Transcript Communities
Populations & Communities I. Population Ecology A. Density B. Factors affecting density II. Community Ecology A. Trophic Relationships 1) Trophic pyramid 2) Food web B. Structure & Local Species Assemblages C. Species Interactions Population Ecology A. Population density = # of individuals of a species per unit area or volume Intrinsic rate of increase = maximum rate at which a population of a given species can increase under ideal conditions Factors affecting density 1) Density dependent factors 2) Density independent factors Governing forces: • Species interactions (comp/pred) • Abiotic factors (tolerances) • Historical factors ~ dispersal 2-5x increase in densities of Batrachoseps attenuatus on islands in SF Bay over those on mainland due to lower predation pressure on the islands P. teyahalee P. metcalfi Chihuahuan Desert - precipitation ranges from 50 to 400 mm Lizard species’ densities studied over 5 year period, exhibited fluctuation correlated with precipitation. Schall & Pianka 1978 Compared Aus & N.A. Turtle & frog richness pos. correlated to annual rainfall, neg. mean annual hrs sunshine Lizards had opposite pattern Snakes positively correlated w/ mean annual temp & rain Community ecology Community = All of the organisms that inhabit a particular area; an assemblage of populations of different species living close enough for potential interaction A. Trophic Relationships 1) Trophic pyramid 2) Food web B. Structure & Local Species Assemblages C. Species Interactions Structuring forces Gause’s Principle 1960’s & ’70’s: Competition structures communities Ecological niche Current view: physical environment predicts composition trophic pyramid trophic structure trophic levels quaternary tertiary secondary consumer primary consumers primary producers Detritovores - decomposers Guild = energy flow through ecosystems: much is lost at each trophic level 2) food web: The structure of a community & assemblages 1. Structure = species diversity, species richness – Diversity = – Richness = – Species assemblages - Subset of species being considered in a community • General trends Gradients in Species Richness Latitude/ Altitude – more species found lower (taxon. widespread) Tropical lowland herp diversity peaks around 150-200 Some exceptions (salamanders, turtles, lizards(!)) Why the trends? 1) Climactic features vary with increases in latitude 2) Habitat heterogeneity HABITAT HETEROGENEITY • The greater the range of environmental conditions, the more kinds of topography, soil conditions, microclimate and habitat – the greater the heterogeneity, the greater the biodiversity of a landscape – Physical or Biotic – Spatial or Temporal – Fixed or Dynamic • Higher heterogeneity = What drives these gradients? Many proposed mechanisms: Productivity Historical (lag) Structural diversity Climate (seasonality) Pattern, Process, Mechanism Mechanism- factors affecting individuals (e.g. competition) Process- population-level effects of individual interactions (neg b/w species) Pattern- community level (presence/ absence, resource/microhabitat use, etc.) PPM, cont’d Cause and effect relationships among these 3 levels often assumed Attribute negatives between 2 spp to using same prey (interference) or aggression (exploitative) However, different mechanisms can produce indistinguishable patterns PPM, cont’d Differences in prey use b/w spp competition, or independent evolution? Process may be difficult to detect Resource-limited years (Dunham 1980) ‘Ghost of competition past’ • Differentiation, displacement Species Interactions - 1) Competition 2) parasitism predation S1 - S2 + S1 S2 - 3) mutualism + S1 S2 + 1) Interspecific Competition for limited resource • Interactions lead to either decrease in abundance or some component of fitness • the 2 species diverge in their use so the co-existence is possible Interspecific competition results in microhabitat differences • Microhabitat parapatry results from intense competition between P. cinerus & P. shenandoah • P cinerus, more aggressive – P. shenanadoah becoming restricted to dry slopes Species Interactions: Competition Williams (1983) examined microhabitat use of 9 Anolis Sun/shade Perch diameter Perch height Reduces interactions – but a result of comp? Ex: Anolis lizard sp. perching sites in the Dominican Republic Alternative explanations? Environmental tolerances (T, water loss) Preferred prey Other researchers have shown determinism in Anolis composition on islands Determinants of Community Structure Often many interacting factors both abiotic and biotic Competition in Caribbean Anolis: 2-9 species on islands; few predators, lizards abundant- resources limiting Interspecific competition may be alleviated by resource partitioning Anolis competition Removal of an aggressive species showed habitat expansion by another (Jenssen 1973) Enclosure experiments revealed greater partitioning b/w a species pair from a more resource-limited island (Pacala & Roughgarden 1985) Predation • Negatively affects prey (they are consumed) • Most predators feed on more than one prey species (dependent upon abundance) Predator and prey populations follow a series of synchronized fluctuations The prey population grows exponentially, and reproduction in the predator population is a function of the number of prey consumed As a single predator population increases, the single prey population decreases to a point at which the trend is reversed The two populations rise and fall, oscillating in a predictable manner Lotka-volterra model -simplest model of predator-prey interactions, predict greater stability w/more “links” Species Interactions: Predation Predation- easier to establish than c Often, more abundant prey species are taken – modify community structure Submergent behavior- prey respond by reducing activity Restrictive activity times, places promote prey-switching in predator mediate competitive coexistence Species Interactions: Parasitism ~1/2 all animals are parasites often specialized; differential effects Shift P-P, competitive interactions Rarely studied at the community level Pathogens may cause local extinction: B. boreas in Colorado by Aeromonas Abiotic factors: Habitat complexity Pianka (1967) Plant height diversity best predicted species richness of flatland desert lizards Abiotic factors: Physiological tolerances Each species has different tol/ pref Trade-offs Distinct vs. shared preferences Temp, precipitation, ET are often correlated with a species’ range, and its abundance therein Weather-induced community changes Whitford & Creusere (1977) – fecundity and composition in Chihuahuan Desert lizard community enhanced in wet years Pechmann et al. (1989) # species metamorphosing from ponds related to hydroperiod Longer-lived spp’s may endure poor weather conditions better Anthropogenic effects Dispersal: Polynesians– gecko introduction Anglers– introduced salamander bait Seri- Sauromalus Landscape changes: Maya cultivated much of Yucatan, favoring open-habitat species Local scale: Single-tree harvest in Amazonia promoted heliothermic lizards; forest species retreated (Vitt et al 1998) Prey of Brazilian Colubridae Vitt & Vangilder (1983) explained resource-use with present-day ecological factors Cadle & Greene (1993) assert that much of observed prey use was due to ancestry No invertebrate specialists Vitt & Vangilder (1983) Too much competition w/ insectivorous mammals, too many small snake predators, suitable microhabitat lacking Cadle & Greene (1993) Insect eating snakes are rare/ absent in neotropics Anuran-eating snakes are speciose V&V: Convergence on this resource due to frog abundance and year-round availability C&G: Large proportion of the frog-eaters have common ancestry Salamander competition Hairston (1949) Distributional patterns in GSM suggest interspecific comp Density manipulations confirmed geog variation in degree of competition Larval amphibian assemblages Extremely dynamic systems Size-dependent predation/competition Composition turnover Hydroperiod; nutrient flux Spatial partitioning In Thailand, different tadpole species use distinct aquatic zones Heyer (1973) Trade-offs- puddle vs. pond Selective forces differ depending on larval environment Fast-drying ponds- promote quick development Permanent ponds- predatory contingent promotes submergent behavior Predation & Competition 6 tadpole species at different newt densities (Morin 1983) Low predation, 4 comp dominant species suppressed 2 inferior tadpoles High predation, inferiors survived better 3 species developed faster with heavy predation Priority Effects Competitors/ predators arriving first can influence late-comers Hyla pseudopuma avoids ovipositing where predators/competitors are present Ambystoma opacum oviposit in fall If they survive winter, they feast on other amphibian larvae Tropical anuran assemblages Up to 80 anuran spp in sympatry Diverse reproductive modes Aquatic larvae Eggs carried on back (terr & aq) Oviposit on land, carry eggs to water Oviposit on leaves over water Direct development Why does aquatic egg & larval development seem to be avoided? Magnusson & Hero (1991) Looked at water quality, desiccation, predation and competition Experiments revealed heavy predation by other anuran larvae and invertebrates probably favors more terrestrial reproductive modes Vocalization partitioning Tropical choral aggregations may prohibit effective sexual selection Responses to distinct environmental cues among species may affect species at a breeding site Temporal partitioning Call-character displacement: Allopatric species pairs have more similar calls than in sympatry Selection favors species recognition Mutualism • Beneficial symbiosis – 2 species dependent upon each other and both benefit from the association • Examples: Red-backed Salamanders help make soil better for plants and animals when they tunnel through it. Nutrients in the soil get mixed and plants can pull them into their roots more easily. Small animals such as mites and beetles find it easier to move around in the soil.