Rodolfo Dirzo

The causes of land-use and land-cover change: moving beyond the myths

Common understanding of the causes of land-use and land-cover change is dominated by simplifications which, in turn, underlie many environment-development policies. This article tracks some of the major myths on driving forces of land-cover change and proposes alternative pathways of change that are better supported by case study evidence. Cases reviewed support the conclusion that neither population nor poverty alone constitute the sole and major underlying causes of land-cover change worldwide. Rather, peoples’ responses to economic opportunities, as mediated by institutional factors, drive land-cover changes. Opportunities and

Defaunation in the Anthropocene

We live amid a global wave of anthropogenically driven biodiversity loss: species and population extirpations and, critically, declines in local species abundance. Particularly, human impacts on animal biodiversity are an under-recognized form of global environmental change. Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25% average decline in abundance. Invertebrate patterns are equally dire: 67% of monitored populations show 45% mean abundance decline. Such animal declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this “Anthropocene defaunation”; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet’s sixth mass extinction and also a major driver of global ecological change.

Deforestation of seasonally dry tropical forest: a national and local analysis in Mexico

Abstract Seasonally dry tropical forests in the neotropics reach their northernmost distribution in Mexico. This vegetation type has both a high diversity and endemism, yet information about its conservation situation is scarce. This study analyzes the loss of this forest at the national level, comparing its potential coverage with that of the early 1990s; and at the local, using a time-series of the potential vegetation and coverage in 1973 and 1989 in the state of Morelos (central Mexico). At the national level we found that only 27% of the original cover remained as intact forest by 1990. At the local level, close to 60% of the original vegetation has been lost, and only 19% remains in a forested condition. These remnant forests are restricted to areas with steep slopes. An annual deforestation rate of 1.4% was calculated and remaining areas are heavily fragmented and somewhat disturbed. If the trends detected continue, these remaining forests will be heavily reduced and degraded in the near future. Urgent measures to promote their conservation are required.

Collapse of the world's largest herbivores.

The collapsing populations of large herbivores will have extensive ecological and social consequences. Large wild herbivores are crucial to ecosystems and human societies. We highlight the 74 largest terrestrial herbivore species on Earth (body mass ≥100 kg), the threats they face, their important and often overlooked ecosystem effects, and the conservation efforts needed to save them and their predators from extinction. Large herbivores are generally facing dramatic population declines and range contractions, such that ~60% are threatened with extinction. Nearly all threatened species are in developing countries, where major threats include hunting, land-use change, and resource depression by livestock. Loss of large herbivores can have cascading effects on other species including large carnivores, scavengers, mesoherbivores, small mammals, and ecological processes involving vegetation, hydrology, nutrient cycling, and fire regimes. The rate of large herbivore decline suggests that ever-larger swaths of the world will soon lack many of the vital ecological services these animals provide, resulting in enormous ecological and social costs.

CARBON EMISSIONS FROM MEXICAN FORESTS: CURRENT SITUATION AND LONG-TERM SCENARIOS

Estimates of carbon emissions from the forest sector in Mexico are derived for the year 1985 and for two contrasting scenarios in 2025. The analysis covers both tropical and temperate closed forests. In the mid-1980s, approximately 804,000 ha/year of closed forests suffered major perturbations, of which 668,000 ha was deforestation. Seventy-five percent of total deforestation is concentrated in tropical forests. The resulting annual carbon balance from land-use change is estimated at 67.0 × 106 tons/year, which lead to net emissions of 52.3 × 106 tons/year accounting for the carbon uptake in restoration plantations and degraded forest lands. This last figure represents approximately 40% of the countrys estimated annual total carbon emissions for 1985–1987. The annual carbon balance from the forest sector in 2025 is expected to decline to 28.0 × 106 t in the reference scenario and to become negative (i.e., a carbon sink), 62.0 × 106 t in the policy scenario. A number of policy changes are identified that would help achieve the carbon sequestration potential identified in this last scenario.

Effects of life history, domestication and agronomic selection on plant defence against insects: Evidence from maizes and wild relatives

Plant domestication and agronomic selection for increased yield may have an associated effect of reducing plant defence against herbivorous insects. This hypothesis is based on evidence for a metabolic cost associated with defence, and on evidence that increases in yield generally come from the re-partitioning of photoassimilates rather than from fundamental increases in photosynthetic rates. We propose that for plants in which domestication and crop development constitute strong selection for increased growth and reproduction, reallocation of resources may result in lower defence against insects. We examine this hypothesis by means of comparative studies of growth, reproduction and resistance in a complex of maizes and closely related wild taxa, the teosintes. The results of these studies are consistent with assumptions of differential investment in growth and reproduction between wild and domesticated plants. A wild perennial grew slowest and had lowest grain production, while a modern cultivar grew fastest and had the highest grain yield. A wild annual and a land-race cultivar were intermediate. Damage from a diverse assemblage of folivorous insects, and from a specialist stemboring lepidopteran larva, fit the defence predictions closely. A gradient of attack levels suggests that the wild perennial is most defended, followed in descending order by the wild annual, the land-race cultivar and the modern high-yielding variety. Alternative hypotheses for this pattern are consistent with some, but not all, of our data.

Analysis of a hyper‐diverse seed dispersal network: modularity and underlying mechanisms

Mutualistic interactions involving pollination and ant-plant mutualistic networks typically feature tightly linked species grouped in modules. However, such modularity is infrequent in seed dispersal networks, presumably because research on those networks predominantly includes a single taxonomic animal group (e.g. birds). Herein, for the first time, we examine the pattern of interaction in a network that includes multiple taxonomic groups of seed dispersers, and the mechanisms underlying modularity. We found that the network was nested and modular, with five distinguishable modules. Our examination of the mechanisms underlying such modularity showed that plant and animal trait values were associated with specific modules but phylogenetic effect was limited. Thus, the pattern of interaction in this network is only partially explained by shared evolutionary history. We conclude that the observed modularity emerged by a combination of phylogenetic history and trait convergence of phylogenetically unrelated species, shaped by interactions with particular types of dispersal agents.

EXPERIMENTAL STUDIES ON SLUG-PLANT INTERACTIONS IV. THE PERFORMANCE OF CYANOGENIC AND ACYANOGENIC MORPHS OF TRIFOLIUM REPENS IN THE FIELD

SUMMARY (1) The performance of cyanogenic and acyanogenic morphs of Trifolium repens (white clover) was investigated by means of a field experiment in which cuttings of both morphs were transplanted into a permanent grassland and observed for one year. (2) During the growing season (spring and summer), slightly more acyanogenic plants survived, and their rate of leaf and stolon production was slightly greater than that of cyanogenic plants. These differences were not apparent during the rest of the year. (3) Observations of leaf scars left by several herbivores showed that only molluscs consistently preferred the acyanogenic plants. Weevils appeared not to be deterred by cyanogenesis and sheep appeared to change their preference from one morph to the other at different times of the year. (4) There was damage by pathogens to the leaves of both morphs, but infection by the systemic rust Uromyces trifolii appeared almost exclusively on cyanogenic plants. This association of cyanogenesis and rust susceptibility was very marked in the field in which the studies were concentrated but was absent from another sampled field in the neighbourhood. (5) During the winter, frost damage caused tissue necrosis and death of leaflets; recordings for the early part of the winter showed that frost damage was significantly greater on the cyanogenic leaves. (6) Few of the transplanted cuttings flowered. Observations on plants growing naturally in the field showed that proportionately fewer cyanogenic plants flowered than acyanogenic ones. (7) The host of interacting factors observed suggest that mollusc grazing is only one of the several forces affecting the cyanogenesis polymorphism in white clover. The events responsible for this polymorphism seem to be more complex than can be accounted for simply by mollusc grazing or by any other single factor (for example, winter temperature). The ecological differences observed between the morphs of the single species Trifolium repens may be as significant as are those commonly found between different species growing together in the same area.

Biodiversity and ecosystem processes in tropical forests

1 Introduction.- References.- 2 Plant Species Diversity and Ecosystem Functioning in Tropical Forests.- 2.1 Introduction.- 2.2 The Dependence of Ecosystem Processes on Species Diversity.- 2.3 Plant Species Richness in Tropical Forests.- 2.4 The Primary Productivity of Tropical Forests.- 2.5 The Stability of Tropical Forests.- 2.6 Conclusions.- References.- 3 Consumer Diversity and Secondary Production.- 3.1 Introduction.- 3.2 Secondary Production and Biodiversity.- 3.3 Evolutionary Effects of Consumers on Ecosystem Properties.- 3.4 Ecological Effects of Consumers on Ecosystem Properties.- 3.4.1 Influence of Consumers on Plant Productivity.- 3.4.2 Influences of Consumers on Plant Diversity.- 3.5 Conclusion.- References.- 4 Biodiversity and Biogeochemical Cycles.- 4.1 Introduction.- 4.1.1 Definitions and Concepts.- 4.2 Species Richness and Biogeochemical Cycling.- 4.3 Functional Diversity and Biogeochemistry.- 4.3.1 The Atmospheric-Terrestrial Interface.- 4.3.2 The Biotic Interface.- 4.3.3 The Plant-Soil Interface.- 4.3.4 The Terrestrial-Hydrologic Interface.- 4.4 Evidence from Experimental Studies.- 4.4.1 Plantations versus Natural Forests.- 4.4.2 Experimental Manipulation of Species Composition.- 4.5 Conclusions.- References.- 5 Microbial Diversity and Tropical Forest Functioning.- 5.1 Introduction.- 5.2 The Knowledge Base.- 5.3 Food Chains.- 5.4 Pathogens.- 5.4.1 Control of Herbivores by Pathogens.- 5.4.2 Pathogens as a Source of Distribution.- 5.4.3 Effect of Pathogens on Patterns of Tree Dispersion.- 5.5 Microbial Contributions to Global Biogeochemistry.- 5.5.1 Atmospheric CO2.- 5.5.2 Methane.- 5.5.3 Nitrous Oxide.- 5.5.4 Rock Weathering.- 5. 6 Nutrient Cycling.- 5.6.1 Litter Decomposition and Soil Fertility.- 5.6.2 Symbiotic Nitrogen Fixation Associated with Plant Roots.- 5.6.3 Effects of Microbial Epiphylls and Epiphytes on Nutrient Fluxes.- 5.6.4 Mycorrhizae and Nutrient Uptake.- 5.7 Plant Endophytes.- 5.8 Threats to the Microbiota and the Processes They Mediate.- 5.8.1 Effects of Forest Fragmentation on Plant Symbioses.- 5.8.2 Effects of Forest Fragmentation on Cord-Forming Fungi.- 5.8.3 Effects of Acid Precipitation on Ectomycorrhizae.- 5.8.4 Effects of Air Pollution and Climate Change on Epiphyte Nitrogen Fixation.- 5.8.5 Are Decomposers Redundant in a Heterogeneous Environment?.- 5.9 Conclusions.- References.- 6 Plant Life-Forms and Tropical Ecosystem Functioning.- 6.1 Introduction.- 6.1.1 Functional Significance of Life-Forms.- 6.1.2 Assessing the Consequences of Life-Form Diversity.- 6.2 Classification.- 6.2.1 Stature.- 6.2.2 Longevity.- 6.3 Biogeographical Patterns.- 6.4 Environmental Correlates of Life-Form Diversity.- 6.4.1 Rainfall.- 6.4.2 Altitude.- 6.4.3 Soil Fertility.- 6.5 Episodic Impacts on Life-form Diversity.- 6.5.1 Wind.- 6.5.2 Fire.- 6.5.3 Animals.- 6.5.4 Climate change.- 6.6 Life-Forms and Succession.- 6. 7 Implications of Loss of Life-Forms.- 6.8 Conclusions.- References.- 7 Functional Group Diversity and Recovery from Disturbance.- 7.1 Introduction.- 7.2 Functional Groups Affecting Tropical Forest Dynamics.- 7.2.1 Pioneer Herbs and Shrubs.- 7.2.2 Large-Leaved Understory Herbs and Shrubs.- 7.2.3 Small-Leaved Understory Herbs and Shrubs.- 7.2.4 Pioneer Trees.- 7.2.5 Understory Treelets.- 7.2.6 Emergent and Canopy Trees.- 7.2.7 Canopy Palms.- 7.2.8 Canopy Legumes.- 7.2.9 Vines and Lianas.- 7.2.10 Epiphytes.- 7.2.11 Seed Dispersers.- 7.2.12 Herbivorous Insects and Pathogens.- 7.2.13 Decomposers.- 7.2.14 Mycorrhizal Fungi.- 7.2.15 Soil-Churning Animals.- 7.3 Functional Groups and Natural Disturbance Processes in Tropical Moist Forests.- 7.4 Anthropogenic Disturbances to Tropical Forests.- 7.4.1 Functional Groups Affect Successional Patterns.- 7.4.2 Causes of Depauperate Regeneration Pools.- 7.5 Functional Groups in Tropical Dry Forests.- 7.6 Redundancy within Functional Groups.- 7.7 Conclusions.- References.- 8 Species Richness and Resistance to Invasion.- 8.1 Diversity vs. Stability.- 8.2 Global Patterns.- 8.3 Intentional Introductions.- 8.4 Invasions into Undisturbed Tropical Forests.- 8.5 Speculations.- References.- 9 The Role of Biodiversity in Tropical Managed Ecosystems.- 9.1 Introduction.- 9.2 Examples of Tropical Managed Ecosystems.- 9.2.1 Managed Forests.- 9.2.2 Home Gardens.- 9.2.3 Swidden Agriculture.- 9.2.4 Intensive Annual and Perennial Crops.- 9.2.5 Traditional Rice Systems.- 9.3 Plant Diversity and Primary Productivity.- 9.3.1 Comparisons Between Natural and Managed Ecosystems.- 9.3.2 Productivity of Diverse Cropping Systems.- 9.3.3 Stability of Diverse Cropping Systems.- 9.4 Plant Diversity and Primary Consumers.- 9.5 Plant Diversity and Secondary Consumers.- 9.5.1 Ants in Diverse Cropping Systems.- 9.6 Conclusions.- References.- 10 Synthesis.- 10.1 Introduction.- 10.2 Environmental Gradients.- 10.2.1 Moisture.- 10.2.2 Fertility.- 10.2.3 Elevation.- 10.3 Biodiversity and Functioning of Tropical Forests.- 10.4 Energy Flow.- 10.4.1 Carbon Allocation and Consumption.- 10.4.2 Animal-Animal Interactions.- 10.4.3 Detritus-Detritivores.- 10.5 Materials Processing.- 10.5.1 Atmosphere-Organism.- 10.5.2 Biotic Interface.- 10.5.3 Plant-Soil.- 10.5.4 Atmosphere-Soil.- 10.5.5 Soil-Water Table.- 10.6 Functional Properties over Longer Temporal Scales.- 10.6.1 Provision and Maintenance of Structure.- 10.6.2 Resistance to Invasions.- 10.7 Functional Properties Over Larger Spatial Scales.- 10.7.1 Movement of Materials by Physical Agents.- 10.7.2 Movement of Materials and Energy by Animals.- 10.8 Biodiversity and Responses to Disturbances.- 10.9 Research Agenda.- 10.10 Conclusions.- References.- Species Index.- Topical Index.

Deforestation in Lacandonia (southeast Mexico): evidence for the declaration of the northernmost tropical hot-spot

To assess the conservation status of Lacandonia, a megadiversity area in Mexico, rates of deforestation were calculated for the periods 1974–1981 and 1981–1991, using a random sample of 38 5 × 5 km sites. We evaluated: (i) the overall magnitude of, and spatial and temporal variation in deforestation; (ii) how spatial variation relates to human population density, terrain slope and the presence of the Montes Azules Biosphere Reserve; (iii) the magnitude of potential plant species loss associated with deforestation. Overall deforestation was greater in the former than in the second period (1412  vs. 744 ha/year), although mean rates (2.1 and 1.6%/year) were statistically indistinguishable due to a considerable spatial variation. The greatest spatial variation was related to the presence of the Montes Azules Reserve: deforestation outside the reserve was 20 and 6 times greater in the first and second period, respectively. Population density and terrain slope were related to deforestation but the relationship was considerably poor. Estimates of plant species committed to extinction (out of the expected total flora of 4314 species) were as high as 22% by year 2035, and 55% by year 2135. Such levels of potential species extinction associated to deforestation, and the great biological diversity of Lacandonia provide evidence to declare it as the northernmost tropical hot-spot and a priority goal in conservation efforts.