP and suggested that NPP of forests could be estimated by multiplying annual leaf litter production by a factor of 3.3. Based on a larger data set on tropical forests, Brown & Lugo (1982) found that the ratio of leaf litter production on total NPP varies, depending upon the life zone, and suggested that estimates of NPP based on leaf litter should be revised. Also, without a knowledge of other functional parameters of the ecosystem, the use of leaf fall as an index of NPP should be taken with caution, since losses due to natural mortality and predation by herbivores are usually not considered, and allocation patterns differ among biomes. Improved measurements of NPP of tropical deciduous forests have included data on annual litterfall and forest biomass increments. For example, at the

REPRODUCTIVE BEHAVIOUR AND ITS INDIVIDUAL VARIABILITY IN A TROPICAL PALM, ASTROCARYUM MEXICANUM

SUMMARY (1) The reproductive characteristics of a tropical understorey palm, Astrocaryum mexicanum, and the causes of individual variability were observed for 414 adults on six plots during 5 yr. (2) The probability of reproduction is the most plastic of the reproductive characteristics analysed, since it varied between plots, age and years. (3) Individuals appear to respond to an environment that is heterogeneous in time and space since reproductive events were not randomly distributed among them. This probably accounts for the unsynchronized supra-annual seeding behaviour. (4) The reproductive output was correlated with both the spatial location and the leaf area of individual palms which suggests that light may play an important role in controlling seed output. (5) Light gaps and seed predation appear to be the most important selective pressures acting on this species.

Direct and indirect estimates of neighborhood and effective population size in a tropical palm, Astrocaryum mexicanum

To estimate the relative importance of genetic drift, the effective population size ∗∗∗(Ne) can be used. Here we present estimates of the effective population size and related measures in Astrocaryum mexicanum, a tropical palm from Los Tuxtlas rain forest, Veracruz, Mexico. Seed and pollen dispersal were measured. Seeds are primarily dispersed by gravity and secondarily dispersed by small mammals. Mean primary and secondary dispersal distances for seeds were found to be small (0.78 m and 2.35 m, respectively). A. mexicanum is beetle pollinated and pollen movements were measured by different methods: a) using fluorescent dyes, b) as the minimum distance between active female and male inflorescences, and c) using rare allozyme alleles as genetic markers. All three estimates of pollen dispersal were similar, with a mean of approximately 20 m. Using the seed and pollen dispersal data, the genetic neighborhood area (A) was estimated to be 2,551 m2. To obtain the effective population size, three different overlapping generation methods were used to estimate an effective density with demographic data from six permanent plots. The effective density ranged from 0.040 to 0.351 individuals per m2. The product of effective density and neighborhood area yields a direct estimate of the neighborhood effective population size (Nb). Nb ranged from 102 to 895 individuals. Indirect estimates of population size and migration rate (Nm) were obtained using Fst for five different allozymic loci for both adults and seeds. We obtained a range of Nm from 1.2 to 19.7 in adults and a range of Nm from 4.0 to 82.6 for seeds. We discuss possible causes of the smaller indirect estimates of Nm relative to the direct and compare our estimates with values from other plant populations. Gene dispersal distances, neighborhood size, and effective population size in A. mexicanum are relatively high, suggesting that natural selection, rather than genetic drift, may play a dominant role in patterning the genetic variation in this tropical palm.

The Costs of Reproduction in a Tropical Palm, Astrocaryum Mexicanum

(1) A combined demographic and energetic approach was used to try to elucidate the possible cost associated with reproduction in the tropical palm, Astrocaryum mexicanum. (2) Survival and reproduction were recorded for about one thousand palms over a 3-yr period. (3) Dry matter allocation calculations were carried out on the annual growth rates of 140 individuals for 3 yr and from harvesting fifty palms of a wide range of ages. (4) There was an increase in reproductive effort with age and an inverse correlation between various measures of reproductive cost and fecundity. (5) The 3-60% variation in reproductive effort for different ages, years and plots seems to be mainly controlled by the probability of reproduction rather than the amount of reproductive structures produced. (6) The physiological reproductive effort appears to be constant over a wide range of environments and ages.

The Importance of Opuntia in Mexico and Routes of Invasion and Impact of Cactoblastis cactorum (Lepidoptera: Pyralidae)

The appearance of the cactus moth Cactoblastis cactorum in Florida has roused concern over its possible effects on the Opuntia-rich areas of Mexico and the southwestern United States. In this paper we discuss the economic importance of Opuntia in Mexico and propose a method to predict the invasion of the alien species C. cactorum. In Mexico, the products derived from Opuntia are mainly human food and fodder for livestock. Both cultivated and wild populations of Opuntia are currently used for these two purposes. By using bioclimatic modeling, we predicted the potential distribution of C. cactorum and overlaid this on the actual distribution of Opuntia species. The resulting maps indicate that the possible routes of invasion to Mexico are 1) along the northern border through Texas (most likely) and 2) via southeastern Mexico (less likely). The impacts of an invasion of C. cactorum on Opuntia products could be significant as well as being a threat to endemic species. Bioclimatic modeling can help to predict the areas of highest probability of attack and facilitate planning to mitigate future impacts.