The Effects of Density Dependence and Immigration on Local Adaptation and Niche Evolution in a Black-Hole Sink Environment

Abstract. We consider the evolution of ecological specialization in a landscape with two discrete habitat types connected by migration, for example, a plant-insect system with two plant hosts. Using a quantitative genetic approach, we study the joint evolution of a quantitative character determining performance in each habitat together with the changes in the population density. We find that specialization on a single habitat evolves with intermediate migration rates, whereas a generalist species evolves with both very low and very large rates of movement between habitats. There is a threshold at which a small increase in the connectivity of the two habitats will result in dramatic decrease in the total population size and the nearly complete loss of use of one of the two habitats through a process of ''migrational meltdown.'' In some situations, equilibria corresponding to a specialist and a generalist species are simultaneously stable. Analysis of our model also shows cases of hysteresis in which small transient changes in the landscape structure or accidental demographic disturbances have irreversible effects on the evolution of specialization. Phytophagous insects feeding on different plant species and parasites exploiting different hosts face conflicting selection pressures due to the heterogeneous nature of their habitats. It is therefore not surprising that the study of such systems has led to so many fruitful insights (Futuyama and Moreno 1988;Jaenike 1990;Via 1991) and raised so many theoretical questions about broad issues, such as the evolution of source-sink dynamics (Pulliam 1988;Holt 1996b) and the evolution of ecological specialization (Levins 1968;Berenbaum 1996;Fry 1996). The nature of the genetic and ecological factors that limit the niche breadth of a species in a heterogeneous habitat still remains largely an open question. In this paper, we suggest that host range evolution may be very sensitive to demographic disturbances affecting both population densities and migration patterns in a heterogeneous habitat.The present theoretical study is inspired by the dynamics and rapid evolution of a heterogeneous metapopulation of the checkerspot butterfly, Euphydryas editha Thomas et al. 1996;Boughton 1999). Before 1989, larvae of E. editha were found on two host plants. Populations on Collinsia torreyi behaved as source populations exporting migrants to pseudo-sink populations on Pedicularis semibarbata. The accidental extinction of populations on Collinsia, due to an unusual summer frost in the early 1990s, was followed by a complete reversal in source-sink dynamics and a dramatic narrowing of the butterfly diet (Boughton 1999). Since the disturbance, local populations have not regained the ability to use their former host, Collinsia. This example suggests that demographic disturbances may play an important role in the evolution of ecological specialization leading to rapid shifts in host use.Several models have explored how the interactions between demography and local adaptation may shape the nic...

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“…9a,b). This situation is very similar to the source-sink system with one-way migration described by Gomulkiewicz and Holt (1999).…”

Section: Asymmetric Equilibriasupporting

confidence: 55%

When Sources Become Sinks: Migrational Meltdown in Heterogeneous Habitats

2001

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“…23). The beneficial effect of intermediate dispersal can be stronger in onelocus models, because in the absence of recombination, dispersal only increases crowding in the new habitat (31) and flow of the advantageous mutants back to the old habitat (32) while bringing new targets for mutation and positively selected mutants. Increased dispersal can thus enhance fitness at the range edge even in the absence of inbreeding depression (cf.…”

Section: Resultsmentioning

confidence: 99%

Limits to adaptation along environmental gradients

Polechová

Barton

2015

Proc. Natl. Acad. Sci. U.S.A.

191

293

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Why do species not adapt to ever-wider ranges of conditions, gradually expanding their ecological niche and geographic range? Gene flow across environments has two conflicting effects: although it increases genetic variation, which is a prerequisite for adaptation, gene flow may swamp adaptation to local conditions. In 1956, Haldane proposed that, when the environment varies across space, "swamping" by gene flow creates a positive feedback between low population size and maladaptation, leading to a sharp range margin. However, current deterministic theory shows that, when variance can evolve, there is no such limit. Using simple analytical tools and simulations, we show that genetic drift can generate a sharp margin to a species' range, by reducing genetic variance below the level needed for adaptation to spatially variable conditions. Aided by separation of ecological and evolutionary timescales, the identified effective dimensionless parameters reveal a simple threshold that predicts when adaptation at the range margin fails. Two observable parameters determine the threshold: (i) the effective environmental gradient, which can be measured by the loss of fitness due to dispersal to a different environment; and (ii) the efficacy of selection relative to genetic drift. The theory predicts sharp range margins even in the absence of abrupt changes in the environment. Furthermore, it implies that gradual worsening of conditions across a species' habitat may lead to a sudden range fragmentation, when adaptation to a wide span of conditions within a single species becomes impossible.W hy a species' range sometimes ends abruptly, even when the environment changes smoothly across space, has interested ecologists and evolutionary biologists for many decades (1-8). Haldane (2) proposed that, when the environment is spatially heterogeneous, a species may be unable to adapt and expand its range because gene flow from the center swamps the populations at the range margins, preventing their adaptation. Theory showed that, when genetic variance is fixed, adaptation indeed fails if the environment changes too steeply across space (9), and a sharp margin to the species' range forms. The population remains well adapted only in the center of the range, and gene flow swamps variants adapted to the margins, preventing range expansion. This result also elucidates range margins in the presence of competitors: then, interspecific competition in effect steepens the environmental gradient (10). However, this limit to adaptation assumes that local genetic variation is fixed. Current deterministic theory states that, when genetic variance can evolve, there is no sharp limit to a species' range (11). The genetic mixing caused by gene flow inflates the genetic variance and facilitates further divergence. Gene flow across a phenotypic gradient maintained by the environment can generate much more variance than would be maintained by mutation alone (12,13). This rise of genetic variance with environmental gradient can allow species to adapt to ...

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“…Theoretical studies have clarified the manifold effects of movement on the likelihood of niche evolution (73,76,77). The rate of immigration influences population size in the sink; depending on the nature of density dependence, immigration can hamper adaptation (78) or facilitate it (Allee effects; ref. 79).…”

Section: The Evolutionary Nichementioning

confidence: 99%

Bringing the Hutchinsonian niche into the 21st century: Ecological and evolutionary perspectives

Holt

2009

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

755

717

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G. Evelyn Hutchinson more than a half century ago proposed that one could characterize the ecological niche of a species as an abstract mapping of population dynamics onto an environmental space, the axes of which are abiotic and biotic factors that influence birth and death rates. If a habitat has conditions within a species' niche, a population should persist without immigration from external sources, whereas if conditions are outside the niche, it faces extinction. Analyses of species' niches are essential to understanding controls on species' geographical range limits and how these limits might shift in our rapidly changing world. Recent developments in ecology and evolutionary biology suggest it is time to revisit and refine Hutchinson's niche concept. After reviewing techniques for quantifying niches, I examine subtleties that arise because of impacts species have on their own environments, the density-dependent modulation of how individuals experience environments, and the interplay of dispersal and temporal heterogeneity in determining population persistence. Moreover, the evolutionary record over all time scales reveals a spectrum of rates of change in species' niches, from rapid niche evolution to profound niche conservatism. Substantial challenges revolving around the evolutionary dimension of the Hutchinsonian niche include quantifying the magnitude of evolved intraspecific and clade-level variation in niches and understanding the factors that govern where along the spectrum of potential evolutionary rates any given lineage lies. A growing body of theory provides elements of a conceptual framework for understanding niche conservatism and evolution, paving the way for an evolutionary theory of the niche.Allee effects ͉ ecological niche ͉ landscape texture ͉ niche conservatism ͉ niche evolution T he niche concept is central to ecology (1, 2). G. EvelynHutchinson in a seminal essay Ͼ50 years ago (1) proposed a formalization of the niche that still resonates in the minds of ecologists. After years of quiescence, there has been a recent upsurge of interest in species' niches (2, 3), driven in part by the urgency of predicting ecological responses to rapid environmental change. In this article, after summarizing Hutchinson's niche concept and sketching empirical approaches to quantifying niches, I will point out limitations in standard articulations of the Hutchinsonian niche. Some arise from its focus on a species solely when it is rare, which can belie the impact of positive density dependence and feedback processes on population persistence; others arise from its relative neglect of temporal variation and spatial dynamics; and yet others arise from genetic variation and evolution in the niche itself. These reflections will lead to alternative and complementary niche definitions, enriching and extending Hutchinson's concept.

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The Effects of Density Dependence and Immigration on Local Adaptation and Niche Evolution in a Black-Hole Sink Environment

Cited by 201 publications

References 17 publications

When Sources Become Sinks: Migrational Meltdown in Heterogeneous Habitats

When Sources Become Sinks: Migrational Meltdown in Heterogeneous Habitats

Limits to adaptation along environmental gradients

Bringing the Hutchinsonian niche into the 21st century: Ecological and evolutionary perspectives

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