Stuart L. Pimm

ON THE RISK OF EXTINCTION

Well-known theoretical predictions are that the risk of extinction should decrease with maximum population size (K) and should increase with the temporal coefficient of variation in population size (CV). In an unvarying environment, where extinction is caused solely by demographic accidents, the risk of extinction should decrease steeply with K; the greater the contribution of environmental variability to the risk of extinction, the less steep should be the dependence on K. Large-bodied species tend to have long lifetimes but low rates of increase, which have opposite effects on the risk of extinction per year. We show that in comparisons of a large- and small-bodied species at the same average population size (N), the large-bodied species should be at less risk at low N but at greater risk at high N. We test these predictions using a data base of short-term survivals (up to a few decades) of 355 populations belonging to 100 species of British land birds on 16 islands. The mean N and risk of extinction are known for these populations, and we can calculate CVs for 39 of the species. To identify how factors other than N affect the risk of extinction, we devise a means of correcting that risk for much of the effect of N. We make the following observations. (1) Risk of extinction does decrease sharply with N. (2) After correcting for much of the effect of N, we confirm the theoretical prediction that the relative susceptibility to extinction of large- and small-bodied species reverses with increasing population size. Above seven pairs, larger-bodied species are at greater risk than smaller-bodied species; the reverse is true below seven pairs. (3) Migratory species are at greater risk of extinction than resident species. (4) Finally, after accounting for the effects of N, body size, and migratory status, we show that the risk of extinction does increase with the CV.

Biodiversity: Extinction by numbers

Habitat destruction, especially of the humid forests in the tropics, is the main cause of the species extinctions happening now. New work documents the uneven, highly clumped distribution of vulnerable species on the Earth, and pinpoints 25 so-called ‘biodiversity hotspots’. Seventeen of them are tropical forest areas, and here reduction of natural habitat is disproportionately high. Nonetheless, identification of this pattern should enable resources for conservation to be better focused.

The biodiversity of species and their rates of extinction, distribution, and protection

Background A principal function of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) is to “perform regular and timely assessments of knowledge on biodiversity.” In December 2013, its second plenary session approved a program to begin a global assessment in 2015. The Convention on Biological Diversity (CBD) and five other biodiversity-related conventions have adopted IPBES as their science-policy interface, so these assessments will be important in evaluating progress toward the CBD’s Aichi Targets of the Strategic Plan for Biodiversity 2011–2020. As a contribution toward such assessment, we review the biodiversity of eukaryote species and their extinction rates, distributions, and protection. We document what we know, how it likely differs from what we do not, and how these differences affect biodiversity statistics. Interestingly, several targets explicitly mention “known species”—a strong, if implicit, statement of incomplete knowledge. We start by asking how many species are known and how many remain undescribed. We then consider by how much human actions inflate extinction rates. Much depends on where species are, because different biomes contain different numbers of species of different susceptibilities. Biomes also suffer different levels of damage and have unequal levels of protection. How extinction rates will change depends on how and where threats expand and whether greater protection counters them. Different visualizations of species biodiversity. (A) The distributions of 9927 bird species. (B) The 4964 species with smaller than the median geographical range size

Ecological networks and their fragility

Darwin used the metaphor of a ‘tangled bank’ to describe the complex interactions between species. Those interactions are varied: they can be antagonistic ones involving predation, herbivory and parasitism, or mutualistic ones, such as those involving the pollination of flowers by insects. Moreover, the metaphor hints that the interactions may be complex to the point of being impossible to understand. All interactions can be visualized as ecological networks, in which species are linked together, either directly or indirectly through intermediate species. Ecological networks, although complex, have well defined patterns that both illuminate the ecological mechanisms underlying them and promise a better understanding of the relationship between complexity and ecological stability.

Body sizes of animal predators and animal prey in food webs

Summary 1. We measured the body sizes (weights or lengths) of animal species found in the food webs of natural communities. In c. 90% of the feeding links among the animal species with known sizes, a larger predator consumes a smaller prey. 2. Larger predators eat prey with a wider range of body sizes than do smaller predators. The geometric mean predator size increases with the size of prey. The increase in geometric mean predator size is less than proportional to the increase in prey size (i.e. has a slope less than 1 on log-log coordinates). 3. The geometric mean sizes of prey and predators increase as the habitat of webs changes from aquatic to terrestrial to coastal to marine. Within each type of habitat, mean prey sizes are always less than mean predator sizes, and prey and predator sizes are always positively correlated. 4. Feeding relations order the metabolic types of organisms from invertebrate to vertebrate ectotherm to vertebrate endotherm. Organisms commonly eat other organisms with the same or lower metabolic type, but (with very rare exceptions) organisms do not eat other organisms with a higher metabolic type. Mean sizes of prey increase as the metabolic type of prey changes from invertebrate to vertebrate ectotherm to vertebrate endotherm, but the same does not hold true for predators. 5. Prey and predator sizes are positively correlated in links from invertebrate prey to invertebrate predators. In links with other combinations of prey and predator metabolic types, the correlation between prey and predator body sizes is rarely large when it is positive, and in some cases is even negative. 6. Species sizes are roughly log-normally distributed. 7. Body size offers a good (though not perfect) interpretation of the ordering of animal species assumed in the cascade model, a stochastic model of food web structure. When body size is taken as the physical interpretation of the ordering assumed in the cascade model, and when the body sizes of different animal species are taken as log-normally distributed, many of the empirical findings can be explained in terms of the cascade model.

On the protection of "protected areas".

Tropical moist forests contain the majority of terrestrial species. Human actions destroy between 1 and 2 million km2 of such forests per decade, with concomitant carbon release into the atmosphere. Within these forests, protected areas are the principle defense against forest loss and species extinctions. Four regions—the Amazon, Congo, South American Atlantic Coast, and West Africa—once constituted about half the worlds tropical moist forest. We measure forest cover at progressively larger distances inside and outside of protected areas within these four regions, using datasets on protected areas and land-cover. We find important geographical differences. In the Amazon and Congo, protected areas are generally large and retain high levels of forest cover, as do their surroundings. These areas are protected de facto by being inaccessible and will likely remain protected if they continue to be so. Deciding whether they are also protected de jure—that is, whether effective laws also protect them—is statistically difficult, for there are few controls. In contrast, protected areas in the Atlantic Coast forest and West Africa show sharp boundaries in forest cover at their edges. This effective protection of forest cover is partially offset by their very small size: little area is deep inside protected area boundaries. Lands outside protected areas in the Atlantic Coast forest are unusually fragmented. Finally, we ask whether global databases on protected areas are biased toward highly protected areas and ignore “paper parks.” Analysis of a Brazilian database does not support this presumption.

Rates of species loss from Amazonian forest fragments

In the face of worldwide habitat fragmentation, managers need to devise a time frame for action. We ask how fast do understory bird species disappear from experimentally isolated plots in the Biological Dynamics of Forest Fragments Project, central Amazon, Brazil. Our data consist of mist-net records obtained over a period of 13 years in 11 sites of 1, 10, and 100 hectares. The numbers of captures per species per unit time, analyzed under different simplifying assumptions, reveal a set of species-loss curves. From those declining numbers, we derive a scaling rule for the time it takes to lose half the species in a fragment as a function of its area. A 10-fold decrease in the rate of species loss requires a 1,000-fold increase in area. Fragments of 100 hectares lose one half of their species in <15 years, too short a time for implementing conservation measures.

Global patterns of terrestrial vertebrate diversity and conservation

Significance Identifying priority areas for biodiversity is essential for directing conservation resources. We mapped global priority areas using the latest data on mammals, amphibians, and birds at a scale 100 times finer than previous assessments. Priority areas have a higher—but still insufficient—rate of protection than the global average. We identify several important areas currently ignored by biodiversity hotspots, the current leading priority map. As the window of opportunity for expanding the global protected area network begins to close, identifying priorities at a scale practical for local action ensures our findings will help protect biodiversity most effectively. Identifying priority areas for biodiversity is essential for directing conservation resources. Fundamentally, we must know where individual species live, which ones are vulnerable, where human actions threaten them, and their levels of protection. As conservation knowledge and threats change, we must reevaluate priorities. We mapped priority areas for vertebrates using newly updated data on >21,000 species of mammals, amphibians, and birds. For each taxon, we identified centers of richness for all species, small-ranged species, and threatened species listed with the International Union for the Conservation of Nature. Importantly, all analyses were at a spatial grain of 10 × 10 km, 100 times finer than previous assessments. This fine scale is a significant methodological improvement, because it brings mapping to scales comparable with regional decisions on where to place protected areas. We also mapped recent species discoveries, because they suggest where as-yet-unknown species might be living. To assess the protection of the priority areas, we calculated the percentage of priority areas within protected areas using the latest data from the World Database of Protected Areas, providing a snapshot of how well the planet’s protected area system encompasses vertebrate biodiversity. Although the priority areas do have more protection than the global average, the level of protection still is insufficient given the importance of these areas for preventing vertebrate extinctions. We also found substantial differences between our identified vertebrate priorities and the leading map of global conservation priorities, the biodiversity hotspots. Our findings suggest a need to reassess the global allocation of conservation resources to reflect today’s improved knowledge of biodiversity and conservation.

Community assembly and food web stability

Abstract The ecological assembly of food web is considered as a process of predator colonizations and extinctions. The results of computer simulations using predator-prey equations allow us to identify three types of food web stability and examine how they may change through development of food webs. Species turnover stability increases, stability to extensive species extinction remains constant, and local stability to population fluctuations decreases as food web assembly proceeds.

What we know and don't know about Earth's missing biodiversity.

Estimates of non-microbial diversity on Earth range from 2 million to over 50 million species, with great uncertainties in numbers of insects, fungi, nematodes, and deep-sea organisms. We summarize estimates for major taxa, the methods used to obtain them, and prospects for further discoveries. Major challenges include frequent synonymy, the difficulty of discriminating certain species by morphology alone, and the fact that many undiscovered species are small, difficult to find, or have small geographic ranges. Cryptic species could be numerous in some taxa. Novel techniques, such as DNA barcoding, new databases, and crowd-sourcing, could greatly accelerate the rate of species discovery. Such advances are timely. Most missing species probably live in biodiversity hotspots, where habitat destruction is rife, and so current estimates of extinction rates from known species are too low.