■ Abstract The literature on effects of habitat fragmentation on biodiversity is huge. It is also very diverse, with different authors measuring fragmentation in different ways and, as a consequence, drawing different conclusions regarding both the magnitude and direction of its effects. Habitat fragmentation is usually defined as a landscape-scale process involving both habitat loss and the breaking apart of habitat. Results of empirical studies of habitat fragmentation are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking apart of habitat after controlling for habitat loss. Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. Therefore, to correctly interpret the influence of habitat fragmentation on biodiversity, the effects of these two components of fragmentation must be measured independently. More studies of the independent effects of habitat loss and fragmentation per se are needed to determine the factors that lead to positive versus negative effects of fragmentation per se. I suggest that the term “fragmentation” should be reserved for the breaking apart of habitat, independent of habitat loss.

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Effects of Habitat Fragmentation on Biodiversity

Human alteration of the global environment has triggered the sixth major extinction event in the history of life and caused widespread changes in the global distribution of organisms. These changes in biodiversity alter ecosystem processes and change the resilience of ecosystems to environmental change. This has profound consequences for services that humans derive from ecosystems. The large ecological and societal consequences of changing biodiversity should be minimized to preserve options for future solutions to global environmental problems.

Consequences of changing biodiversity

Although ecologists commonly talk about the impacts of nonindigenous species, little formal attention has been given to defining what we mean by impact, or connecting ecological theory with particular measures of impact. The resulting lack of generalizations regarding invasion impacts is more than an academic problem; we need to be able to distinguish invaders with minor effects from those with large effects in order to prioritize management efforts. This paper focuses on defining, evaluating, and comparing a variety of measures of impact drawn from empirical examples and theoretical reasoning. We begin by arguing that the total impact of an invader includes three fundamental dimensions: range, abundance, and the per-capita or per-biomass effect of the invader. Then we summarize previous approaches to measuring impact at different organizational levels, and suggest some new approaches. Reviewing mathematical models of impact, we argue that theoretical studies using community assembly models could act as a basis for better empirical studies and monitoring programs, as well as provide a clearer understanding of the relationship among different types of impact. We then discuss some of the particular challenges that come from the need to prioritize invasive species in a management or policy context. We end with recommendations about how the field of invasion biology might proceed in order to build a general framework for understanding and predicting impacts. In particular, we advocate studies designed to explore the correlations among different measures: Are the results of complex multivariate methods adequately captured by simple composite metrics such as species richness? How well are impacts on native populations correlated with impacts on ecosystem functions? Are there useful bioindicators for invasion impacts? To what extent does the impact of an invasive species depend on the system in which it is measured? Three approaches would provide new insights in this line of inquiry: (1) studies that measure impacts at multiple scales and multiple levels of organization, (2) studies that synthesize currently available data on different response variables, and (3) models designed to guide empirical work and explore generalities.

Impact: Toward a Framework for Understanding the Ecological Effects of Invaders

▪ Abstract Deer have expanded their range and increased dramatically in abundance worldwide in recent decades. They inflict major economic losses in forestry, agriculture, and transportation and contribute to the transmission of several animal and human diseases. Their impact on natural ecosystems is also dramatic but less quantified. By foraging selectively, deer affect the growth and survival of many herb, shrub, and tree species, modifying patterns of relative abundance and vegetation dynamics. Cascading effects on other species extend to insects, birds, and other mammals. In forests, sustained overbrowsing reduces plant cover and diversity, alters nutrient and carbon cycling, and redirects succession to shift future overstory composition. Many of these simplified alternative states appear to be stable and difficult to reverse. Given the influence of deer on other organisms and natural processes, ecologists should actively participate in efforts to understand, monitor, and reduce the impact of deer on ...

https://www.kora.ch/malme/05_library/5_1_publications/C/Cote_et_al_2004_Ecological_impacts_of_deer_overabundance.pdf

Ecological Impacts of Deer Overabundance

In a dynamic world, mechanisms allowing prediction of future situations can provide a selective advantage. We suggest that memory systems differ in the degree of flexibility they offer for anticipatory behavior and put forward a corresponding taxonomy of prospection. The adaptive advantage of any memory system can only lie in what it contributes for future survival. The most flexible is episodic memory, which we suggest is part of a more general faculty of mental time travel that allows us not only to go back in time, but also to foresee, plan, and shape virtually any specific future event. We review comparative studies and find that, in spite of increased research in the area, there is as yet no convincing evidence for mental time travel in nonhuman animals. We submit that mental time travel is not an encapsulated cognitive system, but instead comprises several subsidiary mechanisms. A theater metaphor serves as an analogy for the kind of mechanisms required for effective mental time travel. We propose that future research should consider these mechanisms in addition to direct evidence of future-directed action. We maintain that the emergence of mental time travel in evolution was a crucial step towards our current success.

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The evolution of foresight: What is mental time travel, and is it unique to humans?

In eastern U.S. oak forests, defoliation by gypsy moths and the risk of Lyme disease are determined by interactions among acorns, white-footed mice, moths, deer, and ticks. Experimental removal of mice, which eat moth pupae, demonstrated that moth outbreaks are caused by reductions in mouse density that occur when there are no acorns. Experimental acorn addition increased mouse density. Acorn addition also increased densities of black-legged ticks, evidently by attracting deer, which are key tick hosts. Mice are primarily responsible for infecting ticks with the Lyme disease agent. The results have important implications for predicting and managing forest health and human health.

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Chain reactions linking acorns to gypsy moth outbreaks and Lyme disease risk.

popular characterization of ecological systems is that everything is connected to everything else. Although in some metaphysical sense it may be true that influences experienced by a particular species are transmitted to all other species in the ecosystem, such pervasive connections have not been demonstrated scientifically. Instead, the focus of community ecology has been on pairwise interactions between species. The paradigm has been that the determinants of community structure can be elucidated by studies of direct effects of populations on one or a few other populations (Kareiva 1994). To identify the role of predation or competition in structuring ecological communities, typically a predator or competitor is removed, and the responses of prey or other competitors are monitored (Connell 1983, Menge and Sutherland 1976, Schoener 1983, Sih et al. 1985). These studies reveal the importance of direct effects in ecological communities. However, such experiments often produce unexpected results, In oak forests, both

Of Mice and Mast Ecological connections in eastern deciduous forests

I used a 14-year dataset on dynamics of mast-consuming rodents from an eastern deciduous forest to determine the relationship between population fluctuations of white-footed mice ( Peromyscus leucopus ), deer mice ( P. maniculatus ), and eastern chipmunks ( Tamias stria-tus ) and production of acorn mast. Mast (acorn) production was episodic with four excellent crops produced in 14 years. Densities of rodents in summer ranged from 3 to 103 animals/ha and correlated positively with production of mast the previous autumn (all r 2 > 0.56). During years of high production of mast, stores of acorn lasted throughout winter, whereas in most years, acorns were gone by January. During years of high production of mast, mice bred all winter, which resulted in high densities the following summer. Episodic production of mast and resulting fluctuations in consumers of mast have implications for the predator-satiation hypothesis and other community processes.

Population Fluctuations of Mast-Eating Rodents Are Correlated with Production of Acorns

Multi-male mating (MMM) by females is relatively common among mammals, occurring in at least 133 species and several evolutionary benefits of MMM have been proposed. The most convincing explanation is that MMM confuses paternity, thereby deterring infanticide by males. A second explanation for females that are unlikely to experience infanticide is that MMM is a consequence of sexual harassment. Mate guarding and, perhaps even in some cases, behavioral monogamy, might have evolved in response to the threat of infanticide and the subsequent tendency for females to mate multiply. Benefits relating to improved genetic fitness of offspring do occur in some species, but do not provide a widespread explanation for the evolutionary origin of MMM; if cryptic female choice through sperm competition is adaptive to females it probably evolved as a consequence of, rather than a precursor to, female promiscuity. Here, we provide support for the original hypothesis of paternity confusion for MMM, rather than for the more popular good genes or sperm competition hypotheses.

Promiscuous females protect their offspring

I question the widely accepted view that female small mammals are territorial to defend food, and suggest that theoretical and empirical evidence are more compatible with a pup-defense hypothesis to protect young from infanticide. The fact that females are territorial during the time of greatest food abundance and not during food limitation (such as winter) contradicts a food-defense hypothesis. Energy conservation through huddling does not appear sufficient to explain shared use of space during nonbreeding seasons. Aggression and territorial defense are most intense during lactation and are directed toward other females, those most likely to commit infanticide, and not toward males and other food competitors

Jerry O. Wolff

Papers

Chain reactions linking acorns to gypsy moth outbreaks and Lyme disease risk.

Of Mice and Mast Ecological connections in eastern deciduous forests

Population Fluctuations of Mast-Eating Rodents Are Correlated with Production of Acorns

Promiscuous females protect their offspring

Why are female small mammals territorial