Howard V. Cornell

Energy, water, and broad-scale geographic patterns of species richness

It is often claimed that we do not understand the forces driving the global diversity gradient. However, an extensive literature suggests that contemporary climate constrains terrestrial taxonomic richness over broad geographic extents. Here, we review the empirical literature to examine the nature and form of the relationship between climate and richness. Our goals were to document the support for the climatically based energy hypothesis, and within the constraints imposed by correlative analyses, to evaluate two versions of the hypothesis: the productivity and ambient energy hypotheses. Focusing on studies extending over 800 km, we found that measures of energy, water, or water-energy balance explain spatial variation in richness better than other climatic and non-climatic variables in 82 of 85 cases. Even when considered individually and in isolation, water/ energy variables explain on average over 60% of the variation in the richness of a wide range of plant and animal groups. Further, water variables usually represent the strongest predictors in the tropics, subtropics, and warm temperate zones, whereas energy variables (for animals) or water-energy variables (for plants) dominate in high latitudes. We conclude that the interaction between water and energy, either directly or indirectly (via plant productivity), provides a strong explanation for globally extensive plant and animal diversity gradients, but for animals there also is a latitudinal shift in the relative importance of ambient energy vs. water moving from the poles to the equator. Although contemporary climate is not the only factor influencing species richness and may not explain the diversity pattern for all taxonomic groups, it is clear that understanding water-energy dynamics is critical to future biodiversity research. Analyses that do not include water-energy variables are missing a key component for explaining broad-scale patterns of diversity.

Navigating the multiple meanings of β diversity: a roadmap for the practicing ecologist.

A recent increase in studies of β diversity has yielded a confusing array of concepts, measures and methods. Here, we provide a roadmap of the most widely used and ecologically relevant approaches for analysis through a series of mission statements. We distinguish two types of β diversity: directional turnover along a gradient vs. non-directional variation. Different measures emphasize different properties of ecological data. Such properties include the degree of emphasis on presence/absence vs. relative abundance information and the inclusion vs. exclusion of joint absences. Judicious use of multiple measures in concert can uncover the underlying nature of patterns in β diversity for a given dataset. A case study of Indonesian coral assemblages shows the utility of a multi-faceted approach. We advocate careful consideration of relevant questions, matched by appropriate analyses. The rigorous application of null models will also help to reveal potential processes driving observed patterns in β diversity.

Niche conservatism as an emerging principle in ecology and conservation biology.

The diversity of life is ultimately generated by evolution, and much attention has focused on the rapid evolution of ecological traits. Yet, the tendency for many ecological traits to instead remain similar over time [niche conservatism (NC)] has many consequences for the fundamental patterns and processes studied in ecology and conservation biology. Here, we describe the mounting evidence for the importance of NC to major topics in ecology (e.g. species richness, ecosystem function) and conservation (e.g. climate change, invasive species). We also review other areas where it may be important but has generally been overlooked, in both ecology (e.g. food webs, disease ecology, mutualistic interactions) and conservation (e.g. habitat modification). We summarize methods for testing for NC, and suggest that a commonly used and advocated method (involving a test for phylogenetic signal) is potentially problematic, and describe alternative approaches. We suggest that considering NC: (1) focuses attention on the within-species processes that cause traits to be conserved over time, (2) emphasizes connections between questions and research areas that are not obviously related (e.g. invasives, global warming, tropical richness), and (3) suggests new areas for research (e.g. why are some clades largely nocturnal? why do related species share diseases?).

Disentangling the Drivers of β Diversity Along Latitudinal and Elevational Gradients

Increases in species turnover of woody plants at low latitudes and elevations are explained by the size of species pools alone. Understanding spatial variation in biodiversity along environmental gradients is a central theme in ecology. Differences in species compositional turnover among sites (β diversity) occurring along gradients are often used to infer variation in the processes structuring communities. Here, we show that sampling alone predicts changes in β diversity caused simply by changes in the sizes of species pools. For example, forest inventories sampled along latitudinal and elevational gradients show the well-documented pattern that β diversity is higher in the tropics and at low elevations. However, after correcting for variation in pooled species richness (γ diversity), these differences in β diversity disappear. Therefore, there is no need to invoke differences in the mechanisms of community assembly in temperate versus tropical systems to explain these global-scale patterns of β diversity.

PREDATORS, PARASITOIDS, AND PATHOGENS AS MORTALITY AGENTS IN PHYTOPHAGOUS INSECT POPULATIONS

We compiled life tables for 78 holometabolous herbivorous insect species to quantify levels of apparent enemy-induced mortality of immature insects. Enemies were classified by type (predator, parasitoid, or pathogen), and mortalities caused by each type in each herbivore immature stage were tested with Analysis of Deviance for differences associated with four ecological characteristics of preadult herbivores: feeding biology, invasion status, and the cultivation status and latitudinal zone of the habitat. Total enemy- induced mortality is higher in the late developmental stages, and overall, parasitoids kill more herbivores than do either predators or pathogens. Among the ecological variables, both feeding biology and latitude showed significant enemy effects in at least one late developmental stage, whereas neither cultivation status nor invasion status was associated with enemy-induced mortality in any stage. Bonferroni adjustment of probabilities for multiple comparisons resulted in few significant interactions between enemy type and the ecological variables. However, raw probabilities and comparisons across herbivore im- mature stages suggest several patterns that deserve attention in future studies: (1) endophytic herbivores suffer lower mortality by predators and pathogens than exophytics, and endo- phytic leaf miners suffer the greatest parasitoid-induced mortality, while endophytic gallers/ borers/root feeders suffer the least; (2) overall enemy-induced mortality is similar in natural and cultivated habitats; (3) exotic insects do not suffer lower enemy-induced mortality rates than natives; and (4) predation and disease may be greater in tropical/subtropical habitats, whereas parasitism is greater in the temperate zone. These results identify several general patterns in insect demographics that should be useful for hypothesis testing.

Theoretical Approaches to Biological Control

Preface Part I. Biological Control Theory: Past and Present: 1. The theoretical foundations of biological control Alan A. Berryman 2. Recent developments in theory for biological control of insect pests by parasitoids Cheryl J. Briggs, William W. Murdoch and Roger M. Nisbet 3. Biological control models: a field guide Nigel D. Barlow Part II. Ecological Considerations: 4. The uniformity and density of pest exploitation as guides to success in biological control Michael E. Hochberg, and Robert D. Holt 5. Biological control of insect pests: a tritrophic perspective Nick J. Mills and Andrew P. Gutierrez 6. The case for generalists in biological control Gary C. Chang and Peter Kareiva 7. Why is the parasitoid Encarsia formosa so successful in controlling whiteflies Joop C. van Lenteren and Herman W. J. van Roermund 8. Parasitoid adult nutritional ecology: implications for biological control Mark A. Jervis and Neil A. C. Kidd 9. Coexistence of multiple attractors and its consequences for a three-species food chain Liebe F. Cavalieri and Huseyin Kocak Part III. Spatial Considerations: 10. Dynamics of spatially structured spider mite populations Sandra J. Walde and Gosta Nachman 11. Habitat fragmentation and biological control Teja Tscharntke and Andreas Kruess 12. Outbreaks of insects: a dynamic approach Alan Hastings Part IV. Genetic/Evolutionary Considerations: 13. Population dynamics and the evolutionary stability of biological control Robert D. Holt, Michael E. Hochberg and Michael Barfield 14. Genetic conflict in natural enemies: a review, and consequences for the biological control of arthropods Martha S. Hunter 15. Overexploitation and mutualism in plant - herbivore - predator interactions: their evolution and impact on population dynamics Maurice W. Sabelis, Minus van Baalen, Jan Bruin, Martijn Egas, Vincent A. A. Jansen, Arne Janssen and Bas Pels 16. A Darwinian view of host selection and its practical implications Robert F. Luck, and Leonard Nunney Part V. Microbes and Pathogens: 17. The dynamics of insect - pathogen interactions H. C. J. Godfray and Cheryl Briggs 18. Host - pathogen - parasitoid systems Michael Begon, Steven M. Sait and David J. Thompson 19. Persistence of natural enemies of weeds and insect pests in heterogeneous environments David W. Onstad and Edward A. Kornkven 20. Application of insect - pathogen models to biological control Matthew B. Thomas, Simon N. Wood and Veronica Soloranzo 21. Dose - response relationships in biocontrol of plant disease and their use to define pathogen refuge size Kenneth B. Johnson Index.

Accumulation of Native Parasitoid Species on Introduced Herbivores: A Comparison of Hosts as Natives and Hosts as Invaders

Herbivore species newly introduced into foreign locations (hosts as invaders) are often attacked by native parasitoid species. Here we compare the structure and diversity of 87 such parasitoid complexes with those on the same herbivore species in their native regions (hosts as natives). Overall parasitoid attack rates are generally lower on hosts as invaders than on hosts as natives. Also, parasitoid complexes on hosts as invaders are generally less rich and contain a higher proportion of generalists than those on hosts as natives. Overall richness shows a weak tendency to increase with duration in the region of introduction over the first 150 yr, but the ratio of generalists to specialists does not change over this time period. These results, in part, parallel those for herbivore complexes on introduced host plants and suggest that common theoretical principles may apply to both trophic levels. The herbivores were also categorized by level of concealment and taxon (order) to determine whether life-style or phylogeny influenced parasitoid richness in native or foreign locations. No strong influences emerged. Our most novel result is a vulnerability-to-parasitism regression; the numbers of parasitoids attacking host species in invaded regions are correlated with the numbers in native regions. The biological characteristics of the herbivore as well as extrinsic region-specific factors may play important roles in setting parasitoid richness levels on hosts as natives and on hosts as invaders.

Toward a better understanding of the regional causes of local community richness

Despite widespread acknowledgement that local ecological communities are profoundly shaped by regional-scale influences, including evolutionary and biogeographic processes, this perspective has yet to be widely incorporated into ecological research. Drawing on recent research, we propose four steps towards making regional influences a stronger part of research on the richness of local communities: (1) identifying the regional-scale causes of variation in species richness in the systems ecologists study; (2) testing for effects of regional richness on local richness, using improved observational and experimental analyses to overcome earlier problems; (3) simultaneously analysing environmental influences on regional and local species richness as well as the influence of regional richness on local richness and (4) considering the potential reciprocal effects of local processes on regional richness. In conclusion, we suggest some ways that similar approaches could be applied to other aspects of community structure beyond species richness.

Herbivore responses to plant secondary compounds: a test of phytochemical coevolution theory.

Literature data were collected on the floristic distribution and toxicity of phytochemicals to herbivores and on herbivore specialization in order to test phytochemical coevolution theory. The theory makes four predictions that can be tested with this information. Herbivores can adapt to novel, more toxic chemicals by becoming specialists, or they can become generalists but at the cost of lower feeding success on any particular host. Thus, the first two predictions are as follows: herbivores should do better on chemicals that are present in their normal host, and this pattern should be stronger for specialists than for generalists. The “escape and radiation” aspect of the theory holds that if a plant taxon with a novel defense chemical diversifies, the chemical will become widespread. Eventually, herbivores will adapt to and disarm it. So the third prediction is that more widespread chemicals are less toxic than more narrowly distributed ones. Because generalists should not do as well as specialists on chemicals disarmed by the latter, the fourth prediction is that the third prediction should be more true for generalists than specialists and should depend on presence/absence of the chemical in the normal host. Multiple regressions of toxicity (herbivore mortality and final weight) on three predictor variables (chemical presence/absence in the normal host, specialism, and chemical floristic distribution) and relevant interactions were used to test these predictions. Chemical presence/absence in the normal host, the interaction between this variable and specialism, and chemical floristic distribution had significant effects on both measures of toxicity, supporting the first three predictions of the model. Support for the fourth prediction (a three‐way interaction among all predictor variables) was evident for final weight but not mortality, perhaps because growth is more responsive to toxicity differences than survival. In short, the phytochemistry literature provides broad support for the phytochemical coevolution model.

SURVIVAL PATTERNS AND MORTALITY SOURCES OF HERBIVOROUS INSECTS: SOME DEMOGRAPHIC TRENDS

Five hundred thirty life tables for 124 holometabolous, herbivorous insects were compiled from the literature and studied to uncover demographic patterns. Survival and sources of mortality of the preadult stages were correlated with their ecological setting, and several patterns emerged. Exophytic herbivores suffer a 5%-10% greater risk of preadult mortality than endophytics but are also nearly twice as large (pupal length, 11.8 mm vs. 6.4 mm; adult body length, 13.5 mm vs. 5.5 mm) and twice as fecund (206.1 vs. 99.3 eggs per female). Attack by natural enemies is the most frequent mortality source for immature herbivores (frequency = 48%). Plant factors are not nearly as frequent (9%) but have probably been underestimated in past life tables. The influence of plant factors is greatest in the early stages, and the influence of enemies is greatest in the later stages of development. Plant factors kill endophytic species more frequently than exophytic species (frequencies = 15% vs. 2%), whereas enemies kill exophytic species more frequently than endophytic species (frequencies = 51% vs. 44%). Most of the above patterns were consistent with conventional wisdom; the following were not. The frequency of competition and enemies did not vary with latitude or successional status of the habitat. Also, successional status, cultivation, and invasion status of the herbivore had no direct effects on herbivore survival. All of the patterns documented here should be viewed as guidelines for hypothesis testing with the understanding that much unexplained variation in survival exists among herbivorous insect species.